FEATURES OF GEOSYNCLINAL VOLCANISM AS ILLUSTRATED BY THE SEVAN TECTONIC ZONE, LESSER CAUCASUS
Titel:
FEATURES OF GEOSYNCLINAL VOLCANISM AS ILLUSTRATED BY THE SEVAN TECTONIC ZONE, LESSER CAUCASUS
Auteur:
Mkrtchyan, K. A.
Verschenen in:
International geology review
Paginering:
Jaargang 6 (1964) nr. 1 pagina's 94-98
Jaar:
1964-01
Inhoud:
Material on the Sevan tectonic zone of the Lesser Caucasian system makes it possible to discern a pattern in the evolution of its volcanics, which may possibly be of general importance. The zone passed through a complete geosynclinal cycle in the Paleogene. Lower (?) - to middle Eocene effusive andesite-basalt, rhyolite, and spilite-keratoplayre thicken to 3,000 m in the axial portion of the trough, accompanied by gabbro-porphyrite and quartz-porphyrite subintrusives (sills), gabbropyroxenite stocks, and granite intrusions. Upper Eocene deposits are marked by effusive anciesitebasalt and trachyandesite, subvolcanic (subintrusive) gabbro-porphyrite and trachydacite, and intrusive complexes of syenite and gabbro. Stratigraphic relations of a rather complete section of associated, sediments and cross-cutting age relations of igneous rocks permit of rough estimates of formation depths of intrusive bodies. Together these indicate: 1) "subintrusive" and "abyssal" massifs are formed at comparable depths; 2) there is little difference in the fabric of intrusives of the sama series with depth differences of over 1,000 m-1,500 m, regardless of the absolute depth; 3) the Dzoraget trachyandesite, with abundant volcanic glass, was emplaced at depths of 1,200m -1,300 m, whereas the Garnassar alkaline syenite, fully crystallized, was emplaced at depths of 600m - 700m. It would appear that in addition to depth, other factors must affect the fabric of intrusives, such as volatiles, morphology, and conductivity of enclosing rocks. The two main igneous series show a definite order of magmatic evolution from acid to alkaline. This order, along with the simultaneous appearance of acid and basic effusives, suggest the existence of independent sources of acid and basic magmas. Rudy C. Epis
Tijdschrift beschrijving