High-and low-Cr chromitite in a Tibetan Ophiolite: Evolution from Mature Subduction System to Incipient Forearc in the Neo-Tethyan Ocean
摘要：The microstructures, major-and trace-element compositions of minerals and electron backscattered diffraction（EBSD） maps of high-and low-Cr# [spinel Cr# = Cr3+/(Cr3++Al3+)] chromitites and dunites from the Zedang ophiolite in the Yarlung Zangbo Suture（South Tibet） have been used to reveal their genesis and the related geodynamic processes in the Neo-Tethyan Ocean.The high-Cr#（0.77-0.80） chromitites（with or without diopside exsolution） have chromite compositions consistent with initial crystallization by interaction between boninitic magmas, harzburgite and reaction-produced magmas in a shallow, mature mantle wedge. Some high-Cr# chromitites show crystal-plastic deformation and grain growth on previous chromite relics that have exsolved needles of diopside. These features are similar to those of the Luobusa high-Cr#chromitites, possibly recycled from the deep upper mantle in a mature subduction system. In contrast,mineralogical, chemical and EBSD features of the Zedang low-Cr#（0.49-0.67） chromitites and dunites and the silicate inclusions in chromite indicate that they formed by rapid interaction between forearc basaltic magmas（MORB-like but with rare subduction input）and the Zedang harzburgites in a dynamically extended,incipient forearc lithosphere. The evidence implies that the high-Cr# chromitites were produced or emplaced in an earlier mature arc（possibly Jurassic）, while the low-Cr# associations formed in an incipient forearc during the initiation of a new episode of Neo-Tethyan subduction at ～130-120 Ma. This two-episode subduction model can provide a new explanation for the coexistence of high-and low-Cr# chromitites in the same volume of ophiolitic mantle.
The 4th IGCP-649 Diamonds and Recycled Mantle Workshop and Field Trip
地质学; 地质学; 矿业工程