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Trachyandesites of the island Vis (Adriatic Sea, Croatia)

The island Vis is dominately composed of the carbonate rocks. However, in its western part near the small town Komiža occur volcanic rocks having trachyandesitic to andesitic compositions. This trachyandesite rocks are highly crystalline consisting of many plagioclase and augite phenocrysts of variable size. The groundmass contain the same minerals as microphenocrysts, titanomagnetite, cryptocrystalline chlorite aggregates and fine crystallized K-feldspar, albite and quartz. The augite phenocrysts are homogeneous in compositions (En_42Fs_20Wo_38) except for the 50 μm narrow rims (En_34Fs_32Wo_34) showing resorption phenomena. Plagioclase phenocrysts are characterized by the wide cracked, mostly replaced, inclusion-filled core and clear rims of 20-80 μm thickness. The core (An_82 to An_75) is replaced by sericite, albite and pumpellyite and contains inclusions of titanomagnetite and augite (En_34Fs_32Wo_34). The clear rims show normal zoning (An_68 to An_45). The plagioclase microphenocrysts of the groundmass are fresh and zoned ranging between An_84 and An_49. The groundmass augite has almost identical composition (En_34Fs_31Wo_35) as resorbed rims of augite phenocrysts. The described textural and chemical relationships may point to the intermittent rising of small batches of magma causing interaction with the prexisting crystal mush. On the diagram of alkalies versus wt.% SiO_2 studied trachyandesites plot along the dividing line between alkalic and subalkalic magma. This rocks are characterized by high contents of Al_2O_3 (18.65-19.89 wt%) and alkali elements (Na_2O-3.95 wt%, K_2O-2.13 wt%). The MgO contents (3.02-3.35 wt%) are low and (Fe_2O_3)_T is less than 10 wt%. MORB normalized trace element patterns of studied rocks display enrichments in LIL elements and slightly depletion in HFS elements. Except of a remarked Eu anomaly the REE pattern show strong similarity with the lower crust chemical signature.

trachyandesite; Vis

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