geochemistry, tectonics, and crustal evolution of basement rocks in the eastern rhodope massif,...

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Geochemistry, tectonics, and crustalevolution of basement rocks in theEastern Rhodope Massif, BulgariaNikolay Bonev a b , Robert Moritz c , Istvan Márton c , MassimoChiaradia c & Peter Marchev da Department of Geology and Palaeontology, Sofia University ‘St.Kliment Ohridski’, 1504 Sofia, Bulgariab Department of Geology, Miami University, Oxford, OH 45056,USAc Section des Sciences de la Terre, University of Geneva, CH‐1205Geneva, Switzerlandd Department of Geochemistry and Petrology, Geological Institute,1113 Sofia, BulgariaVersion of record first published: 04 Jun 2009.

To cite this article: Nikolay Bonev , Robert Moritz , Istvan Márton , Massimo Chiaradia & PeterMarchev (2010): Geochemistry, tectonics, and crustal evolution of basement rocks in the EasternRhodope Massif, Bulgaria, International Geology Review, 52:2-3, 269-297

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Geochemistry, tectonics, and crustal evolution of basement rocks in theEastern Rhodope Massif, Bulgaria

Nikolay Boneva,b*, Robert Moritzc, Istvan Martonc, Massimo Chiaradiac and

Peter Marchevd

aDepartment of Geology and Palaeontology, Sofia University ‘St. Kliment Ohridski’, 1504

Sofia, Bulgaria; bDepartment of Geology, Miami University, Oxford, OH 45056, USA;cSection des Sciences de la Terre, University of Geneva, CH-1205 Geneva, Switzerland;dDepartment of Geochemistry and Petrology, Geological Institute, 1113 Sofia, Bulgaria

(Accepted 13 November 2008 )

Orthogneisses derived from granitoids with Variscan protolith ages dominate the

lower unit of high-grade metamorphic basement of the Eastern Rhodope Massif

in south Bulgaria. We present whole-rock geochemistry and Sr–Pb isotopic

composition of these orthogneisses, which are compared with Pb isotopes of

parametamorphic rocks, and hydrothermal ore deposits and associated rocks, to

better constrain their composition, origin, and contribution to late Alpine

hydrothermal processes. The igneous mineral assemblage is partly preserved, and

the field textures and microstructures of the orthogneisses are consistent with a

ductile, amphibolite-grade tectono-metamorphic overprint during Alpine time,

when they were involved in the metamorphic nappe stack. Whole-rock

geochemistry revealed compositions of the orthogneisses largely unaffected by

the amphibolite-grade metamorphism, displaying a magmatic differentiation

trend of the igneous protoliths. The protoliths are peraluminous medium-K calc-

alkaline S-type granitoids, whose tectono-magmatic setting discrimination

consistently indicates a continental volcanic arc origin. The orthogneisses present

trace element and rare-earth elements (REE) patterns based on which a group of

high-field strength elements-depleted and REE fractionated orthogneisses and a

group of LREE-enriched orthogneisses can be distinguished. Both geochemical

groups show compositions similar to the bulk and upper continental crust and its

sedimentary counterparts. Crustal Pb isotope ratios (206Pb/204Pbi518.24–18.66)

of the orthogneisses are comparable to the paragneisses (206Pb/204Pbi518.31–

18.93) and uniform in both (207Pb/204Pbi515.64–15.72) and 208Pb/204Pbi ratios in

the paragneisses (38.23–38.60) and the orthogneisses (38.32–38.56). The trace

element data and 87Sr/86Sri isotopes of the orthogneisses (0.7050–0.7117) overlap

those of the parametamorphic rocks (0.7039–0.7144), and confirm the supra-

crustal origin of the igneous precursors. A heterogeneous crustal source region is

suggested in which melting and crustal contamination during magma genesis

with subsequent fractional crystallization was involved in the petrogenesis.

Comparative Pb isotope systematics suggests that a significant crustal Pb input to

ore-forming hydrothermal fluids was derived primarily from the metamorphic

basement, implying that the brittlly deformed basement during crustal extension

acted as an immediate environment for fluid leaching during late Alpine

hydrothermal ore-forming processes.

*Corresponding author. Email: [email protected]

International Geology Review

Vol. 52, Nos. 2–3, February–March 2010, 269–297

ISSN 0020-6814 print/ISSN 1938-2839 online# 2010 Taylor & FrancisDOI: 10.1080/00206810802674493http://www.informaworld.com

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Keywords: basement tectonics; microstructures; whole-rock geochemistry; Sr-Pb

istotopic chemistry; Eastern Rhodope Massif; Bulgaria

Introduction

The Alpine collisional system in the northern Aegean region exposes crystalline

basement in several internal metamorphic massifs including, from west to east, the

Serbo-Macedonian Massif, the Rhodope Massif and the Strandja Massif (Figure 1,

inset; e.g. Jacobshagen et al. 1978; Papanikolaou 1997), which represent pre-Alpine

crustal domains that were actively involved during Mesozoic-Cenozoic tectonic

evolution along the south margin of the Eurasian plate (e.g. Robertson et al. 1996;

Stampfli and Borel 2002, 2004; Stampfli and Kozur 2006). Continuity with other massifs

in the northern-central Aegean, where Variscan metamorphic and igneous basement are

exposed in the Sakarya Zone of western Turkey and the Pelagonian Zone in northern

Greece, was also suggested on the basis of comparable lithology, metamorphic grade,

and radiometric age constraints (Okay et al. 1996, 2001; Lips et al. 1998; Vavassis et al.

2000). Only recently was it recognized, based on radiometric dating, that Late

Proterozoic and Early Palaeozoic Gondwana-derived and Variscan (referring to Late

Palaeozoic tectono-metamorphic and magmatic cycle known in Europe) crustal

fragments in the Serbo-Macedonian Massif and the Pelagonian Zone metamorphic

basement (Anders et al. 2006, 2007; Himmerkus et al. 2006), which are comparable to

the metamorphic basement of the Menderes Massif (Hetzel and Reischmann 1996; Loos

and Reischmann 1999), were amalgamated within the Alpine orogen.

The Rhodope Massif constitutes a major tectonic zone dominated by high-grade

basement metamorphic rocks that are intruded by unmetamorphosed Late

Cretaceous to Miocene granitoids (Meyer 1968, Soldatos and Christofides 1986;

Dinter et al. 1995; Peytcheva et al. 1999; Marchev et al. 2006) and covered by

widespread Tertiary sedimentary, sedimentary-volcanic and volcanic rocks (Ivanov

and Kopp 1969; Innocenti et al. 1984; Del Moro et al. 1988; Harkovska et al. 1989;

Zagorchev 1998; Boyanov and Goranov 2001). It is interpreted as a south-directed

nappe complex resulting from the ductile syn-metamorphic thrusting in the hanging

wall of a north-dipping Cretaceous subduction setting located in the Vardar Zone in

the south (Ricou et al. 1998). The ductile thrusting, leading to crustal thickening

created gravitational instability within the massif, and was accompanied and

followed by crustal extension. Recent detailed geochemical, isotopic and radiometric

studies of the gneissic basement in the Central Rhodope revealed the presence of

Late Carboniferous and Late Jurassic continental arc I-type magmatic suites

(Peytcheva et al. 2004; Cherneva and Georgieva 2005; Turpaud 2006) that were

subsequently sliced within the nappe stack during the Alpine tectono-metamorphic

construction of the orogen (Burg et al. 1990; Koukouvelas and Doutsos 1990).

In the eastern Rhodope Massif, regionally extensive gneiss complexes occur in

systematically lower structural levels of the metamorphic basement (Figure 1),

whose origin was the subject of controversy for years. Most of the previous workers

have suggested a Precambrian age and sedimentary protoliths for the gneisses,

assuming metasedimentary stratigraphically ordered successions (Ivanov 1961;

Boyanov et al. 1963; Kozhoukharov et al. 1988). Recently their igneous origin was

imprecisely documented either as syn- to post-collisional or volcanic arc I and S-type

affinity metagranitoids (Macheva and Kolcheva 1992; Ovtcharova and Sarov 1995),

whereas U–Pb zircon and Rb–Sr dating rather consistently indicates a Permo-

Carboniferous intrusion age (Peytcheva and Quadt 1995; Mposkos and Wawrzenitz

270 N. Bonev et al.

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Figure 1. Synthetic tectonic map of the Eastern Rhodope Massif in southern Bulgaria(simplified after Bonev 2006), showing locations of the studied metamorphic samples and thehydrothermal mineralizations and ore deposits mentioned in the text. Inset: tectonic frameworkof the Alpine system in the northern Aegean region of the eastern Mediterranean domain.

