P–T path of high‐pressure/low‐temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

Abstract

The Chinese western Tianshan high‐pressure/low‐temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta‐sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km2 in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro‐glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre‐peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T path through pre‐peak lawsonite/epidote‐blueschist to eclogite facies conditions. The retrograde part of the P–T path is represented by the transition of epidote‐blueschist to greenschist facies conditions. The P–T path indicates that the high‐pressure rocks formed in a B‐type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili‐central Tianshan plates.

Paleozoic tectonics of the southern Chinese Tianshan: Insights from structural, chronological and geochemical studies of the Heiyingshan ophiolitic mélange (NW China)

“…1B). This belt consists of greenschist-, blueschist-and eclogite-facies rocks, which were probably transformed from sea-floor deposits and basaltic rocks (Gao et al, , 1999Ai et al, 2006). Isotopic ages of 415-343 Ma (phengite 40 Ar/ 39 Ar plateau and Sm-Nd isochron) were obtained for eclogite and blueschist (Dobretsov et al, 1987;Xiao et al, 1992;Gao et al, 1995bGao et al, , 2000Gao et al, , 2006Gao and Klemd, 2003), and the peak HP metamorphism was considered to have occurred at ~345 Ma (Gao and Klemd, 2003;Klemd et al, 2005).…”

Tracing the effects of high-pressure metasomatic fluids and seawater alteration in blueschist-facies overprinted eclogites: Implications for subduction channel processes

“…The HP/LT sequences occur discontinuously within a Paleozoic accretionary wedge (Gao et al, 1995(Gao et al, , 1998) and predominantly comprise eclogite-, blueschist-, and greenschist-facies metasedimentary rocks. Subordinate metavolcanic rocks, ultramafic slices and lenses of Silurian marble occur as exotic blocks and point towards an origin as tectonic mélange (Gao et al, 1999;van der Straaten et al, 2008). The blueschists occur as lenses, bands or layers within greenschist facies metasediments and are intercalated with pods (15-25 cm in diameter), boudins, layers (2-25 cm thickness) or large blocks (2 km 2 ) of eclogite (Gao et al, 1999).…”

“…Subordinate metavolcanic rocks, ultramafic slices and lenses of Silurian marble occur as exotic blocks and point towards an origin as tectonic mélange (Gao et al, 1999;van der Straaten et al, 2008). The blueschists occur as lenses, bands or layers within greenschist facies metasediments and are intercalated with pods (15-25 cm in diameter), boudins, layers (2-25 cm thickness) or large blocks (2 km 2 ) of eclogite (Gao et al, 1999). Blueschist occurrences include prograde and retrograde varieties (Gao and Klemd, 2001;Gao et al, 2007;van der Straaten et al, 2008;Beinlich et al, 2010).…”

Integrated Provenance Analysis of a Complex Orogenic Terrane: Mesozoic Uplift of the Bogda Shan and Inception of the Turpan-Hami Basin, NW China

“…The North Tian Shan fault, a Cenozoic feature with significant rightlateral strike-slip displacement, corresponds approximately to the inferred suture between these elements (Nelson et al 1987;Allen et al 1992;Laurent-Charvet et al 2002). In contrast, ranges to the south of the TurpanHami basin include late Paleozoic granitoids, metamorphic sequences, Precambrian basement, ophiolite fragments, and Paleozoic passive-margin sequences (Chen et al 1985;Chen et al 1999;Coleman 1989;Wen 1991;Allen et al 1992;Hopson et al 1998;Gao et al 1999;Zhou et al 2001).…”