Evolving Mantle Sources in Postcollisional Early Permian‐Triassic Magmatic Rocks in the Heart of Tianshan Orogen (Western China)

Abstract

Magmatism postdating the initiation of continental collision provides insight into the late stage evolution of orogenic belts including the composition of the contemporaneous underlying subcontinental mantle. The Awulale Mountains, in the heart of the Tianshan Orogen, display three types of postcollisional mafic magmatic rocks. (1) A medium to high K calc‐alkaline mafic volcanic suite (∼280 Ma), which display low La/Yb ratios (2.2–11.8) and a wide range of ɛNd(t) values from +1.9 to +7.4. This suite of rocks was derived from melting of depleted metasomatized asthenospheric mantle followed by upper crustal contamination. (2) Mafic shoshonitic basalts (∼272 Ma), characterized by high La/Yb ratios (14.4–20.5) and more enriched isotope compositions (ɛNd(t) = +0.2 – +0.8). These rocks are considered to have been generated by melting of lithospheric mantle enriched by melts from the Tarim continental crust that was subducted beneath the Tianshan during final collisional suturing. (3) Mafic dikes (∼240 Ma), with geochemical and isotope compositions similiar to the ∼280 Ma basaltic rocks. This succession of postcollision mafic rock types suggests there were two stages of magma generation involving the sampling of different mantle sources. The first stage, which occurred in the early Permian, involved a shift from depleted asthenospheric sources to enriched lithospheric mantle. It was most likely triggered by the subduction of Tarim continental crust and thickening of the Tianshan lithospheric mantle. During the second stage, in the middle Triassic, there was a reversion to more asthenospheric sources, related to postcollision lithospheric thinning.

Tectonic Switch From Triassic Contraction to Jurassic-Cretaceous Extension in the Western Tarim Basin, Northwest China: New Insights Into the Evolution of the Paleo-Tethyan Orogenic Belt

“…Some of Latest Triassic post-collisional magmatism and bimodal magmatism have been reported in West Kunlun and South Tienshan Mts. in recent years (Liao et al, 2012;Jiang et al, 2013;Tang et al, 2017). It indicates the post-orogenic delamination could have occurred.…”

Orogenic Root Delamination Induced by Eclogitization of Thickened Lower Crust in the Chinese Western Tianshan: Constraints From Adakites

“…Therefore, the geodynamic evolution of the Chinese Western Tianshan from subduction to collision occurred during the late Carboniferous, and the subsequent magmatic activity in the region was postcollision related. Evidence for postcollisional crustal delamination in the Chinese Western Tianshan includes the following points: (1) Geophysical data from the Chinese Western Tianshan and Tarim and Junggar blocks indicate that they have similar crustal thicknesses (Lei et al, ; J M. Zhao et al, ), which implies that the thickened crust formed by the collisional orogeny was removed; (2) significant changes in mantle chemistry (from enriched to depleted) occurred during the Permian (Tang, Cawood et al, ), which is induced by intraplate crustal delamination (e.g., Gutierrez‐Alonso et al, ).…”

Ordovician to Early Permian accretionary tectonics of Eastern Tianshan: Insights from Kawabulak ophiolitic mélange, granitoid, and granitic gneiss

“…Different models have been applied to the Palaeozoic geological evolution of the Eastern Tianshan (Allen, Windley, & Chi, ; Ge et al, ; Shi et al, ; Windley, Allen, Zhang, Zhao, & Wang, ; Xiao et al, ), and numerous studies have been conducted on the Palaeozoic magmatic rocks recently (Cao et al, ; Du et al, ; Tang, Cawood, Wyman, Wang, & Zhao, ; Zhang et al, ). However, different interpretations including subduction‐related (Ao, Xiao, Han, Mao, & Zhang, ; Mao et al, ; Mao et al, ; Zhao et al, ) and post‐orogenic settings (Cao et al, ; Tang et al, ; Zhang et al, ) were proposed for the magmatic rocks. In addition, some critical issues, such as the exact position and formation age of the final suture zone of the Paleo‐Asian Ocean in the Eastern Tianshan, are still unclear or in controversy (Tang et al, ; Xiao et al, ; Xiao et al, ).…”

“…However, different interpretations including subduction‐related (Ao, Xiao, Han, Mao, & Zhang, ; Mao et al, ; Mao et al, ; Zhao et al, ) and post‐orogenic settings (Cao et al, ; Tang et al, ; Zhang et al, ) were proposed for the magmatic rocks. In addition, some critical issues, such as the exact position and formation age of the final suture zone of the Paleo‐Asian Ocean in the Eastern Tianshan, are still unclear or in controversy (Tang et al, ; Xiao et al, ; Xiao et al, ). Thus, in this paper, we have performed detailed studies of the Kawabulak ophiolite mélange, a key part of the southern suture of the CAOB (Figure ).…”