ultrahigh pressure metamorphism || orogenic garnet peridotites

16Orogenic Garnet Peridotites:Tools to Reconstruct Paleo-Geodynamic Settings of FossilContinental Collision Zones

Cong Zhang1,2, Herman van Roermund2 and

Lifei Zhang1

1Key Laboratory of Orogenic Belts and Crustal Evolution, School ofEarth and Space Sciences, Peking University, Beijing, China2Department of Earth Sciences, Structural Geology and Tectonics,Utrecht University, Utrecht, The Netherlands

16.1 Introduction

Twenty-five years ago, the high-pressure (HP) SiO2 polymorph coesite was found

for the first time in metamorphic crustal rocks (Chopin, 1984; Smith, 1984). Since

then many other ultrahigh-pressure (UHP) metamorphic index minerals were found

in eclogite- and/or garnet peridotite-bearing UHP terranes around the world

(Sobolev & Shatsky, 1990; Yang et al., 1993, 1994; Dobrzhinetskaya et al., 1995;

Van Roermund & Drury, 1998; Zhang et al., 2000; Song et al., 2004). These UHP

terranes record geodynamic environments in which lithospheric subduction and

exhumation processes, driven by plate collision, mantle convection and/or buoy-

ancy, provide us with a natural laboratory to study different types of interaction

between crust and mantle. Two fundamentally different types of subduction zones

have been proposed in the literature, here called Alpine and Pacific type (Ernst,

1988, 2001; Maruyama et al., 1996; Liou et al., 2004). Pacific-type subduction

zones involve collisions between two oceanic plates or an oceanic and a continental

plate. In contrast, the Alpine type refers to collisions between two continental

plates, often predated by subduction of an oceanic plate (Coleman & Wang, 1995).

Fossil Pacific-type subduction zones are dominated by ophiolite melanges, island-arc

magmatic rocks, and deep-sea sediments, mostly metamorphosed during subduction

into relatively low-temperature and (U)HP metamorphic rocks, including perido-

tites. Examples can be found in the European western Alps and the North Qilian

and Western Tianshan UHP belts in western China (Chopin, 1984; Philippot &

Ultrahigh-Pressure Metamorphism. DOI: 10.1016/B978-0-12-385144-4.00015-1

© 2011 Elsevier Inc. All rights reserved.

http://dx.doi.org/10.1016/B978-0-12-385144-4.00015-1

Van Roermund, 1992; Zhang, L. et al., 2003; Song et al., 2006). In contrast, fossil

Alpine-type continental subduction/collision zones are dominated by granitic/tona-

litic gneiss with minor intercalated metapelite, eclogite, and/or peridotite of (U)HP

origin, for example the WGR in southwestern Norway and the Dabie�Sulu and

North Qaidam orogens in China (Krogh, 1977; Yang et al., 1994; 2002; Zhang

et al., 2004a).

Although volumetrically minor, orogenic peridotites (especially garnet-bearing

peridotite) play an important role in understanding the physical and chemical pro-

cesses, including metasomatism, mantle�crust interactions, and fluid/melt�rock

reactions that take place along convergent plate boundaries (Brueckner, 1998;

Brueckner & Van Roermund, 2004; Scambelluri et al., 2008; Spengler et al.,

2009). Garnet-bearing peridotite occurs as a few to numerous lensoid masses within

HP and/or UHP continent�continent collision zones, including the Caledonian,

Variscan, and Alpine orogens in Europe, the early Paleozoic Kokchetav Massif in

Kazakhstan, and the Triassic Dabie�Sulu orogen and early Paleozoic North

Qaidam orogen in China (Yang et al., 1994; Medaris, 1999; Brueckner & Medaris,

2000; Brueckner & Van Roermund, 2004; Song et al., 2004; Zhang et al., 2004b).

How these deep-seated mantle rocks were transported to shallow crustal levels,

including how they became incorporated in subducted continental slabs, remains

intriguing. The crustal emplacement mechanism of mantle-derived garnet peridotite

has been traditionally described by deep-level ductile imbrication of lower parts of

the continental crust (Cuthbert et al., 1983; Cuthbert & Carswell, 1990; Medaris &

Carswell, 1990). This mechanism may explain crustal emplacement of orogenic

spinel peridotite, but it does not apply to garnet peridotite present in deeper levels

of the subcontinental lithospheric mantle (SCLM; Van Roermund, 2009b).

The recognition of diamond and subduction-related majoritic garnet in garnet

peridotite and its host continental crust demonstrated that, during collisional

tectonics, continental subduction can be deep enough to reach the garnet stability

field in the overlying mantle wedge above a continental subduction zone (Ye et al.,

2000; Van Roermund et al., 2002). Subsequent exhumation of the subducted conti-

nental crust, driven by buoyancy after oceanic slab break off, can bring the garnet

peridotite, after its incorporation in the subducted continental crust (Brueckner,

1998), to lower crustal levels where it may be exposed after erosion. This type of

garnet peridotite is called mantle wedge garnet peridotite (Van Roermund, 2009b),

replacing the relict peridotite subtype of Brueckner and Medaris (2000).

Alternatively, garnet peridotite formed by prograde metamorphism of lower-

pressure ultramafic protoliths (mafic/ultramafic intrusions or Fe�Ti type peridotite

spinel peridote, spinel peridote and/or serpentinite), will be called subduction zone

garnet peridotite (Figure 16.1A).

During the continental collision and subduction process, when the mantle wedge

garnet peridotite becomes incorporated in the subducted continental crust, minerals,

structures, textures, mineral fabrics, and geochemical characteristics of the mantle

wedge garnet peridotite may become partly or completely overprinted by features

typical of the subduction zone type. The latter, when complete, may obviously

erase all former mantle wedge evidence.

502 Ultrahigh-Pressure Metamorphism

In this chapter, we propose a simplified conceptual model that is able to illus-

trate the mantle and/or subduction zone evolution of an orogenic garnet peridotite.

Moreover, the model predicts four fundamentally different types of mantle wedge

garnet peridotite. The latter can be used to reconstruct the Paleo-geodynamic set-

ting of an ancient orogenic collision belt. Recognition of the mantle wedge subtype

involved in the fossil continental collision process can be achieved by making a

study of the field, petrological, mineralogical, structural, geochemical, and/or isoto-

pic characteristics of the orogenic garnet peridotite. To illustrate this model and to

test the applicability of the proposed geodynamic classification model, we present

published petrological, mineralogical, geochemical, and/or isotopic characteristics

of some well-known garnet peridotites from the Scandinavian Caledonides and the

Figure 16.1 Diagrams illustrating the two fundamentally different types of orogenic garnet

peridotite in nature. (A) Genetic diagram illustrating the two possible origins of the protolith

of a subduction zone (garnet peridotite) type; The term mantle wedge type is used here only

for garnet-olivine bearing assemblages. (B) Range of P�T conditions in which the

metamorphic garnet�olivine mineral assemblage of a mantle wedge garnet peridotite is

formed (see text for further explanations). Four different geodynamic settings, roughly

outlined by boxes A, B, C, and D, are outlined and further described in the text. (C) Range

of P�T conditions in which the metamorphic garnet�olivine mineral assemblage of a

subduction zone garnet peridotite is formed/reequilibrated. Shown for reference are

equilibria for coesite�quartz (Bohlen & Boettcher, 1982) and diamond�graphite (Bundy,

1980). Also shown are a dry lherzolite solidus (Hirschmann, 2000), the spinel�garnet

transition (O’Hara et al., 1971), the 100 and 200 km thick continental lithosphere geotherms

(Medaris 1999) and the approximate position of the 1% majoritic garnet-in phase boundary

line (Van Roermund 2009a).

503Orogenic Garnet Peridotites

Sulu�Dabie and North Qaidam orogenic belts in China. Finally, in Section 16.4,

we present the results of this test.

16.2 A Conceptual Model Illustrating How Orogenic GarnetPeridotite Can Be Used to Reconstruct FossilGeodynamic Environments

Orogenic garnet peridotites vary in structure, texture, mineralogy, chemistry, and

age from orogen to orogen, within an orogen, and even within an individual terrane

(Brueckner & Medaris, 2000). There are many reasons for these variabilities: (1)

derivation from different mantle sources; (2) perturbations by fluids/melts from

subducting oceanic/continental crust; and (3) different types of continental host

rocks after incorporation in the continental crust. Based on research performed in

several orogenic systems around the world, numerous origins and/or classification

systems for garnet peridotite in (U)HP belts have been proposed in the last decade

(Brueckner & Medaris, 1998, 2000; Medaris, 1999; Zhang et al., 2000).

Detailed analysis of the physical and chemical processes involved in the forma-

tion and exhumation (eduction) of orogenic garnet peridotite indicates a predomi-

nance of the following major processes: (1) formation and evolution of an SCLM;

(2) a process responsible for the formation of a mantle wedge geometry positioned

above a subducting continental crust; (3) a crustal emplacement mechanism that

brings the mantle wedge garnet peridotite into the subducting continental crust; (4)

processes related to exhumation (eduction) of the previously subducted continental

crust, including its garnet peridotite cargo, back to subcrustal levels; and (5) inter-

actions with subduction zone fluids/melts. Evidence of all these major geodynamic

processes are simply stored (and can be recognized) in the geological record of an

orogenic garnet peridotite, provided detailed field and additional laboratory studies

are carried out to elucidate the structural, mineralogical, textural, chemical, spatial,

and temporal characteristics.

In our conceptual model, we have included the major physical (and chemical)

processes described previously by grouping them into the following two geody-

namic scenarios:

1. A process/processes leading to formation of an SCLM that predates (.200 Ma) the plate

convergence processes responsible for scenario 2.

2. A continental collision�subduction scenario that leads to the formation of an SCLM

wedge positioned above a continental subduction zone, crustal incorporation of the man-

tle wedge garnet peridotite, and subsequent exhumation.

These two principle geodynamic scenarios are schematically illustrated in the

P�T diagrams of Figure 16.1B and C. Figure 16.1B illustrates the range of P�T

conditions that can be expected to be operative in a garnet peridotite that is formed

during formation of an SCLM, assuming lithospheric mantle formation and growth

took place by rising hot asthenospheric mantle. In contrast, Figure 16.1C illustrates

the range of P�T conditions that are operative within a continental subduction


zone, assuming the protolith of a subduction zone garnet peridotite consists of a

crustal type that is progressively metamorphosed in a continental subduction zone

(Figure 16.1A).

