Trachity magnezowe z Zawiercia

Abstract

 Autorzy wykonali badania mikroskopowe, rentgenograficzne, spektroskopowe w podczerwieni i chemiczne wybitnie wzbogaconych w potas trachitów nawierconych w Zawierciu, dostarczając nowych faktów do ciągle otwartego problemu tzw. kalifikacji niektórych skał magmowych obszaru krakowsko-śląskiego.

POTASSIUM TRACHYTES FROM ZAWIERCIE

Two trachyte dykes characterized by exceptionally high content of potassium (up to about 16 weight % K₂O) and negligible content of sodium (up to about 0.6 weight % Na₂O) were encountered in the Devonian section by the borehole ZMZ-87 in Zawiercie (about 70 km NW of Cracow). The enrichment in potassium appears related to marked predominance of potassium feldspars in mineral composition of both praecrysts and ground mass of trachytes. The feldspars are accompanied by subordinate amounts of biotite and quartz and accessory minerals apatite, magnetite and titanium oxides only. Secondary components mainly include mixed-layer illite-montmorillonites chalcedony and quartz. Plagioclases are not present among praecrysts, in groundmass, and also in the form of pertite ingrowths in K-feldspars. Chemical composition of the feldspars does not differ much from theoretical composition of K-feldspars and they are characterized by low ordering of Si/AI in crystalline lattice, corresponding to “low-temperature sanidine”, and optical inhomogeneity related to their aggregate structure.

Taking into account chemical and mineral composition (predominance of K-feldspars, lack of plagioclases, and low content of quartz), the rocks may be assigned to sanidine trachytes in the modern international subdivision or potassium trachytes, i.e. orthophyres in the old one.

The studied rocks may represent a product of exceptionally high enrichment in potassium magma differentiate originally rich in that element due to additional saturation, e.g. at the deuteric stage. However, the origin of these rocks due to potassium hydrothermal metasomatosis of rocks of the rhyolite-dacite group can not be excluded.