Volcanic Arcs, Chapters 16 and Volcanic Arcs, Chapters 16 and 1717

Ocean-ocean convergence Ocean-ocean convergence Island ArcIsland Arc (IA) (IA) Ocean-continent convergence Ocean-continent convergence Continental ArcContinental Arc

Figure 16-1. Principal subduction zones associated with orogenic volcanism and plutonism. Triangles are on the overriding plate. PBS = Papuan-Bismarck-Solomon-New Hebrides arc. After Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.

Arcuate volcanic chains above subduction Arcuate volcanic chains above subduction zoneszones

Distinctly different from mainly basaltic Distinctly different from mainly basaltic provinces thus farprovinces thus far– Compositions more diverseCompositions more diverse– Basalt generally subordinateBasalt generally subordinate– More explosive: viscous, cool, magmas trap More explosive: viscous, cool, magmas trap

gas gas – Strato-volcanoes most common volcanic Strato-volcanoes most common volcanic

landformlandform

Arcs are:Arcs are:

Chapter 16. Island Arc Chapter 16. Island Arc MagmatismMagmatism

Structure of an Island ArcStructure of an Island Arc

Figure 16-2. Schematic cross section through a typical island arc after Gill (1981), Orogenic Andesites and Plate Tectonics. Springer-Verlag. HFU= heat flow unit (4.2 x 10-6

joules/cm2/sec)

Volcanic Rocks of Island ArcsVolcanic Rocks of Island Arcs Complex tectonic situation and broad spectrum of Complex tectonic situation and broad spectrum of

rock typesrock types High proportion of High proportion of Basaltic - andesiteBasaltic - andesite and and AndesiteAndesite

– Most Andesites occur in subduction zone settingsMost Andesites occur in subduction zone settingsTable 16-1. Relative Proportions of Quaternary Volcanic

Locality B B-A A D RTalasea, Papua 9 23 55 9 4Little Sitkin, Aleutians 0 78 4 18 0Mt. Misery, Antilles (lavas) 17 22 49 12 0Ave. Antilles 17 42 39 2Ave. Japan (lava, ash falls) 14 85 2 0After Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite

A = andesite, D = dacite, R = rhyolite

Island Arc Rock Types

Recall Major Magma SeriesRecall Major Magma Series

AlkalineAlkaline series (OIA series (OIA ocean island ocean island

alkalinealkaline)) Sub-alkaline types:Sub-alkaline types:

– TholeiiticTholeiitic series (MORB, OIT) series (MORB, OIT)– Calc-Alkaline seriesCalc-Alkaline series (IA island (IA island

arcs) arcs)

C-A ~ restricted to magmas C-A ~ restricted to magmas generated near subduction generated near subduction zones, but keep in mind other zones, but keep in mind other series occur there tooseries occur there too

Major Magma Series Major Magma Series visualized with Major visualized with Major

ElementsElements

Figure 16-3. Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90, 349-370.

a.a. Alkali vs. silica all Alkali vs. silica allb.b. AFM for subalkaline AFM for subalkalinec.c. FeO*/MgO vs. silica FeO*/MgO vs. silica

Diagrams for 1,946 analyses Diagrams for 1,946 analyses from ~ 30 volcanic island from ~ 30 volcanic island arcs arcs and continental arcsand continental arcs

Figure 16-6. b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series.

Not all volcanic arcsabove a subduction zone are calc-alkaline.

Sub-series Calc-AlkalineSub-series Calc-AlkalineKK22OO is an important discriminator is an important discriminator Gill (1981) Gill (1981)

recognized threerecognized three Andesite sub-series Andesite sub-series

Figure 16-4. The three andesite series of Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. Contours represent the concentration of 2500 analyses of andesites stored in the large data file RKOC76 (Carnegie Institute of Washington).

Figure 16-6. a. K2O-SiO2 diagram distinguishing high-K, medium-K and low-K series. Large squares = high-K, stars = med.-K,

diamonds = low-K series from Table 16-2. Smaller symbols are identified in the caption. Differentiation within a series (presumably dominated by fractional crystallization) is indicated by the arrow. Different primary magmas (to the left) are distinguished by vertical variations in K2O at low SiO2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag.

