Petrogenesis and emplacement of the Paleoproterozoic A-type Marajoara granite in the Carajás province, southeastern Amazonian craton: Constraints from geochemistry, zircon geochronology, NdHf isotopes, coexistence of reduced and oxidized magmas and new insights from a composite dike

Abstract

The Marajoara granite is a Paleoproterozoic A-type stock that intruded into Mesoarchean tonalitic granitoids in the Rio Maria domain. It is composed of two compositionally and texturally distinct facies: equigranular (eBMzG) and heterogranular (hBMzG) biotite monzogranites. A rapakivi texture and the occurrence of microgranular enclaves (ME) and porphyritic ME (pME) are restricted to the hBMzG facies. The magnetic susceptibility values and the presence of magnetite indicate that the hBMzG facies is akin to granites from the magnetite series, whereas the eBMzG variety shows affinity with the granites of ilmenite series. These granites are high-silica, peraluminous and similar to ferroan granites. The hBMzG and pME show affinities with oxidized and the eBMzG with reduced A-type granites in the Jamon and Velho Guilherme suites, respectively. The ME have affinities with magnesian granites and the calc-alkaline series. UPb zircon analyses (SHRIMP) yield a crystallization age of 1884 ± 11 Ma for the hBMzG. LuHf and SmNd isotope data Ɛ_{Hf(1.88 Ga)} values range from −11 to −18 (hBMzG), while Ɛ_{Nd(1.88 Ga)} values vary from −9 to −11. The Hf-T_DM^C model ages are between 3.2 and 3.6 Ga, whereas the Nd-T_DM^C ages range from 2.9 to 3.6 Ga, indicating that the Marajoara granite-forming magma originated from a Meso- to Paleoarchean crustal source. Whole-rock geochemical data reveal compositional gaps, in terms of major and trace elements, between the two varieties that constitute the Marajoara granite, supporting the interpretation that they are not cogenetic. Geochemical modeling indicates that the original magmas were generated from partial melting of tonalitic rocks, with assimilation of metasedimentary rocks in the case of eBMzG. The contrasting redox states and modeling outcomes support distinct petrogenetic paths for the two facies. Felsic–mafic magma mixing played an important role in the crystallization history of the granite. The enclaves represent a basic magma from the lithospheric mantle that was injected into the magma chamber during underplating, where it interacted with the Marajoara pluton. This hypothesis is reinforced by the occurrence of a 1.88 Ga porphyritic granite–diabase composite dike in the Rio Maria area, provides independent evidence of bimodal magmatism. Microgranular and porphyritic enclaves formed due to the mixing of the felsic and mafic magmas. The results presented in this work highlight the importance of the Archean crust for the origin of Paleoproterozoic granites, whose emplacement at shallow crustal levels occurred through a dike feeder system resulting from extensional tectonics.