Episodic Paleozoic Adakitic Magmatism in the Eastern Tianshan,Southern Altaids:Implications for Petrogenesis and Geodynamics

The adakites rocks are typically considered as direct diagnostic evidence for subduction and continental growth,as well as the important evidence to determine the location of the major oceanic basin and evolution history of lost oceanic basins,which are longstanding conundrums for understanding the tectonics of the Paleo-Asian Ocean(PAO).As the processor of the Altaids,the PAO was a long-lived ocean,and its multiple subductions led to substantial continental growth in the Central Asian continent.Here we report our newly discovered Paleozoic adakitic rocks(granite and dacite)developed in Jijitaizi in the Eastern Tianshan of the southern Altaids to address the above issues.The Jijitaizi granite and dacite contain high concentrations of Sr(203-343 ppm)and low concentrations of Y(3.00-6.36 ppm)and Yb(0.23-0.62 ppm),indicative of the adakitic affinity.Zircon U-Pb dating results show that the adakitic rocks were formed in three episodes at 372.26±1.51 Ma,356.02±1.11 Ma and 334.51±0.68 Ma from the Late Devonian to Carboniferous.The rocks are depleted in Nb,P and Ti,and possess variable Mg#values(28.51-57.22),high Na_(2)O/K_(2)O ratios(1.39-4.94),as well as relatively depleted Hf isotopic compositions,all of which point to a subduction origin from slab melting.Our new finding with the date from the Jijitaizi complex(ca.321 Ma)suggests continuing subduction of the Paleo-Asian oceanic slab beneath the Central Tianshan from the Late Devonian to Carboniferous.Our work provides a solid line of key evidence demonstrating that continuous oceanic subduction of the major PAO evolved into its mature stage following subducting beneath the Central Tianshan from ca.372 to 321 Ma.