Geology, petrogenesis, and geochronology of the Rio Salitre Complex: Implications for the Paleoproterozoic evolution of the northern Sao Francisco Craton, Brazil

摘要

Greenstone belts and volcano-sedimentary sequences are essential features to understand the tectonic framework of Precambrian terranes worldwide. The Sa?o Francisco Craton, eastern Brazil, hosts several of these supracrustal belts related to its Archean-Paleoproterozoic evolution. The Rio Salitre Complex (RSC) is a Paleoproterozoic greenschist-metamorphosed volcano-sedimentary sequence deposited over Archean orthogneisses of the northern Sa?o Francisco Craton. Here, we present a tectonic model for the Rio Salitre Complex, integrated with regional and global contexts, based on geology, structures, whole-rock geochemistry, U?Pb zircon geochronology, and Lu?Hf isotopes. The RSC comprises the Baixo Vale do Rio Salitre (BVRS), formed by Lower and Upper Subunits, and the Sobradinho Unit. The BVRS Lower Subunit is composed of ultramafic, mafic, and felsic metavolcanic rocks, chemical, and minor clastic metasedimentary rocks. The BVRS Upper Subunit comprises clastic metasedimentary rocks, while the Sobradinho Unit consists of chemical and clastic metasedimentary rocks. The RSC units display evidence of shallow to deep marine platformal deposition in an intra-cratonic rift (abundance of pelitic facies, presence of phosphorites, iron formations, graywackes, arkoses, and cross-bedded quartzites). RSC phosphorites are a register of a global Paleoproterozoic phosphogenic event. The volcanics of the Rio Salitre Complex compose a bimodal suite. The mafic and ultramafic metavolcanics exhibit komatiitic, tholeiitic, and alkaline affinities. Enriched REE, trace elements patterns, and incompatible element ratios indicate a continental rift environment for the mafic and ultramafic metavolcanic rocks. These features also suggest enriched mantlederived melts, with a variable crustal contribution. The porphyritic rhyolites and aphyric high-silica rhyolites have subalkaline to alkaline features, with enriched REE and trace element profiles and negative Eu, Nb, and Ti anomalies. U?Pb SHRIMP dating of porphyritic rhyolite resulted in a concordia age of 2161.1 ? 4 Ma (MSWD: 1.1). Archean Lu?Hf TDM ages with negative ?Hf(t) values (from -8.90 to -2.70), and an isolated Paleoproterozoic TDM age (2440 Ma) with positive ?Hf(t) value (+3.96), related to the geochemical features, indicate that RSC felsic metavolcanics were originated from melting of multiple Archean crustal sources (continental and slab), with minor contributions of Paleoproterozoic juvenile material, and emplaced in a continental rift environment. 2.0 Ga foliated metagranites and metagranodiorites (locally augen) intrude the RSC rocks, resulting in local hornblende-hornfels metamorphism. The RSC supracrustal lithotypes register D1?, D1?, and D2 structures. D1? structures are mainly represented by N?S high-dip angle schistosity, N and SSE plunge folds, and high-rake mineral stretching lineation (Lx?); while D1?corresponds to NE-SW transpressional shear zones, NE-SW low-dip angle crenulation cleavage, and medium to low-rake mineral stretching lineation (Lx1?). The D2 structures are related to, locally quartz-filled, fractures, in addition to NE-SW and ENE-WSW oriented faults. The structural framework reflects a two-phase deformation, with a ductile to ductile-brittle, W-E frontal to NW-SE oblique D1 phase, and a late brittle, NE-SW D2 phase. Late hydrothermal activity resulted in quartz veins and pyrite pyrrhotite-(chalcopyrite) concentrations.The documented aspects of the Rio Salitre Complex are consistent with evolution in a Rhyacian-Orosirian continental rift basin, inverted by the influence of orogenic belts, approaching from northwest and southeast.