Petrography, mineral chemistry and thermobarometry of amphibolites from the Allahyarlu metamorphic complex -Ardebil, NW Iran

The pre-Eocene Allahyarlu ophiolitic- metamorphic complex is exposd in the core of the Allahyarlu anticline in the northwest of Iran and northeast of Meshginshahr. This complex consists of a variety of schist, gneiss, amphibolite and metadibase rocks with serpentinite and tectonic slices including gabbro, dunit, marble and diabase. Amphibolites can be divided into garnet-free amphibolite and garnet amphibolite. Hornblende and plagioclase are the main phases in the garnet-free amphibolite. Titanite, calcite, rare biotite, epidote, Fe-oxides and zircon constitute the minor phases. In garnet amphibolites, in addition to these minerals, garnet is also present. Porphyroblastic and nematoblastic are the common textures in these rocks. The study of mineral chemistry and determination of temperature and pressure conditions of amphibolites are the subject of this study. Based on the results of microprobe analyses in garnet-free amphibolites and garnet amphibolites, the amphiboles are of magnesium hornblende and tschermakite types, plagioclase composition ranges between andesine and albite, and in garnet amphibolites garnet has an almandine-rich nature. Based on the variations of Ti versus total Al (Alt), the studied amphiboles in the amphibolites of the Allahyarlu metamorphic complex are medium pressure amphiboles. The results of thermometry and barometry calculations on these rocks, based on the chemistry of amphibole, garnet and plagioclase, were obtained. The P-T estimates indicate an average pressure of 8Kbar and temperature of 600C for the peak of metamorphism. This is in accordance with Barrovian metamorphic gradients, indicative of crustal thickening (in volcanic or island arcs) or a collisional tectonic setting.