Sérgio P. Neves

Successive: mixing and mingling of magmas in a plutonic complex of Northeast Brazil

Field and petrographic evidence together with major element geochemistry suggest that mixing and mingling of magmas of contrasting compositions were important petrogenetic processes in the Fazenda Nova/Serra da Japeganga plutonic complex of Northeast Brazil. The complex was emplaced at pressures of 300–500 MPa in amphibolite facies metamorphic rocks of Neoproterozoic age and consists of three main rock types: (1) coarse-grained granite; (2) porphyritic granite and (3) diorite to quartz-monzodiorite. The latter two make up the Fazenda Nova batholith which is located on the northwestern side of the sinistral, NE-trending, Fazenda Nova strike-slip shear zone. NE-plunging stretching lineations in the shear zone suggest that this batholith represents an uplifted, and therefore deeper, portion of the complex. The structure of the complex reflects the stratigraphy in a magma chamber, with the porphyritic granite above the diorite and below the coarse-grained granite. The porphyritic granite has a uniform composition, intermediate in mafic mineral content, quartz, and majorelements between the coarse-grained granite and the diorite. It is free of disequilibrium mineral assemblages, and locally displays gradational contacts with the overlain coarse-grained granite. Most elements display linear correlation with SiO2 in Harker diagrams. These features are interpreted as resulting from mixing of almost crystal-free felsic and intermediate magmas. Fluid dynamic calculations using the coarse-grained granite and the silica-poorest diorite as end-members in the mixing process show that mechanical mixing was possible, and thermal modelling suggests that the formation of an homogeneous hybrid may have been achieved in less than 50,000 yr. The diorites contain corroded K-feldspar megacrysts, and range in composition from low to relatively high silica contents, partly overlapping with the porphyritic granite. This suggests that a new mixing event occurred during the crystallisation of the porphyritic granite, this time producing a heterogeneous, xenocryst-bearing, dioritic hybrid. Abundant enclaves of diorite in the porphyritic granite, despite their textural diversity, are typically devoid of chilled margins, and were therefore formed relatively early in the crystallisation history of the granite. They are interpreted as liquid droplets separated from the heterogeneous hybrid magma through convection currents and incorporated in the, crystallising granitic magma. Subsequently, during the crystallisation of the porphyritic granite, mafic magma supply to the batholith continued at a declining rate, probably assisted by the development of the Fazenda Nova shear zone. This leads to the production of stromatitic-like structures, with alternating bands of mutually contaminated granite and diorite, then to the intrusion of contorted synplutonic dykes, and, finally, of late-stage dykes, some of which with chilled finer-grained margins.

Shear zone-controlled magma emplacement or magma-assisted nucleation of shear zones? Insights from northeast Brazil

Abstract In the Borborema province of northeast Brazil, neoproterozoic granitoids and large-scale transcurrent shear zones are spatially associated, suggesting a genetic link between magma bodies and shear zones. In some cases magma emplacement was clearly favored by shear zone activity, but for several plutons this model is not satisfactory. In these plutons, pre-full crystallization strike-slip deformation, evidenced by parallelism of magmatic foliations and lineations with the solid-state mylonitic fabric, and by a transition from magmatic to solid-state flow, is restricted to the vicinity of the shear zones. Evidence of shear zone activity prior to magma emplacement is lacking, and the magmatic foliation away from the shear zones is in most cases shallowly dipping and concordant with the slightly older, gently-dipping, regional gneissic foliation. Field and anisotropy of magnetic susceptibility mapping, together with petrographic and geochemical studies performed in one of the magmatic complexes of the Borborema province have revealed a structure and a magmatic fabric incompatible with the shear zone-controlled emplacement model. Away from the shear zones, this complex has retained a stratification inherited from the mixing of crystal-poor magmas of contrasting composition, and a magmatic fabric characterized by low-to moderate-dip magmatic foliations bearing a NW-trending lineation, which contrast with vertical foliations bearing NE- or E-W-trending stretching lineations in the shear zones and indicates that crystallization started prior to shear zone development. Based on evidence that magma emplacement predated strike-slip shearing and on information about the transition from magmatic to solid-state deformation observed in the studied plutons, we suggest that incompletely solidified plutons within the crust represent rheological heterogeneities that may induce strain localization and favor shear zone nucleation. We propose that in the studied cases deformation first concentrated within the plutons, where shear zones nucleated and grew. Subsequently, as the solid phase component increased, the crystal mush began to behave as a solid, and the shear zones propagated into the country rocks.

