Petrologia da sucessão magmática do Arquipélago de Abrolhos

Abstract

The Magmatic Succession of Abrolhos s Archipelago is composed by four units: Olivine-Plagioclase Basalt (b1), Pyroxene-Plagioclase-Olivine Basalt (b2), Pyroxene-Plagioclase Basalt (b3) and Cumulado (b4). The units are not stratigraphically correlated. They are stacked from the basis to the top in units: b1, b2, b3 and b4. The units of the magmatic succession are different in terms of texture, structure and mineralogy. The litogeochemical analyses of twenty three samples of basaltic rocks discriminated a transitional series of alkaline affinity to the studied rocks. The values of Nb/Y ratios higher than unity of the studied rocks corroborate the alkaline affinity of the series. The rocks were classified as basalts, tracky-basalts, basanites and tephrites (units b3 and b4), basalts and tracky-basalts (units b1 and b2). The interpretation of variation diagrams to major and trace elements together with the trace elements ratios of the representative sample of the parental and the more evaluated liquids indicated fractional crystallization without changing of fractionated assemblage as the evolutionary process of the studied transitional series. The incongruity between the fractionated assemblage and porphyries assemblage (recognized by the petrography) in addition to the indicative features of crystal-liquid disequilibrium discard fractional crystallization as the evolutionary process in the generation of the basaltic rocks of Abrolhos s Archipelago. More complex evaluative models should be considered as, for example, a refueling model of crustal chambers with subsequent pulses of basaltic magmas. The La/Yb (N) ratio >1 and La/Nb (N) ratio <1 indicated that the mapped units of Abrolhos s Archipelago are associated to a fertile mantellic source (plume-like type).