Accreted seamounts in North Tianshan, NW China: Implications for the evolution of the Central Asian Orogenic Belt

Abstract

A B S T R A C T The Carboniferous Bayingou ophiolitic mélange is exposed in the North Tianshan accretionary complex in the southwestern part of the Central Asian Orogenic Belt (CAOB). The mélange is mainly composed of serpentinised ultramafic rocks (including harzburgite, lherzolite, pyroxenite, dunite and peridotite), pillowed and massive basalts, layered gabbros, radiolarian cherts, pelagic limestones, breccias and tuffs, and displays blockin-matrix structures. The blocks of ultramafic rocks, gabbros, basalts, cherts, and limestones are set in a matrix of serpentinised ultramafic rocks, massive basalts and tuffs. The basaltic rocks in the mélange show significant geochemical heterogeneity, and two compositional groups, one ocean island basalt-like, and the other mid-ocean ridge-like, can be distinguished on the basis of their isotopic compositions and immobile trace element contents (such as light rare earth element enrichment in the former, but depletion in the latter). The more-enriched basaltic rocks are interpreted as remnants/fragments of seamounts, derived from a deep mantle reservoir with low degrees (2-3%) of garnet lherzolite mantle melting. The depleted basalts most likely formed by melting of a shallower spinel lherzolite mantle source with ∼15% partial melting. It is probable that both groups owe their origin to melting of a mixture between plume and depleted MORB mantle. The results from this study, when integrated with previous work, indicate that the Junggar Ocean crust (comprising a significant number of seamounts) was likely to have been subducted southward beneath the Yili-Central Tianshan block in the Late Devonian-Early Carboniferous. The seamounts were scraped-off and accreted along with the oceanic crust in an accretionary wedge to form the Bayingou ophiolitic mélange. We present a model for the tectonomagmatic evolution of this portion of the CAOB involving prolonged intra-oceanic subduction with seamount accretion

Accreted seamounts in the South Tianshan Orogenic Belt, NW China

“…The mafic rocks display pronounced LREE‐enriched patterns (La N /Yb N = 4.6–17.4) with no obvious Eu anomalies which are similar to that of OIB (Figure a) and do not show obvious Nb, Ta, and Ti negative anomalies in the primitive mantle‐normalized spidergram (Figure b). These features are markedly different from those of island arc volcanic rocks (Qian et al, ) but similar to those reported for Hawaii seamount basalts (Hofmann & Jochum, ), as well as the enriched basalts from Bayingou ophiolitic mélange in North Tianshan (Yang et al, ), Mayile and Darbut ophiolitic mélange in West Junggar (Yang, Li, Gu, Yang, et al, ; Yang, Li, Santosh, Gu, et al, ; Yang, Li, Santosh, Yang, et al, ). All mafic rocks plot within OIB and oceanic plateau basalts fields in La/Yb–La/Nb and Nb/Th–Zr/Nb discrimination diagrams (Figures a,b).…”

“…Cenozoic deformation strongly overprinted the Palaeozoic structural architecture of the Tianshan orogeny (Wang et al, ). The Palaeozoic Tianshan orogen formed during southward subduction of the Palaeo‐Tianshan and Junggar oceans and the subsequent amalgamation of various microcontinents, oceanic and continental arcs, seamounts, and accretionary complexes (Charvet et al, ; Gao et al, , ; Gao, Long, et al, ; Yang, Li, Kerr, & Tong, ; Windley et al, ; Xiao et al, ). Geologically, the Tianshan Belt is divided into three tectonic units: north, central, and south Tianshan (Gao et al, ; Gao, Long, et al, ).…”

“…Diagram showing mantle compositional components and fields for basalts from various tectonic settings (Condie, ; Yang et al, ). (a) La/Yb versus La/Nb, (b) Nb/Th versus Zr/Nb, (c) Zr/Y versus Nb/Y.…”

Identifying Early Carboniferous bimodal volcanic rocks and geochemical characteristics in the Atengtao Mountain, Yili Block (Chinese western Tianshan)

“…(a) Overview map showing the Central Asian Orogenic Belt and adjacent cratons (Yang et al, ). (b) The tectonic geological map of the Chinese Tianshan.…”

“…Tectonically, the Chinese Western Tianshan is usually divided into three main tectonic units from north to south (Figure b), including the North Tianshan Terrane, the Yili Block–Central Tianshan, and the South Tianshan (Gao et al, ; Yang et al, ). However, the time of collision is strongly disputed.…”