The tectonic-setting of ophiolites in the western Qinghai-Tibet Plateau, China

摘要

The study of geology, geochemistry, rare earth elements, trace elements, Pb and Sr isotopes of representative ophiolite bodies from four ophiolitic belts in the western Qinghai-Tibetan Plateau, shows that the mantle peridotites of these ophiolites are mainly harzburgite in composition, with minor dunite. They are characterized by high magnesium (MgO) and low aluminum, calcium and alkali oxide contents. Enrichment of light rare earth elements in mantle peridotites may be due to two geological processes: relatively strong partial melting; and later metasomatism by the liquids released during the subduction of oceanic crust. Mantle peridotites are characterized by low contents of the trace elements Sr, Ti and Y and relatively high contents of Rb, Nb, Zr, Hf and Th, similar to metasomatic pyrolite. The isotopic compositions of Sr and Pb show evidence of contamination by a crustal component. All the evidence indicates that the four ophiolite belts in the western Qinghai-Tibetean Plateau have undergone metasomatism by liquids released during the subduction of oceanic crust, suggesting that they were formed in a supra-subduction zone (SSZ) tectonic setting. The mantle peridotites in ophiolite belts located in eastern Qinghai-Tibetan Plateau, e.g. Sanjiang and West Kunlun, may be compared with the Troodos, which is regarded as a typical SSZ complex, having the same geochemical characteristics, i.e. high MgO and LREE-rich. The geochemistry, combined with the occurrence of boninite and adakite rocks, which are associated with subduction magmatism, suggest that ophiolites formed at different times in Qinghai-Tibetan Plateau, including Sanjiang and West Kunlun, are all SSZ-type ophiolites formed in a supra-subduction zone tectonic setting.