The wide spectrum of physical conditions, recorded in terms of individual P�T

conditions, as well as P�T trajectories when their mutual temporal evolution is

taken into account, all represent recorded P�T conditions preserved within individ-

ual garnet peridotite bodies worldwide. The main advantage of our conceptual

model is thus that the P�T conditions recorded in Figure 16.1B and C can be

reconstructed using standard petrochemical and/or mineral�chemical techniques

applied to individual garnet peridotite bodies.

The two fundamentally different garnet peridotite types are also incorporated in

our model: mantle wedge garnet peridotite and subduction zone garnet peridotite

(Figure 16.1A). Mantle wedge garnet peridotite may contain garnet-bearing mineral

assemblages that belong to either (1) SCLM formation events predating the geolog-

ical event responsible for mantle wedge formation and/or continental subduction

(Figure 16.2A1�D1) or (2) rising asthenosphere intruding SCLM positioned above

subducting continental crust formation of the mantle wedge geometry, that is, when

the SCLM becomes positioned above subducting continental crust (Figure 16.2B2

and D2).

Moreover, mantle wedges overlying subducting continental crust may be subdi-

vided into four subtypes (endmembers) due to different types of lithosphere and/or

different thermal structures present in such mantle wedges (Figure 16.1B, squares

A, B, C, and D; see the following section for further explanation). Alternatively,

subduction zone garnet peridotite will contain geochemical, mineralogical and ther-

mal signatures diagnostic of subduction-related processes (Figure 16.1C).

16.2.1 Mantle Wedge Garnet Peridotite

In our model, mantle wedge garnet peridotite forms part of an SCLM produced by

hot rising asthenosphere that subsequently, cooled down isobaricly to form subcon-

tinental (SCLM1) or subcratonic (SCLM2) lithosphere (Figure 16.1B). Such mantle

wedge garnet peridotite can at some point in time be trapped by a subducting

crustal slab that, after oceanic slab break off, educts back to lower crustal levels.

P�T conditions recorded in such mantle wedge garnet peridotites may represent

lithosphere conditions at the time when this lithospheric mantle was formed, or

may represent other geological events that predate the upcoming continental colli-

sion event by hundreds of millions of years (Brueckner et al., 2010). Alternatively,

“young” oceanic or continental lithosphere can form when hot asthenosphere rises

to the base of, or into, existing lithosphere. When hot, rising asthenosphere reaches

such positions, it is no longer buoyant and cools down to become part of the litho-

sphere (Figure 16.2). P�T conditions recorded by such mantle wedge garnet peri-

dotites may thus reflect prevalent P�T conditions that were operative during

ancient lithospheric mantle growth processes (Spengler et al., 2006; Van

Roermund, 2009b). Recorded P�T conditions will strongly depend on the thickness

of the lithosphere and also on the time (t) that elapsed between “arrival” in the


Figure 16.2 Simplified conceptual model for formation and crustal incorporation of

mantle wedge garnet peridotites. The symbols A, B, C, and D correspond to boxes A,

B, C, and D illustrated in Figure 16.1. The left-hand side diagrams (A1, B1, C1, and

D1) correspond to event(s) responsible for the formation of the subcontinental/subcratonic

lithospheric mantle needed in model types A, B, C, and D. The right-hand side diagrams

(A2, B2, C2, and D2) illustrate hypothetical continental collision events during which

mantle wedge garnet peridotite (position marked by symbols) is transferred into the

subducting continental crust, c.q., can be exhumed back to subcrustal levels. Note that

in none of the diagrams reference is made to crustal type peridotites. The latter has

been left out for reasons of simplicity, but can be present theoretically in each of the

individual model types. For the same reason, the fate of the subducted oceanic

lithosphere has been ignored. (A) Old, cold and thick mantle wedge (cratonic) type

collides with “normal” continental crust. The garnet-olivine mineral assemblage of this

type of mantle wedge garnet peridotite will be old ($250 Ma) compared to the age of

the collision. (B) Young, hot and thick mantle wedge collides with “normal” continental

crust. The garnet-olivine mineral assemblage of this type of mantle wedge garnet

peridotite will be old “young” and comparable to the age of the collision event. (C)

Old, cold and thin mantle wedge (continental) type collides with “normal” continental

crust. The garnet-olivine mineral assemblage of this type of mantle wedge garnet

peridotite will be old ($250 Ma) compared to the age of the collision. (D) Young, hot

and thin mantle wedge collides with “normal” continental crust. The garnet-olivine

mineral assemblage of this type of mantle wedge garnet peridotite will be old “young”

and comparable to the age of the collision event.


lithospheric mantle and subsequent mantle wedge formation, collisional tectonics,

and continental subduction (tc250 Ma). There are major differences between the

thickness of oceanic lithosphere (B40 km), continental lithosphere (B100 km)- and

cratonic lithosphere (150�200 km). Garnet peridotite, generated at the base of a cra-

tonic lithosphere will thus store in its mineralogy P�T conditions that are much

higher in terms of pressure than those that were formed at much shallower levels—

for example, below oceanic and/or subcontinental lithosphere. On the other hand, if

there is a long time interval (tc250 Ma) between formation of a newly formed litho-

sphere and the convergent plate tectonic regime leading to development of a subduc-

tion zone, the lithosphere will have had enough time to cool down to P�T

conditions represented by a stable continental (100 km) or cratonic (200 km)

geotherm (represented by boxes A and C in Figure 16.1B). If, however, the hot, ris-

ing asthenosphere is contemporaneous with the collision/subduction process, it will

not have had sufficient time to cool down, c.q., it will still be “relatively” hot

(.1100�C) at the moment of emplacement into the subducting crustal slab (repre-

sented by boxes B and D in Figure 16.1B).

Based on the lithosphere thickness and the “mean” thermal state of the SCLM,

four domains in P�T space can thus be recognized in which a mantle wedge garnet

peridotite may have formed. Rough outlines of the expected theoretical P�T condi-

tions recorded in such mantle wedge garnet peridotite types are indicated in

Figure 16.1B as boxes A, B, C, and D. In Figure 16.1B each hypothetical P�T tra-

jectory starts with the adiabatic upwelling of hot asthenosphere intruding into an

already existing SCLM. Adiabatic temperature conditions, which may vary widely

during this stage (1350�1750�C), are simplified by the use of two distinct isother-

mal decompression trajectories at different temperatures, illustrated in

Figure 16.2 Continued


Figure 16.1B. Note also that dry decompression melting (at HPT) will take place

when the isothermal decompression path intersects the dry peridotite solidus

(Figure 16.1B), providing another easy tool that can be used to unravel the protolith

origin of a mantle wedge garnet peridotite.

After accretion to the lithosphere, subsequently followed by isobaric cooling,

four domains in P�T space can thus be recognized (boxes A, B, C, and D in

Figure 16.1B). Each domain, characterised by its own thermal evolution, represents

a geodynamic setting of a hypothetical mantle wedge garnet peridotite. If the garnet

peridotite was formed at the base of a subcratonic lithospheric mantle (SCLM2) at

depths around 150 km, the following two thermal scenarios may occur:

1. With sufficient time for the newly formed lithosphere to cool down, the P�T conditions

preserved in the garnet peridotite would be similar to the metamorphic conditions repre-

sented by a stable geothermal gradient at a certain depth underneath a craton (200 km

geotherm in Figure 16.1B; type A in Figure 16.2). The garnet-bearing mineral assemblage

would be old in comparison to the age of the continental collision/subduction event (type

A in Figure 16.1B).

2. With insufficient time to cool down before the mantle wedge garnet peridotite becomes

incorporated in a continental subduction zone, the P�T conditions recorded by the gar-

net-bearing mineral assemblage would be much higher than those recorded by a sta-

ble 200-km thick cratonic geotherm. In addition, the age of the garnet-bearing

assemblage would be close to the age of the continental subduction event (type B in

Figure 16.1B); that is, as the temperature is still high ($1200�C), the rising garnet peri-

dotite would transform into a syncollisional “diapir” (illustrated in Figure 16.2B2).

Alternatively, if the garnet peridotite was formed at the base of a “normal,” 100-

km thick, subcontinental lithosphere (boxes C and D in Figure 16.1), we can also

discriminate between the same two possibilities, giving rise to the geodynamic

scenarios illustrated by model types C and D in Figure 16.2.

Altogether four geodynamic environments for the formation of a mantle wedge

garnet peridotite, outlined by boxes A, B, C, and D in Figure 16.1B, are illustrated

in the conceptual model of Figure 16.2 (model types A, B, C, and D). Each model

type (A, B, C, or D) in the cartoons of Figure 16.2 represents a geodynamic sce-

nario that corresponds to a mantle wedge type outlined by boxes A, B, C, and D in

Figure 16.1B. Each geodynamic scenario (model types A, B, C, and D in

Figure 16.2) involves a lithosphere formation event (left side of Figure 16.2) fol-

lowed by a continental collision event (right side of Figure 16.2). The simplest way

for a mantle wedge garnet peridotite to become trapped by subducting continental

crust is for continental crust to follow adjacent oceanic crust down to an established

subduction zone, possibly as a result of pulling from the dense oceanic lithosphere

(Brueckner & Medaris, 2000). This mechanism provides an opportunity for garnet

peridotite to be transferred from the mantle wedge into the subducting crust at vari-

ous depths and then be carried to deeper levels as the crustal slab continues to sub-

duct. Alternatively, the garnet peridotite may be tapped on the way back to the

surface. In addition, the mineralogy, microstructure, structure, and geochemistry of

the garnet peridotite may be modified to varying degrees by recrystallization and

deformation processes, either during its introduction into the subducting continental


plate or during its subsequent evolution in the continental crust (Dijkstra et al.,

2004). The mineralogy and chemistry may also be altered by interaction with

crustal components, particularly hydrous fluids, so that the mantle wedge garnet

peridotite lens may be fully transformed into a hydrous mineral assemblage (e.g.,

serpentinite, chlorite peridotite, and so on).

16.2.2 Subduction Zone Garnet Peridotite

Subduction zone garnet peridotite acquired their garnet-bearing mineral assemblage

as a result of prograde subduction postdating the age of their low-pressure mineral

assemblage. The protolith of subduction zone garnet peridotite can be either pro-

duced by differentiation from mafic magma of lower crustal affinity or are serpenti-

nite, plagioclase or spinel peridotite formed at shallow depths in the lithosphere.

Typically, this kind of garnet peridotite may be interlayered with eclogite of various

compositions that share an evolution similar to other components of the subducting

crustal slab. Some subduction zone garnet peridotites contain well-preserved pro-

grade, lower-pressure mineral assemblages as inclusions in HP and UHP phases.

For example, inclusions of sapphirine, corundum, clinochlore, and amphibole occur

in garnet porphyroblasts from the Maowu area of the Dabie orogen in China (Okay,

1993; Liou & Zhang, 1998). The Fe�Ti type of garnet peridotite in the WGR also

belongs to this type (Carswell et al., 1983; Jamtveit, 1987; Vrijmoed et al., 2006).

