Neoproterozoic and Paleozoic suprasubduction regional metamorphism, granitoid magmatism and geodynamics of the Caucasus

The Caucasus represents complicated polycyclic geological structure involving mountain fold systems of the Greater and Lesser Caucasus and adjacent foredeeps and intermountain troughs. Paleomagnetic, paleokinematic and traditional geological data indicate that within the oceanic area of Tethys in geological past relatively small continental or subcontinental plates (terranes) were situated having various geodynamic nature and characterized by specific lithologic-stratigraphic section and magmatic, metamorphic and structural features. During the Neoproterozoic, Paleozoic and Early Mesozoic they underwent horizontal displacement in different directions within the oceanic area of Proto-, Paleo-and Mesotethys (Neothetys) and as a result of Variscan, Early Kimmerian, Bathonian and Austrian orogeny underwent mutual accretion and ultimately joined the Eurasian continent. South of the Scythian platform (Sp) the Greater Caucasian (GC), Black Sea-Central transcaucasian (BC), Baiburt-Sevanian (BS) and Iran-Afghanian (IA) terranes are identified in the Caucasian segment of the Mediterranean mobile belt, which in geological past represented island arcs or microcontinents.
   In modern structure they are separated by ophiolite sutures of different age, which mark the location of small or large paleooceanic basins. All terranes of the East order (superterranes), as well as the southern edge of Sp are characterized by manifestation of polymetamorphism, though in various terranes, separate stages of regional metamorphism established by geological observation, but mostly confirmed by isotopic-geochronological data (K-Ar, Ar-Ar, Rb-Sr,U-Pb, Sm-Nd), became unequally apparent. For instance, Grenville regional metamorphism is observed only in GC (T - 700-750C, P - 3.2-3.5kbr), BC (T - 600-700°C, P - 3.2-3.5kbr) and IA (T - 500-550°C, P - 3.8kbr) terranes, whereas the Baikalian - only in Sp (T - 300-400°C,) and IA (T - 460°C, P - 4kbr) terrene. Late Baikalian metamorphism took place only in GC (T - 430-540°C, P - 3.3-3.5kbr) and BC (T - 540-570°C, P - 2.5kbr) terranes. Caledonian regional metamorphism strictly characterizes Sp (T - 700 ± 50°C, P - 17.8 ± 4kbr) and GC (T - 500-620°C, P - 2.2-2.8kbr) terranes. Early Variscan (Bretonian) metamorphism is observed almost in all terranes of the Caucasus: GC (T - 350-630°C, P - 1.35-2.7kbr), BC (T - 320-380°C, P - 1.5-1.8kbr), BS (T - 330-550°C, P - 1.5-2.6kbr) excluding IA terrene, and southem edge of Sp. Late Variscan metamorphism also comprises almost the whole Caucasus (T<430°C, P<1.4kbr) excluding Sp and IA terranes. Within separate terranes, as well as in Sp, synchronously or almost synchronously with principal stages of metamorphism (connected with main phases of tectogenesis) formation of pre-synand postmetamorphic granitoids of different type took place. Based on a vast analytical material concerning the petrogenic and rare elements in pre-Alpine granitoids of the Caucasus, it is established that the Neoproterozoic granitoids, occurring in all terranes excepting SP, are represented mainly by the crustal and upper crustal formations of the subduction mantle-crust and mantle island arc categories.
   The Late Baikalian granitoids are developed in GC and BC only. They are represented by the subduction mantle-crust and crust-anatectic categories. The Caledonian granitoids, cropping out only on SP, are represented by the subduction mantle-island arc formations formed with participation of the mantle and lower crustal material, and also subduction granitoids emerging due to melting of the immature continental crust. The Bretonian granitoids appear only in GC and BS. Granitoids of GC as whole correspond to the upper crustal granitoids of the other regions of the world, whereas the granitoids of BS are represented by the subduction formations of the mixed mantle-crust category; sialic part of the continental crust has an insignificant role during their formation. The late Variscan (Sudetian) granitoids are present in all terranes, excluding IA. The Sudetian orophase is the time of post-metamorphic potassium granitoid formation and consequently of true continental crust. The Sudetian granitoids of all exposures are characterized by similar composition, pelrogeochemical parameters and geodynamic conditions of formation. Their major part corresponds to the upper crustal formations, and the minor part - to the common crustal ones. The observed occurrence of different age and diversity of endogenic activity in various terranes of the Caucasus show asynchronism of episodic activity of subduction zones on different sides of oceanic basins separating these terranes.