Silvio Roberto Farias Vlach

Migração de orógenos e superposição de orogêneses: um esboço da colagem brasiliana no Sul do Cráton do São Francisco, SE - Brasil

The Meridional Tocantins Orogen represents a collision nappe pile involving three different tectonic environments. A magmatic arc domain developed in the Paranapanema continental active margin (Socorro-Guaxupe Nappe); a subducted continental domain (Andrelândia Terrane); and a passive margin domain and/or related to the Sanfranciscana plate (Carrancas and Lima Duarte Nappe systems). The magmatic activity related to the subduction started at 670 Ma and its metamorphism took place around 625Ma (active margin geothermal pattern modified in the subducted domain). The magmatic arc was active during flysch-type sedimentation in the Andrelândia terrane. Deformation and metamorphism related to the collisional process migrated from E to NE between 620 and 580 Ma. In the western internal nappe the tectonic process was very short-lived, less than 7 Ma, with some 612 Ma post-tectonic plutonism. On the other hand, in the eastern nappe the same process lasted ca. 20 Ma. Exhumation in the subducted terrane was very fast (ca. 2,2 mm/year). Superposed magmatic activities controlled by extensional regimes (Itu Granite Belt) followed orogen exhumation and preceded installation of continental-marine successor basins at 570 Ma. The Aracuai Orogen produced by convergence between the eastern passive margin of the Sanfranciscana plate and the Juiz de Fora Terrane (a microplate in an active margin environment) exhibits a metamorphic peak in the internal domain at around 560 Ma. The migration of the Aracuai nappes towards the cratonic domain (thermal metamorphic event ca. 530 Ma) reflects the superposition of the Mantiqueira Orogenic System. In the Cambrian the basement of the cratonic border was regenerated and included in the Aracuai orogen which involved the foreland in a thin-skinned thrust system well documented in the Quadrilatero Ferrifero. Oblique collision between the Mantiqueira Orogenic system and the proto-continent formed during the Meridional Tocantins orogeny was responsible for greenschist metamorphism at about 550 Ma in the successor basins. After this, the collisional process migrated northwards until 500 Ma. In the southern part of the Sao Francisco Craton the Brasiliano orogenic collage represents the interaction of several short orogenic steps in a long, convergent tectonic process involving plate consumption with eastward polarity and westward orogenic migration that lasted until the Cambro-Ordovician limit. The latest documented pre-collage metamorphism observed in the cratonic basement occurred between 2.03 and 2.06 Ga, probably under extensional conditions.

Stoichiometry-based estimates of ferric iron in calcic, sodic-calcic and sodic amphiboles: a comparison of various methods

An important drawback of the electron microprobe is its inability to quantify Fe3+/Fe2+ ratios in routine work. Although these ratios can be calculated, there is no unique criterion that can be applied to all amphiboles. Using a large data set of calcic, sodic-calcic, and sodic amphibole analysis from A-type granites and syenites from southern Brazil, weassess the choices made by the method of Schumacher (1997, Canadian Mineralogist, 35: 238-246), which uses the average between selected maximum and minimum estimates. Maximum estimates selected most frequently are: 13 cations excluding Ca, Na, and K (13eCNK - 66%); sum of Si and Al equal to 8 (8SiAl - 17%); 15 cations excluding K (15eK - 8%). These selections are appropriate based on crystallochemical considerations. Minimum estimates are mostly all iron as Fe2+ (all Fe2 - 71%), and are clearly inadequate. Hence, maximum estimates should better approximate the actual values. To test this, complete analyses were selected from the literature, and calculated and measured values were compared. 13eCNK and maximum estimates are precise and accurate (concordance correlation coefficient- rc » 0.85). As expected, averages yield poor estimates (rc = 0.56). We recommend, thus, that maximum estimates be used for calcic, sodic-calcic, and sodic amphiboles.

