Geochemical analysis and amphibole ⁴⁰Ar-³⁹Ar dating for meta-basalts from Tongka ophiolitic mélange, eastern Nujiang belt

The nature and early evolution of the ancient ocean basin represented by the Bangongco-Nujiang suture zone in the central Qinghai-Tibetan Plateau are still controversial.To identify the nature of the Early Jurassic tectono-magmatic events and micro-continental tectonics and orogenic processes along the mid-eastern Nujiang suture, geochemical study on meta-basaltic rocks and ⁴⁰Ar-³⁹Ar dating of amphibole from the Tongka ophiolitic mélange, eastern Nujiang belt, were conducted.The meta-basalts in the Tongka ophiolitic mélange witnessed strong metamorphic deformation, indicating the mineralogical composition of the epidote-amphibolite facies.Rare earth element(REE) patterns of the meta-basaltic rocks on the chondrite-normalized REE diagram display LREE depletions indicative of N-MORB, whereas on the N-MORB-normalized spider diagram, the samples indicate Rb, Ba, Th, U, Pb and Sr enrichments and Nb and Ta depletions, which suggest a mixing characteristic of arc and N-MORB, and an origin from back-arc basin spreading center, indicating a SSZ affinity.The ⁴⁰Ar-³⁹Ar dating of the amphibole from a meta-basalt sample yields a plateau age of 161 ±2 Ma, analogous to ⁴⁰Ar-³⁹Ar cooling ages(165~167 Ma) of biotite from the Tongka and Amdo gneisses, which provides direct evidence from ophiolite emplacement for local closure of the eastern Nujiang Ocean and Early Jurassic collision along the Tongka-Exue suture between the Jiayuqiao and Tongka micro-blocks.On the basis of comprehensive analyses of the early and late Jurassic magmatic rocks north of the Bangong-Nujiang belt, the Bangong-Nujiang belt is assumed to have distinct evolutionary histories, subduction polarities and collisional timing at different segments and ophiolitic branches.The multi-island-accretive orogeny model suggests that the orogeny in the hinterland of the Qinghai-Tibet Plateau might be the result of varying level convergences by diverse-sized terranes/blocks with consumption of different oceanic basins, rather than the re-opening process of the Meso-Tethyan Ocean after the closure of the Paleo-Tethyan Ocean.