Coesite in the eclogite and schist of the Atantayi Valley, southwestern Tianshan, China

Abstract

Coesite is an indicator mineral of ultra-high-pressure metamorphism. Since coesite was reported in the Habutengsu Valley, we have also found it in eclogite and schist from the Atantayi Valley in the southwestern Tianshan, China. Petrographic and micro-Raman analyses were carried out for the Atantayi metamorphic rocks and coesite was recognized in the predominant rock types, i.e. schist and eclogite, from three sections. The coesite-bearing schist consists mainly of garnet, Na-Ca amphibole, quartz, white mica and albite; the coesite-bearing eclogite is mainly composed of omphacite, garnet, glaucophane and zoisite. The coesite occurs as various mineral inclusions within porphyroblastic garnet. Findings of coesite in eclogite and associated schist indicate not only the regional in situ formation of the Atantayi ultra-high-pressure eclogite, but also the large areal extent of ultra-high-pressure metamorphism in southwestern Tianshan, extending up to 10 km north-south and 60-80 km east-west. Coesite is an indicator mineral of ultra-high-pressure (UHP) metamorphism [1]. In well-established UHP belts worldwide, coesite is usually preserved in relatively refractory minerals, such as garnet and zircon. An increasing number of studies show that garnet can preserve abundant information on the pressure-temperature (P-T) evolution of a metamorphic garnetiferous rock, as evidenced by the applicability and reliability of garnet isopleth geothermobarometry [2]. In the UHP field, the occurrence of coesite inclusions in garnet is of significance because it identifies the possible episodic growth of garnet, which may, combined with garnet isopleth thermobarometry, decipher the P-T trajectory of a specific UHP rock. The first reports of UHP metamorphism in the southwestern Tianshan instigated debate that has continued until the recent discovery of coeiste in the Habutengsu Valley of this orogenic belt [3][4][5][6][7][8]. However, until now, coesite had not been optically identified in the rocks of the Atantayi Valley, where the eclogites involved in the "UHP versus HP" controversy were sampled [5,9]. Detailed investigations of UHP minerals such as coesite are critical for elaborating on the spatial distribution and evolutionary characteristics of the UHP belt in the southwestern Tianshan. Recently, we collected a number of samples of different rock types along the Atantayi Valley and conducted detailed petrographic analyses on them. In order to find any evidence of ultra-high pressures in these rocks, we carefully examined tiny inclusions in garnets found in these samples employing Raman spectroscopy. This has provided fundamental information on the metamorphic evolution of the HP-UHP belt of the southwestern Tianshan.

The early exhumation history of the Western TianshanUHPmetamorphic belt, China: New constraints from titanite U–Pb geochronology and thermobarometry

“…The garnet mantle composition corresponds to the pressure peak at ~30 kbar and 540–565°C, in accordance with the presence of quartz exsolution lamellae in omphacite (Figure b; Zhang, Ellis, & Jiang, ; Zhang, Ellis, Williams, et al., ; Zhang et al., ) and coesite in eclogites and their host schists in the same unit (e.g. Lü & Zhang, ; Lü et al., , ; Tan et al., ; Tian, Wei, & Zhang, ; Yang et al., ). The maximum X grs and Mg # near the rims reflect a temperature peak at 28 kbar, 570°C.…”

“…Thus, the uniform P–T–t histories of different lithological units indicate that the whole metamorphic belt constituted a coherent unit during peak metamorphism and exhumation (e.g. Li et al., ; Lü & Zhang, ; Lü et al., , , ; Tan et al., ; Yang et al., ).…”

“…Coesite (e.g. Lü & Zhang, ; Lü, Zhang, & Chen, ; Lü, Zhang, Du, & Buche, , ; Tan et al., ; Yang, Zhang, Tian, & Bader, ) and its pseudomorphs (e.g. Zhang, Ellis, & Jiang, ; Zhang, Song, Liou, Ai, & Li, ) have been discovered in the belt.…”

Preservation of Re‐Os isotope signatures in pyrite throughout low‐T, high‐P eclogite facies metamorphism

“…The Tianshan LT–(U)HP belt is regarded as a suture zone formed around 310–330 Ma by the closure of the Southern Tianshan ocean after collision of the Yili and Tarim blocks between 344 and 331 Ma (Gao et al ., , , , ; Klemd et al ., , , ; Gao and Klemd, ; John et al ., , ; Zhang et al ., ; van der Straaten et al ., ; de Jong et al ., ; Li et al ., ). It is composed of blueschists, HP eclogites, UHP eclogites, lower grade metamorphic rocks of basaltic composition and metasedimentary rocks (Gao and Klemd, ; John et al ., ; Lü and Zhang, ; Lü et al ., ). Peak metamorphic conditions of UHP and HP eclogites are estimated to have reached 460–520 °C and >2.7 GPa (Lü and Zhang, ; Lü et al ., ) and ~570 °C and 2.1 GPa (Klemd et al ., ), respectively.…”

“…It is composed of blueschists, HP eclogites, UHP eclogites, lower grade metamorphic rocks of basaltic composition and metasedimentary rocks (Gao and Klemd, ; John et al ., ; Lü and Zhang, ; Lü et al ., ). Peak metamorphic conditions of UHP and HP eclogites are estimated to have reached 460–520 °C and >2.7 GPa (Lü and Zhang, ; Lü et al ., ) and ~570 °C and 2.1 GPa (Klemd et al ., ), respectively.…”