PETROGENESIS AND MAGMA SOURCE OF THE ULTRAMAFIC ROCKS AT WAJILITAG REGION, WESTERN TARIM PLATE IN XINJIANG

Abstract

Six diatreme breccia pipes and 32 dikes, which occurred in late Permian epoch, were found by predecessors in the Wajilitag region located in the western Tarim plate. The dikes belong to ultra-hypabyssal phase of subvolcano phase. The diatreme breccia pipes belong to volcanic crater phase and volcanic conduit phase, and consist of breccias and binding material. The breccias are mainly slithers (dunite, peridotite, olivine pyroxenite, pyroxenite, et al.) and crystal debrises (olivine, clinopyroxenes and phlogolite), which are mainly comprised of early cumulative phases. While all binding material are volcanic lava, main petrological types are meimechite and picrite. The MgO content of primary magma determined by petrochemical compositions and Ni abundances is 18.78%; on the other hand, by Fo molecular of liquidus olivine is 18.8%. Their average CaO/Al2 O3 is 3.9, and average Al2O3/TiO2 is 2.6. All of ultramafic rocks are enriched in LREE, LILE and Nb, Ta, but HREE are depleted, and show negative Zr and Hf abnormality. The differences of ∑ REE and (La/Yb)N between breccias and binding materials reflect cumulation effects of olivines and clinopyroxenes. According to research results of exeperimental petrology, they should be generated from a high degree partial melting in the majozite staility field with depths ranging from 450km to 650km. Their accessory mineral assemblage shows extreme complication, among them, the crystallization depth of corindon should surpass 575km. εNd(t) of these rocks varies from +3.3 to + 5.4, εSr(t) = -4.27 - -10.78, 206Pb/204Pb = 18.47-22.78, 207Pb/204Pb = 15.52 - 15.90, 208Pb/204Pb = 38.45 ∼39.62. Nd, Sr and Pb isotopic compositions are identical to or similar to some ocean island basalts and mantle plume constituent. It is proved by various research results that these ultramafic rocks were possibly derived from D"layer which lay above core-mantle boundary. This source region shows a long time-integrated depletion and therefore, the infarred enrichment event in the mantle sources must have taken place relatively shortly prior to melt segregation.