Paper No. 81-13
Presentation Time: 11:15 AM
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 (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.