Microstructural evolution of the Serido Belt, NE-Brazil: the effect of two tectonic events on development of c-axis preferred orientation in quartz

The polyphase evolution of the Serido Belt (NE-Brazil) includes D, crust formation at 2.3-2.1 Ga, D-2 thrust tectonics at 1.9 Ga and crustal reworking by D-3 strike-slip shear zones at 600 Ma. Microstructural investigations within mylonites associated with D-2 and D-3 events were used to constrain the tectono-thermal evolution of the belt. D-2 shear zones commenced at deeper crustal levels and high amphibolite facies conditions (600-650 degreesC) through grain boundary migration, subgrain rotation and operation of quartz Q-prism slip. Continued shearing and exhumation of the terrain forced the re-equilibration of high-T fabrics and the switching of slip systems from (c)-prism to positive and negative (a)-rhombs. During D-3, enhancement of ductility by dissipation of heat that came from syn-D-3 granites developed wide belts of amphibolite facies mylonites. Continued shearing, uplift and cooling of the region induced D-3 shear zones to act in ductile-brittle regimes, marked by fracturing and development of thinner belts of greenschist facies mylonites. During this event, switching from (a)-prism to a basal slip indicates a thermal path from 600 to 350 degreesC. Therefore, microstructures and quartz c-axis fabrics in polydeformed rocks from the Serido Belt preserve the record of two major events, which includes contrasting deformation mechanisms and thermal paths. (C) 2003 Elsevier Ltd. All rights reserved.

The link between partial melting, granitization and granulite development in central Ribeira Fold Belt, SE Brazil: New evidence from elemental and Sr-Nd isotopic geochemistry

Elemental and Sr-Nd isotopic data on metatexites, diatexites, orthogneisses and charnockites from the central Ribeira Fold Belt indicate that they are LILE-enriched weakly peraluminous granodiorites. Harker and Th-Hf-La correlation trends suggest that these rocks represent a co-genetic sequence, whereas variations on CaO, MnO, Y and HREE for charnockites can be explained by garnet consumption during granulitic metamorphism. Similar REE patterns and isotopic results of epsilon(565)(Nd) = -5.4 to -7.3 and (87)Sr/(86)Sr(565) = 0.706-0.711 for metatexites, diatexites, orthogneisses and charnockites, as well as similar T(DM) ages between 2.0 and 1.5 Ga are consistent with evolution from a relatively homogeneous and enriched common crustal (metasedimentary) protolith. Results suggest a genetic link between metatexites, diatexites, orthogneisses and charnockites and a two-step process for charnockite development: (a) generation of the hydrated igneous protoliths by anatexis of metasedimentary rocks; (b) continuous high-grade metamorphism that transformed the ""S-type granitoids"" (leucosomes and diatexites) into orthogneisses and, as metamorphism and dehydration progressed, into charnockites. (C) 2011 Elsevier Ltd. All rights reserved.

Thermochronology of central Ribeira Fold Belt, SE Brazil: Petrological and geochronological evidence for long-term high temperature maintenance during Western Gondwana amalgamation

The studied sector of the central Ribeira Fold Belt (SE Brazil) comprises metatexites, diatexites, charnockites and blastomylonites. This study integrates petrological and thermochronological data in order to constrain the thermotectonic and geodynamic evolution of this Neoproterozoic-Ordovician mobile belt during Western Gondwana amalgamation. New data indicate that after an earlier collision stage at similar to 610 Ma (zircon, U-Pb age), peak metamorphism and lower crust partial melting, coeval with the main regional high grade D(1) thrust deformation, occurred at 572-562 Ma (zircon, U-Pb ages). The overall average cooling rate was low (<5 degrees C/Ma) from 750 to 250 degrees C (at similar to 455 Ma; biotite-WR Rb-Sr age), but disparate cooling paths indicate differential uplift between distinct lithotypes: (a) metatexites and blastomylonites show a overall stable 3-5 degrees C/Ma cooling rate; (b) charnockites and associated rocks remained at T>650 degrees C during sub-horizontal D(2) shearing until similar to 510-470 Ma (garnet-WR Sm-Nd ages) (1-2 degrees C/Ma), being then rapidly exhumed/cooled (8-30 degrees C/Ma) during post-orogenic D(3) deformation with late granite emplacement at similar to 490 Ma (zircon, U-Pb age). Cooling rates based on garnet-biotite Fe-Mg diffusion are broadly consistent with the geochronological cooling rates: (a) metatexites were cooled faster at high temperatures (6 degrees C/Ma) and slowly at low temperatures (0.1 degrees C/Ma), decreasing cooling rates with time; (b) charnockites show low cooling rates (2 degrees C/Ma) near metamorphic peak conditions and high cooling rates (120 degrees C/Ma) at lower temperatures, increasing cooling rates during retrogression. The charnockite thermal evolution and the extensive production of granitoid melts in the area imply that high geothermal gradients were sustained fora long period of time (50-90 Ma). This thermal anomaly most likely reflects upwelling of asthenospheric mantle and magma underplating coupled with long-term generation of high HPE (heat producing elements) granitoids. These factors must have sustained elevated crustal geotherms for similar to 100 Ma, promoting widespread charnockite generation at middle to lower crustal levels. (C) 2010 Elsevier B.V. All rights reserved.

