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Carneiro, M.A. & Oliveira, A. H.
Departamento de Geologia da Escola de Minas/UFOP, Ouro Preto – MG, Brasil. mauricio@degeo.ufop.br, arildo@degeo.aluno.ufop.br
ABSTRACT
The dominant rocks-type from Southern São Francisco Craton, nearby to Claudio city, are migmatitic high grade gneisses with schlieren and stromatic structures, which display several amphibolitic enclaves, and supracrustal sequences (relicts of greenstone belt). The gneisses and supracrustal sequences are crosscut by mafic dykes swarm that are positioned into NW-SE structures. The 40Ar/39Ar data obtained from biotite and hornblende, extracted from gneisses, amphibolite enclaves, amphibolite from supracrustal rocks, gabbronorite and gabbro, display imperative information related at three events, around 2.0-1.9 Ga; 1.7 Ga and 0.9 Ga, occurred on the southern of the São Francisco Craton. The 2.0-1.9 Ga event is correlated to the exhumation and retrograde metamorphic processes from the high-grade metamorphic event. Around 1.7 Ga an emplacement and crystallization of the gabbronorite occurred under a mafic dykes swarm. The NW-SE structures into which these swarm of dykes were emplaced can be correlated with the Staterian continental rift scenario (1.8-1.6 Ga). The latest thermal evidence in the Campo Belo Metamorphic Complex is the emplacement of the ~0.9 Ma gabbros, into preexisting NW-SE structures and might be coupled to the Macaubas rift. The results point out in this study stand for the final stabilization of the Campo Belo Metamorphic Complex that happened in the Mesoproterozoic and not in the Archaean.
Palavras-chave: Craton São Francisco Meridional, Complexo Metamórfico Campo Belo, Geocronologia 40Ar/39Ar
INTRODUCTION The continental crust of the Southern São Francisco Craton (Fig. 1a) comprises Archaean medium to high-grade metamorphic rocks (e.g. Campo Belo Metamorphic Complex - CBMC), and granite-greenstone associations (Cordani et al., 2000; Teixeira et al., 1996, 1997; Carneiro, 1998 a, b; Oliveira, 1999, 2004). The geological evolution of the continental crust of Southern São Francisco Craton (SSFC) records a polyphase crustal history ranging from Meso- to Neoarchaean. After the first substantial sialic crust and supracrustal sequences were generated, successive accretion and differentiation stages associated with crustal reworking affected this area during the Archaean and Proterozoic (Teixeira et al., 1996; Oliveira 2004; Oliveira et al. 2004).
Geological setting of CBMCThis study is focused on rocks from CBMC which is composed essentially of TTG gneisses, granitoids and ultramafic-mafic intrusions and greenstone sequences (e.g. Machado & Carneiro, 1992; Teixeira 1993; Teixeira et al., 1996; Carneiro et al., 1998a, b; Noce et al., 1998; Oliveira 2004). The dominant rocks-type from CBMC, nearby to Claudio city (Fig 1), are migmatitic high grade gneisses with schlieren and stromatic structures which display several amphibolitic enclaves, relicts of greenstone sequence and a mafic dykes swarm in which illustrate the geological setting of CBMC. The gneisses and relict of greenstone sequence are crosscut by mafic dykes swarm hat are positioned into NW-SE structures (Oliveira & Carneiro 2001). Mafic dykes swarm is constituted by a distinctive group of not deformed neither metamorphosed rocks, which can be classified as gabbronorite and gabbro. However, an incipient saussuritization of plagioclase and pyroxene uralitized are widespread in both lithotypes and usually the gabbronorite dykes are crosscut by gabbro dykes.Problems to be addressed. The debate surrounding the evolution of continental crust of the SSFC is related to the latest tectonothermal event that occurred in the CBMC. Some of the questions are: 1) which are the Mesoproterozoic and Neoproterozoic influence in the CBMC and which the metamorphic grade is? 2) The mafic dyke swarm that crosscut the gneisses units and greenstone relict are Achaean, Mesoproterozoic or Neoproterozoic? Or there are two or more generations? 3) When the latest high grade metamorphic event took place?
