Rodingite and its geochemistry at the Changawuzi Serpentinite complex, Western Tianshan, China

Tianshan rodingites included in the serpentinite complex were formed at the expense of eclogite enclosed in the ultramafic rocks of Changawuzi ophiolites. The Southwestern Tianshan ophiolitic m"¦lange extends along the NE-SW axis of the tectonic ridges and faults of the Tianshan Mountain Range. Serpentinized ultramafic rocks occur together with interlayered south-dipping blueschists and greenschists in Changawuzi ophiolites suggesting to represent relics of a Silurian oceanic crust. They contain a mineral assemblage of prehnite, clinozoisite, hydrogrossular, diopside, vesuvianite and chlorite, while partial rodingitized rocks still preserve the relict omphacite and Fe-Mg-Al garnet. Complete rodingitized rocks have the simplest mineral assemblage of hydrogrossular, vesuvianite, chlorite and occasionally diopside. Our former study indicated that the rodingitization started at 370~410 ¡aC/6.5~8.5 kbar, while pervasive rodingitization took place under conditions of 200~350¡aC/2~6 kbar. Major Elements of protolithic retrogressive eclogites and two partial rodingitized rocks show a good fit to the basalt compositions for most major components except for Na2O and K2O. The high TiO2 and also melt-like P2O5 contents as well as higher total iron content suggest basalt origin rather than gabbros and mafic dykes containing lots of cumulate minerals low in Ti and P. Rodingites suffered intensive metasomatism, however, mostly contains low TiO2 <0.1%, total iron <5% and P2O5<0.01, suggesting a removal process of these elements in intensive rodingitized rocks. Trace Element data of retrogressive eclogites show moderate LREE enrichment patterns. The mantle normalized spidergrams show a slight depletion of HFSE from Nb, Ta and Ti and enrichment of Th and LILE with Nb/La 0.32~0.48 and Nb/Th 3.52~4.36. These features are different from OIBs and MORBs. Trace element characteristic gives a hint of subduction zone magmatic origin. As the rodingitization progressed, many immobile elements, including Zr, Nb, Y, rare earth elements (REE), show continuous and significant depletion, indicationg fluids produced by serpentinization could leach out even most immobile elements from the protolith of rodingites. Therefore, the geochemical patterns of trace elements including REE from rodingites (at least completely rodingitized rocks) could neither represent the geochemical features of the igneous protoliths, nor be used to indicate tectonic environment where the igneous protoliths were formed. Key Words: rodingite eclogite geochemistry Western Tianshan Xinjiang