Geocronologia U-Pb, classificação e aspectos evolutivos do Granito Marajoara – Província Carajás

Abstract

The Marajoara granite (MjGr) is a stock intrusive in mesoarchean granitoids of the Rio Maria domain, which is formed mainly by leucocratic rocks, represented by equigranular (BMzE) and heterogranular (BMzH) monzogranite facies. Rapakivi texture and occurrences of porphyritic granite (EGp) and microgranular enclaves (EMg) are restricted to BMzH facies. Such varieties have similar mineralogy: microcline, quartz and plagioclase occur as essential minerals; biotite partially altered to chlorite as the only varietal phase; zircon, titanite, opaque, apatite and allanite as primary accessories; and chlorite, sericite-muscovite, epidote, fluorite and clay minerals as secondary phases. The high magnetic susceptibility (SM) values (2.3-6.5 x10-3) and the frequent presence of magnetite show that the BMzH facies is akin to granites with magnetite series, whereas the BMzE variety shows affinity with the ilmenite series considering the modal opaque contents ≤0.5%, low values of SM (<0.15x10-3), and ilmenite as the sole Fe-Ti oxide. These rocks are, in general, peraluminous and have high FeOt/FeOt+MgO ratio, similar to the ferroan granites. In addition, they have geochemical affinities with intraplate A-type granites, which have crustal origin, wherein a significant variation of FeOt/(FeOt + MgO) found for these rocks [EGp (> 0.82); BMzH (> 0.86); BMzE (> 0.97)], allow them to be classified as oxidized (BMzH and EGp) and reduced (BMzE) Atype granites, that are related to the Jamon and Velho Guilherme suites, respectively. Differently from this, the EMg show clear affinity with the magnesian granites and the calcalkaline series. Evidence of magma mixing and geochemical modeling calculations demonstrate that EGp originate from the interaction of EMg (60%) and BMzH (40%) liquids. The compositional gaps existing among the several varieties that constitute the MjGr, as well as their compositional contrasts, suggest that their magmas are not cogenetic. The EMg represents a basic magmatism from the enriched lithospheric mantle that would have been injected into the magma chamber during the underplating process and at different phases of the granitic magma crystallization. The U-Pb zircon isotopic analyzes (SHRIMP) yielded an age of 1885 ±6Ma, interpreted as the crystallization age of MjGr. The MjGr was emplaced at shallow crustal levels (epizone) in an extensional tectonic environment with the effort following the trend NNE-SSW to ENE-WSW. The concentric zoning in the MjGr and the rheological behavior of its country rocks as well as the reduced or no influence of the regional efforts during the emplacement of the pluton indicate that the transport of the magma occurred through dikes. It is suggested that the construction of the MjGr was a result of the vertical rise of magmas through fractures and accommodation along the planes of the regional EW foliation, followed by a change of the vertical flow by a lateral scattering of the magma, analogous to the admitted model for the emplacement of the tabular batholiths of the Jamon Suite.