Neoproterozoic Arc Research Articles

THE ARENÓPOLIS-MARA ROSA GOLD-COPPER BELT, NEOPROTEROZOIC GOIÁS MAGMATIC ARC

The Goiás Magmatic Arc hosts several gold and copper-gold deposits, which were originated during several stages of the tectonic, magmatic and metamorphic evolution of this Neoproterozoic Arc, between ca. 0.9 and 0.6 Ga. The nature of these deposits, their spatial distribution, and timing of mineralization agree with the model involving the continuous evolution of a collisional belt. They also display some of the characteristics of the orogenic gold deposits. The Zacarias Au-Ag-Ba Deposit was originated by volcano-exhalative processes in an intraoceanic subduction setting, whereas the Chapada Cu-Au Deposit (porphyry copper-type mineralization) is connected with plutonic rocks also associated with an intra-oceanic arc. The Posse Au Deposit was formed during the syn-collisional phase, shortly following the metamorphic peak of the Brasiliano Orogeny (ca. 630 Ma). Finally, the genesis of the Mundinho Au-Cu-Bi Mineralization is related with the emplacement of granite intrusions associated with post-orogenic extension.

Superposed Neoproterozoic and Silurian magmatic arcs in central Cape Breton Island, Canada: geochemical and geochronological constraints

Isotopic and geochemical data indicate that intrusions in the eastern Creignish Hills of central Cape Breton Island, Canada represent the roots of arcs active at ∼ 540–585 Ma and ∼ 440 Ma. Times of intrusion are closely dated by (1) a nearly concordant U–Pb zircon age of 553±2 Ma in diorites of the Creignish Hills pluton; (2) a lower intercept U–Pb zircon age of 540±3 Ma that is within analytical error of 40Ar/39 Ar hornblende plateau isotope-correlation ages of 545 and 550±7 Ma in the River Denys diorite; and (3) an upper intercept U–Pb zircon age of 586±2 Ma in the Melford granitic stock. On the other hand, ∼ 441–455 Ma 40Ar/39 Ar muscovite plateau ages in the host rock adjacent to the Skye Mountain granite provide the best estimate of the time of intrusion, and are consistent with the presence of granitic dykes cutting the Skye Mountain gabbro–diorite previously dated at 438±2 Ma. All the intrusions are calc-alkaline; the Skye Mountain granite is peraluminous. Trace element abundances and Nb and Ti depletions of the intrusive rocks are characteristic of subduction-related rocks. The ∼ 540–585 Ma intrusions form part of an extensive belt running across central Cape Breton Island, and represent the youngest Neoproterozoic arc magmas in this part of Avalonia. Nearby, they are overlain by Middle Cambrian units containing rift-related volcanic rocks, which bracket the transition from convergence to extension between ∼ 540 and 505/520 Ma. This transition varies along the Avalon arc: 590 Ma in southern New England, 560–538 Ma in southern New Brunswick, and 570 Ma in eastern Newfoundland. The bi-directional diachronism in this transition is attributed to northwestward subduction of two mid-ocean ridges bordering an oceanic plate, and the migration of two ridge–trench–transform triple points. Following complete subduction of the ridges, remnant mantle upwelling along the subducted ridges produced uplift, gravitational collapse and the high-temperature/low-pressure metamorphism in the arc in both southern New Brunswick and central Cape Breton Island. The ∼ 440 Ma arc magmatism in the Creignish Hills extends through the Cape Breton Highlands and into southern Newfoundland, and has recently been attributed to northwesterly subduction along the northern margin of the Rheic Ocean.

GEOCRONOLOGIA E GEOQUÍMICA DE ORTOGNAISSES DA REGIÃO ENTRE IPORÁ E FIRMINÓPOLIS: IMPLICAÇÕES PARA A EVOLUÇÃO DO ARCO MAGMÁTICO DE GOIÁS

In this study we report new Sm-Nd and Rb-Sr isotopic data and preliminary geochemical results for orhtogneissic and metavolcanic rocks units exposed roughly between the towns of Iporá and Firminópolis, southwestern Goiás. Samples from orthogneisses adjacent to the Iporá volcano-sedimentary sequence yielded a Rb-Sr isochron age of 673 ± 75 Ma, with initial 87 Sr/ 86 Sr ratio of 0.7048 ± 0.0004. ε Nd (T) values for these rocks are between ca. +0.4 and +1.9 and T DM ages are between 1.0 and 1.1 Ga, indicating the juvenile character of the parental magma. Trace element results also indicate the arc affinity of the magma. One felsic metavolcanic rock from the Iporá volcano-sedimentary sequence yielded an U-Pb zircon age of 636 ± 6 Ma and T DM model ages between 0.76 and 1.0 Ga. It represents the youngest volcanic example in the Goiás Magmatic Arc. In the Fazenda Nova area, samples from tonalitic and dacitic units display juvenile Nd isotopic characteristics, with T DM model ages of 0.95 and 0.88 Ga, respectively. Further to the south, in the southern end of the Moiporá-Novo Brasil mylonitic zone, biotite gneisses and mylonites have substantially older T DM ages, ranging from ca. 1.7 to 3.2 Ga, indicating the presence of an older sialic fragment in that area. The gneisses in the Moiporá area together with the Ribeirão gneisses in the vicinity of Arenópolis and displaying Archaean model ages, represent the only two exposures of older terranes known in the Goiás Magmatic Arc. The Firminópolis hornblende gneisses, in the eastern end of the area investigated, show arc-type geochemical signature and Nd isotopic characteristics (Sm-Nd whole-rock "errorchron" age of ca. 630 Ma and T DM ages of 1.1 -1.4 Ga) which indicate that the tonalitic protoliths were still part of the Neoproterozoic arc system. The results indicate that the juvenile rock units belonging to the Goiás Magmatic Arc extend to the east as far as the Firminópolis area where gneisses with very primitive characteristics, similar to the Arenópolis, Matrinxã and Sanclerlândia orthogneisses, are exposed.

