Evidências da acreação crustal riaciana em sequências metavulcanossedimentares do setor noroeste do orógeno do Oeste da Bahia, Brasil: contribuições ao estudo do supercontinente Columbia

Abstract

The study area is located in the northwest region of the São Francisco Craton (SFC), in the western sector of the Minas-Bahia Orogen. Two metavolcanosedimentary sequences were identified from the description of boreholes and systematic geological mapping. The oldest one is the Boqueirão de Baixo Formation (BBF), and the youngest one is the Chapada Grande Formation (CGF), defined here, which represents different stages of a Rhyacian orogenic system. These sequences occur in tectonic contact, through shear zones. In BBF, mainly fine-grained amphibolites were identified, in which nematoblastic microstructure predominates, associated with schists with sedimentary protolith. The amphibolites are high-iron, low-potassium tholeiitic rocks with flat REE patterns, negative Nb and Ta anomalies, and positive Pb anomalies. They have a crystallization age of 2218±18 Ma and a metamorphism age of 2020±7 Ma, with a TDM(t) model age between 2.38 and 2.28 Ga and ƐHf(t) between +3.78 and +5.6. These rocks are interpreted as coming from a Continental Arc installed in a thin crust on the eastern margin of the Bom Jesus da Lapa paleoplate. The CGF was subdivided into four lithostratigraphic units where basic and intermediate to acid metavolcanic rocks are generically intercalated with siliciclastic and chemical metasedimentary rocks. The basic rocks comprise mafic shales and amphibolites, both mainly fine-grained, in which the nematoblastic metamorphic microstructure is predominant. They are transitional to calc-alkaline rocks of low to medium potassium, with LREE enrichment in relation to HREE. They also show pronounced negative Nb and Ta anomalies and positive Pb anomalies. Intermediate to acid rocks were identified in two different units of the CGF, with preserved igneous textures, such as porphyritic and poikilitic, and erosion edges in feldspars grains. In the first, it is represented by a metadacite with a crystallization age of 2096±20 Ma, TDM(t) model age of 3.57 Ga, and ƐHf(t) equal to -18.5. In the second, intermediate to acid volcanism comprises metalatites and metatrachytes subalkaline to alkaline, calc-alkaline, high-potassium, metaluminous to peraluminous, magnesian, and with shoshonitic affinity. They show strong enrichment of LREE in relation to HREE and negative anomalies of Nb, Ta, and Ti. The crystallization age is 2066±6 Ma, with TDM(t) model age between 3.51 and 3.13 Ga and ƐHf(t) between -16.44 and -8.84. Alkaline lithotypes have similar characteristics to sanukitoids. The basic to acid volcanic rocks of the CGF are interpreted as formed in a continental arc developed on the eastern margin of the Bom Jesus da Lapa Paleoplate. In turn, the association of metasedimentary rocks suggests, from SE to NW, deposition in forearc, intra-arc, and passive margin basins, with sea level fluctuations and the presence of graphite shales. After the continental collision, sinistral transcurrent NNW-SSE shear zones are generated. They form a pull-apart basin that accommodates the plutonism of the Guanambi Suite and the metasedimentary rocks that support the mountain ranges in the western portion of the area. The existence of crustal accretion processes during the Rhyacian-Orosirian in the Western Bahia Orogen has been the subject of study in several works, which, with the aid of aerogeophysical, geochronological, and isotopic data, allowing inferring that the area presents a greater diversity of terrains than surface geological maps may indicate. The data presented here indicate that the rocks of BBF and CGF exhibit various chemical and isotopic characteristics that allow them to correlate partially with the Extreme Metavulcanosedimentary Sequence to the west and with the Riacho dos Machados Metavulcanosedimentary Sequence in the Porteirinha Terrain. Thus, improving the geology knowledge of the area can contribute understanding the Minas-Bahia Orogenic System formation processes and its role in the construction of the Supercontinent Columbia.