If partition on basis of K versus If partition on basis of K versus Tholeiitic/calc-alkaline, most common Tholeiitic/calc-alkaline, most common samples are:samples are:

Figure 16-5. Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calc-

alkaline types, resulting in six andesite series, after Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. The points represent the analyses in the appendix of Gill (1981).

– Low-K Low-K tholeiitictholeiitic– Med-K C-AMed-K C-A

– Hi-K mixedHi-K mixed

Tholeiitic vs. Calc-alkaline Tholeiitic vs. Calc-alkaline differentiationdifferentiation

for our three examplesfor our three examples

Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Tholeiitic vs. Calc-alkaline differentiatio

nseems to

depend on K

C-A shows continually increasing SiO2 and lacks dramatic Fe enrichment

High K

Calc-alkaline differentiation WHY?Calc-alkaline differentiation WHY?

– Early (as opposed to late in Early (as opposed to late in Tholeiites) crystallization of an Tholeiites) crystallization of an Fe-Fe-Ti oxideTi oxide phase. phase. Probably related to the high Probably related to the high water content of calc-alkaline water content of calc-alkaline magmas in arcsmagmas in arcs

– Iron is removed early so a middle Iron is removed early so a middle fractionation high iron composition fractionation high iron composition cannot occur as it does in cannot occur as it does in TholeiitesTholeiites

http://www.springerlink.com/content/u383118http://www.springerlink.com/content/u38311872w004w16/72w004w16/

Other TrendsOther Trends SpatialSpatial

Antilles Antilles more alkaline N more alkaline N S S Aleutians segmented with C-A Aleutians segmented with C-A

prevalent in center and tholeiite prevalent in center and tholeiite prevalent at endsprevalent at ends

IDEA: source/collection points for IDEA: source/collection points for high K clays (Illite) near trench?high K clays (Illite) near trench?

TemporalTemporal– Early Tholeiitic Early Tholeiitic later C-A and often later C-A and often

latest alkaline is commonlatest alkaline is common

Trace ElementsTrace Elements REEsREEs

– HREE flat in all, HREE flat in all, – so garnet, which

sequesters the HREEs, not in equilibrium with the melt

– Garnet last to go in partial melting of Lherzolite. If melted, HREE would be high.

– also not from also not from subducted basaltsubducted basalt, which , which becomes eclogite with garnet becomes eclogite with garnet at 110 km.at 110 km.

Figure 16-10

The HREE are flat, implying that garnet, which strongly partitions (holds) the HREE, was not in equilibrium with the melt. Melts derived from eclogite are depleted in HREE (abundant garnet in residue). This causes the characteristic low HREE

Figure 16-11a. MORB-normalized spider diagrams for selected island arc basalts. Using the normalization and ordering scheme of Pearce (1983) with LIL on the left and HFS on the right and compatibility increasing outward from Ba-Th. Data from BVTP. Composite OIB from Fig 14-3 in yellow.

MORB-normalized Spider diagramsMORB-normalized Spider diagrams– IA: high LIL (LIL are hydrophilic), low HFSIA: high LIL (LIL are hydrophilic), low HFS

What is it about subduction zone setting that What is it about subduction zone setting that causes causes fluid-assistedfluid-assisted enrichment? enrichment?

Figure 14-4. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345.

Intraplate OIB has similar humpHFS=High Field-strength

Most incompatible

New Britain, Marianas, Aleutians, and South New Britain, Marianas, Aleutians, and South Sandwich volcanics plot show sediment Sandwich volcanics plot show sediment contamination of DMcontamination of DM

IsotopesIsotopes

Figure 16-12. Nd-Sr isotopic variation in some island arc volcanics. MORB and mantle array from Figures 13-11 and 10-15. After Wilson (1989), Arculus and Powell (1986), Gill (1981), and McCulloch et al. (1994). Atlantic sediment data from White et al. (1985).

