An Early-Cambrian UPb apatite cooling age for the high-temperature regional metamorphism in the Piancó area,Borborema Province (NE Brazil): initial conclusions

The Borborema Province (BP) of northeastern Brazil is a complex crustal assemblage, which has undergone a polycyclic evolution during the Proterozoic. In the Piancó-Alto Br??gida belt, a metamorphosed leucosome vein inserted in amphibolites has a trace element pattern suggesting a T-MORB protolith. Apatites yield a REE pattern indicating growth in equilibrium with garnet, thus pointing to its metamorphic origin. UPb analyses yield an age of 540±5 Ma interpreted as a cooling age following amphibolite facies regional metamorphism associated with granitic emplacement at ca. 580 Ma. The resulting slow cooling rates (ranging from ca. 2.5 to 5 °C Ma^?1) are consistent with underplating of mafic magmas, or crustal thickening caused by nappe stacking, as possible processes governing the metamorphic evolution of the BP. To cite this article: B. Dhuime et al., C. R. Geoscience 335 (2003).     

Isotope geochemistry of Paleoproterozoic metacarbonates from Itatuba,Borborema Province,Northeastern Brazil: Evidence of marble melting within a collisional suture

Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions. Both rocks may present similar texture and foliation to the host rock, or even cross cutting field relationships, which could be interpreted either as igneous or high-grade metamorphosed marble. Diagnostic criteria are even more complex when there is evidence of melting of the metasedimentary carbonate rock, such as has been described in the Himalayas and in the Eastern Ghats, India.In the Alto Moxotó Terrane, a high-grade gneissic domain of the Borborema Province, Northeastern Brazil, there are metacarbonates associated with banded gneisses and different metaplutonic rocks. Field evidence indicates the absence of other metasedimentary rocks associated with these marbles, thus suggesting that these carbonates were separated from other siliciclastic metasedimentary rocks. The presence of marble also suggests that it may represent the initial stage of a crustal carbon recycling into the mantle. These marbles present many field similarities to carbonatites (e.g., fluid-flow structure) and, together with metagranites and metamafic intrusions, may represent a major collisional tectonic suture.A detailed study of the carbon, oxygen and strontium isotopic composition of these marbles is presented. This study aims to identify the origin of the different isotopic components. It is argued that these rocks were subjected to temperature and pressure conditions that were sufficiently high to have melted them. The isotopic data presented here support this interpretation and indicate the mixing of two components: (i) one characterized by radiogenic Sr isotopes and mantle-like carbon isotopes, which is associated with the gneissic and mafic rocks, and (ii) another characterized by low ⁸⁷Sr/⁸⁶Sr ratios and highly positive δ¹³C values. Available geochemical data for the upper Paleoproterozoic indicate that the ⁸⁷Sr/⁸⁶Sr ratio of ocean water, varying between 0.7050 (2.25 ± 0.25 Ga) and 0.7047 (1.91 Ga), falls within the lower range of the samples from Itatuba and thus reinforces the interpretation that these marbles are sedimentary-derived and were partially contaminated by interaction with the host gneissic and mafic rocks.     

From extension to shortening: Dating the onset of the Brasiliano Orogeny in eastern Borborema Province (NE Brazil)

In pre-drift reconstructions, the central and southern parts of the Borborema Province, northeastern Brazil, belong to a large Brasiliano-Pan-African orogenic realm situated to the north of the São Francisco-Congo Craton. In order to better understand the timing and geodynamic setting under which this orogenic system developed, a structural, geochemical and geochronological study was conducted across the east Pernambuco shear zone (EPSZ) system, which separates the Pernambuco-Alagoas Domain (PEAL) from the Central Domain. A sample of the Pinhões orthogneiss (GE-1), in the Central Domain, one sample of a syenitic orthogneiss (CA-34) wrapped by the EPSZ, and one sample of orthogneiss named Altinho (CA-40), in the northern portion of the PEAL, were dated by LA-ICP-MS. The Pinhões orthogneiss yielded an age of 869 ± 9 Ma, interpreted as the emplacement age of the protolith during a late Tonian magmatic episode. Samples CA-40 and CA-34 yielded ²⁰⁶Pb/²³⁸U weighted mean ages of 652 ± 6 Ma and 636 ± 3 Ma, respectively, which are interpreted as dating emplacement and crystallization of the magmatic protoliths. However, it is also possible that these rocks were formed during the same magmatic event in view of the identical ages of 646 ± 13 Ma and 646 ± 11 Ma, respectively, given by the less precise upper intercept of the discordia lines. The metaluminous and magnesian nature of the Altinho orthogneiss is akin to the calc-alkalic suite. However, some samples plot in the intraplate field in tectonic discrimination diagrams and the Nd T_DM model age of 1.36 Ga is unlike that of juvenile magmas in convergent settings. The Altinho orthogneiss is quite similar in terms of trace elements geochemistry to the syenitic orthogneiss, which has a clearer intraplate affinity, and the dated samples have identical initial Sr isotope ratios (0.7047). Therefore, emplacement in an extensional setting is preferred over a convergent one. Two samples of paragneisses (SB-1 and BB-9) from the PEAL were also dated. The ages of the youngest zircon grains in sample BB-9 (655–642 Ma) overlap the crystallization age of the Altinho orthogneiss, implying that sedimentation is younger than or, at best, synchronous with magmatism. The age of low Th/U grains in samples CA-34 (615 ± 8 Ma) and SB-1 (587 ± 12 Ma) are related to a subsequent metamorphic overprint, which is loosely constrained between 580 and 620 Ma. These observations, combined with evidence provided by previous studies, suggest that the change from an extensional to a contracional setting occurred at ca. 640–630 Ma. In contrast with most collisional orogens, where a long period of oceanic subduction precedes collision, the inferred tectonic evolution suggests that the Brasiliano Orogeny resulted from inversion of continental and/or proto oceanic rifts.     

