Proveniência sedimentar do grupo São Fidélis, terreno oriental da Faixa Ribeira com base em dados U-Pb

O Domínio Costeiro integra o Terreno Oriental, no segmento central da Faixa Ribeira e abriga rochas ortoderivadas com afinidade de arcos magmáticos (Complexo Rio Negro, ca. 790-605 Ma). Os ortognaisses deste complexo possuem clara assinatura para ambientes de zonas de subducção, encaixados em rochas metassedimentares de alto grau, integrantes do Grupo São Fidélis. O conjunto acima descrito é ainda intrudido por várias de rochas granitóides sin a tardi- colisionais, relacionadas às várias etapas de desenvolvimento da Orogenia Brasiliana neste setor do orógeno (ca. 605-480 Ma). Idades U-Pb (LA-ICP-MS) em zircões detríticos de rochas quartzíticas do Grupo São Fidélis indicam um amplo espectro com modas significativas no Mesoproterozóico e Paleoproterozoico, além de zircões do Neoproterozóico e do Arqueano. Sinteticamente os resultados obtidos foram: a) Idades concordantes Arqueanas com ca. 2,85, 2,84 e 2,70 Ga; b) zircões Paleoproterozóicos (ca. 2,3 a 1,7 Ga), com máxima concentração em torno de ca. 2,2 Ga, representando a segunda maior moda; c) Idades Mesoproterozóicas (ca. 1,3 -1,1 Ga) com idades de espectro dominantes, com moda em ca. 1,5 Ga; d) Zircões Neoproterozóicos com idades de ca. 0,95-90 Ga e 0,86-0,61 Ga. Em vários grãos detríticos observou-se sobrecrescimento metamórfico em ca. 602-570 Ma. Dados U-Pb (LA-ICP-MS) obtidos para zircões para Ortognaisse Rio Grande e o Biotita Ortognaisse, intrudidos na unidade basal do Grupo São Fidélis, apresentam idades em ca. 620 Ma e são equivalentes ao período pré-colisional de geração de rochas do arco magmático Rio Negro. Combinando estas idades com os núcleos de zircões detríticos mais jovens, com assinatura do Arco Rio Negro em ca. 613 Ma, pode-se definir o intervalo máximo de sedimentação da unidade superior do Grupo São Fidélis no Neoproterozóico. Cristais de monazitas selecionadas para análise U-Pb (ID-TIMS) apresentam relações com os principais episódios tectono-metamórficos da Faixa Ribeira. Dois cristais de uma amostra quartzítica e dois do ortognaisse Rio Grande alinham-se em uma discórdia que gerou idade de 603 Ma, referente ao metamorfismo progressivo descrito na literatura, durante a Orogenia Brasiliana. Enquanto a idade concordante obtida em 535 Ma, adquirida em uma amostra quartzítica, é correspontente ao último metamorfismo colisional da Faixa Ribeira.

Relations between A-type granites and copper mineralization as exemplified by the Machangqing Cu deposit

This paper deals with the relations between the Machangqing rockbody which corresponds to the A-type granites and porphyry copper mineralization in terms of petrochemistry, trace element geochemistry, fluid inclusion geochemistry and isotope geochemistry. The results show that the Machangqing porphyry copper deposit was formed from the fluid predominated by mag-matic fluid. This kind of ore-forming fluid was just differentiated from the magma responsible for the A-type granites. therefore,as viewed from whereer they contain water or not,the A-type granites can,at least,be divided into two types: water-bearing and water-free.The water-bearing A-type granites can serve as the host ofporphyry copper deposits under certain geological conditions.

Evolution of an Archean greenstone belt in the Stormy Lake - Kawashegamuk Lake area (stratigraphy, structure and geochemistry) - Western Wabigoon Subprovince, Northwest Ontario /

