Triassic warm subduction in northeastTurkey: Evidence from theAğvanis metamorphic rocks

Abstract

AbstractWithin theTethyan realm, data for the subduction history of thePermo–TriassicTethys in the form of accretionary complexes are scarce, coming mainly from northwestTurkey andTibet. Herein we present field geological, petrological and geochronological data on aTriassic accretionary complex, the Ağvanis metamorphic rocks, from northeastTurkey. TheAğvanis metamorphic rocks form aSSE–NNWtrending lozenge‐shaped horst, ∼20 km long and ∼6 km across, bounded by the strands of the activeNorthAnatolianFault close to the collision zone between theEasternPontides and theMenderes–TaurusBlock. 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 ofEarlyEocene quartz diorite, leucogranodiorite and dacite porphyry. Metamorphic conditions are estimated to be 470–540°Cand ∼0.60–0.90 GPa.Stepwise40Ar/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, theAğvanis metamorphic rocks represent the vestiges of theLateTriassic or slightly older subduction in northeastTurkey. EstimatedP–Tconditions 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‐dayCascadia. Contact metamorphic mineral assemblages around anEarlyEocene quartz diorite stock, on the other hand, suggest that the present‐day erosion level was at depths of ∼14 km during theEarlyEocene, indicative of reburial of the metamorphic rocks. Partial disturbance of white‐micaAr–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 betweenEasternPontides and theMenderes–TaurusBlock.