Gorki Mariano

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...

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.

Cheese-related listeriosis outbreak, Portugal, March 2009 to February 2012

In Portugal, listeriosis has been notifiable since April 2014, but there is no active surveillance programme for the disease. A retrospective study involving 25 national hospitals led to the detection of an outbreak that occurred between March 2009 and February 2012. The amount of time between the start of the outbreak and its detection was 16 months. Of the 30 cases of listeriosis reported, 27 were in the Lisbon and Vale do Tejo region. Two cases were maternal/neonatal infections and one resulted in fetal loss. The mean age of the non-maternal/neonatal cases was 59 years (standard deviation: 17); 13 cases were more than 65 years old. The case fatality rate was 36.7%. All cases were caused by molecular serogroup IVb isolates indistinguishable by pulsed-field gel electrophoresis and ribotype profiles. Collaborative investigations with the national health and food safety authorities identified cheese as the probable source of infection, traced to a processing plant. The magnitude of this outbreak, the first reported food-borne listeriosis outbreak in Portugal, highlights the importance of having an effective listeriosis surveillance system in place for early detection and resolution of outbreaks, as well as the need for a process for the prompt submission of Listeria monocytogenes isolates for routine laboratory typing.

70 m.y. of synorogenic plutonism in eastern Borborema Province (NE Brazil): temporal and kinematic constraints on the Brasiliano Orogeny

Recent structural, petrological and geochronological work on plutons in eastern Borborema Province (NE Brazil) has helped to constrain its tectonic evolution during the Brasiliano/Pan-African Orogeny. 645-630 Ma-old plutons have been intensely affected by solid-state deformation. They show an early, shallowly to moderately-dipping foliation conformable with the regional fabric of country rocks, which is overprinted by a steeply-dipping foliation related to strike-slip shearing. Two compositional groups are recognized: (i) medium-K, epidote-bearing granodiorites, and (ii) coarse-grained granites and associated diorites. 590-580 Ma-plutons were emplaced before or at the very early stage of movement along large transcurrent shear zones, and include high-K calc-alkaline plutons and metaluminous syenites. The 590-580 Ma-old plutons underpin the change from a low-angle tectonic event to transpressive deformation. Once emplaced, these plutons in turn favored the localization of strain needed to promote nucleation of the spatially associated shear zones. Finally, two-mica leucogranites and dike swarms of widely varying compositions were emplaced around 570 Ma at the advanced stages of motion along transcurrent shear zones.

Geometry and Kinematics of the Afogados da Ingazeira Shear Zone, Northeast Brazil

Numerous transcurrent NE-SW, mainly sinistral, and E-W dextral-trending shear zones transect Borborema Province, northeastern Brazil. The most important kinematic event involving those shear zones reflects deformation related to the Brasiliano (Pan-African) tectonic cycle. The Afogados da Ingazeira shear zone (AISZ) is probably the most important of the NE-SW-trending lineaments, having continuity for over 250 km. Field work associated with petrographic studies (including quartz c-axis patterns), conducted in an area encompassing the north-central part of this shear zone, indicate that sinistral transcurrent deformation was responsible for the main banding/foliation (C-S) observed in the rocks. Mineral assemblages and microstructural features are suggestive of a deformational history starting under low- to medium-amphibolite facies (thermal peak conditions) and decreasing until greenschist facies.

The high‐K calc‐alkaline Alagoinhas pluton: anisotropy of magnetic susceptibility, geochemistry, emplacement setting, and implications for the evolution of Borborema Province, NE Brazil

The Alagoinhas pluton is a member of the widespread high‐K calc‐alkaline association of northeastern Brazil. Some authors suggest that this region represents an amalgamation of distinct tectonic terranes assembled during the Brasiliano (Pan‐African) orogeny. Our work compares geochemical data (major, trace and REE) of the Alagoinhas with other plutons of same petrotectonic association (Caruaru‐Arcoverde batholith). These plutons apparently intrude several distinct tectonic terranes, separated by a major E‐W dextral transcurrent system, the East Pernambuco shear zone (EPSZ). Anisotropy of magnetic susceptibility and structural data for the Alagoinhas pluton are used to compare tectonic regimes across the EPSZ. The results indicate that the Caruaru‐Arcoverde batholith and the Alagoinhas pluton evolved from similar sources and were subjected to the same tectonic regime during emplacement, placing severe restrictions on use of the EPSZ as a suture zone between distinct tectonic terranes.

Heat-producing Elements-enriched Continental Mantle Lithosphere and Proterozoic Intracontinental Orogens: Insights from Brasiliano/Pan-African Belts

Abstract Collisional and accretionary processes have been recognized in several Precambrian orogens, but the formation of intracontinental (ensialic) orogens has remained elusive. The recognition that modern orogenic belts (as, for example, the mountain belts of central Asia) may form in response to far-field stresses transmitted to the interior of continental plates removed much of the objection against Proterozoic intracontinental deformation. However, the causes responsible for the localization of contractional deformation are still uncertain. Due to the strong dependence of lithosphere rheology on temperature, lithospheric thermal weakening prior to contraction must be a required condition. Studies of dioritic rocks from Borborema Province, northeastern Brazil, and Damara belt, southern Africa, indicate that they are derived from ancient, incompatible elements-enriched subcontinental lithosphere. The concentrations of the heat-producing elements K (0.6 wt %), U (0.02–0.2 ppm) and Th (0.15–1.5 ppm) estimated for this source allow calculations of heat production between 0.09 and 0.25 mW m−3. These high values of heat generation may cause increases in temperature of several tens to three hundred degrees Celsius with respect to cratonic mantle lithosphere, leading to great reductions in lithosphere strength. Consequently, continental domains underlain by hot and weak continental mantle may thicken under the action of relatively feeble tectonic forces. High rheological contrasts between these domains and adjacent stiff and cold blocks provide an explanation for formation of intracontinental orogens.