Late Paleozoic magmatic record of East Junggar, NW China and its significance: Implication from zircon U–Pb dating and Hf isotope

Abstract

Late Paleozoic magmatic rocks (including basic dykes, basaltic andesite, rhyolite, keratophyre and syenite-porphyry with minor tuff) are widespread in the western margin of the East Junggar terrane. In-situ zircon U–Pb dating and Hf isotope analyses were carried out for these magmatic rocks from the Baijiangou and Zhangpenggou localities of East Junggar, integrating with geochemical data, to investigate their tectonic evolution as well as crustal accretionary process of this region in the Late Paleozoic. Inherited zircons from basic dykes range in age from 435 Ma to 300 Ma. The Zhangpenggou rhyolite and keratophyre have typical arc-like geochemical signatures and were formed in the Early Carboniferous (332 Ma and 336 Ma, respectively), suggesting they are products of subduction-related magmatism. The Baijiangou rhyolites were formed in the Late Carboniferous (315 Ma and 323 Ma) and their formation ages are similar to those of the syenite-porphyries (307 Ma and 312 Ma). The Hf model ages and the formation ages of zircons from these magmatic rocks are alike, with positive ε Hf(t) values vary from + 0.7 to + 16.6, implying that voluminous growth of juvenile crust happened in the East Junggar terrane during the Late Paleozoic. The absence of Precambrian inherited zircons in basic dykes indicates the lack of Precambrian basement beneath the East Junggar terrane. Taking geochronological studies on regional ophiolites into account, the East Junggar terrane is considered as a Devonian–Carboniferous oceanic island arc which has been continuously accreted to the southern active margin of the Siberian Craton since the Early Carboniferous.