Eclogitisation of gabbroic rocks:: Redistribution of trace elements and Zr in rutile thermometry in an Eo-Alpine subduction zone (Eastern Alps)

The investigation of eclogites and a precursor gabbro from the Austroalpine basement domains Koralpe, Saualpe and Pohorje shows that these mafic rocks are similar to oceanic gabbros that were derived from a depleted mantle source. The chemical variations of the eclogites are related to differences in the magmatic history of the precursor rocks and to seawater alteration. The trace element composition of the rocks has not changed significantly during the gabbro to eclogite transformation because trace elements are redistributed among the newly formed high-pressure major and accessory minerals. As other recent studies indicate, incompatible trace elements are predominantly hosted in zoisite/clinozoisite (Sr, Ph, U, Th, LREE), apatite (Sr, Pb, REE), phengite (Cs, Rb, Ba), garnet (Y, HREE, Sc), rutile (Ti, Nb, Ta) and zircon (Zr, Hf) at eclogite-facies conditions. Omphacite hosts most of the Li in addition to some Sr and major amounts of Sc and V. This argues against significant liberation of LILE and LREE during subduction-related dehydration or fluid infiltration of these mafic rocks. The trace element characteristics of accessory minerals in eclogites help to reconstruct the P-T-t evolution of a subduction complex: U-Pb zircon ages will date the high-pressure event if U-Th characteristics and REE analyses constrain zircon growth to being metamorphic and essentially synchronous with the growth of garnet. Recent studies document that variations in Zr content of rutile grown in the presence of zircon and quartz are mainly attributable to differences in temperature. Zr-in-rutile thermometry of Koralpe, Saualpe and Pohode eclogites yields temperatures of 700-730 degrees C (according to Zack et al. [Zack, T., Moraes, R., Kronz, A., 2004. Temperature dependence of Zr in rutile: an empirical calibration of a rutile thermometer. Contrib. Mineral. Petrol. 148, 471-4881) or between 630-650 degrees C (according to Watson et al. [Watson, E.B., Wark, D.A., Thomas, J.B., 2006. Crystallization thermometers for zircon and rutile. Contrib. Mineral. Petrol. 151, 413-433]). No systematic variation in rutile temperatures was observed for matrix rutile and rutile included in garnet, omphacite or kyanite, suggesting that these temperatures represent peak metamorphic conditions and that this part of the Austroalpine basement behaved as a coherent block during subduction. (c) 2007 Elsevier B.V. All rights reserved.