Zircon U–Pb dating, geochemistry and Sr–Nd–Pb–Hf isotopes of the Wajilitag alkali mafic dikes, and associated diorite and syenitic rocks: Implications for magmatic evolution of the Tarim large igneous province

Abstract

Abstract 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 SiO 2 contents (51.9–57.3 wt.%), and are enriched in total alkalis (Na 2 O + K 2 O = 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 Al 2 O 3 (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 Al 2 O 3 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 87 Sr/ 86 Sr ratios (0.7034–0.7046), and high eNd( t ) values (0.1–4.1) and zircon eHf( t ) values (− 0.9–4.4). All the diorites and syenitic rocks show the 206 Pb/ 204 Pb ratios ranging of 18.0–19.5, 207 Pb/ 204 Pb of 15.4–15.6 and 208 Pb/ 204 Pb 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.