International Geology Review 271

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1995; Liati 2005; Turpaud 2006) of the magmatic precursors. The composition and

geochemical signature of the Eastern Rhodope meta-igneous gneissic basement,

however, remains less well understood, which hampers establishing its origin and

more precisely constraining the affinity to past tectonic environments. Therefore,

details of the geochemistry of gneissic basement rocks are particularly important

because they allow the assessment of the original geodynamic setting of the

continental crust during the Mesozoic-Cenozoic collisional history in the northern

Aegean region. Moreover, the gneissic basement is the source of abundant clasts and

constitutes the footwall of Tertiary sedimentary basins. Intense fluid-rock

interaction during the formation of low-sulfidation gold mineralizations and ore

deposits at basement-sedimentary basin interfaces might be an important source of

different metals and gangue components in these systems (Marchev et al. 2003,

2004a; Marton et al. 2006, 2007). Additionally, the composition of Tertiary magmas

during their ascent through the crust was changed by crustal contamination and

assimilation processes. Therefore, their geochemical composition might reflect a

significant gneissic basement component (Marchev et al. 2004b).

This paper focuses on the orthogneisses in the high-grade metamorphic basement

of the Eastern Rhodope Massif in south Bulgaria, providing new data on their

whole-rock geochemical characteristics, Sr and Pb isotopic composition and

microstructures. It also provides Sr, Nd, and Pb isotopic chemistry of the basement

paragneisses. The aims of this study are: (1) to extend knowledge regarding the

composition of the gneissic basement on a regional-scale and to contribute to a

better understanding of its origin and tectonic setting, (2) to provide a tectonic

template for Late Palaeozoic continental arc magmatism and subsequent Alpine

history, and (3) to compare Pb isotopic compositions of the ortho- and paragneisses

with those of adjacent volcanic rocks and pyrites from non-mineralized

metamorphic basement rocks and sedimentary-strata-hosted hydrothermal gold

mineralization.

Geological setting

The regional tectonic pattern of the Eastern Rhodope Massif is dominated by two

late-Alpine extensional metamorphic domes, the Kesebir-Kardamos and the Byala

reka-Kechros domes, exposing four tectono-stratigraphic units (Figure 1; Bonev

2006, Bonev and Beccaletto 2007). Structurally, from the base to the top, these units

include: (1) a lower high-grade unit, (2) an upper high-grade unit, (3) an overlying

low-grade Mesozoic unit, and (4) a sedimentary and volcanic unit of Tertiary cover

sequences related to late-orogenic extension and collision. The lower high-grade and

the upper high-grade units collectively constitute the high-grade metamorphic

basement. These basement units are bounded by contractional, syn-metamorphic

thrust contacts and predominantly by low-angle extensional detachments which are

related, respectively, to pre-latest Late Cretaceous crustal thickening and Tertiary

extension (Burg et al. 1996; Krohe and Mposkos 2002; Bonev 2006; Bonev et al.

2006a). Both large-scale dome structures expose a lower high-grade basement unit in

their cores within the footwall of the detachments, structurally flanked by the upper

high-grade basement unit and the overlying low-grade Mesozoic unit in the hanging

wall. The complex Alpine metamorphic history of the high-grade basement units

includes preserved relics of ultrahigh-pressure and high-pressure metamorphism and

dominant medium-pressure type amphibolite-upper greenschist-facies metamorph-

ism (e.g. Liati 2005 and references therein).

272 N. Bonev et al.

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The lower high-grade basement unit is mainly composed of various types of

orthogneiss, intercalated with paragneiss, schist and amphibolite layers, and

impregnated by sillimanite-bearing migmatites (Bonev 2004), all having a

continental origin. U–Pb zircon conventional and SHRIMP dating revealed

magmatic protolith ages of 265–319 Ma for the orthogneisses (Peytcheva and

Quadt 1995, Liati 2005; Turpaud 2006). These ages indicate that Variscan crustal

fragments were involved in the basement nappe stack during the Alpine tectono-

metamorphic history.

The upper high-grade basement unit represents a lithologically heterogeneous

succession with poorly constrained ages, consisting of an intimately intercalated

metasedimentary (para-amphibolites, marbles, metapelitic schists, paragneisses) and

meta-igneous rocks (gabbros, quartzofeldspathic gneisses) of mixed continental and

oceanic affinity. Oceanic rocks include upper mantle component of meta-ophiolite

(peridotite, cumulate) bodies and lenses, which display bimodal mid-ocean ridge and

supra-subduction zone tholeiitic-boninitic affinities (Kolcheva and Eskenazy 1988;

Haydoutov et al. 2004; Bonev et al. 2006c).

The Jurassic-Early Cretaceous greenschist-facies rocks (Boyanov and Russeva

1989) form a distinct low-grade Mesozoic unit, which is regarded as an extension of

the Circum-Rhodope Belt from the Chalkidiki Peninsula in Greece (e.g. Kauffmann

et al. 1976; Papanikolaou 1997). It consists of greenschists and phyllites overlain by

arc tholeiitic-boninitic mafic lavas and meta-pyroclastic rocks in turn overlain by

turbiditic-like clastic and carbonaceous successions, which contain reworked Upper

Permian and Middle-Upper Triassic shallow-water carbonates. Petrology and

geochemistry of the arc magmatic suite together with the lithologic context of the

low-grade unit is interpreted as representing an island arc-accretionary assemblage

(Bonev and Stampfli 2003, 2008).

The supracrustal clastic and carbonaceous sedimentary and volcanic unit consists

of syn- and post-tectonic cover sequences that range in stratigraphic age from

Maastrichtian-Paleocene to Miocene (Boyanov and Goranov 2001). Early

Palaeogene sedimentary rocks form part of a syntectonic hanging wall supra-

detachment half-grabens in fault contact with the detachment zones (Bonev et al.

2006a) or unconformably overlie the metamorphic basement. Widespread volcanic

activity is recorded within the late Eocene-Oligocene interval (Harkovska et al.

1989).

Field data, petrography and microstructures

Deformation of the orthogneisses in the lower high-grade basement unit relates to

crustal nappe stacking and extensional events (Bonev 2006, Bonev et al. 2006a). The

orthogneisses record an intense ductile and ductile-brittle shearing that imparted a

distinct structure to the rocks, with planar-linear fabrics (i.e. S-L tectonites) in

proto-mylonites and mylonites. In the field, they show a variety of textures and

fabrics, which are related to the metamorphism and deformation. We, therefore,

refer to distinct rock types as metagranites when igneous textures are partly

preserved, and as orthogneisses whose primary igneous textures are strongly

obliterated by tectonic-metamorphic processes. Because contacts with the overlying

upper high-grade basement unit are tectonic, mostly marked by extensional ductile

shear zones and faults, possible primary intrusive relationships of the orthogneisses

with the upper high-grade unit lithologies are inconspicuous and generally not

observable in the field. Therefore, the relationships between distinct meta-igneous


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rock types can only be examined within the cores of the dome structures. There, the

dominant gneissic rock type is a leucocratic-mesocratic equigranular medium to

coarse grained two-mica orthogneiss that shows textural variations of banded or

layered varieties (Figure 2(a)). Metagranites are commonly mesocratic due to the

predominance of biotite over white mica, contain megacrystic alkali feldspar

(Figure 2(b)), and locally are intrusive into equigranular orthogneisses. The

metagranites locally exhibit primary igneous layering and/or foliation defined by

alignment of the euhedral K-feldspar megacrysts. Another abundant textural variety

is augen orthogneiss, which consists of up to 2 cm feldspar augen and modally

Figure 2. Field and microphotographs of the orthogneisses. (a) Equigranular orthogneiss(sample Ks- 553). Asymmetric boudinage of aplitic vein depicts sinistral, N-directed ductileshearing in extensional shear zone. (b) K-feldspar megacrystic metagranite showingamphibolite facies SW-directed (dextral) ductile S/C fabrics (sample Br-08). (c)Recrystallized K-feldspar porphyroclast forming ‘core and mantle structure’ in a ductilematrix (sample Br-202). Note myrmekite (arrows) in high-stress margins of the porphyroclastand recrystallized quartz-feldspar aggregates in the mantle. (d) Quartz ribbons (R) andrecrystallized quartz-feldspar aggregates in augen orthogneiss (sample Ks-6). Note oblique toribbon outlines grain-shape fabric (Qf) of quartz grains with irregular boundaries. (e) Textureof protomylonite derived from equigranular orthogneiss. Note muscovite ‘fish’ associatedwith incipient shear bands and plagioclase deformation band (arrow) (sample Br-28). (f)Equigranular orthogneiss consisting of different size recrystallized quartz grains with irregularboundaries, together with recrystallized plagioclase and mica (sample Ks-95). Abbreviations:Ms, muscovite; Bt, biotite; Pl, plagioclase; Kfs, alkali feldspar; Qtz, quartz; Ap, apatite.

