Ignez de Pinho Guimarães

Proterozoic links between the Borborema Province, NE Brazil, and the Central African Fold Belt

Abstract The Congo (CC) and the São Francisco (SFC) cratons were joined at about 2.05 Ga; northern parts of Palaeoproterozoic basement subsequently underwent extension at about 1 Ga, forming intracratonic basins. Neoproterozoic metasedimentary rocks in these basins yield detrital zircons as young as 630 Ma. The Brasiliano and Pan-African (c. 620–580 Ma) assembly of West Gondwana extensively altered this system. The Sergipano domain occurs north of the SFC, and the comparable Yaoundé domain occurs north of the CC. Crust north of the Sergipano domain comprises the Pernambuco–Alagoas (PEAL) domain. The NE–SW-striking Tcholliré–Banyo fault in Cameroon may extend southwestwards between the PEAL and Sergipano domains, defining northern limits of abundant SFC/CC basement. The Adamawa–Yadé domain in Africa does not appear to extend into Brazil. The Transverse domain of Brazil is a collage of Palaeoproterozoic crustal blocks, the 1.0 Ga Cariris Velhos orogen (CVO), late Neoproterozoic basins, and Brasiliano granites. The CVO extends ENE for more than 700 km in Brazil, but eastern continuation into Africa has not been identified. North of the Transverse domain contiguous c. 2.15 Ga gneisses comprise basement of Rio Grande do Norte and Ceará domains, which continue eastwards into western Nigeria and western Sahara.

Nd and Sr-Isotopic and U-Pb Geochronologic Constraints for Evolution of the Shoshonitic Brasiliano Bom Jardim and Toritama Complexes: Evidence for a Transamazonian Enriched Mantle Under Borborema Tectonic Province, Brazil

The Bom Jardim and Toritama complexes form elongated bodies between the Pernambuco-Alagoas massif and Pajeu-Paraiba fold belt and comprise multiphase intrusions of monzonitic and syenitic compositions. Low-angle foliation recorded in both complexes, indicated by alignment of euhedral minerals, runs parallel to the main thrust faults within the region. This suggests that emplacement of the complexes could be related to the thrust faults. The whole-rock geochemistry of major and trace elements, and the associated mineral chemistry, have established a shoshonitic affinity for these complexes. The mineral chemistry of the essential minerals within both complexes, the presence of magnetite, and the constant Fe/Fe + Mg ratios in biotites and amphiboles with increasing differentiation indicate trends compatible with a fractionating magma under high f(O2) conditions. The general overlap and similarities in mineral chemistry between the mafic minor facies and the less-evolved members of the major facies indicate t...

Intralithospheric differentiation and crustal growth: Evidence from the Borborema province, northeastern Brazil

Thousands of cubic kilometers of high-K calc-alkalic magmas intruded the Borborema Province (northeastern Brazil) during the Neoproterozoic Brasiliano orogeny. They make up large batholiths in which mantle-derived mafic to intermediate rocks coexist with a larger amount of granitoids. The relatively low silica contents (61‐70 wt% SiO 2 ) and moderate to high compatible element concentrations (0.3‐3.5 wt% MgO, 1.5‐3.8 wt% CaO, as much as 150 ppm of Cr) of the granitoids indicate that they contain an appreciable mantle component. The similar trace element geochemical (high contents of incompatible trace elements) and isotopic (strongly negative eNd values) signatures of mafic and felsic rocks combined with geochemical modeling suggest that (1) the mafic and felsic rocks are genetically linked, (2) the granitic magmas were produced by 20%‐30% partial melting from a source having geochemical characteristics similar to the mafic rocks, and (3) mingling and mixing of felsic magmas with subsequent batches of mafic magmas yielded the silica-poor granitoids. Isotopic data preclude involvement of the asthenosphere in the genesis of the mafic melts and instead indicate their derivation from an old, enriched lithospheric mantle. Therefore, addition of mantle material to the crust occurred through internal lithospheric differentiation, in contrast with conventional crustal-growth models.

Geochemical Signatures of Main Neoproterozoic Late-Tectonic Granitoids from the Proterozoic Sergipano Fold Belt, Brazil: Significance for the Brasiliano Orogeny

The Proterozoic Sergipano fold belt is intruded by various Neoproterozoic late-tectonic granitic plutons of contrasting composition, ranging from (1) metaluminous normal-K calc-alkaline and high-K calc-alkaline suites to (2) peralkaline shoshonitic/ultrapotassic suites and (3) a peraiuminous crustal leucocratic complex. Whole-rock trace-element data, however, show an overlapping LILE enrichment, except for the ultrapotassic suite. HFSE distributions range from 24 to 350 ppm, with the highest values belonging to the ultrapotassic suite and the lowest to the suite derived from the crust. High LILE/HFSE ratios of these rocks suggest their subduction-related character, which may be inherited from orogenic cycles older than the Brasiliano. High LILE and medium to high Cr and Ni contents in some facies from these granites suggest a lower-mafic-crust to metasomatized subcontinental-lithospheric-mantle component. The observed LILE content, degree of alkalinity, and REE polarity toward the northern limit of the Se...

