Carlos J. Archanjo

The Pombal granite pluton: Magnetic fabric, emplacement and relationships with the Brasiliano strike-slip setting of NE Brazil (Paraiba State)

Abstract The Pombal pluton (500 km 2 ), a suite of diorite, syenite and porphyritic granite bodies, is here used to constrain kinematics of Brasiliano-age tectonic episodes in northeast Brazil. The pluton intrudes high-grade to migmatitic gneiss forming the western basement of the Serido belt, and is located at the intersection between two sets of continental-scale dextral strike-slip shear zones. The northern set of shear zone strikes NE-SW and branches, southwards, into the E-W Patos mega-shear zone. A detailed microstructural and low-field magnetic susceptibility study was performed to unravel the relationships between solid-state deformation in the country rocks and magma emplacement. Porphyritic granite and syenite have quite high magnetic susceptibilities (10 −3 –10 −2 SI units) indicative of magnetite as the principal carrier of susceptibility. The magnetic fabric is remarkably homogeneous in orientation throughout the pluton. It is characterized by a shape-preferred alignment of magnetite, itself parallel to the shape fabric of mainly biotite (±amphibole), i.e. to the magmatic fabric. Even close to the contact with the high-temperature mylonites of the Patos shear zone, south of Pombal, no imprint of the E-W-trending structures is observed in the fabrics of either the granite or the host rocks. Granite emplacement and its internal fabric development is concluded to be independent of the movement of the Patos shear zone. In the southwestern border of the pluton, a low-dip foliation bearing a NE-SW-striking lineation is shared in both the magmatic fabric of the pluton and the solid-state fabric. Farther to the north, approaching the NE-SW strike-slip shear zone, the magmatic fabric is characterized by a steeply dipping NE-striking foliation carrying a subhorizontal lineation. Transition from low to steep dips of the planar fabrics is progressive. Two models are proposed for emplacement of the Pombal pluton. One considers magma injection during an early episode of tangential tectonics, responsible for the gently dipping foliations, evolving later to strike-slip deformation. The other model considers that the pluton was emplaced in a pull-apart domain developed in the overlapping sector of a right-hand en echelon system of a dextral shear zone. Compatibility of these models with the tectonic evolution of the Serido belt is discussed.

Magnetic fabric and pluton emplacement in a transpressive shear zone system: the Itaporanga porphyritic granitic pluton (northeast Brazil)

The Itaporanga granitic pluton intruded into the lithological contact separating Neoproterozoic pelitic metasediments from Meso- to Paleoproterozoic rocks that constitute the Cachoeirinha belt of northeastern Brazil. An aureole of pelitic gneisses containing sillimanite and andalusite indicates that emplacement occurred at 12–15 km depth. The objective of this work is a detailed study of its magmatic structures, mainly by means of anisotropy of magnetic susceptibility (AMS). Regularly spaced sampling (61 stations) yielded a magnetic fabric map, which shows two domains: (1) a western lobe, which has lineations close to vertical and E-trending foliations that merge southwestward as the transcurrent Igarassi shear zone is approached (ISZ), and (2) an eastern lobe, showing SE-trending lineations associated with inward-dipping magmatic foliation situated in the center of the lobe. The magnetic fabric is mostly paramagnetic in origin, i.e., related to crystallographic preferred orientation of biotite and hornblende. A low susceptibility magnitude (<0.4×10−3 SI), typically of the ilmenite-series granites, is found in porphyritic granites, diorites or their hybrid products. Hysteresis data indicate that the ferrimagnetic contribution to the bulk susceptibility does not reach 30%. The magma was emplaced into a chamber developed by local WNW–ESE extension induced by an overall N–S shortening. Melt ascent may have been facilitated by the pre-existence of a steep fault zone bounding crustal discontinuities. The magma must have ponded along a mechanical barrier formed by the contact between phyllitic metapelites and its gneissic basement, which controlled the final emplacement structures of the pluton.

Magnetic fabrics and microstructures of the post-collisional aegirine-augite syenite Triunfo pluton, northeast Brazil