274 N. Bonev et al.

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predominant white mica over biotite. The deformation-induced transition from

augen to banded orthogneisses is evident in mylonitic gneiss varieties. All gneiss

types contain angular and rounded enclaves of biotite gneiss, amphibolite, and schist

intercalations, suggesting intrusion of their magmatic precursors into pre-existing

metamorphic basement or country rock (meta-) sedimentary sequence.

The orthogneisses have experienced variable overprint of amphibolite-facies

metamorphism; however, primary mineral phases are generally preserved in most

cases. The mineral assemblage in the studied gneisses consists of quartz + alkali

feldspar + plagioclase + biotite + muscovite. Only feldspars and partly micas reflect

the igneous assemblage. Accessories include apatite, zircon, rare monazite, garnet,

and allanite. Macheva and Kolcheva (1992) reported for the Byala reka dome

metagranitoids primary mineral compositions of orthoclase (Or85–93Ab7–15), plagio-

clase (An29–35), amphibole (ferroedenite-edenite), and biotite of eastonite-side-

rophyilite series, and also metamorphic epidote, garnet (Alm58–63 Gross32–37 Pyr5),

and phengitic white mica. Replacement products at the expense of feldspars are

epidote-zoisite group minerals and secondary white mica and chlorite after micas.

The orthogneisses commonly exhibit variable intensity in foliation, depending on

the degree of a solid-state ductile deformation overprint, which largely obliterated the

original igneous grain sizes. This is a ubiquitous regional foliation in equigranular and

banded orthogneisses, representing compositional layering or gneissic planar fabrics

defined by the alignment of flattened K-feldspar magacrysts and micas in the

metagranites (Figure 2(a) and (b)). This fabric grades to mylonitic foliation in the

ductile shear zones beneath the detachment faults (Bonev 2006). Augen orthogneisses

and metagranites display porphyroclastic texture, characterized by alkali and

plagioclase feldspar porphyroclasts in a ductile quartz-feldspathic-micaceous matrix.

The feldspars display macroscopically deformational features related to the

development of pervasive S/C fabrics in the metagranites and augen orthogneisses

(Figure 2(b)), whereas in thin section they show recrystallized porphyroclasts with

typical ‘core and mantle structure’ (Figure 2(c); e.g. White 1975; Passchier and

Simpson 1986). In the latter, the feldspar cores have highly serrated boundaries

displaying generally weak internal deformation, which is expressed by undulatory

extinction in the larger porphyroclasts. Dynamic recrystallization of the K-feldspar

occurs in the mantle, where decreasing grain-size of the porphyroclasts is

accompanied by recrystallization into small subgrains and rotation-recrystallized

new grains in a mixed-phase aggregate together with quartz. This involves subgrain

rotation and subsequent grain-boundary migration recrystallization, which latter

may have produced some strain-free grains in the mantle (Figure 2(c)). Both

recrystallization mechanisms indicate that a climb-accommodated dislocation creep

regime has operated during feldspar deformation (Tullis and Yund 1985).

The observed symplectic vermicular intergrowth of quartz and plagioclase

resulting in myrmekite relates to deformation-induced replacement of the K-

feldspar (e.g. Simpson and Wintsch 1989). Myrmekites are developed on the high-

stress sides of the porphyroclasts facing the finite shortening direction of ductile

shear flow (Figure 2(c)). In the matrix, the alkali and plagioclase feldspar occurs as

small equant grains with straight boundaries forming polygonal mosaic quartz-

feldspar aggregates, where both phases have the same grain-size (Figure 2(d)). This

suggests that the matrix accommodated a significant portion of the deformation,

consistent with weak internal deformation in the cores of the porphyroclasts.

Plagioclases also show internal deformation expressed by undulatory extinction,


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lattice bending, twinning and recrystallization into small subgrains surrounding the

old grains, mostly in the equigranular orthogneisses (Figure 2(e) and (f)). The

dislocation climb in the feldspars is limited approximately at 450uC (Tullis and

Yund 1987), and observed feldspar microstructures are indicative of the temperature

range of middle amphibolite-grade conditions (e.g. Simpson 1985; Pryer 1993).

The deformation of quartz resulted in the development of foliation-parallel quartz

and quartz-feldspar aggregates giving macro- and microscopic appearance of

‘striped gneiss’ (Figure 2(a) and (d); e.g. Passchier and Trouw 1996), most

pronounced in the equigranular to banded orthogneisses and to a lesser extent in

the augen orthogneisses. Microscopically, the quartz aggregates consist of

recrystallized grains of various sizes and irregular boundaries (Figure 2(e) and (f)).

Strain features in quartz grains in these aggregates, such as undulose extinction and

development of subgrains are common. Quartz also occurs in continuous quartz

ribbons, similar to Type 4 described by Boullier and Bouchez (1978), which are

composed of quartz aggregates consisting of elongated dynamically recrystallized

grains with highly serrated grain boundaries that exhibit grain-shape fabric oblique

to the ribbon outlines, i.e. the foliation (Figure 2(d)). Discontinuous quartz ribbons

that are transitional to various-sized quartz aggregates mentioned above are also

present, and are composed of rectangular strain-free grains and small recrystallized

quartz grains displaying undulose extinction, subgrains and irregular boundaries.

This suggests that the recovery process operated together with the ongoing

deformation after ribbon formation. The large grain-size and highly serrated quartz

boundaries are features characteristic of its mobility and recrystallization dominated

by high-temperature grain boundary migration mechanism (regime 3; Hirth and

Tullis 1992). Relict quartz porphyroclasts are rare and are surrounded by fine-

grained recrystallized grains and subgrains in their mantles (Figure 2(d)). Quartz

grains in the orthogneisses commonly show a strong c-axis lattice preferred

orientation (Bonev et al. 2006a).

Micas show preferred orientation of individual laths aligned parallel to the

foliation in the rocks (Figure 2(e) and (f)). Most mica grains are deformed in a

ductile fashion with bending and weak recrystallization at the grain boundaries,

forming ‘mica fish’ lying at a low-angle to the foliation in deformed orthogneisses

(Figure 2(e)).

In brief, all described microstructures of the main constituent phases and

macroscopic features of the orthogneisses relate to high-temperature fabric

development associated with internal ductile deformation in middle-upper

amphibolite facies that resulted from a ductile nappe stacking tectono-metamorphic

event in pre-latest Late Cretaceous times. Macro- and microscopic features of the

orthogneisses are consistent with characteristic crystal-plastic deformation in

corresponding metamorphic grade described for granitoid rocks (e.g. Tullis 2002).

The subsequent overprinting extension-related semi-brittle and brittle microstruc-

tures and shear fabrics of the orthogneisses record their exhumation path in the

vicinity of detachments as presented elsewhere (Bonev 2006).

