Browsing by Author "Armstrong, Richard"

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    Provenance and paleogeographic reconstruction of a mesoproterozoic intracratonic sag basin (Upper Espinhaço Basin, Brazil).
    (2014) Santos, Marcelo Nascimento dos; Chemale Júnior, Farid; Dussin, Ivo Antonio; Martins, Maximiliano de Souza; Queiroga, Gláucia Nascimento; Pinto, R. T. R.; Santos, A. N.; Armstrong, Richard
    The Mesoproterozoic Conselheiro Mata Group is the uppermost sequence of the Proterozoic intracontinental Espinhaço basin that developed on the Congo-São Francisco Paleoplate. This sequence is represented by amarine shallow-water platform that experienced a sag phase followed by a rift phase in the Upper Espinhaço.We used combined sedimentological-stratigraphic descriptions of sections, whole-rock (WR) geochemistry and U-Pb detrital zircon dating to develop a regional paleogeographic evolution model of the sag phase. The succession corresponds to transgressive-regressive cycles in the following ascending order: 1) offshore to lower shoreface facies represented by quiescent periods and episodic sediment supply (Santa Rita Formation); 2) upper shoreface to foreshore and coastal desert facies with a reworking of the underlying units (Córrego dos Borges Formation); 3) lower shoreface with fallout of suspended fine sediments and a combination of unidirectional and oscillatory flows generated by storm waves (Córrego da Bandeira Formation); 4) tidal-influenced upper shoreface to foreshore facies with the migration of subaqueous dunes, wave swash in a beach environment and cycles of neap-spring tides (Córrego Pereira Formation); and 5) the resumption of lower-shoreface sedimentation and the subsequent development of a stromatolitic carbonate-siliciclastic platform (Rio Pardo Grande Formation). The geochemical data indicate that the studied units contain input from felsic rocks and sedimentary rocks. The basal marine to eolian sediments of the Galho do Miguel Formation are dominated by Rhyacian sources (2.1 Ga). The basal and intermediate units of the Conselheiro Group contain Archean, Rhyacian, Statherian and Calymmian-Ectasian (1.6–1.33 Ga) zircon grains, whereas Orosirian (1.9–2.0 Ga) sources dominate in the upper strata of the group. The study of this Stenian (Mesoproterozoic) intracratonic sequence provides clues to understanding the history of sedimentation and the potential source areas on the São Francisco Craton and adjacent areas, which are very useful for comparison to Phanerozoic intracratonic basins and the reconstruction of Paleoproterozoic and Mesoproterozoic supercontinents.
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    Sedimentological and paleoenvironmental constraints of the Statherian and Stenian Espinhaço rift system, Brazil.
    (2013) Santos, Marcelo Nascimento dos; Chemale Júnior, Farid; Dussin, Ivo Antonio; Martins, Maximiliano de Souza; Assis, Tiago A. R.; Jelinek, Andréa Ritter; Guadagnin, Felipe; Armstrong, Richard
    The Espinhaço Basin in eastern Brazil contains depositional sequences developed in the São Francisco paleoplate and its margins. Detailed mapping was conducted and combined with U–Pb detrital zircon dating to determine the sedimentological-stratigraphic framework, provenance and minimum and maximum ages of the syn-rift-deposits. The two cycles have minimum ages of 1192 and 923 Ma and maximum ages of 1785 and 1685 Ma. The first depositional cycle, represented by the Bandeirinha and São João da Chapada formations, is marked by contributions of Neoarchean and Paleoproterozoic detrital zircons. The second cycle, the diamond-bearing Sopa-Brumadinho Formation, also contains Mesoproterozoic zircons formed between 1300 and 1190 Ma, which suggests an additional external source of Grenvillian age, that was not previously recorded in the São Francisco Craton. The investigation of such Mesoproterozoic intraplate sedimentary records, provides clues to understanding the history of the Rodinia active margins and, therefore, the kinematic reconstruction of its paleoplates.
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    Stabilization of the southern portion of the São Francisco craton, SE Brazil, through a long-lived period of potassic magmatism.
