Indications of volcanic style of the Silurian Nová Ves Volcano (Hemrovy Rocks)

Abstract

Studied locality of Hemrovy Rocks belongs to Nová Ves Volcanic Center, one of the Silurian volcanic centers of Prague Basin of Teplá-Barrandien area (lower Wenlock – lower Ludlow age). The aim of this work is to describe details of the volcanic rock structures and to contribute to the interpretation of the Silurian volcanic style. The macroscopic and microscopic structures of basaltic rocks in and close to the abandoned Kační Quarry in the southern part of Prague and at the adjacent locality – ridge of Hemrovy Rocks were studied. At these sites, volcaniclastics dominate over solid volcanic rocks and sediments. The volcanic rocks are represented by basalts, which form thin lava flows overlying both the volcaniclastics and sedimentary rocks. Massive fine-grained basalts pass to pillow lava facies, autobreccia or hyaloclastite breccia in-situ. These brecciated structures evolve into peperites or to unconsolidated volcaniclastic rocks of a previous eruption. Coarse-grained volcaniclastics with oversized subrounded to angular clasts of vesiculated lava or sediments within glassy matrix, sporadically with sediment admixture, are unsorted and thick-bedded. They were probably formed by gravity-driven mass flow. Basalt in the Kační Quarry probably represents a synvolcanic intrusion, as documented by its hyaloclastite rim and presence of clusters of resedimented volcaniclastics in the adjacent fine-grained sediments. Matrix of the volcaniclastics is mostly unsorted, formed mostly by quenched fragments of altered palagonitized and chloritized basaltic glass, locally scoriaceous, and fragments of chilled basaltic lava. Originally highly porous volcaniclastics, resp. hyaloclastites were secondarily cemented by calcite and silica. Secondary chlorite, calcite and silica also fill vesicles and cavities in fragments of glass and lava clasts. Dense vesicularity of lava fragments in volcaniclastics indicates an effective explosive interaction of lava with water. It is probable that much of the wet volcaniclastic material repeatedly slumped back down into the volcano’s crater to be re-ejected by subsequent phreato-magmatic explosions. Hydroclastic fragmentation was iniciated by repeated subaquatic eruptions in shallow subphotic marine zone. Unsorted crinoidal packstones mostly with trilobites, trepostomate bryozoan and volcaniclastic matrix were deposited in the vicinity of the Nová Ves Volcanic Center.