Antilles (Atlantic) and Banda and New Zealand (Pacific) can be explained by partial melting of a MORB-type source + the addition of the type of sediment that exist on the subducting plate (Pacific sediment has 87Sr/86Sr around 0.715and 143Nd/144Nd around 0.5123)

The increasing N-S Antilles Nd enrichment probably related to the increasing proximity of the southern end to the South American sediment source of the Amazon

Figure 16-13. Variation in 207Pb/204Pb vs. 206Pb/204Pb for oceanic island arc volcanics. Included are the isotopic reservoirs and the Northern Hemisphere Reference Line (NHRL) proposed in Chapter 14. The geochron represents the mutual evolution of 207Pb/204Pb and 206Pb/204Pb in a single-stage homogeneous reservoir. Data sources listed in Wilson (1989).

Pb in some arcs overlap with the MORB data; depleted mantle component is a major reservoir for subduction zone magmas

Majority of data enriched in radiogenic lead (207Pb and 206Pb), trending toward the appropriate oceanic marine sedimentary reservoir

1010BeBe created by cosmic rays + oxygen and nitrogen created by cosmic rays + oxygen and nitrogen in upper atmos.in upper atmos. Earth by precipitation & readily Earth by precipitation & readily clay-rich clay-rich

oceanic sedimentsoceanic sediments

– Half-life of only 1.5 Ma (long enough to be Half-life of only 1.5 Ma (long enough to be subducted, but quickly lost to mantle systems). subducted, but quickly lost to mantle systems). After about 10 Ma After about 10 Ma 1010Be is no longer detectable. Be is no longer detectable. 99Be is stable, natural.Be is stable, natural.

– 1010Be/Be/99Be averages about 5000 x 10Be averages about 5000 x 10-11 -11 in the in the uppermost oceanic sedimentsuppermost oceanic sediments

– In mantle-derived MORB and OIB magmas, & In mantle-derived MORB and OIB magmas, & continental crust, continental crust, 1010Be is below detection limits Be is below detection limits (<1 x 10(<1 x 1066 atom/g) and atom/g) and 1010Be/Be/99Be is <5 x 10Be is <5 x 10-14-14

Boron BBoron B is a stable element is a stable element– Very brief residence time deep in subduction Very brief residence time deep in subduction

zoneszones

– B in recent sediments is high (50-150 ppm), but B in recent sediments is high (50-150 ppm), but has a greater affinity for altered oceanic crust has a greater affinity for altered oceanic crust (10-300 ppm)(10-300 ppm)

– In MORB and OIB it rarely exceeds 2-3 ppmIn MORB and OIB it rarely exceeds 2-3 ppm

1010Be/BeBe/Betotaltotal vs. B/Be vs. B/Betotaltotal diagram (Be diagram (Betotaltotal 99Be since Be since 1010Be is so rare). Be is so rare).

This is the smoking gun, the evidence for the fluids This is the smoking gun, the evidence for the fluids (mostly ion-rich water) squeezed out of the (mostly ion-rich water) squeezed out of the sediments.sediments.

Figure 16-14. 10Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88, 111-123.

Figure 16-15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, 8309-8319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).

The potential source components The potential source components IA IA magmasmagmas1.1. The The crustalcrustal portion of the portion of the subducted slabsubducted slab

1a1a Altered oceanic crust (hydrated by circulating Altered oceanic crust (hydrated by circulating seawater, and metamorphosed in large part to seawater, and metamorphosed in large part to greenschist facies)greenschist facies)

1b1b Subducted oceanic and forearc sediments Subducted oceanic and forearc sediments

1c1c Seawater trapped in pore spaces Seawater trapped in pore spaces

2.2. The The mantle wedgemantle wedge between the slab and the arc crust between the slab and the arc crust

Not 1a the subducted basalt Not 1a the subducted basalt fidefide flat HREEs flat HREEs The trace element and isotopic data suggest that The trace element and isotopic data suggest that bothboth 1b 1b

and 1c, the and 1c, the subducted sediments and watersubducted sediments and water and 2, the and 2, the mantle wedgemantle wedge contribute to arc magmatism. How, and to contribute to arc magmatism. How, and to what extent?what extent?– DryDry peridotite solidus too high for melting of anhydrous peridotite solidus too high for melting of anhydrous

mantle to occur anywhere in the thermal regime shownmantle to occur anywhere in the thermal regime shown– LIL/HFS ratios of arc magmas LIL/HFS ratios of arc magmas waterwater plays a plays a

significant role in arc magmatismsignificant role in arc magmatism

Freezing Point Depression always occurs in a mixture

Even small amounts of water (0.5%) and carbon dioxide (0.5%) strongly depress the temperatures of the solidus, moving it below the geotherm at all depths. This effect dominates in subduction environments, where arc magmas are generated. (Modified from B. M. Wilson (1989) Igneous petrogenesis: a global tectonic approach. Chapman and Hall, London.)

