Late Paleozoic Magmatism And Crustal Growth In The Aqishan-Yamansu Belt Of Eastern Tianshan

In order to better understand the Late Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt?CAOB?,we present systematic chronological and geochemical data for the Late Paleozoic volcanic rocks and intrusive rocks from the Aqishan-Yamansu belt in Eastern Tianshan.The collected samples mainly consist of andesites from the Early Carboniferous Yamansu Formation,basalts and basaltic andesites from the Late Carboniferous Tugutubulak Formation,granodiorites and dykes?including diabase,dioritic and granitic dykes?from the Bailingshan Complex which intruded into the Tugubulak Formation.Zircon U-Pb geochronological data show that the intrusive ages of the andesite,granodiorite,diorite and granitic dyke are 330±3 Ma,321±2 Ma,315±1 Ma and 312±3Ma,respectively.The andesites of the Yamansu Formation display subalkaline and calc-alkaline characteristics with moderate SiO₂ contents?58.5-61.4 wt.%?,relative high Mg^#values?49-54?,suggesting that mantle-derived materials play a dominantly role in the magma source.The low?_Nd?t?values?-0.02⁺0.61?and old two-stage T_DM age of 1.39¹.40 Ga indicate a mixing source of old crustal material and mantle-derived material.Their high Rb/Y?1.82-4.23?and Ba/La?23.6-54.9?,low Nb/Y?0.18-0.21?and Th/Yb?0.37-0.43?ratios suggest that the magma source were probably metasomatised by slab-derived hydrous fluids.In combination with the subduction-related character of depleted HFSE?Nb,Ta,and Ti?and enriched LILE,we suggest that the Yamansu andesites were generated by partial melting of a mantle wedge metasomatized by both subducted sediment-derived melts and slab-derived aqueous fluids.Geochemical characteristics indicate that the basalts of Tugutubulak Formation show tholeiitic characteristics and have high MgO,Mg^#?56-58?and relatively high concentrations of compatible trace elements,such as high Cr?207-211 ppm?,Ni?111-116 ppm?,and Co?40-42 ppm?,relatively high?_Nd?t?values?5.9-6.3?and low initial ⁸⁷Sr/⁸⁶Sr ratios?0.7037-0.7039?,suggesting that the basalts were derived from a depleted mantle source.The basaltic andesites of the Tugutublak Formation have higher SiO₂?53.62-54.11 wt.%?and lower Mg^#?48-50?,Cr?133-168?,Ni?50-57?and?_Nd?t?values?1.26-1.27?than the basalts of this formation.Both the basalts and basaltic andesite are enriched in LREE and have slightly flat HREE,with negative Eu and Nb-Ta-Ti anomalies,implying geochemical characteristics of magmatic rocks formed in subduction zones.These samples have high Ba/Th,Ba/La and low Th/Ta,Th/Yb ratios,indicating an addition of subduction-related fluids.Therefore,we suggest the basalts and basaltic andesites of the Tugutublak Formation were formed by partial melting of mantle wedge which was metasomatised by slab-derived fluids,and then experienced a certain degree of crystallization differentiation.The granodiorite rocks intruding into the Tugutublak formation show a character of typical I-type granite,with low P₂O₅?0.05-0.09 wt.%?,A/CNK?0.93-1.05?and high Na₂O?3.57-3.81 wt.%?.Highly positive?_Hf?t?values?14-17?of magmatic zircons,as well as the presence of inherited zircons?an inherited grain with an age of 352 Ma?,suggest a significant contribution from juvenile basaltic lower crust with minor involvement of old crustal components.The diabase dykes have low SiO₂?46.92-48.15 wt.%?,total alkali?Na₂O+K₂O=2.8-4.0 wt.%?,but high?1.47-1.54 wt.%?,MgO?5.78-6.88 wt.%?and Mg^#?54⁵9?.They are enriched in LREE and show a geochemical character of E-MORB.The relatively high Ba/Th ratios of the diabase dykes and their slight low Th/Ta ratios and negative Nb-Ta anomalies imply a mantle source metasomatised by slab-derived fluids.Thus,these basic dykes were likely generated by partial melting of the upwelling asthenosphere mantle with a slight influence of slab-derived fluids.The dioritic dykes show adakitic characteristics and have high Na₂O and positive?_Hf?t?values?+12 to+17?,which suggest an origin from partial melts of subducted oceanic slab.The granitic dykes have high SiO₂?76.52-77.10 wt.%?and K₂O?4.86-5.26 wt.%?and are characterized by enriched LREE and slightly flat HREE,with negative Eu and Nb-Ta-Ti anomalies.These dykes are alkali-calcic and show geochemical features of highly fractionated I-type granites.Their positive?_Hf?t?values?+16 to+17?suggest that they were derived from a juvenile basaltic crustal source.Based on stratigraphy,synchronic tectonic events and special lithologic assemblages,we consider that the formation of these dykes was triggared by a slab breakoff in the Late Carboniferous.In addition,based on the previous data and the variation of the calculated crustal thickness,we suggested that the Aqishan-Yamansu belt was formed by south-dipping subduction of the Kanggur oceanic plate in the Early Carboniferous,and then the arc-basin transition happened due to the steepening of subduction angle in³35 Ma.The closure of the Kanggur Ocean began after³20 Ma and the Aqishan-Yamansu belt changed to post-collision extension.