Neoproterozoicka vulkanoklastika na uzemi jihozapadnich Brd: produkty submarinni vulkanicke aktivity

Occurrences of Neoproterozoic volcaniclastic rocks in the Brdy area (1: 25,000 map sheet 22-121 Mirošov) were newly discovered in the Barrandian Neoproterozoic volcano-sedimentary successions with prevailing basaltic and basalt andesitic volcanites. They exhibit interesting textures and structures. The matrix in some rock samples consists mostly of fragments of ragged scoria and lava, xenocrysts and opaque minerals. Pyroclastic material has mostly grain size of ash up to lapilli. Some scoria particles are highly vesicular. Peperite and hyaloclastite textures were also observed. They are formed by scoria and lava fragments and palagonite within unsorted matrix of sandstone, greywacke and shale. Other type (facies) of volcaniclastic rocks, probably redeposited, exhibits chaotic structure with clusters (size cm up to dm) of greywacke in scoria or devitrified matrix with admixture of ilmenite and fragments of crystals, and also xenocrysts and xenoliths. Some of the studied volcaniclastic rocks are though to have been formed by redeposition of syn-eruptive volcaniclastics which in the studied area are accompanied by metabasalts, including pillow lavas and dolerites, and by small bodies of silicite of hydrothermal origin with brecciated stromatolitic textures. The newly found and studied volcaniclastic rocks are related to metabasalts and to other volcanic rocks together with hydrothermal silicites. Volcaniclastic rocks are part of associations of primary effusive volcanogenic facies originated within eruptive volcanic and hydrothermal paleoenvironment in the frame of an accretionary wedge.

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