Petrology, geochemistry, and geochronology of mafic rocks from the Taoxinghu Devonian ophiolite, LongmuCo-Shuanghu-Lancang suture zone, northern Tibet: evidence for an intra-oceanic arc-basin system

• Published in: International geology review 2016-03, Vol.58 (4), p.441-454
• Main Authors: Wu, Yan-Wang, Li, Cai, Xu, Meng-Jing, Xie, Chao-Ming, Wang, Ming, Xiong, Sheng-Qing, Fan, Zheng-Guo
• Format: Article
• Language: English
• Subjects: central Qiangtang; Devonian ophiolite; geochemistry; LA-ICP-MS zircon U-Pb dating; Palaeo-Tethys Ocean; Qinghai-Tibet Plateau
• ISSN: 0020-6814; 1938-2839
• DOI: 10.1080/00206814.2015.1089422

A newly discovered Devonian ophiolite located in the Taoxinghu area of central Qiangtang on the Qinhai-Tibet Plateau is described. The ophiolite consists of gabbro and diabasic dikes, and invasive cumulate gabbros-leucogabbros. The ophiolite has undergone greenschist facies metamorphism and minor deformation. Dating of the metagabbro by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb techniques yielded a weighted mean age of 367.2 ± 3.3 Ma (Late Devonian). Whole-rock geochemical analyses show that the rocks belong to the tholeiite series, with weak depletion in light rare-earth elements (LREEs), almost no Eu anomalies, weak enrichment in large-ion lithophile elements (LILEs), depletion in Nb and Ta, and weak negative Hf and Ti anomalies. These characteristics are similar to those of back-arc basin basalts. Together, these characteristics suggest that the rocks of the Devonian ophiolite formed by ~30% partial melting of spinel lherzolite, which was enriched by interaction with aqueous fluids during the late-generation phases; there is no evidence of subduction-related melting. The Devonian ophiolite rocks in the Taoxinghu area were first discovered in the LongmuCo-Shuanghu-Lancang suture zone. Detailed geochemical analyses show that the rocks formed in a back-arc ocean basin environment, indicating that the LongmuCo-Shuanghu-Lancang suture zone in central Qiangtang represents a Late Devonian intra-oceanic subduction zone in the Palaeo-Tethys Ocean. The discovery of the central Qiangtang Devonian ophiolite provides essential data for understanding the evolutionary history of the Palaeo-Tethys Ocean, and for identifying and understanding the roles of the different tectonic units on the Qinghai-Tibet Plateau.