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Peak and post-peak P–T conditions and fluid composition for scapolite-clinopyroxene-garnet calc-silicate rocks from the Takab area, NW Iran

¹ Department of Geology, University of Tabriz, 51664, Tabriz, Iran
² Institut für Geowissenschaften, Universität Potsdam, Postfach 601553, D-14415 Potsdam, Germany
³ Department of Geology, University of Kansas, 1475 Jayhawk Blvd, Lawrence, Kansas, USA
⁴ School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK

^* Corresponding author, e-mail: moazzen{at}tabrizu.ac.ir and mmoazzen{at}yahoo.co.uk

The Takab calcareous rocks of northwest Iran crop out in association with a variety of metamorphic rocks including mafic granulites, amphibolites, granitic gneisses, pelitic schists and meta-ultramafic rocks. They can be divided into marbles and calc-silicate rocks on the basis of the dominance of calcite/dolomite and silicate minerals. Dominant peak metamorphic granulite facies assemblage of calc-silicate rocks is Scp + Grt(I) + Cpx + Cal + Qtz ± Hbl(I). The decrease of temperature and pressure during exhumation produced post-peak metamorphic assemblages. Coronal garnet (Grt II) in the calc-silicate rocks was produced by retrograde reactions consuming plagioclase and clinopyroxene, while peak metamorphic garnet (Grt I) occurs as preserved xenoblastic grains in calcite and/or plagioclase (Pl II).

Regional metamorphism took place at 740 °C and X_CO2 ~0.9. Garnet-clinopyroxene-plagioclase-quartz (GADS) barometry yields a pressure of 8–9 kbar, corresponding to a depth of ca. 24–27 km. This was followed by decompression and hydration during exhumation of the crustal rocks up to the surface. Secondary phases such as garnet (II) hornblende (II), plagioclase (II), zoisite and titanite (II) constrain the temperature and pressure of post-peak metamorphism as ~600 °C and ~6 kbar respectively and a fluid with XCO₂ as low as 0.4.

Halogens were near-absent during the peak metamorphic stage. The scapolite and hornblende crystallized under peak metamorphic conditions contain very low fluorine and chlorine, whereas relatively high fluorine (~0.8 wt%) in the titanite (II) and hornblende (II) suggests a possible infiltration of F-rich fluids into the calc-silicate rocks during retrogression. It is interpreted to be related to external fluids released during crystallisation of granitoid magmas and/or leucosome patches in the adjacent migmatites.

Key-words: scapolite-clinopyroxene-garnet calc-silicate rocks, P–T conditions, fluid composition, Takab, north-western Iran.