Effects of the addition of small amounts of volatiles to mantle Iherzolite. A mantle adiabat with potential temperature of 1280 °C is shown for reference

An upside-down PT diagram

AmphiboleAmphibole-bearing hydrated peridotite should -bearing hydrated peridotite should meltmelt at ~ 120 at ~ 120 kmkm

PhlogopitePhlogopite-bearing hydrated peridotite should -bearing hydrated peridotite should meltmelt at ~ 200 at ~ 200 kmkm

second arcsecond arc behind first? behind first?

Crust and Mantle Wedge

Figure 16-18. Some calculated P-T-t paths for peridotite in the mantle wedge as it follows a path similar to the flow lines in Figure 16-15. Included are some P-T-t path range for the subducted crust in a mature arc, and the wet and dry solidi for peridotite from Figures 10-5 and 10-6. The subducted crust dehydrates, and water is transferred to the wedge (arrow). After Peacock (1991), Tatsumi and Eggins (1995). Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

The data from LIL Large Ion The data from LIL Large Ion Lithophiles and HFS High Field Lithophiles and HFS High Field Strength trace elements underscore Strength trace elements underscore the importance of slab-derived water the importance of slab-derived water and a MORB-like mantle wedge and a MORB-like mantle wedge sourcesource

The flat HREE pattern argues against The flat HREE pattern argues against a garnet-bearing (eclogite) sourcea garnet-bearing (eclogite) source

Thus modern opinion has swung Thus modern opinion has swung toward a non-melting subducted toward a non-melting subducted lithosphere slab model for most lithosphere slab model for most cases of IA genesiscases of IA genesis

Phlogopite is stable Phlogopite is stable in ultramafic rocks in ultramafic rocks beyond the beyond the conditions at which conditions at which amphibole breaks amphibole breaks downdown

P-T-t paths for the P-T-t paths for the wedge reach the wedge reach the phlogopite-2-phlogopite-2-pyroxene pyroxene dehydration dehydration reaction at about reaction at about 200 km depth 200 km depth Figure 16-11b. A proposed model for

subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, 4697-4707 and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.

Island Arc Petrogenesis ModelIsland Arc Petrogenesis ModelMantle here is too shallow to have Garnet. Subducted slab turns to Eclogite with Garnet at 110 km.

Chapter 17: Chapter 17: Continental Continental

Arc Arc MagmatismMagmatism

Figure 17-1. NVZ, CVZ, and SVZ are the northern, central, and southern volcanic zones.

Continental Volcanic ArcsContinental Volcanic Arcs

Potential differences with respect to Potential differences with respect to Island Arcs:Island Arcs:– Assimilation of thick silica-rich crust Assimilation of thick silica-rich crust versus versus

mantle-derived partial melts mantle-derived partial melts more more pronounced effects of contaminationpronounced effects of contamination

– Low density of crust may slow magma Low density of crust may slow magma ascent ascent more potential for differentiation more potential for differentiation

– Low melting point of crust allows for partial Low melting point of crust allows for partial melting and some crust-derived meltsmelting and some crust-derived melts

Figure 17-2. Schematic diagram to illustrate how a shallow dip of the subducting slab can pinch out the asthenosphere from the overlying mantle wedge. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

A subducting slab with shallow dip can pinch out the asthenosphere from the overlying mantle wedge

Lithospheric Mantle too shallow to have garnet

Figure 17-4. Chondrite-normalized REE diagram for selected Andean volcanics. NVZ (6 samples, average SiO2 = 60.7, K2O = 0.66, data

from Thorpe et al. 1984; Geist, pers. comm.). CVZ (10 samples, ave. SiO2 = 54.8, K2O = 2.77, data from Deruelle, 1982; Davidson, pers.

comm.; Thorpe et al., 1984). SVZ (49 samples, average SiO2 = 52.1, K2O = 1.07, data from Hickey et al. 1986; Deruelle, 1982; López-

Escobar et al. 1981). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

SVZSVZ has a flat HREE which suggests a shallow garnet-free source

NVZNVZ and and CVZCVZ have a steep slope with depleted HREE which suggests a deep garnet rich source, (the garnets don’t melt) consistent with a steep slab dip angle and aesthenosphere source.

