PETROGENESIS OF KYANITE-BEARING METAGABBRO (BERIT METAOPHIOLITE, SE ANATOLIA)

Mafic granulite consisting of garnet + cpx + plagioclase ± kyanite ± opx ± corundum ± sapphirine occurs as m-scale pods in amphibolite in a mafic-ultramafic complex interpreted as a metaophiolite in SE Turkey. The granulite pods contain significant evidence for textural and chemical disequilibrium, including symplectites, corona structures, and zoned phases. To determine P-T conditions, path, reaction history, and tectonic evolution, a combination of textural analysis and pseudosection modeling was used.

At least two distinct granulite pod lithologies occur: 1) Plagioclase-rich granulites contain Grt + Cpx + Pl + Opx with minor sapphirine, corundum, and secondary amphibole. These granulites have corona structures consisting of a polygonal aggregate of Opx (core), surrounded by a polygonal aggregate of Cpx (middle) that is surrounded by Grt (outer shell) in a matrix of coarse anorthite. Crn-Cpx symplectite locally separates Grt and Pl; and 2) Kyanite granulites contain Grt + Cpx + Ky + Pl ± Opx with minor corundum, clinozoisite, and amphibole. Ky occurs in symplectites with Cpx that commonly separates Grt and Pl and also occurs as coarse-grained laths. Pod-hosting amphibolites contain Amp + Ep ± Pl ± Crn ± Qz with minor rutile, titanite, and chlorite.

Equilibrium assemblage diagrams indicate that the granulites equilibrated at 700-900 °C, 1.1-1.5 GPa (=40-50 km), consistent with previous work. Textural analysis combined with modeling suggests that Cpx and Grt coronas on Opx could have formed along a prograde path from intermediate- to high-P granulite facies. Crn- and Ky-Cpx symplectites suggest rapid isobaric cooling of ~150 °C following peak conditions. Hbl-Pl thermobarometry for pod-hosting amphibolites indicates retrograde metamorphism and hydration of the mafic granulite at 615-700 °C and 0.5-0.8 GPa. If the interpretation of the sequence of reactions is correct, textural evidence and thermobarometric results for granulites and amphibolites are consistent with an anticlockwise P-T path. Similar paths have been proposed for the deep mafic roots of arc systems, however it is difficult to reconcile the P-T path with the interpretation of the complex as a metaophiolite. Other ophiolites in the same belt are not metamorphosed, indicating an unusual petro-tectonic history for the Berit complex.