JUXTAPOSITION OF HP AND HT METAMORPHIC ROCKS IN TRANSTENSIONAL EXHUMATION, SIVRIHISAR BELT, WEST-CENTRAL TURKEY

The Sivrihisar massif has been described as part of the Turkish blueschist belts (HP-LT) that formed by Late Cretaceous subduction, collision, and exhumation of a continental margin during closure of a Neo-Tethyan seaway. In the massif, however, HP-LT rocks are juxtaposed with high T (sillimanite-bearing) rocks. The HP-LT unit consists of sodic amphibole rocks occurring as meter scale lenses in marble with fibrous texture similar to marble described in other HP-LT terrains of the Aegean belt. The fibrous texture in marble gradually decreases and vanishes from north to south over ~1km of section towards the HT rocks; the HT zone consists of non-fibrous marble, schist, and andalusite+kyanite+sillimanite micaceous quartzite. A late-kinematic Eocene granite is emplaced and developed a narrow metamorphic aureole (< 4 m). Therefore, contact metamorphism may not account for the strongly lineated sillimanite that exists ~1km away from the contact. The Sivrihisar massif provides an excellent opportunity to understand the thermal/tectonic evolution of continental rocks at a converging plate boundary. Metasedimentary rocks of the paleo-margin are extensively deformed with a foliation complexly folded around a prominent lineation uniformly oriented ~N-S to NE-SW. This strong lineation characterizes both the HP-LT and HT rocks throughout the southern Sivrihisar massif. Outcrop-scale kinematic indicators in the schist and quartzite confirm a normal / left lateral sense of shear (top-to-north). The transition between HP-LT and HT domains occurs over the chloritoid, staurolite-garnet, and Al2SiO5 isograds that are compressed in a few hundred meters of structural section. These structural and metamorphic relations are interpreted as a transtentional system, possibly developed in the extensional jog of the metamorphic belt. This model explains the ubiquitous normal/sinistral sense of shear and the development of HT metamorphism in transtensional exhumation.