Petrology, geochemistry and U -Pb zircon and baddeleyite ages of the alkaline rocks from the central- southern Guyana Shield, northern Amazonian Craton

CAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORThe Apiau Alkaline Complex (AAC) represents a rift-related intracontinental magmatism and occurs as intrusions within the Precambrian basement of the central-southern portion of the Guyana Shield, Amazonian Craton. The complex extends for over 200 km and86461474CAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORsem informaçãoWe thank the Brazilian agency CAPES/PROEX for the financial support and the fellowship granted to the first author. We are very grateful to the SGM mining company, specially Marcelo Almeida, Claumilde Vasconcelos and Paulo Orcioli for their valuable assi

The São Pedro Tectonostratigraphic Terrane: West of the Transversal Zone – Borborema Province

With the present paper, we are completing the description of the basement fractions of the Borborema Province with a discussion of the tectonostratigraphic terrane of the central part of the aforementioned province (so-called Transversal Zone), which is partially covered by the Gondwanic sediments of the Parnaíba Syneclise, on most of the western part of the province and of its basement (which is partially composed of paleo-cratonic segments, posing metacratonic behavior, detected by geopysical methods). The basement of this wide tectonostratigraphic terrane (“São Pedro Terrane”) is characterized by a complex setting of Archean rock units (western part of it, sheets of Patos do Piauí and Simões, Piauí State) with fast passage to Paleoproterozoic rock units that dominate in the whole eastern domains (Sheets of Ouricuri and Bodocó, Pernambuco State). To the south and southeast of this pre-Paleozoic terrane it is possible to see the development of a straight and long linear belt (SW-NE) the Ouricuri-Bodocó belt. This orogenic belt (of evolution in the Brasiliano cycle) is presenting different linear bands of metasedimentary and volcano-sedimentary rock assemblages at green schist facies (northern band) and amphibolite fáceis (southern band), and they are both intruded by various Ediacaran stocks and batholiths. This orogenic belt, to the south, is limited by another Pre-Brasiliano tectonostratigraphic terrane, so-called Icaiçara. Both limits of this Ouricuri–Bodocó belt (northwest and southeast) are strongly marked by shear zones, associated with thrusts. South of Exu, a small perpendicular paleo- rift zone is exposed (as a paleo-aulacogen), now presenting an interesting example of local impactogenic structure.Com a presente publicação, completa-se a descrição das frações de embasamento da Província Borborema com o terreno tectonoestratigráfico da parte central da referida província (usualmente chamada de Zona Transversal), que é parcialmente coberta pelos sedimentos gondwânicos da Sinéclise do Parnaíba, na maior parte ocidental da província e de seu embasamento (supostos meta-crátons detectados por geofísica). O embasamento deste terreno tectonoestratigráfico “São Pedro”, na sua parte mais ocidental é composto por rochas arqueanas (meso e neoarqueanas, com alguns traços paleoarqueanos) na sua porção mais ocidental (folhas de Patos do Piauí e Simões, no Estado do Piauí). Há passagem para rochas paleoproterozoicas que predominam nas folhas de Ouricuri e Bodocó (Estado de Pernambuco). Como limite sul e sudeste destes terrenos pré-paleozoicos, encontra-se uma faixa orogênica longa e linear (SW-NE), chamada de Ouricuri-Bodocó. Esta faixa orogênica de evolução no ciclo Brasiliano apresenta uma faixa de rochas na fácies anfibolito (situada mais ao sul), metassedimentares e metavulcânicas, e uma outra faixa de rochas metassedimentares clásticas e calciossilicáticas na fácies xisto verde (situada mais ao norte, bordejando o Terreno São Pedro). Estas faixas são intrudidas por diferentes conjuntos de granitos brasilianos. A faixa orogênica brasiliana, ao sul, encontra-se circundando outro terreno tectonoestratigráfico já conhecido e muito discutido em várias teses e trabalhos prévios (Terreno Icaiçara). Ambos os limites norte e sul da faixa orogênica Ouricuri-Bodocó são bem-marcados por zonas de cisalhamento policíclicas, associadas com empurrões. Ao sul de Exu (Pernambuco), um pequeno paleorifte pode ser observado (como um antigo aulacógeno), agora protagonizando interessante e possível exemplo de impactógeno

Depósitos de escarnitos mineralizados em ferro e cobre do arco magmático de Santa Quitéria, Ceará, Provincia Borborema do nordeste do Brasil