Metamorphic P-T evolution of granulites in the central Ribeira Fold Belt, SE Brazil

Pseudosections, geothermobarometric estimates and careful petrographic observations of gneissic migmatites and granulites from Neoproterozoic central Ribeira Fold Belt (SE Brazil) were performed in order to quantify the metamorphic P-T conditions during prograde and retrograde evolution of the Brasiliano Orogeny. Results establish a prograde metamorphic trajectory from amphibolite facies conditions to metamorphic peak (T = 850 +/- 50 A degrees C; P = 8 +/- 1 kbar) that promoted widespread dehydrationmelting of 30 to 40% of the gneisses and high-grade granitization. After the metamorphic peak, migmatites evolved with cooling and decompression to T a parts per thousand 500 A degrees C and P a parts per thousand 5 kbar coupled with aH2O increase, replacing the high-grade paragenesis plagioclase-quartz-K-feldspar-garnet by quartz-biotite-sillimanite-(muscovite). Cordierite absence, microtextural observations and P-T results constrain the migmatite metamorphic evolution in the pseudosections as a clockwise P-T path with retrograde cooling and decompression. High-temperature conditions further dehydrated the lower crust with biotite and amphibole-dehydration melting and granulite formation coupled with 10% melt generation. Granulites can thus be envisaged as middle to lower crust dehydrated restites. Granulites were slowly (nearly isobarically) cooled, followed by late exhumation/retrograde rapid decompression and cooling, reflecting a two step P-T path. This retrograde evolution, coupled with water influx, chemically reequilibrated the rocks from granulite to amphibolite/greenschist facies, promoting the replacement of the plagioclase-quartz-garnet-hypersthene peak assemblage by quartz-biotite- K-feldspar symplectites.

P-T-Fluid evolution and graphite deposition during retrograde metamorphism in Ribeira Fold Belt, SE Brazil: Oxygen fugacity, fluid inclusions and C-O-H isotopic evidence

