Metamorphic evolution of the Hırkadağ and Kırşehir massifs, Central Turkey, based on geothermobarometry of HT metapelitic rocks

Abstract

The Central Anatolian Crystalline Complex (CACC) in Turkey is a continental domain of Pan-African origin that collided with the Eurasian continent during the Alpine orogeny. The crystalline rocks are characterized by intense Late Cretaceous deformation, high-temperature metamorphism and intrusion. This study focuses on the metamorphic history recorded in regionally metamorphic garnet-sillimanite schists from the Hırkadağ massif in the centre of the CACC and garnet-sillimanite schists and biotite-clinopyroxene schists from the Kırşehir massif in the northwest of the CACC. The principal aim is to elucidate PTt histories of selected samples of pelitic rocks from both massifs on the basis of microstructurally controlled mineral chemistry data. Petrography and mineral and bulk rock major element compositions are used to calculate and interpret TheriakDomino PT-pseudosections. The general mineral assemblage observed in the pelitic schists from the Hirkadag massif is Grt + Bt + Sil + Plag + Qtz + Spl ± Kfs ± Crd ± Wh mica + Ilm. The mineral assemblage of the garnet-sillimanite schist of Kırşehir is similar to the metapelitic rocks of the Hirkadag massif, but the microstructure is finer grained and lacks spinel: Grt + Bt + Sil + Plag + Qtz + Kfs + Wh mica. The biotite-clinopyroxene schists of the Kirsehir massif show a banded structure with pelitic bands, calc-silicate bands and intermediate zones, showing the mineral assemblage: Bt + Sil + Plag + Kfs + Qtz ± Cpx ± Wh mica ± Spl. Important reactions include replacement of garnet by cordierite (decompression) and growth of hercynite (high temperature). The two assemblages can be linked in a clockwise PT path. Peak temperature conditions are best preserved in the cordierite-bearing garnet-sillimanite schists and less well in the biotite-clinopyroxene schists. PT domains calculated with TheriakDomino show approximately isothermal decompression from ~9.5 kbar to 3 kbar at temperatures of 800 °C for the samples of the Hırkadağ massif and decompression from about 8 kbar down to 5 kbar at 700 °C for the Kırşehir samples. Peak metamorphic conditions calculated with the garnet-biotite and Ti-in-biotite thermometers and GASP and GPMB barometers show lower peak temperatures: 620-740 °C at about 8 kbar for the Hırkadağ massif and 620 °C at 6 kbar for the Kırşehir massif. The presence of melt as inferred by the high-temperature low-pressure metamorphic conditions and shown in TheriakDomino pseudosections is supported by microstructural relationships. Attempts to date the metamorphism of the Hırkadağ and Kırşehir massifs using monazite U-Pb dating are unfortunately unsuccessful, because concentrations of radiogenic Pb are below the detection limits of the available electron microprobe at Utrecht University. These low concentrations are likely due to the relative young (Cretaceous, <100 Ma) age commonly assigned to the metamorphism and intrusion of the CACC. The inferred HT/LP history calls for a highly anomalous thermal state of the pertinent crust, possibly indicating ascent of asthenospheric mantle to shallow levels.