Navegando por Autor "Schulz, Bernhard"

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Contrasting provenance and timing of metamorphism fromparagneisses of the Araçuaí-Ribeira orogenic system, Brazil : hints for Western Gondwana assembly.
(2017) Degler, Reik; Soares, Antônio Carlos Pedrosa; Dussin, Ivo Antonio; Queiroga, Gláucia Nascimento; Schulz, Bernhard
The Araçuaí orogen and the Ribeira belt make up a complex Neoproterozoic-Cambrian orogenic system, the Araçuaí-Ribeira orogenic system(AROS) located fromthe eastern to southeastern Brazil. Along the AROS, the Ediacaran Rio Doce magmatic arc represents a geotectonic connection between the Araçuaí and the Ribeira orogenic domains. Although the nature and evolution of the Rio Doce plutonic rocks is regionally well established, it lacks detailed studies on the paragneisses found along the western and central regions of this magmatic arc. Besides information on the nature and provenance of their sedimentary protoliths, the paragneisses provide data to unravel the palaeogeographic scenario fromthe precursor to arc-related basins. Six samples of Al-rich gneisses covering a large AROS region were selected for electron microprobe (EMP) mineral analyses in order to obtain geothermobarometric data and monazite ages, as well as for Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) isotopic analyses on zircon (U-Pb, Lu-Hf) and monazite (U-Pb). The different age spectra fromdetrital zircon grains and contrasting Hf isotopic signatures suggest a complex sedimentary history. Located in thewestern sector of the study region, the samples RC-02 and RC-34, with an 80% age peak of detrital zircon grains from 2158 Ma to 1830 Ma, εHf(t) from −2.2 to −22.7, and Hf TDM model ages from 3530 Ma to 2440 Ma, suggest sediment sources located in the São Francisco craton basement. The samples RC-03, also fromthewestern sector, and RC-46 fromthe southern sector, have a more complex assemblage of detrital zircon grains with an 87% age peak from 987 Ma to 592 Ma, εHf(t) from +14.9 to −2.9, and Hf TDM model ages from 2220Ma to 720 Ma, indicating provenance from mainly juvenile sources of distinct ages. Candidates to be juvenile sources for RC-03 and RC-46 sedimentary protoliths are the Rhyacian Juiz de Fora and Pocrane complexes in the basement of the Rio Doce arc, the Neoproterozoic Rio Negro arc systemof the Ribeira belt, and AROS ophiolite complexes. Samples RC-30 and RC-38 from the eastern sector of the study region, with most detrital zircon ages between 650 Ma and 552 Ma and very negative εHf(t) (−25.3 to −16.5), suggest main sediment sources in the Rio Doce arc. By extending U-Pb analyses on metamorphic zircon and monazite, we have identified a complex timing of metamorphism, represented by metamorphic ages ranging from 621 Ma to 480 Ma, with the main collisional activity between 580Ma and 540 Ma. Geothermobarometric studies on garnet porphyroblasts, syn-kinematic to the D2 regional foliation, show a retrograde metamorphic path typical of continental collision belts, starting with P-T conditions of Tmax = 733 °C and Pmax = 6.43 kbar. Our data also suggest: i) the studied paragneisses represent distinct Neoproterozoic basin stages, shifting from passive to active margin settings; ii) if the Rio Negro arc system really provided sediments for the basin stage represented by the RC-03 and RC-46 paragneisses, it would have amalgamated with the AROS before 614 Ma; iii) the final amalgamation of Western Gondwana took place around 540 Ma in the focused region; iv) an important re-heating period (520–480 Ma) can be related to the AROS gravitational collapse, afterWestern Gondwana assembly.
Electron microprobe Th-U-Pb monazite dating and metamorphic evolution of the Acaiaca Granulite Complex, Minas Gerais, Brazil.
(2016) Medeiros Júnior, Edgar Batista de; Evangelista, Hanna Jordt; Queiroga, Gláucia Nascimento; Schulz, Bernhard; Marques, Robson de Abreu
The Acaiaca Complex (AC) is located in southeastern Minas Gerais state, and comprises felsic, mafic, ultramafic, and aluminous granulites as well as lower grade gneisses and mylonites. The complex is distributed over an area of ca. 36 km by 6 km, surrounded by amphibolite facies gneisses of the Mantiqueira Complex (MC). The discrepancy in the metamorphic grade between both complexes led to the present study aiming to understand the metamorphic history of the AC by means of geothermobarometric calculations and electron microprobe Th-U-Pb monazite dating. Estimates of the metamorphic conditions of the granulites based on conventional geothermobarometry and THERMOCALC resulted in temperatures around 800 ºC and pressures between of 5.0 and 9.9 kbar and a retrometamorphic path characterized by near-isobaric cooling. Part of the granulites was affected by ana¬texis. The melting of felsic granulites resulted in the generation of pegmatites and two aluminous lithotypes. These are: i) garnet-sillimanite granulite with euhedral plagioclase and cordierite that show straight faces against quartz, and is the crystallization product of an ana¬tectic melt, and ii) garnet-kyanite-cordierite granulite, which is probably the restite of anatexis, as indicated by textures and high magnesium contents. Th-U-Pb monazite geochronol¬ogy of two granulite samples resulted in a metamorphic age around 2060 Ma, which is similar to the age of the MC registered in the literature. The similar Paleoproterozoic metamorphic ages of both complexes lead to the conclusion that the Acaiaca Complex may be the high grade metamorphic unit geochronologically related to the lower grade Mantiqueira Complex.
