A MESOPROTEROZOIC SOURCE AT SOUTHERN SÃO FRANCISCO CRATON

 

 Valladares, C.S.¹; Machado, N.²; Heilbron, M. ¹; Gauthier, G.³

 

1. Tektos – DGRG-Faculdade de Geologia – UERJ, 20550-013, Rio de Janeiro - RJ, Brasil. vallada@uerj.br

2. GEOTOP-Sc. de la Terre, UQAM - Canada.

3. Dept. Géologie, Université de Montréal, Montréal, Canada

 

ABSTRACT

 

²⁰⁷Pb/²⁰⁶Pb LA-ICPMS ages of detrital zircons from lower greenschist quartzites from Proterozoic basin successions of the southern border of the São Francisco Craton, southern Minas Gerais State, are discussed. These are the intracratonic São João del Rei basin, the intraplate continental margin Andrelândia basin, and the Serra do Ouro Grosso sequence, developed on a crystalline basement older than 1.8 Ga that was deformed and metamorphosed during the Brasiliano Orogeny, ca. 0.59-0.50 Ga.

 

Keywords: Detrital zircon ²⁰⁷Pb/²⁰⁶Pb ages, laser ablation ICPMS, Proterozoic basins, southern São Francisco Craton

 

INTRODUCTION AND OBJETIVES

     The determination of detrital zircon populations present in metasedimentary successions can be used to constrain both the provenance and minimum age for deposition. One possible methodology, is the determination of ²⁰⁷Pb/²⁰⁶Pb ages by (LA-ICPMS). The methodology and results were discussed by Feng et al. (1993) and Machado & Gauthier (1996), and in principle can be interpreted as indicative of minimum ages.

     The short paper reports ages of detrital zircon from lower greenschist quartzites published by Valladares et al. (2004) of the following Proterozoic basin: a) the São João del Rei basin; b) the Andrelândia basin, and c) The Serra do Ouro Grosso sequence. Constraints of the probable source rocks as well as the sedimentation ages of these successions are discussed.

 TECTONIC SETTING AND SELECT SAMPLES

     The studied metasedimentary successions crop out within the Neoproterozoic Ribeira and Brasília belts, located, respectively, east/southeast and west/southwest of the São Francisco Craton, southeastern Brazil.

Stratigraphic, metamorphic and structural analysis of Proterozoic sedimentary successions occurring in the southernmost São Francisco Craton and in its southern border led to the recognition of several depositional sequences separated by unconformities. One of them is the Serra do Ouro Grosso sequence, that comprises quartzite and conglomeratic quartzite deposited on an Archaean to Paleoproterozoic basement. The others successions are located in the autochthonous and allochthonous domains of the external portion of the Brasília and Ribeira belts. Recently, Ribeiro et al., (1995) and Trouw et al. (2000) recognized the sedimentary record of three superposed basins named São João del Rei, Carandaí and Andrelândia basins. The first two are considered to be related to an intracratonic setting, whilst the Andrelândia basin is thought to represent a continental margin (Ribeiro et al., 1995; Paciullo et al., 1998). All three sequences lie on poorly dated basement (greenstone belts and gneiss complexes) which is the southern extension of an Archean terrain intruded by Paleoproterozoic granitoids with ages of  as 1.98 Ga (Rb-Sr age; Heilbron et al., 1989) and 2.22-2.12 Ga (²⁰⁷Pb-²⁰⁶Pb, Ávila, 2000). The Proterozoic successions were deformed and metamorphosed during the Brasiliano Orogeny, in a ductile nappe system with top-to-east tectonic transport. The associated metamorphism varies from middle greenschist to granulite facies. The extent to which the 750-450 Ma Brasiliano Orogeny affected the different sequences is poorly constrained. Sm-Nd, Rb-Sr and K-Ar ages indicate that the Andrelândia sequence was metamorphosed at ca. 0.50-0.66 Ga (Heilbron et al., 1989; Söllner & Trouw, 1997). Therefore, the deposition in the three basins must have occurred between ca. 1.98 Ga and 0.5-0.7 Ga.

 

GEOLOGY AND RESULTS

Serra do Ouro Grosso sequence

An isolated body composed of quartzites with approximately 200m of thickness crops out at Serra do Ouro Grosso ridge. The Serra do Ouro Grosso sequence has been correlated to the basal quartzites of the Tiradentes Platform of the São João del Rei basin (Trouw et al., 2000). Zircons from a green-mica-quartzite are mainly colorless and yellowish and a few are of pale brown color. Their size ranges from smaller crystals of ca.  40mm to prismatic bigger ones that reach 200mm. They are elongated 1:2 and euhedral, and the majority show one visible overgrowth that was not large enough to be analyzed by this method. The ²⁰⁷Pb-²⁰⁶Pb analyses (Fig. 1a) yielded ages from 3.22 to 2.4 Ga defining two modal classes at 2.8-2.7 and 2.6-2.5 Ga. A single grain yielded an age of 2,245 ± 85 Ma which is interpreted as the maximum age for sedimentation.