Upper-lower crust thickness of the Borborema Province,NE Brazil,using Receiver Function

The thickness estimates of the upper-crust and the V_p/V_s ratios are essential in order to detail the structures and geologic features. They also corroborate the regional tectonic evolution models. The crust study using the Receiver Function is performed with teleseismic P-waves that reaches the interface, under the station, with an almost vertical angle. The Receiver Function is obtained through the deconvolution of the horizontal component from the vertical one. The synthetic seismogram of the Receiver Function has a higher peak of direct P-waves, followed by minor peaks of Psc waves, where P converted into S in the upper-lower crust limit and multiple reflections. It was used the phase weighted slant-stacked method for the estimate of upper-crustal thickness and m_b ratio. The best estimates are correctly stacked. The Normal Moveout correction for the Psc phase was used for the upper-crust thickness, simulating a vertical incidence. A total of 8 stations on the Borborema Province in the Brazilian Northeast was analyzed. The results show that there is an upper-lower crust limit in all the analyzed signs. The thickness estimates showed two thickness regions. The first one in Coast Region, that indicates of the thin upper-crust and the second, continental regions characterized by uniform thicker upper-crust. East station data suggests underplating of mafic bodies.     

U/Pb age and origin of supergene uranophane-beta from the Borborema Pegmatite Mineral Province,Brazil

Uranophane-beta of supergene origin formed in the Borborema Pegmatite Mineral.Province, which is situated in the north-easternmost part of Brazil. The sampling site lies in the topmost parts of the Quintos Pegmatite, about ten kilometers north of the town of Equador. The uranyl silicate was investigated for its age and physical chemical regime of formation. Age dating yielded a U/Pb age of 6.77 ± 0.61 Ma. Uranophane has been derived together with autunite from weathering of brannerite in a tropical climate under alternating wet and dry seasons, when the pH was below 8. This canary-yellow well-crystallized uranyl silicate can be used as a physical chemical marker as well as a clock for supergene alteration.     

Late-Neoproterozoic ferroan granitoids of the Transversal subprovince,Borborema Province,NE Brazil: petrogenesis and geodynamic implications

ABSTRACT

Ferroan granites (585–530 Ma) have been described in the Transversal subprovince of the Borborema Province (BP) and in Pan-African counterparts. They comprise two groups: Group 1 – slightly peraluminous to metaluminous, alkali-calcic rocks, with low Fe# mica and crystallized under intermediate fO₂ (Aroeiras Complex and Serra Branca – Coxixola dike swarms); Group 2 metaluminous to slightly peraluminous, alkalic to alkali-calcic rocks, with high Fe# mica and crystallized under low fO₂ (Queimadas and Prata intrusions). Group 1 marks transition from collision to transcurrence (ca. 585 Ma), or from transcurrence to uplift and transtension (ca. 545 Ma). Group 2 – represents granitoids intruded during extensional tectonics in transcurrent setting (ca. 550 Ma), or coeval with deposition of transtensional intracratonic basins (ca. 530 Ma). Hf and Nd model ages are older than 2.0 Ga, suggesting that the ferroan granitoids involved partial melting of Paleoproterozoic rocks. The data presented in this paper show that the ferroan magmatism was widespread in the BP and its counterparts in Africa in pre-drift reconstructions.     

