Petrogenesis And Deep Dynamic Process Of Basic-ultrabasic Dykes In The Tarim Large Igneous Province

The Permian Tarim large igneous province is different from other typical large igneous provinces because of its complex rock assemblage,long-term magmatism and extensive iron and alkali rich basalt.This paper selects the basic-ultrabasic dike in Bachu area（Wajilitag and Xiaohaizi）on the northwest edge of Tarim Large Igneous Province as the research object,and tries to reveal the characteristics and formation mechanism of its mantle source area through the insitu geochemical composition of olivine and pyroxene phenocryst minerals in the dike,combined with the analysis of major and trace elements and Sr-Nd-Pb homotopy composition of the whole rock,it provides important information for magmatism,mineralization and deep dynamic process in this large igneous province.The results show that the ultrabasic dike in Bachu area have high Mg O（6.61-18.0wt.%）and low Si O₂（43.9-44.8wt.%）,Fe₂O₃(12.9-15.7wt.%,（Na₂O+K₂O）（0.80-3.88wt.%）,while the basic dike have low Mg O（4.59-9.58wt.%）and high Si O2（45.3-48.5 wt.%）,Fe₂O₃(10.8-15.2wt.%,（Na₂O+K₂O）（1.75-6.96wt.%）.They all show a right inclined distribution patterns in the rare earth elements and trace elements normalized diagrams,and have a weak Nb-Ta positive anomaly.The La/Nb（1.1-1.4）,high Nb/U（39.4-43.9）,Ce/Pb（19-25.6）ratio and the initial value of appropriately depleted Sr-Nd-Pb isotope[(⁸⁷Sr/⁸⁶Sr)_i=0.7039-0.7058;ε_Nd（t）=-3.60-1.90;(²⁰⁶Pb/²⁰⁴Pb)_i=17.8018-18.4347],indicating that they may originate from OIB type enriched mantle source and are not obviously affected by crustal contamination in the rising process.Olivine phenocrysts in ultrabasic dike have low100*Mn/Fe（1.11-1.26）,Mn/Zn（11.4-14.2）and high Ni/（Mg/Fe）/1000（0.94-1.20）,suggesting that their source is heterogeneous mantle peridotite containing small amounts of pyroxenite.At the same time,these olivines have high Ca and Ti characteristics,indicating that the source may be metasomatized by sedimentary carbonate.The trace element inversion results of clinopyroxene phenocrysts in basic-ultrabasic dike in Bachu area show that their equilibrium melts have similar distribution patterns,which are similar to Emeishan high Ti picrite,suggesting that basic and ultrabasic dike may be the products of different evolution stages of the same parental magma.Combined with the low（La/Yb）_N（0.98-5.80）and high Ti/Eu（5180-8501）ratio in clinopyroxene phenocrysts,it shows that the basic-ultrabasic dike in Bachu area are formed in the heterogeneous peridotite source area metasomatized by silicate melt and carbonate melt.In this study,we therefore propose that the abundant rock assemblages in Tarim Large Igneous Province may be the result of the interaction of mantle plume-lithosphere mantle-subducted slab.