EARLY EOCENE OVERPRINT OF A PERMO-TRIASSIC ACCRETIONARY COMPLEX IN THE EASTERN PONTIDES (Ağvanis MASSIF)

The Ağvanis metamorphic massif, ~20 km long and ~6 km across, forms a SSE-NNW trending lozenge-shaped horst bounded by the strands of the North Anatolian Fault in the southern margin of the Eastern Pontides close to Neo-Tethyan Izmir-Ankara-Erzincan suture. The metamorphic rocks are made up of metabasite, phyllite, marble and minor metachert and serpentinite, and are intruded by Early Eocene quartz diorite, leucogranodiorite and dacite. The metabasites comprise hornblende/±actinolite, albite, chlorite, epidote, titanite, ±magnetite, quartz and ±biotite, indicative of the albite-epidote-amphibolite facies conditions. The phyllites contain quartz, muscovite, chlorite, albite, ±garnet, ilmenite, rutile, apatite and tourmaline. The peak metamorphic conditions are estimated as ~530 ºC and ~7 kbar. Stepwise ⁴⁰Ar/³⁹Ar dating of muscovite-phengite separates outside the contact metamorphic aureoles yielded steadily increasing disturbed age spectra with the highest incremental stage corresponding to age values between ~180 and 207 Ma, suggesting that the albite-epidote amphibolites facies metamorphism occurred at ≥ 207 Ma (Late Triassic or earlier). Contact metamorphic mineral assemblages, on the other hand, suggest that the present-day erosion level of the Ağvanis Massif was at depths of ~14 km during the Early Eocene. Given a nominal geothermal gradient of 25-30 ˚C/km, the ambient temperatures outside the contact aureoles would correspond to 350-425 ˚C. We therefore suggest that the partial resetting of the Ar-Ar muscovite ages are related to Paleocene-Early Eocene reburial coupled with the igneous activity. The Early Eocene reburial is probably related to thrusting due to the continental collision between Eastern Pontides and Anatolide-Tauride block. Foliation observed in many of Eocene dacite dykes and sills and in one granitic body also indicates major burial and shortening during the Early Eocene