Geochemistry and geochronology of pre-Brasiliano rocks from the Transversal Zone, Borborema Province, Northeast Brazil

Proterozoic metamorphosed sequences are identified in the Transversal Zone (TZ) domain of the Borborema geological province, Northeast Brazil. This TZ domain is located between the well-known E–W Patos and Pernambuco continental shear zones. In its eastern part, in the Taquaritinga region, a large mass of augen gneisses with a conspicuous horizontal to subhorizontal tectonic foliation forms one of the most important rock types in the region that displays U–Pb zircon ages ca. 1.52 Ga. Paleoproterozoic orthogneisses dated by U–Pb on zircon at ca. 1.97 Ga and older paragneisses and banded gneisses represent basement rocks, which were cross-cut by these Mesoproterozoic augen gneisses, and have been in turn intruded by plutonic rocks in upper Neoproterozoic (U–Pb and Rb–Sr, ca. 0.6 Ga) times.Chemical analyses of major, minor, and trace elements (including REE) for the basement orthogneisses indicate calcalkaline affinities and a signature very similar to volcanic arc granites, representing crustal accretion during the Paleoproterozoic Transamazonian/Eburnean orogenesis in the region. In turn, the chemical data for augen gneisses indicate that they are relatively homogeneous and evolved metaluminous metaplutonic rocks with characteristics very similar to A-type granites generated and emplaced in an extensional anorogenic setting. Relatively high ⁸⁷Sr/⁸⁶Sr initial ratio and negative _Nd(t) are signatures of crustal components in these rocks.Based on geochemical, geochronological, and structural data, the Taquaritinga region is composed of Paleoproterozoic (>1.97 Ga) rocks intruded by Mesoproterozoic (ca. 1.5 Ga) anorogenic granites and Neoproterozoic granites (ca. 0.6 Ga). These data also suggest that the tectonometamorphic structures displayed by Meso and Neoproterozoic suites were developed by the Brasiliano/Pan-African orogeny and that the record of Transamazonian/Eburnean orogeny is restricted to basement rocks. This means that there is no evidence for a compressional event in Mesoproterozoic times (Cariris Velhos=Grenville) as suggested for the central and western part of the TZ. It is important to remark that the Taquaritinga augen gneisses are, up to now, the only unit that represents magmatic pulses associated with extensional episodes with this age (ca. 1.5 Ga) in the TZ and in the whole Borborema Province.

Resumen

A zona Transversal da Província Borborema está geologicamente situada entre os lineamentos Pernambuco e Patos. Na sua parte oriental, na região de Taquaritinga (PE), dominam augen gnaisses com uma penetrativa e proeminente foliação tectônica subhorizontal, com idade de cristalização de 1.52 Ga obtida em zircão. Estas rochas são intrusivas em ortognaisses e paragnaisses do embasamento com idade mínima em torno de 1.97 Ga, e são intrudidas por rochas plutônicas associadas ao Ciclo Brasiliano (ca. 0.6 Ga).Análises de elementos maiores, menores e traços (incluindo terras raras) nos augen gnaisses mostram caráter metaluminoso para esta suíte ígnea e fortes semelhanças com os granitos tipo-A gerados em ambiente anorogênico. Razões iniciais ⁸⁷Sr/⁸⁶Sr relativamente altas e _Nd(t) negativos são assinaturas indicadoras de forte contribuição crustal nestes augen gnaisses.A conjunção dos dados geoquímicos, geocronológicos e de campo indicam que na região de Taquaritinga onde são identificadas rochas de idades paleo (>1.95 Ga), meso (ca. 1.5 Ga) e neoproterozóicas (ca. 0.6 Ga), o ciclo orogênico Transamazônico é restrito às rochas do embasamento, e que todas as estruturas dúcteis identificadas nas rochas meso e neoproterozóicas foram desenvolvidas durante o Ciclo orogênico Brasiliano. Não foram encontradas evidências que atestem a existência do Ciclo Cariris Velhos nesta parte da Província Borborema. Contudo, está bem marcado que nesta Zona Transversal e na Província Borborema, os augen gnaisses de Taquaritinga são, até o momento, os únicos marcadores de pulsos magmáticos associados com eventos extensionais crustais datados do Mesoproterozóico inferior.     