Figure 17-5. MORB-normalized spider diagram (Pearce, 1983) for selected Andean volcanics. NVZ (6 samples, average SiO 2 = 60.7, K2O

= 0.66, data from Thorpe et al. 1984; Geist, pers. comm.). CVZ (10 samples, ave. SiO2 = 54.8, K2O = 2.77, data from Deruelle, 1982;

Davidson, pers. comm.; Thorpe et al., 1984). SVZ (49 samples, average SiO2 = 52.1, K2O = 1.07, data from Hickey et al. 1986; Deruelle,

1982; López-Escobar et al. 1981). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

LILs are very soluble in aqueous fluids. LIL enrichment of the mantle wedge via aqueous fluids from dehydration of the subducting slab and sediments. Similar to Island Arcs

Figure 17-6. Sr vs. Nd isotopic ratios for the three zones of the Andes. Data from James et al. (1976), Hawkesworth et al. (1979), James (1982), Harmon et al. (1984), Frey et al. (1984), Thorpe et al. (1984), Hickey et al. (1986), Hildreth and Moorbath (1988), Geist (pers. comm), Davidson (pers. comm.), Wörner et al. (1988), Walker et al. (1991), deSilva (1991), Kay et al. (1991), Davidson and deSilva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Recall low 143Nd/144Nd and high 87Sr/86Sr is due to an isotopically enriched source such as continental crust contamination.

The CVZ exhibits substantial crustal contamination

Assimilation

Figure 17-7. 208Pb/204Pb vs. 206Pb/204Pb and 207Pb/204Pb vs. 206Pb/204Pb for Andean volcanics plotted over the OIB fields from Figures 14-7 and 14-8. Data from James et al. (1976), Hawkesworth et al. (1979), James (1982), Harmon et al. (1984), Frey et al. (1984), Thorpe et al. (1984), Hickey et al. (1986), Hildreth and Moorbath (1988), Geist (pers. comm), Davidson (pers. comm.), Wörner et al. (1988), Walker et al. (1991), deSilva (1991), Kay et al. (1991), Davidson and deSilva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Andean Pb enrichments are not much greater than OIBs, and could be derived almost solely from a subducted sediment

Figure 17-9. Relative frequency of rock types in the Andes vs. SW Pacific Island arcs. Data from 397 Andean and 1484 SW Pacific analyses in Ewart (1982) In R. S. Thorpe (ed.), Andesites. Wiley. New York, pp. 25-95. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Andean chemistry is similar to Island Arcs. They also have as their main source the depleted mantle above the subducted slab.

However, Andean volcanics are more evolved, as they must pass through continental lithosphere, which has a lower melting point than the rising magma.

Figure 17-11. Schematic cross sections of a volcanic arc showing(a)an initial state followed by(b) trench migration toward the continent resulting in a destructive boundary and subduction erosion of the overlying crust. (c)Alternatively, trench migration away from the continent results in extension and a constructive boundary. In this case the extension in (c) is accomplished by “roll-back” of the subducting plate. An alternative method involves a jump of the subduction zone away from the continent, leaving a segment of oceanic crust (original dashed) on the left of the new trench. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

http://geoweb.princeton.edu/events/abstract_talk_Princeton.pdf

Figure 17-10. Map of the Juan de Fuca plate-Cascade Arc systemAlso shown are the approximate locations of the subduction zone as it migrated westward to its present location.

