MINERALOGICAL, PETROLOGICAL AND GEOCHEMICAL ASPECTS OF ALKALINE AND ALKALINE-CARBONATITE ASSOCIATIONS FROM BRAZIL

A general description of Mesozoic and Tertiary (Fortaleza) Brazilian a lkaline and alkaline-carbonatite districts is presented with reference to mineralogy, petrology, geochemistry and geochronology. It mainly r efers to scientific results obtained during the last decade by an Ital o-Brazilian research team. Alkaline occurrences are distributed across Brazilian territory from the southern (Piratini, Rio Grande do Sul St ate) to the northeastern (Fortaleza, Ceara State) regions and are main ly concentrated along the borders of the Parana Basin generally coinci ding with important tectonic lineaments. The most noteworthy character istics of these alkaline and alkaline-carbonatite suites are: (i) prev alence of intrusive forms; (ii) abundance of cumulate assemblages (min or dunites, frequent clinopyroxenites and members of the ijolite serie s) and (iii) abundance of evolved rock-types. Many data demonstrate th at crystal fractionation was the main process responsible for magma ev olution of all Brazilian alkaline rocks. A hypothesis is proposed for the genesis of carbonatite liquids by immiscibility processes. The inc idence of REE and trace elements for different major groups of lithoty pes, belonging both to carbonatite-bearing and carbonatite-free distri cts, are documented. Sr and preliminary Nd isotopic data are indicativ e of a mantle origin for the least evolved magmas of all the studied o ccurrences. Mantle source material and melting models for the generati on of the Brazilian alkaline magma types are also discussed.