Combined fluid inclusion (FI) microthermometry, Raman spectroscopy, X-ray diffraction, C-O-H isotopes and oxygen fugacities of granulites from central Ribeira Fold Belt, SE Brazil, provided the following results: i) Magnetite-Hematite fO(2) estimates range from 10(-11.5) bar (QFM + 1) to 10(-18.3) bar (QFM - 1) for the temperature range of 896 degrees C-656 degrees C, implying fO(2) decrease from metamorphic peak temperatures to retrograde conditions; ii) 5 main types of fluid inclusions were observed: a) CO(2) and CO(2)-N(2) (0-11 mol%) high to medium density (1.01-0.59 g/cm(3)) FI; b) CO(2) and CO(2)-N(2) (0-36 mol%) low density (0.19-0.29 g/cm(3)) FI; c) CO(2) (94-95 mol%)-N(2) (3 mol%)-CH(4) (2-3 mol%)-H(2)O (water phi(v) (25 degrees C) = 0.1) FI; d) low-salinity H(2)O-CO(2) FI; and e) late low-salinity H(2)O FI; iii) Raman analyses evidence two graphite types in khondalites: an early highly ordered graphite (T similar to 450 degrees C) overgrown by a disordered kind (T similar to 330 degrees C); iv) delta(18)O quartz results of 10.3-10.7 parts per thousand, imply high-temperature CO(2) delta(18)O values of 14.4-14.8 parts per thousand, suggesting the involvement of a metamorphic fluid, whereas lower temperature biotite delta(18)O and delta D results of 7.5-8.5 parts per thousand and -54 to -67 parts per thousand respectively imply H(2)O delta(18)O values of 10-11 parts per thousand and delta D(H2O) of -23 to -36 parts per thousand suggesting delta(18)O depletion and increasing fluid/rock ratio from metamorphic peak to retrograde conditions. Isotopic results are compatible with low-temperature H(2)O influx and fO(2) decrease that promoted graphite deposition in retrograde granulites, simultaneous with low density CO(2), CO(2)-N(2) and CO(2)-N(2)-CH(4)-H(2)O fluid inclusions at T = 450-330 degrees C. Graphite delta(13)C results of -10.9 to -11.4 parts per thousand imply CO(2) delta(13)C values of -0.8 to -1.3 parts per thousand suggesting decarbonation of Cambrian marine carbonates with small admixture of lighter biogenic or mantle derived fluids. Based on these results, it is suggested that metamorphic fluids from the central segment of Ribeira Fold Belt evolved to CO(2)-N(2) fluids during granulitic metamorphism at high fO(2), followed by rapid pressure drop at T similar to 400-450 degrees C during late exhumation that caused fO(2) reduction induced by temperature decrease and water influx, turning carbonic fluids into CO(2)-H(2)O (depleting biotite delta(18)O and delta D values), and progressively into H(2)O. When fO(2) decreased substantially by mixture of carbonic and aqueous fluids, graphite deposited forming khondalites. (C) 2010 Elsevier Ltd. All rights reserved.

Boron isotope composition of tourmalinite and vein tourmalines associated with gold mineralization, Serra do Itaberaba Group, central Ribeira Belt, SE Brazil

Important concentrations of tourmaline occur as gold-bearing stratiform tourmalinites and in mineralized quartz-tourmaline veins at the Tapera Grande and Quartzito gold prospects in the Mesoproterozoic Serra do Itaberaba Group, central Ribeira Belt (Sao Paulo State, SE Brazil). The main rock types in both prospects constitute the volcanic-sedimentary Morro da Pedra Preta Formation, which formed in a submarine back-arc setting. At Tapera Grande, the volcanic-sedimentary sequence is composed of metabasic and metavolcaniclastic rocks, graphitic and sulfur-rich metapelites, banded iron formation, metandesite, metarhyolite, calcsilicates, tourmalinites and metahydrothermalites derived from mafic and felsic rocks. The Mesoproterozoic rocks at Quartzito prospect are lithologically similar but they have been affected by Neoproterozoic faulting and shearing and by the emplacement of granitic rocks, resulting in the formation of tourmaline-rich quartz-carbonate veins with gold and base metal mineralization. We conducted a chemical and B-isotope study of tourmalines in order to better understand the origin of the stratiform tourmalinites in the Morro da Pedra Preta Formation and their relationship with gold mineralization. The overall range of delta(11)B values obtained for the tourmalinite and vein tourmalines is between - 15%. and -5 parts per thousand, with the tourmalinites failing at the low end of this range (-15 to -8 parts per thousand). Such values are typical for continental crust and inconsistent with a primary marine boron signature as expected from the submarine-exhalative model for the gold prospects. We conclude from this that tourmaline formed or recrystallized from crustal fluids related to the amphibolite-grade metamorphism which affected the Serra do Itaberaba Group and that gold deposition occurred syn- to post-peak metamorphism by phase immiscibility, as attested by fluid inclusions in Tapera Grande tourmalinite tourmaline and quartz. The vein-hosted tourmalines at Quartzito have isotopically variable boron signatures, with heavier delta(11)B values of -5 parts per thousand to -8 parts per thousand for acicular green tourmalines and lighter values (-15 parts per thousand to -7 parts per thousand for light blue, Ti-firee tourmaline from quartz-carbonate veins). We attribute the heavier boron to fluids derived from the volcano-sedimentary rocks of marine affinity whereas the lighter boron was contributed by crustal fluids related to the granitoids or metasediments in the continental crust. (c) 2009 Elsevier B.V. All rights reserved.

