Brasilia Orogen Research Articles

In-sequence tectonic evolution of Ediacaran nappes in the southeastern branch of the Brasília Orogen (SE Brazil): Constraints from metamorphic iterative thermodynamic modeling and monazite petrochronology

The metamorphic and kinematic evolution of medium-high grade rocks of the Andrelândia Nappe System (ANS), the orogenic wedge of the Southern Brasilia Orogen (SBO), was investigated in this work. Field and microstructural observations were combined with metamorphic petrology (i.e., iterative thermodynamic modeling) and monazite petrochronology to reconstruct the tectono-metamorphic history of the ANS rocks. The Liberdade Nappe experienced prograde metamorphism at ca. 610 Ma, achieving peak metamorphic conditions of ca. 650 °C and 9.5–10 kbar. This stage was followed by isothermal decompression linked to tectonic transport toward SE, at ca. 570 Ma. On the contrary, the Andrelândia Nappe experienced prograde metamorphism later, at ca. 580 Ma, reaching peak metamorphic conditions of ca. 680 °C and 11–12 kbar. The obtained results indicate that each nappe of the Andrelândia System records a single metamorphic cycle of burial and decompression, although it took place at different ages over a period of ca. 60 myr, from 630 to 570 Ma. The nappes experienced prograde and retrograde metamorphism whose ages progressively decreased toward the bottom of the nappe stack. We attribute this pattern to propagation of older buried material from the orogenic wedge (i.e., Liberdade Nappe), via thrust-and-fold, upon recently accreted rocks (i.e., Andrelândia Nappe), conducting a younger metamorphism event on the footwall of the ductile thrusted nappes. This mechanism is consistent with the ANS in-sequence fold-and-thrust architecture.

Metamorphism of retroeclogites from the Passos Nappe, Southern Brasília Orogen

High-pressure metamafic rocks from the Passos Nappe, Southern Brasília Orogen, have been traditionally interpreted as highly retrogressed eclogites, even though no reliable P-T conditions were available thus far. In this work, samples from the least retrogressed sample were studied using petrography, mineral and whole-rock chemical analysis, thermodynamic modelling, and rutile U–Pb geochronology, with the objective of unravelling its metamorphic history. The studied rocks are, descriptively, hornblende-garnet-clinopyroxene schists. Garnet and hornblende occur in a matrix composed of clinopyroxene-plagioclase symplectite, with garnet being surrounded by plagioclase-hornblende coronas. A combination of thermodynamic modelling, Zr-in-rutile and Ti-in-quartz thermobarometry suggests peak temperature to be 725 ± 30 °C, attained at a minimum pressure of ≈16 kbar, and data from rutile and quartz crystals included in garnet qualitatively indicate an earlier stage at higher P and lower T, constraining a clockwise P-T path. No evidences of UHP metamorphism were found thus far. A lower intercept age of 579 ± 23 Ma was calculated from rutile data, which is identical, within uncertainty, to rutile ages obtained previously in units immediately above the rocks studied here. The data suggests that these rocks were submitted to a different P-T path compared to the adjacent units, and that they were subducted to depths of at least ≈60 km and tectonically emplaced amid the surrounding metapelitic rocks before 579 Ma, which is compatible with current models for the evolution of the Southern Brasilia Orogen.

The role of deformation-assisted by water influx on partial melting of a granite protolith, and its role in the evolution of southwestern São Francisco Craton margin, Brazil

The Areado Granite-Migmatite Unit is part of the basement from the Southern Brasilia Orogen at southwestern São Francisco Craton margin, outcropping between Alfenas and Areado regions, Minas Gerais State, Brazil. This unit is composed of garnet meta-syenogranite, hornblende-garnet metatexite, and stromatic diatexite with different degrees of partial melting. This study characterizes the garnet meta-syenogranite, its migmatitic counterparts and investigates the different patterns of deformation and partial melting involved to produce the observed migmatites within the unit. The connection between the garnet meta-syenogranite, hornblende-garnet metatexite, and biotite diatexite is inferred from field observations and assemblage of accessory minerals. Petrography associated with modal analysis, structural data, quartz c-axis analysis, zircon U–Pb (LA-MC-ICP-MS and SHRIMP) dating, as well as trace elements chemical analysis in quartz and titanite to determine metamorphic, partial melting, and deformation evolution were used as tools in this investigation. The garnet meta-syenogranite has a crystallization age of 2069.8 ± 5 Ma. Lower intercepts from zircon indicate a metamorphic and partial melting age between 671 ± 58 and 657 ± 40 Ma. Melt crystallization ages are inferred between 622 ± 7 Ma and 604 ± 6 Ma. Different rates of partial melt are structurally connected, being higher at the base of the unit, and are related with contrasting amounts of water influx and deformation. The deformation during metamorphism/partial melting was the main cause of the differential aspect of the produced rocks. Quartz c-axis thermometer yielded deformation temperatures of 765–935 °C and other pulses occurred between 590 and 715 °C, both between 11 and 15 kbar. Those values match the Zr-in-titanite thermobarometer data, which is 670–710 °C for low-temperature titanite and 785–815 °C for high-temperature crystals. These data indicate that the protolith was generated at a Rhyacian late magmatic stage of the Mineiro Belt and passed by a Neoproterozoic deformational-metamorphic event of high temperature.

