The Study On Magmatic Sequence And Chronology For The Pobei Basic And Ultrabasic Complex In The Northeastern Margin Of The Tarim Plate

Pobei basilica-ultrabasic complex is located in the Beishan rift zone the northeastern margin of the Tarim plate.Which is controlled by the NE trending Baidiwa and Yunihe deep fault.The exposed area is about165Km~2,and the exposed part of the complex is composed by two north-east lopolith.The long axis of the complex is same as the direction of the fracture zone in the region.The rock types mainly conclude gabbro,gabbro-norite,olivine-norite,olivine gabbro,pyroxenite,Iherzolite,olivine pyroxenite,and pure olivite.At the same time,numerous diabase veins and granitic veins are distributed in the complex.Based on the 1:5million thematic geological mapping of the Pobei basilica-ultrabasic complex,this paper studies the magma sequence and the chronology of the Pobei complex,the genesis of the intrusion and the rock veins of complex.We give the result as follows:1.Through the outwork of 1:50000 geological mapping and measured profiles,this paper divides the basilica-ultrabasic complex into one magma sequence,two magma subsequences and five magma units.The first subsequence includes the first unit gabbro facies,second unit gabbro-norite facies,third unit olivic gabbro facies,and second subsequence of fourth unit olivic facies,mainly composed by ultrabasic rocks,and the fifth unit is small grained gabbro.Second magmatic subsequence Cu-Ni sulfide deposit,mainly hosted in peridotite.2.The chronological data show that the formation age of each lithofacies in the Pobei complex is close,the age of the olidic facies is 280+2Ma,the age of the olivine gabbro is 275.5+1.5Ma,the age of the gabbro rock facies is 277.1+2.2Ma and 280.2+1.7Ma,about 280Ma,and the age of gabbro-norite is 273.5+2.9Ma.The formation age of each lithofacies in the complex is concentrated at 270-280Ma,with an average age of 275.1+4.2Ma,which has a certain coupling relationship with the time and space of the Tarim mantle plume events,which the age is~275Ma.At the same time,the study of rock geochemistry and isotopes shows that the rock facies of Pobei complex have similar characteristics,and should have the same source material composition.3.The formation age of diabase dykes is 271.2+2.9Ma,and its chronology is similar to the complex,and the upwelling of the mantle plume provides the heat source,which leads to the melting of the asthenosphere mantle to form a large amount of magma,and then the magma has been contaminated by the crust,forming the diabase dykes in the Pobei complex.The formation age of the granitic vein is about280Ma,which is similar to the chronology of each rock phase and diabase vein in the basilica-ultrabasic complex.And the granitic veins should be the product of partial melting from the ancient crust after the mantle derived magma undergression.