Subduction initiation of Proto-Tethys Ocean and back-arc extension in the northern Altun Mountains, northwestern China: Evidence from high-Mg diorites and A-type rhyolites

Abstract Subduction initiation and sequent evolution of oceanic lithosphere at convergent plate boundaries have important implications for our understanding of plate tectonics. Using zircon U-Pb and Lu-Hf isotopic and whole-rock geochemical data of newly identified Baijianshan diorites and Dawanbei and Dabanxi rhyolites from the center of the North Altun Orogenic Belt, we provide the time constraint for the initial subduction of the Proto-Tethys Ocean lithosphere and back-arc extension in the northern Altun Mountains. Zircon U-Pb ages show that the Baijianshan diorites emplaced at ~535 Ma and the Dawanbei and Dabanxi rhyolites erupted at ~495 Ma. The Baijianshan diorites share most features of high-K calc-alkaline high-Mg andesites, characterized by moderate SiO2 contents (56.34-57.65 wt.%), high MgO contents (4.72-5.26 wt.%) and Mg# (57-65) values, and LREE and LILE enrichment relative to HREE and HFSE (Nb/La = 0.46-0.78). Their elemental geochemistry and coupled negative whole-rock ɛNd(t) (-9.1 to -8.5) and zircon ɛHf(t) (-15.4 to -10.9) values suggest that the diorites were formed by interaction between the subducted sediment-derived melts and peridotites in the mantle wedge. The Dawanbei and Dabanxi rhyolites are metaluminous to weakly peraluminous, and have high SiO2 (70.07-78.65 wt.%) and Na2O (2.27-5.97 wt.%) contents, and low K2O (0.95-4.72 wt.%) contents. The rhyolites are enriched in most incompatible trace elements (e.g. Rb, Th, Zr, Hf and REE), and show affinity with A2-type granites. Isotopically, they have whole-rock ɛNd(t) values of -0.7 to -0.2 and zircon ɛHf(t) values of -2.3 to 6.4. Geochemical and isotopic compositions suggest that these rhyolites were derived through shallow (1-3 kbar) dehydration melting of basaltic rocks. Our analysis together with the regional geology of the northern Altun Mountains indicates that subduction of the Proto-Tethys Ocean lithosphere began started at ~535 Ma in the northern Altun Mountains. The juvenile A-type rhyolites (~495 Ma) are spatially and temporally associated with the Dawan high-Mg andesites and diorites (480-495 Ma), indicating that they were likely formed in a back-arc basin that was extending due to rollback of subducted oceanic lithosphere. The subduction process most likely continued until ~460 Ma and the Proto-Tethys Ocean finally closed at ~440 Ma. As a result, the central Altun terrane, as well as Tianshuihai-Mazar, Central Qilian, and North Qinling docked to the northern margin of Eastern Gondwana at ~440 Ma.