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

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 degrees 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.

Automated summary

The study of the Kezier mafic-felsic complex and Tuwu granitic pluton in the Chinese Eastern Tianshan orogen provides new geochronological and geochemical data for the Late Devonian period. These data reveal a transition from subduction to back-arc tectonic features. The Kezier gabbro is classified as high-Al basalt and originated from a depleted mantle under hydrous conditions. The Kezier and Tuwu granites show geochemical characteristics of A-type granites, likely generated by high temperature melting of crustal rocks.