 

São João del Rei basin

The sedimentary record in this basin consists mainly of a quartzitic succession up to 1000 m thick unconformably overlain by the Carandaí basin (Ribeiro, 1997, Ribeiro et al. 1995). The previous estimation for basin filling was based on Nd T_DM model ages varying from 1.8 to 1.3 Ga for five metamafic dykes intruded in São João del Rei units (Ribeiro et al., 1990).

Zircons extracted from one of the lower basal quartzite units, Tiradentes sequence, are all rounded and mostly spherical, but a minor proportion are elongated 1:2; they vary in color from colorless to pink to dark brown and all have characteristic percussion marks. The ²⁰⁷Pb/²⁰⁶Pb ages (Fig. 1b) vary between 2.86 and 1.81 Ga and define a modal class at 2.2-2.1 Ga followed by the 2.1-2.0 Ga class as the next most abundant. A smaller group of zircons yield ages in the 2.4-2.9 Ga interval. The age of the youngest zircon, 1,809±41 Ma provides the best estimate for the maximum age of deposition in this basin.

 

Andrelândia basin

The metasediments of the Andrelândia sequence are interpreted as deposited on a Neoproterozoic intraplate continental margin that surrounded the southern São Francisco paleocontinent (Paciullo et al., 1998; Trouw et al., 2000). Following the Brasiliano Orogeny, the successions are found far away from the foreland, in the internal zones of the Brasília and Ribeira belts located, respectively, southwestward and southeastward of the São Francisco Craton. The regional distribution of the main lithofacies associations suggests, for the Brasília belt side, a NW-SE coastline and a depocenter to the WSW. The main main lithofacies associations are described by Paciullo (1997) and Paciullo et al. (1988).

Two samples of quartzite beds interlayed with gray phyllite/schist deposited during a marine-flooding event were analyzed: one from the autochthonous domain and the other one from the basal thrust sheet of an allochthonous domain of the Brasília belt. Detrital zircons extracted from both quartzites (samples ITA-01 and ITA-03) display low to medium sphericity and are mainly rounded and sub-rounded, although some preserve prismatic faces. They are mainly pinkish and yellowish, although a few are colorless. All have characteristic percussion marks.

The zircons from the autochthonous quartzite yielded ages in the range 1.0-2.2 Ga, with a modal class at 1.2-1.3 Ga (Fig. 1b). Only two out of thirty-six grains yielded Archean ages. The youngest zircon yielded an age of 1,086±85 Ma. This distribution indicates the presence of Mesoproterozoic zircon-bearing source rocks not well documented from the southern São Francisco Craton.

The zircons from the allochthonous quartzite yielded ages in the range 1.0-2.7 Ga, with a 2.1-2.2 Ga modal class Ga (ITA-03, Fig. 1c).  Many zircons have ages in the range 1.0-1.5 Ga and only five of forty three grains yielded Archean ages. The youngest zircon yielded an age of 1,047±77 Ma.

 

PROTEROZOIC BASIN EVOLUTION

The ²⁰⁷Pb/²⁰⁶Pb LA-ICPMS data (Valladares et al. 2004) combined with the detailed geological data (Trouw et al., 2000; Paciullo, 1997; Ribeiro, 1997; Ávila, 2000; Teixeira et al., 2000) reported for the southernmost region of the São Francisco Craton support the proposed Proterozoic basin evolution:

 

Episode 1,  ca. 2.25 Ga (The Serra do Ouro Grosso intracratonic basin)

The Serra do Ouro Grosso sequence was derived predominantly from the erosion of a 2.5-2.8 Ga Neoarchean crust with subordinate contribution of Paleoproterozoic rocks. The existence of overgrowths indicates that the source area underwent some kind of metamorphism or magmatic overgrowth during the Paleoproterozoic. The ca. 2.25 Ga age is the best estimate for the beginning of the sedimentation for this sequence. It is also interesting to stress that, until now, only U-Pb and ²⁰⁷Pb-²⁰⁶Pb ages around 2.2 to 2.0 Ga (Ávila, 2000; Teixeira et al., 2000) were reported for the “Transamazonian” granitoids of this sector along the southern border of the São Francisco Craton.