CARACTERIZAÇAO E EVOLUÇÃO DE (HEDENBERGUITA)-HASTINGSITA-BIOTITA-GRANADA ORTOGNAISSE NA REGIÃO DA ZONA DE PALEO-SUTURA DE ALTEROSA, PORÇÃO SUL DO ORÓGENO BRASÍLIA MERIDIONAL

No sudoeste do estado de Minas Gerais, região do distrito de Santa Cruz do Prata, inserido num contexto de rochas de alta pressão atribuídas ao Grupo Araxá, na porção sul do Orógeno Brasília Meridional, foi registrada a ocorrência de um corpo de (hedenbergita)-hastingsita-biotita-granada ortognaisse, cuja associação mineral é pouco comum. Neste trabalho são apresentados dados de petrografia, química mineral, geoquímica, geotermobarometria e idade modelo da ocorrência. O ortognaisse configura um corpo lenticular, orientado segundo a direção E-W, paralelo a sub-paralelo à direção das zonas de cisalhamento regionais. Apresenta estrutura isotrópica a anisotrópica, podendo exibir domínios bem mais foliados e orientados, em consequência do maior grau de deformação. As porções não deformadas preservam feições e mineralogia anatética reliquiar (andesina + ortoclásio + quartzo + granada + hedenberguita), em aparente equilíbrio com o auge metamórfico (fácies granulito). As transformações retrogressivas registram o reequilíbrio metamórfico das condições de pressão e temperatura em fácies anfibolito médio a superior (em torno de 11,0 kbar e 750ºC). A idade modelo TDMNd indica elevado tempo de residência crustal

U[sbnd]Pb zircon ages and metamorphic conditions of mafic granulites from the basement of the southern Brasilia Orogen, Campinas-SP region

In the southern Brasília Orogen, slices of an intensively reworked Paleoproterozoic basement occur. These are partially covered by a Neoproterozoic sequence of high-metamorphic grade, corresponding to the Socorro–Guaxupé Nappe and intruded by several Neoproterozoic plutonic rocks. In this contribution, we present thermobarometric and geochronological data of the basement rocks in the Campinas city, southern part of the Brasilia Orogen. The occurrence of mafic granulites included as lenses in mylonitic gneisses is described for the first time in a quarry located in the southern part of Campinas. Petrographic studies revealed high temperature paragenesis composed of orthopyroxene, clinopyroxene, garnet, biotite, and plagioclase. We present thermobarometric results with conditions close to the metamorphic peak, at ca. 8.7–10 kbar and 820–890 °C, which characterize these rocks as medium-pressure mafic granulites. UPb zircon ages of 2134 ± 6 Ma and 2156 ± 6 Ma were obtained for the host gneisses and the mafic granulite, respectively and interpreted as the timing of their igneous crystallization. Metamorphic zircon overgrowths in the mafic granulites show a concordia age of 608 ± 3 Ma. This age is coeval with one of the several episodes of melt crystallization reported in the southern Brasilia Orogen. The Paleoproterozoic ages of ca. 2.15 Ga suggest that the protoliths of the gneisses and granulites in the Campinas region are similar to the rocks of the Pouso Alegre Complex, which represents part of the lower São Francisco plate overthrusted by the Brasilia Orogen during the Neoproterozoic orogenic collage in southeastern Brazil.

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.

