Blueschists of the Amassia-Stepanavan Suture Zone (Armenia): linking Tethys subduction history from E-Turkey to W-Iran

Abstract

International audienceThe 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

Late Cretaceous eclogitic high-pressure relics in the Bitlis Massif

“…However, these two domains fundamentally differs from each other on the following aspects: (a) The Tavşanlı Zone was buried into a very cold subduction zone (Okay, 2002;Whitney & Davis, 2006), whereas peak metamorphic conditions were relatively "warmer" for the Bitlis complex; (b) the HP metamorphic rocks of the Bitlis complex were strongly overprinted by collision-type metamorphism, whereas the Tavşanlı Zone HP rocks were cooled down during their exhumation, as indicated by widespread pristine lawsonite; (c) the Bitlis complex represents a continental passive margin fragment, including neritic sedimentary cover and substratum, whereas the Tavşanlı Zone consists mainly of metamorphosed mafic and pelagic sedimentary rocks (Okay, 1984) and (d) the evolution of the Bitlis complex is related to the closure of a oceanic domain once located between the Arabian continental to the south, and the Anatolide-Tauride block to the north, whereas the Tavşanlı Zone stems from the northern edge of the later related then to the closure of another oceanic realm further north (Barrier & Vrielynck, 2008;Okay & Tüysüz, 1999;Şengör & Yılmaz, 1981), and likely are connected eastwards to the Amassia-Stepanevan blueschists (Rolland, Billo, Corsini, Sosson, & Galoyan, 2008).…”

Subductions, obduction and collision in the Lesser Caucasus (Armenia, Azerbaijan, Georgia), new insights

“…The HP-LT metamorphism was dated 90-80 Ma by K -Ar methods (Aghamalyan 1998). Further, it was found to be a two-stage evolution using Ar/Ar dating technique (on phengites), with HP-LT stage at 94 -90 Ma followed by a retrograde Green Schist/epidote amphibolite metamorphism during exhumation in the Late Cretaceous (71-74 Ma, Ar/Ar on phengites, Rolland et al 2007Rolland et al , 2010. The blueschist unit is overthrusted by unmetamorphosed ophiolites, Late Jurassic in age (Danelian et al 2007).…”

“…From the overall geometry, we can suspect a subduction towards the NW beneath the Eurasian margin, but the preceding stages of intra-oceanic subduction may be featured by different orientations. Based on stretching lineations, mineral HP-LT mineral lineations and senses of shear in the Stepanavan Blueschist unit, Rolland et al (2007) suggest an east-verging intra-oceanic subduction zone. However this direction could have been rotated during collision.…”

“…2) is characterized by tectonic units outcropping within a nappe anticline in which the core is made of blueschist slices (Aghamalyan 1981(Aghamalyan , 2004Galoyan et al 2007;Galoyan 2008;Rolland et al 2007). The blueschist units protoliths correspond to unroofed sediments along a subduction zone.…”

Genetic relationships between skarn ore deposits and magmatic activity in the Ahar region, Western Alborz, NW Iran

“…This magmatic activity was responsible for the contact metasomatic mineralization as well as the porphyry-type copper occurrences in NW Iran. As Figure 1 shows, the Gharah Dagh Formation is one of the important metallogenic provinces of the East Mediterranean Copper-Molybdenum belt (Bazin & Hübner 1969;Superceanu 1971;Rolland et al 2009;Jamali et al 2010). The associated intrusive bodies occur as batholiths as well as stocks, ranging in composition from quartz monzodiorite to granite, and the Ahar Batholith is the largest early Oligocene intrusion in the Gharah Dagh-Tarom plutonic belt (Lescuyer 1976).…”