Ophiolitic mélanges in crustal-scale fault zones: Implications for the Late Palaeozoic tectonic evolution in West Junggar, China

Abstract

The Baijiantan and Darbut ophiolites in West Junggar are exposed in steep fault zones (>70°) containing serpentinite mélange, in contact on either side with regionally distributed Upper Devonian-Lower Carboniferous ocean floor peperitic basalts and overlying sedimentary successions. The ophiolitic mélanges show classic structural features created by strike-slip faulting and consistent shear sense indicators of left-slip kinematics. Sandstone blocks within the mélanges resemble the surrounding sediments in lithology and age, indicating that the ophiolitic mélanges consist of locally derived rocks. The ophiolitic mélanges therefore originated from left-slip fault zones within a remnant basin and are not plate boundaries nor subduction suture zones. Sandstone is the youngest lithology involved in the mélange and provides a maximum age for the mélange of 322 Ma, whereas stitching plutons are younger than 302 Ma. Multiple clusters in zircon ages from single gabbro blocks in the mélange at~375,~360,~354, and~340 Ma are inconsistent with accretionary incorporation of subducting ocean crust but rather suggest that episodic movement of the faults provided pathways for magma from the mantle into magma chambers. Late Paleozoic tectonic evolution of West Junggar involved Late Devonian to Carboniferous relative motion between the Junggar block and West Junggar ocean basin, which triggered the left-slip fault zones within a remnant ocean basin, along which the oceanic crust was disrupted to form linear ophiolitic mélanges. Final filling of this remnant ocean basin and its dismemberment by strike-slip faulting occurred in the late Carboniferous, followed by crustal thickening by juvenile granites at the Carboniferous-Permian boundary.

Petrogenesis of pillow basalts in West Junggar, NW China: Constraints from geochronology, geochemistry, and Sr–Nd–Pb isotopes

“…As an important part of the CAOB, West Junggar, which is located at the eastern margin of the Kazakhstan orocline (Figure b; Tang et al, ; Windley et al, ; Xiao et al, ; Xiao et al, ), has attracted extensive attention of the geological community. In the last decade, researchers from different countries have carried out extensive and intensive investigations on this region and produced a large amount of new data and competing interpretations (Chen & Arakawa, ; Chen, Pe‐Piper, Piper, & Guo, ; Chen & Zhu, ; Choulet et al, , ; Geng et al, 2011; He, Liu, Zhang, & Xu, ; Li et al, , Li, He, Ma, et al, ; Liu et al, ; Ren et al, ; Shen et al, , Shen, Shen, Liu, et al, ; Wang et al, ; Shen, Shen, Pan, et al, , Shen et al, c; Yin et al, ; Zhang, Xiao, Han, Ao, et al, , Zhang et al, ; Zhu, Xu, Chen, & Xue, ; Zhu, Xu, Wei, Song, & Guo, ). However, the mechanism of tectonic evolution in West Junggar remains controversial, with models of a single subduction–accretion processes (Wang et al, ), arc–arc collisions (Buckman & Aitchison, ; Zhang, Huang, & Zhai, ), remnant ocean filling (Chen, Guo, Pe‐Piper, & Zhu, ; Xu et al, ), and intraoceanic subduction or oceanic‐ridge subduction (Geng et al, ; Li, He, Qi, & Zhang, ; Liu et al, ; Ma, Xiao, et al, ; Tang et al, ; Xiao et al, ; Yang et al, , Yang, Li, Santosh, Gu, et al, ; Yin et al, ;).…”

The Ediacaran to Early Palaeozoic evolution of the Junggar–Balkhash Ocean: A synthesis of the ophiolitic mélanges in the southern West Junggar terrane, NW China

“…These two ophiolitic mélanges are composed mainly of ~414–385 Ma N‐MORB, E‐MORB, and arc‐like gabbros (e.g., Xu et al, ; Yang, Li, Gu, et al, ; Zhu, Chen, & Qiu, ) and Late Devonian radiolarian‐bearing cherts (Zong, Wang, Jiang, & Gong, ). These blocks of diverse tectonic origins were thought to have been welded together during the final oceanic subduction, probably within a remnant Junggar–Balkhash Ocean (Chen et al, ; Li et al, ; Liu, Han, Chen, et al, ; Q. Q. Xu et al, ).…”

“…Some of the ophiolitic fragments were finally incorporated into accretionary complexes during the consumption of the PAO, representing a real suture zone, such as the Char suture zone in NE Kazakhstan (Li et al, ) and the North Tianshan (Han, Guo, & He, ; Han, Guo, Zhang, et al, ) and South Tianshan (Han, He, Wang, & Guo, ) suture zones in NW China. Others, however, could reflect merely regional convergent events that predate the eventual accretion/collision events and may not have formed so‐called sutures (Chen, Pe‐Piper, Piper, & Guo, ; Xiao et al, ). This kind of ophiolite, generally featured by supra‐subduction zone (SSZ) geochemical fingerprinting, does not often show a linear distribution and was suggested to be either substrata of oceanic arcs or limited survivals of closed/collapsed arc‐related basins (Xiao et al, and references therein).…”

Late Carboniferous to Early Permian adakitic rocks and fractionated I‐type granites in the southern West Junggar terrane, NW China: Implications for the final closure of the Junggar–Balkhash Ocean

“…The central WJT is separated from the northern WJT by the Chagantaolegai ophiolitic mélange (Figure a) and composed principally of Devonian ophiolitic mélanges (e.g., Karamay and Darbut; Xu et al, ; Yang et al, ), fragments of seamounts, and overlying Late Devonian to Early Carboniferous volcano‐sedimentary strata. Based on structural observations and occurrences of peperite‐bearing successions, the ophiolites in the central WJT were interpreted as uplifted oceanic crust within a remnant Junggar–Balkhash Ocean (Chen, Guo, Pe‐Piper, & Zhu, ; Chen, Pe‐Piper, Piper, & Guo, ; Xu, Han, Ren, Zhou, & Su, ). Recent studies of sedimentary rocks indicate that this remnant ocean basin was almost filled with sediments in the Middle Pennsylvanian (Chen et al, ; Liu, Han, Chen, et al, ; Zong, Gong, & Wang, ).…”