Petrogênese dos granitos Manda Saia e Marajoara: contribuições para a definição da natureza do magmatismo paleoproterozóico da Província Carajás.

Abstract

The Marajoara (MJG) and Manda Saia (MSG) granites are located in southeastern Pará State, Brazil, and represent circular intrusions with stock dimensions embedded in Mesoarchean rocks of the Rio Maria Domain, in the central-southern portion of the Carajás Province. These rocks outcrop as extensive pavements, exhibiting no solid-state deformation features (isotropic aspect) and frequently containing angular enclaves of the surrounding host rocks. The MJG comprises equigranular biotite monzogranite (eBMzG) and heterogranular (hBMzG) varieties, as well as porphyritic (pME) and microgranular enclaves (ME) restricted to the hBMzG facies. Quartz content and plagioclase/microcline ratios vary significantly, allowing these rocks to be classified from syenogranitic to monzogranitic, and even granodioritic in the case of microgranular enclaves. They are peraluminous granites, similar to ferroan granites with high K2O+Na2O/CaO and FeOt/(FeOt+MgO) ratios, enriched in Rb, Zr, Y, Nb, F, and heavy REEs, with more evolved facies displaying low Sr and Ba contents. In REE patterns, negative Eu anomalies are prominent, and heavy REEs show a gradual increase with magmatic differentiation. These granites fall within the intraplate granite field and exhibit geochemical affinities with A-type granites. Their FeOt/(FeOt+MgO) ratios align with typical oxidized (hBMzG and pME) and reduced (eBMzG) A-type granites, while the MSG displays a moderately reduced character. The ME, however, show affinity with magnesian and calcalkaline series granites. According to biotite mineral chemistry, MSG and the hBMzG facies of MJG fall within the magnetite series field, while eBMzG rocks are similar to ilmenite series rocks. SHRIMP zircon U–Pb analyses provide crystallization ages of 1884 ± 11 Ma for MJG and 1866 ± 10 Ma for MSG (LA–SF–ICP–MS). Lu–Hf isotopic data indicate ƐHf(t) between - 11 and -18 and Hf-TDMC from 3.2 to 3.6 Ga for MJG; and ƐHf(t) between -13 and -19 and Hf-TDMC from 3.3 to 3.6 Ga for MSG. The compositional gaps among the various MJG varieties suggest that their magmas are not cogenetic. Geochemical modeling suggests that MJG and MSG were generated by partial melting of tonalitic rocks, with occasional metasedimentary contributions, at a melting rate ranging from 16 to 18%, with a residual assemblage of plagioclase, quartz, biotite, magnetite, and ilmenite. Felsic and mafic magma mixing played an important role in the emplacement. The enclaves represent enriched lithospheric mantle-derived magmatism injected into the magma chamber during the subduction process, interacting to varying degrees with the magma forming the Marajoara granite. This hypothesis may be reinforced by the occurrence of a 1.88 Ga diabase-porphyritic granite composite dyke in the Rio Maria region. The proposed model suggests that the granitic magma initially formed a magma chamber, followed by repeated mafic magma injections, resulting in small-scale convection. Subsequently, large volumes of hot mafic magma entered the chamber, leading to mixing processes. Microgranular and porphyritic enclaves were formed due to magma mixing in areas where there were temperature contrasts between felsic and mafic magmas. The results presented in this study highlight the importance of the Archean crust in the origin of Paleoproterozoic granites, which were emplaced in shallow crustal levels through a dyke feeder system as a result of extensional tectonics.