Geochemistry

Whole-rock geochemistry

Twenty-four representative samples of orthogneisses and one migmatite sample

from the lower high-grade basement unit within the cores of both metamorphic

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domes were selected for analyses together with seven samples (six paragneisses and

one metabasite) from the upper high-grade unit on their flanks (see Figure 1, for

sample locations). Whole-rock chemical analyses were performed by analytical

facilities at the University of Lausanne (Switzerland) and calibrated against both

international and internal standards (e.g. MFTH, AGV, QLO, GA, SY-2, NIM-G,

GSP-1, SDC-1, and BHVO-1). Major- and trace element concentrations were

determined by X-ray fluorescence (XRF) on fused discs and pressed pellets,

respectively, using a Philips PW 2400 automated spectrometer. Accuracy of the

analyses for the major elements is ,1%, and ,2% for the trace elements. Rare-earth

elements (REE) concentrations were analysed by laser ablation inductively coupled

plasma mass spectrometry (LA-ICP-MS) on a Perkin-Elmer 6100 ELAN instrument

on fused discs, using NBS612 standards. The chemical analyses of selected samples

are listed in Tables 1 and 2. The low loss on ignition (LOI) values in the samples,

which is generally used as a crude measure of alteration, attests for a minimal

influence of this process in the studied rocks.

Because major element and possibly some trace element mobility is expected to

induce changes in the whole-rock composition due to high-grade metamorphism of

the meta-igneous rocks, standard geochemical diagrams have been used to: (1)

evaluate element mobility, and (2) to define geochemical trends (Figure 3). The

orthogneisses display well-defined linear trends in variation diagrams of major oxides

plotted against SiO2 used as an index of differentiation. The exceptions are Na2O and

K2O with less pronounced correlation on the diagrams, suggesting their mobility or

reflecting inheritance from the magmatic process. The trace elements present linear

trends and a good correlation with decreasing SiO2 indicating immobile behaviour of

the high-field strength elements (HFSE, e.g. Nb, Hf). Large-ion-lithophile elements

(LILE) such as Ba and Rb show two distinct trends, suggesting either their mobility or

influence of magmatic differentiation processes. Because of the similar trends of Sr

and Na2O vs. SiO2 and the trend of Ba (Figure 3), this implies that the differentiation

processes particularly involved feldspar as a fractionating phase, and is responsible for

the observed patterns for these elements. We, therefore, consider that the observed

distribution patterns demonstrate a conservative nature of the major and trace

elements in the studied rocks, representing igneous differentiation trends and

reflecting also magmatic abundances.

The orthogneisses have SiO2 abundances in the range of 66.7–76.5 wt-%. They are

characterized by low TiO2, MgO, and Fe2O3 concentrations, and all these oxides

display a negative correlation with the SiO2 that is characteristic for the normal

trend of magmatic differentiation (Figure 3, Tables 1 and 2). Alkali contents are

variable (av. total alkali ,7 wt-%), with slightly high K/Na ratio (av. 1.15). The

orthogneisses are peraluminous (A/CNK.1, molecular Al2O3/CaO + Na2O + K2O)

and have normative corundum . 1 wt%, both features together with Na2O ,3.2 wt-

% at K2O.4 wt-% (Tables 1 and 2) defined as pertinent for S-type granitoids

(Chappell and White 1974), displaying a calc-alkaline affinity (Figure 4). In terms of

the trace elements, the orthogneisses have low HFSE abundances (e.g. Nb, Y), low

concentrations of some compatible elements (e.g. Cr, Ni, V) characteristic of

granitoids and also low Ba/Rb ratio (Tables 1 and 2) reflecting the evolved nature of

the granitic protoliths. Major and trace element characteristics of the orthogneisses

classify their plutonic protoliths as medium-K calc-alkaline granitoids (Figure 4;

Table 1 and 2), that have retained their initial compositions, and were largely

unaffected by the experienced high-grade metamorphism.


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Migmatite associated with the orthogneisses in the lower high-grade basement

unit and the paragneisses from the upper high-grade basement unit at the Kesebir

dome shows comparable major- and trace elements and REE abundances to those

displayed by the orthogneisses in both domes (Tables 1 and 2). Details concerning

the geochemistry of the migmatite and paragneisses are beyond the scope of the

study, and these rock types are used below only for the comparison with the meta-

igneous rocks.

Chondrite-normalized REE patterns of the orthogneisses present two distinct

profiles characterized respectively by light rare earth element (LREE) enrichment

relative to the heavy rare-earth elements (HREE) with a pronounced negative Eu

anomaly (same pattern normalized to N-MORB, not shown), or fractionated REE

patterns without Eu anomaly (Figure 5(a) and (b)). The migmatite and paragneiss

samples also display fractionated REE patterns similar to that shown by the

orthogneisses (Figure 5(b)). These distinct patterns are presented in a similar manner

Figure 3. Variation diagrams of selected major and trace elements for the basementorthogneisses.

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Table 1. Selected chemical analyses of the basement ortho- and parametamorphic rocks fromthe Kesebir dome. Location of the samples in Figure 1.

Sample Ks-55 Ks-100 Ks-6 Ks-18 Ks-160 Ks-688 Ks-693 Ks-M Ks-206 Ks-H1

Rocktype agn agn agn egn bgn bgn mgr mig msgn msgn

SiO2 72.80 72.53 73.67 72.59 70.27 66.72 75.49 73.17 77.17 77.04TiO2 0.28 0.17 0.09 0.20 0.33 0.46 0.17 0.17 0.26 0.22Al2O3 14.43 14.02 14.53 14.25 14.80 15.83 12.95 14.35 12.18 12.16Fe2O3 2.19 1.34 1.31 2.52 2.77 3.56 1.76 1.43 1.77 2.40MnO 0.04 0.01 0.02 0.05 0.04 0.04 0.03 0.04 0.03 0.03MgO 0.52 0.26 0.20 0.48 0.71 1.00 0.39 0.41 0.83 0.80CaO 1.99 0.35 1.29 1.81 1.66 2.81 0.56 1.06 1.54 0.42Na2O 3.24 2.44 3.79 3.16 3.19 3.88 2.62 2.95 2.42 0.05K2O 3.13 7.61 3.62 4.19 4.47 2.22 4.66 4.72 2.59 4.15P2O5 0.08 0.12 0.11 0.09 0.25 0.10 0.12 0.19 0.07 0.28LOI 0.75 0.25 0.58 0.21 0.53 1.92 0.63 0.84 1.06 2.51Total 99.44 99.10 99.21 99.55 99.01 98.53 99.37 99.32 99.94 100.06

Nb 12.4 9 5 10 13 19.3 8 14.2 6 13Zr 171 13 41 126 176 110 92 91 101 86Y 14.7 19 22 18 20 8.1 15 17.6 13 23.7Ta 9.59 n.a. n.a. n.a. n.a. 9.58 n.a. 363.71 n.a. 365.47Rb 107.9 259 90 141 151 126.9 142 213.3 89 115.2Sr 305 240 138 250 177 239 82 95 112 16Ba 993 2285 329 962 873 210 322 406 532 372U ,2, 2 4 6 4 ,2, 4 7 ,2, ,2,Th 13 5 4 11 17 7 11 7 9 ,2,Pb 9 55 38 24 36 20 35 31 14 ,2,Hf 4.02 7 7 7 7 3.84 8 17.59 6 20.41Sc n.a. ,2, 2 4 4 n.a. 3 n.a. 7 n.a.Cr 3 9 10 4 14 5 9 5 63 8V 16 12 8 25 32 58 15 12 42 20Ni ,2, 3 ,2, 3 4 ,2, 2 ,2, 8 ,2,Ga 19 14 15 18 19 22 16 20 15 19Zn 50 37 19 34 61 77 41 47 27 66Cu n.a. 4 9 11 12 n.a. 8 n.a. 7 n.a.Co n.a. 4 ,2, 5 4 n.a. 4 n.a. 6 n.a.

