Physical volcanology and petrogenesis of the Archean Quebra Osso komatiite flow field, Rio das Velhas greenstone belt, Quadril?tero Ferr?fero (Brazil).

The Quebra Osso komatiite flow field is part of the 2.9?2.7 Ga Rio das Velhas greenstone belt, in the Quadril?tero Ferr?fero province, located in the Southern S?o Francisco craton (Brazil). It is mostly composed of metamorphosed komatiites with a minor association of pyroclastic and metasedimentary rocks. Through a detailed and systematic study, the komatiite flows were subdivided into coherent facies (massive, layered, and pillowed facies) and autoclastic facies (autobreccias and hyaloclastites), formed by effusive flows; and minor pyroclastic facies (tuffs and lapilli-tuffs) that are related to explosive flows. The igneous textures are still preserved, although their primary minerals (olivine and pyroxene) were completely pseudomorphosed by serpentine and/or chlorite. The major proportion of the komatiites of Quebra Osso Group is massive and cumulate-textured, which suggests channelized flow systems. The layered flows are subordinate and encompass an upper zone of random spinifex that grades to a lower cumulate zone. They are restricted to the marginal zones and interpreted as breakouts of lava from the main pathway. The occurrence of pillow lavas, hyaloclastites, and chemical sediments suggests that lavas erupted in a submarine environment. The abundance of autobreccias and local hybrid rocks point out to turbulent percolation of komatiites and/or multiple influxes of melts that caused the fragmentation of the solidified lava and magma mixing/mingling. The studied komatiites were variably affected by hydration due to seafloor alteration, regional metamorphism, and hydrothermal alteration. The mineral assemblage is secondary and comprises serpentine (mostly antigorite), chlorite, tremolite, talc, carbonate, and opaque minerals as accessory phases. The chemical composition of chromites suggests metamorphic overprint up to lower amphibolite facies conditions. Some komatiites are enriched in LREE (La, Sm, Nd), Rb, Ba, Th, Zr, Ti, and Y, and exhibit low [Nb/Th]MN, suggesting contamination by continental crustal. The bulk-rock and the mineral chemistry of amphibole, serpentine/chlorite, and chromite show enrichment of Al and decrease in Si contents in the spinifex-textured komatiites, as a result of crustal material assimilation and/or hybridization processes. The partial melting model for the HREE composition refers to a 50% batch melting of a garnet-free source with a composition similar to the pyrolytic mantle. The formation of Quebra Osso komatiite lavas is associated with the ascension of a mantle plume that was subsequently erupted onto the Archean sialic crust of the Santa B?rbara Complex and was later reworked by subsequent metamorphic and deformational events

Geologia da regi?o de Jequeri-Vi?osa (MG), Or?geno Ara?ua? Meridional.

