The Tectono-magmatic Evolution Of The Late Paleozoic Balkhash-Yili Active Continental Margin

The Balkhash-Yili orocline is located in the southwest part of the Central Asia Orogenic Belt(CAOB)and forms in an arc shape,Late Paleozoic magmatic rocks are widely distributed throughout this region.The Yili block is an important part that forms the eastern Balkhash-Yili orocline and late Devonian to late Carboniferous volcanic rocks are widespread along its northern and southern margins.However,their genesis and tectonic setting remain controversial and various hypotheses have been put forward,including post-orogenic continental rift,large igneous province probably related to a mantle plume,back-arc or intracontinental extension and continental arc.In order to further prove the tectono-magmatic evolution of Yili block,we conducted field investigations,zircon U-Pb dating,whole-rock geochemical and Sr-Nd isotopic studies on these volcanic rocks from southern Yili block.These data,combined with the literature data of the magmatic rocks from Yili block and Balkhash active margin,as well as the ophiolitic melange zones distributed to the north and south margins of the Yili block and other geological evidences put constraints on the the dynamic mechanism of the late Paleozoic accretionary tectonics of the SW CAOB.Whether the subduction direction is unidirectional or bidirectional,if it is unidirectional,which model is reasonable,the northward subduction of the South Tianshan Ocean(STO)or southward subduction of the Junggar-North Tianshan Ocean.The late Paleozoic volcanics in the southern Yili block mainly comprises undeformed andesite,trachy-andesite,rhyolite,basalt and pyroclastic rocks,these basic-intermediate-acid volcanics show geochemical features consistent with that of calc-alkaline series.They are characteristically enriched in LILEs(Rb,Th,Ba,and K)and LREEs and are depleted in HFSEs(Nb,Ta,and Ti)and HREEs which shows geochemical feature of island-arc volcanic rocks.They have positive sNd(t)values(+0.9?+5.6),variable initial 87Sr/86Sr ratios(0.7041-0.7059)and Neo-to Mesoproterozoic single-stage Nd model ages(0.80?1.33 Ga)suggesting that these magmatic rocks are likely derived from partial melting of a moderately depleted mantle wedge with minor involvement of a Precambrian continental crust.The OIB-like geochemical features of some volcanic rocks indicate incipient marginal rifting occurred along the southern Yili Block during Late Carboniferous.The chronological data reveal that Late Paleozoic arc magmatism initiated from-386 Ma along the northern Yili Block,and migrated southward to have occurred since-340 Ma along its southern margin.Subsequently,the arc-related magmatism moved back northward and terminated at 310 Ma in the south and at-295 Ma in the north.Taking into account the regional structural and tectonic data,the late Devonian to late Carboniferous Yili magmatic arc may be laterally correlated with the Balkhash-Yili active margin of the Kazakhstan microcontinent,which was likely resulted from the southward subduction of the Junggar-North Tianshan Ocean,back-arc extension setting occurred in southern Yili during late Carboniferous.The Junggar Ocean closed in early Permian and the magmatism ceased.Although Carboniferous arc-releated volcanic rocks distributed along the southern Yili block which is located to the north of the HP metamorphic belt,taking into account the fact that contemporaneous island-arc magmatism is not developed in the Kyrgyzstan Tienshan,hence,we thinks the late Paleozoic Balkhash-Yili active continental margin might be unrelated to accretion of the South Tianshan Ocean.We also conduct systematic analysis on the Carboniferous magmatite from the Balkhash active margin.The granites from inner edge possess high ?Nd(t)values and younger Nd model ages than the arc-related granites from outer edge.These characteristics combined with chronological data demonstrate that the magma source of these Carboniferous granites from inner edge mainly contain input of mantle materials mixed with crust-derived material,however,the magma source of the granites from outer edge consists mainly of crust-derived material as well as part mantle derived material.The magmatic activity of inner edge initiated earlier and terminated later than the outer edge.The above characteristics favor a southward subduction model of oceanic basin.Taking into account the feasures and spatial and temporal distribution of the late Paleozoic volcanism,the Bayingou ophiolitic melanges distributed along northern margin of Yili block and the Kekesu-Akeyazi HP metamorphic belt and associated ophiolitic melanges distributed along southern margin of Yili block,and the feasures of the Carboniferous magmatic rocks from Balkhash active continental margin.We infer that the magmatism originateds from northern margin of the Balkhash-Yili active margin which was caused by the southward subduction of the Junggar-North Tianshan Oceanic basin beneath the Kazakhstan microcontinent.Back-arc extensional setting occurred in southern during late Carboniferou,the Junggar-North Tianshan Ocean closed during early Permian and lead to the cease of arc-related magmatic activity.Although Carboniferous island-arc volcanics outcroppted to the north of the Kekesu-Akeyazi HP metamorphic belt,considering that contemporaneous arc-related volcanism is not developed in the Central Tianshan of Kyrgyzstan,hence,we infer that the accretionary tectonics of late Paleozoic Balkhash-Yili active continental margin have no connection with the South Tianshan Ocean.