La 31.79 9 7 24 31 25.33 17 52.52 15.57 19.44Ce 71.32 ,3, 23 61 69 43.83 30 39.20 29.83 14.90Pr 7.19 n.a. n.a. n.a. n.a. 4.95 n.a. 33.60 3.66 15.37Nd 27.66 ,4, 10 24 29 19.82 12 23.50 14.63 12.10Sm 5.05 n.a. n.a. n.a. n.a. 4.00 n.a. 17.71 3.08 13.12Eu 0.96 n.a. n.a. n.a. n.a. 0.79 n.a. 4.42 0.67 4.10Gd 3.31 n.a. n.a. n.a. n.a. 2.83 n.a. 13.67 2.55 12.16Tb 0.52 n.a. n.a. n.a. n.a. 0.43 n.a. 12.22 0.33 12.38Dy 2.39 n.a. n.a. n.a. n.a. 2.39 n.a. 7.88 2.29 14.53Ho 0.34 n.a. n.a. n.a. n.a. 0.42 n.a. 8.69 0.45 14.94Er 1.19 n.a. n.a. n.a. n.a. 0.77 n.a. 6.69 1.28 11.25Tm 0.10 n.a. n.a. n.a. n.a. 0.14 n.a. 7.64 0.22 13.52Yb 0.85 n.a. n.a. n.a. n.a. 0.53 n.a. 5.73 1.34 13.49Lu 0.23 n.a. n.a. n.a. n.a. 0.15 n.a. 6.66 0.18 10.12

Note: Major elements (wt-%) determined by XRF, trace and rare earth elements (ppm) analysedby XRF and LA-ICP-MS.Abbreviations: bgn, banded orthogneiss; agn, augen orthogneiss; egn, equigranular orthogneiss; mgr,K-feldspar megacrystic metagranite; mig, migmatite; msgn, muscovite paragneiss; n.a. not analysed.


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Table 2. Selected chemical analyses of the basement ortho-metamorphic rocks from the Byalareka dome. Location of the samples in Figure 1.

Sample Br-28 Br-31 Br-Or2 Br-112 Br-202 Br-Pk Br-P32 Br-5 Br-8 Br-13

Rocktype bgn bgn bgn agn agn egn egn egn mgr mb

SiO2 69.58 68.27 73.51 70.12 73.39 71.39 76.47 67.52 68.63 46.03TiO2 0.42 0.49 0.33 0.37 0.24 0.23 0.02 0.49 0.30 1.85Al2O3 14.29 15.00 13.07 15.04 13.86 14.39 14.78 15.48 13.85 14.42Fe2O3 3.91 3.73 2.97 2.49 1.90 1.75 0.62 3.80 2.69 12.06MnO 0.09 0.06 0.05 0.04 0.03 0.03 0.01 0.07 0.06 0.20MgO 0.77 0.88 0.57 0.77 0.46 0.45 0.11 1.18 0.81 7.70CaO 2.70 2.15 2.66 2.20 1.88 1.67 0.97 2.79 1.93 9.33Na2O 3.54 3.26 4.33 3.42 3.12 3.06 4.10 3.51 3.60 2.44K2O 1.77 3.29 0.62 3.36 3.20 3.95 2.15 3.54 4.02 0.77P2O5 0.10 0.23 0.07 0.16 0.11 0.11 0.05 0.16 0.12 0.22LOI 1.99 1.75 0.93 1.13 1.23 1.87 0.92 0.36 1.07 2.25Total 99.16 99.12 99.12 99.10 99.41 98.90 100.21 98.89 97.07 97.20

Nb 9.5 18.1 9.4 11.7 11 12.5 5 12 12 18Zr 195 202 206 155 146 123 15 158 134 143Y 32.5 35.6 37.3 12.6 15.8 17.5 7 16 20 32Ta 7.68 8.76 215.03 9.32 13.06 445.03 n.a. n.a. n.a. n.a.Rb 61.8 122.5 18.8 131.7 93.6 155.9 79 153 160 27Sr 158 169 155 277 281 236 131 303 234 168Ba 497 556 293 733 910 790 307 709 582 89U 1.42 4 2 2.2 2.6 2 2 3 3 2Th 7 19 9 9 11 9 8 12 15 5Pb 8 13 4 25 21 9 42 19 20 5Hf 6.88 7.26 34.06 4.24 4.19 28.8 6 7 7 6Sc n.a. n.a. n.a. n.a. n.a. n.a. ,2, 5 6 48Cr 4 2 3 5 3 3 4 16 21 192V 34 46 19 34 17 17 ,2, 63 36 339Ni ,2, ,2, 3 ,2, ,2, ,2, ,2, 5 6 65Ga 17 20 15 20 19 20 18 19 18 20Zn 63 60 34 66 46 50 24 66 45 154Cu n.a. n.a. n.a. n.a. n.a. n.a. 5 11 7 45Co n.a. n.a. n.a. n.a. n.a. n.a. 3 8 7 41

La 29.38 40.76 69.62 32.28 39.68 95.06 ,4, 33 13.83 19Ce 51.89 73.84 53.69 60.23 67.16 59.28 6 64 31.87 31Pr 6.38 8.82 44.14 7.25 8.71 53.46 n.a. n.a. 4.08 n.a.Nd 29.53 35.81 34.59 30.12 32.45 42.14 ,4, 27 19.57 18Sm 6.20 8.01 22.33 6.04 6.96 25.63 n.a. n.a. 5.06 n.a.Eu 0.92 1.01 9.67 0.72 1.22 11.91 n.a. n.a. 1.59 n.a.Gd 7.32 8.37 20.25 5.25 5.22 15.98 n.a. n.a. 5.54 n.a.Tb 0.99 1.29 16.53 0.57 0.71 13.18 n.a. n.a. 0.86 n.a.Dy 7.46 7.72 17.45 2.67 4.05 9.61 n.a. n.a. 5.54 n.a.Ho 1.68 1.55 15.30 0.53 0.78 9.17 n.a. n.a. 1.10 n.a.Er 4.47 4.98 18.52 1.45 1.60 7.13 n.a. n.a. 3.42 n.a.Tm 0.78 0.71 15.79 0.22 0.18 5.43 n.a. n.a. 0.43 n.a.Yb 4.80 5.60 17.45 1.24 1.24 6.12 n.a. n.a. 2.88 n.a.Lu 0.80 0.89 18.81 0.17 0.23 5.13 n.a. n.a. 0.47 n.a.

Note: Major elements (wt-%) determined by XRF, trace and rare earth elements (ppm)analysed by XRF and LA-ICP-MS.Abbreviations: bgn, banded gneiss; agn, augen gneiss; egn, equigranular orthogneiss; mgr,metagranite; mb, metabasite; n.a. not analysed.

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on a bulk continental crust normalized REE diagram (Figure 5(c)). In the latter, the

LREE-enriched group of orthogneisses shows a nearly flat REE pattern with

concentrations close to those of continental crust, whereas the group of

orthogneisses with fractionated REE patterns deviate significantly from the

continental crust, showing partly comparable to LREE-enriched group concentra-

tions of LREE (e.g. La, Ce, Nd), middle rare earth elements (MREE) (e.g. Dy, Er)

and Yb. Similarly, the REE patterns normalized to average European shale of both

groups are comparable to those displayed for the bulk continental crust, in which

significant overlap with the shale composition particularly for the LREE-enriched

group is observed (Figure 5(d)). Both latter diagrams strongly suggest involvement

of shallow-level crustal components in the composition of the studied rocks. In

multi-element diagrams normalized to upper crust composition, the LREE-enriched

group has composition very close to the upper crust and display slightly high LILE/

HFSE ratio (higher when normalized to N-MORB, not shown), whereas the REE-

fractionated group of samples is similar to the former group and the upper crust

with regard to LILE and LREE, having higher HFSE and HREE concentrations

(Figure 5(e) and (f)). In the multi-element diagrams all studied samples exhibit

negative Nb anomalies, and in addition, a negative Tb anomaly for the LREE-

enriched group and negative Sr, P and HFSE (e.g. Zr, Y) anomalies for the group of

REE fractionated samples.

Figure 4. Classification diagrams for the basement orthogneisses. (a) Zr/TiO2 vs. Nb/Y plot(adapted after Winchester and Floyd 1977). (b) ANK vs. ACNK diagram. (c) AFM diagram(after Irvin and Baragar 1971). (d) ACNK-normative corundum diagram (fields afterChappell and White 1974). Symbols as in Figure 3.