Proterozoic c-type eclogites hosting unusual TiFe±Cr±Cu mineralization in northeastern Brazil

Mafic to ultramafic rocks occur at Bodoco, about 640 km west of Recife, cofolded with Early Proterozoic gneisses and migmatites, close to the tectonic contact with Middle to Late Proterozoic metasedimentary rocks of the Salgueiro-Cachoeirinha Group. The jadeite content of clinopyroxenes, the Fe/Mg partitioning between clinopyroxene and garnet, the Si activity in phengite, and the Na content of amphiboles in retrograde symplectites in gabbroic members, suggest a progressive metamorphic evolution up to C-type eclogites at 13 kbar/525° C, followed by a retrograde path of medium P/T ratio, to the greenschist facies. The final event transformed parts of these mafic rocks into banded amphibole gneisses and migmatites. The metagabbroic members contain up to 12% rutile. Metapyroxenitic members bear two types of massive ores: (a) cumulatic CrTi spinel bands and; (b) network ilmenomagnetite. The CrTi spinel is unusual in that it has a composition between ulvospinel, magnetite, and chromite and has a zonation of CrMg enrichment from core to rim which may be explained by peripheral Ti loss during high-pressure metamorphism. Several Ti occurrences in the Floresta area, 400 km west of Recife, including the 20 Mt Riacho da Posse deposit show striking similarities with the Bodoco occurrences: (1) Massive ores with high Ti/Fe ratios related to ultramafic rocks (olivine cumulates); (2) gabbroic intercalations composed of only garnet and amphibole, with the same mineral compositions recorded in Bodoco, lacking plagioclase but rich in rutile; and (3) migmatitic-gneissic wall rocks in the vicinity of the contact with younger metasedimentary rocks. A similar tectono-stratigraphic setting, a few km from the contact with the Late Proterozoic Macururo Group, and high pressure metamorphic host rocks (mafic granulites), has been described in the literature for the Serrote da Lage deposit of massive titanomagnetite with a 60 Mt interstitial copper mineralization. The only other eclogite occurrence recorded in Brazil, at Pouso Alegre, state of Minas Gerais, has an identical geological setting. Chemistry of the metagabbroic rocks of Bodoco and Floresta indicate an oceanic, tholeiitic-picritic character while the rare earth element patterns of the ultramafic rocks suggest a “modified ocean-floor peridotite” nature. Positive Co/Ni and very high Cu/Pd ratios in the sulphides also suggest an oceanic origin. All these features, with exception of the high Ti/Fe ratio, support an ophiolitic character of the mafic rocks. A detailed geochronological study may help to determine whether the mafic-ultramafic rocks represent relics of a true oceanic crust of the same age as the Salgueiro-Cachoeirinha Group, obducted onto older continental crust on an active Late Proterozoic continental margin, or if the proximity to the contact between basement and metasedimentary rocks is coincidental.

A-Type Post-Collisional Granites in the Borborema Province - NE Brazil: The Queimadas Pluton

Abstract The Queimadas Pluton constitutes an E-W elongated tabular intrusion, exposed over an area of 50 km 2 . It is intruded in Paleoproterozoic gneiss-migmatite of the Alto Pajeu Terrane in the Central Tectonic Domain of the Borborema Province, and has a zircon U-Pb age of 570±20 Ma. It is cut by later shear zones with NNE direction, which provoked necking and disruption in the body. A suite of biotite±amphibole monzogranites, associated with quartz diorite and later leucogranite and diabase, dominates in these alkaline granites that are metaluminous to slightly peraluminous and show high FeOt/(FeOt + MgO) and K 2 O/Na 2 O ratios. Their REE patterns are moderately fractionated, with (Ce/Yb) N ratios of 10-16 and significant negative Eu anomalies (Eu* = 0.40-0.67). Geochemical and Nd isotopic data indicate that the Queimadas Pluton magma originated by partial melting of a crustal fertile granodioritic rocks. The studied granites crystallized under relatively low f O 2 and are A-type post-collisional granitoids.

Shoshonitic and ultrapotassic Proterozoic intrusive suites in the Cachoeirinha-Salgueiro belt, NE Brazil: a transition from collisional to post-collisional magmatism

Abstract The Cachoeirinha-Salgueiro belt is located in the Borborema Province, NE Brazil. It has been intruded by several syenitic and granitic plutons and by dykes, which can be grouped into four magma series. This study deals with the petrology, geochemistry and the tectonic setting of two Neoproterozoic and one Cambrian intrusive suite from the Cachoeirinha-Salgueiro belt, namely the Terra Nova, Salgueiro and Serra do Livramento plutons, and associated dyke swarms. They belong to three distinct magma series: high-K calc-alkaline, shoshonitic and ultrapotassic. They show high LIL/HFS ratios. Increase in initial 87 Sr 86 Sr and Rb Sr ratios, with decreasing age of the intrusives, fits a genetic model in which crustal thickening occurred in the belt at the end of the Proterozoic. The intrusive rocks studied in this investigation show a clear overall geochemical signature of a subduction-related origin, although there are also some subtle differences. The authors believe these plutons both to represent the last major magmatic stage and to record a transitional tectonic setting between the collisional and post-collisional phases in the evolution of an active continental margin, now represented in part by the Cachoeirinha-Salgueiro belt.

