Alkaline rocks from continental Brazil

Abstract

The present literature review on Brazilian alkaline rocks stresses mainly the petrographic and geologic aspects. At least eight different rock associations are represented. Agewise, four groups are found among southern Brazilian outcrops: an older one, about 240 m.y. only, that is still poorly known; the massifs clustered around Jacupiranga, about 130 m.y. old; a few occurrences 100–110 m.y. old (Tunas, Itapirapua); and the youngest and most important one, with an age spread from about 80 m.y. to about 53 m.y. Recent research shows a clear tectonic control — both on local and on regional scales — of alkaline occurrences in southern Brazil. Alkaline magmatism was set off by pre-drift and drift activity and is, at its onset, geologically related to basaltic activity. Emplacement of many massifs is also locally controlled by old lineaments (e.g., in the Minas Gerais-Goiás belt), as is emphasized by geophysical data. On a petrographic and chronologic basis, several alkaline provinces are distinguished in southern Brazil. Preliminary data on the Cambrian-Precambrian rocks from southern Bahia State (a province with undersaturated syenites and litchfieldites) and from cratonic areas in northern Brazil are also presented.