Magmatismo mafico nella regione di Carajàs (Parà Meridionale, Brasile).

This study examines the petrological characteristics of the basaltic dykes of the Carajás region and of a few samples of the older Archean flood basalts (Grão Pará). The age of the mafic dykes is unknow, although they are Proterozoic. They have predominant trends NE-NNE (NED) and NW-NNW (NWD); more rare is N-S (NSD). Moreover, basaltic sills (SB) occur at the Salobo mine in drill holes. NED and SB dykes are tholeiites and andesite basalts respectively; NWD and NSD dykes are principally transitional basalts, two NWD dykes are trachyandesites. Alltogheter, the dykes span a fractionation range between mg = 0.63 and mg = 0.36, dominated by clinopyroxene, plagioclase and olivine. Consistent compositional differences between various dykes groups not caused by fractionation as shown by incompatible element ratios (e.g. Ba/Nb ane Ba/Rb) and element patterns. These differences derive in part from different melting degrees of a garnet peridotite source and in part reflect complementars processes of crustal contamination and/or mantle metasomatism. The Grão Pará basalts are intercalated with rhyolites, dacites and metasediments and have an U/Pb age of 2758±39 Ma (WIRTH, GiBBS & OLSZEWSKI, 1986) on a rhyolites zircon. They have suffered green-schist metamorphism, which has partially trasformed the mafic minerals into chlorite and actinolite. Compositionally they are latibasalts and andesite-basalts and have suffered fractionation of plagioclase and clinopyroxene; their mg number varys from 0.61 to 0.55. They are LILE and LREE enriched, expecially in Ba (2850 ppm) and show negative Nb anomalies. Their overall chemical trends and characteristics indicate calcalcaline affinity.