矿床地球化学国家重点实验室知识库

The Early Permian Tarim large igneous province (Tarim LIP) consists mainly of basaltic lavas, mafic–ultramafic intrusions including dikes and, syenite bodies in the Tarim Basin, NW China. A major unit of the Tarim LIP, the Wajilitag intrusive complex, consists of olivine pyroxenite, clinopyroxenite and gabbro units (from bottom to top), diorite and syenite rocks occurred in the upper part of the complex and alkali mafic dikes intrude the clinopyroxenite phase. Here we report the zircon U–Pb age and Hf isotopes, geochemical characteristics and Sr–Nd–Pb isotopic data of the alkali mafic dikes, and diorite, aegirine–nepheline syenite and syenite porphyry units in the Wajilitag intrusive complex. 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. The alkali mafic rocks and syenitic rocks have high Al₂O₃ (19.4–21.1 wt.%), Zr, Hf, Ba contents, total rare earth element abundances and LREE/HREE ratios and low Mg# value, K, P and Ti contents. 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). All the diorites and syenitic rocks show the ²⁰⁶Pb/²⁰⁴Pb ratios ranging of 18.0–19.5, ²⁰⁷Pb/²⁰⁴Pb of 15.4–15.6 and ²⁰⁸Pb/²⁰⁴Pb of 38.0–39.9. 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 basalt and syenite elsewhere in the Tarim LIP indicate 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.