Datação Th-U-PbT com microssonda eletrônica, parte I. Monazita: procedimentos analíticos e tratamento de dados

Dating methodology by the electron probe microanalyser (EPMA) of (Th, U)-bearing minerals, highlighting monazite, acquired greater than ever importance in literature, particularly due to its superior spatial resolution, as well as versatility, which allow correlating petrological processes at times registered only in micro-scales in minerals and rocks with absolute ages. Although the accuracy is inferior to the one achieved with conventional isotopic methods in up to an order of magnitude, EPMA is the instrument that allows the best spatial resolution, reaching a few µm³ in some conditions. Quantification of minor and trace elements with suitable precision and accuracy involves the own instrumental and analytical set-ups and data treatment strategies, significantly more rigorous when compared with those applied in conventional analyses. Th-U-PbT dating is an example of these cases. Each EPMA is a unique machine as for its instrumental characteristics and respective automation system. In such a way, analytical procedures ought to be adjusted for laboratory specificities. The analytical strategies and data treatment adopted in the Electronic Microprobe Laboratory from Instituto de Geociencias of Universidade de Sao Paulo, Brazil, with a JEOL JXA8600S EPMA, and a ThermoNoran-Voyager 4.3 automation system, are presented and compared with the ones used in other laboratories. The influence of instrumental factors and spectral overlaps on Th, U, and Pb quantification is discussed. Applied procedures to interference correction, error propagation, data treatment, and final chemical age presentation as well as to sampling and analyses are emphasized. Some typical applications are discussed, drawing attention to the most relevant aspects of electron microprobe dating.

Early Paleozoic Structural and Metamorphic Evolution of Western Sierra de San Luis (Argentina), in Relation to Cuyania Accretion

Abstract Structural, metamorphic and isotopic data obtained from the Nogoli Metamorphic Complex of western Sierra de San Luis indicate that the Early Paleozoic Famatinian Orogeny overprinted an already structured and metamorphosed older basement. The older geological features are relict NW trending fabric associated with high-grade (amphibolite facies) regional metamorphism preserved within thin strips of schists and paragneisses and in the core of mafic to ultramafic lenses. Arc magmatism, medium P (Barrovian type)/high T (amphibolite to granulite facies) regional metamorphism and penetrative NNE to NE trending foliation are related to the building of the Famatinian orogenic belt. The P-T conditions of the Famatinian prograde metamorphism reached a pressure peak of ca. 8 kb, with a thermal peak from -750°C up to -820°C. U-Pb conventional and chemical dating and Ar-Ar plateau ages constrain the peak of the main orogenic phase related to the Famatinian belt to 470–457 Ma (Early to Mid-Ordovician). Greenschist facies retrograde metamorphism closely associated with shear zones and secondary Ar-Ar plateau and Sm-Nd ages suggest that a late to post-orogenic phase of the Famatinian belt was active at least since -445 Ma. This phase continued during the Silurian to Late Devonian times through multiple reactivation of early shear zones. The Famatinian Orogeny reset a previous thermal history and therefore, the timing of the relict fabric could not be constrained conclusively with radiometric dates. Despite this difficulty, a range of 520 to 490 Ma suggests some inheritance from Pampean events registered by the older NW-SE fabric. The Early to Mid-Ordovician regional metamorphism and ductile deformation of the western Sierra de San Luis is interpreted as the orogenic effects of the collision of the allochthonous Cuyania terrane with the autochthonous proto-Pacific margin of Gondwana during the Famatinian Orogeny.

Micro-structural and compositional variations of hydrothermal epidote-group minerals from a peralkaline granite, Corupá Pluton, Graciosa Province, South Brazil, and their petrological implications