Figure 1. Panel 1a: Geological map of the southern São Francisco Craton (modified from Teixeira et al. 1996). Key: 1- Neoproterozoic mobile belt; 2 – São Francisco Supergroup; 3- Minas Supergroup; 4- Rio das Velhas Supergroup; 5 – Archean metamorphic complexes. Panel 1b: Simplified geological map of the Campo Metamorphic Complex (modified from Oliveira 1999; Oliveira & Carneiro 2001) showing the following features: 1) Mafic dykes swarm; 2) Supracrustal sequences (greenstone belt relicts); 3) Candeias Gneisses; 4) Itapecerica Gneisses; 5) Cláudio Gneisses; 6) Inferred Contact; 7) Foliation; 8) Cláudio Shear Zone, and 9) Outcrops dated by 40Ar/39Ar: A –Kinawa dimension stone quarry; B - Corumbá dimension stone quarry; C – Amphibolite outcrop; D – Gabbronorite outcrop
40Ar/39Ar Data, Sample Selection And Analytical Procedures In order to investigate the evolution and the potential problems of the CBMC, we will present 40Ar/39Ar step-heating analyses for biotites and amphiboles from three different rock-type of the CBMC: gneisses 2 samples; Kinawa quarry (Fig 1b - outcrop A) and Corumbá (Fig 1b - outcrop B) and its amphibolites enclaves (1 sample; (Fig 1b - outcrop B); amphibolites from the greenstone sequence (1 sample: (Fig. 1, outcrop C), gabbronorite (1 sample; (Fig. 1, outcrop D) and gabbros (2 samples Fig. 1, outcrops E and F) from the mafic dyke swam. The samples were analyzed by the laser incremental heating 40Ar/39Ar method at the Argon Geochronology in Earth Science Laboratory (UQ-AGES) of the University of Queensland. Each rock sample weighed 10-15 kg. From each sample 100 grams of fragments containing amphiboles and biotites were crushed, sieved to 0.2-2mm, and washed in absolute ethanol in an ultrasonic bath. Approximately 10 single crystals of biotite and amphibole from each sample were hand-picked under a binocular microscope. The minerals were loaded into Al-irradiation disks, as illustrated in Vasconcelos et al. (2002), and irradiated, together with Fish Canyon sanidine neutron fluency monitors, for 14 hours at the CLIC-Facility at the Oregon State University Triga reactor. After a two-month cooling period, the samples were loaded into a copper disk, placed in an ultra-high vacuum extraction line, and incrementally heated by an Ar ion laser beam. Each fraction of gas released was cleaned by a cold trap operated at –140ºC, two C-40 SAES getter pumps, and analyzed by a MAP-215-50 mass spectrometer. Details of the analytical procedures and correction factors are presented in Vasconcelos (1999 a, b) and Vasconcelos et al. (2002).
