Gabbroic pluton in the Dahalajunshan Formation volcanic rocks from northern Zhaosu, western Tianshan: Age, geochemistry and geological implications

The gabbroic rocks intruded in the upper part of the Dahalajunshan volcanic formation from northern Zhaosu, western Tianshan, are characterized by enrichment in large-ion-lithophile elements ( LILE) such as Rb, Sr and Ba, strongly depletion in high-field-strength elements (HFSE) such as Nb and Ta, and fractionation of rare earth elements (REE). The gabbroic rocks are analogues to the surrounding volcanic rocks in terms of incompatible elements and Sr-Nd isotopes. Their parental magmas were probably produced by partial melting of enriched lithospheric mantle that had been metasomatized by subduction fluids due to the northward subduction of the South Tianshan oceanic crust beneath the Yili-Central Tianshan block before the early stage of the Late Carboniferous. Geochemical modeling using incompatible race elements indicates that the gabbroic rocks are composed of 50% similar to 80% cumulated crystals of clinopyroxene and plagioclase and 50% similar to 20% basaltic magma. Cameca zircon U-Pb dating yielded a weighted mean Pb-206/U-238 age of 311. 3 +/- 2. 3Ma, representing the formation age of the gabbro pluton. The age is similar to that of the volcanic rocks from the Yishenjilike Formation of the Yili-Central Tianshan, and is slightly younger than the peak metamorphic age of the western Tianshan eclogites. The dating result of the gabbroic rocks indicates that the volcanic activity of the Dahalajunshan Formation ended by the end of the Early Carbonaceous, and the Akeshake Formation sediments were formed between 320 similar to 311Ma. Magmatic rocks coeval with the gabbroic rocks are widespread within the Yili-Central Tianshan block, and were probably formed in a tectonic setting of transition from compression to extension following the termination of the northward subduction.