Figure 17-15a. Major plutons of the North American Cordillera, a principal segment of a continuous Mesozoic-Tertiary belt from the Aleutians to Antarctica. After Anderson (1990, preface to The Nature and Origin of Cordilleran Magmatism. Geol. Soc. Amer. Memoir, 174. The Sr 0.706 line in N. America is after Kistler (1990), Miller and Barton (1990) and Armstrong (1988). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Hundreds to thousands of individual intrusionsThe range of volcanics from basalts to rhyolites is matched by the plutonics:Gabbro -> diorite -> tonalite -> granodiorite -> granite

Quartz-richGranitoid

9090

6060

2020

QuartzSyenite

QuartzMonzonite

QuartzMonzodiorite

Syenite Monzonite Monzodiorite10 35 65 90

5

Quartzolite

Granite Grano-diorite

Tonalite

Alka

li Feld

spar

Gra

nite

Q

A P

Figure 17-15b. Major plutons of the South American Cordillera, a principal segment of a continuous Mesozoic-Tertiary belt from the Aleutians to Antarctica. After USGS.

Figure 17-16. Schematic cross section of the Coastal batholith of Peru. The shallow flat-topped and steep-sided “bell-jar”-shaped plutons are stoped into place. Successive pulses may be nested at a single locality. The heavy line is the present erosion surface. From Myers (1975) Geol. Soc. Amer. Bull., 86, 1209-1220.

Granitoid magmas rise to, and freeze at, similar shallow subvolcanic levels of the crust.

Figure 17-17. Harker-type and AFM variation diagrams for the Coastal batholith of Peru. Data span several suites from W. S. Pitcher, M. P. Atherton, E. J. Cobbing, and R. D. Beckensale (eds.), Magmatism at a Plate Edge. The Peruvian Andes. Blackie. Glasgow.

Consistent with fractional crystallization of plagioclase and pyroxene +/- magnetite, later giving away to hornblende and biotite , from initial gabbroic, tonalitic, or quartz diorite parental material

Notice that the great majority of Peruvian samples are calc-alcaline

Figure 17-18. Chondrite-normalized REE abundances for the Linga and Tiybaya super-units of the Coastal batholith of Peru and associated volcanics. From Atherton et al. (1979) In M. P. Atherton and J. Tarney (eds.), Origin of Granite Batholiths: Geochemical Evidence. Shiva. Kent. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Coastal Peru batholiths have the same REE profiles as coastal Peru volcanics

Figure 17-19. a. Initial 87Sr/86Sr ranges for three principal segments of the Coastal batholith of Peru (after Beckinsale et al., 1985) in W. S Pitcher, M. P. Atherton, E. J. Cobbing, and R. D. Beckensale (eds.), Magmatism at a Plate Edge. The Peruvian Andes. Blackie. Glasgow, pp. 177-202. . b. 207Pb/204Pb vs. 206Pb/204Pb data for the plutons (after Mukasa and Tilton, 1984) in R. S. Harmon and B. A. Barreiro (eds.), Andean Magmatism: Chemical and Isotopic Constraints. Shiva. Nantwich, pp. 235-238. ORL = Ocean Regression Line for depleted mantle sources (similar to oceanic crust). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Lima segment intruded into younger, thinner crust so radiogenic 87Sr low, reflecting the mantle derived parent. Arequipa intrudes and assimilated old thick crust so 87Sr high.

Lima segment has high 206Pb reflecting minor assimilation of Pacific sediments

Figure 17-20. Schematic diagram illustrating (a) the formation of a gabbroic crustal underplate at an continental arc and (b) the remelting of the underplate to generate tonalitic plutons. After Cobbing and Pitcher (1983) in J. A. Roddick (ed.), Circum-Pacific Plutonic Terranes. Geol. Soc. Amer. Memoir, 159. pp. 277-291.

Experiments show Tonalites(granitoids with low K-spar) can be formed by the partial fusion remelting of gabbroic magmas under hydrous conditions.a.Up-arched mantle results in partial melting and underplate gabbros.b.During later compression, heat added by more underplate magmas remelts the underplate gabbros to produce tonalites.

Why are granitoids so abundant?

Figure 17-23. Schematic cross section of an active continental margin subduction zone, showing the dehydration of the subducting slab, hydration and melting of a heterogeneous mantle wedge (including enriched sub-continental lithospheric mantle), crustal underplating of mantle-derived melts where MASH processes may occur, as well as crystallization of the underplates. Remelting of the underplate to produce tonalitic magmas and a possible zone of crustal anatexis is also shown. As magmas pass through the continental crust they may differentiate further and/or assimilate continental crust. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.