Lower and Upper Neoproterozoic magmatic records in Itaiacoca Belt (Parana-Brazil): Zircon ages and lithostratigraphy studies

The Itaiacoca Belt is a sequence of metavolcanic and metasedimentary rocks that crop out east of Parana and southeast of Sao Paulo states, in southern Brazil. This geologic-geochronologic study supports division of the Itaiacoca Belt into two major lithologic sequences. The older is a carbonate platform sequence (dolomitic meta-limestones/metamarls/calc-phyllites/ carbonate phyllites) with minimum deposition ages related to the end of the Mesoproterozoic/beginning of the Neoproterozoic (1030-908 Ma:U-Pb, zircon of metabasic rocks). The younger sequence contains mainly clastics deposits (meta-arkoses/metavolcanics/metaconglomerates/metapelites) with deposition ages related to the Neoproterozoic (645-628 Ma:U-Pb,zircon of metavolcanic rocks). These ages are quite close to K-Ar ages (fine fraction) of the 628-610 Ma interval, associated with metamorphism and cooling of the Itaiacoca Belt. The contact between the dolomitic meta-limestones and meta-arkoses is marked by intense stretching and high-angle foliation, suggesting that the discontinuity between these associations resulted from shearing. It is proposed here that the term Itaiacoca Sequence, should represent the dolomitic meta-limestones, and the term Abapa Sequence represents the meta-arkoses/metavolcanics/phyllites. in a major tectonic context, these periods are related to the break-up of Rodinia Supercontinent (1030-908 Ma) and the amalgamation of the Gondwana Supercontinent (645-628 Ma). (C) 2008 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Magnetic fabric and zircon U-Pb geochronology of the Itaoca pluton: Implications for the Brasiliano deformation of the southern Ribeira Belt (SE Brazil)

The Itaoca pluton consists of porphyritic monzogranite that intruded the upper crust into low-grade metasedimentary rocks of the Apiai Dornain (Ribeira Belt). Anisotropy of magnetic susceptibility and zircon U-Pb (Shrimp) geochronology were combined to determine pluton emplacement mechanisms and its chronology relative to the collision structures of the Paranapiacaba (Brasiliano II) orogenic system. Magnetic susceptibility ranges between 4 and 38 x 10(-3) SI, and thermomagnetic measurements indicate multidomain magnetite is the main carrier of anisotropy. The pluton shows an ""onion-skin"" structure roughly elongated to the northeast with its hinge zone including kilometer-wide roof-pendants. Magnetic lineations are variable in orientation in consistency with the dominant oblate symmetry of the magnetic fabric. A distinct NE-trending point-maxima, however, indicates the mean lineation is parallel to the stretching direction of the transpressive deformation that affected the regional host rocks. Prismatic zircon from the monzogranite, both in the core and in the finely-zoned margins, yielded an age of 623 +/- 10 Ma. These results suggest the magmatic fabric recorded the earlier strain increments of the regional shear deformation. It may correspond to the transition from continental arc to collision tectonics of the southern Ribeira Belt. (C) 2008 Elsevier Ltd. All rights reserved.

Constraining the age of the iporanga formation with SHRIMP U-Pb zircon: Implications for possible ediacaran glaciation in the Ribeira Belt, SE Brazil

The Ribeira belt in SE Brazil is a Neoproterozoic to Early Palaeozoic orogen, whose architecture and history is not yet fully understood. The depositional age of many of the sedimentary sequences in the Ribeira Belt remains unconstrained, and with debate concerning their depositional environment and tectonic setting. In this paper we present SHRIMP zircon U/Pb age constraints for one such problematic unit in the Ribeira Belt the lporanga Formation - and discuss the significance of this age with regards to the timing of Neoproterozoic glacial events in southeast Brazil. Using a felsic volcanic unit immediately under the lporanga Formation and granite cobbles from breccias in its basal parts a reconnaissance SHRIMP U/Pb zircon maximum depositional age of 580 Ma is assigned for the base of this unit. This age is marginally younger than the 625605 Ma ages for intrusions into the Lajeado and Ribeira subgroups, with which the lporanga Formation is in tectonic contact. This indicates that the Lajeado and Ribeira subgroups are not stratigraphically equivalent to the lporanga Formation, as thought previously by some workers. The maximum depositional age of 580 Ma also places a maximum time constraint on the tectonic juxtaposition of the lporanga Formation with other supracrustal units, and on the greenschist facies metamorphism and isoclinal folding that affected it. The potential glacial origin for the lporanga Formation, if correct, would place it in the late Ediacaran - provisionally equivalent to the Gaskiers glaciation. (c) 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Tectonic setting and U/Pb zircon dating of the plutonic Socorro Complex in the Transpressive Rio Paraíba do Sul Shear Belt, SE Brazil

