Metamorphism, magmatism, and exhumation history of the Tavşanlı zone, NW Turkey: New petrological constraints

The Tavşanlı Zone (TZ) is a high-pressure/low-temperature metamorphic belt representing subduction and exhumation between the Sakarya Zone and the Afyon-Bolkardağ Zone in western Anatolia. This paper provides new and precise geological data including whole-rock and mineral chemistry, phengite⁴⁰Ar/³⁹Ar ages, zircon laser ablation ICP-MS U-Pb, and apatite (U-Th)/He thermochronological information for a region of the Sivrihisar metamorphic complex that has been less studied than other regions of the TZ. This region comprises Permo-Carboniferous metamorphic units, Eocene granodiorite and microgranodiorite, terrestrial clastics, and Holocene alluvium. The mineral assemblage of the granite-gneiss contains quartz, plagioclase, K-feldspar, microcline, muscovite, and rare biotite and garnet, while the blueschist comprises plagioclase, white mica, biotite, Na-amphibole, and garnet that are considered to represent greenschist and blueschist facies metamorphism, respectively. We estimate the peak metamorphic conditions for schists of the Permo-Carboniferous metamorphics and associated rocks as T = 303-484 °C and P ≥10 kbar. This high-pressure event occurred at ca. 83 Ma, indicated by a well-defined ⁴⁰Ar/³⁹Ar plateau age for phengite. Laser ablation ICP MS U-Pb analyses of euhedral or subeuhedral magmatic zircons from previously unknown microgranodiorite, intruding the metamorphic rocks, yield an Eocene (50 Ma) age. The new results are interpreted to indicate that lithospheric collision and northward subduction beneath the Sakarya Zone occurred between 83 and 50 Ma. Apatite crystals separated from microgranodiorite yield (U-Th)/He ages consistent with cooling of the Tavşanlı Zone to ~65 °C by ~38 Ma.