The Mineralogy, Geochemistry And Source Region Of Agpaitic Lamprophyre In Wajilitage, Xinjiang

In recent year, the Tarim LIP has been attentioned by more and more geologistswho try to explain its formation progress by studying the igneous and intrusive rocksin this area. The lamprophyres occurring at Wajilitage area in the Tarim LIP arespecial lamprophyres species, which including volhynite, odinite and camptonite.The lamprophyres are typical porphyritic texture. The compositions of major darkmineral phenocrysts are clinopyroxene, amphiboles and biotite, while thecompositions of matrix are plagioclase and fine-grained dark minerals. The contentsof SiO2are41.5%~44.9%, Na2O+K2O=5.5%~8.5%, which show the rocks arealkalic series. The geochemical characteristics of the major elements are deficit inMgO (3.8%~5.9%) and Mg#(39.4~47.9), which imply the evolution of maga, rich inNa and Ti, K2O/Na2O=0.49, which mean that the lamprophyres are sodium-richseries. In lamprophyre classification, the rocks belong to agpaitic lamprophyres. Thetotal amount of REE is high (414ppm). Relatively, LREE is enriched and HREE isdefective.(∑LREE)/(∑HREE)=4.8,(La/Sm)N=3.0,(Gd/Yb)N=3.97, It is obviousthat the fractionation is existence not only between LREE and HREE, but alsobetween inner of themselves. The analysis of trace elements show that Nb, Ta, Baare enriched while Lu, Y, Yb are depleted. Both of the trace elements and rare earthelement are similar to characteristic of ocean island basalt (OIB). The ratios of thetrace elements are closed to Primitive mantle, which means the magma might not bemixed with the material from the crust. The initial87Sr/86Sr is0.703732~0.705505,the initial143Nd/144Nd isotope ratio is0.512241~0.512577and εNd(t) is-0.5.960~+5.598. Compared with the initial isotope ratio of ocean island basalt(stand for Hawaii igneous rocks) and the continental crust, we can find out thelamprophyres in Wajilitage is extremely closer to the former. Combined with theclinopyroxene-melt equilibrium thermobarometer and Melting Curve, the source oflamprophyres is garnet phase, which stands for a environment of high temperatureand pressure. However, the formation depth of the lamprophyres was too shallow toform the magma. So, there must be thermal anomaly in the area, such as the mantle plume. Thank for the heat and power of the mantle plume, the upwelling ofasthenosphere took place metasomatism with lithospheric mantle which formed thespecial resource of agpaitic lamprophyres. The interaction progress of the Tarim LIPand mantle plume had been almost sustained30Ma, starting at300Ma and ending at270Ma. The lamprophyres are the symbol of the end.