330 km 2 of the easter-n part of the Archean Manitou Lakes - Stormy Lake metavolcanic - metasedimentary belt have been mapped and sampled. A large number of rocks ~.vere analyzed for the major and trace constituents including the rare-earth elements (REE). The Stormy Lake - Kawashegamuk Lake area may be subdivided into four major lithological groups of supracrustal rocks 1) A north-facing mafic assemblage, consisting of pillowed tholeiitic basalts and gabbro sills characterized by flat REE profiles, is exposed in the south part of the map area and belongs to a 8000 m thick homoclinal assemblage outside the map area. Felsic pyroclastic rocks believed to have been issued from a large central vent conformably overlie the tholeiites. 2) A dominantly epiclastic group facing to the north consists of terrestrial deposits interpreted to be an alluvial fan deposit ; a submarine facies is represented by turbiditic sediments. 3) The northeastern part of the study area consists of volcanic rocks belonging to two mafic - felsic cycles facing to the southuest ; andesitic flows with fractionated REE patterns make up a large part of the upper cycle, whereas the lower cycle has a stronger chemical polarity being represented by tholeiitic flows, with flat REE, which a r e succeeded by dacitic and rhyolitic pyroclasti cs. iii 4) A thick monotonous succession of tholeiitic pillmled basalt f lows and gabbro sills with flat REE represent the youngest supracrustal rocks. TIle entire belt underwent folding, faulting and granitic plutonism during a tectono-thermal event around 2700 Ma ago. Rocks exposed in the map area were subjected to regional greenschist facies metamorphism, but higher metamorphic grades are present near late granitic intrusions. Geochemical studies have been useful in 1) distinguishing the various rock units ; 2) relating volcanic and intrusive rocks 3) studying the significance of chemical changes due to post magmatic processes 4) determining the petrogenesis of the major volcanic rock types. In doing so, two major volcanic suites have been recognized : a) a tholeiitic suite, mostly represented by mafic rocks, was derived from partial melting of upper mantle material depleted in Ti, K and the light REE ; b) a calc-alkalic suite which evolved from partial melting of amphibolite in the lower crust. The more differentiated magma types have been produced by a multistage process involving partial melting and fractional crystallization to yield a continuum of compos i t i ons ranging from basaltic andesite to rhyolite. A model for the development of the eastern part of the Manitou Lakes - Stormy Lake belt has been proposed.

Physical volcanology, sedimentology, stratigraphy and petrochemistry of the Berry Creek metavolcanics: an Archean calc-alkaline complex, Lake of the Woods, Ontario

The steeply dipping, isoclinally folded early Precambrian (Archean) Berry Creek Metavolcanic Complex comprises primary to resedimented pyroclastic, epiclastic and autoclastic deposits. Tephra erupted from central volcanic edifices was dumped by mass flow mechanisms into peripheral volcanosedimentary depressions. Sedimentation has been essentially contemporaneous with eruption and transport of tephra. The monolithic to heterolithic tuffaceous horizons are interpreted as subaerial to subaqueous pumice and ash flows, secondary debris flows, lahars, slump deposits and turbidites. Monolithic debris flows, derived from crumble breccia and dcme talus, formed during downslope collapse and subsequent gravity flowage. Heterolithic tuff, lahars and lava flow morphologies suggest at least temporary emergence of the edifice. Local collapse may have accompanied pyroclastic volcanism. The tephra, produced by hydromagmatic to magmatic eruptions, were rapidly transported, by primary and secondary mechanisms, to a shallow littoral to deep water subaqueous fan developed upon the subjacent mafic metavolcanic platform. Deposition resulted from traction, traction carpet, and suspension sedimentation from laminar to turbulent flows. Facies mapping revealed proximal (channel to overbank) to distal facies epiclastics (greywackes, argillite) intercalated with proximal vent to medial fan facies crystal rich ash flows, debris flows, bedded tuff and shallow water to deep water lava flows. Framework and matrix support debris flows exhibit a variety of subaqueous sedimentary structures, e.g., coarse tail grading, double grading, inverse to normal grading, graded stratified pebbly horizons, erosional channels. Pelitic to psammitic AE turbidites also contain primary stru~tures, e.g., flames, load casts, dewatering pipes. Despite low to intermediate pressure greenschist to amphibolite grade metamorphism and variably penetrative deformation, relicts of pumice fragments and shards were recognized as recrystallized quartzofeldspathic pseudomorphs. The mafic to felsic metavolcanics and metasediments contain blasts of hornblende, actinolite, garnet, pistacitic epidote, staurolite, albitic plagioclase, and rarely andalusite and cordierite. The mafic metavolcanics (Adams River Bay, Black River, Kenu Lake, Lobstick Bay, Snake Bay) display _holeiitic trends with komatiitic affinities. Chemical variations are consistent with high level fractionation of olivine, plagioclase, amphibole, and later magnetite from a parental komatiite. The intermediate to felsic (64-74% Si02) metavolcanics generally exhibit calc-alkaline trends. The compositional discontinuity, defined by major and trace element diversity, can be explained by a mechanism involving two different magma sources. Application of fractionation series models are inconsistent with the observed data. The tholeiitic basalts and basaltic andesites are probably derived by low pressure fractionation of a depleted (high degree of partial melting) mantle source. The depleted (low Y, Zr) calc-alkaline metavolcanics may be produced by partial melting of a geochemically evolved source, e.g., tonalitetrondhjemite, garnet amphibolite or hydrous basalt.

