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

Abstract

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 calcalkaline 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 supracrustal 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

Late Cretaceous UHP metamorphism recorded in kyanite–garnet schists from the Central Rhodope Mountains, Bulgaria

“…The current interpretation indicates that the RM is a south directed nappe complex formed as a result of a north dipping Cretaceous subduction zone associated with the closure of the Vardar Ocean (Ricou et al, 1998). By the Late Eocene-Miocene, the RM experienced post-orogenic extension, which led to the emplacement of a series of large-scale metamorphic domes (core complexes) that dominate the regional geology (Bonev et al, 2006(Bonev et al, , 2010Burg et al, 1996;Ivanov et al, 1985).…”

“…This upper highgrade basement unit, or uppermost allochthon, is composed of greenschists and phyllites overlain by arc tholeiitic and boninitic lavas, which are in turn overlain by meta-pyroclastic rocks, turbidites, and carbonates (Bonev et al, 2010). The entire sequence is overlain by a series of syn-and post-tectonic sequences.…”

Alpine tectonic evolution of a Jurassic subduction-accretionary complex: Deformation, kinematics and 40Ar/39Ar age constraints on the Mesozoic low-grade schists of the Circum-Rhodope Belt in the eastern Rhodope-Thrace region, Bulgaria-Greece

“…1, Bonev, 2006b): (i) a lower high-grade basement unit of continental affinity composed of orthogneisses with Variscan protolith ages (Peytcheva and Quadt, 1995;Liati, 2005;Cornelius, 2008;Turpaud and Reischmann, 2010) having an origin mostly from a continental arc S-type granitoids (Bonev et al, 2010b), (ii) an upper lithologically heterogeneous (intercalated various gneisses, schists, amphibolites and marbles) high-grade basement unit of continental-oceanic affinity, enclosing lenses of metaophiolites and metamafic rocks of oceanic ridge and supra-subduction zone tholeiitic-boninitic signature (Kolcheva and Eskenazy, 1988;Mposkos et al, 1989;Haydoutov et al, 2004;Bonev et al, 2006c). The radiometric determinations of the metamafic rocks revealed Neoproterozoic (572 Ma) and latest Paleozoic protolith ages (288 Ma) and Variscan metamorphic ages (350-300 Ma) Bauer et al, 2007), while gneisses have Late Carboniferous and Late Jurassic igneous ages (Cornelius, 2008), (iii) a low-grade (i.e.…”

Geochemistry and petrology of the Early Miocene lamproites and related volcanic rocks in the Thrace Basin, NW Anatolia

“…In order to quantitatively evaluate the petrogenesis of these volcanic rocks we have, therefore, used an average of available chemical data from the felsic Rhodope crustal basement (RCB) (Bonev et al, 2010) to represent this end member. We further assume that the RCB (average mineral assemblage of K-feldspar (40%) + plagioclase (30%) + biotite (10%) + garnet (10%) ) underwent high-degree melting, and that a melt composition obtained by 40% modal batch melting was added to a depleted mantle with by simple mixing.…”

“…11. RCB: Average composition of the Rhodope Continental Basement (fromBonev et al, 2010). (2) Obtained by 40% modal melting of RCB.…”