Studia petrograficzne wulkanitów z Wisznic k. Włodawy

Abstract

Otwór wiertniczy Wisznice IG 1 stanowi pierwszy punkt odkrycia przejawów wulkanizmu zasadowego na Wschodzie Polski. Wulkanity spoczywają tu bezpośrednio na utworach eokambryjskich, a przykryte są osadami jury środkowej i górnej. Badania optyczne i oznaczenia chemiczne wskazują, żę są to bazalty o charakterze melanokratczym i należą do najstarszych skał wulkanicznych tego obszaru. Wulkanity wisznickie wykazują znaczną analogię z wulkanitami Białorusi i zachodniego przedpola masywu ukraińskiego, co pozwala sądzić, że skały z Wisznic stanowią fragment formacji trappowej rozwiniętej na tych obszarach.

PETROGRAPIUC STUDIES OF VOLCANIC ROCKS FROM WISZNICE NEAR WŁODAWA

The presence of basic volcanism in eastern part of the Polish Lowlands has been found for the first time in the borehole Wisznice IG l. In that borehole, volcanic rocks overlaying thermically changed sandy-conglomeratic Precambrian rocks were recorded beneath the Jurassic at the depth from

375.3 to 416.5 m (Figs. 1, 2).

Tuffites underlaying the Jurassic (375.3-379.2 m) are mainly built of pelitic deposits (Table 1) with kaolinite (Fig. 3) as one of components. Sandy fraction of tuffites comprises feldspars, dipyramidal quartz, glass, mica, pyrite, and fragments of strongly alterated rocks of the porphyry and orthofelsite character.

Tuff (379,2 - 390,5 m) displays compact texture and in several places, slightly marked bedding. It mainly consists of fine-grained material. Kaolinite predominates here but chlorites, celadonite, quartz, volcanic glass, olivine, and fine fragments of blister lava are also present Small fragments of eruptive breccia, porous green-coloured basalt as well as slaggy breccias were found in lower part of the tuff. Tables 2 - 5 show chemical and mineral (normative) composition of the tuff, ratios of characteristic oxides as well as the results of spectral analyses.

In depth interval 390.0 -403.6 m, there were found agglomerates, and eruptive breccias strongly cemented with tuff and slaggy material. Porous, green coloured basalt and brown-black basalt were recorded. They strongly vary in both shape and size. Green basalt consists of olivine, diopside augite, feldspars, glass, magnetite and large group of chlorites and it is strongly alterated.

Black basalt, occurring at the depth from 403.6 to 416.5 m, is varying in texture. It is porous in the upper part and compact and fine-grained at the contact with underlaying sedimentary rocks. Mineral components are the same as in green basalt but better preserved. The content of olivine ranges from 3.9 to 11.1 volume per cent, post-gas voids - from 6.1 to 37.7%, and cement (interstices) from 60.4 to 82.3%, and the amount of chlorite is lower than in green basalt Tables 2, 3, 4, 6 show the chemical and mineral (normative) composition, characteristic values and the results of spectral analyses.

ChIorophaeite occurring at surfaces of fractures in black basa1t (depth 408.3 m) is fine-crystalline, with bulk density equaI 2.29 Gjcm3,and variable refraction coefficient (1.549-1.577). X-ray measurements showed that the most intense lines include 4.52, 3.02 and 1.545. Chlorophaeite from Wisznice corresponds to that of Volhyn basalts, being also close to saponite from Markowice in Czechoslovakia. It is of hydrothermal origin (Fig. 6, Table 6).

The comparison of volcanic rocks from the borehole Wisznice IG I with those known from other boreholes in the Polish Lowlands showed that the former correspond only to those from the borehole Kapłonosy IG I (depth 1720-1773 m), also resting on the Precambrian crystalline platform. Basalts are melanocratic; they have crystallized from juvenile magma and after that they were subjected to hydrothermal processes. They represent the oldest member of effusive facies. The results of geochemical studies of basalts from these boreholes (H. Szczepanowski, 1977) made possible to assign them to the oldest variety. In both boreholes two facies occur - effusive and eruptive.

Volcanic rocks from Wisznice correspond to those from Byelorussia (boreholes Miednoj in NE part of the Sławatycze elevation and Bor in the Ratno elevation) in chemistry but in the latter area the two facies are accompanied by the intrusive one (gabbro, gabbro-diabases) and volcanic vent is of the diatreme type, not known from the Wisznice area.

In comparison with volcanic rocks from Volhyn, Wisznice basalt corresponds to the oldest melanocratic basalt from the Mutwica stream near Wielkie Mydzko whereas tuff appears to be related to that from Berestowiec (Table 2). Volcanic rocks from Wisznice belong to the trap formation described by Soviet geologists from western part of the Volhynian - Ukrainian Massif and the Prypet' Swell.