The Armenian Ophiolite: insights for Jurassic back-arc formation, Lower Cretaceous hot spot magmatism and Upper Cretaceous obduction over the South Armenian Block

Abstract

Similar geological, petrological, geochemical and age features are found in various Armenian ophiolitic massifs (Sevan, Stepanavan and Vedi). These data argue for the presence of a single large ophiolite unit obducted on the South Armenian Block (SAB). Lherzolite Ophiolite type rock assemblages evidence a Lower–Middle Jurassic slow-spreading rate. The lavas and gabbros have a hybrid geochemical composition intermediate between arc and Mid Ocean Ridge Basalt (MORB) signatures which suggest they were probably formed in a back-arc basin. This oceanic sequence is overlain by pillowed alkaline lavas emplaced in marine conditions. Their geochemical composition is similar to plateau-lavas. Finally, this thickened oceanic crust is overlain by Upper Cretaceous calc-alkaline lavas likely formed in a supra-subduction zone environment. The age of the ophiolite is constrained by 40Ar/39Ar dating experiments provided a magmatic crystallization age of 178.7±2.6 Ma, and further evidence of greenschist facies crystallization during hydrothermal alteration until c. 155 Ma. Thus, top-to-the-south obduction likely initiated along the margin of the back-arc domain, directly south of the Vedi oceanic crust, and was transported as a whole on the SAB in the Coniacian times (88–87 Ma). Final closure of the basin is Late Cretaceous in age (73–71 Ma) as dated by metamorphic rocks

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

“…New evidence for an intra-oceanic subduction is described in detail in a companion paper (Rolland et al 2010), consequently we point out and summarize here the main results.…”

“…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).…”

“…The main parts of the ophiolitic units are very well preserved excepted in some shear zones, especially at the base of the obducted sequence, in which some low greenschist facies metamorphic crystallisations occur. The ophiolites record a low metamorphic grade imprint related to heterogeneous hydrothermal alteration dating from oceanic stages (Rolland et al 2010). Even in the Stepananvan area, the main ophiolitic body is unmetamorphosed.…”

“…2). They have slightly different structures as a result of their lithologic and structural settings, but are thought to be remnants of a single obducted sequence (see Rolland et al 2010). The ophiolitic series of the North Sevan Lake, Amassia and Stepanavan (Fig.…”

Alteration Zones Associated With Eocene Magmatism in the Olur (Erzurum) 9 Area, Eastern Pontides and Their Significance

“…and in the Minor Caucasus (Galoyan et al, 2007(Galoyan et al, , 2009Rolland et al, 2009bRolland et al, , 2010Sosson et al, 2010), the formation and settlement of ophiolites is very important. The presence of metallogenic provinces along the Alpine Orogenic Belt has been described by scientists (Moore et al, 1980;Yiğit, 2006Yiğit, , 2009.…”