The age of the garnet-bearing assemblage formed in a subduction zone garnet perido-

tite is always approximately equal to the age of the collision event leading to subduc-

tion. The restricted P�T conditions recorded by the mineralogy of subduction zone

garnet peridotite: can be used to discriminate the prograde subduction zone type from

a syncollisional mantle wedge garnet peridotite type (Figure 16.2B2 and D2) which,

by definition, must have had a much higher P�T evolution.

16.3 A Test of the Feasibility of the Model in Scandinaviaand China

It is possible to test the feasibility of our conceptual model (Figures 16.1 and 16.2)

using field and laboratory data. In order to do so, we have chosen three well-

studied garnet peridotite-bearing (U)HP terranes in Scandinavia and China. P�T

conditions, ages of garnet-bearing mineral assemblages, and isotopic data from for-

mer studies performed on these garnet peridotites will be used to make a compari-

son with the predicted theoretical P�T conditions illustrated in Figure 16.1B

and C. The literature data used for the text are presented in Tables 16.1 and 16.2.

Note that the symbols used in Tables 16.1 and 16.2 also correspond to those used

in Figures 16.4 and 16.6. If the published geological data are in agreement with our

conceptual model, it is taken as first-order evidence that our conceptual model can

be used to reconstruct the geodynamic settings of an orogenic garnet peridotite.


Table 16.1 Summary Illustrating Location Name, Age, and P�T Conditions of Orogenic

Garnet Peridotites in the Scandinavian Caledonides

Location Symbol Age T (�C) P (GPa) References

Western Gneiss RegionOtrøy,

Flemsøy

M1a Archean ca. 1800 .12 Van Roermund (2009b)

Otrøy,

Flemsøy

M1b Archean ca. 1500 ca. 4

Kalskaret M2c Proterozoic 891 4.4 Medaris (1984, 1999)

Aldalen Proterozoic 898 4.94

Lien Proterozoic 877 3.56

Sandvika Proterozoic 947 4.42

Raudhaugene Proterozoic 885 4.28

Rødskar Proterozoic 871 4.37

Ugelvik Proterozoic 708 3.65

Kalskaret Proterozoic 949 4.16 Jamtveit et al. (1991);

Medaris et al. (1999)

Lien Proterozoic 895 3.91 Medaris (1980);


Rødhaugen Proterozoic 850 3.21 Carswell (1981);


Rødhaugen Proterozoic 737 2.73



Sandvika Proterozoic 934 5.02 Jamtveit (1984);


Raudhaugene Proterozoic 744 2.32 Carswell (1986);


Ugelvik Proterozoic 789 2.78

Raudhaugene M3c Caledonian 668 1.36 Jamtveit et al. (1991);


Fjørtoft Caledonian 771 2.6

Fjørtoft Caledonian 680 1.8 Krogh and Carswell (1995);


Bardane M2a Proterozoic 1300�1500 3�4.5 Van Roermund et al.

(2002);

Brueckner et al. (2002)

Bardane M3a Caledonian 840�900 3.4�4.1

Bardane M2b Proterozoic 1410 3.2 Carswell and

Van Roermund (2005)

M3b Caledonian 8756 25 4.16 0.2

M3d Caledonian 840�900 3.4�4.5 Van Roermund (2009a)

Caledonian 850�950 5.5�6.5

(Continued)


16.3.1 Orogenic Garnet Peridotites in the Scandinavian Caledonides

The Caledonides in Scandinavia are a deeply eroded, early to mid-Paleozoic orogen

that evolved during opening and closure of the northern Iapetus, though early phases

of closure may have involved a different ocean (Ægir Sea; Hartz & Torsvik, 2002). It

was formed as a result of a Paleozoic continent�continent collision between

Laurentia and Baltica (forming Laurussia). The mountain belt consists of a pile of tec-

tonic nappes called allochthons (Figure 16.3), translated eastwards onto the continen-

tal margin of Baltica. Baltica underthrusted Laurentia, resulting in the thrusting, or

renewed thrusting, of allochthons to the east over the Baltic shield (Roberts & Gee,

1985; Brueckner & Van Roermund, 2004). The Baltic shield is exposed in windows

through the allochthons, in which basement complexes of high-grade gneisses and

supracrustal rocks are exposed. Most of these complexes give Precambrian crystalli-

zation ages, similar to those in the Baltic shield that is exposed along the eastern side

of the allochthons. However, some complexes, especially those in the west, also give

Table 16.1 (Continued)


Bardane M3f1 Caledonian 750�800 4�4.5 Scambelluri et al.

(2008, 2010)Bardane M3f2 Caledonian 1000 6.5�7

Otrøy M2d1 Proterozoic 740�770 3.3�3.7 Spengler et al. (2009a)

Fjørtoft M2d2 Proterozoic 700�750 2.6�3.1

Otrøy,

Flemsøy

M3e1 Caledonian 870�920 5.9�6.5

Otrøy,

Flemsøy

M3e2 Caledonian 820�880 3.8�4.0

Svartberget Fc1 Caledonian 800 3.4 Vrijmoed et al. (2006)

Svartberget Fc2 Caledonian 800 5.5

Eiksunddal Fc3 Caledonian 750 2 Jamtveit (1987)

Seve NappeErtsekey Sc1 Caledonian 623 1.99 Van Roermund (1989);

Medaris (1999)

Tjeliken Caledonian 676 1.95

Tjeliken Caledonian 796 2.13

Jamtland Sp Proterozoic 690�800 1.1�1.6 Brueckner et al. (2004)

Jamtland Sc2 Caledonian 700�800 2.0�3.0

Lindas NappeBergen Arcs Lp Proterozoic 840�992 Kuhn et al. (2000)

Bergen Arcs Lc Caledonian 650�700 1.6�2.1 Austrheim and Griffin

(1985)

Tromsø NappeTromsø T Proterozoic 675 1.4 Ravna et al. (2006)

Tromsø Tc Caledonian 740 .2.4


Caledonian recrystallization ages representing Baltica remobilized to varying degrees

by Caledonian metamorphic events (Figure 16.3), that is, the WGR and Lofoten. The

allochthons are usually subdivided into four parts: lower, middle, upper, and upper-

most allochthons (Figure 16.3). Slices of Baltica occur in the lower and middle

allochthons as imbricated thrust sheets of Precambrian crystalline rocks and upper

Proterozoic and/or lower Paleozoic platform sediments deposited on top. The two

overlying allochthons (upper and uppermost) are more complicated and probably rep-

resent composite terranes that were assembled during closure of Iapetus prior to the

Scandian Orogeny (430�390 Ma). The upper allochthon, which is a composite of tec-

tonic units, is derived from both Iapetus and the outermost edge of Baltica. In

Sweden it is called the Seve�Koli Nappe complex and records the evolution of the

Iapetus�Baltica margin. Its uppermost part, the Koli Nappe complex, is composed of

ophiolites, volcanic-arc terranes, marginal basins, and the like and records part of the

Table 16.2 Summary Illustrating Location Name, Age, and P�T Conditions of Orogenic

Garnet Peridotites in Sulu�Dabie and North Qaidam Orogenic Belts, China


Sulu�DabieRongcheng R1 Triassic 820�900 4�6 Zhang et al. (1994);

Hiramatsu and Hirajima

(1995)

Rizhao � Triassic .820 .3 Zhang et al. (1994)

Junan J Triassic 850 4 Zhang et al. (1995a)

Maobei � Triassic 750 .3.6 Zhang et al. (1994, 2000)

Xugou X1 Triassic 770 6.4 Zhang et al. (2000)

X2 Triassic 7551 33 5.46 0.3 Spengler et al. (2009b)

Triassic 7321 33 4.36 0.3

C3 Triassic 780�870 5�7 Zhang, R.Y. et al. (2003)

Zhimafang Z1 Triassic 800�900 4�6.5 Yang et al. (1993);

Zhang et al. (1994)

Z2 1000 5.1 Yang and Jahn (2000)

Z3 Triassic 760 4.2

Z4 800 5 Yang et al. (2007)

� .850 ,5 Zhang et al. (2008)

Z5 Triassic 800 6.8

Z6 Triassic 810�880 5.6�6.1 Ye et al. (2009)

Bixiling B Triassic 820�970 4.7�6.7 Zhang et al. (1995a, 2000)

Maowu M Triassic 7501 50 4�6 Liou and Zhang (1998);

Zhang et al. (1999)

Yangkou � Triassic 750�850 .2.7 Zhang et al. (2000)

� Triassic 7501 50 .4 Zhang et al. (2004a)

North QaidamLuliangshan L1 960�1040 5�6.5 Song et al. (2004, 2005b)

L2 800�1000 4.6�6.6 Song et al. (2007, 2009a,b)

L3 700 3�3.5 Yang and Powell (2008)


closure history of eastern Iapetus (Stephens & Gee, 1989). The uppermost allochthon

is composed of units, including thick carbonate complexes and crystalline basement,

which are generally interpreted as the eastern margin of Laurentia, as well as terranes

that accreted to this margin during closure of western Iapetus, that is, the Tromsø

Nappe complex (Krogh et al., 1990; Pedersen et al., 1992; Melezhik et al., 2000;

Yoshinobu et al., 2002; Ravna & Roux, 2006).

Garnet peridotites have been reported from four major terranes in the

Scandinavian Caledonides. Garnet-bearing peridotite localities are summarized in

Figure 16.3 and comprise: (1) the WGR, a basement window exposed along the west

coast of southwest Norway; (2) the northern Jamtland terrane, of central Sweden

(upper allochthon); (3) the Tromsø Nappe in the North (uppermost allochthon); and

(4) the Lindas Nappe of the Bergen Arcs in the south (middle allochthon).

16.3.1.1 Western Gneiss Region

The WGR is the lowermost tectonic unit of the Scandinavian Caledonides and

interpreted to form the western, outermost edge of Baltica during Scandian conti-

nental collision (Krogh, 1977). There are hundreds of orogenic peridotite bodies of

subcontinental lithosphere affinity. Some bodies contain garnet-bearing

Figure 16.3 Simplified

geological map of the

Scandinavian Caledonides

showing the major

allochthons (remobilized),

basement, garnet peridotite

locations and Devonian-

Carboniferous sediments and

plutonic rocks of the Oslo

area.

Interpretation after Gee et al.

