The Comparison Of Petrogenesis Of Ultramafic Intrusions In The Western Pobei Complex,nw China

The Pobei Permian layered mafic-ultramafic complex in Beishan,Xinjiang,NW China is composed of a large gabbro intrusion and more than 20 ultramafic intrusions,The noble gases and carbon isotopic compositions of the Poyi?#1?ultramafic intrusion revealed information of mantle plume.The types and mixing mechanism of contaminated crust materials,petrogenesis and contribution of the mantle plume need to be constrained by Li-C-He-Ne-Ar isotope systems.In present study the abundance and isotopic compositions of noble gases,carbon and lithium were analyzed for magmatic minerals from Poyi?#1?,Posi?#4?,Poshi?#10?and Podong?#14?ultramafic intrusions in the western end of Pobei complex.The source of fluid volatiles during the magmatic action of the intrusion have been discussed,the type and mixing mechanism of contaminated crust source materials were determined,the dynamic background,the characteristics of the magma source area,petrogenesis of ultramafic intrusions and the contribution of the mantle plume have been revealed.The following main understandings were obtained:1.The carbon isotope composition of the ultramafic intrusion magmatic minerals in the western end of Pobei complex are determined:?¹³C_CO2O2 values ranged from-25.6 to 0.6‰,the?¹³C_CO2O2 values of Poyi ranged from-21.8 to 0.3‰,Poshi ranged from-22.9 to 0.6‰,Posi ranged from-25.6 to-0.4‰,Podong ranged from-21.9 to-1.8‰;?¹³C_CH4H4 ranged from-15.6 to-56.3‰,the?¹³C_CH4H4 values of Poyi ranged from-41.8 to-19.4‰,Poshi ranged from-47.6 to-34.7‰,Posi ranged from-56.3 to-21.2‰,Podong ranged from-46.1 to-27.5‰,located within the range among crust,mantle and organic matter thermal genesis,the carbon isotope composition of CH₄homologues in most samples has the characteristics of positive sequence distribution and some samples show reverse order characteristics.2.The noble gas isotope composition characteristics of the ultramafic intrusion magmatic minerals in the western end of Pobei complex are determined:³He/⁴He ratios range from-25.6 to 0.6‰,the ³He/⁴He ratios of Poyi intrusion range from 0.38to 2.79Ra,Poshi range from 0.39 to 0.63Ra,Posi range from 0.26 to 0.55Ra,Podong range from 0.38 to 0.75Ra,the ³He/⁴He ratios distributed between of crust and mantle,²⁰Ne/²²Ne ratios range from 8.1 to 11.7,av.10.3,²¹Ne/²²Ne ratios range from 0.0250 to0.0342,av.0.0287,distributed between MFL and L-K lines,⁴⁰Ar/³⁶Ar ratios range from 295 to 598,slightly higher than atmospheric ⁴⁰Ar/³⁶Ar ratio.3.The lithium content and lithium isotopic composition of the ultramafic intrusion magmatic minerals in the western end of Pobei complex are determined:Li content range from 2.07 to 8.48×10^-6,av.4.25×10^-6.Li content of Poyi intrusion range from 2.07 to 8.48×10^-6,av.4.26×10^-6,Poshi range from 2.79 to 5.64×10^-6,Podong range from 3.43 to 4.82×10^-6.?⁷Li ratios range from-3.43‰to+2.94‰,av.1.00‰,lower than lithospheric mantle and MORB.The?⁷Li ratios decreases sequentially from Poshi and Poyi to Podong intrusion.The?⁷Li ratios in olivine range from 2.63‰to 2.64‰,av.2.785‰are lower than pyroxene which range from-3.43‰to 2.43‰,av.0.53‰.4.The C-He-Ar isotope composition of ultramafic intrusives indicates that the fluid volatiles have contributions from different end-members of the mantle?mantle plume?,altered oceanic crust and atmospheric composition.The carbon isotope composition of the CO₂ and CH₄ homologues indicates that the fluid volatility is divided into mantle sources and mixed with crust sources and thermal cracking components of sedimentary organic matter;through He-Ar isotope,the proportions are estimated of the SCLM of Poyi,Poshi,Podong and Posi are estimated to be 22.0%,16.8%,9.7%and 6.6%respectively,the proportions of the AOC are estimated to be10.9%,9.0%,7.5%and 8.3%,respectively,the proportions of the ASF are estimated to be 67.1%,74.2%,82.8%and 85.1%,respectively.5.The magmatic source area of Pobei ultramafic intrusion was superimposed by deep mantle plume on lithospheric mantle that was reformed by subducted fluids and subduction of plate melt,Lithium isotope indicates that the subducted recirculating oceanic crust enters the magmatic mantle source area.The C-He-Ne-Ar isotope reveals that the magmatic source area of the Poyi intrusion is deeper,while the magmatic source area of the Poshi,Posi and Podong intrusions are the lithospheric mantle metasomatic by subduction fluid;The positive correlation of³He and³⁶Ar contents of magmatic minerals indicates that the atmospheric components are mixed in the source area,during the process of mineral crystallization,the surrounding rock material with heavier?¹³C value were added to the magma system.6.Evidence of carbon isotopes and noble gas isotopes indicates that the petrogenesis of the ultramafic intrusions in the western end of Pobei complex may be different and the contributions from the mantle plume are different.The Poyi intrusion is caused by the magmatism of the mantle plume,and the other three intrusions?Poshi,Posi and Podong intrusions?are mainly formed by the lithospheric mantle magmatism which is subducted by the subduction fluid,and the contribution of the mantle plume is not significant.