    (2012) Romano, Rafael Cotta; Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de; Stevens, Gary; Armstrong, Richard
    Although the southernportion of the SãoFranciscocraton in southeast Brazil encompasses one of the largest segments of Meso- to Neoarchaean crust in the South American plate, there is little understanding of the processes leading to cratonization of this region. Our combined field and U-Pb age data show that the crust experienced massive intrusions of potassic (K-rich) granitoids between 2750 and 2600 Ma, and that this magmatic event marked the beginning of a fundamental change in the tectonics of the terrain, with the ensuing billion years being dominated by the deposition of a series of thick clastic and chemical sedimentary successions. Thus, deep crustal partial melting and subsequent transport and intrusion of these granites in the upper crust is a likely mechanism by which this segment of the SãoFrancisco crust attained buoyancy to survive recycling. Regional mapping indicates that the potassic granitoids are distributed over an area of ∼25000 km2, and are related to one of the most prolific periods of potassicmagmatism in the craton. LA-ICP-MS and SHRIMP U-Pb data from 16 samples of potassic granitoids together with published TIMS ages suggest alonglivedperiod of magmatism from 2750 to 2700 Ma, and a small pulse of granite crystallisation at ca. 2612 Ma. The U-Pb SHRIMP and LA-ICP-MS data also indicate that the Archaean granitoids do not record evidence of U-Pb zircon resetting. Our interpretation is that partial melting and transport of granitoid melts (rich in heat producing elements) to the upper crust gave rise to a thermally stable lower crust and that this lower crustal segment (in the craton area) became sufficiently refractory and resistant to further partial melting.
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    The ancestry and magmatic evolution of Archaean TTG rocks of the Quadrilátero Ferrífero province, southeast Brazil.
    (2013) Lana, Cristiano de Carvalho; Alkmim, Fernando Flecha de; Armstrong, Richard; Cipriano, Ricardo Augusto Scholz; Romano, Rafael Cotta
    Combined LA-ICP-MS and SHRIMP U–Pb ages from Quadrilatero Ferrifero (QF) province reveals for the first time three main periods of TTG magmatism in the southern part of the Sao Francisco Craton (Brazil). These periods – described here as the Santa Barbara (SB), Rio das Velhas I (RVI) and Rio das Velhas II (RVII) events – embody a significant part of a protracted tectonomagmatic history of the Craton, spanning from 3220 to 2770 Ma. The initial stages of TTG magmatism (the SB event) led to formation of a core of Palaeoarchaean TTG crust via magmatic additions of juvenile felsic rocks into mafic–ultramafic rocks, at ca. 3212–3210 Ma. The following event (RVI event) saw the growth of this Palaeoarchaean core into a more composite, polydefomed continental segment through magmatic additions of juvenile TTG rocks and tectonic accretion of mafic–ultramafic greenstone belt terrains. Our U–Pb data and regional constraints suggest that much of the TTG crust exposed in the QF region today was produced during the RVI event, between 2930 and 2900 Ma. The final stages of TTG crust production (RV II event) were marked by convergence-related magmatism, affecting an extensive Palaeo- to Mesoarchaean continental block (The RVI block). The timing and duration of the latest TTG event coincides with the felsic volcanism and deposition of turbiditic wackes of the main greenstone belt sequence.
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    Unravelling a Proterozoic basin history through detrital zircon geochronology : the case of the Espinhaço Supergroup, Minas Gerais, Brazil.
    (2012) Chemale Júnior, Farid; Dussin, Ivo Antonio; Alkmim, Fernando Flecha de; Martins, Maximiliano de Souza; Queiroga, Gláucia Nascimento; Armstrong, Richard; Santos, Marcelo Nascimento dos
    New U–Pb in situ zircon dating provides a new approach for the study of the Espinhaço Supergroup units exposed in the São Francisco Craton and Araçuaí Orogen. Located in Southern Espinhaço, Minas Gerais, the following two basins were formed in intraplate conditions: (i) the Lower Espinhaço Basin is marked by a volcano-sedimentary sequence with alluvial, fluvial and eolian deposits that were formed from 1.68 Ga to 1.80 Ga; (ii) the overlying sequence, called the Upper Espinhaço Basin, is a rift-sag basin that presents basal diamond-bearing continental deposits (the Sopa–Brumadinho Formation) with a younger zircon peak at 1192 Ma, which is the maximum age for deposition of the upper units. The obtained U–Pb detrital zircon data of the studied units show age groupings related to the orogenic cycles of Jequié (Neoarchean) and Transamazonian (Paleoproterozoic), which are associated with the stability period from 1.8 Ga to 0.91 Ga in the São Franscico Craton, and record deposition during at least the three basinal cycles. The Stenian–Tonian ages of the Upper Espinhaço Basin units (906 Ma to 1192 Ma) suggest a direct link with the evolution of the Rodinia Supercontinent and represent a marker for Mesoproterozoic to Early Neoproterozoic evolution in the São Franscico Craton and adjacent areas.