Abstract Anisotropy of magnetic susceptibility (AMS) and microstructures of the Neoproterozoic Triunfo alkaline pluton, situated in northeast Brazil, have been investigated in order to characterize fabrics formed during its emplacement. Bulk magnetic susceptibility in the pluton varies between 0.15 × 10 −3 and 25 × 10 t-3 SI, and the mean magnetic anisotropy ratio is around 1.04. Aegirine-augite and secondary magnetite are the main carriers of the magnetic susceptibility, with the former controlling the directions of the anisotropy. Magnetic foliation and lineation are consistently oriented throughout the pluton: foliation typically dips gently to the northeast and lineation plunges gently to the east. These structures were mostly acquired at the magmatic and sub-magmatic stages. The shape of the magnetic ellipsoid is dominantly oblate, a feature that may reflect either the uniaxial oblate AMS ellipsoid of the aegirine-augite crystals and/or a vertical compaction of the magma in its final stages of crystallization. The foliation pattern suggests the pluton has a tabular shape within the basement rocks. Magma emplacement is inferred to have occurred by its lateral migration along a flat-lying crustal structure after fracture propagation and ascension of the magma from a lower crust-mantle source. Late, fine-grained, vertical dykes with the same mineral composition as the Triunfo syenite cross-cut the wall rocks of the pluton and close the alkaline magmatism in the area. The NE-trending alignment of alkaline plutons and the NNE-trending syenitic dykes indicate that the emplacement of such bodies occurred along a crustal extension event with a minimum compressive stress horizontal trending roughly E-W.

Magma flow inferred from preferred orientations of plagioclase of the Rio Ceará-Mirim dyke swarm (NE Brazil) and its AMS significance

Abstract Low-field magnetic and plagioclase fabrics were compared in Mesozoic mafic dykes of the Rio Ceará-Mirim swarm. Coarse titanomagnetites with pervasive ilmenite lamellae constitute the main carrier of the magnetic anisotropy. The hysteresis parameters of the mafic dykes fall in the pseudo-single domain field. The resulting AMS ellipsoid is usually oblate and has a very low anisotropy (<3%). Textures indicate that the oxi-exolution processes and size reduction of the ferrimagnetic domains occurred at subsolidus temperatures on cooling of the dykes. The magmatic fabric was determined by the shape preferred orientation of plagioclase laths. It rarely matches the magnetic fabric. Besides their contrasting shape ellipsoids, prolate and oblate respectively, their corresponding principal directions diverge from each other or exchange their positions depending on the symmetry of the ellipsoids. These discrepancies are attributed principally to small differences in the net shape of Ti-poor magnetite after exsolution of ilmenite and in the inherently oblique fabric of grains with different shapes. These results draw attention to the need to use independent methods to confirm the conclusions about flow fabrics of weakly anisotropic mafic dykes based only in AMS.

Brasiliano crustal extension and emplacement fabrics of the mangerite-charnockite pluton of Umarizal, North-east Brazil

Abstract Shear zone kinematics and anisotropy of magnetic susceptibility (AMS) data have been used in a Brasiliano/Pan-African fayalite-clinopyroxene (±orthopyroxene) syenite of Umarizal (NE Brazil) in order to reveal its emplacement mechanisms. The pluton is located at the intersection of two major shear zones, a NE-trending transcurrent Portalegre shear zone (PSZ) and a NW- to E-trending Frutuoso Gomes shear zone (FGSZ). The FGSZ dips gently NE in its NW-trending branch and N in the E-trending branch. Asymmetric porphyroclasts and S C′ shear planes observed in mylonitic gneisses along the FGSZ point to a top to the East (extensional) movement. Hysteresis and low-field susceptibility measurements on mangerites and charnockites indicate that magnetic susceptibility is principally carried by multidomain magnetite. In the core of the pluton, the magnetic anisotropy is weaker (P F) AMS ellipsoids. The magnetic anisotropy increases south-westward (P>1.10) approaching the FGSZ and the AMS ellipsoid becomes dominantly disc shaped (F>L). Magnetic lineations plunge gently to NW in the western part of the pluton and to ENE on the East. The absence of steep lineations, except around a large xenolith in the center of the pluton, exclude the presence of magmatic feeding zones at the actual level of exposure. Foliations are roughly parallel to the pluton contact, but most of them dip moderately towards North-east. In the SW border the magnetic foliation is parallel to the mylonitic foliation displayed in the FGSZ. Although emplaced within migmatized host rocks, the kinematics of the shear zones and the magmatic fabric pattern rule out a buoyancy-driven diapiric emplacement of the pluton. Syn-magmatic shear deformation associated with EW crustal stretching accounts for the fabrics of emplacement recorded in the Umarizal pluton.

Late Jurassic to Early Cretaceous plutonism in the Colombian Andes: A record of long-term arc maturity

Integrated geochemical, isotopic, and geochronological constraints from Jurassic plutonic rocks of the Central Cordillera in Colombia were used to determine the tectonic setting and long-term tectonomagmatic evolution of the Northern Andes. We examined three plutonic units with compositions that vary from diorite to granite and with U-Pb zircon crystallization ages from 165 Ma to 129 Ma. These units are interpreted as subduction-related magmas, as indicated by their K 2 O, Na 2 O contents, light to heavy rare earth element (LREE/HREE) ratios, and Pb isotope signatures. The Nd and Hf isotope compositions of these magmatic events become more juvenile (radiogenic) with time. This compositional record suggests an arc maturity trend in which partial melting of basaltic and peridotitic sources becomes more significant than radiogenic subducted sediments or the ancient continental crust. Global-scale tectonic reconstructions suggest that Late Jurassic to Early Cretaceous subduction of the Farallon oceanic plate under the Andean continental margin became highly oblique. The consequence was a reduction of the fusible sedimentary budget commonly incorporated during subduction into the mantle, leaving a more refractory mantle with a more primitive compositional signature, and a major decrease in magmatic activity in the Early Cretaceous. In addition, the magmatic evolution recorded in the North Andean Jurassic arc shows that long-term source evolution and regional-scale plate-tectonic processes also play an important role in the compositional evolution and volumes of magmatic products in convergent settings.

