Niektóre problemy chemizmu wulkanitów dolnośląskich

Kazimierz Dziedzic

Abstract


Przedstawiono wybrane zagadnienia dotyczące wulkanitów hercyńskich i alpejskich obszaru dolnośląskiego.

Wulkanizm hercyński w przeciwieństwie do alpejskiego przyrównuje się do grzbietowych partii obszarów ryftowych z wysokim strumieniem cieplnym.

Rozmieszczenie K20 w skałach wylewnych wiąże się ze sposobem wznoszenia i akumulacji stopów.

Wzrost saliczności wyrażony indeksem dyferencjacji (Dl) wpływa na obniżenie wytrzymałości mechanicznej skał.

 

SOME PROBLEMS CONNECTED WITH CHEMISTRY OF LOWER SILESIA VOLCANIC ROCKS

 

Volcanic rocks of the Lower Silesia region are related to Hercynian and Alpine tecto-magmatic cycles. Hercynian volcanic rocks, represented by intermediate and acid varieties, were usually termed as melaphyres and porphyries and the Alpine ones – as basalts. Figures 1 and 2 give basic characteristics of these rocks as implied by their chemical composition. The sum of alkalis versus Si02 in; dicates that a marked part of these rocks fall within the field of alkaline ones (Fig. 2). In regard to the degree of saturation with silica, Hercynian and Alpine volcanic rocks may be compared with the Coombs and Kennedy trends (see A. Miyashiro, 1978), respectively, presumably characterizing nonuniformities in development in accordance with distribution of these rocks in volcanic areas.

With reference to regions of recent volcanic activity, the rift model is adopted for the Hercynian volcanism as the most close to the geological setting of the area. It is suggested that magmas of that cycle were originating due to heat induction by uplifted subcrustal masses in areas roughly corresponding to crest part of a rift. Both a gradual change in composition of volcanic rocks and a close correlation with coeval geological structures suggest that the magmas in their rise to the surface were passing through the stage of shallow-seated magma chambers, above which subsidence areas controlled by volcanism were situated.

In time of Alpine volcanism, the zone of high thermal gradient was situated beyond the extent of occurrence of basalts. The stage of magma chambers was presumably reduced here also. Magmas of that cycle were presumably migrating along pre-existing deep crustal fractures of the fracture zone type and, therefore, they appear not related to shallower geological structures.

Some regularities in distribution of K20 values in vertical section of layered magma bodies may be noted. The content of potassium increases upwards the section in melaphyres whereas the opposite trend may be noted in basalts (Fig. 4). Changes in content of potassium are due to migration of that element along with volatite components at the place of depogition of magma. Fluctuation of potassium in intervals equal 20 m and 18 m at the average in melaphyres and basalts, respectively, corresponds to stages in rise of a portion of magma, i.e. to individual cooling units. The recorded increase in content of potassium upwards the melaphyre section is related to intrusive nature of these rocks, for which siltstones were providing a screen impeding release of that element. Basalts were usually subaerial which made the release of potassium much easier. In infillings of volcanic necks, the content of potassium was found to be much lower than the average, implicating that distribution of this element also depends on the mode of migration and accumulation of magma.

The strength of rocks decreases along with increase in DI in the case of both individual groups and the whole population of volcanic rocks (Fig. 6). Taking into account negative correlation of the

Dl/W relation, it was shown that the rocks become progressively “weaker” (Fig. 7) along with increase in distance from their base, in accordance with Dl progression (Fig. 5). This is mainly determined by original agents acting in time of origin and solidification of magma.

Distribution of the studied volcanic rocks in A. Streickeisen (1978) double classification triangle is also given (Fig. 8).


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