GEOQUÍMICA E PETROGÊNESE DE ROCHAS METAVULCÂNICAS DE GREENSTONE BELTS ARQUEANOS: REGIÃO DE RIO MARIA SUDESTE DO PARÁ BRASIL

Abstract (English)

The Rio Maria region, located at about 250 km south of the Carajás Ridge, is a well preserved typical Archaean (> 2.87 Ga) granite - greenstone terrane. Supracrustal sequences are made up of meta-ultramafic rocks (UM - talc-tremolite schists with relict spinifex textures) at the base, followed by metabasic rocks (BAS - massif basalts with quenched textures; GB - porphyritic gabbros) and, at the top, metadacitic rocks (DAC) with plagioclase, quartz, hornblende and sphene phenocrysts. Terrigenous (graywackes, siltstones) and volcano-chemical metasediments (cherts, iron formations) are intercalated within the basal and intermediate stratigraphic levels. The volcanic rocks define three geochemical series, namely komatiite (UM), low-K tholeiite (BAS, GB) and sodic calc-alkaline (DAC). These series mainly evolved by low pressure fractional crystallization giving respectively olivine + orthopyroxene, clinopyroxene + plagioclase and plagioclase + quartz + alkali feldspar + hornblende + biotite + sphene cumulates. Modeling based on major and REE indicate a Iherzolitic source with 1-5% garnet; the degree of partial melting is from 10 to 25% in order to generate the komatiitic and tholeiitic magmas. The source of dacitic volcanism could be a metasomatised tholeiitic crust transformed into garnet arnphibolite, melted from 10 to 15% and leaving a residue with hornblende, plagioclase, garnet and clinopyroxene. The source types and the trace elements characteristics allow to consider that the magma generated in marginal basins and island arcs geodynamic environments. Consequently, the calc-alkaline magmatism (volcanism at ca. 2.97-2.90 Ga and plutonism at ca. 2.87 Ga) would represent a stage of tectonic stabilization and welding of crustal blocks, forming the continental crust which acted as cratonic basement for the Neoarchaean evolution of the northern portion of the Carajás Mineral Province.