Magmatismo granitóide arqueano da área de Canaã dos Carajás: implicações para a evolução crustal da Província Carajás.

Abstract

Geological mapping, geochemical, and geochronological studies undertaken in the Archean granitoids of the Canaã area in the Carajás province, Amazonian craton, Brazil, led to the definition of new granitoid units that entirely replace the Xingu complex in the area. Four major magmatic events are indentified: three of Mesoarchean age and one of Neoarchean age. The succession of events is: (1) at 3.05-3.0 Ga, it occurred the formation of the protolith of the Pium complex and of rocks with similar ages only indicated by inherited zircons found in different units; (2) at 2.96-2.93 Ga, occurred the crystallization of the Canaã dos Carajás granite and the formation of the older rocks of the Rio Verde trondhjemite; (3) at 2.87-2.83 Ga, the Bacaba tonalitic complex, the Rio Verde trondhjemite, and the Cruzadão, Bom Jesus and Serra Dourada granites were formed; (4) in the Neoarchean, at 2.75-2.73 Ga, the Planalto and Pedra Branca suites and charnockite rocks were originated. Geochemically, two groups of granitoid units were distinguished: (1) The tonalitic-trondhjemitic units, which encompass the Bacaba tonalitic complex and the Pedra Branca suite, which are geochemically distinct of typical Archean TTG series, and the Rio Verde trondhjemite, akin to the TTG series; (2) the granitic units which cover more than 60% of the Canaã surface and include five distinct granites. The Mesoarchean Canaã dos Carajás, Bom Jesus, Cruzadão, and Serra Dourada granites are composed dominantly of biotite leucomonzogranites whereas the dominant rocks in the Neoarchean Planalto suite are biotite-hornblende monzogranites to syenogranites with total mafic content between 5% and 20%. The Canaã dos Carajás and Bom Jesus granites and the variety of the Cruzadão granite with higher La/Yb are geochemically akin to the calcalkaline granites, whereas the other varieties of the Cruzadão granite are transitional between calc-alkaline and alkaline granites. The Serra Dourada granite has an ambiguous geochemical character with some features similar to those of calc-alkaline granites and other to peraluminous granites. The Canaã dos Carajás and Bom Jesus granites of Canaã are similar to the High-Ca granites, whereas the Cruzadão and Serra Dourada are more akin to the Low- CaO granites of the Yilgarn craton. The geochemical characteristics of the Mesoarchean Canaã granites approach those of the biotite granite group of the Dharwar craton but the latter are enriched in HFSE and HREE compared to the Mesoarchean granites of Canaã. The accentuated variation of the Sr/Y and (La/Yb)N ratios observed in the Canaã granites should reflect dominantly compositional differences in the sources of the granite magmas with a subordinate effect of pressure. Geochemical modeling suggests that partial melting of a source similar in composition to the average of Early Proterozoic basalts or to the average lower continental crust could be able to give origin to the Bom Jesus granite and to the variety of the Cruzadão granite with higher (La/Yb)N. The residue of melting will contain variable proportions of plagioclase, amphibole, garnet, clinopyroxene ± orthopyroxene, and ilmenite. In the other Canaã granites, plagioclase was dominant, garnet was probably an absent phase in the residue of melting and the influence of amphibole was also apparently limited. A crustal environment and a pressure of 8 to 10 kbar is estimated for the generation of the Bom Jesus and similar granite magmas that left garnet as a residual phase. The Neoarchean Planalto granites have ferroan character and are similar geochemically to reduced A-type granites. The tectonic setting and the association between the Planalto suite and charnockitic series led us to propose classifying these biotite-hornblende granites as hydrated granites of the charnockitic series. The Planalto suite was derived by partial melting of mafic to intermediate tholeitic orthopyroxene-bearing rocks similar to those of the Pium complex. The Archean granitoid magmatism in Canaã significantly differs of that found in most classical Archean cratons, including the Rio Maria terrane, because TTG magmatism is not abundant, sanukitoid rocks are absent and granitic rocks are dominant. The Neoarchean Planalto suite granite has no counterpart in the Mesoarchean Rio Maria terrane of the Carajás province, neither apparently in the Yilgarn and Dharwar cratons. The contrasts between Canaã and the Rio Maria granitegreenstone terrane do not favor a common tectonic evolution for these two domains of the Carajás province. The Archean crust of Canaã has not a juvenile character and the Nd evolution paths suggest the existence of a little older crust in the Canaã area compared to that of Rio Maria. The crust of the Canaã area existed at least since the Mesoarchean (ca. 3.2 to 3.0 Ga) and was strongly reworked during the Neoarchean (2.75 to 2.70 Ga). A similar terrane to that represented by the Canaã Mesoarchean crust or even an extension of it was probably the substratum of the Carajás basin formed during the Neoarchean. Probably there is no an effective transition between Rio Maria and the Carajás basin and the denominated 'Transition' subdomain had more probably an evolution distinct of that of Rio Maria. The Neoarchean evolution of the Carajás province is marked by the upwelling of the asthenospheric mantle in an extensional setting that propitiated the formation of the Carajás basin. Later on, at ca. 2.73-2.70 Ga, the heat input associated with underplate of mafic magma induced the partial melting of mafic to intermediate lower crustal rocks originating the Planalto and Pedra Branca suites, and charnockite rocks. The close association between the Planalto suite and charnockitic rocks suggests similarity between its evolution and that of the high temperature granite magmatism commonly found near the limits between distinct tectonic blocks or in their zone of interaction.