Eclogites from the south Tianshan, NW China: petrological characteristic and calculated mineral equilibria in the Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O system

Abstract

Eclogites from the south Tianshan, NW China are grouped into two types: glaucophane and hornblende eclogites, composed, respectively, of garnet + omphacite + glaucophane + paragonite + epidote + quartz and garnet + omphacite + hornblende (sensu lato) + paragonite + epidote + quartz, plus accessory rutile and ilmenite. These eclogites are diverse both in mineral composition and texture not only between the two types but also among the different selected samples within the glaucophane eclogite. Using thermocalc 3.1 and recent models of activity–composition relation for minerals, a P–T projection and a series of P–T pseudosections for specific samples of eclogite have been calculated in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O (NCFMASH) with quartz and water taken to be in excess. On the basis of these phase diagrams, the phase relations and P–T conditions are well delineated. The three selected samples of glaucophane eclogite AK05, AK11 and AK17 are estimated to have peak P–T conditions, respectively, of 540–550 °C at c. 16 kbar, c. 560 °C at 15–17 kbar and c. 580 °C at 15–19 kbar, and two samples of hornblende eclogite AK10 and AK30 of 610–630 °C and 17–18 kbar. Together with H2O‐content contours in the related P–T pseudosections and textural relations, both types of eclogite are inferred to show clockwise P–T paths, with the hornblende eclogite being transformed from the glaucophane eclogite assemblage dominantly through increasing temperature.

Palaeozoic tectonics of the south-western Chinese Tianshan: new insights from a structural study of the high-pressure/low-temperature metamorphic belt

“…All diagrams are equiareal Schmidt net, lower hemisphere. Sporadic localities of eclogites are after Gao et al (1999), Zhang et al (2002Zhang et al ( , 2005Zhang et al ( , 2007a, Gao and Klemd (2003), Klemd et al (2002Klemd et al ( , 2005, Klemd (2003), Wei et al (2003) and our field survey. 3: Nalati Fault or Nikolaiev tectonic line…”

“…Mineralogical and petrological characteristics of blueschists and eclogites from the SW Chinese Tianshan have already been described in detail (Gao et al , 1999Zhang et al 2002Zhang et al , 2005Wei et al 2003;Gao and Klemd 2003;Lin and Enami 2006 and references therein; Fig. 1).…”

Tectonic evolution of the South Tianshan orogen and adjacent regions, NW China: geochemical and age constraints of granitoid rocks

“…Eclogites are interlayered with the blueschist layers as pods, boudins, thin layers or as massive blocks. The transition between blueschist and eclogite is locally gradual, and both underwent prograde metamorphic conditions of 480-580°C at 1.9-2.1 GPa (e.g., Klemd et al 2002;Wei et al 2003;Lin and Enami 2006). UHP-conditions for some eclogites were reported and discussed (cf.…”

New 40Ar/39Ar age constraints on the Late Palaeozoic tectonic evolution of the western Tianshan (Xinjiang, northwestern China), with emphasis on Permian fluid ingress

“…1) that forms a suture zone (Wang et al 2008b, c). The belt principally comprises eclogite-blueschist-and greenschist-facies, in part metagraywacke-like, metasediments that contain numerous lenses and blocks of ultramafic rocks, marbles and eclogites with blueschist layers (Gao et al 1995;Gao and Klemd 2003;Wei et al 2003;Klemd et al 2005;Lin and Enami 2006). The Changawuzi-Kekesu belt continues westward to Kyrgyzstan (Atbashy belt: Tagiri et al 1995;Stupakov et al 2004) and to Tajikistan (Fan-Karategin blueschist belt: Volkova and Budanov 1999).…”