AMS and grain shape fabric of the Late Palaeozoic diamictites of the Southeastern Paraná Basin, Brazil

Diamictites interbedded with marine shales and turbidites onlap the eastern border of the Paraná Basin (Southern Brazil). These poorly sorted sediments were deposited during the Permo-Carboniferous glaciation, and their matrix-supported clasts show no preferred orientation. These massive rocks have been studied using anisotropy of magnetic susceptibility (AMS) and grain shape fabric. Hysteresis loops and thermomagnetic measurements show that AMS depends mostly on the paramagnetic clays, but fine ferromagnetic particles also contribute to the anisotropy. The coarse silt to sand grain preferred orientation study supports the use of AMS in describing the diamictite fabric, at least regarding the orientation of the foliation. AMS and grain shape data reveal subhorizontal to weakly inclined magnetic and grain shape foliation parallel to the regional bedding. The magnetic lineations are normally scattered within the foliation plane in agreement with the oblate AMS ellipsoids found in these rocks. Both fabric patterns are consistent with deposition by subaqueous mudflows that were resedimented downslope, with clastic supply from continental sources. The off-vertical grain shape foliation poles suggest that the deposition of diamictites was controlled by the depocentre topography of the Rio do Sul sub-basin.

U-Pb Ages and Hf Isotopes in Zircons from Parautochthonous Mesozoic Terranes in the Western Margin of Pangea: Implications for the Terrane Configurations in the Northern Andes

U-Pb laser ablation inductively coupled plasma mass spectrometry ages and Hf isotopes in zircons were used to constrain the nature of two geological units representative of the basement of the Central Cordillera of Colombia. Graphite-quartz-muscovite schists from the Cajamarca Complex show inherited detrital zircons supplied mostly from Late Jurassic (ca. 167 Ma), Ediacaran (ca. 638 Ma), and Tonian (Grenvillian; ca. 1000 Ma) sources. These marine volcanosedimentary deposits form an N-trending metamorphic belt in fault contact to the east with orthogneisses and amphibolites of the Tierradentro unit. Zircon U-Pb determinations of the Tierradentro rocks—previously interpreted as Grenvillian basement slices—yielded crystallization ages between 271 and 234 Ma. Initial Hf data reveal that the Tierradentro unit shares isotopic characteristics similar to other Permo-Triassic rocks of the Central Cordillera. In contrast, inherited detrital zircons from the Jurassic metasedimentary rocks suggest that their sources are distinct from the plutonic rocks that crop out in the Central Cordillera with Jurassic crystallization ages. Large xenoliths of the Tierradentro unit within the Ibagué batholith indicate that the granodioritic magma mostly intruded a Permo-Triassic basement possibly by exploiting the Otú-Pericos fault. The Jurassic metasedimentary belt is correlated further south with a similar sequence in the Ecuadorian Andes named Salado terrane.

Evidence of An Early Cretaceous Giant Dyke Swarm in Northeast Brazil (South America): A Geodynamic Overview