RESULTS AND DISCUSSION The results obtained in this study can be set basically in three groups of rocks: The 40Ar/39Ar isotopic data for three biotite single crystals from the grey gneiss of Kinawa quarry (Fig 1b - outcrop A) shows an integrated age of 1,937 ± 4 Ma, 1,898 ± 3 Ma, and 1,886 ± 3 Ma. The analogous data was obtained for the three-biotite single crystals from dark gneiss of the Corumbá quarry (Fig 1b - outcrop B) which display well-correlated integrated ages of 1,920 ± 2 Ma, 1,864 ± 2 Ma, and 1,838 ± 3 Ma. 40Ar/39Ar isotopic data for three biotites and three amphiboles single crystal from amphibolites enclaves on the Gneisses Claudio (Fig 1b - outcrop B) displayed well-defined plateaus ages of 1965 ± 2 Ma; 1987 ± 2 Ma and 1969 ± 3 Ma for the biotite and, 2027 ± 14 Ma, 2003 ± 8 Ma and 2016 ± 7 Ma for the amphiboles. The analyses of three amphibole single crystals for the amphibolite from Supracrustal Unit (Fig. 1, outcrop C) showed well defined plateaus. The 40Ar/39Ar step-heating spectra for these sample displayed integrated ages of 2031 ± 13 Ma, 2040 ± 20 Ma and 2032 ± 9 Ma. For the gabbronorite (Fig. 1, outcrop D) three amphibole crystals and three biotite single crystals were taken. The 40Ar/39Ar isotopic data for the three amphibole single crystals exhibit plateaus at 1,530 ± 9 Ma, 1,752 ± 15 Ma and 1,690 ± 40 Ma. The three biotite single crystals yield three disturbed plateaus at 1,332 ± 3Ma, 1,611 ± 13Ma and 1,340 ± 30Ma respectively. Two gabbros (Fig. 1, outcrops E and F), that represent the “second” dyke swarm generation, were analyzed. Three amphibole crystals from each gabbro were chosen and analyzed. The first one (Fig 1 - outcrop E) shows good plateaus at 940 ± 50 Ma, 990 ± 30 Ma and 1080 ± 40 Ma. The second one (Fig 1 - outcrop F), also shows good correlations and displays good plateaus at 960 ± 50 Ma, 952 ± 7 Ma and 864 ± 14 Ma. The 40Ar/39Ar results describe two of the latest events of the CBMC evolution: the first one show that the Transamazonian orogeny intensely affected the CBMC, and the second one evidences the intense breaking-up of the sialic crust after post-Transamazonian orogeny. From the considerations above, the gneisses, amphibolites, gabbronorite and gabbro of the CBMC have undergone wide disturbances since the earliest event that affected the surrounding areas. The gneisses and amphibolites from both units (gneiss and supracrustal) record the same peculiar closing temperature at about 2.0 Ga. The gabbronorite, which represents the first group of the mafic dykes swarm rocks, emplaced at about 1.7 Ga and the gabbro represent the second group of the mafic dykes swarm rocks that emplaced at about 1.0 Ga. Oliveira (2004) recognized the high-grade metamorphic event at 2.05 Ga, as attested by U-Pb data for charnockites elsewhere (Oliveira 2004). As a consequence, the ages at about 2.0 Ga can be correlated not only with the age of exhumation of the CBMC but the age of the retrograde metamorphic processes from this high-grade metamorphic event at about 2.05 Ga. The data suggests that the CBMC was largely and quickly exhumed after the 2.05Ga metamorphic event. The ages at about 1.7 and 1.0 Ga can be correlated to the emplacement and crystallization of the two swarm dykes. The NW-SE structures into which these mafic dykes were emplaced can be correlated with the Staterian continental rift scenario (1.8 - 1.6 Ga, Brito Neves 1995). A similar episode, the Uruguayan dyke swarm, occurred in the Rio de la Plata Craton about 1.7 Ga (Teixeira et al. 1999). Another record, at ca. 1.7 Ga, is related to the Espinhaço rift, affecting the São Francisco Craton; the emplacement of the gabbronorite and gabbro cannot be earlier than ~2.0 Ga. This assertion is based on geochronological records for the gneisses and amphibolites from the gneisses and supracrustal rocks, which are strongly deformed and metamorphosed and they were cooled at about 2.0 Ga. Hence, the NW-SE dykes are neither deformed or metamorphosed, and crosscut the gneissic and supracrustal rocks, as be evidenced from field relations (Oliveira & Carneiro 2001). The latest thermal evidence in the CBMC is the emplacement of the ~1.0 Ma gabbro, emplaced in preexisting NW-SE structures and can be correlated to the Macaubas rift. The results displayed in this study represent the final stabilization of the CBMC that happened in the Mesoproterozoic and not in the Achaean as postulated previously. Finally, this study demonstrates the need for comprehensive Ar-Ar datasets. Only limited 40Ar/39Ar data for high-grade metamorphic terrains is available in the literature.
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