Early Palaeozoic Syn-Collisional Granitic Magmatism in the North Altun and Its Implication for Accretionary Orogenic Processes

This paper present a set of new petrography, major and trace element geochemistry, zircon U-Pb geochronology and in situ Hf isotopic compositions for Kaladawan porphyritc granite, Goukouquan porphyritc monzogranite, Zhuoerbulake granodiorite and Muzisayi muscovite granite in North Altun. The Kaladawan porphyritc granite are characterized by relatively high contents of silicon and sodium, but low contents of iron, magnesium and calcium, with moderate alumina saturation index (A/CNK = 1. 03 - 1. 04), belonging to slightly peraluminous I -S transition type granite. Goukouquan porphyritc monzogranite and Zhuoerbulake granodiorite exhibit relatively high sodium contents and metaluminous to slightly peraluminous calc-alkaline I -type granitoids features. Muzisayi muscovite granite show relatively high Si02, ALK (Na20 + K2 0, enrichment Rb, Th and LREE, depletion Ba, Sr, Ti, Eu, and yields alumina saturation index values of > 1. 1, showing S -type granite features. Zircon U-Pb dating suggest that four granitic plutons were emplaced during Early Silurian (439 432Ma). Integrating the regional tectonic evolution and discrimination diagrams, we propose the closure time of North Altun Ocean occurred during ca. 445 - 440Ma. Goukouquan porphyritc monzogranite and Zhuoerbulake granodiorite exhibit relatively high zircon e f (t) values (+ 4. 72 + 7. 55 and + 6. 33 - + 9. 96, respectively), suggest that they were generated by partial melting of the pre-existing juvenile crustal rocks. Geochemical characteristics show that the Kaladawan porphyritc granite was derived from partial melting of metagreywackes rocks, whereas the Muzisayi muscovite granite was derived from low degree partial melting of metapelitic rocks. However, the relatively positive EHf (t) values (+ 0. 18 - + 5. 88 and - 6. 47 - +4. 52, respectively) strongly suggest that the involvement of some juvenile crust components in different proportions. Above these features are similar to the Early Palaeozoic syn-collision granitoids from the North Qilian, which further support that two orogenic belts have undergone similar geological and tectonic processes involving continental rifting, ocean floor spreading, seafloor subduction, accrationary orogenesis in the time period fromthe Neoproterozoic to the Paleozoic.