Tantalite-(Mn) from the Borborema Pegmatite Province,northeastern Brazil: conditions of formation and melt- and fluid-inclusion constraints on experimental studies

Carbon dioxide-rich fluid and carbonate-rich aluminosilicate melt inclusions in tantalite-(Mn) from the Alto do Giz pegmatite in the Borborema Pegmatite Province, northeastern Brazil were investigated to constrain the formation of the host crystals. The results demonstrate that in the Alto do Giz pegmatite, water- and alkaline carbonate-rich fluids and melts are responsible for the transport and deposition of tantalite-(Mn) at temperatures around 600°C and about 4 kbar. Moreover, evidence is presented to show that during crystallization of the tantalite-(Mn), three different components coexisted, which are now trapped as separate inclusions: two immiscible silicate melts (types A and B melt inclusions) and a CO₂-rich aqueous fluid. We hypothesize that immiscible fluid separation may have been a critical factor in producing the water- and alkaline carbonate-rich fluids and melts necessary for Ta and Nb transport. Since the tantalite-(Mn) crystallized during pegmatite formation, this mechanism must also have implications for pegmatite genesis in general.     

Isotopic and geochemical constraints for a Paleoproterozoic accretionary orogen in the Borborema Province,NE Brazil: Implications for reconstructing Nuna/Columbia

The Alto Moxotó Terrane of the Borborema Province presents a wide exposure of Paleoproterozoic crust, but unlike other continental blocks of South America, its orogenic history is strongly obliterated by late Neoproterozoic deformation. New isotopic and geochemical studies were conducted in mafic-ultramafic (Fazenda Carmo Suite) and granitic-gneissic rocks (Riacho do Navio Suite) within the terrane. The former present zircon U-Pb crystallization ages at ca. 2.13 Ga, whereas Sm-Nd data suggests a juvenile origin via melting of early Paleoproterozoic to Archean peridotitic sources. Geochemical data for these rocks are compatible with tholeiitic magmas with some degree of crustal contamination and trace element distribution points to a continental-arc related setting interpreted as remnants of the early stages of subduction. In contrast, the Riacho do Navio Suite was emplaced at ca. 2.08 Ga and has highly negative ε_Nd(t) values indicating crustal reworking. The suite displays calc-alkali to alkali-calcic and ferroan geochemical signatures compatible with Cordilleran magmas. In addition, trace-element distribution as well as discriminant diagrams suggest that the precursor magmas were generated during the later stages of a continental arc or in a syn-collisional setting. Based on our results, we suggest that the studied units might represent missing pieces of a Paleoproterozoic accretionary orogen that formed the crustal framework of the Alto Moxotó Terrane, and that this represents a block associated with assembly of the Nuna/Columbia supercontinent, which is now largely hidden within the Neoproterozoic orogenic belts of West Gondwana.     

High-pressure metamorphic rocks in the Borborema Province,Northeast Brazil: Reworking of Archean oceanic crust during proterozoic orogenies

We present the first evidence of Archean oceanic crust submitted to Proterozoic high-pressure (HP) metamorphism in the South American Platform. Sm–Nd and Lu–Hf isotopic data combined with U–Pb geochronological data from the Campo Grande area, Rio Grande do Norte domain, in the Northern Borborema Province, reflect a complex Archean (2.9 ​Ga and 2.6 ​Ga) and Paleoproterozoic (2.0 ​Ga) evolution, culminating in the Neoproterozoic Brasiliano/Pan-African orogeny (ca. 600 Ma). The preserved mafic rocks contain massive poikiloblastic garnet and granoblastic amphibole with variable proportions of plagioclase ​+ ​diopside in symplectitic texture, typical of high-pressure rocks. These clinopyroxene-garnet amphibolites and the more common garnet amphibolites from the Campo Grande area are exposed as rare lenses within an Archean migmatite complex. The amphibolite lenses represent 2.65 ​Ga juvenile tholeiitic magmatism derived from depleted mantle sources (positive ε_Hf(t) values of +3.81 to +30.66) later enriched by mantle metasomatism (negative ε_Nd(t) values of –7.97). Chondrite and Primitive Mantle-normalized REE of analyzed samples and discriminant diagrams define two different oceanic affinities, with E-MORB and OIB signature. Negative Eu anomalies (Eu/Eu1 ​= ​0.75–0.95) indicate depletion of plagioclase in the source. Inherited zircon cores of 3.0–2.9 ​Ga in analyzed samples indicate that the Neoarchean tholeiitic magmatism was emplaced into 2923 ​± ​14 ​Ma old Mesoarchean crust (ε_Nd(t) ​= ​–2.58 and Nd T_DM ​= ​3.2 ​Ga) of the Rio Grande do Norte domain. The age of retro-eclogite facies metamorphism is not yet completely understood. We suggest that two high-grade metamorphic events are recognized in the mafic rocks: the first at 2.0 ​Ga, recorded in some samples, and the second, at ca. 600 Ma, stronger and more pervasive and recorded in several of the mafic rock samples. The Neoproterozoic zircon grains are found in symplectite texture as inclusions in the garnet grains and represent the age of HP conditions in the area. These zircon grains show a younger cluster of concordant analyses between 623 ​± ​3 ​Ma and 592 ​± ​5 ​Ma with ε_Hf(t) values of +0.74 to –65.88. Thus, the Campo Grande rock assemblage is composed of Archean units that were amalgamated to West Gondwana during Neoproterozoic Brasiliano orogeny continent-continent collision and crustal reworking.     

