Late Jurassic-Early Cretaceous geodynamic setting in middle-northern Gangdese; New insights from volcanic rocks

Distribution, volcanism-age constrained by stratigraphy, paleontology and isotopic dating for the Late Jurassic-Early Cretaceous volcanic rocks on Gangdese, and their tectonic setting, petrogenesis and subducted polarity have been preliminarily studied based on regional geological survey (1: 250000) and our data. Our study shows that the Late Jurassic-Early Cretaceous volcanic rocks are not only distributed on southern Gangdese, but also widely exposed on middle, northern Gangdese and southern margin of Bangong Co-Nujiang suture zone. They started to erupt at about 120 similar to 130 Ma and likely lasted to the Late Cretaceous. However, the maximum volcanic activity occurred in middle-northern rather than southern Gangdese. Present geochemical data indicate that the Zenong Group volcanic rocks on middle Gangdese, which are mainly composed of partly intermediate-basic rocks of medium-K calc-alkaline series and dominant intermediate-acid rocks of high-K calc-alkaline series, are not consistent with traditional island-arc volcanic rocks but similar to volcanic rocks erupted in a thick crust from Central Andes, whereas the Duoni Formation volcanic rocks on northern Gangdese consist of mainly medium-K calc-alkaline series, resembling traditional island-arc volcanic rocks. It should be noted that all of these volcanic rocks have affinities with island-arc volcanic rocks. The Zenong Group intermediate-basic volcanic rocks on middle Gangdese are most likely derived from the partial melting of mantle wedge materials induced by fluid from subducted sediments and/or subducted basaltic crust and then experienced assimilation-fractional crystallization (AFC process) during ascending. The Zenong Group felsic rocks are dominantly related to crustal remelting, however, fractional crystallization of mafic magma cannot be entirely ruled out. The compositional variation, crustal thickness, distance from subducted zone revealed by present geochemical data of the Late Jurassic-Early Cretaceous volcanic rocks on middle-northern Gangdese, together with the distributions of contemporaneous volcanic rocks on Gangdese, indicating that the Late Jurassic Early Cretaceous volcanic rocks on middle-northern Gangdese are most likely related to southward subduction of Bangong Co-Nujiang oceanic crust.