Triassic crust–mantle interaction in the Eastern Tianshan, southern Altaids: Insights from microgranular enclaves and their host Tianhu granitoids

Abstract

Triassic magmatism has been increasingly identified in the Eastern Tianshan and played a crucial role in the tectonic evolution of the Tianshan Orogenic Belt. The petrogenesis and geodynamic setting of the Triassic magmatism, however, are poorly constrained. In this study, we integrate new field and petrological observations, zircon UPb ages and Hf isotopic compositions, and whole-rock geochemical data for the microgranular enclaves (MEs) and their host Triassic Tianhu granitoids to investigate their origin and the tectonic regime of the Eastern Tianshan during the Triassic. Based on zircon UPb geochronology, the Tianhu granitoids and MEs have indistinguishable Early Triassic crystallization ages of 247.1 ± 1.8 Ma, 249.5 ± 2.0 Ma, and 251.0 ± 3.2 Ma. The MEs in the Tianhu granitoids are typically sub-rounded to ellipsoidal in shape. They contain abundant acicular apatite, plagioclase that has been partially resorbed, and megacrysts of feldspar and quartz. They are characterized by high Mg# (50–52), positive εHf(t) values of 3.41–5.83, and young one-stage Hf model ages, all of which are indicative of crystallization from mantle-derived melts. The host Tianhu granitoids, which belong to I-type granites, contain higher SiO2 contents (68.3–69.8 wt%) and lower Mg# (38.3–45.4) than the MEs, are metaluminous (A/CNK = 0.97–0.98), and are characterized by high εHf(t) values of 2.26–5.46 and young two-stage Hf model ages. These geochemical characteristics indicate that the Tianhu granitoids and the MEs likely formed through mixing between mafic and felsic magmas. The mafic magma was originally derived from partial melting of a depleted mantle source, and subsequently evolved via fractional crystallization and modification by the felsic magma. In contrast, the felsic magma was generated by partial melting of the juvenile continental lower crust. The Tianhu granitoids would likely form in an intracontinental extensional environment, in which upwelling of the asthenospheric mantle led to partial melting of the lithospheric mantle to form the mafic magmas. And the felsic magmas were produced by partial melting of the deep juvenile crust with a heat source either from the ascending mafic magmas or the upwelling asthenospheric mantle along the thinned orogenic lithosphere. In either case, there would be significant Triassic crust–mantle interaction in the Eastern Tianshan and its adjacent Beishan area. This interaction was responsible for the formation of widespread Triassic granites and related ore deposits in this intracontinental region.