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RESEARCH PRODUCT

Rhyolitic dykes of Paros Island, Cyclades

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Abstract The discovery of rhyolitic dykes from the NE part of the island of Paros is described here for the first time. The dykes that can be mapped for a length of ca. 1.1 km are striking about 6° and 38° NE. The width reaches up to 11m maximum. The rhyolitic rocks are porphyric with a fine-grained matrix of mainly feldspar, quartz, some biotites and opaques. K-feldspar, biotite and plagioclase occur as phenocrysts, and mafic-intermediate enclaves are common. At the chilled margin in contact to the gneissic country rocks the dykes are vitric. There, the enclaves have been preserved from contact reaction with the melt and alteration effects and show primary igneous minerals such as clinopyroxene and plagioclase. This implies that the enclaves are no xenoliths from the basement gneisses, but cogenetic with the dykes, probably from less differentiated parts of the magma. The dykes are leucocratic with 68.20-74.03 wt. % SiO 2 . They display fractionation trends in variation diagrams implying that the compositional variation is largely based on crystal fractionation from a parental magma. Since no other Neogene volcanics are known from Paros, we compare the dykes with rhyolites from Antiparos. The dykes are slightly less silicic than the Antiparos rhyolites. This corresponds to higher concentrations of Al 2 O 3 , Fe 2 O 3 (t), MgO, CaO and TiO 2 . However, because of higher concentrations of HFSE such as Zr, Nb, Y, and LILE such as Ba and LREE the dykes are unlikely to be comagmatic with the Antiparos rhyolites. In commonly used discrimination diagrams for granites ( Pearce et al., 1984 ) the Paros rhyolithic dykes are classified as WPG (within-plate granites). According to this discrimination the relation to the South Aegean Volcanic arc appears to be more complex and might involve melting or assimilation of continental material in the back-arc region.