T81 Bridging Two Continents: Diamonds, Zircons and Native Elements in the Mantle: New Discoveries and Models on the Properties and Petrogenesis of Oceanic Mantle Lithosphere
Monday, 2 November 2015
8:00 AM - 12:00 PM
Room 347/348
MID-OCEAN RIDGE (MOR) AND SUPRASUBDUCTION ZONE (SSZ) GEOLOGICAL EVENTS IN THE YARLUNG ZANGBO SUTURE ZONE: EVIDENCE FROM THE MINERAL RECORD OF MANTLE PERIDOTITES
Paper number: T81-14
Program: Technical Sessions
Time: 11:15 AM – 11:30 AM
Authors
Abstract
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 (Fo 90–92 ) 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.