Late Palaeozoic Tectonic Evolution Of Xinjiang West Junggar

Central Asian orogenic belt formed through a series complicated tectonic evolution of Paleo-Asian Ocean.The west Junggar region,as one of the important sections of central Asian orogenic belt,has great study significance to central Asian orogenic belt of tectono-magatic evolution process.This paper studied petrology and geochemistry of granites in this region and got the following understandings:1.Six granite intrusions LA-ICP-MS zircon U-Pb ages distribute in 324-278 Ma.This result determine the chronology framework of granite magmatism in west Junggar.According to these zircon U-Pb age data,late Paleozoicwest magmatism in west Junggar region can be divided into four stages:(1)Late Silurian-Early Devonian(405-425Ma);(2)the late Early Carboniferous(324.3 ± 8.2Ma?319.6 ± 3.2Ma);(3)Late Carboniferous(309.4 ±2.5Ma?301.1±3.4Ma);(4)Early early Permian(290.8±6.2Ma?277.7 ± 8.1Ma).2.Late Early Carboniferous granitoids characterize peraluminous,high potassium,calcium alkaline,with high Si(Si02=73.93-78.35%),rich in alkali and K(K20 = 4.31-4.90%),depleted Na(Na2O = 3.27-3.91%),depleted Ca(CaO = 0.23-1.16%),low Al(A1203 = 11.66-14.10%),Mg(MgO = 0.02-0.25%)and Ti(Ti02 = 0.04-0.20%).As to the trace elements,Ba(15.03-365.10X10-6)?Sr(3.08-65.29x10-6)?Cr(0.32-2.45×10-6)?Ni(0.32-1.62×10-6)are low,Y(39.85-83.43×10-6)and Yb(4.46-8.27x1O-6)are high,the ratio of Rb/Sr(1.38-44.70)are high.The trace elements spider diagram shows deficit of Ba?Nb?Ta?Sr?P?Ti and enrichment of Rb?Th?K.In the are earth element distribution diagram which has A type granite geochemistry character,negative Eu anomaly exhibit rightist "Seagull" type.The tectonic setting is thought to be post collision extensional collapse derived from partial melt of crust material combined with regional geology background.3.Late carboniferous Selden granite has the characters of peraluminous high alkaline potassium calcium,with high Si(SiO2=69.82-70.11%),highAl(Al2O3= 15.00-15.30%),rich in alkali and Na(Na2O=4.02-4.10%),depleted K(K20 = 3.28-3.48%),low Mg(MgO =0.86-0.92%),Ca(CaO = 2.24-2.27%),Ti(TiO2 = 0.37-0.39%).As to trace elements,Sr(319.70-339.20X10-6),Cr(4.43-4.90×10-6)and Ni(3.22-4.47×10-6)are low,Yb(2.04-2.22x10-6),Y(20.40-22.62x10-6),Ba(738.40-757.70×10-6)and ratio of Rb/Sr(0.18-0.21)are high.The trace elements spider diagram shows deficit of Nb?Ta?P?Ti,enrichment of Th,K and relative enrichment of LREE.In the are earth element distribution diagram,HREE flat distribution and slightly negative Eu anomaly exhibit the character of I-type granite.Combine with regional geology background,it is considered that late carboniferous Selden granite derived from remelt of juvenile crust(product of island arc magmatism)at compression to extension transition environment in post collision stage.Late carboniferous Kezilekeya granite has the characters of peraluminous high alkaline potassium calcium,with high Si(SiO2= 74.26-76.15%),middle Al(Al2O3=12.59-13.34%),rich in alkali,depleted Ca(CaO=0.25-0.33%),low Mg(MgO=0.13-0.18%,Mg#=16.32-20.30)and Ti(Ti02 =0.24-0.32%),relative rich in K and poor Na(K2O/Na2O=1.20-1.49).As to trace elements,Sr(33.9-59.2x1O-6)is very low,Ba(180-237×10-6)is low,Y(27.4-38.4x10-6),Yb(3.70-4.48×10-6),Nb(31-43.9×10-6)and ratio of Rb/Sr(2.41-4.20)is high.The primitive mantle standardized spider diagram of trace element is slightly to the right of serration,with Ba?U?Nb?Ta?Sr?P?Ti depetion and Rb?Th?K enrichment which reflect strong fractional crystallization.Meteorite standardized REE distribution patterns figure shows Eu loss,LREE relative enrichment.Distribution curve is typical rightist "Seagull" type which is similar to A-type granite.Combine with regional geology background,late carboniferous Kezilekeya granite formed of partial melting metasomatite vein(wall)in lithospheric mantle,after post collision extensional collopse crustal highly thinning environment.4.Permian Kulumusu quartz diorite have the characters of peraluminous calc-alkaline SiO2=58.08-61.57%,high Al(A1203 = 16.78-17.42%).Ca(CaO=4.11-5.23%).TFe(Fe2O3=6.28-7.70%),rich in Na and poor K(K2O/Na2O=0.29-0.36).The trace elements have the characters of high Sr(396-444×10-6)?Y(31.6-43.5×10-6)?Yb(2.99-4.24×10-6).Ba(650.72-773.69×10-6),rich in Nb(9.04-11.2×10-6)and ratio of Rb/Sr(0.06-0.13),low Cr(7.36-12.25×10-6)?Ni(6.29-10.37×10-6).The trace elements spider diagram shows deficit of Nb?Ta?P?Ti,enrichment of Ba?Rb?K?U?Zr?Hf.The REE distribution patterns figure shows relative enrichment of LREE,flat distribution of HREE and slightly negative Eu anomaly.Combine with regional geology background,Kulumusu quartz diorite formation influenced by subducted fluid metasomatic asthenosphere upwelling and underplating induced partial melting of residual oceanic crust which be beneath the bottom of crust with migmatization.Monoclinic pyroxene and other basic mineral fractional crystallization and felsic materials contamination happened during magma upwelling.Permian Kulumusu western granite have the characters of peraluminous high alkaline potassium calcium,alkali rich,A1(Al2O3=12.82-14.38%).Ca(CaO=0.65-1.25%)?Mg(MgO=0.24-0.34%)?Ti(TiO2 =0.22-0.30%)low,K rich and Na(K2O/Na2O=l.01-1.17)poor.Trace elements have characters of low Ba(409-549x1O-6)?Sr(49.60-75.30×10-6)and high Y(43.40-52.20×10-6).Yb(4.83-5.62×10-6)and ratio of Rb/Sr(1.18-2.15).The trace elements spider diagram shows Ba?Nb?Ta?Sr?P?Ti deficit,Rb?Th?K enrichment.Meteorite standardized REE distribution patterns figure shows Eu loss,LREE relative enrichment.Distribution curve is typical rightist "Seagull" type which is similar to A-type granite.It is considered that Kulumusu western granite formed of crust material partial melting in post collision extensional environment.5.The tectonic evolution of west Junggar can be divided into three stages:Late Silurian ?Early Devonian(425Ma-405Ma)oceanic subduction environment;Early Carboniferous(325Ma-319Ma post collision extensional collapse tectonic setting;Late Carboniferous(318Ma-300 Ma)crust(lithosphere)thinning after post collision collapse tectonic environment.6.West Junggar region in late palaeozoic happened extensive crust-mantle interactions.Into post collision evolution stage,asthenosphere mantle showed heterogeneity maybe because of structural processes of multiple subduction zone.The dynamic mechanic of crust-mantle interaction is tectonic regimes transitions during plate tectonic evolution.