Lithological research on the Purang mantle peridotite in western Yarlung-Zangbo suture zone in Tibet

The Purang ophiolite in the Yarlung-Zangbo suture zone in Tibet is characterized by containing a large mantle peridotite massif of ca.600km2 in area.The mantle peridotite consists of dominant harzburgite,minor lherzolite and dunite,and contains some dikes or veins of olive clinopyroxenite,gabbro and diabase in the massif.The Fo values of olivine in mantle peridotite vary in the range of 90 ～ 93,and in which relatively higher Fo values of olivine are from the inclusions within other minerals in the rocks.The orthopyroxenes in the rocks are enstatite(En 88～90),and the clinopyroxenes are endiopside and diopside with low Al2O3(0.48%～3.96%) contents and high Mg#(91～96) values.Chrome spinels have various Cr# values from 18 to 69,and in which aluminum-rich spinels are from harzburgite and lherzolite,but chromium-rich spinels from dunite.In olivine clinopyroxenites,olivines have uniformly low Fo values of around 88 and orthopyroxenes have En values around 87.Clinopyroxenes in the rocks are dominated by diopside,and chrome spinels contain both aluminum-rich and chromium-rich types with Cr# values varied in the range of 45～69.The mantle peridotite and olivine clinopyroxenite in Purang have the similar distribution patterns of rare-earth elements and trace elements,characterized by their slightly enrichment in LREE and weakly depleted in Eu,relatively lower content of large ion lithophile element(LILE),and either depleted or enriched in high filed-strength element(HFSE).These features imply a depleted mantle source,which were overlapped by fluid alteration in a subduction zone.It concludes that the Purang ophiolite formed in a MOR setting and was modified by fluids in a SSZ setting,similar to the Luobusa ophiolite in the eastern Yarlung-Zangbo suture zone.