http://journals.cambridge.org Downloaded: 06 Dec 2011 IP address: 130.204.35.6 Geol. Mag. 149 (1 ), 2012, pp. 146–152. c  Cambridge University Press 2011 146 doi:10.1017/S0016756811000938 RAPID COMMUNICATION Nd–Sr–Pb isotopic composition and mantle sources of Triassic rift units in the Serbo-Macedonian and the western Rhodope massifs (Bulgaria–Greece) NIKOLAY BONEV ∗ †, YILDIRIM DILEK ‡, JOHN M. HANCHAR§, KAMEN BOGDANOV ¶ &LASLO KLAIN ∗ ∗ Department of Geology, Palaeontology and Fossil Fuels, Sofia University St Kliment Ohridski, BG-1504 Sofia, Bulgaria ‡Department of Geology, Miami University, Oxford, OH 45056, USA §Department of Earth Sciences, Memorial University of Newfoundland, St John’s, NL A1B 3X5, Canada ¶Department of Mineralogy, Petrology and Economic Geology, Sofia University St Kliment Ohridski, BG-1504 Sofia, Bulgaria (Received 6 April 2011; accepted 24 August 2011; first published online 26 September 2011) Abstract We report on the field occurrence and isotopic compositions of metamafic rocks exposed in the Serbo-Macedonian (Volvi and Therma bodies) and western Rhodope (Rila Mountains) massifs of Bulgaria and Greece. These metamafic units consist of high- and low-Ti gabbroic and basaltic rocks, whose Nd–Sr–Pb isotopes are compatible with mantle- derived MORB and OIB components with a small amount of crustal material involved in their melt source. These isotopic features combined with the field observations are consistent with an intra-continental rift origin of the metamafic rocks protolith, and are comparable to those of the Triassic rift- related mafic rocks in the northern Aegean region. Keywords: metamafic rocks, whole-rock Nd–Sr–Pb iso- topes, Triassic rifting, Serbo-Macedonian–Rhodope massifs, Bulgaria, Greece 1. Introduction The Serbo-Macedonian and Rhodope metamorphic massifs constitute the crystalline basement of the Alpine orogenic belt in the Balkan Peninsula and display a structural record of a late Mesozoic contractional deformation episode overprinted by an early Cenozoic extensional deformation (e.g. Ricou et al. 1998). Both massifs comprise amphibolite- facies metamorphic rocks of continental and oceanic origin (e.g. Liati, 2005; Himmerkus, Reischmann & Kostopoulos, 2009) that are intruded by a series of Late Cretaceous to Miocene granitoids (Zagorchev, Moorbath & Lilov, 1987; Dinter et al. 1995; Christofides et al. 2001). The S-vergent, imbricate crustal architecture of the Serbo-Macedonian and Rhodope massifs was assembled during a contractional phase in the hanging wall of a N-dipping Cretaceous– Tertiary subduction zone that was located within the Vardar Ocean farther to the SSW (in the present-day coordinate system) (Ricou et al. 1998). This shortening and nappe- stacking event resulted in crustal thickening, amphibolite- † Author for correspondence: niki@gea.uni-sofia.bg facies regional metamorphism and topographic build-up, reminiscent of many other collision zones (Dilek, 2006). The subsequent extensional collapse of this young orogenic belt led to metamorphic core complex formation starting in early Cenozoic time (e.g. Bonev & Beccaletto, 2007 and references therein). The high-grade metamorphic units of the Serbo- Macedonian and Rhodope massifs are locally intercalated with mafic-ultramafic rocks, which are pervasively deformed and metamorphosed up to amphibolite facies. The same structural and metamorphic fabric elements observed in these mafic-ultramafic rock assemblages and in the surrounding rocks of the two massifs indicate that they all experienced the same deformational events, and that they were already part of the same crustal mosaic in the Balkan Peninsula by latest Cretaceous time. However, how these mafic-ultramafic rocks were incorporated into the protoliths of the Serbo- Macedonian and Rhodope massifs and the nature of their melt source(s) and the tectonic setting of their formation, are not well understood. The existing models consider them either as ophiolitic allochthonous tectonic sheets, representing the remnants of a Tethyan oceanic lithosphere, or as late-orogenic intrusive-extrusive complexes (Dixon & Dimitriadis, 1984; Robertson et al. 1996; Himmerkus et al. 2005; Himmerkus, Reischmann & Kostopoulos, 2009). These different interpretations have major implications for the crustal evolution of the Balkan Peninsula and need to be validated with field-based structural, geochemical, isotopic and geochronological studies. In this paper, we report on the field occurrence and Nd– Sr–Pb radiogenic isotope geochemistry of the Volvi and Therma mafic-ultramafic units in the Serbo-Macedonian Massif and a series of other mafic complexes in the western Rhodope Massif exposed in southern Bulgaria and northern Greece (Fig. 1). Our data and field observations suggest that these mafic-ultramafic rock units represent para-rift assemblages emplaced during rifting of the Serbo- Macedonian and Rhodope massifs during Triassic time. This inferred rifting was aborted, however, before the onset of continental break-up and seafloor spreading, and thus no ocean basin was created between these rifted blocks. In the first part of the paper, we present a brief account of the