Geochemical characteristics of mineral inclusions in the Luobusa chromitite (Southern Tibet): Implications for an intricate geological setting

Abstract

The Luobusa chromitite and ophiolite present a captivating geological feature marked by peculiar mineralogical and geochemical characteristics. Abundant platinum-group minerals (PGM), base-metal sulfides (BMS), and PGE-sulfides and alloys in the chromitite unveil a multi-stage genesis, encompassing partial mantle melting, melt-rock interactions, and dynamic shifts in oxygen and sulfur fugacity (fO2, fS2). This study explores the geochemical signatures and PGE patterns of these mineral inclusions to elucidate the evolutionary process of the Luobusa ophiolite, tracing its transition from a sub-ridge environment to a sub-arc setting. The variable ΣPGE values (40 - 334 ppb) in chromitite, coupled with notably lower ΣPGE values (10 - 63 ppb) in dunite imply extensive melt fractionation and melt-rock interactions. Coexisting well-crystalline Os-Ir alloys alongside interstitial BMS likely reflect low fS2 and high temperatures during the early formational stages, whereas abundant anhedral sulfarsenide and pyrite inclusions in chromite point to lower temperatures and higher fS2 during the late stages. The trace element composition of pyrite inclusions resonates with the characteristics of mid-ocean ridge (MOR) and oceanic island rocks, manifesting interplay of diverse magmatic sources during the evolution of the Luobusa ophiolite.