(1985) and Brueckner and

Van Roermund (2004).


assemblages that equilibrated at about 700�950�C and 2.0�6.0 GPa in southern

and/or northern parts of the WGR (Medaris & Carswell, 1990; Jamtveit et al.,

1991; Krogh & Carswell, 1995; Carswell and Cuthbert 2003; Van Roermund,

2009a; Brueckner et al., 2010). The discovery of Scandian microdiamonds in both

orogenic garnet peridotite and associated subducted crustal rocks in northern parts

of the WGR (Dobrzhinetskaya et al., 1995; Van Roermund et al., 2002; Vrijmoed

et al., 2008) implies that both rock suites have had a shared subduction (.120 km,

in the diamond stability field) evolution during the Scandian orogeny (Spengler

et al., 2009a; Van Roermund, 2009b). However, the occurrence of Archean�mid-

Proterozoic pyroxene exsolution microstructures from garnet after majorite in gar-

net peridotite indicates an initial deeper origin for the WGR garnet peridotite

(B350 km) than for the surrounding crustal gneiss (Van Roermund & Drury, 1998;

Van Roermund et al., 2001; Spengler et al., 2006).

Two compositionally distinct types of garnet peridotite (Fe�Ti type and Mg�Cr

type) occur in the WGR (Carswell et al., 1983). The Fe�Ti garnet peridotite was

interpreted to have had a prograde metamorphic history that started as the ultra-

mafic portions of layered, low-pressure (LP), mafic crustal intrusive bodies were

metamorphosed during Scandian subduction (Jamtveit, 1987; Jamtveit et al.,1991).

Jamtveit et al. (1991) was unable to obtain a meaningful Sm�Nd age for the

Fe�Ti garnet peridotite of the Eiksunddal complex of WGR due to isotopic dis-

equilibrium. However, they obtained a Scandian Sm�Nd metamorphic age

(4126 12 Ma) of the eclogite in the same complex, which is in agreement with

other Scandian Sm�Nd ages from the WGR (Griffin & Brueckner, 1980). The

recent finding of microdiamonds (Vrijmoed et al., 2008) within mineral assem-

blages dated as 390�380 Ma (Vrijmoed et al., 2006, 2008), using Grt�Cpx mineral

pairs, in Fe�Ti garnet peridotites at Svartberget in the northernmost UHP domain

of the WGR, further demonstrates a subduction zone origin for Fe�Ti garnet peri-

dotite. We consider all Fe�Ti peridotites of the WGR as subduction zone garnet

peridotites in our classification system.

Mg�Cr garnet peridotite occurs in larger bodies of recrystallized chlorite peri-

dotites that are isofacial with surrounding amphibolites facies gneiss (Carswell,

1973, 1981, 1986; Medaris, 1980; Jamtveit, 1984). Their characteristic upper-

mantle mineral assemblages, whole-rock chemistries, and isotopic ratios demon-

strate that they are derived from highly depleted SCLM (Brueckner, 1977).

Sm�Nd dating of Mg�Cr garnet peridotites and associate olivine-free garnet pyr-

oxenites showed that the earliest garnet-bearing assemblages in the mantle-derived

peridotite formed in the mid-Proterozoic or older times (Jamtveit et al., 1991;

Brueckner et al., 1996, 2002, 2010; Lapen et al., 2005), long before the Scandian

continental collision event leading to (U)HP. Based on a detailed petrological, min-

eralogical, geochemical and geochronological study of Mg�Cr garnet peridotites in

the northernmost domain of the WGR, two major evolutionary stages have been

recognized: Archean (M1) and Proterozoic (M2) mantle events versus Scandian

(M3) crustal emplacement (Van Roermund, 2009b). Archean garnet assemblages

(M1) in the garnet peridotite consist of megacrysts of olivine, garnet, and orthopyr-

oxene, confirmed by Archean Re�Os ages of sulfide grains included in megacrysts


of orthopyroxenite (Brueckner et al., 2002) and ca. 2.53 and 2.90 Ga Sm�Nd

model ages (initial 143Nd/144Nd5 0.547566 0.00013) of M1 mineral assemblages

in garnetite (Spengler, 2006; Spengler et al., 2006). Spengler (2006) also presents

Archean Re�Os model ages of sulfide-free rocks in Otrøy, interpreted as the age

of Archean melting of the garnet peridotite. These ages clearly indicate that the

original, highly depleted, refractory garnet peridotites have an Archean signature.

The discovery of pyroxene exsolution lamellae in garnet megacrysts of Mg�Cr

garnet peridotite bodies on Otrøy (Van Roermund & Drury, 1998; Van Roermund

et al., 2001), and apparently similar pyroxene exsolution textures from peridotite

bodies exposed on Fjørtoft and Flemsøy (Terry & Robinson, 1999; but see

Scambelluri et al., 2010), indicates that these garnets contain a significant (e.g.,

5�8%) amount of majorite component, implying minimum pressures of

6.0�6.5 GPa at high temperatures (Van Roermund et al., 2000, 2009b). The most

likely explanation for the origin of the garnet peridotites at Otrøy, Flemsøy, and

Fjørtoft is that they were part of a rising mantle diapir and/or an upward-moving

asthenospheric convection cell that crossed the dry peridotite solidus, resulting in

high P�T decompression melting (Van Roermund & Drury, 1998; Drury et al.,

2001; Spengler et al., 2006). Decompression melting was followed by accretion of

the refractory peridotite to the subcratonic lithosphere (SCLM2) at depths well

within the garnet�olivine stability field (1% majorite stable; Van Roermund,

2009a) and a subsequent cooling of the refractory peridotite body to a stable, local

geotherm. A Proterozoic garnet-bearing assemblage (M2) in the garnet peridotite

was formed by dynamic recrystallization of former Archean minerals. They are

composed of granoblastic Cpx, Opx, Grt, Ol, and Sp, formed at 1300�1500�C at

3�4.5 GPa (Carswell, 1973; Carswell & Van Roermund, 2005) during the mid-

Proterozoic (Sm�Nd method, 16516 47 Ma, Brueckner et al., 2002;

14056 13 Ma, Spengler et al., 2006). Medaris (1999) gave a summary of P�T con-

ditions for M2 assemblages in garnet peridotite of the WGR in the range

750�950�C at 2.5�5 GPa. These temperatures are much lower than the newly pub-

lished data and are interpreted to be due to cooling, as this event records much

younger ages (ca. 600 Ma). This illustrates clearly that WGR garnet peridotites

formed as part of a former cratonic lithospheric mantle that had enough time to

cool before final crustal emplacement during the Scandian (Spengler et al., 2009a).

The emplacement of Mg�Cr garnet peridotite into deeply subducted continental

crust at 840�900�C at 3.4�4.1 GPa (Jamtveit, 1987; Van Roermund et al., 2002)

during continental subduction is supported by the occurrence of M3 microdiamonds

(Van Roermund et al., 2002), whereas a third generation (M3) of majoritic garnet

implies conditions of 850�950�C at 5.5�6.5 GPa (Spengler et al., 2009a; Van

Roermund, 2009a; Scambelluri et al., 2010). Spengler et al. (2009a) obtained a

weighted mean Sm�Nd age of 429.56 3.1 Ma for the microdiamond-bearing M3

mineral assemblage formed at P�T conditions of 870�C at 6.3 GPa. This age is

30 Ma older than the Sm�Nd, Rb�Sr and U�Pb ages from eclogites in WGR

(Cuthbert et al., 2000; Carswell et al., 2003; Root et al., 2004; Glodny et al., 2008;

Spengler et al., 2009a), which may indicate earlier peak UHP metamorphism

(UHPM) for some diamond eclogite facies rocks during collision/subduction.


16.3.1.2 Seve Nappe Complex

Based on lithological association and “detrital” zircon provenance ages, the Seve

Nappe complex is classically interpreted as the continent�ocean transition zone

between the western edge of Baltica and the neighboring Iapetus Ocean (Andresen &

Steltenpohl, 1994; Andreasson et al., 1998; Brueckner & Van Roermund, 2007).

It includes at least two HP metamorphic terranes characterized by the presence

of eclogites and/or garnet peridotites/pyroxenites, for example, Norrbotten

(Figure 16.3; Stephens and Van Roermund,1984; Van Roermund, 1989; Andreasson &

Albrecht, 1995) and N.Jamtland regions (Figure 16.3; Van Roermund & Bakker,

1984; Van Roermund, 1985, 1989). The garnet peridotite is only found in the

N.Jamtland/S.Vasterbotten regions.

The Seve Nappe complex in N.Jamtland/S.Vasterbotten (Figure 16.3) consists of

medium- and high-grade pelitic to quartzofeldspathic schist and gneiss, amphibo-

lite, eclogite, and subordinate marble, with scattered ultramafic bodies, which are

locally garnet�olivine bearing. Eclogite and garnet peridotite occur within the cen-

tral and eastern belts (Van Roermund & Bakker, 1984; Van Roermund, 1985,

1989). The field relationships in N.Jamtland/S.Vasterbotten seem remarkably simi-

lar to those in Norrbotten, and the two areas were earlier considered together. P�T

conditions of eclogite formation are 550�780�C and 14�22 kbar (Van Roermund,

1985). Zircons from eclogite and associated host rocks give 423�453 Ma

(Claesson, 1987; Williams & Claesson, 1987), interpreted to represent the age of

HP metamorphism. Two additional analyses demonstrate an inheritance of ca.

1700 Ma zircon for eclogite protoliths of the eastern belt. Similar ages

(425�444 Ma and 435�440 Ma) were obtained by Sm-Nd (grt-cpx; Brueckner

et al., 2004; Brueckner and Van Roermund 2007) and U�Pb analyses of titanites

and monazites from associated metamorphic rocks (Gromet et al., 1996).

Four lenses of garnet-bearing peridotites occur within this region. Metamorphic

conditions are estimated to 620�796�C at 1.9�2.1 GPa (Figure 16.4; Van

Roermund, 1989). A Sm�Nd mineral isochron age of 452.96 5.3 Ma indicates that

the garnet-bearing assemblage crystallized at the same time as the adjacent eclogites

(Brueckner et al., 2004; Brueckner and Van Roermund 2007). Re�Os, Sm�Nd iso-

tope, and trace-element studies of two garnet-bearing peridotite bodies in N.Jamtland

provide evidence that the garnet peridotite originates from an old SCLM rather than

from the previously suggested Iapetus-related suboceanic lithosphere (Bucher-

Numinen, 1988, 1991). Re�Os isotope studies of sulfides indicate that the protolith

of the garnet peridotite is at least early Proterozoic, and possibly as old as late

Archean (Brueckner et al., 2004). In addition, Sm�Nd isotopes of clinopyroxene

grains give scattered but largely middle Proterozoic model ages for the garnet perido-

tites. The clinopyroxenes also show enrichment in trace-element patterns that are con-

sistent with a subduction zone fluid overprint affecting the subcontinental lithosphere.