The Precambrian Rio Paraíba do Sul Shear Belt comprises a 200-km-wide anastomosing network of NE-SW trending ductile shear zones extending over 1000 km of the southeastern coast of Brazil. Granulitic, gneissic-migmatitic, and granitoid terrains as well as low- to medium-grade metavolcanosedimentary sequences are included within it. These rocks were affected by strong contractional, tangential tectonics, due to west-northwestward oblique convergence of continental blocks. Subsequent transpressional tectonics accomodated large dextral, orogen-parallel movements and shortening. The plutonic Socorro Complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic, magmatic, and metamorphic structures. These relationships point to a continuous magmatic evolution related to regional thrusts and strike slip, ductile shear zones. The tectonic and magmatic structural features of the Serra do Lopo Granite provide a model of emplacement by sheeting along shear zones during coeval strike-slip and cross shortening of country rocks. Geochronological data indicate that the main igneous activity of Socorro Complex spanned at least 55 million years, from the late stage of the northwestward ductile thrusting (650 Ma), through right-lateral strike slip (595 Ma) deformation. The country rocks yield discordant age data, which reflect a strong imprint of the Transamazonian tectono-metamorphic event (1.9 to 2.0 Ma). We propose a model for the origin of calcalkaline granites of the Ribeira Belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins, which were bounded by deep shear zones. The transpressional regime also seems to have focused granite migration from deeper into higher crustal levels along these shear zones.

U-Pb geochronology of the southern Brasília belt (SE-Brazil): Sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

The Brasília belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U-Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana. Ages of detrital zircons (by ID-TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the São Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxá Group in the Áraxá and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperança Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean- Paleoproterozoic ages (3.4-3.3, 3.1-2.7, and 2.5-2.4Ga); Paleoproterozoic ages attributed to the Transamazonian event (2.3-1.9Ga, with a peak at ca. 2.15Ga) and to the ca. 1.75Ga Espinhaço rifting of the São Francisco Craton; ages between 1.6 and 1.2Ga, with a peak at 1.3Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the São Francisco Craton; and ages between 0.9 and 1.0Ga related to the rifting event that led to the individualisation of the São Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxá Group yields a rutile age of ca. 0.9Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin. Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the São Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6-1.2Ga source rocks must be present in the São Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambuí Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasília belt. Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630Ma mark the orogenic event that started with westward subduction of the São Francisco plate and ended with continental collision against the Paraná block (and Goiás terrane). Continuing collision led to the exhumation and cooling of the Araxá and Passos metamorphic nappes, as indicated by monazite ages of ca. 605Ma and mark the final stages of tectonometamorphic activity in the southern Brasília belt. Whilst continent-continent collision was proceeding on the western margin of the São Francisco Craton along the southern Brasília belt, eastward subduction in the East was generating the 634-599Ma Rio Negro magmatic arc which collided with the eastern São Francisco margin at 595-560Ma, much later than in the Brasília belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasília belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588Ma, as indicated by monazite age. The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545-500Ma in the Paraguay belt and ca. 500Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Paraná block and Goiás terrane with the São Francisco Craton along the Brasília belt preceded the accretion of the Amazonian craton by 50-100 million years. © 2003 Elsevier B.V. All rights reserved.

Carbon and oxygen isotope geochemistry of Ediacaran outer platform carbonates, Paraguay Belt, central Brazil

Após a glaciação do final do Criogeniano, a região central do Brasil foi palco de extensa deposição de carbonatos plataformais do Grupo Araras. Este grupo inclui na sua base uma seqüência de capa carbonática sucedida por depósitos transgressivos de calcilutitos betuminosos e folhelhos de plataforma profunda. Dados de fácies e estratigráficos combinados com análises isotópicas de carbono e oxigênio da seção mais completa desses depósitos transgressivos, expostos no sinclinal da Guia, foram empregados para avaliar o paleambiente deposicional e para testar a correlação desses depósitos ao longo da faixa e também com outras unidades de outros continentes. A sucessão estudada consiste de 150 m de espessura de camadas tabulares de calcilutitos e folhelhos cinza a pretos com valores de d13CPDB negativos, entre -2,5 e -1 . O perfil de d13CPDB do sinclinal da Guia mostra clara correlação com a porção superior da Formação Guia da região de Cáceres, 200 km a sudoeste. O perfil de d13CPDB do Grupo Araras é comparável aos perfis de d13CPDB de outras unidades ediacaranas da parte sul da Faixa Paraguai, oeste do Canadá, e crátons do Congo e do Kalahari. Além disso, a distribuição de fácies, estratigrafia e o perfil isotópico do Grupo Araras são equiparáveis à porção média do Subgrupo Tsumeb na Namíbia, o que reforça a atribuição de idade Ediacarana para o Grupo Araras.

