Study On Magma Dynamics And Ore Potential Of The Early Permain Tarim Large Igneous Province

The Early Permian Tarim Large Igneous Province (TLIP), located in the southern part of Xinjiang Uygur Autonomous Region of the northwestern China, is another most important Large Igneous Province genetically linked with a mantle plume after the finding and study of the Emeishan Large Igneous Province in China by the geologists. It has an important scientific significance and a good support on the exploration of mineral resources and oil and gas. Systematic studies of Hf isotopes on various rock types of the TLIP and Early Permian magmatic zircons in the Tarim continental flood basalts (CFBs), platinum-group elements (PGE) on the Tarim CFBs in different areas of the TLIP, and geneses on the Wajilitag intrusive complex containing the Fe-Ti-V oxide ore deposit in the Bachu area have been carried out in this study; and the results can provide some good information to better understand the magma dynamics and ore potential of the TLIP.Through the study of geological, petrological and geochemical characteristics, the TLIP can be subdivided into five major igneous rock units (three earlier-formed basaltic units and two later-formed intrusive rock units). There is an evolutional trend of the Sr-Nd-Hf isotopes from enriched mantle components in the earlier basalts (87Sr/86Sri>0.705, εNd(t)<1, εHf(t)<2.5) towards depleted mantle components in the later intrusive rocks (87Sr/86Sri<0.705, εNd(t)>1, εHf(t)>2.5), indicating a remarkable change of the magma source materials. Combined with Hf isotope studies of the Early Permian magmatic zircons in the Tarim CFBs, it is considered that successive interactions between the plume and lithospheric mantle probably played an important role for the generation of the TLIP. A new and improved magmatic evolution model is proposed, which a rising mantle plume was continuously injecting depleted mantle components to the magma source region of the TLIP at the bottom of the sub-continental lithospheric mantle under the Tarim Block, changing its isotopic compositions and finally producing the various igneous rock units in the TLIP. Besides, the Early Permian magmatic zircons in the Tarim CFBs have lower εHf(t) values (-6.8～-1.1) than either their host basalts (-2.3～2.1) or other known igneous rocks (0.5～8.8) of the TLIP. These zircons were probably formed in the concealed pluton shortly prior to the extrusion of basalts, and were captured by the latter as xenocrysts. The Tarim CFBs in different areas of the Tarim Basin are extremely depieted in PGE (∑PGEs<1ppb, Cu/Pd>105), suggesting that their parental magmas were likely to have been S-saturated before the final eruption. Our studies denote that the S-saturation of the basaltic magmas was mainly attributed to the low-degree (ca.5%) partial melting in their mantle source; and the degree of partial melting played an important role on the forming of magmatic sulfide deposit in the TLIP. Although the trace elements geochemical proxies (Th-Nb and Nb-La) indicate that the Tarim CFBs in different areas have suffered variable degrees of crustal assimilation by the Tarim basement rocks, this process did not trigger S-saturation for the basaltic magmas. It is probably because that the Tarim basement rocks do not contain enough sulfur. However, the magma mixing by magma chamber replenishment during the basalt eruptions in the Keping area may induce secondary S-saturation for the basaltic magmas in the crust, causing PGE enrichment in some parts of the magma conduit. Therefore, it is necessary to further strengthen the exploration and research on the potential magmatic sulfide deposit in the TLIP.Through the study of geology, petrology and geochemistry on the Wajilitag Fe-Ti-V oxide ore-bearing layered mafic-ultramafic intrusion and kimberlitic brecciated rocks in the Bachu area. we argue that these rocks were probably derived from a common depleted mantle source at the boundary between the lithospheric and asthenospheric mantle. The Fe-Ti-V oxide ore-bearing layered mafic-ultramafic intrusion was produced by low-degree (<10%) partial melting from the mantle source and formed in a slowly cooling fractional crystallization process in the crust level. The kimberlitic brecciated rocks are highly enriched in rare earth elements (∑REEs>960ppm) and incompatible elements (e.g., Th and U), and contain lots of hornblendes and phlogopites. indicating that their parent magma may be affected by fluid metasomatism of lithospheric mantle. The relatively thin lithospheric thickness in the Bachu area is probably the main reason to explain poor potentiality for diamonds in the kimberlitic brecciated rocks.