Controls on development and diversity of Early Archean stromatolites

The ≈3,450-million-year-old Strelley Pool Formation in Western Australia contains a reef-like assembly of laminated sedimentary accretion structures (stromatolites) that have macroscale characteristics suggestive of biological influence. However, direct microscale evidence of biology—namely, organic microbial remains or biosedimentary fabrics—has to date eluded discovery in the extensively-recrystallized rocks. Recently-identified outcrops with relatively good textural preservation record microscale evidence of primary sedimentary processes, including some that indicate probable microbial mat formation. Furthermore, we find relict fabrics and organic layers that covary with stromatolite morphology, linking morphologic diversity to changes in sedimentation, seafloor mineral precipitation, and inferred microbial mat development. Thus, the most direct and compelling signatures of life in the Strelley Pool Formation are those observed at the microscopic scale. By examining spatiotemporal changes in microscale characteristics it is possible not only to recognize the presence of probable microbial mats during stromatolite development, but also to infer aspects of the biological inputs to stromatolite morphogenesis. The persistence of an inferred biological signal through changing environmental circumstances and stromatolite types indicates that benthic microbial populations adapted to shifting environmental conditions in early oceans.

Major- and trace-element composition of REE-rich turkestanite from peralkaline granites of the Morro Redondo Complex, Graciosa Province, south Brazil

Turkestanite, a rare Th- and REE-bearing cyclosilicate in the ekanite-steacyite group was found in evolved peralkaline granites from the Morro Redondo Complex, south Brazil. It occurs with quartz, alkali feldspar and an unnamed Y-bearing silicate. Electron microprobe analysis indicates relatively homogeneous compositions with maximum ThO(2), Na(2)O and K(2)O contents of 22.4%, 2.93% and 3.15 wt.%, respectively, and significant REE(2)O(3) abundances (5.21 to 11.04 wt.%). The REE patterns show enrichment of LREE over HREE, a strong negative Eu anomaly and positive Ce anomaly, the latter in the most transformed crystals. Laser ablation inductively coupled plasma mass spectrometry trace element patterns display considerable depletions in Nb, Zr, Hf, Ti and Li relative to whole-rock sample compositions. Observed compositional variations suggest the influence of coupled substitution mechanisms involving steacyite, a Na-dominant analogue of turkestanite, iraqite, a REE-bearing end-member in the ekanite-steacyite group, ekanite and some theoretical end-members. Turkestanite crystals were interpreted as having precipitated during post-magmatic stages in the presence of residual HFSE-rich fluids carrying Ca, the circulation of which was enhanced by deformational events.

ASSOCIATED A-TYPE SUBALKALINE AND HIGH-K CALC-ALKALINE GRANITES IN THE ITU GRANITE PROVINCE, SOUTHEASTERN BRAZIL: PETROLOGICAL AND TECTONIC SIGNIFICANCE

The 590-580 Ma Itu Granite Province (IGP) is a roughly linear belt of post-orogenic granite plutons similar to 60 km wide extending for some 350 km along the southern edge of the Apia-Guaxupe Terrane in southeastern Brazil. Typical components are subalkaline A-type granites (some with rapakivi texture) that crystallized at varied, but mostly strongly oxidizing conditions, and contrast with a coeval association of also oxidized high-K calc-alkaline granites in terms of major (e. g., lower Ca/Fe) and trace elements (higher Nb, Y, Zr). Mantle-derived magmas (such as those forming the LILE-rich Piracaia Monzodiorite, with epsilon(Nd(t)) = -7 to -10, (87)Sr/(86)Sr((t)) = 0.7045-0.7055) are inferred to derive from enriched subcontinental lithosphere modified during previous subduction, and may have played a role in the generation of the A-type granites, adding melts or fluids or both to the lower crust from which the latter were generated. The IGP is interpreted as a reflection of crust uplift and increased heat flux during ascent of hot, less dense asthenosphere after continental collision, probably reflecting breakoff of an oceanic slab coeval to the right-lateral accretion of a terrane related to the Mantiqueira Orogenic System.

