Petrography and geochronology of the Furquim Quartzite, an eastern extension of the Itacolomi Group (Quadrilátero Ferrífero, Minas Gerais)

This paper presents the results of a petrographic and geochronological investigation of the Furquim Quartzite (FQ) to establish its stratigraphic correlation to quartzitic units of the Quadrilatero Ferrifero (QF) province. The Quartzite comprises a ca. 20km long and 1-6km wide ridge overlying discordantly the Archean to Paleoproterozoic gneissic basement and rocks of the Archean Rio das Velhas Supergroup between the city Mariana and the town Furquim, southeast of the QF. Despite the discordant contacts, previous field-based stratigraphic studies considered the Furquim Quartzite as part of the Archean Maquine Group – top unit of the Rio das Velhas Supergroup. U-Pb zircon geochronology via LA-ICP-MS identified several detrital populations ranging from Paleoproterozoic to Archean age. The youngest population of 2087±19 Ma defines the maximum age for the sedimentation of the precursor sandstone. This age can be correlated to be the age of the youngest zircon population of the Itacolomi Group quartzites in the QF. Thus, in contrast to previous studies, the results indicate that the FQ is an eastern extension of the Itacolomi Group, the youngest unit of the Paleoproterozoic Minas Supergroup.


Introduction
The Furquim Quartzite is exposed along a ca.20 km long and 1 to 6 km wide ridge some 10 to 20 km southeast of the Quadrilátero Ferrífero (Fig. 1a).In the investigated area (Fig. 1b) the Furquim Quartzite trends NE-SW and inflects to a NNE-trend in the region of the town of Furquim.According to the geological map of Baltazar et al. (1993), the quartzite ridge separates the western Archean Santa Bárbara Complex from the eastern Proterozoic Mantiqueira Complex (Fig. 1a).Baltazar et al. (1993) correlated this quartzite unit to the Maquiné Group of the Archean Rio das Velhas Supergroup.However, the postulated correlation with the Rio das Velhas quartzite is not corroborated by the mode of occurrence, since in a great part of the area, the quartzite ridge overlies discordantly the Rio das Velhas schists and the Santa Bárbara gneisses, thus suggesting an allochtonous origin.This paper presents the results of the petrographic and geochronological investigation of the Furquim Quartzite in order to enable the discussion of its stratigraphic correlation to quartzite units of the wellknown Maquiné, Moeda and Itacolomi quartzites in the Quadrilátero Ferrífero.

Geological context
The main quartzitic units in the region of the Quadrilátero Ferrífero belong to the Archean Rio das Velhas Supergroup and to the Paleoproterozoic Minas Supergroup.
The Rio das Velhas Supergroup is subdivided into Nova Lima and Maquiné groups (Dorr, 1969).The Nova Lima Group is composed mostly of metaultramafic, metamafic and metasedimentary pelitic to ruditic rocks.Felsic volcanic rocks mark the final deposition of the Nova Lima Group at ca. 2.75 Ga (Machado et al., 1992(Machado et al., , 1996;;Noce et al., 2005).The overlying Maquiné Group is a clastic unit comprised of mainly quartz-ites.U-Pb age determinations of detrital zircons indicate 3.2 to 2.9 Ga for the main sources of the Maquiné sediments (Machado et al., 1996).
The Minas Supergroup overlies the Rio das Velhas Supergroup and surrounding TTG-gneiss terrains.From bottom to top, it is subdivided into the Tamanduá, Caraça, Itabira, Piracicaba, Sabará, and Itacolomi Groups (Dorr, 1969).Zircon U-Pb detrital age data suggest that the maximum age of deposition of the sediments of the Caraça quartzite is ca.2.6 Ga (Machado et al., 1996;Hartmann et al., 2006).The Sabará and Itacolomi groups are the youngest units of the Minas Supergroup.The Sabará Group comprises metassedimentary rocks such as metadiamictites and metatuffs and metaturbidites.The Itacolomi Group comprises quartzites derived from sediments with a maximum deposition age of around 2.1 Ga (Machado et al., 1996, Hartmann et al., 2006).Similar ages were also obtained for the deposition of the Sabará sediments (Machado et al., 1996).Table 1 presents the ages of the youngest zircons found in the main quartzitic units of the Quadrilátero Ferrífero.These ages correspond to the maximum age for the deposition of the sediments.
Two other geological units that would be potential sources for the detrital zircons of the Furquim Quartzite are the TTG-gneiss complexes, including the Santa Bárbara and the Mantiqueira complexes (Fig. 1b).The Santa Bárbara Complex corresponds to an Archean TTG gneiss terrain considered to be the basement of the Rio das Velhas Supergroup in the eastern portion of the Quadrilátero Ferrífero.Geochronological U-Pb SHRIMP and LA-ICP-MS dating by Lana et al. (2013) indicate crystallization ages of 3.2 Ga.The Mantiqueira Complex is composed of TTG ortogneisses thrusted over the southern margin of the São Francisco Craton during the 2.1 Ga Transamazonian event (Silva et al., 2002, Noce et al., 2007).U-Pb zircon age determinations by SHRIMP (Silva et al., 2002;Noce et al., 2007) resulted in paleoproterozoic crystallization ages of around 2180-2041 Ma for ortogneisses of the Mantiqueira Complex and two metamorphic events at 2100 Ma and 560 Ma.