Resumo Depósitos de escarnitos mineralizados em W-Mo da Província Borborema são conhecidos desde os anos 40. Entretanto, até a execução deste trabalho, não se tinha registros desses depósitos mineralizados em Fe-Cu. No Ceará, as primeiras ocorrências e/ou depósitos de escarnitos ricos em ferro e cobre foram identificadas recentemente no Arco Magmático Continental de Santa Quitéria, considerado produto de evolução de vários arcos magmáticos, variando de arco juvenil (870 - 800 Ma) a colisão continental (625 - 600 Ma). Os escarnitos estão associados às rochas calcissilicáticas e mármores dolomíticos e/ou calcíticos, recortadas por quartzo monzonitos neoproterozóicos cedo a tardi-colisionais. São encontrados todos os tipos do sistema escarnítico como endoescarnitos, hornfels, escarnóides e exoescarnitos. Os exoescarnitos apresentam duas associações minerais, que refletem protólitos carbonáticos distintos: uma composta por clinopiroxênio-granada (estágio progradante) e magnetita-pirrotita-calcopirita-pirita-biotita (estágio retrogradante), característica de protólito calcítico, e outra formada por olivina-espinélio-diopsidio na fase progradante e serpentina na fase retrogradante, representativa de protólito dolomítico. As granadas, em parte zonadas, exibem composição dominada por andradita (82 - 52 mol%), seguida por grossularita (16 - 45mol%) e, em proporção menor, piropo (2 - 3 mol%). O piroxênio do protólito calcítico é típico da série diopsídio-hedenbergita, com composição média Diopsídio46Hedenbergita52Johannsenita2. Três tipos de minério magnetítico são encontrados: i) magnetito bandado; ii) magnetito disseminado; iii) magnetito maciço filoneano. Ocorre ainda um minério sulfetado de cobre com e sem magnetita associado, e blocos de gossans. Os dados geológicos e metalogenéticos obtidos indicam que a gênese destes escarnitos está associada à influência dos granitos monzoníticos tardi-brasilianos e, embora haja dois tipos de escarnito, predomina o mineralizado em ferro e cobre

Evidence for Neoproterozoic continental arc magmatism in the Santa Quitéria Batholith of Ceará State, NW Borborema Province, NE Brazil: Implications for the assembly of West Gondwana

Recent field investigations and geochronological studies of Neoproterozoic rocks in the northwestern part of the Borborema Province, Ceará State, NE Brazil provide important clues pertaining to the nature of convergence between the Borborema Province and the West African-São Luis craton during the assembly of West Gondwana. U-Pb zircon data indicate that the earliest evidence of convergent magmatism along the northwest margin of the Borborema Province occurred around 777 Ma, and was followed by the development of a large continental arc batholith (Santa Quitéria batholith) between ca. 665 and 591 Ma within the central part of Ceará State. These findings, along with supporting geophysical data, suggest that convergence between the Borborema Province and the West African-São Luis craton involved closure of an oceanic realm with subduction polarity to the southeast beneath the northwestern part of the province. Consequently, it seems likely that the Pharusian Ocean was continuous from the Hoggar Province in West Africa into South America during the late Neoproterozoic and additional data suggests that it may have even been connected with the Goianides Ocean of the Brasília Belt farther to the southwest

Lithogeochemistry and zircon U-Pb geochronology of the Granjeiro Complex and associated units, Curral Novo do Piauí, SW-Borborema Province, Brazil: implications for Archean to Paleoproterozoic crustal evolution

The Curral Novo Iron District, located in the center-east of the state of Piauí, is hosted in the Granjeiro Complex in its southwestern section, located in the Rio Piranhas-Seridó Domain, northern subprovince of Borborema Province. In this district, the banded iron rocks have an average content of approximately 27% Fe, with an estimated potential of more than one million tons of ore. Through the integration of petrographic, isotopic, geochemical and geochronological field data, a comprehensive characterization of the lithotypes present in the complex (orthogneisses, migmatites and a metavolcano-sedimentary unit) and associated units (syenogranitic orthogneisses and pegmatite granites) was carried out, focusing on the evolutionary discussion of this Archean complex, as well as the origin of metasomatism present in the iron district.  Tonalitic and granodioritic orthogneisses include calcic to alkali-calcic rocks with low mafic mineral content. These rocks have low concentrations of lithophile elements and high field potential elements, significant fractionation of light rare earth elements and slightly negative Eu anomalies. U-Pb geochronology in tonalitic orthogneiss zircon from the Granjeiro Complex indicates the age of the protolith in the Paleoarchean at 3349 Ma. The metavolcano-sedimentary unit is made up of amphibolites, magnesian schists, serpentinites and banded iron rocks. Such rocks derived from mafic and ultramafic protoliths have a komatiitic and basaltic composition and have a genesis associated with the subduction phase of oceanic plates, presenting a geochemical affinity similar to that of modern island arc rocks. The iron rocks present sedimentary facies of the oxide and silicate types, with strong impoverishment of high field potential elements and rare earth elements and negative Ce anomalies. These units are intruded by syenogranitic plutons, collisional powders, type A2, aged 2651 Ma. Chemically, these granites are metaluminous to peraluminous, alkali-calcic and ferrous types, having a slight enrichment of light rare earth elements in relation to heavy rare earth elements and high contents of lithophile elements and high field potential elements.  The rocks of the Granjeiro Complex show records of strong crustal reworking in the Paleoproterozoic, starting from the Riacian, with a metamorphic peak in metavolcanosedimentary rocks indicated by U-Pb dating in amphibolite zircon at 2200 Ma. An important magmatic event related to the global process of extension of continental masses during the Staterian, aged 1759 Ma, represented by several intrusive plutons in all units of the area, promoted hydrothermal alteration in the surrounding amphibolites and iron rocks, causing silicification, albitization, potassification, carbonatization, sulfidation and reconcentration of iron. δ 34S values = -3.11 to 2.3 per mil in hydrothermal sulfides suggest magmatic sources for the sulfur associated with the hydrothermal system. The incorporation of the analysis of these diverse geological data, the focus of this study, made it possible to obtain a detailed understanding of the geological evolution of the Granjeiro Complex in the area, as well as the origin of metasomatism in the iron district