Blueschists of the Amassia-Stepanavan Suture Zone (Armenia): linking Tethys subduction history from E-Turkey to W-Iran |
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Authors: | Yann Rolland Sandra Billo Michel Corsini Marc Sosson Ghazar Galoyan |
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Institution: | (1) Géosciences Azur, UMR 6526, Université de Nice-Sophia Antipolis CNRS, Parc Valrose, 06108 Nice cedex 2, France;(2) DTP, UMR 5562, Observatoire Midi-Pyrénées, 14, avenue Edouard Belin, 31400 Toulouse, France;(3) Institute of Geological Sciences, National Academy of Sciences of Armenia, 24a Baghramian avenue, Yerevan, 375019, Armenia |
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Abstract: | 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,
metaconglomerates, quartzites, gneisses and metabasites, suggesting a tectonic mélange within an accretionary prism. This
blueschist mélange 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°C. 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°C. This LP–MT retrogression
is coeval with exhumation and nappe-stacking of lower grade units over higher grade ones. 40Ar/39Ar 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. |
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Keywords: | Armenia Lesser Caucasus Blueschists HP metamorphism 40Ar/39Ar dating |
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