Petrogenesis of the Late Triassic diorites in the Hoh Xil area, northern Tibet: Insights into the origin of the high-Mg# andesitic signature of continental crust

Abstract

An integrated petrologic, geochronologic, major and trace element geochemical, and Sr-Nd-Hf isotopic study of Late Triassic (~215Ma) diorites from the Hoh Xil area, northern Tibet, provides new constraints on the genesis of intermediate magmas and insights into the origin of the high-Mg# andesitic signature of continental crust. These dioritic rocks are characterized by high MgO contents (3.3–5.0wt%) and Mg# values (50–57) comparable to the estimates for the bulk continental crust at the same level of SiO2 contents (61.1–64.5wt%). They also display continental crust-like trace element distribution patterns and uniformly enriched isotope compositions ([87Sr/86Sr]i=0.7081 to 0.7094, ɛNd[t]=−8.0 to −6.9, and ɛHf[t]zircon=−10.1 to −5.0). Combining our results with published data from crystallization experiments, we propose that they were probably produced by fractional crystallization from a primitive andesite parent, rather than a primitive basalt parent. This parental magma may be geochemically similar to the roughly contemporaneous primitive andesites in the adjacent Malanshan area of northern Tibet. Our compilation of modern arc lavas shows that progressive fractional crystallization of primitive andesites is also required to reproduce the Mg# versus SiO2 array for natural arc magmas, in addition to differentiation of mantle-derived primitive basaltic magmas and/or mixing of basaltic with felsic magmas. Therefore, we emphasize that fractional crystallization of primitive andesitic magmas is potentially a frequent occurrence in arc crust and hence may play an important role in producing the high-Mg# signature of intermediate magmas comprising the continental crust.