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Isotope chemistry

Three samples of augen orthogneisses in the cores of both basement metamorphic

domes, together with two paragneiss samples from the mantle of the Kesebir dome

and one metabasite sample in the mantle of the Byala reka dome were analysed for

Sr, Nd, and Pb isotopic composition (see Figure 1 for sample locations). Pb isotopes

Figure 5. Chondrite-normalized REE patterns and trace elements normalized diagrams forthe orthogneisses, migmatite and paragneisses. (a,b) Chondrite-normalized REE patterns.Normalization values after Taylor and McLennan (1985). (c) Samples normalized to bulkcontinental crust. Normalization values after Hofmann (1988). (d) Samples normalized toaverage European shale. Normalization values after Haskin and Haskin (1966). (e,f) Samplesnormalized to upper crust composition. Normalization values after Taylor and McLennan(1981).

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of the basement rocks were compared with the Pb isotopic composition of samples

containing pyrite in non-mineralized metamorphic and hydrothermal rocks of

nearby low-sulfidation gold mineralizations and ore deposits.

For lead chemical separation, whole-rock samples were prepared at the

Department of Mineralogy, University of Geneva. Between 100 and 150 mg of

powdered rock fractions (,70 mm) were dissolved in closed Teflon vials during 7

days on a hot plate at 140uC with a mixture of 4 ml conc. HF and 1 ml HNO3 15 M.

The sample was then dried on a hot plate, and re-dissolved in 3 ml of 15M HNO3 in

closed Teflon vials at 140uC and dried again. Sr, Nd, and Pb separation was carried

out using cascade columns with Sr-spec, TRU-spec and Ln-spec resins following a

modified method after Pin et al. (1997). Pb was further purified with an AG-MP1-M

anion exchange resin in a hydrobromic medium. Pb isotope ratios were measured on

a Thermo TRITON mass spectrometer on Faraday cups operating in static mode.

Pb was loaded on Re filaments using the silica gel technique and all samples as well

as the SRM981 standard were measured at a pyrometer controlled temperature of

1220uC. Sr and Nd isotopes were measured at the University of Geneva on a

multicollector Finningan MAT 262 thermal ionization mass spectrometer in semi-

dynamic mode; 87Sr/86Sr and 143Nd/144Nd ratios were corrected to mass fractiona-

tion and normalized to international standards.

The Sr and Pb isotopic data for orthogneisses were corrected to an average age of

300 Ma, based on the known intrusion age of the granitic protolith from U–Pb

zircon geochronology. The Sr–Nd–Pb isotopes of the metabasite sample were

corrected to the same average age on the basis of U–Pb zircon SHRIMP 288¡6 Ma

protolith age of a metabasite in the mantle of the Kesebir dome, in Greece (Bauer

et al. 2007). The same isotopic data for the paragneiss samples were also corrected to

an age of 300 Ma, according to the U–Pb SHRIMP detrital zircon record for the age

of sedimentation (Liati and Gebauer 2001), and because paragneisses host the

metabasite bodies (see also Bauer et al. 2007), they approximate this time interval

reasonably. The whole-rock Sr, Nd and Pb isotopic compositions are given in

Table 3.

The initial 87Sr/86 Sr ratios of the orthogneiss samples (0.7050–0.7117), together

with those of the paragneiss and metabasite samples (0.7039–0.7144), display an

extended range of overlapping values. 143Nd/144Ndi ratios of the paragneiss and

metabasite samples fall in the range 0.5120–0.5123, with eNdt values varying from

24.11 to 2.33. The time-integrated 143Nd/144Nd isotopic compositions of the

paragneiss samples are similar to isotopic composition of global subducting sediment

(GLOSS), especially overlapping those for terrigenous sediments

(143Nd/144Nd50.5120, 147Nd/144Nd50.11–0.14; Plank and Langmuir 1998) and are

closest to compositions of cherts and volcaniclastic turbidites (143Nd/144Nd50.5124;

Elliot et al. 1997). The 143Nd/144Ndi isotopic composition of the metabasite sample

suggests significant crustal influence. The initial 87Sr/86Sr isotopic ratios of the

paragneiss sample Ks-73 are comparable to GLOSS composition (87Sr/86Sr50.7173;

Plank and Langmuir 1998), whereas the isotopic composition for the quartz-rich

sample Ks-206 is significantly distinct from GLOSS (Table 3).

Initial 206Pb/204Pb ratios of the augen orthogneiss samples fall into a relatively

narrow range (18.24–18.66), comparable to the range of 206Pb/204Pbi ratios of 18.31

and 18.93 respectively in the two paragneiss samples, and differ from the lower value

of the matabasite sample (206Pb/204Pbi517.55). Both, augen orthogneisses and

paragneisses display a similar 207Pb/204Pbi ratio (15.64–15.72); this ratio is lower


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(207Pb/204Pbi515.61) in the metabasite. Similarly, uniform 208Pb/204Pbi ratios in

both paragneisses (38.23–38.60) and orthogneisses (38.32–38.56) differ from208Pb/204Pbi537.95 of the metabasite. These data indicate that augen orthogneisses

and paragneisses were derived from a similar continental crust source with coherent

time-integrated Pb isotopic ratios. In contrast, the metabasite has a different origin,likely from a distinct mantle-derived source. The Pb isotopic compositions of

hydrothermal pyrites overlap the metamorphic para- and orthogneiss samples

(Figure 7).

Discussion

Geochemical types, origin and tectonic setting of the orthogneisses

The petrography, microstructures and geochemistry of the orthogneisses give clear

evidence that they represent magmatic products of an intrusive suite that

experienced high-grade metamorphism and related textural modifications

(Figures 2–4), during an Alpine-phase metamorphic event. Two geochemical types

can be distinguished within this suite on the basis of the trace elements and REE

characteristics of the meta-igneous rocks (Figure 5): (1) a group of HFSE-depleted

and REE fractionated orthogneisses, represented mostly by equigranular andbanded orthogneisses, and (2) a group of LILE and LREE-enriched orthogneisses in

turn represented by augen orthogneisses and K-feldspar megacrystic metagranites.

Table 3. Pb, Sr, and Nd isotopic chemistry of the basement ortho- and parametamorphicrocks from the Eastern Rhodope. Locations of the metamorphic samples are depicted inFigure 1.

Sample Ks-73 Ks-2 pgn 06 Br-13 Ks-6 Ks-18 Br-8

Rock type pgn mb augn augn augn

Pb (ppm) 11 14 5 38 24 20U (ppm) ,2, ,2, 2 4 6 3Th (ppm) 6 9 5 4 11 15206Pb/204Pb 19.496 18.746 18.777 18.556 19.030 19.124207Pb/204Pb 15.751 15.682 15.669 15.654 15.696 15.687208Pb/204Pb 39.149 38.868 38.940 38.425 39.018 39.237206Pb/204Pbi 18.931 18.310 17.552 18.237 18.261 18.660207Pb/204Pbi 15.722 15.659 15.605 15.637 15.656 15.663208Pb/204Pbi 38.600 38.230 37.947 38.321 38.560 38.485Rb 102 89 27 90 141 160Sr 89 112 168 138 250 23487Sr/86Sr 0.728611 0.713819 0.708428 0.719855 0.715932 0.71348487Rb/86Sr 3.3226 2.3005 0.4650 1.8891 1.6331 1.979487Sr/86Sri 0.714426 0.703998 0.706443 0.711790 0.708960 0.705034Nd 11.07 14.63 19.57Sm 2.43 3.08 5.06143Nd/144Nd 0.512647 0.512290 0.512677147Sm/144Nd 0.1323 0.1267 0.1557143Nd/144Ndi 0.512387 0.512041 0.512371et

CHUR 2.64 24.11 2.33

Note: Recalculated Pb isotopic initial ratios of orthogneisses, paragneisses, and metabasite(300 Ma) based on U, Th, and Pb contents.Recalculated Sr and Nd isotopic initial ratios of orthogneisses, paragneisses and metabasite(300 Ma) based on Rb, Sr, and Sm, Nd contents, respectively. Abbreviations: pgn, paragneiss;augn, augen gneiss; mb, metabasite. See also Table 1 and 2, and Figure 1.

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Field data indicate a sequential intrusion history of the porphyritic igneous facies

of K-feldspar megacrystic metagranites and augen orthogneisses into the

more homogeneous equigranular orthogneisses with respect to these geochemical

groups.

