Petrogenesis of Ultramafic Rocks from the Ultrahigh-pressure Metamorphic Kimi Complex in Eastern Rhodope (NE Greece)

JOURNAL OF PETROLOGY VOLUME 49 NUMBER 5 PAGES 885^909 2008 doi:10.1093/petrology/egn010 Petrogenesis of Ultramafic Rocks from the Ultrahigh-pressure Metamorphic Kimi Complex in Eastern Rhodope (NE Greece) I. BAZIOTIS1, E. MPOSKOS1 AND P. D. ASIMOW2* 1 NATIONAL TECHNICAL UNIVERSITY OF ATHENS, DEPARTMENT OF MINING AND METALLURGICAL ENGINEERING, SECTION OF GEOLOGICAL SCIENCES, HEROON POLYTECHNIOU 9, 15780, ATHENS, GREECE 2 CALIFORNIA INSTITUTE OF TECHNOLOGY, DIVISION OF GEOLOGICAL AND PLANETARY SCIENCES, PASADENA, CA 91125, USA RECEIVED JANUARY 25, 2007; ACCEPTED FEBRUARY 13, 2008 ADVANCE ACCESS PUBLICATION MARCH 24, 2008 Widespread bodies of garnet^spinel metaperidotites with pyroxenitic layers occur in the ultrahigh-pressure metamorphic Kimi Complex. In this study we address the origin of such peridotite^pyroxenite associations in the context of polybaric melting regimes. We conduct a detailed geochemical investigation of major and trace element relations and compare them with a range of major element modelling scenarios. With increasing bulk-rock MgO content, the garnet^spinel metaperidotites exhibit decreasing CaO, Al2O3, TiO2, and Na2O along with increasing Ni and a gradually increasing Zr/Zr
anomaly, consistent with an origin as residues after variable degrees of melt extraction.The major element modelling further suggests a polybaric adiabatic decompression melting regime beginning at high to ultrahigh pressure, with an intermediate character between pure batch and fractional melting and a mean extent of melting of 9^11%. The pyroxenites exhibit major element compositions that cannot be reproduced by experimental or calculated melts of peridotite. Moreover, the Kimi pyroxenites have highly variable Ni and Sc contents and a wide range of Mg-number (0 76^0 89), inconsistent with an origin as frozen melts or the products of melt^peridotite interaction. However, both the major element systematics and the observed rare earth element patterns, with both convex and concave shapes, can be explained by an origin as clinopyroxene-rich, highpressure cumulates involving garnet and/or Cr-spinel. I N T RO D U C T I O N KEY WORDS: peridotite; pyroxenite; partial melting; UHP metamorphism; cumulate The current consensus among observational, experimental, and theoretical geochemists and petrologists is that upper mantle processes are dominated by adiabatic, polybaric, decompression-induced partial melting (McKenzie, 1984; Klein & Langmuir, 1987; Asimow, 2002), leading to the formation of new crust from the aggregated liquids (McKenzie & Bickle, 1988; Kinzler & Grove, 1992; Langmuir et al., 1992; Asimow et al., 2001). Data regarding upper mantle mineralogy, element partitioning, partial melting processes and the generation of primary basaltic magmas are interpreted within this broad paradigm. However, a range of important variations on the theme of polybaric melting exist, including equilibrium vs fractional melt production (Niu, 1997; Johnson et al., 1990) and melt migration (Spiegelman & Kenyon, 1992; Iwamori, 1993), the pressure range of melt production (Salters & Hart, 1989; Hellebrand et al., 2002; Presnall et al., 2002; Weyer et al., 2003; Brunelli et al., 2005), the importance of deep (i.e. subcrustal) differentiation (e.g. Grove et al., 1992), and the significance of lithological heterogeneity in the mantle (Hirschmann & Stolper, 1996; Sobolev et al., 2005). The origin of peridotites and related rocks and variations in their chemical composition reflect processes such as partial melting and melt^rock interaction; the petrogenetic history of such rocks is, therefore, invaluable for *Corresponding author. Telephone: þ 1-626-395-4133. Fax: þ 1-626568-0935. E-mail: ß The Author 2008. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org JOURNAL OF PETROLOGY VOLUME 49 understanding upper mantle processes. Pyroxenites are commonly associated with peridotites; they have been the focus of several studies leading to various hypotheses for their origin and geochemical significance, including frozen melts (e.g. Pearson et al., 1993), melt^rock interaction (e.g. Kelemen et al., 1998; Litasov et al., 2000; Hermann et al., 2006) and/or cumulate processes (e.g. Hirschmann & Stolper, 1996; Kopylova et al., 1999; Xu, 2002; Dantas et al., 2007). Here we study the geochemistry of garnet^spinel metaperidotites and associated spinel^garnet clinopyroxenites tectonically emplaced into the crustal rocks of the ultrahigh-pressure (UHP) metamorphic Kimi Complex in eastern Rhodope, Greece. We compare whole-rock compositions of the peridotites with residues generated in polybaric melting models (Asimow, 1999) to define the detailed melting history of the suite. We infer constraints about the nature of the mantle source and melt extraction process that affected the metaperidotites and the processes controlling the formation of the spinel^garnet clinopyroxenites. In particular, we attempt to give answers to the following questions, using geochemical evidence from whole-rock major and trace element concentrations. (1) Are the peridotites formed as residues from a batch, fractional or mixed polybaric melting regime? It should be noted that we use batch melting here as a proxy for physical settings in which, despite melt mobility, melts remain in equilibrium with residues throughout the melting column; the equivalence of this case to batch melting in terms of residue composition has been demonstrated by several workers (Ribe, 1985; Spiegelman & Elliott, 1993; Asimow & Stolper, 1999). (2) Which processes are responsible for the formation of the clinopyroxenites? Geological framework of the UHP Kimi Complex: general The Rhodope high-pressure (HP) province in the easternmost part of the Hellenic Orogen is an Alpine synmetamorphic thrust and nappe complex (Burg et al., 1996; Ricou et al., 1998; Liati & Gebauer, 1999; Mposkos & Krohe, 2000; Krohe & Mposkos, 2002) that incorporates several tectonic slivers of UHP and HP metamorphic rocks (Mposkos & Krohe, 2000, 2006; Mposkos & Kostopoulos, 2001; Mposkos et al., 2004). The Rhodope HP province is subdivided into several tectonometamorphic units that are bounded by thrust and normal faults. In eastern Rhodope, the Kimi Complex, representing the structurally uppermost metamorphic unit, records an alpine UHP metamorphism followed by an HP granulite- to upper amphibolite-facies event. It was exhumed between 65 and 48 Ma (Mposkos & Wawrzenitz, 1995; Liati et al., 2002; Mposkos & Krohe, 2006). NUMBER 5 MAY 2008 The UHP Kimi Complex in eastern Rhodope The Kimi Complex (Fig. 1) is a tectonic mixture of crustal and mantle-derived rocks. The crustal rocks comprise amphibolitized eclogites, orthogneisses, marbles and migmatitic pelitic gneisses. The presence of diamond inclusions in garnet and needle-like rutile exsolution in Na-bearing garnet in migmatitic pelitic gn (...truncated)