engleski

Dolerites from the islet of Brusnik (Adriatic Sea, Croatia)

The islet Brusnik, located about 30 km southwest of island Vis in Adriatic Sea, is completely formed by subvolcanic rocks of basaltic andesite composition. Conglomerates containing only pebbles of studied rocks are situated at the top of the islet. Subvolcanic rocks are medium grained and mostly characterized by groundmass having ophitic to subophitic texture. The main mineral phases are augite, zoned plagioclase ranging from anorthite to andesine and rarely pigonite. In the groundmass present amoeboid shapes of magnetite, titanomagnetite, ilmenite and sphene, interstitial micrographic patches of K-feldspar and quartz, interstitial albite and apatite as also secondary amphiboles varying from actinolite, ferro-actinolite to magnesio-hornblende all point to late stage crystallization. The occurrence of chlorite (brunsvigite, pycnochlorite to diabantite), sericite, prehnite and zeolite can be ascribed to circulation of low grade metamorphic fluids. The investigated rocks are characterized by silica content varying from 52.67 to 53.22 wt%, high Al_2O_3 (16.73-19.77 wt%) and low K_2O (0.44-1.12 wt%) content. Generally, the TiO_2 (~ 0.9 wt%) and MgO contents (2.55-4.35 wt%) are low, and (Fe_2O_3)T is less than 10 wt%. K_2O/SiO_2 ratio (wt%) suggest that the studied rocks are product of subalkaline magma. High contents of Al_2O_3 and alkali elements point to their calc-alkaline character. MORB normalized trace element patterns of studied rocks display enrichments in LIL elements and slightly depletion in HFS elements. Major and trace elements analyses of studied rocks are very similar to other triassic magmatic rocks of Alps, Dinaride and Hellenide which originated in an extensional environment without development of the real rift.

dolerite; Brusnik

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