Tectono-thermal evolution, magma emplacement, and shear zone development in the Caruaru area (Borborema Province, NE Brazil)

The Neoproterozoic Borborema Province (BP) in northeastern Brazil is characterized by a regional flat-lying foliation and by abundant magmatic rocks commonly spatially associated with large transcurrent shear zones. Combined field, micropetrographic, thermobarometric and magmatic fabric studies carried out on plutons, country rocks, and shear zones in the eastern domain of the province (the Caruaru area) reveal that: (1) an early episode of regional deformation, with a top-to-the-NE displacement, was followed by the development of conjugate strike‐slip shear zones; (2) similar low-pressure ( 650°C ) metamorphic conditions occurred during the two events; (3) plutons intruded the flat-lying foliation; (4) magma emplacement slightly predated strain localization in transcurrent shear zones; but (5) plutons underwent strike‐slip deformation before complete crystallization. Therefore, regional deformation, pluton emplacement and shear zone development were successive events occurring over a relatively short time span. 40Ar/39Ar laser dating of amphibole (584 Ma) and biotite (545‐553 Ma) single grains supports slow cooling (#5°C/Ma) of country rocks through the argon closure temperatures of these minerals. Complex, sometimes discordant, amphibole ages bracketed between 552 and 575 Ma for shear zones may be attributed to episodic activity and/or dike swarm intrusions into them, which disturbed the Ar system. Biotite ages for the shear zones (533‐545 Ma) also imply a relatively low cooling rate. These data show that the massive injection of magmas in the continental crust of the BP during the Brasiliano/Pan-African orogeny produced a long-lived thermal anomaly of regional extent. The 40Ar/39Ar ages in the Caruaru area are older than in other segments of the BP with similar geological characteristics. This indicates a tectono-thermal history for the BP that is more complex than previously recognized, with either diachronous deformation, magmatism and associated low-P, high-T metamorphism, or contrasting cooling of the diVerent domains, possibly due to diVerential vertical movements.

High-K Calc-Alkalic Plutons in Northeast Brazil: Origin of the Biotite Diorite/Quartz Monzonite to Granite Association and Implications for the Evolution of the Borborema Province

High-K calc-alkalic plutons represent a significant proportion of the abundant magmatic bodies that intruded Borborema province (BP) of northeastern Brazil during the Neoproterozoic Brasiliano (Pan-African) orogeny. They consist of an association of mafic to intermediate (diorites to granodiorites) and felsic rocks (coarse-grained to porphyritic quartz monzonites to granites). Field and petrographic evidence indicates that the felsic and mafic rocks coexisted as contemporaneous melts, and major- and trace-element data favor magma mixing over fractional crystallization as the main petrogenetic process responsible for the petrographic and geochemical variability of these rocks. Major- and trace-element, oxygen-isotope, and radiogenic-isotope (Sr and Nd) data suggest that (1) the main source rocks of the granitoids are lower-crustal amphibolites having rare-earth-element (REE) and isotopie characteristics similar to the associated mafic rocks and (2) the source region of the diorites is the metasomatized sub...

Magma emplacement and shear zone nucleation and development in northeast Brazil (Fazenda Nova and Pernambuco shear zones; State of Pernambuco)

Abstract The Fazenda Nova and Serra da Japeganga batholiths and the Toritama pluton form a large granitoid massif located in the eastern part of the Borborema Province of Northeast Brazil. The sinistral, NE-SW trending, Fazenda Nova transcurrent shear zone (FNSZ) cuts across the contacts, and the deformation is localized within and in the immediate vicinity of the three magmatic bodies. The southern boundary of the Serra da Japeganga batholith is marked by the dextral, east-west trending, East Pernambuco shear zone (EPESZ). The internal structure in the Fazenda Nova/Serra da Japeganga complex, with increasingly more felsic units upwards, reflects the original stratigraphy in a vertically stratified magma chamber. Together with local strong discordant contacts, the common absence of shear zone-related deformation in the country rocks, and the lack of evidence for shear zone control on the internal magmatic fabric of the plutons away from the FNSZ and EPESZ, this suggests that magma emplacement predated shear zone development. Magma crystallization, however, was contemporaneous with shear zone evolution, as indicated by the rotation of the magmatic foliation approaching the shear zones, and by a continuous transition from magmatic to solid-state deformation fabric. During and after crystallization in the plutons, shear zones controlled the ascent and emplacement of subsequent magma batches which then formed dike swarms in both the FNSZ and EPESZ. Predominance of mafic dikes suggest that the shear zones extended downward to the upper mantle during their development. It is often suggested that fault zones control the ascent and styles of pluton emplacement. The results of this study, however, suggest an alternative point of view to explain the common association of granites and strike-slip faults. It is proposed that several shear zones in the intracontinental Borborema Province were nucleated due to thermal anomalies associated with preexisting magma chambers. Incompletely crystallized magmatic bodies represent significant rheological heterogeneities in the crust, that may have triggered strain localization and nucleation of these shear zones.