Geochemical Characterization of Granites of Distinctly Different Ages in the Alto Pajeu Terrane,Borborema Province,Northeast Brazil

This work presents geochemical data for granitic plutons that intruded the Alto Pajeu terrane, Borborema Province, northeastern Brazil, during several distinct tectonic cycles. The Alto Pajeu terrane is represented in the study area by metasedimentary (São Caetano Formation) and metavolcanosedimentary (Irajai Formation) sequences overlying a gneissic-migmatitic basement. Several granitic plutons of different ages, mineralogical proportions, and chemical compositions have invaded the supracrustal and basement rocks. Two deformational events, one tangential (older) and the other transcurrent (younger), were identified. The metamorphic grade determined for the metasedimentary rocks varies from amphibolite to high amphibolite facies, with temperatures ranging from 600 to 680°C and pressures from 6.0 to 7.5 kbar. Three magmatic events with distinct ages were identified for the tectonic evolution of the Alto Pajeu terrane in the study area. The older magmatic event is related to the Transamazonico orogenic cycle (～ 2.0 Ga), the intermediary magmatic event to the Cariris Velhos cycle (～0.97 Ga), and the younger magmatic event to the Brasiliano orogenic cycle (～0.55 Ga). The Cariris Velhos- and Brasiliano-age granites are geochemically classified as peraluminous, subalkalic granites of the calc-alkalic series, whereas the Transamazônico-age granite is metaluminous and alkaline. Geochemical data presented and discussed here suggest that the magmatism related to the three tectonic events had distinctly different sources and petrogenetic evolutions.     

Diorites of the High-K Calc-Alkalic Association: Geochemistry and Sm-Nd Data and Implications for the Evolution of the Borborema Province,Northeast Brazil

Accretion of exotic terranes during the Cariris Velhos (late Mesoproterozoic to early Neoproterozoic) and Brasiliano (late Neoproterozoic) events has been proposed as an important process of continental growth in Borborema province, northeastern Brazil. If this hypothesis holds true, it is reasonable to assume that the roots of these accreted blocks have variable chemical and isotopic compositions. Here, we investigate this possibility using lithospheric-derived high-K calc-alkalic diorites. These rocks are widespread throughout the province and occur in areas proposed to represent distinct tectonic terranes. The similar geochemical and Sm-Nd isotopic signatures of the diorites (T _DM from 1.7 to 2.0 Ga), except for one pluton, impose serious limitations to the terrane accretion model in Borborema province. On the contrary, these data strongly suggest that a large portion of the province is underlain by a LILE-enriched metasomatized lithospheric mantle formed around 2.0 Ga.     

Age,composition, and source of continental arc- and syn-collision granites of the Neoproterozoic Sergipano Belt,Southern Borborema Province,Brazil