This geochemical fingerprint of Jamtlandian garnet peridotite appears to indicate that

the peridotite bodies of the Seve Nappe complex in N.Jamtland are derived from an

SCLM1 (with thickness B60 km) underneath a microcontinent that previously had

rifted away from Baltica (Brueckner & Van Roermund, 2007). Taken together, garnet


peridotites in N.Jamtlanda belong to the C-type mantle wedge garnet peridotites of

our classification system; their protoliths formed part of a relatively thin SCLM.

16.3.1.3 Tromsø Nappe

The Tromsø Nappe defines the uppermost part of the uppermost allochthon in

northern Scandinavia (Figure 16.3) and consists of a sequence of polymetamorphic

Figure 16.4 Summary of relevant literature data on orogenic garnet peridotites from the

Scandinavian Caledonides. For explanation of symbols, see Table 16.1. (A) Summary of

reported P�T conditions for/during (subcontinental/cratonic) lithosphere formation plotted

against the background conditions predicted by our conceptual model (Figure 16.1B) for

mantle wedge lithosphere formation. (B) Summary of reported P�T conditions of

subduction zone garnet peridotites plotted against background conditions predicted by our

conceptual model (Figure 16.1C) for a subduction zone garnet peridotite. In the diagram,

recorded subduction zone overprint conditions in mantle wedge garnet peridotite are also

included, but Fe�Ti peridotite of crustal origin is excluded. (C) Summary of reported P�T

conditions for subduction zone Fe�Ti garnet peridotites of crustal origin. (D) Summary of

the reported P�T paths. Numbers 1, 2, 3, and 4 correspond to: (1) Van Roermund (2009b);

(2) Ravna et al. (2006); (3) Jamtveit (1987); 7and (4) Brueckner et al. (2010).


high-grade metasediments (garnet-bearing mica schist, marble, calc-silicate rocks)

with numerous bodies of mafic (eclogite and garnet amphibolite) and less

abundant ultramafic rocks like garnet peridotite and dunite (Krogh et al., 1990;

Ravna & Roux, 2006; Ravna et al., 2006). Peridotite in the Tromsø Nappe occurs

as small bodies with a variable bulk-rock composition, ranging from olivine-poor

garnet peridotite to dunite. Evidence for a prograde metamorphic evolution for the

garnet�olivine assemblage in the peridotite was obtained from mineral inclusions

in garnet (Ravna et al., 2006). Hornblende and chlorite inclusions occur in coarse-

grained garnet cores, clinopyroxene and Cr-poor spinel in rims, succeeded by the

matrix assemblage garnet, forsterite, diopside and a Cr-rich spinel. This sequential

mineral assemblage was used to demonstrate a prograde metamorphic evolution

from 675�740�C at 1.4�2.4 GPa (Ravna et al., 2006) during subduction; hence,

the garnet peridotite is of the prograde subduction zone type. Much higher meta-

morphic conditions (735�C at 3.36 GPa) were calculated using the mineral assem-

blage Grt1Cpx1Phe in eclogite bodies of the same area, showing that the

Tromsø Nappe experienced UHPM (Ravna & Roux, 2006). Zircons separated

from the eclogite give an age of 4526 2 Ma (Corfu et al., 2003), which is identi-

cal to the HP metamorphic age of eclogite and garnet peridotite in the Seve Nappe

complex of N. Jamtland/S.Vasterbotten. However, Brueckner and Van Roermund

(2007) recently proposed that both these (U)HP terranes were evolved indepen-

dently in subduction systems located on opposite sides of Iapetus. The Tromsø

Nappe is a crystalline terrane that lies structurally above the Lyngen Nappe, a

large ophiolite complex overlain by carbonate shelf sediments similar to

Laurentian carbonates (Andresen & Steltenpohl, 1994). In addition, Pb isotopes

from titanites from the Tromsø eclogites have elevated 207Pb/204Pb ratios, indicat-

ing derivation from old crust, apparently more consistent with Laurentia than with

Baltica (Corfu et al., 2003).

16.3.1.4 Lindas Nappe, Bergen Arcs

The Lindas Nappe belongs to the Bergen Arcs, southern Norway (Figure 16.3),

and is composed of igneous rocks that belong to the anorthosite�granodiorite suite

metamorphosed under granulite facies conditions at 920�1230 Ma (Bingen et al.,

2001b). Recent dating by Glodny et al. (2002) gives B930 Ma for the granulite

facies, and 979 Ma for the crystallization of the magmatic precursor. Peridotite

bodies (lherzolite, wehrlite) in the Lindas Nappe occur as lenses intercalated with

the anorthosites. They share similar granulite facies mineral assemblages with

their host rocks (Austrheim, 1990), indicating that the spinel peridotites formed

part of a continental lithosphere during the Proterozoic at ca. 840�992 Ma (Kuhn

et al., 2000). The transition from spinel to garnet lherzolite, located within meso-

scale ductile shear zones, records a stage of HP metamorphism related to

Caledonian continent�continent collision (including crustal thickening), which

brought the granulite facies crustal rocks into HP metamorphic conditions at

650�700�C and 1.6�2.1 GPa (Austrheim & Griffin, 1985; Krogh et al., 1990).

The Caledonian HP metamorphism was first dated to have occurred at


ca. 440�460 Ma (Boundy et al., 1996, 1997; Bingen et al., 2001a). More recent

dating suggests, however, this occurred during the early Scandian (425 Ma;

Glodny et al., 2002). Based on the above-mentioned petrographic and geochrono-

logical data, the peridotites in the Lindas Nappe are interpreted to belong to the

prograde subduction zone garnet peridotite type.

16.3.1.5 Summary of Orogenic Garnet Peridotite in Scandinavian Caledonides

We have summarized particular characteristics, illustrated in Figure 16.4, of garnet

peridotites exposed at different structural levels of the Scandinavian Caledonides

(Figure 16.3). Metamorphic conditions, registered by the mineral�chemistry of

the orogenic garnet peridotites, are summarized in Table 16.1. When the metamor-

phic conditions are plotted in the theoretical P�T diagram of our model

(Figure 16.4) and reported ages of the lithosphere protoliths are taken into account

(Table 16.1), we can clearly see that mantle wedge protolith types (Figure 16.4A)

follow a distinct trend. Thin SCLM1 wedges (T, SP, and LP; Figure 16.4A) are all

exposed within the allochthonous units that are now positioned within the tecto-

nostratigraphy of the mountain belt at high structural levels (Figure 16.3). In addi-

tion, in all cases they represent early Caledonian plate collisions (.425 Ma)

involving crustal fragments that are underlain by relatively thin, old, cold litho-

spheres. Associated subduction zone overprints (with associated subduction-type

garnet peridotite formation) are all relatively shallow (Figure 16.4B). In contrast,

thick, old, cold SCLM2 wedges are found only in the WGR (Figure 16.4A; M2c,

M2d1, M2d2), representing the lowermost and structurally deepest tectonic unit of

the Scandinavian Caledonides. The latter mantle wedge type also corresponds to

much deeper subduction depths (150�200 km; Figure 16.4B), which occurred at

the end of the orogeny when Baltica collided with Laurentia; the latter was most

probably underlain by thick, old, cold subcratonic lithosphere in accordance with

our model (Spengler et al., 2009a; Van Roermund, 2009a).

16.3.2 Orogenic Garnet Peridotites in Sulu�Dabie and North QaidamOrogens of China

In the past two decades, new HP/UHP metamorphic belts have been identified in

China in addition to the well-studied Sulu�Dabie terranes in eastern China

(Figure 16.5A), for example, the North Qaidam�Altyn (Yang et al., 1994, 2002;

Song et al., 2003), Western Tianshan (Zhang, L. et al., 2003; Lv et al., 2008) and

North Qinling orogenic belts. Detailed mapping has discovered garnet peridotite,

garnet pyroxenite and eclogite in many localities of these (U)HP metamorphic ter-

ranes. Like the orogenic garnet peridotites in Scandinavia, the Chinese mantle

slices also reveal a dual origin, referred to in the literature as mantle-derived ver-

sus crustal peridotites (Zhang et al., 2000, 2004b). Garnet peridotite and garnet

pyroxenite are found as isolated lenses in gneiss or occur as layers within larger

eclogite bodies. Most of these are serpentinized to various degrees, so that fresh


garnet-bearing assemblages are so far found only in the core of the bodies or in

drill cores (CCSD Project). To test the feasibility of our conceptual model, we

have restricted our study to five well-studied orogenic garnet peridotite bodies

exposed in the Sulu�Dabie and North Qaidam orogenic belts. They are the

Zhimafang and Xugou garnet peridotites in Sulu, the Bixiling and Maowu com-

plexes in Dabieshan, and the Luliangshan garnet peridotite of the North Qaidam

orogen.

16.3.2.1 Sulu�Dabie Orogen

The Sulu�Dabie orogenic belt in eastern China was formed by subduction of the

Yangtze Craton beneath the Sino-Korean Craton. It consists of three fault-bounded

metamorphic terranes called the northern Dabie high-temperature, amphibolites�granulite facies unit, the central Dabie�Sulu UHP metamorphic unit, and the

southern HP unit (Figure 16.5B; Hacker et al., 2000; Zhang et al., 2000; Zheng

Figure 16.5 Simplified geological map of Sulu�Dabie and North Qaidam orogenic belts in

China. (A) Map of northern and central China. Rectangular blocks refer to locations of maps

illustrated in B and C. (B) Geological map of Sulu�Dabie orogen, eastern China, showing

important metamorphic terranes and locations of peridotite described in the text. (C)

Geological map of North Qaidam orogen, western China.

Part B modified after Zhang et al. (2000) and Part C modified after Song et al. (2006).


et al., 2005). Coesite and diamond have been identified in various container miner-

als, most commonly in zircon extracted from eclogite, garnet peridotite, and their

host gneiss, but also in marble of the Dabie�Sulu central metamorphic unit

(Wang et al., 1989; Yang et al., 1993; Schertl & Okay, 1994; Zhang et al., 1995a;

Zhang & Liou, 1996; Liu et al., 2001). Thermodynamic calculations on mafic and

ultramafic lithologies indicate UHP metamorphic conditions of ca. 760�970�Cand 4�6.7 GPa (Yang et al., 1993; Yang & Jahn, 2000; Zhang et al., 2000; Ye

et al., 2009). Relict subduction related majoritic garnet microstructures in garnet

peridotite provide further evidence that pressures in the subducted continental slab

are around ca. 7 GPa, and indicates that the edge of the Yangtze Craton was sub-

ducted to .200 km into the mantle (Ye et al., 2000). Coesite-bearing eclogite in

Sulu�Dabie has been dated with different methods. Rims of zircon, extracted

from both eclogite and surrounding gneiss, yielded an UHP peak metamorphic age

of 240�220 Ma (Hacker et al., 1998), subsequently confirmed by 230�220 Ma

Sm�Nd mineral isochron ages and additional U�Pb zircon dating (Li et al., 1993;

Ames et al., 1996; Chavagnac & Jahn, 1996; Liu et al., 2001, 2004), whereas

several Secondary High Resolution Ion Microprobe (SHRIMP) U�Pb ages of zir-

con cores yielded ca. 770 Ma for the eclogite protolith age.