A caracter?stica fundamental da regi?o de Jequeri-Vi?osa, situada no extremo sul do Or?geno Ara?ua?, ? a abund?ncia de rochas metam?rficas, ortoderivadas e paraderivadas, de f?cies anfibolito alto e granulito. O embasamento paleoproteroz?ico ? representado, a oeste, por ortognaisses tonal?ticos a gran?ticos do Complexo Mantiqueira e, a leste, por ortognaisses charno-enderb?ticos do Complexo Juiz de Fora. Ambos os complexos incluem anfibolitos e exibem intensidades vari?veis de migmatiza??o. O contato entre eles ? marcado pela zona de cisalhamento transpressiva destral de Abre Campo, interpretada como uma sutura paleoproteroz?ica reativada no Neoproteroz?ico. O Anfibolito Santo Ant?nio do Grama e rochas meta-ultram?ficas associadas (C?rrego do Pimenta) representam restos ofiol?ticos ediacaranos, colocados ao longo da Zona de Cisalhamento de Abre Campo. Assentada sobre o embasamento, na parte oeste da ?rea, ocorre uma associa??o metavulcano-sedimentar neoproteroz?ica do Grupo Dom Silv?rio, composta por xistos diversos e quartzito. Na por??o leste da ?rea mapeada, a cobertura metassedimentar neoproteroz?ica ? atribu?da ao Grupo Andrel?ndia que inclui paragnaisse migmat?tico e raro quartzito. Corpos de hidrotermalito quartzoso, indiscriminadamente associados ?s unidades do embasamento e da cobertura neoproteroz?ica, ocorrem ao longo de zonas de cisalhamento. Hidrotermalitos ferruginosos associam-se ao Complexo Mantiqueira na Zona de Cisalhamento de Ponte Nova. O granito foliado a milonitizado da Serra dos Vieiras parece ser um produto de fus?o parcial do paragnaisse Andrel?ndia. Completam o quadro geol?gico os pegmatitos da Su?te Paula C?ndico e diques de diab?sio mesoz?icos.This paper focuses on the southwestern sector of the Ara?ua? orogen in a region located close to the boundary with the northern Ribeira orogen. This region is rich in ortho- and para-derived metamorphic rocks of the high amphibolite and granulite facies. The Paleoproterozoic basement includes, to the west, tonalitic to granitic orthogneisses of the Mantiqueira Complex and, to the east, enderbitic to charnockitic orthogneisses of the Juiz de Fora Complex. Both complexes also include amphibolite enclaves and show several rates of partial melting. The contact between them is marked by the dextral transpressional Abre Campo shear zone, considered to be a Paleoproterozoic suture reactivated during the Neoproterozoic Era. The Santo Ant?nio do Grama Amphibolite and associated meta-ultramafic rocks (C?rrego do Pimenta) are Ediacaran ophiolite slivers emplaced along the Abre Campo shear zone. In the western part of the region, the Paleoproterozoic basement is locally covered by a metavolcano-sedimentary assemblage composed of amphibolite facies schist and quartzite of the Neoproterozoic Dom Silv?rio Group. To the east, the Neoproterozoic cover comprises the migmatized paragneiss and rare quartzite of the Andrel?ndia Group. The Serra dos Vieiras foliated to mylonitic granite seems to be formed from the partial melting of the Andrel?ndia paragneiss. Pegmatites of the Paula C?ndido Suite and Mesosozic diabase dikes complete the geologic framework of the mapped area

Late Neoproterozoic?Cambrian granitic magmatism in the Ara?ua? orogen (Brazil), the eastern brazilian pegmatite province and related mineral resources.

The Ara?u? orogen extends from the eastern edge of the S?o Francisco craton to the Atlantic margin, in southeastern Brazil. Orogenic igneous rocks, formed from c. 630 to c. 480 Ma, cover one third of this huge area, building up the Eastern Brazilian Pegmatite Province and the most important dimension stone province of Brazil. G1 supersuite (630?585 Ma) mainly consists of tonalite to granodiorite, with mafic to dioritic facies and enclaves, representing a continental calc-alkaline magmatic arc. G2 supersuite mostly includes S-type granites formed during the syn-collisional stage (585?560 Ma), from relatively shallow two-mica granites and related gem-rich pegmatites to deep garnet-biotite granites that are the site of yellow dimension stone deposits. The typical G3 rocks (545?525 Ma) are non-foliated garnet-cordierite leucogranites, making up autochthonous patches and veins. At the post-collisional stage (530?480 Ma), G4 and G5 supersuites were generated. The S-type G4 supersuite mostly consists of garnet-bearing two-mica leucogranites that are the source of many pegmatites mined for tourmalines and many other gems, lithium (spodumene) ore and industrial feldspar. G5 supersuite, consisting of high-K?Fe calc-alkaline to alkaline granitic and/or charnockitic to dioritic/noritic intrusions, is the source of aquamarine-topaz-rich pegmatites but mainly of a large dimension stone production

The Ediacaran Rio Doce magmatic arc revisited, Ara?ua?-Ribeira orogenic system, SE Brazil.

