Magmatic Event In The Late Stage And Its Evolution Of Tarim Large Igneous Province

The Early Permian Tarim Large Igneous Province (Tarim LIP) is regarded as an important large igneous province in China. The Tarim LIP consists mainly of basaltic lavas, mafic-ultramafic intrusions including dikes and syenite bodies in the Tarim Basin, NW China. Previous studies almost focus on magma source and magmatic evolusion of continental flood basalt and mafic-ultramafic intrusions, which ignore the relationship between intermediate-felsic rocks and Tarim LIP. Here I report the zircon U-Pb age and Hf isotopes, geochemical characteristics and Sr-Nd-Pb isotopic data of alkali rock units in the Wajilitag intrusive complex, granitic intrusions in Halajun and Bashisuogong area and relative rocks in Keping area. Based on this study, there is some good evidence to explain the magma dynamics of Tarim LIP.Wajilitag intrusive complex belongs to the product of late stage of Tarim LIP magmatic event. The geochemical evidence reflects a FOZO-like mantle source and there are intruded mafic dikes at different times. Zircons from the diorite and alkali mafic rocks yield concordant crystallization ages of 275.2 ±1.2 Ma and 281.4 ± 1.7 Ma, respectively. The diorite and syenitic rocks in Wajilitag area have a narrow range of SiO2 contents (51.9-57.3 wt.%), and are enriched in total alkalis (Na2O+K2O= 8.3-14.3 wt.%), among which the aegirine-nepheline syenite and syenite porphyry have the geochemical affinity of A-type granites. Alkali mafic dikes have similar characteristics with syenite. Diorites have lower Al2O3 contents, total REE abundances and LREE/HREE ratios and higher Mg# values than the alkali mafic rocks and syenitic rocks. The diorites and syenitic rocks have low initial 87Sr/86Sr ratios (0.7034-0.7046), and high εNd(t) values (0.1-4.1) and zircon εHf(t) values (-0.9-4.4). Sr-Nd isotopic ratios indicate a FOZO-like mantle source for the diorite and syenitic rocks, similar to that of the mafic-ultramafic rocks in the Wajilitag complex, In contrast, zircon Hf isotopes of diorite are different to basalt and syenite elsewhere in the Tarim LIP, which indicated a FOZO-like component may also contribute to Tarim LIP magmatism. Geochemical and Sr-Nd-Pb-Hf isotopic features reflect that diorites and syenitic rocks are probably derived from a FOZO-like mantle source, consistent with a plume mantle origin and then underwent crystal fractionation process.Through the analysis of four granitic intrusions occurred around Tarim Basin Margin, the intrusive age are identical to the late stage of Tarim LIP magmatic events and geochemical analyses reflect the granite record the interaction of mantle component and crust. The zircon U-Pb dating of alkali feldspar granite in Bashiguogong intrution is yield 277±2.1 Ma, which similar to the Halajun granites. The four granites in this study are enriched in K2O, Na2O, total alkalis, high field-strength elements, large ion lithophile elements and Ga/Al×104 and have Eu negative anomaly. The granite outcropped around the northwest margin of Tarim Basin show the A-type affinity, whitch may be caused by OIB-type magma from asthenosphere. The granites have lower high εNd(t) values and old model age of Nd, which indicates the magma mixing with lithosphere mantle. The granites show similar characteristics of Wajilitag and Xiaohaizi intrusion, but under more crust assimilation and fractional crystalization. The granites intruded into the northwest margin of Tarim Basin are parts of the Tarim LIP magmatic products and record the interaction of mantle component and crust.The samples from the sedimentary rocks in Keping area yielded the youngest peak age ranging between 284 and 291Ma, coincided with the geological event of the Early Permian Tarim LIP reported previously. In addition, there are other age clusters which are corresponding to breakup and assembly of Rodinia supercontinent, and assembly of Columbia supercontinen. The εHf(t) value of Permian zircons was-10-4 with the average value of -4.8, and the TDMHf (depleted-mantle model age) of Hf isotopes between 1365 and 802 Ma were yielded, illustrating that detrital zircons may be from the old enrichment lithosphere mantle. The study of zircon U-Pb geochronology and Hf isotopic compositions from these sedimentary rocks provided the better understanding of the Early Permian TLIP magmatic evolution.Based on the study of different rock units in Tarim LIP, it is helpful to understand the change between two period magmatic activities and figure out the interaction between plume mantle and lithospheric mantle source. Tarim plume went through the magmatic vent and emplaced the margin of Tarim block to make a series of mafic-ultramafic intrusive rocks, syenite and granite, which belongs to the products of Tarim LIP. These different types of rock underwent crustal contamination and fractional crystalisation.