Geology and geochronology of Monte de Trigo island alkaline suite, southeastern Brazil

Abstract

The undersaturated syenite-gabbroid alkaline suite of Monte de Trigo Island lies in the northern sector of the Serra do Mar Alkaline Province in southeastern Brazil, and intrudes Precambrian gneissic and migmatitic rocks of the Ribeira Belt. It is made up by a cumulate mafic-ultramafic body, dominantly showing nepheline-bearing olivine melagabbros, melatheralites and clinopyroxenites, in association with synplutonic microtheralite and microessexite dykes, and by a miaskitic nepheline syenite stock genetically related to nepheline microsyenite dykes of miaskitic and agpaitic affinity. A magmatic breccia pipe having fragments derived from the mafic-ultramafic body and from the Precambrian country-rocks crops out in the eastern side of the island. The last magmatic alkaline event is represented by a series of lamprophyre-to-phonolite dykes. Geological evidences suggest a low emplacement level for the Monte de Trigo Island intrusion. Ar-Ar geochronological data for the different rock types yielded an average value of 86.6 Ma (Late Cretaceous), which is comparable to the age of other alkaline intrusions occurring in the northern sector of the Serra do Mar Province. The alkaline magmatism probably took place in a short time interval, lower than 0.5 Ma. Moreover, age incompatibility makes difficult to support the Trindade mantle plume model proposed for its origin. Thus, other factors such as pressure release and the influence of volatile phases in the source region might have contributed to the generation of the alkaline magmas.