Meso-Neoproterozoic Magmatic-metamorphic Events And Their Geological Significance In Southern Beishan Orogenic Belt

The Beishan Orogenic Belt,is situated at the southernmost edge of the Central Asian Orogenic Belt,being bordered by the Central Tianshan to the west,the Solonker sutures to the east,and the Dunhuang block to the south.It's a critical site to explore the formation and evolution of the Central Asian orogenic belt and the Paleo-Asian Ocean.It bears a long and complex geological history,developing a complex rock assemblages with multiple stages of magmatic-metamorphic events.Basing on detailed field survey,I will systematically disscuss the petrological,geochemical,chronological analyses and temperature-pressure calculations on different Precambrian rock units,especially the medium to high grade metamorphic rocks exposed in the study.A spatial and temporal sequence of Precambrian tectonothermal events has been established,through analyzing the genesis and tectonic background of major rock units of different ages,and determined the metamorphic features of major complexes at Jiujing and Heishantou localities.The following results were achieved through this study:(1)In this study,a new set of post-orogenic A-type granitic gneisses with zircon U-Pb age of 1408±9 Ma(206Pb/238U age)was determined at Jiujing.The Mesoproterozoic granitic gneisses belong to the medium-high K-calc-alkaline,weakly peraluminous series with high SiO2(69.56-77.01 wt%),K2O(1.744.77 wt%),10000Ga/Al(2.4-3.2)and Zr+Nb+Ce+Y(312.2-656.9 ppm),poor Fe(Fe2O3T=1.16?5.06%)and Mg(MgO=0.15-0.9%).The total rare earth elements(TREE)is high(?TREE=219.76?323.88 ppm),the fractionation between the light and heavy REE is weak(LREE/HREE=3.65?6.41),with a flat distribution of HREE and an substantially negative anomaly of Eu.There are also some of the contemporaneous rocks displaying the characteristics of calc-alkaline type I granite,with enrichment of LILE(large ionic lithophile elements)such as Rb,Th and U,loss of high field strength elements such as Nb,Ta,Ti and P,and lower Y and Yb contents;the rare earth elements(?TREE=148.13?176.35 ppm)are relatively low,and the light and heavy rare earths are significantly differentiated(LREE/HREE=10.93?10.93),with obvious depletion of heavy rare earths and negative anomalies of Eu,which are characteristic of island-arc magmatism.It is suggested that the tectonic background of this period was the post-arc extensional environment,and the spatio-temporal association between Type ? and Type A rocks may be related to the delayed magmatism in the subduction zone.Positive zircon ?Hf(t)values for magmatic rocks indicate significant production of juvenile crustal material during this period.(2)A new set of augen gneisses with zircon U-Pb age of 877±5 Ma was found in southwestern Liuyuan town.Geochemical characteristics indicate that the augen gneisses are meta-aluminous-peraluminous,high K-calcium-alkaline type ? granites,formed by partial melting of the crust in the post-collisional environment.There are some fractional crystallization and mantle-derived material mixing existed during the magmatic evolution.The Guboquan paragneiss was formed in an active continental margin environment with a maximum depositional age of 863 Ma.The material source may be the early Neoproterozoic magmatic rocks in this area.(3)High-grade metamorphism in the early Neoproterozoic was identified in the Jiujing and Heishantou areas.Amphibolite and metamorphic supracrustal rocks with amphibolite facies-granulite facies metamorphism are exposed in Jiujing.The P-T conditions of peak stage of the Jiujing amphibolite given by traditional geothermometer is T=682?,P=0.62 GPa,belonging to amphibolite facies.The peak mineral assemblage of sillimanite-garnet-biotite mica schist is Grt+Bt+Pl+Qtz+Sil,with later superposed muscovite containing early sillimanite.Phase equilibrium modeling combined with traditional geothermometer give peak metamorphic P-T conditions of 790?797? and 0.63?0.65 GPa,representing granulite facies metamorphism.The maximum sedimentary age of sillimanite-garnet-biotite mica schist is 1206±51 Ma.The formation age of syntectonic gneissic granodiorite produced along schistosity at the same location is 915±9 Ma,constraining the time of this metamorphic event.