Charakterystyka petrograficzna melanokratycznej skały bazaltowej z Różanej (Góry Kaczawskie)

Czesław Juroszek

Abstract


W Różanej stwierdzono nek ankaratrytu, bezplagioklazowej skały bazaltowej. Obecność enklaw Iherzolitowych sugeruje powiązanie ankaratrytu z górnym płaszczem Ziemi. Powiązanie to potwierdzają istotne różnice składu mineralnego enklaw oraz ankaratrytu. Skład chemiczny wskazuje na duże podobieństwo do skał bazaltowych okolic Lubania ŚI. i przynależność do czesko-śląskiego szeregu dyferencjacyjnego. Różnice zaznaczają się w zwiększonej zawartości AI203 i mniejszej K20 oraz Na20.

PETROGRAPHIC CHARACTERISTICS OF MELANOCRATlC BASALT ROCK FROM RÓŻANA (GÓRY KACZAWSKIE MTS)

A melanocratic basalt rock identified as ankaratrite forms a chimney vein discordantly cutting metamorphic basement, Rotliegendes sediments and Permian eruptive series. In the north and south its marginal parts display chimney breccia consisting of numerous fragments of cover rocks as well as crystals of basalt hornblende, up to 1.5 cm in size. A columnar jointing with a dip of 10° t0 15° to the west is marked in western part of quarry, where the basalt rock contacts Permian trachybasalts or their tuffs. In that part of the quarry the rock is rich in olivine bombs.

Ankaratrite shows porphyritic structure with cryptocrystalline or (in marginal parts) hypocrystalline groundmass. The groundmass mainly consists of monoclinic pyroxene microlites and iron oxides. Its subordinate components include olivine, nepheline, and glass. The share of the latter markedly in-creases in marginal parts of ankaratrite body. Praecrystals are represented, by olivine and augite, accopanied by single fine grains of strongly corroded chromium spinel (picotite). Olivine, predominating here, is poor in foyaite particle so its secondary alterations were not accompanied by usual separation of iron oxides.

The rock displays enclaves of lherzolite with olivine content equal 72.3%, and rhombic (16.5%) and monoclinic pyroxene. Such composition suggests its affinity with Earth mantle rocks. The enclaves do not represent cumulates related to differentiation of magma as they clearly differ from ankaratritc minerals. Olivine present in the enclaves does not form authimorphic forms but shows twinnings. Monoclinic pyroxenes arc strongly fractured, do not show banded structure but rhombic (unknown in ankaratrite), and react with rock melt, which is evidenced by rims. The rims display needle-like ore minerals which mark extensions of traces of pyroxene cleavage.

Chemical composition of ankaratrite indicates its affinity to the Czech-Silesian differentiation series. The rock appears similar to basalts from the vicinities of Lubań Sląski, from which it differs in higher content of AI2O3 and smaller of K20 and Na20.


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