Compositional diversity in chromitites from Eastern Rhodopes (SE Bulgaria): petrogenesis and tectonic implications Vanessa Colás, Isabel Fanlo Dpt. Earth Sciences, University of Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain. Fernando Gervilla Dpt. Mineralogy & Petrology & IACT (UGR-CSIC), University of Granada, Fuentenueva s/n, 18002 Granada, Spain José María González-Jiménez GEMOC ARC National Key Centre, Dpt. Earth and Planetary Sciences, Macquarie Univ., Sydney, NSW 2109, Australia Thomas Kerestedjian Geological Institute. Bulgarian Academy of Sciences, Georgi Bonchev 24, 1113 Sofia, Bulgaria Abstract. In the Eastern Rhodopes (SE Bulgaria) the low-Al and low-Ti chromite forming chromitite bodies at Yakovitsa and Avren West massifs, and the high-Al and variable Ti chromite forming chromitite bodies at Golyamo Kamenyane (low-Ti) and Chernichevo (high-Ti) occur hosted within a representative ophiolitic mantle sequence formed in a mid-oceanic ridge, which evolved to an island arc/back-arc setting in a supra-subduction zone. Such conclusion is drawn from the diversity of MORB and boninitic affinity of the unaltered chromite cores, deduced from major element distribution in chromite and lithology of the ultramafic massifs. Keywords. chromitites, major elements, MORB-ZSS affinity, Eastern Rhodopes, Bulgaria 1 Introduction Chromite is a mineral highly sensitive to alteration under metamorphic and hydrothermal environments (Gervilla et al., 2012). However, even strongly altered grains of chromite may preserve their cores which retain the primary igneous composition. This “unmodified” composition can be used as an indicator for the petrogenesis of the chromitites (e.g. Barnes & Roeder, 2001; Kamenetsky et al., 2001). Bazilev et al. (1999) use the composition of chromite from different ultramafic massifs scattered in the metamorphic basement of the Rhodope Crystalline Massif in SE Bulgaria (see Georgiev et al. 2006) (Fig.1) to interpret the tectonic setting in which they were formed. However, in their study they did not accurately distinguish the different patterns of alteration in the chromite produced during the metamorphism of the chromitites that recently observed Gervilla et al. (2012). With the aim to decipher the true primary composition of chromite that can be used to constrain the tectonic type of parental melts and the tectonic setting of formation we have re-examinated those Eastern Rhodopean chromitites. Through the careful examination of the chemistry of different textural types of chromite we have been able to discriminate between unaltered and modify cores, providing only major (Cr 2 O 3 , Al 2 O 3 , FeO and MgO) and minor elements (TiO 2 ) useful for petrogenetic interpretations. Figure 1: Simplified geological map of the Eastern Rhodope (Bonev, 2006). The metamorphic basement is made up of two tectonostratigraphic complexes, the Gneiss–Migmatite Complex at the bottom and the Variegated Complex at the top, outcropping in two domal structures: the Kesebir–Kardamos Dome and the Byala Reka–Kechros Dome (Georgiev, 2006). The studied Ultramafic Massifs (stars: Yakovitsa, Avren West, Golyamo Kamenyane and Chernichevo) are located in the limbs of the Kesebir–Kardamos Dome, scattered into the Variegated Complex. 2 Geology of the ultramafic massifs The chromitites that have been re-examinated in this study are located in the ultramafic massifs near the villages of Yakovitsa, Avren, Golyamo Kamenyane and Chernichevo in the eastern Rhodope Mountains, south Bulgaria (Fig.1). These ultramafic massifs are suggested to be portions of a dismembered ophiolite affected by poly-phase metamorphism [from UHP/HT eclogite- facies down to amphibolite/greenschist facies; Mposkos & Krohe, 2006; Haydoutov et al., 2004] related to the evolution of a Metamorphic Core Complex (i.e. Rhodope Complex or Rhodope Crsytalline Massif). These ultramafic massifs are scattered in upper tectonostratigraphic unit of the core complex (i.e., the Variegated Formation; Kolcheva et al., 2000; Haydoutov et al., 2004) (Fig.1), and mainly consist of serpentinized