Described half a century ago, the Galil eia tonalite represents a milestone in the discovery of plate margin magmatic arcs in the Ara?ua?-Ribeira orogenic system (southeastern Brazil). In the 1990's, analytical studies on the Galil eia tonalite finally revealed the existence of a Late Neoproterozoic calc-alkaline magmatic arc in the Ara?ua? orogen. Meanwhile, the name Rio Doce magmatic arc was applied to calc-alkaline plutons found in the Ara?ua?-Ribeira boundary. After those pioneer studies, the calc-alkaline plutons showing a pre-collisional volcanic arc signature and age between 630 Ma and 585 Ma have been grouped in the G1 supersuite, corresponding to the Rio Doce arc infrastructure. Here, we revisit the Rio Doce arc with our solid field knowledge of the region and a robust analytical database (277 lithochemical analyses, and 47 UePb, 53 SmeNd, 25 87Sr/86Sr and 7 LueHf datasets). The G1 supersuite consists of regionally deformed, tonalitic to granodioritic batholiths and stocks, generally rich in melanocratic to mesocratic enclaves and minor gabbroic to dioritic plutons. Gabbroic to dioritic enclaves show evidence of magma mixing processes. The lithochemical and isotopic signatures clearly reveal a volcanic arc formed on a continental margin setting. Melts from a Rhyacian basement form the bulk of the magma produced, whilst gabbroic plutons and enclaves record involvement of mantle magmas in the arc development. Tonalitic stocks (UePb age: 618e575 Ma, ?Nd(t): 5.7 to 7.8, Nd TDM ages: 1.28e1.68 Ga, 87Sr/86Sr(t): 0.7059e0.7118, and ?Hf(t): 5.2 to 11.7) form the northernmost segment of the Rio Doce arc, which dies out in the ensialic sector of the Ara?ua? orogen. At arc eastern and central zones, several batholiths (e.g., Alto Capim, Baixo Guandu, Galil eia, Muniz Freire, S~ao V?tor) record a long-lasting magmatic history (632e580 Ma; ?Nd(t): 5.6 to 13.3; Nd TDM age: 1.35e1.80 Ga; 87Sr/86Sr(t): 0.7091 e0.7123). At arc western border, the magmatic evolution started with gabbro-dioritic and tonalitic plutons (e.g., Chaves pluton, UePb age: 599 ? 15 Ma, ?Nd(t): 4.8 to 6.8, Nd TDM ages: 1.48e1.68 Ga, 87Sr/86Sr(t): 0.7062e0.7068, and ?Hf(t): 4.3 to 9.7; and Brasil^andia pluton, UePb age: 581 ? 11 Ma, ?Nd(t): 8.2 to 10.2, Nd TDM ages: 1.63e1.68 Ga, 87Sr/86Sr(t): 0.7088e0.7112, ?Hf(t): 12.3 to 14.9),followed by late granodioritic intrusions (e.g., Guarataia pluton, UePb age: 576 ? 9 Ma, ?Nd(t): 12.52 to 13.11, Nd TDM age: 1.74e2.06 Ga, 87Sr/86Sr(t): 0.7104e0.7110, ?Hf(t): 12.9 to 21.6). The Muria e batholith (UePb age: 620e592 Ma, ?Nd(t): 8.2 to 13.6, Nd TDM age: 1.41e1.88 Ga) and the Concei?~ao da Boa Vista (586 ? 7 Ma) and Serra do Valentim (605 ? 8 Ma) stocks represent a segment of the Rio Doce arc correlated to the Serra da Bol?via and Marceleza complexes, making the link between the Ara?ua? and Ribeira orogenic domains. We suggest three phases of arc development: i) eastward migration of arc front (632e605 Ma), ii) widespread magma production in the whole arc (605e585 Ma), and iii) late plutonism in the western arc region (585e575 Ma). Usual processes of volcanic arc development, like subduction of oceanic lithosphere under a continental margin, followed by asthenosphere ascent related to slab retreating and break-off may explain the Rio Doce arc evolution