The Sergipano belt is the outcome of collision between the Pernambuco-Alagoas Domain (Massif) and the São Francisco Craton during Neoproterozoic assembly of West Gondwana. Although the understanding of the Sergipano belt evolution has improved significantly, the timing of emplacement, geochemistry and tectonic setting of granitic bodies in the belt is poorly known. We recognized two granite age groups: 630–618 Ma granites in the Canindé, Poço Redondo and Macururé domains, and 590–570 Ma granites in the Macururé metasedimentary domain. U–Pb SHRIMP zircon ages for granites of first age group indicated ages of 631 ± 4 Ma for the Sítios Novos granite, 623 ± 7 Ma for the Poço Redondo granite, 619 ± 3.3 Ma for the Lajedinho monzodiorite, and 618 ± 3 Ma for the Queimada Grande granodiorite. These granitoids are dominantly high-K calc-alkaline, magnesian, metaluminous, mafic enclave-rich (Queimada Grande and Lajedinho), or with abundant inherited zircon grains (Poço Redondo and Sitios Novos). Geochemical and isotope data allow us to propose that Sítios Novos and Poço Redondo granites are product of partial melting of Poço Redondo migmatites. Sr-Nd isotopes of the Queimada Grande granodiorite and Lajedinho monzodiorite suggest that their parental magma may have originated by mixing between a juvenile mafic source and a crustal component that could be the Poço Redondo migmatites or the Macururé metasediments. Other 630–618 Ma granites in the belt are the mafic enclave-rich Coronel João Sá granodiorite and the Camará tonalite in the Macururé sedimentary domain. These granites have similar geochemical and isotopic characteristics as the Lajedinho and Queimada Grande granitoids. We infer for the Camará tonalite and Coronel João Sá granodiorite that their parental magmas have had contributions from mafic lower crust and felsic upper crust, most probably from underthrust São Francisco Craton, or Pernambuco-Alagoas Domain. The younger 590–570 Ma granite group is confined to the Macururé metasedimentary domain. Although these granites do not show typical features of S-type granites, their U–Pb age, field relationships, geochemical and Sr-Nd data suggest that their parental magmas have originated from high degree melting of the Macururé micaschists. Field observations support a model in which the Macururé domain, limited by the Belo Monte-Jeremoabo and São Miguel do Aleixo shear zones, behaved as a ductile channel flow for magma migration and emplacement during the Neoproterozoic, very much like the channel flow model proposed for emplacement of leucogranites in the Himalayas.     

Geochemistry of orthogneisses and metasedimentary rocks across a proposed terrane boundary in the Central Domain of Borborema Province,NE Brazil: Geodynamic implications

The Borborema Province, northeastern Brazil, occupies a central position in pre-drift reconstructions of western Gondwana, making an understanding of its geological evolution crucial for Neoproterozoic reconstructions. In recent years, it has been proposed that the Borborema Province grew by accretion of distinct tectonic terranes. In order to test this hypothesis, we compare here the geochemistry of orthogneisses and metasedimentary rocks across a proposed terrane boundary in the Central Domain of the province. Orthogneiss samples show smooth trends in Harker diagrams and similar rare earth element (REE) patterns, characterized by sharp decreases from La to Sm (chondrite-normalized La/Sm = 3–6) and flat heavy REE profiles (chondrite-normalized Tb/Yb = 1.5–2.5), with small or no Eu anomalies. In primitive mantle-normalized multi-element diagrams, all samples show parallel patterns characterized by sharp negative anomalies of U, Ta, Nb, P and Ti. The metasedimentary samples show little scatter of the major elements in Harker diagrams, suggesting that their chemistry was little affected by post-depositional diagenesis and metamorphism. They have indistinguishable chondrite-normalized REE patterns, characterized by light REE enrichment, flat heavy REE (normalized Tb/Y = 1–2) and small or no negative Eu anomalies, and similar ratios of immobile trace elements (e.g., Th/Sc, Zr/Sc). The geochemistry of the metasedimentary samples is comparable in many ways to those of the orthogneisses, suggesting that these may have been an important source of the precursor sedimentary rocks. These data do not support the terrane accretion hypothesis, rather suggesting the existence of a continuous basement that became available for erosion during intraplate continental extension in the late Neoproterozoic. Comparisons of the studied sequences with those present in the Northern Domain suggest that most, if not all of the Neoproterozoic geodynamic evolution of Borborema Province, occurred in an intracontinental setting.     

Tectonostratigraphic and geochronologic reappraisal constraining the growth and evolution of Singhbhum Archaean craton, eastern India

Reappraisal of field relationships between the different lithological ensembles supported by available geochronological data, and taking due note of the tectono-metamorphic, magmatic and sedimentation history helped to build up a coherent crustal evolutionary history of the Singhbhum Archaean craton, eastern India. The evolution of the earliest sialic crust, as the isotope ages suggest, was around 3700 Ma or even earlier. Deposition of the oldest, dominantly metasedimentary supracrustals, the Older Metamorphic Group (OMG), was initiated at around 3380 Ma, i.e. after a gap of about 320 million years. The closing of OMG basins synchronously with the emplacement of a granitoid phase was at ca.3285 Ma. No other fabric-forming ductile deformation and metamorphism associated with the development of foliation and mineral lineation is known in the rocks of the Singhbhum Archaean craton subsequent to this event. Formation of the succeeding geological ensembles including the deposition of BIF-bearing Iron Ore Group (IOG) and the emplacement of the post-IOG granitoids at ca.3100 Ma can be described as ??lsnon-orogenic?? event taking place during the phase of tectonic quiescence. Supracrustals like the Dhanjori and Simlipal mafic volcanics with intercalated beds of arenite evolved later during the phase of Plume outburst at around 2800 Ma. The end-Archaean intrusion of Newer Dolerite dykes in conjugate sets and the deposition of Kolhan Group in an N-S oriented basin during an E-W stress system mark the culmination of the Archaean crust-building activity in the Singhbhum Archaean craton.     

