Petrologia magnética e química mineral dos granitoides mesoarqueanos de Ourilândia do Norte (PA)

Abstract

The Ourilândia do Norte area is located in the midwestern portion of the Carajás province, whose tectonic segment is comprised between the Rio Maria (RMD) and Carajás domains (CD), where outcrop three main Mesoarchean (2.92-2.88 Ga) granitoids: (i) leucogranite and associated high-Ti granodiorite – are constituted by equi- to heterogranular monzogranites and associated granodiorite with high titanite contents. Both of them have biotite as an essential mafic phase, lack amphibole and host TTG basement; (ii) sanukitoids – are represented by granodiorites (equigranular, heterogranular and porphyritic), with subordinate tonalite, quartz monzodiorite and quartz diorites occurrences. These rocks are characterized by the presence of hornblende as an essential mineral and associated mafic enclaves; and (iii) trondhjemite – represented by porphyritic granitoids with fine-grained mafic enclaves. Based on magnetic susceptibility (MS) studies these granitoids were distinguished into three magnetic populations: (i) lowest magnetic values (A; MS ranging from 0.05x10-3 to 0.57x10-3 SI) – characterized by rare opaques phases, predominant sanukitoids and trondhjemite; (ii) moderate magnetic values (B; MS 0.59x10-3 to 2.35x10-3 SI) – the modal ilmenite contents prevails over magnetite, with variable proportions of sanukitoids and leucogranites; and (iii) high magnetic values (C; MS 2.35x10-3 to 17.0x10-3 SI) – characterized by the highest magnetite contents, with subordinate ilmenite occurrence as trellis and composite textural types. The leucogranites and associated high-Ti granodiorite predominate over porphyritic sanukitoid. Amphiboles classify as magnesio-hornblende, characterized by Mg/(Mg+Fe+2) ratio ≥ 0.70, and minor occurrence of ferropargasite e actinolite-hornblende. For the trondhjemite, amphibole occurs as an acessory mineral and corresponds to magnesio- hornblende and tschermakite. Biotite crystals have Fe+2/(Mg + Fe+2) ratio down 0.6 in leucogranites and associated high-Ti granodiorite, and lower values (≤ 0.4) in sanukitoids and trondhjemite. Plagioclase crystals are predominantly oligoclase, without significant compositional variations between phenocrysts and matrix, and are frequently replaced by sericite. Although there are four distinct textural types of epidote and titanite, were investigated two main textural types: the former has magmatic origin and the latter indicate late-magmatic origin. In terms of the pistacite molecule contents in epidote {Ps = [Fe+3/(Fe+3 + Al)]*100}, Ps values of 25 to 36 %, 26 to 36 % and 22 to 30 % were estimated for the leucogranites, sanukitoids and trondhjemite, respectively, along with TiO2 contents ≤ 0.137, suggesting magmatic origin. Temperature estimative based on zircon (TZr) and apatite saturations (TAp) are in the range TZr 841-990 °C and TAp 884-979 °C (leucogranite and associated high-Ti granodiorite), TZr 826-972 °C and TAp 864-886 °C (sanukitoids) and, finally, TZr 853-977 °C and TAp 909 °C (trondhjemite), which all of them are interpreted as close to liquidus, with pressure estimates in general between 100 and 280 MPa, akin to upper crust conditions. Geothermometers and barometers based on aluminium contents in amphibole indicate temperature around 738-811 °C (sanukitoids) and 779-892 °C (trondhjemite). The lower temperatures(< 800 °C) is assumed as resulted of opening magmatic system related to deformation, and denote that dynamic recrystallization may have occurred at temperatures close to solidus, indicative of a syntectonic nature. Although the leucogranites and associated high-Ti granodiorite present relative enrichment of #Fe in whole- rock, are calc-alkaline affinity granitoids, overlapping the Cordilleran granites with SiO2 > 70%, low HFSE (high field strength elements) abundances, magmatic magnetite and high MS, pointing to oxidized crystallization conditions (problably at ∆NNO+2.8). Sanukitoids varieties have FeOt/(FeOt + MgO) < 0.7 in whole-rock, amphibole and biotite, low MS, indicating less oxidized crystallization conditions (∆NNO+1.0); however, the equigranular varieties and trondhjemite probably are formed in conditions relatively close to the ΔFMQ+0.5. The lower MS and lower magnetite contents reported for the sanukitoids and trondhjemite can also be attributed to the earlier epidote formation and to the late magmatic processes, which are responsible by the magnetite instability. It is concluded that the precursor magma of the sanukitoids are hydrated (H2O > 4-7 %), whereas lower water contents in melt (H2O < 4-7 %) are suggested for the leucogranites and associated high-Ti granodiorite; and trondhjemite, as indicated by their amphibole absence and acessory amphibole contents, respectively, along with low contents of hydrated mineralogy. These results are compared to those estimated for calc-alkaline rocks of the Rio Maria suite from the Carajás province and oxidized members from other Archean to Paleoproterozoic terrains such as Baltic Belt, Sarmatian orogen (Western Europe), Closepet-type granites and high-Mg granodiorites of the Matok pluton (Limpopo Belt – South Africa).