Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications

• Published in: Chinese science bulletin Vol. 58; no. 1; pp. 92 - 98
• Main Authors: Huang, QiShuai, Shi, RenDeng, Liu, DeLiang, Zhang, XiaoRan, Fan, ShuaiQuan, Ding, Lin
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer-Verlag 2013; SP Science China Press
• Subjects: aluminum oxide; chemical composition; Chemistry/Food Science; chromium; Earth Sciences; Engineering; evolution; geochemistry; Humanities and Social Sciences; Indexing in process; isotopes; Life Sciences; magnesium oxide; multidisciplinary; osmium; Physics; Science; Science (multidisciplinary); zirconium; 同位素组成; 幔源; 碳质球粒陨石; 蛇绿岩; 西藏; 那曲地区; 锆石U-Pb年龄; China; Nagqu; ophiolite; spinel; Tibet; Re-Os isotopes
• ISSN: 1001-6538; 1861-9541
• DOI: 10.1007/s11434-012-5384-8

Early-crystallizing chromian spinel （Cr-spinel） in the Nagqu ophiolite has high Os and low Re contents, and it is resistant to alteration during serpentinization, weathering and metamorphism. The chemical composition of primitive magma is preserved in Cr-spinel, which makes it suitable for determining the initial Os-isotope composition of the mantle source. This study presents Cr-spinel Os isotopes and zircon U-Pb ages for cumulate dunite and gabbro, respectively, in the same cumulate section of the ophiolite at Nagqu in Tibet. The results shed light on the formation and evolution of lithospheric mantle. The Nagqu ophiolite is located in the central part of the Bangong-Nujiang suture zone. It is a remnant of the Neotethyan oceanic crust, and contains cumulate dunite and gabbro. Zircon from the gabbro yielded a weighted mean 206pb/238U age of 183.7±1 Ma. Cr-spinel exhibits γos values of -0.2 to -0.3, suggesting that the mantle source for the dunite is similar to that of carbonaceous chondrites. Thus, the Tibetan lithosphere is primarily a relic of Tethyan oceanic lithosphere, which has formed by the transformation of the normal asthenospheric mantle in the Mesozoic. This is the first study to combine the spinel Os isotopes with accurate zircon U-Pb ages to constrain the geochemical characteristics of the mantle source for the ophiolite.