Extruding the Borborema Province (NE‐Brazil): a two‐stage Neoproterozoic collision process

We propose that the development of the Borborema Province from 620 to 570 Ma resulted from two discrete collisional events. Collision I, along the West Gondwana Orogen on the west side of the Province, took place at c. 620–610 Ma as the result of collision between the Parnaíba Block, as the forefront of the much larger Amazonian‐West Africa Craton, and the old basement of the Borborema Province. The suture zone related to this collision was reactivated by a dextral transform zone (the Transbrasiliano Lineament), allowing the Borborema Province to approach and collide against the São Francisco Craton in the south at c. 590–580 Ma marking collision II along the Sergipano Orogen. The combined stresses related to eastward push from collision I and northward push from the cratonic indentation into a thickened lithosphere gave rise to an extensive network of strike‐slip shear zones across the Province, forcing its northeastward extrusion.     

Petrogenesis of A-type Granitoids from the Alto Moxoto and Alto Pajeu Terranes of the Borborema Province, ne Brazil: Constraints from Geochemistry and Isotopic Composition

A-type granitoids (512 Ma) either intruded into Paleoproterozoic gneiss of the Alto Moxoto Terrane (Prata Complex and Serra da Engabelada Pluton) or into Early Neoproterozoic metavolcanic metasedimentary sequence of the Alto Pajeu Terrane (Serrote Santo Antonio Pluton), constitute a small proportion of Brasiliano (= Pan-African) granitoids in the Central Tectonic Domain of the Borborema Province, northeastern Brazil. The Prata Complex consists of syenogranites, monzogranites, mafic enclaves of diorites and norites. The felsic and mafic members are not genetically related through fractionation. Mingling and mixing were extensive processes within the Prata Complex. The granites evolution appears to have involved fractionation of alkali feldspar, biotite, apatite and sphene without significant wall-rock assimilation. The Serra da Engabelada and Serrote Santo Antonio plutons consist of biotite syenogranites, with rare mafic enclaves. The studied granitoids are dominantly metaluminous, characterized by Fe-rich biotite and Fe-hornblende. High total alkalis, Y, Nb and REE and low CaO, MgO and Sr abundances and high FeO/(FeO+MgO) ratios characterize these granitoids. Chondrite-normalized REE patterns show enriched LREE, moderate to strong negative Eu anomalies and more or less flat heavy REE.The studied granitoids and diabase from dykes and enclaves show negative eNd _(512Ma) values (-14 to -10), high incompatible elements such as LILE, HFSE and REE, suggesting important contribution of Paleoproterozoic crust. The origin of the granites is thought to have involved partial melting of granodioritic or tonalitic lower crust. Such isotopic signature of the diabase from the dykes also reflects a Paleoproterozoic enriched lithospheric mantle in the area. The intrusion of the studied granitoids contemporary with sub-volcanic bimodal magmatism and deposition of many Cambrian "pull-apart" basins in the north and central tectonic Domain of the Borborema Province, suggest intrusion during post-tectonic relaxation of the Brasiliano orogeny following the assembly of West Gondwana.     

Perspectives for Li- and Ta-Mineralization in the Borborema Pegmatite Province,NE-Brazil: A review

