Mid-ocean ridge (MOR) and suprasubduction zone (SSZ) geological events in the Yarlung Zangbo suture zone: Evidence from the mineral record of mantle peridotites

The Purang ophiolite in the westernmost Yarlung-Zangbo suture zone of southern Tibet shows a two-stage evolution, on the basis of its mineralogy and geochemistry, having formed originally at a mid-ocean spreading ridge and later been modified in a suprasubduction zone (SSZ) environment. Olivine from the Purang peridotites is highly magnesian (Fo90-92) with NiO contents of 0.32-0.48 wt%, consistent with formation in a fore-arc environment. Magnesiochromite in the mantle peridotites has a large range in Mg# [100Mg/(Mg + Fe)] (43.6-67.7) and Cr# [100Cr/(Cr + Al)] (22.3-67.8), reflecting different degrees of partial melting and melt-rock reaction. The Cr#s increase and the Mg#s decrease systematically from lherzolite to harzburgite to dunite. Magnesiochromites in the harzburgites and lherzolites are analogous to those from abyssal peridotites and MORB-like ophiolites, whereas those in clinopyroxenites and cumulate orthopyroxenites are typical of SSZ ophiolites, suggesting refertilization of the peridotites by metasomatic fluids/melts. This interpretation is supported by the “U-shaped”, chondrite-normalized rare earth element (REE) patterns of Purang mantle peridotites. Plots of TiO2 and Al2O3 vs. Mg# show that both the orthopyroxenes and clinopyroxenes overlap the fields of abyssal and SSZ peridotites indicating a transition from a MOR to a SSZ setting for the Purang peridotites.