Trace element geochemistry and Sr-Nd characteristics of Mesoproterozoic mafic intrusive rocks from Rondonia, Brazil, SW Amazonian Craton: Petrogenetic and tectonic inferences

The Amazonian Craton comprises an Archean domain surrounded by four successively younger Proterozoic tectonic provinces. Within the Rio-Negro-Juruena province the Serra da Providencia Intrusive Suite (1.60 and 1.53 Ga) consists of A-type rapakivi granites, charnockites and mangerites genetically associated with diabase dikes, gabbros and amphibolites lites. The original mafic melts were derived from a depleted mantle source (epsilon(Nd(T)) + 2.5 to +2.8; epsilon(Sr(T)) - 12.1). Underplated mafic magma induced melting of a short-lived fielsic crust, thus originating coeval felsic-inafic magmatism in a continental intraplate setting. The Colorado Complex, assigned to the Rondonian-San Ignacio province, comprises 1.35-1.36 Ga intrusive bimodal magmatism represented by monzonite gneisses associated with amphibolite, gabbro and metadiabase dikes intercalated with metasediments with detrital zircon that yield U-Pb ages of 1.35 to 1.42 Ga. Mafic samples display juvenile signatures (epsilon(Nd(T)) 0.0 to +5.2; epsilon(Sr(T)) -5.0 to -30.7) and are less contaminated than the Serra da Previdencia and Nova Brasiladndia ones. The generation of the basaltic magma is related to the subduction of an oceanic slab below the peridotite wedge (intraoceanic arc setting). Fluids and/or small melts from the slab impregnated the mantle. The Nova Brasilandia Sequence (Sunsas-Aguapei province) comprises a metasedimentary sequence intruded by 1.10-1.02 Ga metadiabases, gabbros, meta-gabbros, and amphibolites associated with granitic plutons (bimodal magmatism). The original tholeiitic magmas, derived from a depleted source (epsilon(Nd(T)) = +3.1 to +5.0), in a proto-oceanic setting, underwent subsequent contamination by the host rocks, as indicated by the isotopic and trace element data.

Sr and Nd isotopic geochemistry of the early ultramafic-mafic rocks of the Mako bimodal volcanic belt of the Kedougou-Kenieba inlier (Senegal)

The Mako bimodal volcanic belt of the Kedougou-Kenieba inlier is composed of volcanic basalts and peridotites interbedded by quartzites and limestones intruded by different generations of granitoids. The early volcanic episode of the belt is constituted of submarine basalts with peridotite similar to those of the oceanic abyssal plains. It is intruded by the Badon Kakadian TTG-granitic batholite dated around 2200 Ma. The second volcanic phase is constituted of basaltic, andesitic, and felsitic flows exhibit structures of aerial volcanic rocks. It is intruded by granites dated between 2160 and 2070 Ma. The general pattern of trace element variation of submarine volcanic rocks is consistent with those of basalts from oceanic plateaus which are the modern equivalent of the Archean greenstones belts. The Nd and Sr isotopic systematics typical of juvenile material indicates that the source of these igneous rocks is derived from a depleted mantle source. These results are consistent with the idea of a major accretion within the West African Craton occurring at about 2.1 Ga and corresponding to an important process of mantle-oceanic lithosphere differentiation.

Limites e evolução geodinâmica do Sistema Jaguaribeano, Província Borborema, Nordeste do Brasil

