Identification of the Late Devonian back-arc magmatism in the Chinese Eastern Tianshan

Abstract

Identifcation of subduction to back-arc tectonic transitions is critical to the study of accretionary orogens. Some new geochronological and geochemical data for the Late Devonian (380–361 Ma) Kezier mafic–felsic complex and Tuwu granitic pluton in this study have well recorded such transition feature in the Chinese Eastern Tianshan orogen. The Kezier gabbro is classified as high-Al basalt given their high Al contents (A12O3 > 17 wt%). They were originated from a depleted mantle fluxed by fertile mantle components at shallow depths under hydrous condition. Both the Kezier and Tuwu granites show geochemical characteristics of high total alkali, Ga/Al, HFSE, low CaO, and strongly negative Ba, Sr, and Eu anomalies, demonstrating a close affinity with A-type granites. The granites were probably generated by high temperature melting (zircon saturation temperature up to 916 °C) of crustal rocks under a low-pressure (<0.8–1.0 GPa) condition. These rocks can help identify the Late Devonian back-arc extensional event in the Eastern Tianshan. This extensional event contemporaneous with the onset of the earliest arc magmatic records southward rejuvenating. The back-arc extensional with crustal high-temperature melting event can be reconciled with the Kangguer Ocean rollback, which forced the arc rifting occur on the Dananhu fore-arc side during the Late Devonian.