P–T–t Path of Garnetites in South Altyn Tagh, West China: A Complete Record of the Ultradeep Subduction and Exhumation of Continental Crust

Abstract

We report here a detailed investigation of mineral chemistry, phase modelling and geochronology for garnetites from the Bashiwake ultrahigh pressure (UHP) terrane, South Altyn Tagh, West China. Three generations of mineral assemblages including two stages of melting were identified. The first generation is represented by the zoning with decreasing grossular and increasing pyrope and multiphase inclusions in garnet core. It records an early‐stage prograde evolution from amphibolite facies to eclogite facies (1.7–2.5 GPa/710–850 °C) where the first stage of melting occurs. The second generation is characterized by the zoning in garnet mantle with increasing grossular and nearly constant pyrope, defining a later‐stage prograde evolution to UHP peak with the minimum condition of 6.5–7.0 GPa/990 °C. The pressure is consistent with that constrained from the pigeonite exsolution particularly along (401) planes in clinopyroxene. The third generation includes the presence of perthite, plagioclase along with amphibole, suggesting the decompression from eclogite facies to HP–ultrahigh‐temperature (UHT) granulite facies (~3.3–1.2 GPa/960–1020 °C) coupled with the second stage of melting. It is for the first time to get two metamorphic ages about 500 Ma and 486 Ma using in situ SIMS zircon dating which represent the prograde eclogite facies and decompressional granulite facies stages, suggesting the rocks staying at mantle depths for ~14 million years. The garnetites have recorded a complete metamorphic P–T–t history of continental ultradeep subduction to mantle depths of ~200 km or more followed by diapiring exhumation to the crust bottom.

Multistage metamorphism of eclogite in the South Altyn HP–UHP belt, Northwest China: Deep subduction and exhumation process of continental crust

“…The early exhumation (perhaps a few Myr) was very rapid, leading to the preservation of well-zoned garnet (Carlson, 2006;Caddick, 2010) and symplectites of Cpx + Pl, Cpx + Amp + Pl and Amp + Pl. The occurrence of rapid exhumation is also supported by both numerical modelling (Sizova, Gerya, & Brown, 2012) and studies of other HP-UHP terranes (Dong et al, 2018(Dong et al, , 2020Gai et al, 2021, in press;Haifler & Kotkova, 2016;Perraki & Faryad, 2014).…”

“…Metamorphic facies and metamorphic facies series are from Spear (1993) and O'Brien and Rtzler (2003). The provenance of each P – T path has been marked in the figure, including eclogite‐facies granitic gneiss in the Younusisayi from Ma et al (2018), eclogite in Danshuiquan from Gai (2018) and Gai et al (2021), HP pelite granulite in Munabulake from Cao et al (2013), UHP eclogites in Jianggalesayi from Liu et al (2012), Pelitic gneisses in Jianggalesayi from Liu et al (2007), UHP felsic granulite in Yinggelisayi from Dong, Wei, Clarke, and Zhang (2018), and UHT mafic granulites in Yinggelisayi from Dong et al (2019, 2020) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Increasing temperature and pressure during subduction cause minerals in mafic and surrounding felsic rocks to dehydrate and release fluid, locally saturating eclogite lenses with water (Dong et al, 2020). The studied eclogites are considered to have been water saturated during prograde metamorphism (Figure 9).…”

“…Results from detailed petrography, mineral chemistry, and pseudosection modelling reveal the following stages of metamorphic evolutions recorded in the eclogites from the Younusisayi area (Figure 19): prograde stage from upper-amphibolite to eclogite-facies, peak HP-UHP eclogite-facies, early decompression from eclogite-facies to HP granulite-facies conditions with melting, and cooling and decompression from HP granulite-facies to amphibolite facies. Increasing temperature and pressure during subduction cause minerals in mafic and surrounding felsic rocks to dehydrate and release fluid, locally saturating eclogite lenses with water (Dong et al, 2020). The studied eclogites are considered to have been water saturated during prograde metamorphism (Figure 9).…”

“…This crust contains oriented Al‐ and Fe‐bearing oxide inclusions in quartz interpreted as having replaced stishovite (Liu, Zhang, Green, et al, 2007), as well as well‐preserved paramorphs after stishovite (Liu et al, 2018). However, within the South Altyn belt, UHP rocks with preserved index minerals are restricted to Jianggalesayi in the west and Yinggelisayi in the east after many years of research (Dong et al, 2020; Gai et al, 2017; Liu et al, 2005; Liu, Zhang, Cao et al, 2018; Liu, Zhang, Chen et al, 2007; Zhang et al, 2002), limited by challenging natural conditions and logistical difficulties (Figure 1b). In previous studies of the western part of South Altyn belt (WSA), index minerals such as coesite (Gai et al, 2017) and/or stishovite paramorphs (Liu et al, 2018) in eclogites have been used to constrain subduction depths (Gai et al, 2017; Liu et al, 2018), and zircons and their inclusions have been used to detemine peak and retrograde ages (Liu et al, 2012).…”