The studied area is geologically located in the Northern Domain of the Borborema Province (Northeast Brazil), limited to the south by the Patos shear zone. Terranes of the Jaguaribeano system are dominant, flanked by the Piranhas (E and S sides) and Central Ceará (NE side) terranes. Its basement comprises gneiss -migmatite terrains of Paleoproterozoic to Archean age (2.6 to 1.9 Ga old), overprinted by neoproterozoic to cambrian tectonotherma l events. Narrow supracrustal belts ( schist belts) display a 1.6 to 1.8 Ga age, as shown by whole - rock Rb-Sr and zircon U-Pb and Pb/Pb dates in acid metavolcanics which dominate in the lower section of these sequences, and in coeval metaplutonics (granitic augen gneisses). From the stratigraphic point of view, three Staterian belts are recognized: 1. Orós Belt - made up by the Orós Group, subdivided in the Santarém (predominantly pure to impure quartzites, micaschists and metacarbonates) and Campo Alegre (metandesites, metabasalts, metarhyolites and metarhyodacites, interlayered with metatuffs and metasediments) formations, and by the Serra do Deserto Magmatic Suite (granitic augen gneisses). 2. Jaguaribe Belt - its lithostratigrahic-lithodemic framework is similar to the one of the Orós Belt, however with a greater expression of the volcano -plutonic components (Campo Alegre Formation and Serra do Deserto Magmatic Suite). The Peixe Gordo Sequence, separately described, is also related to this belt and contain s metasedimentary, metavolcanic (with subordinated volcanoclastics) and metaplutonic units. The first one correlated to the Orós Group and the latter the Serra do Deserto Magmatic Suite. 3. Western Potiguar Belt - represented by the Serra de São José Gro up, subdivided in the Catolezinho (biotite -amphibole gneisses with intercalations of metacarbonates, calcsilicate rocks, amphibolites and quartzite beds to the top) and Minhuins (quartzites, micaschists, metaconglomerates, calcsilicate rocks, acid to the b asic metavolcanics and metatuffs) formations. Its late Paleoproterozoic (Staterian) age was established by a Pb/Pb date on zircons from a granitic orthogneiss of the Catolezinho Formation. The petrographic characteristics and sedimentary structures of the Santarém Formation of the Orós Group point to deltaic to shallow marine depositional systems, overlain by deep water deposits (turbidites). The geodynamic setting of this region encompassed a large depositional basin, probably extending to the east of the Portalegre shear zone and west of the Senador Pompeu shear zone, with possible equivalents in the Jucurutu Formation of the Seridó Belt and in the Ceará Group of central Ceará. The Arneiróz Belt, west Ceará, displays some stratigraphic features and granito ids geochemically akin to the ones of the Orós Belt. The evolutionary setting started with an extensional phase which was more active in the eastern part of this domain (Western Potiguar and part of the Jaguaribe belts), where the rudite and psamite sedime ntation relates to a fluviatile rift environment which evolved to a prograding deltaic system to the west (Orós Group). The basaltic andesitic and rhyolitic volcanics were associated to this extensional phase. During this magmatic event, acid magmas also crystallized at plutonic depths. The Orós Group illustrates the environmental conditions in the western part of this domain. Later on, after a large time gap (1.6 to 1.1 Ga), the region was subjected to an extensional deformational episode marked by 900 Ma old (Sm-Nd data) basic rocks, possibly in connection with the deposition of the Cachoeirinha Group south of the Patos shear zone. In the 800 to 500 Ma age interval, the region was affected by important deformational and metamorphic events coupled with in trusion of granitic rocks of variable size (dykes to batholiths), related to the Brasiliano/Pan -African geotectonic cycle. These events produced structural blocks which differentiate, one from the other, according to the importance of anatectic mobilizatio n, proportion of high-grade supracrustals and the amount of neoproterozoic -cambrian granitoid intrusions. On this basis, a large portion of the Jaguaretama Block/Terrane is relatively well preserved from this late overprint. The border belts of the Jagua retama Block (Western Potiguar and Arneiroz) display kyanite-bearing (medium pressure) mineral associations, while in the inner part of the block there is a north-south metamorphic zoning marked by staurolite or sillimanite peak metamorphic conditions. Regarding the deformations of the Staterian supracrustal rocks, second and third phases were the most important, diagnosed as having developed in a progressive tectonic process. In the general, more vigorous conditions of PT are related to the interval tardi - phase 2 early-phase 3, whose radiometric ages and regional structuring indicators places it in the Brasiliano/Pan-African Cycle. In the Staterian geodynamic setting of Brazilian Platform , these sequences are correlated to the lower Espinhaço Supergroup (p.ex., Rio dos Remédios and Paraguaçu groups, a paleproterozoic rift system in the São Francisco Craton), the Araí and Serra da Mesa groups (north of Goiás, in the so -called Goiás Central Massif), and the Uatumã Group (in the Amazonian Craton). Granitic ( augen gneisses) plutonics are also known from these areas, as for example the A-type granites intrusive in the Araí and Serra da Mesa groups, dated at 1.77 Ga. Gravimetric and geological data place the limits of the Jaguaribeano System (terranes) along the Senador Pompeu Shear Zone (western border) and the Portalegre- Farias Brito shear zone (eastern and southern). However, the same data area not conclusive as regards the interpretation of those structures as suture of the terrane docking process. The main features of those shear zones and of involved lothological associations, appear to favour an intracontinental transpressional -transcurrent regime, during Neoproterozoic-Cambrian times, marking discontinuities along which different crustal blocks were laterally dispersed. Inside of this orogenic system and according to the magnetic data (total field map), the most important terrane boundary appears to be the Jaguaribe shear zone. The geochronological data, on some tectonostratigraphic associations (partly represented by the Ceará and Jucurutu groups), still at a preliminary level, besides the lack of granitic zonation and other petrotectonic criteria, do not allow to propose tectonic terrane assembly diagrams for the studied area