Comparison of Redox States between the Ultramafic Bodies of Xigaze and Luobusha Ophiolites, Tibet, China

Abstract

AbstractThe tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity (fO2) (e.g., Ballhaus, 1993; Dare et al., 2009; Parkinson and Arculus, 1999). Here we present results of fO2 of chromites determined by Mössbauer spectroscopy from both the Luobusha and Dazhuqu areas along Yarlung Zangbo suture zone, Southern Tibet.The fO2 values (−1.02∼0.04 log units against the FMQ buffer) and Cr# (22∼54%) in chromites from lherzolites and harzburgites of both areas are similar to those of abyssal peridotites, indicating that they may be residues after partial melting at spreading centers. However, both dunite envelopes and chromitites from Luobusha have high fO2 values (0.04∼2.25 log units) and Cr# (73∼84%), showing an affinity to boninitic melts, and thus form in a suprasubduction zone. Dazhuqu dunites show diverse fO2 values (−0.22∼2.19 log units) and Cr# (22∼82%), indicating that they form in distinct settings. Chromitites and chromite dunites from Dazhuqu have low fO2 values (−0.3∼0.71 log units) and Cr# (16∼63%), both of which are similar to those of MORB‐like basalts, inferring that they form in an extensional setting. Both high‐Cr and high‐Al chromitites from other typical podiform chromite ore deposits, such as Kempirsai, Oman, and Albania ophiolites, also show high fO2 values (e.g., Chashchukhin and Votyakov, 2009; Melcher et al., 1997; Quintiliani et al., 2006; Rollinson and Adetunji, 2015), while the distribution‐limited small chromitites and chromite dunites from Dazhuqu exhibit low fO2 values. The phenomenon infers that the suprasubduction zone is more beneficial to the formation of podiform chromitites.