115 macla nº 9. septiembre ‘08 revista de la sociedad española de mineralogía Geochemistry of Platinum Group Elements in Chromitites from The Rhodope Massif (Bulgaria) / FERNANDO GERVILLA (1), JOSÉ Mª GONZALÉZ-JIMÉNEZ (1), THOMAS KERESTEDJIAN (2) BORISLAV GLAVEV (2) (1) Departamento de Mineralogía y Petrología, Facultad de Ciencias and Instituto Andaluz de Ciencias de la Tierra (CSIC-Universidad de Granada). Avda. Fuentenueva s/n, 1800 Granada (Spain) (2) Geological Institute, Bulgarian Academy of Sciences, 24 Georgi Bonchev Str., 1113 Sofia (Bulgaria) INTRODUCTION. Ophiolite chromitites are usually enriched in platinum-group elements (PGE) with respect to other mantle rocks. The origin of such enrichment is still controversial, and not completely understood. It is widely accepted that concentration of PGE depends on the degree of partial melting, the nature of the source rock, and the sulphur content of the parent partial melt. All these parameters are closely linked to the geotectonic setting where chromitite forms. Thus, Cr-rich chromitites formed in suprasubduction zones are rich in PGE, while Al-rich chromitites formed in spreading zones are usually poor in PGE (Prichard et al., 1986; Proenza et al., 1999; Ahmed & Arai 2002, 2003; Proenza et al., 2004; Gervilla et al., 2005). Furthermore, some mechanisms, such as hydrothermal activity (Prichard et al., 1986), serpentinization (Thalhammer et al., 1990), metamorphism (Zaccarini et al., 2005), or weathering (Augé & Legendre 1994) can modify the concentration of PGE in chromitites. In the current communication, we report whole rock PGE analyses of a set of chromitites from three metamorphosed meta-ophiolite bodies, scattered in the Rhodope Massif (Southern Bulgaria). The degree of metamorphic alteration varies among these bodies as well as their PGE contents and their distribution among the different chromitites. This will allow us to discuss the origin of the geochemical distribution of PGE in the chromitites of the Rhodopean massif, its geodynamic implications and the role played by metamorphism. GEOTECTONIC SETTING. The studied meta-ophiolite bodies (Dobromirtsi, Jakovitsa and Golyamo Kamenyane) occur scattered within the easternmost part of central Rhodopes which is a crystalline nappe, composed of granitic and metamorphic rocks, extending along south Bulgaria and north Greece (Marchev et al., 2005). At Dobromirtsi and Jakovitsa massifs, the ophiolite sequence is dominated by mantle rocks (harzburgites and dunites) metamorphosed up to lower- temperature amphibolite facies. These rocks are strongly serpentinized although in some areas they also show chloritization, carbonatization and talcization (González-Jiménez et al., 2008). At Golyamo Kamenyane the sequence comprises harzburgites and dunites, grading upward to alternating dunite-pyroxenite. Rodingite bodies are also recognized at the top of the sequence. This massif is spatially associated with amphibolitized gabbros. All these rocks are highly metamorphosed. The alteration includes, serpentinization, tremolitization, chloritization, talcization, carbonatization, phlogopitization and listvenitization (Zhelyaskova-Panayotova & Economou-Eliopoulous 1994). In fact, the studied chromitites occur scattered in a wide area mined for asbestos. PLATINUM-GROUP ELEMENTS. All the studied chromitites are of podiform-type, hosted by mantle rocks (dunites in harzburgites). The PGE contents are low, mainly below 2 ppm. The highest values are recorded in chromitites from Dobromirtsi (up to 1.6 ppm). As expected in ophiolite chromitites, they are enriched in Ir- subgroup PGE (IPGE: Os, Ir, Ru) relative to Pt-subgroup (PPGE: Rh, Pt, Pd) (Table 1). The Pd/Ir ratio is consequently low (0.04-1). Their chondrite-normalized PGE patterns plot between 0.002 and 1.10 times the chondritic values showing variable, negative-sloped patterns. These are relatively flat from Os to Ir, with Ru anomalies, followed by strong negative slopes from Ru to Pt. Most samples show significant enrichment in Pd relative to Pt. Total PGE contents in chromitites from Jakovitsa are lower than those from Dobromirtsi (<0.5 ppm). Their chondrite-normalized PGE patterns plot between 0.013 and 0.33 times the chrondritic values and show variable shapes, with slight positive anomalies in Ru. The patterns of the samples with the lower PGE content show a nearly flat, horizontal shape. Nevertheless, two samples show a Pd positive anomaly relative to Pt. These variations of PGE contents causes strong variation of Pd/Ir ratios (0.1-2.5). The studied chromitites from Golyamo Kamenyane have the lowest PGE contents (<0.2 ppm) (Table 1). Their chondrite-normalized PGE patterns plot between 0.004 and 0.08 times the chondritic values and show relatively flat horizontal shape. Most samples show negative slope from Os to Ir, positive anomalies in Ru relative Os and Ir, and smooth negative slope from Ru to Pt. Some samples also show a relative enrichment in Pd compared with Pt, and one sample shows a smooth positve- sloped pattern from Os to Pd, with a slight, positive anomaly in Ru. palabras clave: Elementos del grupo del platino, Metamorfismo, Cromititas key words: Platinum-group elements, Metamorphism, Chromitites. resumen SEM/SEA 2008 * corresponding author: gervilla@ugr.es PGE Dobromirtsi Jakovitsa Golyamo Kamen. Os 6-299 7-84 6-34 Ir 9-370 10-97 5-32 Ru 33-765 35-229 18-58 Rh 4-94 6-19 2-10 Pt 2-42 14-51 5-36 Pd 2-7 8-42 6-17 Table1. PGE contents (in ppb) of the analyzed chromitites.