Geochronological And Geochemical Characteristics Of Volcanic Rocks From Shangkuli Formation In Tulihe Region,Northern Great Xing'an Range

In this study,the new petrographical,zircon U-Pb ages,whole-rock geochemical data,and zircon Lu-Hf isotope compositions for volcanic rocks from Shangkuli Formation of the Tulihe area in the northern Great Xing'an Range(GXR)have been reported,with the aim of determining their precise formation age,petrogenesis,and magma source,inferring crust-mantle interaction,and further restricting the geodynamic processes between the late Mesozoic lithospheric extension and the evolution of the Palaeo-Pacific plate in the NE China.Field surveys and petrographic observations reveal that the Shangkuli Formation in the Tulihe area comprises rhyolite,trachyte and trachyandesite.The zircons from three volcanic rock samples display euhedral or subhedral with distinct oscillatory growth zoning and exhibit high Th/U ratios(0.19?3.04),indicating an apparent magmatic origin.LA-ICP-MS zircon U-Pb ages show that the volcanics form the Shangkuli Formation formed during the late Early Cretaceous(130?126 Ma).Based on their petrological and geochemical characteristics,the volcanic rocks of Shangkuli Formation are divided into two groups:rhyolites and trachytic volcanics.Geochemically,most of the rhyolites contain high concentrations of SiO₂(75.97?81.73 wt.%)and Na₂O+K₂O(6.30?8.72 wt.%),but low concentrations of total Fe₂O₃(0.86?2.07 wt.%),CaO(0.13–0.35 wt.%)and MgO(0.15?0.17 wt.%),with corresponding Mg^#values of 14?31,belonging to high-K calc-alkaline series.These rhyolite samples yield A/CNK ratios of 0.98?1.21 and are classified as metaluminous to weakly peraluminous rocks.They are enriched in large-ion lithophile elements(LILEs;e.g.,Rb,Th,U,and K)but depleted in high-field-strength elements(HFSEs;e.g.,Nb,Ta,Ti,and P),and exhibit relatively high zircon saturation temperatures(731?787?),obvious negative Eu(?Eu=0.18?0.49)anomalies and high differentiation index(DI=95.44?97.49),showing that our studied rhyolites belong to I-type fractionated rhyolites.Zircon Lu-Hf isotopic compositions of the rhyolites have?_Hf(t)values of+4.1?+9.0 and their two-stage model ages(T_DM2)ranging from 919?609Ma,suggesting that they were derived by the partial melting of juvenile crustal components accreted during the Nesoproterozoic.In addition,the studied rhyolites have high K₂O/Na₂O ratios(>1),and K-feldspar was a fractionated phase,suggesting that the magma source exhibit enrichment in K.Notably,two rhyolite samples exhibit relatively high zircon saturation temperatures(816?841?)and Mg^#values(43?48),implying that mantle materials had an influence on the partial melting of the continental crust.These features suggest that the rhyolites belong to highly fractionated I-type rhyolites that originated from the partial melting of a dominantly juvenile,K-rich basaltic lower crust source,and subsequently experienced significant fractional crystallization.Compared with the rhyolites,the trachytic volcanics contain moderate concentrations of SiO₂(57.81?65.14 wt.%),relatively high concentrations of total Fe₂O₃(3.43?6.55wt.%),CaO(2.28?4.57 wt.%)and MgO(0.76?2.36 wt.%),with corresponding Mg^#values of 31?46,belonging to subalkaline to alkaline Transition series.They yield A/CNK ratios of 0.88?0.99,indicating metaluminous characteristics.They display enrichment of LILEs(e.g.,Rb,K,Ba and Sr)but depletion of HFSEs(e.g.,Nb,Ta,P and Ti),and their trace element ratio(e.g.,Rb/Sr=0.05?0.37<0.5,Lu/Yb=0.14?0.15,Nb/Ta=14.61?17.34,and Zr/Hf=39.50?42.85)coincide with those of mantle-derived magma,indicating that they were derived predominantly from lithospheric mantle.Of note,some magmatic zircons with high?_Hf(t)values(+8?+13)were found in the trachytic rocks,implying that a more depleted asthenospheric mantle existed within their source region.These trachytic volcanics exhibit clear linear trends in Harker diagrams,indicating a non-negligible degree of fractional crystallization during magma evolution.The trachytic rocks exhibit negative Eu anomalies and have lower MgO,Cr,Co and Ni contents compared with primary-mantle-derived magma,indicating that the primary magma experienced some degree of fractional crystallization processes.In addition,three analysis spots with negative?_Hf(t)values(-17.8?-7.4)in the trachytic volcanic samples,together with the presence of captured zircons with ages of c.762 Ma,and obvious positive Zr-Hf and Nb-Ta anomalies,suggest that ancient crustal rocks were involved in the petrogenesis these rocks.The study of volcanic rocks from Shangkuli Formation indicates that intense Early Cretaceous crust-mantle interaction occurred within the northern GXR,which is very closely to the interval of widespread extension in NE China.By systematically summarizing the spatiotemporal distribution of the late Mesozoic magmatic rocks in NE China,we believe that Early Cretaceous lithospheric extension and crust–mantle interaction within the NE China were influenced by rollback of the Palaeo-Pacific flat-subducting slab.During the late Early Cretaceous time,large-scale upwelling of the asthenosphere resulted from continuous rollback of the Palaeo-Pacific flat-subducting slab heated the metasomatized lithospheric mantle and overlying crust,leading to high-degree partial melting and the formation of widespread magmatism in NE China.