Garnet peridotite occurs as lenses or boundins within amphibolite- to granulite

facies gneiss or as thin layers in large eclogite bodies. Most of the garnet perido-

tites are partially or completely serpentinized and often strongly deformed.

Diamond has been reported from heavy mineral separates from lherzolites in

Donghai county (Xu et al., 1998). The discovery of diamond, in combination with

detailed petrological, mineral�chemical, geothermobarometric, and isotope studies,

may suggest that garnet peridotites in Sulu�Dabie were recrystallized in a subduc-

tion zone setting at depths within the diamond stability field (Yang et al., 1993;

Okay, 1994; Zhang et al., 1994, 1995b; Liou & Zhang, 1998; Yang & Jahn, 2000).

Based on field appearance and bulk-rock geochemistry, two distinct garnet perido-

tite types were recognized in the Sulu�Dabie orogen (see review of Zhang et al.,

2000). One type is interpreted to originate from the mantle wedge above a subduct-

ing continental plate and contains rare lenses of eclogite (Zhang et al., 2000; Yang

et al., 2007). This type of garnet peridotite is found in Rongcheng, Rizhao, Junan,

and Donghai in the Sulu region (Figure 16.5B), and shows petrological and geo-

chemical characteristics that are similar to the Mg�Cr type garnet peridotite of the

WGR (Carswell et al., 1983; Medaris & Carswell, 1990). The other type is a

crustal-derived garnet peridotite interlayered with garnet pyroxenite and eclogite

and surrounded by orthogneiss. The protolith of this type of garnet peridotite was

formed by cumulate fractional crystallization of LP basaltic magma that intruded

the continental crust prior to Triassic subduction (Zhang et al., 2000). This type of

garnet peridotite includes the well-investigated Bixiling mafic�ultramafic com-

plexes, the Maowu garnet pyroxenite (Zhang et al., 1995a; Zhang & Liou, 1996),

and Yangkou ultramafic body (Figure 16.5B) with lherzolitic, wehrlitic and/or

harzburgitic compositions. Both types of garnet peridotite experienced Triassic

UHPM at conditions of 760�970�C and 4.0�6.5 GPa (Yang & Jahn, 2000; Zhang,

R.Y. et al., 2000, 2003).


16.3.2.1.1 Zhimafang Garnet PeridotiteThe well-studied garnet peridotite in Zhimafang is a good example of the mantle-

derived garnet peridotite type in the Sulu UHP metamorphic terrane

(Figure 16.5B). It consists of garnet lherzolite, garnet pyroxenite, garnet-free harz-

burgite and dunite (Yang & Jahn, 2000; Zhang et al., 2000; Yang et al., 2007; Ye

et al., 2009). The bulk composition of the garnet peridotite is characterized by high

MgO, low CaO, Al2O3 and total REE, indicating a depleted residual mantle origin.

High Mg# (91�93) in olivine, various degrees of depletion in middle rare-earth

elements (MREE), and high field strength elements (HFSE) are interpreted to indi-

cate an origin from a depleted mantle source (Yang & Jahn, 2000; Zhang et al.,

2000, 2008; Yang et al., 2007). Detailed petrological, mineral�chemical, and isoto-

pic studies demonstrate, however, that the source rock of the Zhimafang peridotite

body was overprinted by (1) an UHP metamorphic event, with P�T conditions of

800�1000�C at 5�6.5 GPa (Figure 16.6A), and (2) various degrees of metasoma-

tism induced by crustal-derived fluid/melt during Triassic subduction and/or exhu-

mation of the Yangtze continental crust (Yang & Jahn, 2000; Zhang et al., 2000,

2005a,b, 2007; Yang & Enami, 2003; Zheng et al., 2006; Ye et al., 2009).

SHRIMP U�Pb dating of zircon rims in garnet peridotite yield ages of ca. 220 Ma,

interpreted as the age of the UHPM that overprinted the peridotite mantle source

rock (Zhang et al., 2005b; Zheng et al., 2006). Thick kelyphitic rims around garnet

are always present and interpreted to be the result of partial to complete consumption

of garnet during exhumation postdating UHPM (Yang & Jahn, 2000; Zhang et al.,

2000; Ye et al., 2009). The consumption of garnet rims by two pyroxene�spinel

kelyphite is also assumed to have erased many important aspects of the metamorphic

record and, together with the lack of dedicated geochronological data, is probably the

cause of the conflicting metamorphic histories and P�T estimates reported in the lit-

erature (Yang & Jahn, 2000; Zhang et al., 2000; Yang et al., 2007; Ye et al., 2009).

The Zhimafang garnet peridotite has been interpreted as a LP mantle slice that

formed part of the SCLM beneath the Sino-Korean Craton (Zhang et al., 2000,

2005b, 2008). Before ca. 220 Ma, the SCLM was thinned, and the Zhimafang peri-

dotite became part of a thin, cold mantle wedge environment, which was then sub-

sequently brought to depths again by Triassic subduction of the Yangtze Craton

underneath the Sino-Korean Craton (Yang & Jahn, 2000; Zhang et al., 2005b). The

dominant harzburgitic bulk-rock composition (high MgO (Mg#5 90�92) and low

Al2O3 and CaO content) of the Zhimafang mantle protolith source rock is

interpreted to be due to HP (.5 GPa) partial melting at an unknown age. Hf iso-

tope compositions, .1.4 Ga model ages of zircons, and an Os isotope age

of .1 Ga indicate that the protolith of the Zhimafang garnet peridotite apparently

formed during Meso-Proterozoic times (Zheng et al., 2006; Yang et al., 2007;

Zhang et al., 2008). P�T estimates, using Mg-rich core compositions of porphyro-

blastic garnet in garnet peridotite, point toward initial temperatures around

B1000�C and pressures .5.1 GPa (Z2 in Figure 16.6A). It is unclear, however,

whether these calculated P�T conditions represent an early stage of asthenospheric

mantle flow related to lithosphere formation or are correlated with the UHP


metamorphic event related to continental subduction. P�T conditions of 760�C at

4.2 GPa, calculated using matrix mineral assemblages in peridotite and websterite,

are assumed to result from subduction zone metamorphism (Z3 in Figure 16.6A).

These results are compatible with UHP metamorphic conditions of the surrounding

coesite-bearing eclogite and gneiss. In addition, stable isotope compositions and

petrochemical analyses demonstrate that the hydrous UHP garnet peridotite experi-

enced at least three stages of metasomatism, interpreted to have occurred in the

Figure 16.6 Summary of relevant literature data on orogenic garnet peridotites from

Sulu�Dabie and North Qaidam orogenic belts in China. For explanation, see Table 16.2.

(A) Summary of reported P�T conditions in Sulu�Dabie, plotted together with the

background conditions predicted by our conceptual model (Figure 16.1C) for subduction

zone garnet peridotites. (B) Summary of reported P�T conditions in North Qaidam plotted

together with the background conditions predicted by our conceptual model (Figure 16.1C)

for subduction zone garnet peridotites. (C) Summary of reported P�T paths of Sulu�Dabie

garnet peridotites, eastern China. Numbers 1�5 correspond to: (1) Zhang et al. (2000);

(2) Yang and Jahn (2000); (3) Zhang et al. (2008); (4) Ye et al. (2009); and (5) Zhang et al.

(2000). (D) Summary of recorded P�T paths in Luliangshan garnet peridotite, North Qaidam

orogen.

Data after Song et al. (2005b).


mantle as well as in the continental crust (Zhang et al., 2005a). This model corre-

sponds to type C in Figure 16.1B. Competing models consider, however, that the

garnet peridotite was never subjected to UHPM in a subduction zone, but was sim-

ply brought from mantle depths to lower crustal levels directly by eduction of the

continental crust (Yang et al., 2007). This model also corresponds to type C in

Figure 16.1B, but the ultramafic rocks were caught up by the continental crust dur-

ing its way back to the surface.

A more coherent interpretation for the Zhimafang garnet peridotite has recently

been proposed based on detailed petrological and mineralogical investigations (Ye

et al., 2009). This model describes a two-stage metamorphic evolution for the peri-

dotite, involving (1) an early mantle stage followed by (2) a subduction zone evolu-

tion. In this model, an initially depleted peridotite, formed part of a rising, hot

asthenosphere, intruded the lithospheric mantle wedge above an active subducted

oceanic plate. Then it transformed into a fertile garnet lherzolite by fluid/melt

metasomatism initiated by the subducting oceanic crust at high temperatures. As

this hot, rising garnet peridotite entered the spinel stability field, it underwent a

high degree of LP decompression melting, which changed the bulk-rock composi-

tion into refractory spinel harzburgite and spinel dunite. The refractory peridotite

was refertilized again by subduction zone fluid/melt in shallow parts of the mantle

wedge. The peridotite, still in the mantle wedge, was then transported by corner

flow processes in the mantle wedge to depths of B200 km (P�T path; see 4 in

Figure 16.6C) which could explain the higher temperature estimated for the perido-

tite than for the surrounding host gneiss (Ye et al., 2009). In deeper parts of the

upper mantle (ca. 200 km), the garnet peridotite was trapped by the subducted con-

tinental crust and subsequently exhumed back to crustal levels under amphibolite

facies conditions (Ye et al., 2009). This model corresponds to type D in

Figure 16.1B.

These contrasting interpretations for the Zhimafang garnet peridotite are all

based on petrological, mineralogical and/or geochemical evidence. Nevertheless,

various researchers reached different models/conclusions. In the absence of sup-

porting geochronological data, the geochemical, mineral�chemical and petrologi-

cal data apparently can be interpreted in different ways. Each proposed endmember

model corresponds, however, to one of our mantle wedge subtypes.