Granite rocks are conventionally subdivided into I-, S-, M-, A-types (Chappell

and White 1974; Pitcher 1983). In terms of rock series, the studied orthogneiss

precursors have calc-alkaline affinity and a peraluminous character with a high silica

content (.70 wt-%) and a high K/Na ratio, containing Al-rich minerals such as

muscovite, biotite and garnet, and contain mostly meta-sedimentary pelitic enclaves

rather then mafic hornblende-bearing xenoliths. Thus, they present field evidence

and mineralogical and chemical characteristics that fulfil criteria for an origin as S-

type granites (Figure 4; Tables 1 and 2; e.g. Chappell and White 1974). This

supracrustal origin is further substantiated by comparing the trace elements and

REE patterns of the lower high-grade basement unit orthogneisses with the

overlying upper high-grade basement unit paragneisses, with the latter showing

significant overlap with the patterns of the group of HFSE-depleted and REE

fractionated orthogneisses (Figure 5(b) and (f)). This, together with the close

similarity of LILE and LREE-enriched group of orthogneisses to shale composi-

tions and similar Sr and Pb isotope chemistry of both the para- and orthogneisses

(Table 3), implies an upper crustal origin of the meta-igneous orthogneiss precursors

Figure 6. Tectono-magmatic discrimination diagrams for the orthogneisses. (a) Rb vs.Nb + Y diagram (after Pearce et al. 1984). (b) Nb vs. Y diagram (after Pearce et al. 1984). (c)Rb/Zr vs. SiO2 diagram (after Harris et al. 1986). (d) Ce/P2O5 vs. Zr/TiO2 diagram (afterMuller et al. 1992). Shaded fields in (a) and (c) refer to the Kesebir dome orthogneiss samplesfrom Ovtcharova and Sarov (1995). Symbols as in Figure 3.


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that involves melting of sedimentary rocks in their source region (Figure 5). The

migmatite sample has trace elements and REE pattern indistinguishable from the

paragneisses and HFSE-depleted and REE fractionated group of the orthogneisses,

in turn, indicating continuous remelting of the lower high-grade basement unit at

middle crustal levels. Additionally, whole-rock 87Sr/86Sr isotope ratios of the Byala

reka dome orthogneisses ranging between 0.70799 and 0.72034 with initial 87Sr/86Sr

ratios .0.7076 (Peytcheva and Quadt 1995; Peytcheva 1997), correspond to the

defined wide scatter of 87Sr/86Sr isotope ratios in S-type granitoids (Chappell and

White 2001). This provides additional support for an S-type origin of the

orthogneiss precursors that have incorporated compositionally various crustal

components. The high 87Sr/86Sri ratios for samples Ks-6 and Ks-18 imply input from

pre-existing crust and thus fit S-type origin, whereas lower 87Sr/86Sri ratio in sample

Br-8 that represents a granitoid protolith later in the intrusion sequence, in turn

deviates toward ratios characteristic for I-type granitoids and therefore suggest

juvenile material contribution.

Using relatively immobile trace elements, the tectonic setting of orthogneisses has

been defined on several discriminative diagrams (Figure 6). In the tectono-magmatic

setting diagrams for granitoids of Pearce et al. (1984), the orthogneiss samples plot

in the field of volcanic arc granites (VAG) and straddle also the field of syn-collision

granites (Syn-COLG) (Figure 6(a) and (b)). A continental volcanic arc tectonic

setting is further demonstrated in the tectono-magmatic discriminative diagrams of

Harris et al. (1986) and Muller et al. (1992) (Figure 6(c) and (d)).

The geochemical data allow us to place constraints on the petrogenesis of the

magmatic protoliths of the orthogneisses (Figure 5). The presence of negative Nb

anomalies in trace element profiles of the orthogneisses is a feature characteristic of

arc-related petrogenesis and the contribution of this tectonic environment to the

crustal growth, also compatible with the tectonic setting discrimination. Negative

Ce, Sr, Zr, Y anomalies are notable for the HFSE-depleted and REE fractionated

group of the orthogneisses. The negative Ce anomaly ascribed to a sedimentary

input in the subduction zones suggests contribution of recycled sediments in the

crustal source region, in which garnet was likely present during partial melting in

view of its Y-depleted nature and Zr retained in the source during melting as well.

The positive Eu anomalies in this group of orthogneisses likely reflect processes in

the source region where plagioclase and K-feldspar were present to account for the

Ba and Sr depleted character.

The negative Eu anomaly coupled with lower magnitude negative Sr anomaly of

LREE-enriched group implies differentiation involving mostly feldspar fractional

crystallization and, to a lesser extent, apatite and zircon fractionation during magma

genesis. The variable Zr contents of the orthogneisses also suggest fractional

crystallization or incorporation of crustal material with a different Zr content by the

magma during partial melting (Tables 1 and 2). Thus, the trace and REE patterns are

Figure 7. Comparison of lead isotopic compositions of the metamorphic rocks, volcanicrocks, and the pyrites from the hydrothermal and non-mineralized metamorphic rocks fromthe Eastern Rhodopes, Bulgaria. (a) 206Pb/204Pb vs. 208Pb/204Pbt diagram. (b) 206Pb/204Pb vs.207Pb/204Pbt diagram. Isotopic ratios recalculated at 35 Ma. Data sources for Serbo-Macedonian Massif in Chalkidiki Peninsula after Frei (1995), alkaline basalts after Marchevet al. (1998), calc-alkaline volcanics after Marchev et al. (2004b), potassium-alkaline basalts(KAB) after Marchev et al. (2005), data for Ada tepe after Marchev et al. (2004a), Jurassicbasalts after Bonev and Stampfli (2008).


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consistent with the magmas from which the orthogneisses originated, attesting to the

diverse, crustal contamination source regions that were responsible for their origin.

All together, it is suggested that the effects of partial melting and source compositions

mainly control the compositional variations of the magmatic protoliths of the

orthogneisses, which underwent also subsequent crystal fractionation.

Overall, the field and petrographic data coupled with the geochemical and isotope

results consistently indicate an origin of the orthogneiss precursors as intimately

related to a calc-alkaline S-type granite suite in a continental volcanic arc tectonic

setting, in which remelting and/or assimilation of sedimentary rocks took place at

upper crustal levels in a virtually heterogeneous source region of the subduction

system.

Comparative Pb isotope systematics

There are several Tertiary low-sulfidation sedimentary rock-hosted occurrences

along the hanging wall of the detachment fault system in the northern Kesebir dome

and the central Byala reka dome in the Eastern Rhodopes (Figure 1; Marchev et al.

2003, 2004a; Marton et al. 2007; Marton 2008) with 40Ar/39Ar ages for adularia

between 34.71¡0.16 and 36.46¡0.26 Ma. Lavas of the Iran tepe paleovolcano, in

close proximity to the Ada tepe deposit yielded 40Ar/39Ar biotite and amphibole

ages between 33.97¡0.38 and 34.69¡0.25 Ma (Marton 2008), thus implying

temporarily close hydrothermal and volcanic activity.

The Pb isotopic compositions of pyrites from both the hydrothermal and non-

mineralized metamorphic rocks are compared with 35 Ma time-integrated Pb

isotopic compositions of the studied metamorphic rocks and Iran tepe volcanic

rocks in Figure 7. The 206Pb/204Pb vs. 208Pb/204Pbt plot displays a nearly linear

distribution trend of these isotopic ratios in all aforementioned rocks (Figure 7(a)).

When plotted on a 206Pb/204Pb vs. 207Pb/204Pbt diagram, the trend lines of the

isotopic ratios in the metamorphic samples and pyrites are very close to each other,

especially in the case of the orthogneiss samples, and the trend line of the Iran tepe

lavas are displaced to slightly higher 207Pb/204Pbt ratios but still parallel with their

linear distribution (Figure 7(b)). In this diagram, a comparison of these isotopic

ratios in the metamorphic rocks, the Iran tepe lavas and the pyrites show a close

chemical similarity with previously documented Pb isotopic compositions for the

Ada tepe deposit and calc-alkaline lavas from other palaeovolcanic structures in the

Eastern Rhodope region (Marchev et al. 2004b, 2005).