(4)The first granulite facies metapelite was found at Heilenhead Mount.The spinel-bearing sillimanitecordierite-biotite gneiss suffered from granulite facies metamorphism and anatexis under water-absent condition,which produced a microstructure with a distinct separation of felsic component Qtz+Pl+Kfs and mafic-rich component Sil+Crd+Ilm+Mt+Spl.Metamorphic mineral assemblages are associated with the differential migration of components in open or locally open system.The phase equilibrium modeling results indicate peak metamorphic conditions of 912? and 0.62 GPa.Homogeneous composition of cordierinebearing garnet-biotite-plagioclase gneiss exposed at adj acent locations,with enriched felsic components,may represent the metamorphic characteristics before anatexis."The results of the traditional geothermometer and phase modeling show that,the P-T conditions of peak stage are as follows:715?725?,0.37?0.40 GPa.Furthermore,they demonstrate a collision-related clockwise metamorphic P-T trajectory on the region.Combined with zircon features and previous research results,it is assumed that this metamorphism may also have taken place in the early Neoproterozoic and was modified by magmatic intrusion(?443 Ma)during the Early Paleozoic.The above rock assemblages all indicate the existence of orogenic activities in the early Neoproterozoic in Beishan Belt.Combined with the relevant magmatic rock characteristics in the literature,the early Neoproterozoic of the Beishan orogen has experienced the subduction-convergence-collision orogenic process firstly and then extensional background by 870 Ma.(5)Sedimentary and magmatic records of middle-late Neoproterozoic have been found in the Lebaquan,Liuyuan and Heishantou areas,that is the schist(maximum depositional age 710 Ma)and gneiss(crystallization age 648 Ma,peak age 781 Ma)in Lebaquan,granitic gneiss(743 Ma)in Liuyuan,and metapelite(maximum depositional age 555 Ma)in Heishantou.The age spectrum shows a primary peak at 745 Ma and a secondary peak at 781 Ma,apparent peaks at 649 Ma,550 Ma,876 Ma,1393 Ma and 1711 Ma.Except for the protolith record of 1711 Ma,which was not found,all other peaks can be found in Beishan belt with dentical or similar age magmatic records,and suggest that there were multiple stages of magmatic activity in Beishan orogen during the middle-late Neoproterozoic(780-550 Ma),complementing and completing the chronological framework of Neoproterozoic tectonothermal events in Beishan orogen.Accordingly,we have established a geochronological framework of Precambrian tectonothermal events in the Beishan orogenic belt.The ancient continental crust in Beishan was mainly formed in Mesoproterozoic(?1.4 Ga)and was strongly modified in the early Neoproterozoic(930-870 Ma).It's likely that the middlelate Neoproterozoic(780?550 Ma)may be under the background of continental crustal extension.The Mesoproterozoic(?1.4 Ga)magmatic events within the Beishan and other microcontinental blocks along the southern CAOB actually record a transition from an orogenic compressional to anorogenic extensional environment on the margin of the Columbia supercontinent during its later evolution.Two tectonothermal events in the Neoproterozoic can correspond to Rodinia amalgamation and breakup processes,respectively.A comparison with the ages and properties of Precambrian rocks exposed in the surrounding blocks suggests that the Precambrian geological bodies in Beishan orogenic belt differs from that of the Tarim Craton.From Mesoproterozoic to early-middle Neoproterozoic,it has obvious affinity with the Middle Tianshan and Yili blocks,and has experienced a similar formation process of early continental crust.They entered into different evolutionary processes in the middle and late Neoproterozoic.By complementing and refining the chronological framework of Neoproterozoic magmatic events in Beishan orogen,we also deduce that the belt was still part of an mobile orogenic belt during the Precambrian,and there was no stable microcontinental block.In this study,we propose that the Beishan orogenic belt was still part of the orogenic belt rather than a stable block in the Precambrian.