2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

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


GUO, Guolin, CARMA, State Key Laboratory of Continental Tectonic and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, No. 26 Baiwanzhuang Street, Beijing, 100037, China; Fundamental Science on Radioactive Geology and Exploration Technology Laboratory, East China Institute of Technology, No. 418 Guanglan Road, Nanchang, 300013, China, gglrobin@163.com

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.