STRATIGRAPHY AND PETROLAGY OF THE LOW-GRADE REGIONALLY METAMORPHOSED ROCKS OF THE EOCENE FORMATION IN THE ALBORZ RANGE, NORTH OF TEHRAN, IRAN

Abstract

Field investigation in 1965 and subsequent petrographic observations established the following stratigraphic division, in ascending order, of the Eocene submarine volcanic and pyroclastic complex called “Karaj Formation” (formerly “Green Series”): 1, Sarband Basic Volcanic Member; 2, Lower Tuff Member; 3, Middle Tuff Member; 4, Astara Shale Member; 5, Upper Tuff Member. These units form a thick pile of continuous accumulation, without stratigraphic breaks, of volcanic, muddy, and marly sediments, in which dacitic pyroclastic rocks are the major constituents of the upper and middle horizons and basic volcanics predominate in lower the horizons. In spite of the great total thickness of the Karaj Formation (more than 10, 000m), there is no distinct evidence of bathyal sedimentation and extrusion. Convolution and double-graded bedding of the acid pyroclastics are common. The volcanic rocks and pyroclastics have undergone low-grade regional metamorphism deriving from deep burial. The metamorphic rocks of the upper stratigraphic horizons correspond to a zeolitic facies. With transition of metamorphic features, they are underlain by rocks in which epidote and or prehnite are characteristic and pumpellyite is absent; petrographic features differ from those of greenschist facies. They can be regarded as a subfacies in the prehnite-pumpellyite-metagreywacke facies group. Laumontite predominates in pyroclastics only in the metamorphic halo coating the monzonite sills near Karaj dam. The Karj Formation is rather similar in major metamorphic features to the largest part of the “Green Tuff” Formation of Miocene age in Japan, but seems to have been metamorphosed under a lower geothermal gradient.