P-T path reconstruction in neoproterozoic garnet-bearing paragneisses from a metasedimentary succession of the south western Araçuaí Orogen, Minas Gerais, Brazil.
(2015) Degler, Reik; Novo, Tiago Amâncio; Schulz, Bernhard; Queiroga, Gláucia Nascimento
Este estudo foca analises de caminhos de pressão e temperatura em paragnaisses de sucessão metassedimentar (SMS) da região sudoeste do Orógeno Araçuaí. A área de amostragem é limitada pela zona de cisalhamento de Abre Campo a oeste e pelo Arco Magmático Rio Doce a leste. A região é rica em rochas metamórficas de alto grau ortoderivadas (complexos Mantiqueira e Juiz de Fora do embasamento) e paraderivadas; incluindo paragnaisses intercalados a quartzitos, que formam o SMS. As rochas amostradas são compostas por quartzo, plagioclásio, granada, feldspato potássio, ortopiroxênio e sillimanita. Perfis de pontos de análise em porfiroblastos de granada mostram certo zoneamento caracterizado pela redução de piropo e por aumento de almadina do núcleo para borda; isto implica em crescimento de cristal sob condições de metamorfismo retrógrado. Condições metamórficas são de fácies anfibolito alto a granulito com um máximo de pressão de ca. 6 kbar e com temperatura máxima de ca. 700°C. Os dados sugerem que porfiroblastos de granada da SMS têm nucleação e início de crescimento durante o final da etapa deformacional principal compressiva do Orógeno Araçuaí (região sudoeste do orógeno); o evento de descompressão captado em bordas dos porfiroblastos foi relacionando ao colapso gravitacional no Cambriano.
Pseudosection modeling and U-Pb geochronology on Piranga schists : role of Brasiliano Orogeny in the Southeastern Quadrilátero Ferrífero, Minas Gerais, Brazil.
(2019) Queiroz, Yanne da Silva; Queiroga, Gláucia Nascimento; Moraes, Renato de; Fernandes, Victor Matheus Tavares; Medeiros Júnior, Edgar Batista de; Evangelista, Hanna Jordt; Schulz, Bernhard; Schmiedel, Julia; Martins, Maximiliano de Souza; Castro, Marco Paulo de; Lana, Cristiano de Carvalho
In the Southeastern Quadrilátero Ferrífero, a package of metapelitic rocks previously attributed to the Archean Rio das Velhas Supergroup crops out in Piranga locality. This study presents the mineral chemistry and U-Pb-Hf zircon geochronology on foliated staurolite-garnet mica schists. Garnet and staurolite index minerals are syn- to post-kinematic towards the main schistosity. Garnet porphyroblasts display well-developed compositional zoning of Mg-Fe-Mn-Ca, with increase of almandine and pyrope and decrease of spessartine towards the rim, implying in prograde metamorphic pattern. Estimates of P-T values for the metamorphic peak resulted in temperatures between 630 to 650ºC and pressure around 7 kbar. Pseudosections show well-defined stability fields in amphibolite facies, with a metamorphic path displaying progressive increase in P-T conditions. Maximum depositional age of 1,875 ± 51 Ma is established for the Piranga mica schists pointing to a depositional history that is younger than those previously described. Metamorphic Cambrian ages characterize the strong influence of deformational processes related to the final stages of Brasiliano Orogeny in the Southeastern Quadrilátero Ferrífero.
Thermobarometry and electron-microprobe Th-U-Pb monazite dating in garnet metapelites from the Capelinha Formation, Araçuaí Orogen, Brazil.
(2016) Queiroga, Gláucia Nascimento; Schulz, Bernhard; Martins, Maximiliano de Souza; Castro, Marco Paulo de; Soares, Antônio Carlos Pedrosa; Evangelista, Hanna Jordt; Silva, Ana Lúcia da
The Capelinha Formation (Macaúbas Group) consists of a lower quartzitic unit with metamafic intercalations and an upper metapelitic sequence. It occurs in a complex tectono-metamorphic sector of the Araçuaí orogen, where post-collisional collapse-related structures superimposed collisional structures. The garnet-bearing assemblages started crystallization in the collisional deformation stage that formed the main regional foliation around 570 Ma. Garnet porphyroblasts display a well-developed growth zonation of Fe-Mg-Ca-Mn and show, from core to rim, increasing almandine and pyrope contents in contrast with decreasing grossular and spessartine contents. Mineral relations and microstructures provide criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation ex¬change and net transfer reactions. The P-T values calculated from cores to rims of gar¬nets, aligned along clockwise trends, resulted in increasing temperatures (from 500 ºC up to 620 ºC) under decompression conditions (from 8.0 kbar to 4.5 kbar). The Th-U-Pb dating of homogeneous monazites by electron microprobe revealed a recrystalliza¬tion period at around 490 – 480 Ma. These ages can be related to the tectono-thermal event associated with the gravitational collapse, constraining the youngest time limit for metamorphic processes in the Araçuaí orogen.