Metasediments of the neoproterozoic Mara Rosa arc Brazil: Sm-Nd constraints for the tectonic setting of deposition and timing of metamorphism

Metasediments of the neoproterozoic Mara Rosa arc Brazil: Sm-Nd constraints for the tectonic setting of deposition and timing of metamorphism

The Mara Rosa Arch in the Tocantins Province: further evidence for Neoproterozoic crustal accretion in Central Brazil

The Mara Rosa volcano-sedimentary sequence consists of several NNE-trending belts of metavolcanic (metabasalts to metarhyolites) and metasedimentary rocks (micaschists, quartzites, cherts), exposed over large areas of the Tocantins Province, a Neoproterozoic (Brasiliano) orogenic region in the central part of Brazil. The supracrustal belts are separated by terrains dominated by metatonalites and metadiorites and intruded by several post-orogenic granitic and dioritic bodies. These volcano-sedimentary and associated metaplutonic rock associations have been previously interpreted as a typical granite-greenstone terrain of the Archaean basement. In this paper we present geochronological and isotopic data for rocks of the metavolcanic-metaplutonic rock association. Samples of a felsic metavolcanic rock from the Posse gold mine, and a metatonalite yielded UPb zircon ages of 862±8 Ma and 856 −7 +13 Ma, respectively. These are interpreted as crystallization ages of the igneous protoliths. Titanites from the same metavolcanic rock sample yielded a concordant recrystallization age of 632±4 Ma. SmNd isotopes for these rocks indicate primitive compositions, with ϵ Nd(T) of +4.6 and +3.7 and T DM model ages of ∼1.0 Ga. A syn-tectonic dioritic intrusion has an UPb crystallization age of 630±6 Ma, and ϵ Nd(t) of +1.9 and TDM of ∼1.0 Ga. Additional SmNd analyses on metasedimentary rocks and post-orogenic granites of the Mara Rosa region yield T DM ages between ∼1.2 and 1.0 Ga. The isotopic data, combined with preliminary trace element results, suggest that the protoliths of the metavolcanic and metaplutonic rocks investigated were formed in an island arc system, off the coast of the São Francisco-Congo continent at around 860 Ma. Recrystallization and deformational ages of ∼630 Ma may represent the timing of final ocean closure and continental collision. The rock associations, structural pattern, geochronological and isotopic characteristics of the rocks from the Mara Rosa region are very similar to those of Neoproterozoic arc terrains exposed ∼ 300 km to the southwest, in the Arenspólis area, proving the regional importance of the Neoproterozoic crustal accretion event in the central part of Brazil.

O ARCO MAGMÁTICO DE MARA ROSA, GOIÁS: GEOQUÍMICA E GEOCRONOLOGIA E SUAS IMPLICAÇÕES REGIONAIS

The Mara Rosa volcano-sedimentary sequence consists of several NNE-trending belts of metavolcanic (metabasalts to metarhyolites) and metasedimentary rocks (micaschists, quartzites, cherts), exposed over large areas of the Tocantins Province, a Neoproterozoic (Brasiliano) orogenic region in central Brazil. The supracrustaJ belts are separated by terranes dominated by metatonalites and metadiorites and intruded by several post-orogenic granitic and dioritic bodies. This metavolcanic/metaplutonic rock association has been previously interpreted as a typical Archaean granite-greenstone terrane and, therefore, part of the sialic basement to the Brasiliano supracrustal fold belt. In this paper we present the first reliable geochronological and isotopic data for rocks of the Mara Rosa sequence and associated metaplutonic rocks. Samples of a felsic metavolcanic rock from the Posse gold mine, and of a metatonalite yielded U-Pb zircon ages of 862 ± 8 Ma and 856 +13/-7 Ma, respectively. These are interpreted as crystallization ages of the igneous protoliths. Titanite from the metavolcanic rock sample yielded a concordant recrystallization age of 632 ± 4 Ma Rb-Sr geochronological data for orthogneissic, metasedimentary and mylonitic rocks indicate the closure of Rb-Sr isotopic system at ca. 600 Ma after isotopic re-homogeneization. Sm-Nd isotopes for these rocks indicate primitive compositions, with e Nd (T) of +4.6 and +3.7 and T DM model ages of ca. 1.0 Ga. A syn-tectonic dioritic intrusion has an U-Pb crystallization age of 630 ± 6 Ma, and e Nd (T) of +1.9 and T DM of ca. l .0 Ga. Additional Sm-Nd analyses of metasedimentary rocks and post-orogenic granites of the Mara Rosa region showed T DM ages between ca. 1.2 and 1.0 Ga. The isotopic data, combined with preliminary trace element results, suggest that the protoliths of the investigated metavolcanic and metaplutonic rocks formed at ca. 860 Ma in an island arc system, off the coast of the Sao Francisco-Congo continent. Recrystallization and deformation ages of ca. 630 Ma could represent the timing of final ocean closure and continental collision. The rock associations, structural pattern, geochronological and isotopic characteristics of the rocks from the Mara Rosa region are very similar to those of Neoproterozoic arc terrains exposed ca. 300 km to the southwest, in the Arenopolis area, proving the regional importance of the Neoproterozoic crustal accretion event in the central part of Brazil.