Abstract
The rocks that constitute the Arvoredo Island are alkaline granites (Arvoredo Granite) that are cut by basaltic andesite dykes with variable thickness, striking N10°-20°E. This dykes belongs to Florianopolis Dyke Swarm (Cretaceous age, gotten by Ar 40 /Ar 39 data in previous paper - Tomazzoli et al., 2005) and are associated to rhyodacites and dacites that can occur as simple dykes, composite dykes or as hypabissal bodies of irregular geometry. The composite dykes are constituted by rhyodacite in the center with borders of basaltic andesite. Composite dykes and rounded enclaves of basaltic andesite in rhyodacite are features that indicate magma mingling process. The basaltic andesites and rhyodacite are high-K tholeiitic rocks with high Ti, P and uncompatible trace element contents. Geochemical modeling shows that rhyodacites can be derived from basaltic andesites by fractional crystalization of 61% of solids, represented by andesine (45.1%), edenite (16.5%), augite (18.7%) magnetite (9.8%) and ilmenite (6.8%). The magma mixing/mingling process could have occurred after the fractional crystallization. The geochemistry of rhyolite dykes and aplite veins differ from that of the tholeiitic rhyodacites/dacites and is similar to Arvoredo Granite.
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