Plume-modified collision orogeny: The Tarim–western Tianshan example in Central Asia

Abstract

Plume-modified orogeny involves the interaction between a mantle plume and subducting oceanic lithosphere at accretionary margins. We propose that a plume can also be involved in collisional orogeny and accounts for the late Paleozoic geological relations in Central Asia. Continental collision between the Tarim and Central Tianshan–Yili blocks at the end Carboniferous resulted in an orogeny lacking continental-type (ultra)high-pressure [(U)HP] rocks and significant syncollision surface erosion and uplift, features normally characteristic of continent-continent interactions. Their absence from the Tianshan region corresponded with the arrival of a mantle plume beneath the northern Tarim. Elemental and isotopic data reveal an increasing influence of the mantle plume on magmatic petrogenesis from ca. 300 to 280 Ma, immediately after collision at 310–300 Ma. The rising mantle plume interrupted the normal succession of collisional orogenic events, destroying the deeply subducted continental crust and hence preventing slab break-off–induced continental rebound. Plume-modified continental collision thus limited continental (U)HP rock exhumation and associated surface uplift. INTRODUCTION Oceanic convergent zones have been shown to be impacted by mantle plumes. The resultant “plume-modified orogeny” is exemplified by ancient orogens in North America and eastern Australia (Murphy et al., 1998, 1999; Betts et al., 2012), and the present Tonga subduction zone and Samoa plume (Chang et al., 2016). Plume-slab interaction can cause prominent tectonic changes such as flattening of the subducting slab, development of a slab window or slab break-off due to plume erosion, arcto plumerelated magmatic transition, and supercontinent fragmentation (e.g., Murphy et al., 1998, 1999; Dalziel et al., 2000; Betts et al., 2012). In contrast to growing understanding of the interplay between a mantle plume and an oceanic subduction zone, the influences of a plume at a zone of continental collision have not been investigated. This study proposes such a case by providing age, geochemical, and isotopic evidence that the late Paleozoic continental collision orogeny in the western Tianshan and Tarim region in Central Asia was profoundly affected by the impingement of the Tarim mantle plume. In particular, such plume-collision interaction explains previously enigmatic features of the collisional orogen, i.e., the lack of continental-type (ultra) high-pressure [(U)HP] rock suites and the absence of significant surface uplift during collision, and it extends our understanding of plumemodified orogeny to collisional settings. GEOLOGICAL OVERVIEW AND KEY ISSUES The northern margin of the Tarim craton and the adjoining Tianshan region in Central Asia are an accretionary orogen that recorded the consumption of the Paleozoic Paleo-Asian Ocean (Fig. 1; e.g., Windley et al., 2007; Cawood et al., 2009; Charvet et al., 2011; Xiao et al., 2015; Zhao et al., 2018). In the late Paleozoic, oceanic subduction and closure resulted in collision between the Tarim craton and the Central Tianshan (CTS)–Yili block, forming the South Tianshan (STS) suture zone, which contains ophiolite relics and (U)HP metamorphic rocks (Fig. 1B; e.g., Gao et al., 2011; Han et al., 2011, 2016a; Klemd et al., 2011; Xiao et al., 2013; Bayet et al., 2018; Zhang et al., 2019). A final stage, involving north-directed oceanic subduction is evidenced by Carboniferous passive-margin deposition along the STS–northern Tarim and intense arc magmatism in the CTS-Yili block. The collision time is commonly suggested at ca. 325–310 Ma, coinciding with the (U)HP metamorphism (e.g., Gao et al., 2011; Han et al., 2016b; Loury et al., 2018; Zhang et al., 2019). A large igneous province (∼4 × 105 km2) in the Tarim and STS region includes ca. 300 Ma kimberlites near Bachu, ca. 295–285 Ma flood basalts, and 285–265 Ma (ultra-)mafic intrusions and/or dikes, as well as rhyolites, granites, and syenites (Fig. 1B). These early Permian bimodal magmatic suites have been ascribed to the incubation of a mantle plume beneath the Tarim lithosphere (e.g., Zhou et al., 2009; Xu et al., 2014), as manifested by crustal uplift centered in Tarim at the Carboniferous-Permian transition (Li et al., 2014). Current models for the Tarim and CTS-Yili collision invoke a classic Alpine-Himalayan–type belt, but this interpretation fails to reconcile the lack of continental-type (U)HP rock suites and the lack of significant upper-plate uplift in the CTSYili region during collision. Such collision zones are normally characterized by intense and rapid surface uplift and exhumation of continental-type (U)HP rocks, mainly induced by break-off of the lower plate around the ocean-continent boundary and prompt continental slab rebound, as the subducting dense oceanic slab fails to resist the *E-mail: [email protected] Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/10/1001/4830278/1001.pdf by Northwest Univ Dept of Geology user on 06 January 202