Contact relationships between mafic and syenitic alkaline rocks in Jabaquara Beach, northern sector of São Sebastião Island, SP

Abstract

Contact relationships between syenitic rocks of the Serraria stock and plutonic to hypoabisal mafic rocks of the northern sector of the alkaline suite of São Sebastião Island are described here for the first time, allowing the indication of temporal relationships between different magmatic batches. The outcrops show a hololeucocratic syenitic variety hosting mega-, macro- and microxenoliths from different rocks forming agmatites. Five main rock units were described: alkali feldspar syenites, melasyenites, diabases, mafic cumulates (melagabbro/clinopyroxenite), and heterogeneous gabbros. Geochemically, the bimodal alkaline magmatism found on the island is well represented by the studied rocks. The sequence of magmatic events is as follows: emplacement and crystallization of basic alkaline magma batches in a magmatic chamber, forming a varieties of gabbroic cumulates, mainly olivine-bearing melagabbros and clinopyroxenites, and breccias with gabbroic matrix with evidences of textural reequilibrium by thermal action; new intrusions of alkaline basic magma batches, represented by diabase fragments, which also present textural reequilibrium (predominant granoblastic texture); syenitic magma intrusion in the mafic chamber, which fragmented the already partially or fully crystallized gabbro, carrying a large proportion of xenocrysts and xenoliths, and forming melasyenite; new syenitic magma intrusion, which fragmented the whole system forming an agmatite with megaxenoliths and microxenoliths of cumulative gabbro, diabase, agmatite with gabbroic matrix (heterogeneous gabbros) and melasyenite; pegmatite veins and dykes of quartz syenites crosscutting all lithotypes. All types sampled as xenoliths have some degree of recrystallization and chemical reaction with the final syenitic host magma.