Identification of the Earliest (Late Devonian) Back-Arc Magmatism in the Chinese Eastern Tianshan

Abstract

Identifcation of subduction to back-arc tectonic transitions is the key to understanding the evolution of accretionary orogens. In order to characterize such a transition in the Chinese Eastern Tianshan orogen, we report new geochronological and geochemical data for the Late Devonian (380–361 Ma) Kezier mafic–felsic complex and Tuwu granitic pluton of Eastern Tianshan. The gabbros from the Kezier mafic–felsic complex have relatively high A12O3 (>17 wt.%) contents, akin to high-Al basalts. Both the Kezier and Tuwu granites have diagnostic geochemical characteristics (high total alkali, Ga/Al, HFSE, low CaO, and strongly negative Ba, Sr, and Eu anomalies), demonstrating a close affinity with A-type granites. Formation of the high-Al gabbros attributed to hydrous pressure-release partial melting at a relatively shallow depth. Generation of the A-type granites required high-temperature melting (up to 916 °C) of crustal rocks under a low-pressure (<0.8–1.0 GPa) condition. These rocks not only identify the earliest back-arc extensional magmatism in the Eastern Tianshan, but also indicate an extensional event contemporaneous with the onset of the earliest arc magmatic records southward rejuvenating, and development of the crustal high-temperature melting event northward migration. The back-arc extensional and crustal high-temperature melting events can be reconciled with the Kangguer oceanic lithosphere rollback forced the initial arc rifting occur on the Dananhu fore-arc side since the Late Devonian, and then the back-arc spreading center move to the Bogda area.

Keywords: Accretionary orogen, Eastern Tianshan, Back-arc, High-Al gabbro, A-type granite