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Autori: Constantina C., Szakács A., Pécskay Z.
Editorial: Carpathian Journal of Earth and Environmental Sciences, 4, 1, p.31-47, 2009.
This paper proves for the first time the existence of a complex distribution of volcanic rock types occurring in the Gurasada region. At the base of these a bentonite deposit crops out in the Gurasada open pit, followed by pyroclastic deposits represented by pyroclastic breccia and rare tuff levels the youngest rocks are lava flows exposed in the Runcşor Hill. Thermometamorphic products represented by hornfels are described here for the first time within the outcrop area of the pyroclastic breccia. The site observations, petrographic, XRD, RAMAN spectroscopy and chemical analyses indicated the formation of the bentonite deposit by weathering of a vitroclastic tuff, probably of dacitic composition, belonging to a first (more felsic) eruptive stage consisting of Plinian-type explosive volcanism. The pyroclastic breccia, tuffs and lava flows are mainly of calc-alkaline andesitic composition and belong to a second stage of volcanism, which developed intermittently, over a longer period of time. Radiometric dating (K/Ar method) of the rocks indicates the formation of the volcanic-volcaniclastic deposits within a period of 69 to 80 million years ago. These ages confirm that these volcanics belong to the laramic cycle as inferred previously place them at the level of Upper Cretaceous (Senonian).
Cuvinte cheie: Andesite, volcaniclastics, Laramian, bentonite