Epidote-group minerals, together with albite, quartz, fluorite, Al-poor and Fe-rich phyllosilicates, zircon, and minor oxides and sulphides, are typical hydrothermal phases in peralkaline alkali-feldspar granites from the Corupa Pluton, Graciosa Province, South Brazil. The epidote-group minerals occur as single crystals and as aggregates filling in rock interstices and miarolitic cavities. They display complex recurrent zoning patterns with an internal zone of ferriallanite-(Ce), followed by allanite-(Ce), then epidote-ferriepidote, and an external zone with allanite-(Ce), with sharp limits, as shown in BSE and X-ray images. REE patterns show decreasing fractionation degrees of LREE over HREE from ferriallanite to epidote. The most external allanite is enriched in MREE. LA-ICP-MS data indicate that ferriallanite is enriched (>10-fold) in Ti, Sr and Ga, and depleted in Mg, Rb, Th and Zr relative to the host granite. Allanite has lower Ga and Mn and higher Zr, Nb and U contents as compared to ferriallanite, while epidote is enriched in Sr, U and depleted in Pb, Zr, Hf, Ti and Ga. The formation of these minerals is related to the variable concentrations of HFSE, Ca, Al, Fe and F in fluids remaining from magmatic crystallization, in an oxidizing environment, close to the HM buffer. L-MREE were in part released by the alteration of chevkinite, their main primary repository in the host rocks.

Geology, petrography and geochemistry of the A-type granites from the Morro Redondo Complex (PR-SC), southern Brazil, Graciosa Province

The Morro Redondo Complex is one of the most important occurrences of the Graciosa A-type Province, southern Brazil. It consists of the Papanduva and Quiriri granitic plutons and a contemporaneous bimodal volcanic association. The Papanduva Pluton includes massive and deformed peralkaline alkali-feldspar granites with Na-Ca and Na-amphiboles and clinopyroxenes. The deformed types are the most evolved rocks in the province and carry rare ‘agpaitic’ minerals, some being described for the first time in granites from Brazil. The larger Quiriri Pluton comprises massive, slightly peraluminous, biotite syeno- and monzogranites with rare Ca-amphibole. Biotite compositions are relatively homogeneous, whereas sodic amphiboles and clinopyroxenes show increasing Na and Fe3+ evolving paths. The Morro Redondo granites are ferroan, with high SiO2, alkalis and HFSE contents; the peralkaline types registering the highest fe#. LILE and HFSE abundances increase with the agpaitic index and the most evolved are HHP granites, with radiogenic heat production up to 5.7 µWm–3. Geothermobarometric estimates indicate emplacement under low pressures (∼100 MPa), at temperatures up to 850-800 °C, and relatively reduced (QFM) and oxidized (+1 < ΔQFM < +3) environments for the Papanduva and Quiriri Plutons, respectively. In both cases, melts evolved to relatively high oxidation states upon crystallization progress.

Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications

The Pocos de Caldas Alkaline Massif, located in southeast Brazil, is composed of tinguaites, as well as phonolites and nepheline syenites (NeS), together with assorted pyroclastic rocks. The latter constitute deposits within the Vale do Quartel, and show fragments of alkaline rocks (NeS, tinguaites), basaltic and ultramafic volcanics/subvolvanics, sandstones, and quartz-feldspathic rocks from the crystalline basement. NeS fragments with melanite, previously unknown, and cm-sized fragments of phlogopite megacrysts from related conduit breccia were chosen for detailed studies. The NeS fragments are medium-to coarse grained and contain K-feldspar (Or 95-83), secondary albite (Ab 97), altered nepheline, clinopyroxene (diopside, minor hedenbergite) and partially altered, zoned, idiomorphic melanite. These NeS fragments are typically metaluminous, and point to the existence of similar intrusions hidden at depth, an important input to the massifs petrology. The phlogopite megacrysts are compositionally homogeneous, with high Ti, Mg and Al contents; Ba and F are minor components. Two phlogopite aliquots were dated and yielded identical Ar-40/Ar-39 plateau ages of 87.1(+/- 0.5) Ma and 86.6(+/- 0.5) Ma, which represent minimum and maximum values for phlogopite crystallization and breccia emplacement, respectively. Previously obtained ages for the Pocos de Caldas rocks are concentrated in the 75-86 Ma range interval.

