Petrogenesis and tectonic implication of the Late Triassic A1-type alkaline volcanics from the Xiangride area, eastern segment of the East Kunlun Orogen (China)

Abstract

A series of alkaline volcanic rocks with pronounced Nb contents, were identified in the Xiangride area of the eastern segment of the East Kunlun Orogen (EKO). An integrated investigation, involving mineral composition, whole-rock elemental and Sr − Nd isotopic geochemistry and zircon UPb dating and LuHf isotopes was conducted on the alkaline volcanics as well as a spatially associated andesite. The Xiangride alkaline volcanics are mainly composed of trachyandesite, quartz trachyte, rhyolitic ignimbrite, and rhyolite. Zircon UPb dating reveals that the Xiangride alkaline volcanics were formed at 212–209 Ma, apparently postdating the associated andesite (231 Ma) and the volcanic rocks of the Elashan Formation (232–218 Ma) as well as the widespread I-type granitoids in the eastern EKO (265–217 Ma). The Xiangride alkaline volcanics display variably high total alkali, (Zr + Nb + Ce + Y) content, Ga/Al ratio, and zircon saturation temperature, typical of A1-type granite. They are also characterized by high HFSEs and LREEs contents, along with strong depletion of Ba, Sr, P, Ti and Eu, similar to the OIB-type alkaline trachytic to rhyolitic rocks and obviously different from the coeval arc-related magmatic rocks in the eastern EKO. In contrast, the associated Xiangride andesite shows similar chemical signatures to the Elashan andesites, resembling the IAB-series rocks. Notably, the Xiangride alkaline volcanics all contain high Nb contents (up to 120 ppm) as compared to other Triassic granitoids and volcanic equivalents of the EKO (Nb = 5–60 ppm). The dominate Nb-hosts include primary titanite and rutile and secondary ilmenorutile and euxenite, indicating both contributions of magmatic and hydrothermal processes. The Xiangride alkaline volcanics yield slightly higher ɛNd(t) values (−3.7 to −2.4) and zircon ɛHf(t) values (−0.6 to 2.6) than the associated Xiangride andesites (−4.2 to −4.1 and − 4.7 to −3.5, respectively) and the widespread Elashan andesites (−6.4 to −3.4 and − 7.4 to −2.0, respectively).Based on our results, we suggest that the Xiangride A1-type alkaline volcanics were likely generated via partial melting of a metasomatized lithospheric mantle followed by prolonged fractional crystallization and minor crustal contamination, whereas the associated Xiangride andesite might have originated from partial melting of the juvenile mafic lower crust, similar to the Elashan andesites. We propose that the Xiangride A1-type alkaline volcanics may represent a within-plate anorogenic extensional magmatic pulse, whereas the associated andesite and the Elashan andesites recorded an earlier period of post-collisional volcanism. Therefore, the Triassic continental collision orogeny of the EKO is likely to have ended before ca. 212 Ma, followed by a transition from post-collisional to intracontinental extension.