Geochronology and Petrogenesis of Late Paleozoic Volcanic Rocks from the Dashizhai Formation in Songnen Terrane, China: Constraints on the Tectonic Evolution of the Central Asian Orogenic Belt

Abstract:

The Central Asian Orogenic Belt is located between the Siberia, Tarim, and North China cratons. It considered to have been the site of the largest Phanerozoic continental growth on Earth, and to have formed by the closure of the Paleo-Asian Ocean and the amalgamation of various terranes. However, it remains debated whether the southward subduction of the Paleo-Asian oceanic plate beneath the Songnen terrane happened during late Paleozoic time. In this paper, we undertook zircon U-Pb dating and geochemical analyses of late Paleozoic volcanics from the Dashizhai Formation exposed along the subduction zone to the north, with the aim of resolving the above question.

Zircons from six samples are euhedral–subhedral, and display oscillatory growth zoning in CL images, indicating a magmatic origin. The analytical results demonstrate an early Permian magmatism in the Songnen terrane, with ages of 283 to 295 Ma. The Permian volcanic rocks are composed of rhyolite tuff, rhyolite, dacite, andesite, basaltic andesite, basaltic trachyandesite, and basalt. Geochemically, they belong to the mid-K calc-alkaline series, and are characterized by enrichment in light rare earth elements and large ion lithophile elements, and depletion in high field strength elements (such as Ni, and Ta). Moreover, the in situ Hf isotopic data of zircons from the rhyolites show positive εHf(t) values and two-stage model ages of 1396–551 Ma. Taken together, the acidic volcanic rocks were most likely derived from the partial melting of accreted Meso–Neoproterozoic lower crust. In contrast, the basic to intermediate volcanics were derived from the partial melting of a depleted lithospheric mantle that had been metasomatized by fluids derived from a subducted slab. Therefore, we conclude that the Permian volcanic rocks in the Songnen terrane could be generated under an active continental margin related to the southward subduction of the Paleo-Asian oceanic plate beneath the Songnen terrane.