The increasing strategic importance of Li- and Ta-ores during the last decades due to the strong consumption growth for rechargeable batteries and high temperature and corrosion resistant capacitors reactivated the interest of studies in pegmatite fields around the world, because these rocks supply respectively 25% and 100% of the world consumption in these elements. Research on petrogenetic issues and major and accessory mineral chemistry variations in rare element (REL)-pegmatites of the Borborema Pegmatite Province in Northeast Brazil were tested as tools for the diagnosis of the metallogenetic potential of rare metals in individual pegmatites and in the province as a whole along the last dozen of years. The results allowed to establish the nearly isobaric (3.8 kbar) crystallization conditions of the REL-pegmatites between approximately 580 °C (liquidus) and 400 °C (solidus) from a peraluminous melt saturated in an aquo-carbonic medium to low salinity volatile phase and an immiscible peralkaline flux-enriched (H₂O, CO₂, F, B, Li etc.) melt fraction, based on melt and fluid inclusion studies. Mineral-chemistry data from 30 selected REL-pegmatites in the province allowed to classify three of them as being of the complex-spodumene or -lepidolite subtype in Černý's classification. Both subtypes are supposed to be potentially fertile, (highly fractionated, and with good chances to bear Li- and Ta-ore concentrations). It was also possible to identify several pegmatitic granite intrusions with textural and lithogeochemical characteristics also found in source granites of REL-pegmatite provinces elsewhere. Preliminary chemical Pb/U/Th geochronological determinations in uraninite and xenotyme crystals of these granites indicate an age of 520 ± 10 Ma and match recently published Ar/Ar in mica and U/Pb ages in columbite-group minerals (CGM) of the REL-pegmatites between 509 and 525 Ma. Mineral-chemistry data from grains of the outer zones of the pegmatites do not allow to distinguish potentially fertile from barren pegmatites. This discrimination is possible only if samples of the inner intermediate zone, replacement pockets or quartz core are used. From the tested minerals trace-element determinations (mainly Li, Al, Ti, Ge, B among 14 tested elements) by LA-ICP-MS technique in quartz seem to be more efficient than the classical approach (of Rb, K, Cs, Ga, Sr Ta) in K-feldspar or micas, due to the susceptibility to hydrothermal or supergene alteration of the latter. Mineral-chemistry variations in CGM, tourmalines, garnet and gahnite turned out to be efficient discriminators but all of them have the disadvantage of an eventual and, if present, random distribution, typical for accessory minerals in pegmatites, not allowing a regular sampling in most cases. Additional tests are recommended to confirm respectively the preliminary results of mineral-chemistry as exploration tools on a larger number of pegmatites and geochronological data to confirm the existence of another, older, synorogenetic generation of REL-pegmatites in the BPP.     

Ar/Ar geochronology of gold mineralization in Brasiliano strike-slip shear zones in the Borborema province, NE Brazil

⁴⁰Ar/³⁹Ar geochronology of muscovite and biotite grains genetically related to gold and Be–Ta–Li pegmatites from the Seridó Belt (Borborema province, NE Brazil) yield well-defined, reliable plateau ages. This information, combined with data about paragenetic and field relationships, reveals Cambro-Ordovician mineralization ages (520 and 500–506 Ma) for the orogenic gold deposits in the Seridó Belt. Biotite ages of 525±2 Ma, which represent the mean weighted results of the incremental heating analysis of six biotite single crystals, record the time of pegmatite emplacement and reactivation of Brasiliano/Pan-African strike-slip shear zones. These results, along with previous structural evolution studies, suggest that shear zones formed during the Brasiliano/Pan-African event were reactivated in the Upper Cambrian–Lower Ordovician. Mineralization occurs late in the history of the orogen.     

Relics of eclogite facies assemblages in the Ceará Central Domain, NW Borborema Province, NE Brazil: Implications for the assembly of West Gondwana

The Borborema Province, in the NE of Brazil, is a rather complex piece in the Brazil–Africa puzzle as it represents the junction of the Dahomeyide/Pharusian, Central African, Araçuai and Brasilia fold belts located between the West-African/São Luis, Congo/São Francisco and Amazonas craton. The correlation between the Dahomeyides from W-Africa (Ghana, Benin, Togo, and Mali) and the Borborema Province involves the Médio Coreaú and Central Ceará domains. The inferred continuation of the main oceanic suture zone exposed in the Dahomeyides of W Africa is buried beneath the Phanerozoic Parnaíba Basin in Brazil (northwest of the Médio Coreaú domain) where some high density gravity anomalies may represent hidden remnants of an oceanic suture. In addition to this major suture a narrow, nearly continuous strip composed of mainly mafic pods containing relics of eclogite-facies assemblages associated with partially migmatized granulite-facies metapelitic gneisses has been found further east in the NW Borborema Province. These high pressure mafic rocks, interpreted as retrograded eclogites, are located between the Transbrasiliano Lineament and the Santa Quitéria continental arc and comprise primitive to evolved arc-related rocks with either arc- or MORB-type imprints that can indicate either deep subduction of oceanic lithosphere or roots of continental and oceanic magmatic arcs. Average peak P–T conditions under eclogite-facies metamorphism (T = 770 °C and P = 17.3 kbar) were estimated using garnet–clinopyroxene thermometry and Jd content in clinopyroxene. Transition to granulite-facies conditions, as well as later widespread re-equilibration under amphibolite facies, were registered both in the basic and the metapelitic rocks and suggest a clockwise P–T path characterized by an increase in temperature followed by strong decompression. A phenomenon possibly related to the exhumation of a highly thickened crust associated with the suturing of the Médio Coreaú and Central Ceará domains, two distinct crustal blocks separated by the Transbrasiliano Lineament.