Estudo do vulcano--plutonismo paleoproterozoico ocorrente na região de Vila Mandi (PA), Cráton Amazônico

Abstract

The Amazon Craton is part of the South American Platform and represents one of the most expressive pre-Cambrian lands in the world. This tectonic mega-unit is covered by volcanic effusive and explosive Paleoproterozoic sequences that cover approximately 1.500.000 km2. Near the district of Vila Mandi, extreme south of the municipality of São Félix do Xingu (PA), this event is represented by predominantly felsic volcano-plutonic sequences, with subordinate intermediate types and associated sediments, little studied, but which can be correlated to other portions of the Amazonian Craton. In this way the objective of the work is to contribute to the characterization of the volcano-plutonic system that occurs in the region, with respect to its petrographic, geochemical and geochronological aspects. Two effusive and explosive vulcano-plutonic units, named Formations Cinco Estrelas and Vila Mandi, were identified in the study area. The basal unit Cinco Estrelas Formation is composed of basic volcanic rocks to effusive and subordinately explosive intermediates of subalcaline composition and reveals at least two distinct facies: 1) facies of subaerial mass lava flow, represented by intermediate rocks with foliation of horizontal magmatic flow; And 2) Vulcanoclastic facies with laminated ash tuff. The upper unit, Vila Mandi Formation, of subalcaline composition is composed of five facies: 1) lava flow facies of massive rhyolites; 2) facies of equigranular granitoid stocks; 3) felsic composite ignimbrite associated with welded and unalloyed tuffs of ash; 4) Mass polymorphic facies and subordinate lapilli-tuff and tuff crystals; 5) facies of dikes with granite porphyries. The geochemical data of these rocks show that the rocks of the Cinco Estrelas Formation have SiO2 content between 55.49 and 73.31% and K2O / Na2O ratios between 0.56 and 1.64. On the other hand, the volcanic ones of the Vila Mandi Formation comprise a more evolved suite, with SiO2 contents between 69.10 and 78.31% and very high K2O / Na2O (0.24 - 111.66) ratios. The basal unit exhibits a predominantly calcium-alkaline character, slightly transitional between calcium-alkaline to shoshonite, exclusively metal-based composition, A/NK ratios between 1 and 2, as well as geochemical affinity with volcanic arches granitoids. However, the Vila Mandi Formation presents transitional characteristics between metaluminous and peraluminous with A/NK ratio between 1 and 1.5 and tectonic affinity similar to Cinco Estrelas Formation. Although some differences between the rocks of the two formations, marked by the contents of Rare Earth Elements (ETR), there are also many similarities. The rocks of the two formations exhibit an enrichment of the Light Rare Earth Elements (ETRL) relative to the Heavy Rare Earth Elements (ETRP). The facies of Lava Flow and Vulcanoclastic Formation of the Cinco Estrelas Formation exhibit total low to moderate ETR contents (177.6 - 475.9 ppm), with average to non-existent Eu (Eu/Eu*= 0.69-0.93 ) For the two facies, thus characterizing a behavior compatible with the calcium-alkaline series. On the other hand, the lithotypes of the Vila Mandi Formation have more diversified patterns, probably linked to their polyphase evolution, with significant Eu (Eu/Eu*= 0.35-0.71) negative anomalies, pointing to the expressive fractionation of feldspars. In the normalized multi-element diagrams for MORB, the Cinco Estrelas Formation rocks show significant enrichment of the elements Sr, K, Rb, Ba and Th; Expressive impoverishment in Ta and Nb in relation to Th and Ce; Impoverishment of P in relation to Ce and Zr; And strong impoverishment in Ti, Y and Yb. The Vila Mandi Formation lithotypes have negative anomalies of Ta and Nb, although they reveal higher contents of these elements, in relation to the Vila Mandi Formation. They also show remarkable negative anomalies of Sr, Ba, P, Ti and Yb, which may reflect the fractionation of feldspar, apatite and Fe and Ti oxides in these units. The geological evolution of the Cinco Estrelas Formation is linked to phases of effusive volcanism of basic to intermediate composition and other explosive. On the other hand, the Vila Mandi Formation has polyphase evolution generated by crustal cracks that form conjugate pairs oriented in the NE-SW and NW-SE directions. The final phase involved effusive volcanism that allowed the accumulation of rhyolites with foliation of vertical flow and subordinate dikes of granitic porphyries and stocks of equigranular sienogranites that sealed the fissures. The U-Pb zircon ages were obtained in rhyolites of the Vila Mandi Formation of 1889 ± 4 Ma and 1983 ± 18 Ma, suggesting that the magmatism in the area is polyphase, formed by at least two distinct volcano-plutonic events separated by approximately 100 Ma.