16.3.2.1.2 Xugou Garnet PeridotiteThe Xugou garnet peridotite body, enclosed in felsic gneiss, is dominantly com-

posed of serpentinized harzburgite with subordinate lherzolite and garnet clinopyr-

oxenite. Garnet pyroxenite is commonly concordantly enclosed in peridotite, with

thicknesses varying from 0.5 to 15 m (Zhang, R.Y. et al., 2003). Garnet peridotite

consists of variable amounts of olivine, garnet, enstatite, diopside and rare chro-

mite. The bulk composition of the garnet peridotite is characterized by high MgO

and low CaO, Al2O3 and total REE, which indicates a depleted residual mantle ori-

gin comparable to that of the Zhimafang peridotite in the same region. High Mg#

(B92) of the olivine in the matrix further supports this interpretation.


Garnet pyroxenite consisting of garnet, diopside, and small amounts of rutile has

low bulk-rock Mg# (ca. 0.74�0.71; Zhang, R.Y. et al., 2003) and REE abundances.

Recently, the discovery of two generations of exsolution lamellae, involving garnet,

spinel and pyroxenes, within coarse-grained porphyroclastic orthopyroxene and

clinopyroxene crystals in the Xugou garnet pyroxenite, provides evidence that the

garnet pyroxenite has undergone a period of isobaric cooling from T$ 1200�C(Spengler et al., 2009b). Exsolved Cr-spinel having Cr#. 0.3 indicates that the iso-

baric cooling occurred under (minimum) lithostatic pressures of B4 GPa. This two-

stage isobaric exsolution history of the Xugou garnet pyroxenite indicates that the

Xugou garnet peridotite originated from an old, subcratonic lithosphere of unknown

age at depths $135 km (Spengler et al., 2009b). During the Triassic (2316 11 Ma,

Zhang et al., 2003) the subcratonic lithospheric mantle became part of a subcratonic

lithospheric mantle wedge, situated directly above a continental subduction zone.

Subsequently, parts of this mantle wedge were detached and became involved in

subduction-related UHPM at 780�870�C and 5�6.7 GPa (Zhang, R.Y. et al., 2003;

Spengler et al., 2009b). The latter model thus corresponds to our type A in

Figure 16.1B; however, this depends on the age of the garnet pyroxenite. It may

also be interpreted as a type B garnet peridotite in which the garnet pyroxenites are

simply derived from subduction-related rising asthenosphere at deeper levels.

16.3.2.1.3 Bixiling and Maowu Garnet PeridotiteGarnet peridotite and garnet pyroxenite occur as layers within coesite-bearing eclo-

gite bodies in the Maowu and Bixiling terranes of Dabieshan (Figure 16.5B). The

latter forms large-scale lenses within gneiss. It is generally accepted in the litera-

ture that the protoliths of these garnet-bearing mafic�ultramafic bodies crystallized

at lower crustal levels (,1 GPa) under LP and high-temperature conditions (ca.

1000�C; Zhang et al., 2000). Subsequently, the Maowu and Bixiling peridotite,

pyroxenite and eclogite acquired their HP garnet-bearing mineral assemblages dur-

ing prograde subduction within a continental subduction zone at P�T conditions of

700�800�C at 3�5 GPa and 820�970�C at 4.7�6.5 GPa, respectively (B and M in

Figure 16.6A; Okay, 1994; Liou & Zhang, 1998; Zhang et al., 2000). The Maowu

and Bixiling garnet peridotite/pyroxenite thus belong to the subduction zone garnet

peridotite type in our classification system with crustal origin. The host gneiss pro-

vides evidence for the same P�T evolution as the mafic and ultramafic rocks—

coesite included in zircon and garnet—confirm their UHP nature. It is therefore

suggested that all these rocks were subducted to mantle depths underneath the

Sino-Korean Craton during the Triassic continental collision.

16.3.2.2 North Qaidam Orogen

The Luliangshan garnet peridotite massif is exposed in the North Qaidam UHP

metamorphic belt, northwestern China, a 400-km long, early Paleozoic, continental

collision zone exposed along the northern edge of the Tibetan Plateau

(Figure 16.5C). It consists of garnet-bearing dunite, garnet harzburgite, garnet lher-

zolite, garnet pyroxenite and spinel dunite enclosed in granitic gneiss (Yang et al.,


1994; Song et al., 2004, 2005a,b, 2007, 2009a,b; Yang & Powell, 2008). Sm�Nd

isotope studies of three dunite and garnet peridotite samples yield negative εNdvalues ranging from 20.5 to 26.8, that is, substantially different from oceanic

and/or SCLM (Song et al., 2007). SHRIMP dating of cores of magmatic zircon

extracted from garnet lherzolite yields ca. 4576 22 Ma for the protolith of the gar-

net peridotite (Song et al., 2005b). Textural observations, mineral�chemistry, and

geochemical calculations suggest that the Luliangshan garnet peridotite was derived

from middle Ordovician, Alaskan-type, layered, sub-arc, LP cumulate intrusions,

related to ascending melts (Song et al., 2007, 2009a). According to this model, the

Luliangshan garnet peridotite should belong to our crustal type.

Recently, another interpretation for the origin of the Luliangshan garnet perido-

tite was proposed by Shi et al. (2010) with new Re�Os isotope data from the

dunites. They achieved Archean Re�Os model ages (TRD5 2.6 Ga, TMA5 2.8 Ga)

for the refractory dunite (Mg#5 92 �.95) that has high Os contents

(2.90�4.25 ppb) and low 187Os/188Os (up to 0.2847 ppb), which represent mini-

mum estimates for the age of the original melt depletion event. Garnet lherzolite/

peridotite has high bulk contents of Ca, Al, and lower Mg (Mg#, 92), and plots

within the field of Proterozoic SCLM as defined by Griffin et al. (1999). This,

together with the Archean Re�Os ages, suggests that the Luliangshan garnet peri-

dotite can be interpreted as an SCLM fragment, probably derived from beneath the

Sino-Korean Craton, in which ancient Archean residues, after melt extraction

(dunite harzburgite), became refertilized by infiltrating mafic melts that produced

a garnet peridotite bulk composition out of dunite by interactions with the melt

(Shi et al., 2010). This model, depending on the lack of depth information,

corresponds to type A or C in Figure 16.1B. Exsolution lamellae of rutile1 two

pyroxene1 sodic amphibole in garnet and ilmenite1Al-chromite in olivine (Song

et al., 2004, 2005a), as well as diamond inclusions in zircon, demonstrate that the

LP protolith of the Luliangshan peridotite body subsequently experienced UHPM.

In addition, an UHP metamorphic age of 4236 5 Ma, obtained from diamond-

bearing zircon (Song et al., 2005b), and P�T conditions of 900�1000�C at

5.5�6.5 GPa (L1 in Figure 16.6B) (Song et al., 2004, 2005b, 2009a), demonstrate

that the LP protolith of the garnet peridotite was subducted to mantle depths in

excess of 200 km during Silurian continental collision involving subduction of the

Qaidam�Qilian Craton underneath the northern Sino-Korean Craton (Song et al.,

2007). During subsequent exhumation, garnet peridotite experienced four stages of

retrograde metamorphic overprint responsible for (1) formation of exsolution

lamellae, (2) two pyroxene�spinel kelyphites around garnet; (3) amphibolite facies

mineral assemblages in the matrix, and (4) serpentinization of the garnet peridotite

(Song et al., 2009a).

The Luliangshan garnet peridotite thus experienced a mantle evolution event fol-

lowed by a subduction zone metamorphic overprint, in agreement with our mantle

wedge garnet peridotite model. The garnet peridotite can be interpreted as being

derived either by subduction-related magmatism, partial melting in the shallow part

of a spinel-bearing mantle wedge underneath an island arc, or from an Archean

fragment of the SCLM underneath the craton that became refertilized, depending


on the model, by Proterozoic or Paleozoic mafic melts. Subsequent subduction to

deeper mantle depths was responsible for the subduction zone UHP metamorphic

overprint. However, the temporal and spatial relationship between dunite, garnet

pyroxenite and garnet peridotite, the depth levels, and the mechanism responsible

for crustal incorporation are still matters of debate and needs to be studied in more

detail. From the literature it is thus far from clear whether the Luliangshan garnet

peridotite and related rock types represents a LP Paleozoic crustal cumulate (sub-

duction zone type) or an old, cold, depleted, spinel/garnet-bearing mantle wedge

infiltrated by mafic melts during Proterozoic and/or Paleozoic times. In terms of

our classification system the latter will correspond to either type C or D in

Figure 16.1B.

16.3.2.3 Summary of the Orogenic Garnet Peridotite in China

In summary, except for a crustal origin for the Bixiling and Maowu subduction

zone garnet peridotite in Dabieshan, the origin of garnet peridotites from

Zhimafang, Xugou, and Luliangshan remains controversial. Most garnet peridotite

in these areas experienced an UHP subduction zone metamorphic overprint related

to continental collision and subduction. Recent studies (Ye et al., 2009; Shi et al.,

2010; Spengler et al., 2009b) provide evidence that the garnet peridotite in the three

areas described earlier originated from either old, thick/subcratonic, or old thin/

SCLM, corresponding to types A or C in Figure 16.1B. It is evident, however, that

much more isotopic and geochronological data will be needed before a mantle

wedge origin for most of these garnet peridotites can be taken for granted.

16.4 Discussion and Conclusions

Quantitative compositional and structural data on the SCLM provides crucial infor-

mation about realistic large-scale models, describing Earth’s geochemical and tec-

tonic evolution (Griffin et al., 1999). Most of our current knowledge on mantle

composition and associated heterogeneities has been obtained through studies of

xenoliths and xenocrysts from kimberlite and other volcanic rocks of deep origin.

During the last decades, however, detailed integrated studies of volumetrically

minor orogenic garnet peridotite, exposed in ancient orogenic belts all over the

world, have provided us with another important data source that can be used to

obtain quantitative data regarding processes like the formation of subcontinental/

subcratonic lithosphere, evolution of a lithospheric mantle wedge, incorporation of

garnet peridotite into deeply subducted continental crust, subsequent UHPM during

ongoing continental subduction, and final exhumation back to subcrustal levels. In

order to do so, orogenic garnet peridotite on a worldwide scale needs to be subdi-

vided into mantle wedge- versus subduction zone types, allowing the former type

to be used for subcontinental lithosphere growth and evolution studies.

