Chronological,Geochemical Characteristics Of The Kazan Complex.Northern Yili Margin And Its Regional Tectonic Significance

The biezhentao mountain is located in the northwest margin of the Yili plate.Island arc magmas of different ages are widely developed in the area.The deep processes and dynamic mechanisms reflected by these magmatic formations of different ages are geological problems of great concern.In this paper,systematic field observation,petrography,geochronology,major and trace elements and Sr Nd Hf stable isotope geochemistry of monzogranite,diabase and gabbro in Kazan complex,which is the largest outcrop in the region,have been carried out.Monzonitic granite is mainly distributed in the south of Bortala River.Controlled by faults,monzonitic granite is distributed in the form of batholith and vein.It intrudes into the strata of Neoproterozoic Wenquan group.The contact boundary between monzonitic granite and strata is not obvious.It has light gray medium coarse texture and massive structure.The weighted average age of zircon is 906±2.3Ma,which is characterized by high silicon(>70%)and rich alkali(>7%).It belongs to high-K calcalkaline peraluminous rocks,rich in Rb,Th,U,K and poor in P,Ti,Ta,Nb,showing the characteristics of active continental margin rocks.The REE distribution curve shows that the monzonitic granite samples show a right dipping pattern of"steep left and gentle right",which is similar to the typical active continental margin rocks The REE patterns of marginal magmatic rocks are similar.The Sr-Nd isotopic data show that the granites are of typical crustal origin,and the zircon in-situ Hf isotopic data show that the granites are of typical crustal origin,and the?_Hf(t)is(1.51-10.20),with an average of 5.43,and the second-order model age(T^C_DM)is1247-1768 Ma,with an average of 1491 Ma.The early Neoproterozoic granitic intrusions in the study area were formed in the tectonic environment of the active continental margin,which was the product of Greenwell orogeny under the convergence of continental blocks and subduction of oceanic crust,and formed a part of Rodina continent under the polymerization.Diabase is mainly distributed in adamellite of Kazan complex.Controlled by structural faults,diabase is distributed in nearly east-west direction,with grayish green,medium fine grain diabase structure and massive structure.Weighted average age is 775±6 Ma;it belongs to low-K calc alkaline paraaluminous rock;it is rich in Rb,Th,U,La,Nd and other elements,and depleted in Ta,Nb,P,Ba and other elements;light rare earth elements are rich and heavy rare earth elements are relatively depleted;combined with regional tectonic evolution,diabase may be formed in the tectonic environment of mid Neoproterozoic and continental disintegration.Gabbros are mainly distributed in adamellite of the Kazan complex.Controlled by structural faults,gabbros are distributed in nearly EW trending dikes with gray black,medium coarse grain gabbro texture and massive structure.Weighted average age is 302.4±1.7 Ma,belonging to calc alkaline paraaluminous rocks;rich in Rb,Th,U,and La,Ce,Nd,depleted in Ta,Nb,P and Ba,rich in LREE and relatively depleted in Hree,with weak negative Eu anomaly;?_Sr(t)is(97.9117?119.2439),?_Nd(t)is(-1.02?2.41),with the characteristics of depleted mantle source;?_Hf(t)is(5.59?19.64)The two-stage model age(T^C_DM)is 357-917 Ma.Combined with the regional tectonic evolution,gabbros may have formed in the Late Carboniferous intraplate extensional environment after the collision between Yili block and Junggar plate.The northern margin of the Yili block recorded three global scale orogenies,including the formation and disintegration of Rodina continent under the Greenwell orogeny,the evolution of the paleo Asian Ocean,and then the formation of Pangaea continent.The Yili block in the study area was about 0.9 Ga in the early Neoproterozoic,which may form a part of Rodina continent together with Tarim plate.It cracked at about 0.7 Ga and entered the stage of ocean basin expansion.It was in the stage of ocean continent subduction at about 0.46 Ga in Ordovician.At about 0.3 Ga in the Late Carboniferous,the Yili Kazakhstan Plate collided with Junggar plate and closed,forming a part of Pangaea continent Points.