Brazilian Journal of Geology
Latest articles in this journal
Brazilian Journal of Geology, Volume 51; doi:10.1590/2317-4889202120190136
The pegmatites of the São João del Rei Pegmatitic Province are related to the Siderian protoliths of the Cassiterita and Resende Costa orthogneisses and to the Ritápolis metagranitoid of Ryacian age. Chemical analysis of garnet from twelve pegmatites reveal two different types of grains, which were found in the same pegmatitic body in six of these samples. One garnet type has almandine-spessartine composition (Sps11-7-58.8 Alm36.8-86.5 Prp0.1-4.1Grs0.0-1.4Adr0.0-2.6), grains with orange and pink tones, and scarce mineral inclusions. These garnet grains may have been formed at the magmatic stage of pegmatite crystallization. The composition of these grains plot exclusively on the Alm-Sps axis at the Prp+Grs+Adr+Uvr × Alm × Sps diagram, as expected from garnet crystallized in pegmatites, and an expansion of the field associated to pegmatites is proposed. The second type has a distinct chemical composition (Sps26.9-84.8Alm3.6-40.0 Prp0.0-10.4 Grs9.3-45.6 Adr0.1-3.4), displaying enrichment in Ca. This Ca-enriched garnet has irregular shaped colourless grains and abundant mineral inclusions. These grains may have been formed by Ca-metasomatism during the late-stage crystallization of the pegmatites.
Brazilian Journal of Geology, Volume 51; doi:10.1590/2317-4889202120200073
The geochemical behavior of the major, minor, trace and rare earths elements (REEs) in soil profiles from ultramafic volcanoclastic rocks of the Vulcão do Paredão and Morro Vermelho Formation from Trindade Island (TI) was analyzed in this study. Losses and gains of chemical elements were calculated through the mass balance for two profiles along the slope: one located at higher altitude (460 m) and the other at lower altitude (258 m). In all profiles, Al, Fe and Ti accumulate due to their low mobility, whereas Ca, Na, K and Mg are the most intensely leached. Soils located at lower altitude show higher K and Mg values in the surface due to the contribution of saline sprays. Leaching of the REEs from higher to the lower slope led to the enrichment of these elements, especially the light REEs, in the soil at the lowest altitude (258 m). The high altitude profile showed Ce positive anomaly due to longer exposure to weathering. The geochemical balance shows a relative enrichment of Ti, Mn, Fe, Co, Cr, Ni, V, Zr, S related to the loss of mobile elements during the soil formation process, despite the youthful nature of these volcanic rocks.
Brazilian Journal of Geology, Volume 51; doi:10.1590/2317-4889202120200104
The current knowledge on the accretionary evolution of the Borborema Province is evaluated considering recently published data and interpretations. Early to late Neoproterozoic eclogite, ophiolite, and magmatic arc remnants have been documented and point to oceanic crust consumption. Isotopic contrasts as well as geophysical anomalies across the major domain boundaries are suggestive of collisional sutures and a speculative model of terrane accretion is presented here. On the other hand, the lack of concise evidence for some of the proposed deep-seated structures as well as putative lithospheric continuity in both sides of it suggest reworking of previously continuous lithosphere through intracontinental deformation in localized sectors of the province interior. It is patent that one model does not preclude the other and available evidence so far has triggered the emergence of conciliatory proposals for the Neoporterozoic history the Borborema Province.
Brazilian Journal of Geology, Volume 51; doi:10.1590/2317-4889202120190133
Tectonic stylolites are indicators of compressive deformation episodes in rocks and are commonly studied in terms of their geometrical attributes’ length (L), and maximum (D max ) and average (D avg ) amplitudes. These parameters were analyzed for vertical stylolites from a sinistral strike-slip fault zone in limestones from the Guia Formation, Paraguay Belt, and compared mathematically to scarce vertical stylolites away from the fault zone. The study aimed to understand the role of strain-induced stylolites on the thinning of faulted limestone layers. Stylolites range from 2.66 up to 28.15 mm in length and from 0.143 up to 1.378 mm in amplitude. The total contractional strain (ϵ) was calculated for 49 stylolites with maximum amplitude peaks of 0.762 mm, using the Kostrov formula. The strain produced layer thinning and growth of the stylolite population. Regions of high stylolite concentration accommodated 16.92% contractional strain, whereas strain values of 3.29% are present in portions with low stylolite density. Layer thinning is directly proportional to contractional strain (ϵ) values spatially induced by faulting. The stylolite morphology plays an essential role in the permeability structure at the reservoir scale, where it can act as a barrier or a channel for fluid flux.