PETROGRAFIA, QUÍMICA MINERAL E GEOTERMOBAROMETRIA DE METAPELITO DO GRUPO CARRANCAS NA NAPPE DE LUMINÁRIAS (MG)

A Nappe Luminárias corresponde a estrutura alongada de orientação NNE-SSW, situada na porção sul do Orógeno Brasília e de idade neoproterozoica, na borda sul do Cráton do São Francisco. Tal estrutura é composta por metapelitos e quartzitos do Grupo Carrancas. O presente trabalho foca na caracterização metamórfica das suas rochas da, utilizando petrografia, química mineral e termobarometria. A paragênese observada na porção norte é Cld+Chl+Ky+Rt+Qtz+Ms; na porção centro norte é St+Grt+Rt+Qtz+Ms e na porção sul é Grt+Ky+St+Rt+Qtz+Ms. Biotita, clorita e ilmenita retrometamórficas são obervadas no centro-norte e sul. A granada apresenta aumento de almandina e piropo em direção às bordas. As condições obtidas pelo método average PT do THERMOCALC na porção centro-norte são de 591±59 ˚C e 8,3±1,8 kbar para o pico metamórfico e 571±20 ˚C e 10,1±4,8 kbar) para o retrometamorfismo. Na porção sul foi obtido 608±47 ˚C e 11,8±2,8 kbar para o pico metamórfico. Os resultados indicam a presença de gradiente metamórfico com condições variando de fácies xisto-verde na porção norte e centro-norte a fácies anfibolito/eclogito na porção sul.

Pseudo- and real-inverted metamorphism caused by the superposition and extrusion of a stack of nappes: a case study of the Southern Brasília Orogen, Brazil

The Southern Brasilia Orogen is a Neoproterozoic belt that occurs along the southernmost border of the Sao Francisco Craton where the Andrelândia Nappe System represents the subducted sedimentary domain and is divided into three allochthonous groups, of which the ages and P–T conditions of metamorphism are studied here. The basal unit, the Andrelândia Nappe, exhibits an inverted metamorphic pattern. The base of the structure, composed of staurolite, garnet, biotite, kyanite, quartz, and muscovite, marks the metamorphic peak, whereas at the top, the association of the metamorphic peak does not contain staurolite. The Liberdade Nappe, the middle unit, presents a normal metamorphic pattern; its base, close to the Andrelândia Nappe, shows paragneiss with evidence of in situ partial melting, and towards the top, coarse-grained staurolite schist is found. The staurolite-out and melt-in isograds are coincident and parallel to the main foliation. Thus, the shear zone that limits the nappes is syn-metamorphic, reheating the underlying Andrelândia Nappe and influencing the establishment of metamorphic inversion. This suggestion is supported by the monazite chemical ages, which indicates that the Andrelândia Nappe metamorphic peak (586 ± 15 Ma) is younger than that of the Liberdade Nappe (622.3 ± 7.6 Ma). The upper unit, the Serra da Natureza Klippe, bears a typical high-pressure granulite mineral assemblage that is composed of kyanite, garnet, K-feldspar, rutile, and leucosome, as well as a metamorphic peak at 604.5 ± 6.1 Ma. This tectonic assembly, with inverted and non-inverted metamorphic patterns and generation of klippen structures, is consistent with exhumation models and a strong indentor located in the lower continental crust.

A paleoproterozoic intra-arc basin associated with a juvenile source in the Southern Brasilia Orogen: Application of U–Pb and Hf–Nd isotopic analyses to provenance studies of complex areas

Early Proterozoic sedimentary basins are an important record of crust generation processes and consequently a fundamental key to unraveling Earth's evolution through geological time. Sediments within the basins are typically deformed and metamorphosed by subsequent tectonothermal events, which can obliterate their links to source terranes. Nd-whole-rock and detrital zircon U–Pb and Lu–Hf isotopic analyses are among the most reliable tools to be used in provenance investigations, since zircon is a resilient mineral and the Sm–Nd system is not extensively modified during metamorphism. These methods have been applied to a study of the provenance and tectonic setting of the São Vicente Complex, preserved in a Neoproterozoic passive margin related allochthon within the Southern Brasilia Orogen. The complex consists of siliciclastic and calc-silicate gneisses with mafic and minor ultramafic rocks, which were deformed and metamorphosed during late Neoproterozoic collision between the Paranapanema Block and the São Francisco-Congo plate. Detrital zircons indicate derivation from a juvenile Paleoproterozoic source terrane (peaks of crystallization ages of ca. 2130Ma, 2140Ma and 2170Ma; ɛHft between +0.1 and +6.0; NdTDM=2.31–2.21Ga; ɛNdt=+1.6 to +2.8), with a minor contribution from older continental crust. Interlayered amphibolite rocks, with juvenile signatures (ɛHft=+5.8 to +8.2; NdTDM=2.14 and 2.30Ga; ɛNdt=+2.2 and +3.2), yielded similar ages of 2136±17 and 2143±14Ma, suggesting syn-sedimentary magmatism. Thus, the maximum age of deposition at around 2130Ma represents the best estimate of the depositional age of the complex. The dominance of detrital zircons ages close to the age of deposition, along with syn-sedimentary magmatism, imply a convergent margin basin tectonic environment for the São Vicente Complex, with similarities to fore arc basin and trench deposits. Amphibolite and meta-sedimentary rocks point to important juvenile magmatism around 2.14Ga. Juvenile Rhyacian (ca. 2.1Ga) granite-granodiorite-tonalite orthogneisses with arc-related geochemical signatures (Pouso Alegre Complex) that override the São Vicente Complex, are the probable main source of detritus within the complex. Both basin and source were part of the southern edge of the São Francisco plate during the assembly of West Gondwana, and served as sources for early Neoproterozoic passive margin related basins. The age of intrusive anorogenic A-type Taguar granite indicates that by 1.7Ga the São Vicente Complex was in a stable tectonic environment.

