Early Silurian bimodal magmatism in the northeastern Junggar block: Implications for the origin and evolution of the Karamaili Ocean

The origin, subduction-accretion, and closure of the Paleo-Asian Ocean are pivotal for our understanding of the growth of the Asian continent. The Karamaili ophiolite belt is a key tectonic unit in the northeastern Junggar block of the southwestern Central Asian Orogenic Belt, but the origin of the precursor Karamaili Ocean, an important branch of the southern Paleo-Asian Ocean, is controversial. In this study, we present new zircon U-Pb, geochronological, geochemical, and Hf isotope data for the early Silurian magmatic rocks on the southern side of the Karamaili ophiolite belt. These rocks display a classic bimodal association of basalt and rhyolite with contemporaneous granites. Zircon U-Pb dating reveals that the mafic and felsic volcanic rocks have ages of 442−435 Ma and the granites have ages of 439−435 Ma, which indicate they were coeval with the early Silurian volcanism. The mafic rocks have arc-like trace element signatures characterized by enrichment in large-ion lithophile elements (LILEs; e.g., Th and U) and light rare earth elements (LREEs) but with some depletion of high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). They have juvenile zircon εHf(t) isotopic values of +10.0 to +13.6 and high (Gd/Yb)N (1.37−1.88) and Sm/Yb (1.53−2.29) ratios, which suggest that they were likely derived from a depleted spinel-garnet lherzolite mantle source with 10%−15% partial melting. In contrast, the felsic rocks (rhyolite and coeval granite) are geochemically equivalent to A-type granite, with high contents of SiO2 (72.21−77.95 wt%) and Ga (15.3−29.1 ppm) and ratio of 10,000 × Ga/Al (2.34) and strong depletion in Eu, S, P, and Ti. Their positive zircon εHf(t) values (+12.0 to +15.3) indicate that they were most likely generated by partial melting of juvenile mafic lower crust due to underplating by mantle-derived mafic magmas. It is concluded that the early Silurian bimodal magmatism was generated in a backarc setting, associated with the southward subduction of the Aermantai Ocean plate, a branch of the Paleo-Asian Ocean lying to the northeast of the Junggar block. We propose that the Karamaili Ocean opened as a result of backarc extension during the early Silurian. This detailed study on the geochronology and geochemistry of bimodal magmatic rocks provides tighter constraints on the opening and closing of the transient oceans in the southern Asian Ocean, which now constitute the southwestern part of the Central Asian Orogenic Belt.