Evolution of Late Paleozoic Magmatic Arc in the Yili Block, NW China: Implications for Oroclinal Bending in the Western Central Asian Orogenic Belt

Abstract

Subduction transformation (advancing vs. retreating) may be manifested by compositional variations of arc magmas and may result in oroclinal bending. Identifying relevant chemical and physical processes is crucial for understanding accretionary orogenesis and continental crustal evolution. The Northern Yili Block (NYB) was situated on an active margin associated with subduction of the Junggar Ocean (part of the Paleo-Asian Ocean) and underwent perplexing accretionary orogenesis in late Paleozoic. Two episodes of subduction-related granitoid magmatism have been identified, the first in the Late Devonian (374-369 Ma) and the second in the Late Carboniferous (ca. 304 Ma). These two episodes of granitoid magmatism exhibit contrasting features; for example, the first episode shows low ε Nd (t) (−6 to −2) and ε Hf (t) (−12 to +3) values, while the second episode displays relatively higher values (−2 to +7 and 0 to +20, respectively), suggesting increase contribution of juvenile components in the magma sources. The calculated zircon saturation temperatures are mostly <800°C for the first episode but increased to >800°C for the second episode. The (La/Yb) N ratios of the two-episode granitoids vary from 2-17 to 1-9, indicative of the NYB crustal thinning in the Late Carboniferous. In addition, the migrations of the magmatic arc and trench in the NYB took place coevally in the Late Carboniferous and were accompanied by development of an accretionary complex and an immature back-arc basin. We interpret that the arc magmatic variations and migrations reflect geodynamic transition from the Late Devonian advancing subduction to the Late Carboniferous retreating subduction. The retreating subduction of the Junggar oceanic plate possibly resulted in prominent oroclinal bending of the southern Kazakhstan collage system in the Late Carboniferous.

Paleozoic Episodic Magmatism in Western Tianshan: Insight Into Assembling the Northeastern Pangea

“…An outward younging trend for late Paleozoic igneous units was identified (X. Wang et al, 2020), where pre-350 Ma rocks were predominately distributed in the interior of the Yili block, while post-350 Ma rocks were distributed in the northern Yili block and the North Tianshan belt. This phenomenon reveals a trench-ward migration of arc magmatism at ca.…”

“…In contrast to weak magmatic activities in the Neoproterozoic and early Paleozoic, late Paleozoic igneous rocks are widespread in the Yili block (Z. Huang et al., 2013; Wang, Liu, et al., 2014; Wang, Shu, et al., 2014). For example, two magmatic peaks occurring in the late Devonian and the late Carboniferous were identified based on regional pluton data (X. Wang et al., 2020).…”

Diapir Melting of Subducted Mélange Generating Alkaline Arc Magmatism and Its Implications for Material Recycling at Subduction Zone Settings

“… (a) Simplified geological map of Western Tianshan (after Wang et al., 2020), showing that the study areas of the HP–UHP metamorphic rocks and alkaline arc rocks are spatially juxtaposed. Inset denotes the location of Tianshan.…”

“…The Western Tianshan is a portion of the Central Asian Orogenic Belt that was formed by subduction of the Paleozoic Asian Ocean, including the South Tianshan, Yili–Central Tianshan, and North Tianshan subunits (Figure 1a) (Wan et al., 2021; Wang et al., 2020). The South Tianshan exposes an HP–UHP metamorphic belt named the Akesayi Metamorphic Complex (AMC) (Figure S1a in Supporting Information ).…”

Early Permian Syn-Subduction Extension in the South Tianshan (NW China): Insights From A-Type Granitoids in the Southern Altaids

“…The Yili-Central Tianshan arc, separated from the South Tianshan by the Atbashi-Inylchek-South Nalati Fault (Figure 1B), is a Paleozoic magmatic arc located along the Southern Yili block, the Central Tianshan (in China) and the Middle Tianshan (in Kyrgyzstan) (Figure 1B) (Abuduxun et al, 2021a;Han et al, 2011;Xiao et al, 2013); these units record significant accretion-collision events related to closure of the South Tianshan ocean (Alekseev et al, 2009;Gao et al, 2009). This Paleozoic arc, which is mainly underlain by Mesoproterozoic (∼1.4 Ga) to Neoproterozoic (969-708 Ma) basement rocks (e.g., He et al, 2015;Huang et al, 2017), is mostly composed of arc-related calk-alkaline plutonic and volcanic rocks with ages of 490-308 Ma (e.g., Alekseev et al, 2009;Gao et al, 2009;Su et al, 2021;Wang et al, 2020). In addition, throughout the Yili-Central Tianshan there are widely exposed granitoids that intruded into pre-Permian igneous and sedimentary rocks (Figure 1B; Supplementary Table S1).…”