Geologic Characteristics And Petrogenesis Of The Lamprophyres In The Tarim Large Igneous Province

This paper reports the geochemical characteristics of major,trace elements and Sr-Nd,Mg and Zn isotopes in lamprophyre from Wajilitag complexes in the northwestern margin of the Tarim Large Igneous Province.As an important part of the Wajilitag complexes,the determination of the source area of lamprophyre is of great significance for revealing the deep geological process of the Tarim Large Igneous Province.The lamprophyre has porphyritic texture and massive structure.Phenocrysts are composed of olivine,pyroxene,hornblende and biotite.The matrix consists of pyroxene,hornblende,mica and feldspar.Apatite and titanium-magnetite are the main secondary minerals in lamprophyre.According to the petrographic characteristics,we divided lamprophyre in Wajilitag into three types:onidite,kersantite and spessartite.All of them show evolved characteristic(Mg^#36-56)and enrichment in contents of incompatible trace elements,LILE and HFSE,along with depleted Sr-Nd isotopic compositions((⁸⁷Sr/⁸⁶Sr)t=0.70355-0.70611,?Nd(t)=-0.04?+4.43).Geochemical signatures display a strong correlation with OIB type-enriched mantle source.The Mg isotope values exhibit a limited range of?²⁶Mg from-0.36±0.06‰to-0.17±0.03‰(average=-0.27±0.04‰),which is similar to the mantle values(-0.25‰).The Zn isotope values also show a limited range of?⁶⁶Zn from 0.35±0.01‰to 0.46±0.02‰(average=0.39±0.02‰),which is higher than mantle values(0.28‰±0.05‰).The source region is subjected to the metasomatism and enrichment of fluid which is mainly rich in and a small amount of calcite.The Mg and Zn isotopic compositions,combined with the Sr-Nd isotopes and major and trace element contents reveal that the source area of Wajilitag lamprophyres is much shallower than other rocks'in the same area(basalt,pyroxenite,nephelinite,carbonate and kimberlite rock).The lamprophyre may indicate a very small degree(2-5%)of melting of a mixed mantle source zone between amphibole-bearing eclogite and metasomatised iherzolite.The inhomogeneity of source zone is the result of the interaction among the subducted plate,mantle plume and the lithosphere mantle.