Ultrahigh pressure metamorphism and tectonic evolution of southwestern Tianshan orogenic belt, China: a comprehensive review

Abstract

Recently, a huge ultrahigh-pressure (UHP) metamorphic belt of oceanic type has been recognized in SW Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, pelitic garnet phengite schists, marbles and serpentinites. The well-preserved coesite inclusions are commonly found in eclogites, garnet phengite schists and marbles. Ti-clinohumite and Ti-chondrodite have been identified in UHP metamorphic serpentinites. Based on the P–T pseudosection calculation and combined U–Pb zircon dating, the P–T–t path has been outlined as four stages: cold subduction to UHP conditions before c. 320 Ma whose peak ultrahigh pressure is about 30 kbar at 500°C; heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600°C at 22 kbar; then the early cold exhumation from amphibolite eclogite facies to epidote-amphibolite facies metamorphism before 220 Ma; and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn-subduction arc-like 333–326 Ma granitic rocks and 280–260 Ma S-type granites in the coeval low-pressure and high-temperature metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed in this paper.

Petrology of UHP eclogite-facies felsic schist in the Western Tianshan subduction zone, China

“…(OIB) and E-type or N-type mid-oceanic ridge basalt (MORB; Ai et al, 2006;Gao and Klemd, 2003;Klemd et al, 2005) with dismembered seamounts, sediments, and volcanic arc-derived materials (Xiao et al, 2012). Coesite was observed in a variety of rocks, peak metamorphic conditions were estimated at about 550 °C and 2.7-3.2 GPa, and the subsequent exhumation to pressures of, or lower than, 15 kbar took place through near-isothermal decompression (e.g., Lü et al, 2008Lü et al, , 2009Lü et al, , 2013Lü et al, , 2014Lü and Zhang, 2012;Wei, 2013, 2014;Yang et al, 2013;Zhang et al, 2016Zhang et al, , 2018aTan et al, 2017). According to garnet Sm-Nd and Lu-Hf, white mica Ar-Ar and Rb-Sr, and zircon and rutile U-Pb geochronology, the eclogites underwent pressure peak metamorphism during the Carboniferous period, at 315 ± 5 Ma (Figure 1B; e.g., Gao and Klemd, 2003;Klemd et al, 2005Klemd et al, , 2011Zhang et al, 2009;Li et al, 2011;Su et al, 2010;Tan et al, 2017Tan et al, , 2019Yang et al, 2013).…”

“…In addition, the high content of low-density hydrous minerals (e.g., glaucophane, lawsonite, and chlorite) may lead to a lower density of the subducted slab relative to the mantle (Chen et al, 2013;Wang et al, 2019;Zhang and Wang, 2020). Such a positive buoyancy due to hydrous mineral assemblages could occur at depths of up to ~110 km if the geotherm of a cold subduction zone is below 6 °C/km, as is the case in the Western Tianshan (Zhang et al, 2018a). The aforementioned are the main factors leading to the initial exhumation of eclogite.…”

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

“…The South Tianshan exposes an HP-UHP metamorphic belt named the Akesayi Metamorphic Complex (AMC) (Figure S1a in Supporting Information S1). The AMC is made mainly of schistose meta-volcanoclastic rocks hosting mafic metavolcanic rocks, marbles, and rare ultramafic rocks (Bayet et al, 2020;Klemd et al, 2015;Zhang et al, 2018). Bayet et al (2018) and Tan et al (2019) revisited the structural organization of the AMC along a North-South cross-section in the Akesayi and Kebuerte valleys and subdivided it into several subunits according to new lines of evidence of petrology, lithology, and structure (Figure S1c in Supporting Information S1).…”

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

“…Some ophiolites with OIB-type basalts and associated limestones were interpreted as fragments of accreted seamounts (Yang et al, 2018;Sang et al, 2020b;Wan et al, 2020). In addition, eclogites and blueschists in ophiolite mélanges at Akeyazi, NW China (Gao and Klemd, 2000) and at Atbashi, Kyrgyzstan (Hegner et al, 2011) occur in a well-documented (ultra-)-high-pressure metamorphic belt in the South Tianshan (Zhang et al, 2019a).…”