Contrasted crustal sources as defined by whole-rock and Sr-Nd-Pb isotope geochemistry of neoproterozoic early post-collisional granitic magmatism within the Southern Brazilian Shear Belt, Camboriu, Brazil

The early phase of post-collisional granitic magmatism in the Camboriu region, south Brazil, is represented by the porphyritic biotite +/- hornblende Rio Pequeno Granite (RPG; 630-620 Ma) and the younger (similar to 610 Ma), equigranular, biotite +/- muscovite Serra dos Macacos Granite (SMG). The two granite types share some geochemical characteristics, but the more felsic SMG constitutes a distinctive group not related to RPG by simple fractionation processes, as indicated by its lower FeOt, TiO2, K2O/Na2O and higher Zr Al2O3, Na2O, Ba and Sr when compared to RPG of similar SiO2 range. Sr-Nd-Pb isotopes require different sources. The SMG derives from old crustal sources, possibly related to the Paleoproterozoic protoliths of the Camboriu Complex, as indicated by strongly negative epsilon Nd-t (-23 to -24) and unradiogenic Pb (e.g., Pb-206/Pb-204 = 16.0-16.3; Pb-207/Pb-204 = 15.3-15.4) and confirmed by previous LA-MC-ICPMS data showing dominant zircon inheritance of Archean to Paleoproterozoic age. In contrast, the RPG shows less negative epsilon Nd-t (-12 to -15) and a distinctive zircon inheritance pattern with no traces of post-1.6 Ga sources. This is indicative of younger sources whose significance in the regional context is still unclear; some contribution of mantle-derived magmas is indicated by coeval mafic dykes and may account for some of the geochemical and isotopic characteristics of the least differentiated varieties of the RPG. The transcurrent tectonics seems to have played an essential role in the generation of mantle-derived magmas despite their emplacement within a low-strain zone. It may have facilitated their interaction with crustal melts which seem to be to a large extent the products of reworking of Paleoproterozoic orthogneisses from the Camboriu Complex. (C) 2012 Elsevier Ltd. All rights reserved.

Using AMS combined with mineral shape preferred orientation analysis to understand the emplacement fabrics of the Apiai gabbro-norite (Ribeira Belt, SE Brazil)

The Apiai gabbro-norite is a massive fine-grained Neoproterozoic intrusion emplaced in a core of synformal structure that deforms low-grade marine metasedimentary rocks of the Ribeira Belt of south-eastern Brazil. The lack of visible magmatic layering or any internal fabric has been a major limitation in deciding whether the emplacement occurred before or after the regional folding. To assist in the tectonic interpretations, we combine low-field anisotropy of magnetic susceptibility (AMS) and silicate shape preferred orientation (SPO) to reveal the internal structure of the mafic intrusion. Magnetic data indicate a mean susceptibility of about 10(-2) SI and a mean anisotropy degree (P) of about 1.08, essentially yielded by titanomagnetite. The magnetic and silicate foliations for P >= 1.10 are parallel to each other, while the lineations tend to scatter on the foliation plane, in agreement with the dominant oblate symmetry of the AMS and SPO ellipsoids. For lower P values, the magnetic and silicate fabrics vary from coaxial to oblique, and for P <= 1.05, their shapes and orientations can be quite distinct. The crystal size distribution (CSD) of plagioclase for P > 1.05 is log linear, in agreement with a bulk simple crystallisation history. These results combined show that for a strong SPO, corresponding to a magnetic anisotropy above 1.10, AMS is a reliable indicator of the magmatic fabric. They indicate that the Apiai gabbro-norite consists of sill-like body that was inclined gently to the north by the regional folding.