Blueschists Of The Amassia-stepanavan Suture Zone (armenia): Linking Tethys Subduction History From E-turkey To W-iran

摘要

The Amassia-Stepanavan blueschist-ophiolite complex of the Lesser Caucasus in NW Armenia is part of an Upper Cretaceous-Cenozoic belt, which presents similar metamorphic features as other suture zones from Turkey to Iran. The blueschists include calcschists, metaconglomer-ates, quartzites, gneisses and metabasites, suggesting a tectonic melange within an accretionary prism. This blues-chist melange is tectonically overlain by a low-metamorphic grade ophiolite sequence composed of serpentinites, gabbro-norite pods, plagiogranites, basalts and radiolarites. The metabasites include high-P assemblages (glaucophane-aegirine-clinozoisite-phengite), which indicate maximal burial pressure of ～ 1.2 GPa at ～ 550℃. Most blueschists show evidence of greenschist retrogression (chlorite-epidote, actinolite), but locally epidote-amphibolite conditions were attained (garnet-epidote, Ca/Na amphibole) at a pressure of ～0.6 GPa and a temperature of ～500℃. This LP-MT retrogression is coeval with exhumation and nappe-stacking of lower grade units over higher grade ones. ~(40)Ar/~(39)Ar phengite ages obtained on the high-p assemblages range between 95 and 90 Ma, while ages obtained for epidote-amphibolite retrogression assemblages range within 73.5-71 Ma. These two metamorphic phases are significant of (1) HP metamorphism during a phase of subduction in the Cenomanian-Turonian times followed by (2) exhumation in the greenschist to epidote-amphibolite facies conditions during the Upper Campanian/Maastrichtian due to the onset of continental subduction of the South Armenian block below Eurasia.