The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists

Petrographical and chemical studies of the Paleozoic volcanic rocks in the Kitakami mountainland were carried out. The rocks from the lower Carboniferous show the assemblage of greenschist facies and contain secondary albitized low-temperature form plagioclase. The Permian volcanic rocks have remained in the grade lower than greenschist faceis, having primary high-temperature form plagioclase.
Chemically, volcanic rocks of the pre-Upper Devonian Motai Group belong to alkali basalt, those of the Arisu Formation in the lower Carboniferous belong to tholeiite and those of the Ubaishi Formation in the Permian belong to high-alumina basalt. Volcanic rocks of the post Carboniferous in the district are rather rich in K2O and poor in TiO2, and have no oceanic character.

View full abstract

Show abstractHide 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.

View full abstract

Download PDF (1051K)

Register with J-STAGE for free!