Zircon Pb-Pb Geochronology of the Caruaru Area, Northeastern Brazil: Temporal Constraints on the Proterozoic Evolution of Borborema Province

The Caruaru-Arcoverde batholith, with an outcrop area of ~2000 km2, is one of the largest Brasiliano plutons in Borborema Province, northeastern Brazil. The batholith intrudes orthogneisses and supracrustal rocks characterized by flat-lying foliation, and is bounded in the south by a synmagmatic, transcurrent shear zone, interpreted by some as a terrane boundary. In order to constrain the emplacement age of the batholith and the Proterozoic history of the eastern portion of Borborema Province, single zircon grains from seven samples from the Caruaru-Arcoverde batholith and nearby country rocks were analyzed by the Pb-evaporation technique. Zircons from orthogneisses immediately north of the Caruaru-Arcoverde batholith yielded ages of 2098 ± 15 Ma and 2072 ± 3 Ma. One zircon sample from an orthogneiss located south of the batholith gave an age of 2075 ± 7 Ma. Combined with existing zircon U-Pb ages in orthogneisses and Nd model ages of plutons and country rocks in eastern Borborema Province, these data indicate important crustal growth during the Middle Paleoproterozoic (2.2-2.0 Ga). In contrast, a sample from another orthogneiss southeast of the Caruaru-Arcoverde batholith gave an age of 629 ± 9 Ma. Because the flat-lying foliation in different country rocks was formed under the same high-amphibolite-facies conditions, this age suggests that the regional fabric developed during the Late Neoproterozoic. Two samples from the main phase of the Caruaru-Arcoverde batholith yielded mean ages of 591 ± 5 Ma and 587 ± 5 Ma, whereas a sample from a late facies gave an mean age of 583 ± 5 Ma. The oldest age of 591 Ma places an upper bound on the beginning of strike-slip motion, because field evidence indicates that the shear zone developed after emplacement of the Caruaru-Arcoverde batholith. Correlations of the Caruaru area with other sectors of Borborema Province and the Cameroon and Nigerian provinces of western Africa suggest that these areas: (1) were assembled during the Transamazonian/Eburnean orogeny; and (2) formed a coherent unit during the Late Neoproterozoic Brasiliano/Pan-African orogeny, when they were deformed first by a tectonic event, possibly resulting from thrust motion, and then subjected to a strike-slip regime.

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.

Magnetic fabrics in the Cabanas Granite (NE Brazil): interplay between emplacement and regional fabrics in a dextral transpressive regime

The late Neoproterozoic Cabanas Granite, northeastern Brazil, is a tabular body that was emplaced south of the dextral East Pernambuco shear zone system. Anisotropy of magnetic susceptibility (AMS) measurements reveal that the Cabanas Granite has fabric patterns more complex than usually reported in plutons intruded in similar settings. Most structures have EW- to ENE-trending magnetic lineations consistent with the overall regional structural direction. In the north, these structures parallel the mylonitic fabric encountered in shear zones that bound or cross-cut the pluton. In the central and southern portions of the pluton, magnetic foliation trajectories, the overall banded disposition of the iso-susceptibility lines, and shape parameters of the AMS ellipsoids suggest that the Cabanas Granite was subjected to ENE-trending folding, along with its underlying and enclosing country rocks. It is concluded that emplacement took place during regional dextral transpression. A secondary, but well-developed, family of structures consists of N- to NW-plunging lineations, which is interpreted to represent the trace of the vertical conduits that fed the pluton. This study emphasizes the utility of the AMS to unravel both the internal structure of low anisotropy plutons and the interplay between emplacement and regional straining during magma crystallization.

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.

Atlantica revisited: new data and thoughts on the formation and evolution of a long-lived continent

The existence of the continent Atlantica, postulated to encompass the Amazonian, São Luís, West African, São Francisco–Congo, and possibly, Rio de la Plata cratons, as well as the basement of the Neoproterozoic Gurupi, Araguaia, Borborema, Nigeria, and Cameroon belts, is evaluated based on an assessment of a large volume of recently published data. Striking similarities are found in the pre-Brasiliano–Pan-African geological events in these regions, supporting the Palaeoproterozoic formation of Atlantica and its persistence through the end of the Precambrian, when it was incorporated in western Gondwana. Formation of Atlantica involved the amalgamation of Archaean microcontinents, 2.4–2.3 thousand million year-old juvenile crust (extremely rare elsewhere), and 2.25–2.10 thousand million year-old crust, culminating with a great collisional event at 2.10–2.05 Ga. Afterwards, Atlantica continued to grow southwestward by accretion of successive magmatic arcs to the Amazonian craton, and eastward by incorporation of the Tanzania craton and the Bangweulu Block to the Congo craton, whereas its core was subjected to several events involving intraplate continental extension. Major rifting in the early Neoproterozoic led to the formation of a passive margin south of the São Francisco craton, which may be related to separation of the Rio de la Plata craton. This event and successive ones did not produce large ocean basins separating the other components of Atlantica. Rifting and limited drifting followed by convergence in the late Neoproterozoic produced the Brasiliano–Pan-African Gurupi, Araguaia, Borborema, Nigeria, and Cameroon belts. Complete splitting of Atlantica only occurred during the Mesozoic with opening of the Atlantic Ocean.