 

Episode 2 (< ca. 1.8 Ga): São João del Rei basin

Data of the São João del Rei basin successions indicate derivation from a 2.2-2.0 Ga crust, with subordinate contribution from Neoarchean sources. The age of the youngest zircon, 1,809±41 Ma provides the best estimate for the maximum age of deposition in this basin. Ages between 2.2 and 2.0 Ga are regarded as the signature of the so-called Transamazonian orogeny in the São Francisco Craton (Machado et al., 1992; Noce, 1995; Teixeira & Figueiredo, 1991) and represent arc-related rocks, collisional granitoids and metamorphism. These considerations may be taken to suggest that the formation of the São João del Rei basin may postdate a period of erosion of continental crust stabilized after the Transamazonian orogeny.

     The Serra do Ouro Grosso sequence was tentatively correlated with basal quartzites of the Tiradentes Platform of the São João del Rei basin (Trouw et al., 2000), but the data do not corroborate this assumption, and we suggest that the Serra do Ouro Grosso succession is older than the São João del Rei successions, unless a unique provenance pattern reflects different source areas.

 

Episode 3 (ca. 1.3-1.05 Ga): The Carandaí basin

Until now no direct geochronological data for the metasediments of the Carandaí basin successions are avaiable. The time interval of 1.3-1.0 Ga proposed by Ribeiro et al. (1995) is based on Sm/Nd model ages of mafic dykes. The age of 1.05 Ga proposed above is based on the age of the youngest detrital zircon (1,047±77 Ma) of the overlying Andrelândia basin

 

Episode 4 (ca.1.05-0.61Ga): The Andrelândia Passive Margin

The detrital sediments of the Andrelândia basin were derived mainly from the erosion of Mesoproterozoic and Paleoproterozoic rocks. The Archean contribution is minimal, indicating that in the internal sector of the Transamazonian Orogen juvenile rocks predominated. The age of the youngest zircon (1,047±77 Ma) provides the best estimate for the maximum age of deposition of the Andrelândia basin A4 association. The Sm/Nd ages of 604 ± 19 Ma (Trouw & Pankhurst, 1993) and U-Pb of 567±11 Ma (Söllner & Trouw (1997) provide the best estimate for the minimum age of the sedimentation. The data permit proposing the time interval between 1,047 Ma and 604-567 Ma for the deposition in the Andrelândia basin and are in accordance with the time interval of 1.0-0.6 Ga proposed by Trouw et al. (2000). The presence of several Mesoproterozoic source rocks to the north-northeast of the present location, to the interior of the São Francisco Craton, as indicated by provenance indicators, are of particular importance since igneous rocks in the 1.0-1.6 Ga range are not known from the São Francisco Craton, with the possible exception of mafic dykes.

 

REGIONAL CORRELATIONS

The new ²⁰⁷Pb-²⁰⁶Pb data indicate that the Serra do Ouro Grosso succession probably represents the oldest Proterozoic siliciclastic succession of the investigated area. The ca. 2.25 Ga youngest zircon found in this sequence is immediately older than the Transamazonian orogenic rocks, suggesting deposition prior to that orogeny. The Serra do Ouro Grosso sequence could be chronocorrelated with the Piracicaba Group of the Minas Supergroup.

The São João del Rei basal unit presents similar age distribution with the Itacolomi Group, with zircon populations coeval with the main phases of the Transamazonian Orogeny (2.22 to 2.04 Ga). The youngest age obtained, around 1.8 Ga, corresponds to post-tectonic rocks described in the southern border of the São Francisco Craton by Heilbron et al. (1989) and Ávila (2000). Therefore, the results indicate erosion of a stable crust after the Transamazonian Orogeny. Previous regional correlation with the Espinhaço Rift successions (Ribeiro et al.,1990;  Trouw et al. (2000) based on Nd T_DM model ages for metamafic dykes that crosscut the São João del Rei units, are also corroborated by the presented data.

The two samples of the Andrelândia successions display identical detrital signatures, with populations derived from Archean, Tranzamazonian and Mesoproterozoic sources. The first two populations are well constrained in the southern segment of the São Francisco Craton and discussed above. The Mesoproterozoic    source   is  poorly  defined  in   the cratonic region, but as we investigated both autochthonous and allochthonous settings, one might speculate that the zircons could be derived from eroded volcanic successions, only preserved as subvertical feeder dykes that crosscut the São João del Rei successions. As stated before, T_DM model ages of 1.8 to 1.3 Ga are in agreement with this interpretation. The youngest detrital zircon age of 1.05 Ga constrains the beginning of the sedimentation of the Andrelândia successions.

 

Figure 1. Histogram for ages of detrital zircon from quartzites of: a) Serra do Ouro Grosso sequence; b) São João del Rei sequence; c) and d) Andrelândia from autochthonous and allocthonous domains, respectively. N = total number of grains analyzed.

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