The mafic potassic intrusion of Pariquera-Açu, São Paulo, Brazil

Abstract

This paper describes the geological, petrographic, mineralogic and chemical characteristics of the mafic potassic intrusion of Pariquera-Açu (P-A), southern São Paulo. The studied rock assemblage comprises members of the high-K alkalic magma series. Nepheline, orthoclase and clinopyroxene are the essential minerals present in all samples examined microscopically. Olivine, biotite and magnetite are also present. Plagioclase is absent. The series range from nepheline syenite to alkali pyroxenite with strong dominance of shonkinites and malignites. Alkali diabase dykes were also sampled. The alkalic magma intruded Proterozoic phyllites along tensile fractures.The intrusion is conceivably shaped as sub-vertical ring dykes with internal occupancy of sub-horizontal magma sheets. Sharply discordant contacts and occurrence of low to medium angle sheets of shonkinite and of unstrained intrusion breccias, give support to the idea of a pipe-like magmatic body formed by ring-fracture, stoping of a roof slab and final magma injections. It is suggested that small degrees of partial melting of a phlogopite-bearing mantle peridotite could produce the high K2O magma responsible for the generation of the P-A rocks.