Geochemical signatures of metasedimentary rocks of high-pressure granulite facies and their relation with partial melting: Carvalhos Klippe, Southern Brasília Belt, Brazil

Abstract High-grade metasedimentary rocks can preserve geochemical signatures of their sedimentary protolith if significant melt extraction did not occur. Retrograde reaction textures provide the main evidence for trapped melt in the rock fabrics. Carvalhos Klippe rocks in Southern Brasilia Orogen, Brazil, present a typical high-pressure granulite assemblage with evidence of mica breakdown partial melting (Ky + Grt + Kfs ± Bt ± Rt). The metamorphic peak temperatures obtained by Zr-in-Rt and ternary feldspar geothermometers are between 850 °C and 900 °C. The GASP baric peak pressure obtained using grossular rich garnet core is 16 kbar. Retrograde reaction textures in which the garnet crystals are partially to totally replaced by Bt + Qtz ± Fsp intergrowths are very common in the Carvalhos Klippe rocks. These reactions are interpreted as a result of interactions between residual phases and trapped melt during the retrograde path. In the present study the geochemical signatures of three groups of Carvalhos Klippe metasedimentary rocks are analysed. Despite the high metamorphic grade these three groups show well-defined geochemical features and their REE patterns are similar to average compositions of post-Archean sedimentary rocks (PAAS, NASC). The high-pressure granulite facies Grt-Bt-Pl gneisses with immature arenite (wacke, arkose or lithic-arenite) geochemical signatures present in the Carvalhos Klippe are compared to similar rocks in amphibolite facies from the same tectonic framework (Andrelândia Nappe System). The similar geochemical signatures between Grt-Bt-Pl gneisses metamorphosed in high-pressure granulite facies and Grt-Bt-Pl-Qtz schists from the Andrelândia and Liberdade Nappes, with minimal to absent melting conditions, are suggestive of low rates of melt extraction in these high-grade rocks. The rocks with pelitic compositions most likely had higher melt extraction and even under such circumstances nevertheless tend to show REE patterns similar to average compositions of post-Archean sedimentary rocks (PAAS, NASC).

Orogen migration and tectonic setting of the Andrelândia Nappe system: An Ediacaran western Gondwana collage, south of São Francisco craton

Abstract The southern Brasilia Orogen is organized in a pile of nappes that records the Neoproterozoic history of the subduction and collision between passive and active margins, respectively belonging to the Sao Francisco and Paranapanema Plates. The whole pile of allochthons comprises the rootless Andrelândia Nappe System (the upper kyanite-bearing granulite of Tres Pontas-Varginha Nappe, the intermediate high-pressure amphibolite-to eclogite facies of Liberdade Nappe and the lower Andrelândia Nappe) that is located below an Andean-type magmatic arc (Socorro-Guaxupe Nappe) and overrides the Lima Duarte Nappe and the Carrancas Nappe System. The tectonic units of the Andrelândia Nappe System seem to be exotic to the Sao Francisco Plate. The retroeclogite of the Liberdade Nappe yielded a 670 Ma SHRIMP U–Pb age in zircon, that is interpreted as the age of N-MORB-type basic magmatism. Detrital zircon grains of proximal flysh deposits of wackes in the Andrelândia Nappe present similar ages that reflect the crystallization in its source area. Both, rocks present Nd isotopic juvenile signatures with TDM in the range of 1.4 to 1.1 Ga. Rhyacian orthogneisses occur as slices in the Liberdade Nappe and have Nd isotope signature of juvenile source. The building of the collision pile of the whole system of nappes was diachronic and records a continuous outward migration of the orogen. The main structure is a middle crust-level duplex. The propagation of the structure and the metamorphism advanced progressively from the upper to the lower nappes, as is shown by U–Pb monazite ages in the range of 618–595 Ma for the Andrelândia Nappe System and 590–575 Ma for the Carrancas and Lima Duarte nappes.