Brazilian Journal of Geology, Volume 51; doi:10.1590/2317-4889202120200065
The Santa Vitória Formation is a lithostratigraphic unit known for its fossil assemblage of Pleistocene mammals, established in 1973 from sediments exposed along the banks of Chuí Creek, in the southern coastal plain of the state of Rio Grande do Sul (CPRS), Brazil. The original description was presented in a Masters’ dissertation and never formally described in a peer-reviewed publication. Moreover, surveys and fossil collecting efforts developed in the last decade have led to a better understanding of its origin and nature. An updated description of this unit is presented here, which outcrops in areas occupied by barrier-lagoon depositional systems originated by eustatic oscillations. The formation encompasses Middle-Upper Pleistocene fluvial and eolian depositional systems and paleosols formed chiefly of fine to medium quartz sand, with subordinate clay, concentrations of iron and manganese oxides, and caliche nodules of pedogenic origin. It conformably overlies upper shoreface-foreshore marine deposits, and is overlain by loess deposited during the last glacial period. Here is proposed to re-define it as Santa Vitória Alloformation, considering that the use of an allostratigraphic framework to characterize this unit has the practical purpose of grouping depositional systems genetically unrelated to the barrier-lagoon systems.
Brazilian Journal of Geology, Volume 51; doi:10.1590/2317-4889202120190132
The Southeastern Brazilian Margin presents a NE-SW-striking structural framework, which is characteristic of the Neoproterozoic Ribeira Belt. Other important structural trends along the area are associated with E-W and NW-SE-striking structures, being most related to the Mesozoic-Cenozoic tectonic evolution. A succession of Cenozoic tectonic events has been described by many authors in the Southeastern Brazilian. Among those Cenozoic tectonic events, a Neogene to Quaternary E-W dextral strike-slip regime (EW-DT) shows a wide distribution, presenting some variations in the stress field orientation. This research investigates the influence of the preexisting structural framework on the variation of the paleostress field orientation associated with the EW-DT tectonic event in a selected area in the onshore continental margin of the Southeastern Brazil. The study was carried out in three main stages: lineament analysis, paleostress field analysis, and numerical mechanical modeling. The paleostress field related to the EW-DT tectonic event in the studied area presents: NNW-SSE-striking maximum horizontal stress, and counterclockwise rotation influenced by the structural framework. Regarding this, the regional variation of the paleostress field is influenced by a preferential reactivation of NE-SW-striking structures while sets of approximately E-W-striking structures are the main local control of counterclockwise rotation of the stress field.
Brazilian Journal of Geology, Volume 51; doi:10.1590/2317-4889202120200057
Brazilian Journal of Geology, Volume 51; doi:10.1590/2317-4889202120200106
Brazilian Journal of Geology, Volume 51; doi:10.1590/2317-4889202120200049
Differences in tectonic style and kinematics in orogenic belts can result from either multiphase or progressive deformation. In eastern Borborema Province, a regional shallow dipping foliation is multiply folded and crosscut by transcurrent shear zones. Here, we investigate if these fabrics resulted from diachronous deformational events or from a single-phase progressive deformation. The study area, the Feira Nova region, mostly comprises metasedimentary rocks from the Surubim Complex, and is bounded by the NE-trending sinistral Gloria do Goitá (GGSZ) and the sinistral contractional Paudalho (PSZ) shear zones, which separate it from Paleoproterozoic basement rocks. Structures can be grouped into a contractional and a transpressional stage, both related to the Brasiliano Orogeny. The contraction-related structures are represented by a gently dipping foliation (S2) related to a top-to-the-NW tectonic transport. The contact between the metasedimentary and basement rocks is parallel to S2, indicating the fabrics in both lithotypes are of Brasiliano age, with strain localization having produced the PSZ. NW-verging macroscopic folds are consistent with this regime. The following transpression generated steep mylonitic foliation (S3) mainly along the GGSZ, and caused refolding. The data here presented are consistent with a progressive deformation history comprising a gradual transition from contraction to transpression.
Brazilian Journal of Geology, Volume 51; doi:10.1590/2317-4889202120200009
The Aptian Crato Formation is world renowned for its well-preserved fossils in microbially-induced laminated limestones, which are regarded as one of the main Cretaceous Konservat-Lagerstätte of the geological record. Detailed stratigraphic investigation and mapping of the up to 90-m-thick Crato Formation at the eastern border of the Araripe Plateau allowed recognition of a regionally persistent fossil-bearing muddy interval, herein defined as the Caldas Bed. At its type locality, it is defined as an up to 2-m-thick coarsening-upward succession of grey/green mudstone and interbedded sandy siltstone and claystone. The 0.85- to 2-m-thick interval was recognized in several localities along the outcrop belt, and it is bounded by sharp, lower (Konservat-Lagerstätte limestone) and upper (sandstone and heterolithic facies) contacts. Despite previous literature data suggesting the presence of marine mollusks, the bed contains freshwater bivalves, small gastropods, spinicaudatans, plant remains, trace fossils, and rare ostracods. The Caldas Bed records benthic paleocommunities representing a short-term isochronous regional freshening event, marked by abrupt changes in sedimentation pattern, bathymetry, salinity, oxygenation and water chemistry.