2D magnetometric modeling of a basic-intermediate intrusion geometry: geophysical and geological approaches applied to the Limeira intrusion, Paraná Magmatic Province (SP, Brazil)

The Limeira Intrusion is an occurrence related to the Mesozoic continental flood basalts of Parana Magmatic Province, cropping out at the eastern Sao Paulo state. It is made up of basic-intermediate rocks with a great diversity of structures, textures and modal compositions, related to specific evolutionary process in a magma chamber. Previous geological and stratigraphic evidence suggests it has a lopolithic-like geometry rather than a conventional sill-like, assumed for most regional similar occurrences. 2D forward modeling of magnetometric data, based on available aerial and newly obtained ground data integrated with in situ rock magnetic susceptibility measurements, were used to test such hypothesis. The results, even considering intrinsic drawbacks of magnetometric theoretical models, are compatible with the geologic-based presumed intrusion geometry. The determination of intrusive bodies geometry is quite significant to explain their different emplacement mechanisms. In the case of Limeira Intrusion, the results suggest that the emplacement mechanism may occur, apparently, by inner-accretion of fresh magmatic pulses in its central region. Importantly, the magmatic evolution of lopolith-like basic-intermediate intrusions may open the way for extensive differentiation processes, with strong implications to petrology and metallogenesis.

On the crystallization conditions of the Neoproterozoic, high-K calc-alkaline, Bragança Paulista-type magmatism, southern Brasília Orogen, SE Brazil

Syn-orogenic Neoproterozoic granitic magmatism (ca. 630–600 Ma) in southern Brasilia Orogen includes large batholiths of compositionally expanded high-K calc-alkaline granites (lato sensu) and small occurrences constituted by anatetic leuco-granites associated to the regional metamorphism. We present petrographic, mineralogical and geochemical data for a representative sample of the Braganca Paulista-type granite and estimates for magmatic crystallization conditions. The sample corresponds to a porphyritic hornblende-biotite monzogranite (CI ≈ 20) comprising perthitic microcline megacrysts within a medium-grained matrix with plagioclase (An 30 Ab 70 ), quartz, microcline and mafic minerals. The main mafic minerals are biotite (0.51 ≤ mg# ≤ 0.57) and Ca-amphibole (potassic-magnesio-hastingsite, 0.47 ≤ mg# ≤ 0.52), with similar volumes; accessory phases (ca. 2 vol.%) are Fe-Ti oxides (ferrian ilmenite, titanohematite, hematite and magnetite), allanite, apatite, zircon, titanite and sulphides. Our results indicate crystallization at 510 ± 60 MPa and temperatures from ca. 970oC (close-to-liquidus) to 755 ± 45oC (close-to-solidus), under oxidizing conditions, compatible with 0 ≤ ΔNNO ≤ +1. Estimated water and halogen fugacity ratios are ƒ H2O /ƒ HF ≥ 10 5.8 , 10 3.5 ≤ ƒ H2O /ƒ HCl ≤ 10 3.8 and ƒ HF /ƒ HCl ≤ 10 -2.2 . The obtained pressure, integrated with available regional geobarometric data, suggests a relative uplift rate about 0.2–0.3 (km/Ma) from the main regional metamorphism to the post-orogenic magmatism in the area.

Caracterização química de cromitas nos maciços de Cromínia e Mairipotaba, Goiás, Brasil

Metaultramafic bodies tectonically emplaced within the metasedimentary sequence of the Araxa Group are associated with an ophiolitic melange in southeast Goias. In the region of Crominia - Mairipotaba, they occur as lenticular bodies aligned E-W. Cumulate textures and geochemical data indicate that the parent rocks had harzburgitic to dunitic compositions. Relicts of primary crystals of olivine and orthopyroxene are suggestive of amphibolite facies metamorphic re-equilibration fabrics, even though the paragenesis and mineral associations of these metaultramafic rocks are typical of greenschist facies (T < 550oC and P = 5.5 kbar). The chromitites exhibit massive to breccioid structure and pull-apart texture, with chromite crystals around 0.5 mm in size. Chromite concentrations in the chromitite levels reach 70 to 85% by volume of the rock. The crystals are dispersed in the matrix, which is composed essentially of serpentine, and subordinately of chlorite and talc. The textures and geochemical data (Cr2O3 x TiO2 and Mg x Cr ratios present in the chromitite) are similar to those observed in ophiolitic complexes. Hence, they correspond to allochthonous bodies (Alpine type) associated with an ophiolitic melange.