Atlantic margin in the northeast Brazil (South America) is characterized by a modest magmatic activity compared with the Southern Atlantic margin, where the ParanáEtendeka Province is accounted for eruption of millions of cubic kilometers of mafic and acid magmas (flow basalts, dyke swarms). The most significant magmatic activity in the Equatorial Atlantic margin is the Rio Ceará Mirim Dyke Swarm (CMDS), which marks the early stages of rifting in the Cretaceous. The CMDS consists of an arcuate, nearly 800 km-long mafic dykes that crosscut rocks and structures of the Precambrian basement. The swarm can be divided into two laterally continuous segments, one of E-W strike that is parallel to the southern border of the Mesozoic Potiguar basin, and one barely studied NE-SW segment which, after recent high-resolution aeromagnetic anomalies, can be traced for 300 km along the east border of the Paleozoic Parnaíba basin up to the northern boundary of the São Francisco craton. The E-W-trending segment encompasses essentially tholeiitic basalts including plagioclase, clinopyroxene (±olivine), Fe-Ti oxides and pigeonite in their groundmass. The dykes show oneto 150-meters in width and of up to one kilometer in length; a few dykes exceeding 100 kilometers have been inferred from the aeromagnetic anomalies. The tholeiites have been subdivided into three groups: high-Ti olivine tholeiites, evolved high-Ti tholeiites (TiO2≥1.5 wt.%; Ti/Y >360), and low-Ti tholeiites (TiO2≤1.5 wt%; Ti/Y≤360), with all exhibiting distinct degrees of enrichment in incompatible elements relative to Primitive Mantle. Negative Pb anomalies are found in all three groups, while Nb-Ta abundances similar to those of OIB-type magmas are found in the olivine tholeiites, with moderate to high depletions being observed, respectively, in the evolved high-Ti and low-Ti tholeiites. The latter exhibit some contamination with crustal (felsic) materials. The initial isotopic compositions of the olivine tholeiites show uniform and unradiogenic 87Sr/86Sr (~0.7035–0.7039) combined with (in part) radiogenic 143Nd/144Nd and 206Pb/204Pb (>19.1) ratios, which together reveal a contribution of FOZO (FOcalZOne) component in their genesis. The other tholeiite groups show quite variable Sr-Nd initial ratios with relatively consistent 206Pb/204Pb ratios clustering toward an isotopically enriched mantle (EM1) component. Taken in conjunction with the Nb-Ta anomalies, this enriched signature reflects the involvement of a subduction-modified lithospheric mantle in the source of the evolved high-Ti and low-Ti tholeiites. Hence, FOZO and EMI components might to be coexisted and contributed in varying extents to the generation of the CMDS primary melts. Plagioclase dating of one evolved high-Ti tholeiite dyke provided two plateau ages of 127.1 ± 0.2 Ma and 128.2 ± 1.3 Ma, with an integrated mean age of 127.7 ± 0.1 Ma. Plagioclase multigrain fractions from one low-Ti tholeiite dyke provided two plateau ages of 131.6 ±0.7 Ma and 131.0 ± 0.4 Ma, with mean age of 131.2 ± 0.1 Ma. These 40Ar/39Ar ages clearly reveal that the low-Ti and high-Ti magmas encompassing the E-W segment of the CMDS were emplaced as two pulses during the Early Cretaceous. AMS investigations of the E-W-trending segment evidenced (at least) two main feeder zones located at the intersection of the dykes with Cenozoic N-trending volcanic centers referred in NE Brazil as the Macau magmatism. Such feeder zones were characterized by vertical magnetic fabrics (via steep-plunging magnetic Maria Helena B. M. HOLLANDA, Carlos J. ARCHANJO, Paul R. RENNE, Donald E. NGONGE, David L. CASTRO, Diógenes C. OLIVEIRA and Antomat A. MACÊDO FILHO, 2016. Evidence of An Early Cretaceous Giant Dyke Swarm in Northeast Brazil (South America): A Geodynamic Overview. Acta Geologica Sinica (English Edition), 90(supp. 1): 109-110.

Geologia e caracterização química do magmatismo peralcalino ultrapotássico do enxame de diques Manaíra-Princesa Isabel, Província Borborema

The Manaira-Princesa Isabel dike swarm forms one of the most expressive examples of Neoproterozoic (c. 600 Ma) peralkaline magmatism in the Borborema Province (NE Brazil). It consists of about a hundred NE-trending bodies intrusive in older, Neoproterozoic porphyritic granites (Princesa Isabel and Tavares plutons), and orthogneisses and low-grade metasediments of the Eo-neoproterozoic Riacho Gravata complex. The dike swarm includes mostly silica-saturated syenites, with potassic to ultrapotassic, peralkaline affinity, containing microcline and sodic amphibole ± pyroxene as the main mineral assemblage. Amphibole is dominantly Mg-riebeckite (Manaira, Princesa Isabel and Tavares sub-swarms), whereas pyroxene is mainly aegirine-augite (Manaira and Tavares sub-swarms). A minor set of dikes from this swarm is slightly metaluminous with Mg-biotite as the major mafic phase. Geochemical and isotopic signatures indicate strong enrichment in incompatible elements (Rb, Ba, K, Th, U), in association with a negative Nb anomaly, and strongly radiogenic 87Sr/86Sr and 143Nd/144Nd (= negative eNd values) initial ratios. These features suggest a common source, which was probably an enriched lithospheric mantle reservoir, chemically modified by an ancient subduction component inferred to be Paleoproterozoic from T DM model ages. Conversely, Pb isotopic ratios lower than average crustal values indicate the influence of a non-radiogenic component interacting with the enriched mantle source. A SHRIMP U-Pb age of c. 600 Ma obtained for the Manaira-Princesa Isabel dike swarm defines an important crustal exhumation period which was relatively synchronous with intensive tectonomagmatic activity related to the Brasiliano orogeny, which occurred in distinct structural domains within the Transverse Zone of the Borborema Province.