Layered intrusions and volcanic sequences in Central Brazil: Geological and geochronological constraints for Mesoproterozoic (1.25 Ga) and Neoproterozoic (0.79 Ga) igneous associations
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Abstract:

AbstractA 350-km long belt of layered complexes and associated volcano-sedimentary sequences forms a continental-scale feature exposed along the internal portion of the Neoproterozoic Brasília Belt in central Brazil. This study provides new geochronological results and a critical review of the available data of these igneous associations of central Brazil. Precise age dating combined with geological and petrological studies indicate that this belt consists of two distinct igneous rock associations. The 1.25 and 0.79 Ga igneous episodes are constrained by reliable U–Pb zircon ages (SHRIMP and ID-TIMS) obtained in different regions and lithotypes of this belt. Both igneous associations were affected by high-grade metamorphism and tectonism at ca. 0.76 Ga, which partially disrupted the original igneous stratigraphy of the layered complexes and volcano-sedimentary sequences. The present configuration of this belt results from later events, probably caused by final ocean closure and continental collision between the São Francisco and Amazonian continents at ca. 0.63 Ga.The 1.25 Ga magmatic event represents an igneous rock association comprising extensive bimodal volcanic rocks (Palmeirópolis, Juscelândia and Indaianópolis sequences) and associated large mafic layered intrusions (Serra dos Borges and Serra da Malacacheta complexes). Geochemical data of mafic volcanic rocks indicate compositions similar to MORB, suggesting a tectonic setting where continental rifting led to the opening of an ocean basin. Large layered intrusions coeval to the volcano-sedimentary sequence consist mainly of olivine plus plagioclase cumulates, with abundant troctolite and leucotroctolite, associated with olivine gabbro, gabbro, anorthosite and pyroxenite. These layered intrusions are characterized by moderately primitive olivine compositions (Fo 63–76), positive ɛNd(1250Ma) values, low contents of incompatible trace elements and LREE-depleted mantle-normalized-REE patterns. These features suggest an origin from the same depleted mantle source that produced the extensive MORB-type volcanism. Intrusion of layered complexes possibly occurred since the early stages of rifting, being emplaced both at the base of and within the volcanic pile. Different magmatic ages and fractionation trends of layered bodies suggest that they represent a cluster of intrusions that originated during the time span of the continental to oceanic ridge rifting event.The 0.79 Ga igneous association consists of three large layered intrusions (Niquelândia, Barro Alto, and Canabrava complexes). Despite intense tectonism, several striking geological similarities between the three separate bodies have been used to suggest that they originally constituted a single continuous magmatic structure. These layered intrusion are characterized by thick ultramafic zones, highly primitive compositions of olivine (up to Fo 93) and extensive Fe-enrichment trends of pyroxenes. Layered mafic–ultramafic rocks have abundant evidence of crustal contamination, including highly negative ɛNd(800Ma) values, as well as older crustal xenoliths and zircon xenocrysts. Cyclic units consisting of successive layers of dunite, harzburgite, orthopyroxenite, websterite and gabbronorite occur throughout the stratigraphy of these layered intrusions. The Niquelandia Complex ranks among the thickest layered intrusions in the world, and together with the Barro Alto and Canabrava layered intrusions represent an enormous volume of mantle-derived magma emplaced into the crust during a continental rifting event.The 1.25 and 0.79 Ga rifting events are not exposed in the Brasília Belt or in the São Francisco Craton. This supports previous interpretations suggesting that these igneous rock associations are part of an exotic block in the Brasília Belt.Equivalent Mesoproterozoic (ca. 1.25 Ga) and Neoproterozoic (ca. 0.79 Ga) extensional events have been reported in other continents (e.g., North America). These igneous associations described within an allochtonous block in the Brasília Belt provide additional constraints, therefore, for continental reconstruction models before the break-up of Rodinia.

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