LA-ICP-MS zircon U-Pb ages and geochemical characteristics of cumulates in the Riwanchaka area, middle Qiangtang, the Tibetan Plateau

A set of ophiolite melanges, which are composed of cumulate gabbros, gabbro dykes, basalts and some other kinds of rocks, were first found in the Riwancaka area, middle Qiangtang. Geochemical characteristics of cumulutes in Chaka area show that gabbros within the Riwanchaka ophiolite belong to the E-MORB type, similar to features of ophiolites in Taoxinghu and Guoganjiashan areas. LA-ICP-MS U-Pb analyses of twelve zircon samples from this cumulate gabbro are similar to data of magmatic zircon in the chondrite-normalized REE patterns, with an average weighted age of 442.7Ma±3.4Ma, suggesting that the cumulate gabbros were formed in the Early Silurian. The Riwanchaka ophiolite is a new oceanic crust fragment of Early Palaeozoic in the Lungmu Co-Shuanghu-Lancang River suture zone. The results achieved by the authors will provide new evidence for the tectonic evolution of the Lungmu Co-Shuanghu-Lancang River Ocean.