Long-lived Neoproterozoic high-K magmatism in the Pernambuco–Alagoas Domain, Borborema Province, northeast Brazil

The Pernambuco–Alagoas (PE–AL) Domain contains major granitic batholiths typified by a wide range of TDM model ages (Archaean to Neoproterozoic), reflecting the important role of quartzofeldspathic plutons attending the Brasiliano (Pan-African) Orogeny. U/Pb zircon data for eight syn- to post-collision to syn-transcurrent granitic intrusions of the PE–AL Domain allow the studied plutons to be divided into two groups: (1) granitoids with crystallization ages older than 600 Ma (Água Branca, Serra do Catú, Serra da Caiçara, and Mata Grande plutons) and (2) granitoids with ages of ca. 590 Ma (Correntes, Águas Belas, Viçosa, and Cachoeirinha plutons). The intrusions of group 1, except for the Mata Grande Pluton, all show Nd TDM model ages ranging from 1.5 to 1.2 Ga, whereas the granitoids from group 2 and the Mata Grande Pluton have Nd TDM model ages ranging from 2.2 to 1.7 Ga. The studied granitoids are in part high-K, calc-alkaline, shoshonitic, and in part transitional high-K calc-alkaline to alkaline in terms of their bulk chemistry. Volcanic arc signatures associated with the Palaeoproterozoic TDM model ages are interpreted as inherited from the source rocks. The oldest ages and higher Nd TDM model ages recorded in the granitoids intruded in the southwestern part of the PE–AL Domain suggest that these intrusions are associated with slab-tearing during convergence between the PE–AL and Sergipano domains. The investigated plutons are coeval with high-K granitoids intruded within the Transversal Zone Domain of Borborema Province and calc-alkaline granitoids of the Sergipano Domain. This suggests that these geologic realms belonged to the same crustal block during the Brasiliano Orogeny. However, such large volumes of high-K granitoids with crystallization ages older than 600 Ma are not recorded in the Sergipano and Transversal Zone domains, suggesting differences in the crustal evolution of these three areas.

An example of in situ granite formation in the northern boundary of the Proterozoic Sergipano fold belt, NE Brazil: the Xingó Complex

The Xingo Complex occurs as a 1000 km2 NE-SW strip in the northern limit of the Proterozoic Sergipano fold belt within the Pernambuco-Alagoas Median Massif. It is comprised of the following rock types: a) migmatites, metatexites with diatexite pods; b) coarse-grained granites compared to migmatites, as lenses parallel to the migmatite foliation, with restites composed of biotite+zircon+sphene+apatite+epidote and c) as stocks of medium-grained granites and fine-grained dikes, hololeucocratic (biotite < 5%), with minor amounts of tourmaline, epidote, garnet and fluorite. Some dikes, more enriched in biotite, show crenulated borders, suggesting intrusion during a migmatite plastic stage. The granites are peraluminous, and show a narrow range in SiO2 (70–75%). The observed geochemical patterns and variations within the studied granites suggest an origin of succesive restite unmixing followed by homogenization and fractional crystallization. The Xingo Complex is an example of in situ granite formation during the Brasiliano (Pan-African) orogeny, possibly related to the collision between the Sergipano Belt and the Pernambuco-Alagoas Median Massif.

Mineral Chemistry and Tectonic Significance of Neoproterozoic Ultrapotassic Plutonic Rocks from the Cachoeirinha-Salgueiro Fold Belt, Northeastern Brazil

The Terra Nova ultrapotassic igneous rocks of northeastern Brazil consist of two dike swarms (alkali-feldspar syenites to quartz syenites and alkali-feldspar granites) and one elongated E-W syenitic body (the Serra do Livramento pluton), which intruded metasediments of the Cachoeirinha-Salgueiro fold belt from 580 to 514 Ma. Mafic ultrapotassic syenite enclaves are recorded in the Serra do Livramento and Terra Nova shoshonitic plutons, both of which are cut by the dike swarms. Mineralogically, Terra Nova ultrapotassic hypabyssal rocks resemble shoshonitic lamprophyres. Pyroxene is present in all facies; the clinopyroxenes are zoned, SiO2 saturated, and Al2O3 poor (0.12 to 1.15%), and range from earlier diopside to late acmite. Amphiboles are characterized by high SiO2 and low Al2O3 (0.20 to 2.00%) and TiO2 (0.0 to 1.76%) contents; their compositions range from calcic to alkaline. The late amphiboles are riebeckite-arfvedsonites instead of K-richterites, as expected in ultrapotassic rocks, reflecting the e...