Material and methods
Thin sections of representative hand samples collected in the vicinity of the town of Furquim and on the highway BR-356 were described on a Leica DM EP microscope at the Departamento de Geologia (DEGEO), Universidade Federal de Ouro Preto.
One sample (sample JP-F02, UTM: 0682390/7746163, sample locality on Fig. 1b) weighing ca.5kg was collected for LA-ICP-MS U-Pb geochronology.Zircons were concentrated making use of a conventional jaw crusher, milling, manual panning and heavy liquids separation.The zircons were hand-picked under a binocular microscope.Approximately 123 zircon crystals were selected and mounted on a 2.5 cm-diameter epoxy mount.The mount was polished and imaged under SEM-cathodoluminescence to accentuate internal growth zoning.
The laser ablation-ICP-MS (LA-ICPMS) analyses were performed using a single collector Agilent 7700 Quadrupole(Q)-ICP-MS and a 213 nm New Wave laser at the isotope/geochemistry laboratory of Department of Geology, Universidade Federal de Ouro Preto.Acquisitions consisted of a 20 s measurement of the gas blank, followed by a 40 s measurement of U, Th and Pb signals during ablation, and a 30 s washout.All ratios were obtained after averaging the background-subtracted signal (See Romano et al., 2013 andTakenaka et al., 2015, for details on the instrumentation and methodology).Two standards were used during runs: the primary standard GJ-1 zircon (608±1 Ma) and the secondary standard Plesovice zircon (338±1 Ma).The relevant isotopic ratios have been calculated using Glitter data reduction software (van Achterbergh et al., 2001).The U-Pb diagrams were produced using Isoplot 4 software (Ludwig, 2012).The results of the analyses, including data for the primary and secondary standards can be found in Table 2.

Petrography
The main rock type is a strongly folded (Fig. 2a) and sheared muscovite quartzite.Disrupted quartz veins along fold hinges may be confused with pebbles or cobbles of conglomerates (Fig. 2b).Quartz reaches 70 to 90 vol%, muscovite 5 to 20%, while the accessory minerals hematite, magnetite, kyanite, feldspars, garnet, zircon, and tourmaline seldom sum 5% (Fig. 2c to 2f, all samples from outcrop in the town of Furquim).

