Triassic warm subduction in northeast Turkey: Evidence from the Aǧvanis metamorphic rocks

Within the Tethyan realm, data for the subduction history of the Permo-Triassic Tethys in the form of accretionary complexes are scarce, coming mainly from northwest Turkey and Tibet. Herein we present field geological, petrological and geochronological data on a Triassic accretionary complex, the Aǧvanis metamorphic rocks, from northeast Turkey. The Aǧvanis metamorphic rocks form a SSE-NNW trending lozenge-shaped horst, ~20km long and ~6km across, bounded by the strands of the active North Anatolian Fault close to the collision zone between the Eastern Pontides and the Menderes-Taurus Block. The rocks consist mainly of greenschist- to epidote-amphibolite-facies metabasite, phyllite, marble and minor metachert and serpentinite, interpreted as a metamorphic accretionary complex based on the oceanic rock types and ocean island basaltic, mid-ocean ridge basaltic and island-arc tholeiitic affinities of the metabasites. This rock assemblage was intruded by stocks and dikes of Early Eocene quartz diorite, leucogranodiorite and dacite porphyry. Metamorphic conditions are estimated to be 470-540°C and ~0.60-0.90GPa. Stepwise ⁴⁰Ar/³⁹Ar dating of phengite-muscovite separates sampled outside the contact metamorphic aureoles yielded steadily increasing age spectra with the highest incremental stage corresponding to age values ranging from ~180 to 209 Ma, suggesting that the metamorphism occurred at ≥209 Ma. Thus, the Aǧvanis metamorphic rocks represent the vestiges of the Late Triassic or slightly older subduction in northeast Turkey. Estimated P-T conditions indicate higher temperatures than those predicted by steady state thermal models for average subduction zones, and can best be accounted for by a hot subduction zone, similar to the present-day Cascadia. Contact metamorphic mineral assemblages around an Early Eocene quartz diorite stock, on the other hand, suggest that the present-day erosion level was at depths of ~14km during the Early Eocene, indicative of reburial of the metamorphic rocks. Partial disturbance of white-mica Ar-Ar age spectra was probably caused by the reburial coupled with heat input by igneous activity, which is probably related to thrusting due to the continental collision between Eastern Pontides and the Menderes-Taurus Block.