Permskie skały wulkaniczne w rejonie Bolkowa

Abstract

Przedstawiono geologiczną formę wystąpienia skał eruptywnych oraz rozpatrzono ich zmienność, a także relację do utworów podścielających. Na podstawie badań petrograficznych, chemicznych i technologicznych stwierdzono, że wulkanity z obszaru Bolkowa, znane w starszej literaturze jako "porfiry kwarcowe", są skałami o strukturze porfirowej z prakryształami kwarcu, skaleni i biotytu. Kwaśny plagioklaz przeważa nad kwarcem i ortoklazem, zawartość minerałów femicznych nie przekracza 6,5%. Zgodnie z obecnie stosowaną nomenklaturą petrograficzną są to ryodacyty. Wulkanity te porównano z podobnymi skałami rejonu Świerzawy opracowanymi wcześniej. W obszarze Bolkowa wydzielono dwa cykle wulkaniczne. Do pierwszego zaliczono . tufy ryodacytowe, a do drugiego - ryodacyty. Z ryodacytami wiąże się mineralizacja hydrotermalna.

PERMIAN VOLCANIC ROCKS IN THE BOLKOW AREA

The paper presents the results of geological, petrographic, chemical and technological studies on rhyodacite from the Bolków area, related to Hercynian volcanic series from the Lower Silesia. Analysis of borehole data showed that in the studied part the rhyodacite consists of a number of lava flows.

In the borehole column II, there were found five successive lava flows, ranging from 10 to 35 m in thickness. The flows differ from one another in texture, content of pracrystals, plate-like separateness and massiveness of rocks. Taking. into account differences in structure and texture, two varieties of rhyodacite are differentiated: coarse-grained. with random texture, and fine-grained, with fluidal texture. The former characterized by porphyry texture with pracrystals of quartz, feldspars (orthoclase, oligoclase) and biotite, predominates here. Precrystals are altered: quartz is usually fractured and it displays effects of igneous corrosion and feldspars also show hydrothermal alteration. Oligoclase quantitatively predominates on quartz and orthoclase and content of mafic minerals is below 6.5%. According to chemical analyses, the rocks may be classified as rhyodacites in currently used petrographic subdivisions.

The studies showed similarities of volcanic rocks from Bolków and those from the Świerzawa region, suggesting their genetic affinities. The record of two horizons of acid eruptive rocks suggests that we are dealing with two volcanic cycles in the Bolków area, similarly as in the Świerzawa region. Volcanic sequence from Bolków comprises rhyodacite tuffs and rhyodacites. The former are assigned to the cycle I, and the latter - to the cycle II. Rhyodacities are accompanied by hydrothermal mineralization, resulting in origin of barite vein (with barite content equal 98.44% BaSO₄). The studied rhyodacites occur in upper part of the Rotliegendes (Autunian) and they represent a component of eruptive member, assignable to the diastrophic-sedimentary cycle III.