Geochronology
The extracted zircons from sample JP-F02 measure ca.100-200 µm, are yellow to brown, slightly rounded and often fractured.Cathodoluminescence images show that most grains present well-defined oscillatory zoning with some broad zones of intense alteration and radiation damage and no discernible core-rim relationships (Fig. 3).Of the 123 grains mounted on the epoxy disc, 119 LA-ICP-MS analyses were performed on center and rims of 47 translucent to partly translucent grains.The complete geochronological data set can be found in Table 2.The age pattern of 31 zircons reveals a high concentration of ages in the 2.0-2.5 Ga range, with a mode of around 2.2 Ga and an important Archean component.This pattern is similar to that obtained for the Itacolomi Group by Machado et al. (1996), thus suggesting that the Furquim Quartzite detrital sequence belongs to this Group.This interpretation is also supported by the obtained minimum age of 2087±19 Ma that is identical to the minimum ages of 2059±58 Ma obtained by Machado et al. (1996) and that of 2143±16 obtained by Hartmann et al. (2006) for the Itacolomi samples.
The large number of zircon grains of Paleoproteozoic age indicates that the main sediment sources for the Furquim Quartzite are terrains generated during the Transamazonian Orogeny.The Archean ages indicate contribution of the gneissic and greenstone terrains.
The youngest Paleoproterozoic population dated at 2087±19 Ma defines the maximum age of deposition of the precursor sandstones of the Furquim Quartzite.
Quartzite ridges belonging to the Itacolomi Group are more widespread than supposed so far.Besides the locus tipicus near the city of Ouro Preto, the huge mass of quartzite, approximately 1400 m thick, occurring at Serra de Ouro Branco, southern Quadrilátero Ferrífero, previously correlated to the Tamanduá Group or to the Moeda Formation was dated by Machado et al.(1996) and correlated to the Itacolomi Group.
The occurrence of a ridge of quartzite belonging to the Itacolomi Group farther east from its locus tipicus near Ouro Preto is probably due to the action of a tectonic event as indicated by its allochtonous nature.Other studies concerning the structural complexity of Furquim Quartzite are necessary to elucidate its geological evolution.

Concluding remarks
The results of the geological, petrographic and geochronological investigation of the Furquim Quartzite allowed for a conclusion that the ridge located southeast of the Quadrilátero Ferrífero is possibly an allochtonous unit.The postulated correlation to the Archean Maquiné quartzite as presented in the regional geological map published by CPRM (Baltazar et al., 1993) is not supported by the results of U-Pb geochronology on zircon by LA-ICP-MS, which defined several detrital zircon populations ranging from Paleoproterozoic to Archean age.The youngest population dated 2087±19 Ma defines the maximum age for the sedimentation of the precursor sand-stone.This age is similar to the age of the youngest zircon population of the Itacolomi Group in the Quadrilátero Ferrífero dated by Machado et al. (1996) and Hartmann et al. (2006).Therefore, it is possible to conclude that the Furquim Quartzite can be stratigraphically correlated to the quartzites of the Itacolomi Group.

Discussion
Figure 1 a) Geological map of the Quadrilátero Ferrífero (modified from Lana et al., 2013).Rectangle: location of the study area; b) Geological map of the study area of the Furquim Quartzite (modified from Jordt-Evangelista, 1984) and location of dated sample JP-F02.
Figure 2 (a) -Outcrop of the dated folded quartzite (Point JP-F02 on Fig. 1b), view to SSW.See hammer in the center of photo for scale.(b)-Disrupted quartz veins, outcrop in the town of Furquim.See pencil in the center of photo for scale.(c) -Photomicrograph of muscovite quartzite with tourmaline (greenish), kyanite (gray, strong relief) and hematite (black), N//.(d) -Photomicrograph of hematite-rich portion in quartzite, N//.(e) -Photomicrograph showing subgrains in deformed quartz vein in muscovite quartzite.Strongly altered feldspar is seen in the center of figure (arrow), NX.(f) -Photomicrograph of folded muscovite quartzite, NX.Samples of Fig. 2c to e collected in Furquim.(Photomicrographs from Alvarenga, 2013).

Figure 3
Figure 3 Cathodoluminescence images of analyzed zircons, ages in million years (Ma).

Figure 4
Figure 4 shows concordant to subconcordant points (63 analyses > 3% concordant) plotted on the frequency histogram and on the concordia.Several Figure 4 (a) Frequency histogram where two main detrital zircon populations of the Furquim Quartzite can be identified: a Paleoproterozoic population of about 2.2 Ga and an Archean population around 2.8 Ga.(b) U-Pb Concordia diagram of LA-ICP-MS analyses of zircons (see Fig. 1b for sample locality).

Table 1 Youngest
zircon grains in the Quadrilátero Ferrífero quartzites corresponding to the maximum ages for the deposition.

Table 2 -
part I Results of U-Pb LA-ICP-MS analyses for the Furquim Quartzite.

Table 2 -
part II Results of U-Pb LA-ICP-MS analyses for the Furquim Quartzite.