This comparison reveals a significant part of the lead from the pyrites in the

hydrothermal rocks and in non-mineralized host metamorphic rocks was leached

from the basement metamorphic rocks, in particular the orthogneisses, together

with some lead contribution from the Iran tepe palaeovolcano lavas and other

palaeovolcanic edifices and volcanic bodies in the Eastern Rhodope region. The

significant contribution from the metamorphic basement to ore-forming fluids in the

Ada tepe and Rosino deposits was also proposed by Marchev et al. (2004a) based on

whole-rock and carbonate Sr isotope systematics, with 87Sr/86Sr isotopic ratios

similar to those of the orthogneissic basement (e.g. Peytcheva 1997). Overall, these

isotopic data suggest crustal interactions and interplay between distinct processes

involved in the late Alpine history of the Eastern Rhodope Massif, namely the

extensional tectonics, volcanic activity and ore formation, implying high-grade

basement influence to the hydrothermal processes.

288 N. Bonev et al.

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Tectonic significance of arc magmatism as revealed by basement orthogneisses

Neoproterozoic and early Palaeozoic Gondwana-derived blocks were assembled

after the Silurian and subsequently accreted to the Laurasian margin during the

Variscan orogeny in Visean times (von Raumer et al. 2003; Stampfli and Borel

2004). The Variscan tectonism in western-central Europe was accompanied by

widespread syn- and post-collisional calc-alkaline to high-K plutonic (S and I-type

granitoids) and volcanic activity, the magmatism of which encompasses an early

phase that spans 340–325 Ma and a late phase between 315–290 Ma (Harris et al.

1986; Matte 1986; Finger et al. 1997).

Late Carboniferous to Early Permian igneous activity (,319–265 Ma) is recorded

in the Rhodope Massif and within immediately adjacent crustal units (i.e. Serbo-

Macedonian Massif ,293–285 Ma granitoids (Anders et al. 2007); Pelagonian zone

granitoids ,300 Ma (Anders et al. 2006); Strandja Massif 309 Ma and 271 Ma S-type

metagranitoids (Okay et al. 2001)) as already stated in the introduction. Furthermore,

to the north of the Rhodope Massif (see Figure 1, inset), the pre-Mesozoic basement

of the Sredna Gora zone records a Neoproterozoic magmatism (Carrigan et al. 2006)

that is temporarily equivalent to that in the Serbo-Macedonian Massif and

Pelagonian zone, and pronounced Variscan plutonic activity that spans 315–290

Ma and extends within the Balkanides chain further north (Carrigan et al. 2005). All

these radiometric age constraints indicate a widespread, voluminous Late

Carboniferous-Early Permian granitoid magmatism at the vicinity and within the

Rhodope Massif, the magmatism of which can be traced to NW Anatolia eastwards

(e.g. 308 Ma orthogneisses of the Kazdag Massif (Okay et al. 1996)).

Figure 8 depicts tectonic scenarios that account for the Late Carboniferous

continental arc-related magmatism in the Rhodope Massif and its significance for

the Alpine tectono-metamorphic history. The Late Carboniferous-Early Permian

times correspond to the closure of the Palaeotethys Ocean, which was terminated by

Late Triassic time in the eastern Mediterranean domain (Stampfli and Kozur 2006)

following northward Palaeotethyan oceanic crust subduction and development of

magmatic arc on the Laurasian margin (i.e. Variscan orogen). This Late

Carboniferous final stage Palaeotethys subduction arc magmatism is recorded in

the Pelagonian zone (Vavassis et al. 2000; Anders 2007) and NW Anatolia (e.g.

Delaloye and Bingol 2000). In this geodynamic context, we consider precursors of

the orthogneisses in the Eastern Rhodope basement as magmatic products of a

continental magmatic arc, which had developed in Late Carboniferous to Early

Permian times, with plutonic bodies intruding pre-existing Neoproterozoic and

Early Palaeozoic igneous basement and Palaeozoic sedimentary cover (Figure 8(a)).

This intrusive magmatic activity was accompanied by coeval upper plate acid

volcanism and associated upper crustal late stage extensional molasse-type

sediments in the Sredna Gora Zone and Balkanides chain (Yanev 2000;

Cortesogno et al. 2004). The Late Carboniferous regionally extensive and correlative

continental arc intrusive magmatism testifies to a crust-forming event creating a

common Variscan igneous and metamorphic basement for the Rhodope and

adjacent metamorphic massifs. This basement incorporated Variscan magmatic

bodies that were strongly overprinted attending the subsequent Mesozoic history.

During the Alpine orogeny resulting in the closure of the Neotethyan Vardar

Ocean, the Variscan magmatic arc basement units were sliced in pre-latest Late

Cretaceous time and assembled within the Rhodope nappe stack, which led to

crustal thickening that caused gravitational instability and provoked coeval to


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subsequent early Tertiary extension in the region. By the end of Eocene time, the

orthogneissic basement acted as an immediate crustal source region within the

footwalls of extensional domes for the hydrothermal fluids giving birth to ore

deposits and largely concomitant volcanic activity (Figure 8(b)).

Conclusions

(1) Petrographic observations reveal partly preserved igneous mineral assem-

blages, whereas field textures and microstructures of the orthogneisses

indicate a high-temperature ductile tectono-metamorphic overprint that

obliterated igneous structures and grain sizes. These textural features relate

to amphibolite-facies fabrics during the imbrication of the lower high-grade

basement unit orthogneisses within the nappe stack, predating the

subsequent extension-related low-temperature fabric of ductile-brittle over-

print at the vicinity of extensional detachments.

(2) Geochemical data indicate a magmatic origin of the lower unit basement

orthogneiss precursors which present igneous trends and retained the

Figure 8. Tectonic scenarios for the late Palaeozoic and Late Cretaceous-Tertiary evolutionof the Eastern Rhodope basement.

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composition of the plutonic protolith. The protoliths are peraluminous,

medium-K calc-alkaline S-type granitoids that originated in a continental

volcanic arc tectonic setting. Two geochemical types can be distinguished in

the meta-igneous suite; a group of HFSE-depleted and REE fractionated

orthogneisses and a group of LREE-enriched orthogneisses, both showing

compositions similar to the continental crust and its sedimentary counter-

parts. A geochemical comparison with the paragneisses from the overlying

upper high-grade basement unit indicates that the first group of the

orthogneisses has compositions indistinguishable from those of paragneisses,

which further substantiate melting and involvement of sedimentary material

in the magma genesis. The LREE-enriched group of the orthogneisses

displays a less pronounced sediment contribution and chemistry character-

istic involving derivation from a closed system igneous melt fractionation.

We suggest that a heterogeneous crustal source region underwent partial

melting and crustal contamination with subsequent fractional crystallization,

accounting for petrogenesis of the meta-igneous suite.

(3) Sr and Pb isotopic composition of the orthogneisses, fully comparable to that

of the paragneisses, reveals crustal compositions, which differ from the mantle-

like Pb isotopic composition of a metabasite associated with the paragneisses.

This hints at a supracrustal S-type origin of the orthogneisses resulting through

remelting of sedimentary material in a continental arc setting. This setting

relates to regionally extensive Variscan crust-forming processes by develop-

ment of a Late Palaeozoic magmatic arc during closure of the Palaeotethys

Ocean, with a strong overprint of these arc units by distinct Alpine processes

that pertains to the history of latest Cretaceous-early Tertiary closure of the

Neotethyan Vardar Ocean south of the Rhodope Massif. Comparative Pb

isotopic systematics of the metamorphic rocks, together with pyrites from

non-mineralized metamorphic and hydrothermal rocks of ore deposits and

mineralizations and lavas from nearby to the latter palaeovolcanic edifice,

suggest a significant crustal Pb input to the hydrothermal fluids by leaching of

the metamorphic basement rocks and their erosion products, with a possible

subordinate contribution of magmatic Pb.

Acknowledgements

The study was supported by the Bulgarian National Fund for Scientific Research

project no. VU -NZ-02/06, a fellowship from the University of Geneva to N. Bonev,

and the Swiss National Science Foundation project 200020-113510 and the SCOPES

project IB 7320-111046/1. Editorial comments by Y. Dilek helped to improve the

manuscript, which was finalized while NB was holding a Fulbright Research

Scholarship at Miami University, Oxford, OH, USA.

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geochemistry, tectonics, and crustal evolution of basement rocks in the eastern rhodope massif,...

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