Spatial-temporal distribution and tectonic setting of Jurassic magmatism in the Gangdise belt, Tibet, China

The distribution and age-constrained by stratigraphy, paleontology and isotopic age dates of Jurassic igneous rocks, and the nature of Jurassic granitoids that are poorly discussed previously in the Gangdese Belt are preliminarily summarized on the basis of regional geological survey(1∶250000) and related works. At present no Jurassic volcanic rocks have been found in the Gangdise back-arc fault uplift belt; Early Jurassic magmatism in the eastern Gangdise took place almost synchronously in the southern Gangdise (e.g. the Yeba Formation volcanic rocks and Oiyug and Niêmo granitoids), Gangdise back-arc fault uplift belt (e.g. Nyingzhong, Gyinda and Puqu granitoids) and northern Gangdise (Nyainrong granitoids). Mid-Upper Jurassic Jienu Group and Lagongtang Formation volcanic rocks are discontinuously distributed in the northern Gangdise and Late Jurassic magmatism is only sporadically observed to the north of the Shamolei-Maila-Luobadui-Milashan fault. Geochemical analysis combined with spatial and temporal constraints on the Jurassic magmatism indicates that the southern Gangdise and northern Gangdise were primarily affected by subduction during Jurassic time, while the Gangdise back-arc fault uplift belt and central Gangdise were not only affected by subduction but also by westward-propagating collision since Early Jurassic time. Considering paleomagnetic reconstruction and other related new findings, it may be considered that the features of such Jurassic magmatism in the Gangdise may be explained using the bidirectional scissors-style (opening westward) subduction model involving the southward subduction of the Bangong Co-Nujiang Tethyan oceanic crust and northward subduction of the Neo-Tethyan oceanic crust.