In order to get better control on the complex spectra of physical and chemical

processes that are involved in formation and evolution of subcontinental/subcratonic


lithospheric mantle followed by subsequent uptake in subducted continental crust

during continental collision and eduction, we have proposed a simplified concep-

tual model that is able to explain both mantle and crustal evolution of an orogenic

mantle wedge garnet peridotite. Our conceptual model is intended to present a rela-

tively simple framework that is able to pinpoint in space and time the relative order

of the complicated spectrum of physical and chemical processes thought to be

active in orogenic garnet peridotite in general. For garnet peridotite of the

Scandinavian Caledonides, our conceptual model seems to fit well with published

geological, petrological, mineral�chemical, geochemical and isotope characteris-

tics. Different types of peridotites were recognized, as garnet peridotite of the

WGR are compared to those in the Seve, Lindas, and Tromsø Nappes. Mantle

wedge garnet peridotite of the WGR records metamorphic P�T conditions, using

Proterozoic to early Scandian garnet-bearing assemblages (M2c in Figure 16.4A),

that are positioned along a subcratonic lithospheric mantle trend (Figure 16.4A,

types A�C). After Archean decompression melting at HPT conditions, there is

apparently enough time available to cool down from Archean (melting) to

Proterozoic (Figure 16.4A; M2a and M2b) and Paleozoic times predating their

involvement in a continental subduction zone. Thus the garnet peridotites of the

WGR have had enough time (ca. 20 Ga) to cool down to stable “normal” geother-

mal conditions represented by a subcratonic lithosphere of around 200 km thick

(Figure 16.4A, M2c). Then they were transformed into an old, cold, depleted and

thick mantle wedge overlying a continental subduction zone. In the northern part

of the WGR, evidence has been presented (Scambelluri et al., 2008, 2010;

Spengler et al., 2009a; Van Roermund, 2009b) that pieces of the garnet�olivine-

bearing mantle wedge were enclosed in a subducted continental crust and then

partly recrystallized during prograde subduction into a subduction zone garnet peri-

dotite (M3a, M3b, M3d, M3e, M3f1, and M3f2 in Figure 16.4B). However, in gar-

net peridotite exposed in southern parts of the WGR, evidence for prograde

Scandian subduction zone metamorphism is missing and the exhumation path of

the garnet�olivine-bearing mantle wedge is solely characterized by the develop-

ment of retrograde mineral assemblages (Figure 16.4D; Brueckner et al., 2010).

The latter garnet peridotites are interpreted to be transferred from the mantle

wedge into the continental crust when the crust was exhumed to lower crustal

levels (Brueckner et al., 2010).

Garnet peridotite from other, structurally higher nappes in the Scandinavian

Caledonides all originate from microcontinental fragments that were underlain by

much thinner lithospheric mantle keels (Figure 16.2E and F). Recorded mantle

wedge conditions, illustrated in Figure 16.4B, are all defined by spinel-bearing

mineral assemblages and positioned close to a continental geotherm of 100 km

thickness (Figure 16.4A; T, Sp, Lp). Both thick and thin mantle wedge peridotite

types, except for the southern garnet peridotite suite of the WGR, were tapped by

subducted continental crust and subsequently experienced a subduction zone meta-

morphic overprint that differed from each other by the recorded subduction depths

when mutually compared (Figure 16.4B). An exception to this general rule is the

southern garnet peridotite suite of the WGR, in which the mantle wedge


assemblage is defined by a garnet�olivine assemblage but the (subduction and)

exhumation history is so far only documented by retrograde mineral assemblages

(Brueckner et al., 2010).

The subduction zone garnet peridotites from the Seve-, Lindas-, and Tromsø

Nappe complexes record, however, a much lower subduction depth than those from

the WGR, maybe because of a lack in pull strength from previously subducted oce-

anic crust (Figure 16.4B). The garnet peridotites of the Scandinavian Caledonides

thus represent both basic garnet peridotite types: that is, mantle wedge versus sub-

duction zone garnet peridotite, in agreement with our model P�T diagram

(Figures 16.1 and 16.2). However, the mantle wedge involved is always cold, and

can be subdivided into thin and thick types, using the mineral assemblage that was

present in the mantle wedge protolith. Exceptions to this general “Scandinavian

rule” are the Fe�Ti garnet peridotite of the WGR that, by definition, all belong to

the crustal type which obtained their garnet�olivine mineral assemblage by pro-

grade metamorphism in a subduction zone environment (Figure 16.4C).

Similar results from the Chinese orogenic garnet peridotite appear to be less

straightforward (Figure 16.6), except for the crustal origin of the Bixiling and

Maowu subduction zone garnet peridotite in Dabieshan. The origin and evolution

of the garnet peridotite in Zhimafang, Xugou, and Luliangshan is controversial,

maybe because former researchers paid more attention to the subduction zone UHP

metamorphic overprint of the garnet peridotite than to the lithospheric mantle pro-

cesses predating the continental collision and/or subduction. Published models con-

cerning the origin of the Zhimafang garnet peridotite have two contrasting

interpretations: (1) old, cold, depleted (subcratonic) lithospheric mantle (Zheng

et al., 2006; Zhang et al., 2008), and (2) young, hot, fertile, upwelling astheno-

sphere (Ye et al., 2009). Both models are supported by strong petrological,

mineral�chemical, and geochemical evidence. However, the results indicate

completely opposite findings. If the results are mutually compared with our concep-

tual model, we think that the ultimate model interpretation of the Zhimafang garnet

peridotite will depend on the age of the coarse-grained granoblastic garnet-bearing

assemblage. If it can be demonstrated to be Proterozoic (or even Archean), a sub-

cratonic lithospheric mantle wedge origin of the peridotite can be proven (depend-

ing on depth, this will correspond to type A or C; Figure 16.2). If it is of Paleozoic

age, the protolith type of the mantle wedge will correspond to young, fertile, hot,

rising asthenosphere present in an active mantle wedges (type B or D, depending

on depth; Figure 16.2). In the latter case, more attention needs to be paid to

mineral�chemical data and geothermobarometry, as the expected temperature pre-

dicted by our model should be much higher. The same debate arises for the nearby

Xugou garnet peridotite. Recent evidence points toward the idea that the Xugou

garnet peridotite also originated from the SCLM2 underneath the Sino-Korean

Craton (Spengler et al., 2009b). Simultaneously this may be used as evidence for

an old lithospheric mantle origin for the Zhimafang garnet peridotite.

There are also two interpretations for the origin of the Luliangshan garnet peri-

dotite: (1) an Alaskan-type magmatic cumulate (Song et al., 2005b, 2009a,b), or

(2) an Archean fragment from beneath cratonic SCLM2 (Shi et al., 2010). It is clear


from the results presented in Figure 16.6 that the Chinese garnet peridotite still

needs to be studied in much greater detail, especially in the field of (in situ) isotope

dating/geochronology. In addition, much more attention needs to be paid to the

temporal and spatial relationships between dunite, garnet lherzolite, garnet harzbur-

gite and garnet pyroxenite/websterites, including their relative modal proportions.

In the foregoing we have classified mantle wedge garnet peridotite into four fun-

damental subtypes. The most important discriminative parameters are: (1) the mean

bulk temperature (T) that is operative in the mantle wedge at the onset of continen-

tal collision/subduction (i.e., discrimination between hot and cold mantle wedges in

Figure 16.2), and (2) the thickness (P) of the lithosphere underneath the overriding

plate in the collisional belt (i.e., thick (deep) versus thin (shallow) mantle wedges

in Figure 16.2). Finally, it needs to be emphasized that both the age of partial melt-

ing, as well as information concerning the depth at which melting took place

(LP vs. HP), are important parameters that need to be determined in garnet perido-

tite that are affected by various degrees of partial melting.

Decompression melting is illustrated in Figure 16.1B by the widely variable

temperature conditions interpreted to be operative in a rising convective astheno-

spheric mantle cell. Decompression melting will occur at the moment when the

P�T path of the rising asthenosphere intersects the dry peridotite solidus, resulting

in the production of refractory material and liquid melts. The latter may or may not

leave the system. Without, or with little, overprinting by a later subduction zone

process, the depth at which lithosphere accretion takes place can be reconstructed

from the mineral assemblage, stable at the time recorded in the refractory material

(Van Roermund, 2009b). Alternatively, “fertile” subcontinental lithosphere can be

produced by the rise of a relatively cold asthenospheric “plume.” If not, it will pro-

vide evidence that refertilization processes have been related to injection of large

amounts of “basic” dikes into previously depleted mantle material.

It also needs to be realized that young, hot, and thus rheologically “dynamic”

mantle wedges can, in principle, be formed by two alternative mechanisms: (1) ris-

ing of young, hot asthenosphere and (2) by large quantities of basic (hot) magma

that intrudes old, cold, buoyant mechanically strong lithosphere.

In our conceptual model, illustrated in Figure 16.2, we have subdivided mantle

wedge garnet peridotites into four subtypes. Each of the four mantle wedge types is

formed under different P�T conditions and thus represents a different geodynamic

setting. By detailed studies of the geochemical, petrological, mineralogical, and

isotopic characteristics, in combination with the P�T conditions that are operative

in the garnet peridotite, including their mutual time relationships, we are able to

reconstruct the geodynamic setting of an orogenic garnet peridotite, according to

our model. The first thing that needs to be established is whether the garnet perido-

tite represents a mantle wedge or a subduction zone type. In the case of mantle

wedge type garnet peridotite, this fundamental discrimination will allow a recogni-

tion of the following two processes: (1) a lithospheric mantle evolution, and (2) a

crustal incorporation process that may occur at different levels of a subduction

zone, including a shared (U)HP overprint. It is thus very important to recognize in

the rock texture/microstructure mantle wedge- versus overprinted subduction zone


fabrics, including the equilibrated P�T conditions for different garnet peridotite

stages. The most frequently used geothermobarometer is a combination of the

Fe�Mg olivine�garnet exchange thermometer (O’Neill & Wood, 1979; O’Neill,

1980; Brey & Kohler, 1990) with the Al-in-orthopyroxene barometer (Brey &

Kohler, 1990). However, in the case of mantle wedge garnet peridotite, it seems

more appropriate to discriminate first between mineral assemblages and microstruc-

tures that belong to each of the different metamorphic stages. In principle the litho-

spheric mantle evolution and subduction zone metamorphic overprint should

produce different mineral fabrics and mineral�chemical characteristics that can be

recognized at the outcrop scale and in thin sections, for example, mineral grain

sizes (olivine and garnet), mineral inclusions, trace, and REE chemistry of miner-

als. It will be clear that such detailed studies will ultimately lead to a better under-

standing of the garnet peridotites.

Acknowledgment

We are grateful to Professor Erling K. Ravna and Dr. Marian Janak for their critical com-

ments and constructive reviews. Dr. Gill Pennock helped with polishing the English. This

work was financially supported by the Major State Research Development Program of China

(Grant 2009CB825007), National Nature Science Foundation of China (Grant 40821002),

the China Scholarship Council and the HPT laboratory, Department of Earth Sciences,

Utrecht University, the Netherlands.

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ultrahigh pressure metamorphism || orogenic garnet peridotites

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