Petrography and geochemistry of the Pedra Dourada Granulite , southeastern Minas Gerais , Brazil

The Pedra Dourada Granulite (PDG) occurs at the southeastern portion of the Araçuaí Belt, north of the town of Ponte Nova. It comprises bodies up to 45 km2 inserted into amphibolite-facies gneisses of the Mantiqueira Complex. Motivated by the discrepancy of metamorphic grade with surrounding rocks, this paper presents results of the petrographic and geochemical investigation of the PDG. The unit is comprised of meta-igneous and metasedimentary rocks. Meta-igneous rocks dominate and include felsic granulites (biotite ± garnet-bearing and orthopyroxene-bearing) and subordinate mafic granulites. Metasedimentary rocks are aluminous granulites with Al-rich mineral assemblages (garnet, sillimanite, spinel). Geochemical data show that most of felsic protoliths are peraluminous rocks including granites, granodiorites and diorites of calc-alkaline character, chemically similar to granitoids of convergent tectonic settings. Mafic protoliths are metaluminous rocks comprised of gabbros and subordinate diorites of tholeiitic affinity, compositionally similar to plate margin basalts. Aluminous protoliths may be peraluminous pelitic rocks and wackes, analogous to sediments from convergent environments. The mineral assemblages indicate that these rocks were metamorphosed under medium-P granulite-facies conditions. Coronitic garnet textures suggest a near-isobaric cooling (IBC-path) after metamorphic peak.


Introduction
The Pedra Dourada Granulite (PDG) is located in the southeastern portion of the Araçuaí Belt, which was defined by Almeida (1977) as a Brasiliano fold-thrust belt developed along the southeastern edge of the São Francisco Craton.This belt is now viewed as the external domain of the Araçuaí Orogen (Alkmim et al., 2007).
The Araçuaí Orogen encompasses the entire region between the São Francisco Craton and the Brazilian continental margin and is roughly limited by the 15º and 21º S parallels.This orogen displays an arbitrary boundary with the Ribeira Orogen to the south (Pedrosa-Soares & Wiedemann-Leonardos, 2000) (Figure 1a).The Araçuaí Orogen is subdivided by the Abre Campo Shear Zone into two tectonic domains -the internal (eastern) and the external (western) (Figure 1b).The internal domain corresponds to the crystalline core of the orogen and comprises high amphibolite to granulite facies rocks of the Juiz de Fora Complex (Alkmim et al., 2007).
Unlike the AC, the PDG has been little investigated from the petrogenetic point of view.This paper presents and discusses field, petrographic and geochemical data of the PDG.This study aims to contribute to the knowledge of the constitution and evolution of the basement of the Araçuaí Orogen.

Materials and methods
The spatial distribution of the PDG was defined based on 120 geological stations, in which 72 stations were visited during the field studies from this work.The other field data comes from studies of Jordt-Evangelista (1992, 1996), Alcântara & Machado (2010) and Melo & Maia (2010).The petrographic and microstructural characterization was based on the description of 102 thin sections under polarized light microscope.Whole-rock geochemical analyses were carried out on 20 representative samples of the granulites.These samples were crushed and milled at the Departamento de Geologia of the Universidade Federal de Ouro Preto.Major and trace element concentrations were determined using Inductively Coupled Plasma Emission Spectroscopy (ICP-ES) at the ACME Analytical Laboratory LTDA, Canada.The samples from this study were combined with 17 published rock compositions by Jordt-Evangelista (1996).

Field characteristics and lithological constitution
The PDG occurs as a large body in the central part of the study area and also as isolated outcrops located in the northern and southern portions of the study area (Figure 2).The field studies show that the occurrence area of the PDG is larger than originally defined by Brandalise (1991).Peres et al. (2004).
The PDG comprises meta-igneous and metasedimentary rocks of granulite-facies.The meta-igneous granulites dominate and include felsic granulites from granite-tonalitic to charnokite-enderbitic composition and less abundant mafic granulites of gabbroic composition.The metasedimentary rocks are aluminous granulites.
The contacts between felsic and mafic granulites are variable.The occurrence of the mafic granulites as rounded, sub-angular or lens-shaped xenoliths in the felsic granulites is common (Figure 3a).However, dominantly mafic outcrops intruded by felsic rocks are also found.The two lithologies also coexist as alternating felsic and mafic bands of few centimeters thick.These bands occur as folded or showing diffuse contacts (Figure 3b).No contacts between aluminous granulites and orthogranulites were observed.
The felsic and the mafic granulites may show both as well as isotropic textures as millimetric to centimetric mineralogical banding, whilst the aluminous granulite exhibits a prominent banding (Figure 3c).Overall, the granulites show a mylonitic foliation parallel to the compositional banding.Furthermore, centimetric scale S-C shear zones were observed in some outcrops.
The granulite-facies rocks also present partial melting features defined by the presence of quartzfeldspathic leucossomes.The textures vary from schlieren, schollen (Figure 3d), fleck (Figure 3e) to phlebitic (Figure 3f).The last occurs predominantly in aluminous granulites while the others are typically found on metaigneous rocks.The leucossome may also contain mafic anhydrous or hydrous mineral phases, like hornblende in mafic granulites (Figure 3e) or garnet in aluminous varieties.

Felsic granulites
Felsic granulites are the most abundant granulite-facies rocks in the study area.According to the mafic mineral content, these granulites were subdivided into two groups: (i) biotite ± garnet-bearing felsic granulites, compositionally belonging to granite-tonalite series, and (ii) orthopyroxene-bearing felsic granulites, belonging to charnockiteenderbite series.
Orthoclase (15 -55% vol) and plagioclase (10 -45%) occur both as porphyroclasts and as matrix constituent.Porphyroclasts consist of anhedral to subhedral grains with interlobate or ameboid boundaries, suggesting grain boundary migration recrystallisation.They commonly exhibit perthitic (alkali-feldspar) and antiperthitic (plagioclase) exsolutions (Figure 4a) and also show evidences for intracrystalline deformation, like undulose extinction and deformation twins (plagioclase).Matrix grains exhibit anhedral crystal shape and often define core-and-mantle structures in porphyroclasts.Quartz (20 -35%) occurs as anhedral grains and may be equant or elongate.The grains form the matrix or compose monomineralic ribbons that wrap around feldspar porphyroclasts.The main deformation microstructures are undulose extinction, deformation bands and subgrains, which sometimes define chessboard-type extinction.Biotite (1 -15%) is reddish-brown (X Mg = 0.57 and 0.33 apfu of Ti) and displays a weak orientation.Smaller secondary flakes are light-green and occur filling fractures in garnet or surrounding this mineral.Garnet (0-5%) may occur as two generations.The primary is characterized by rounded porphyroblasts of alm 69,1 prp 23,0 grs 4,6 sps 3,3 and contains quartz and feldspars inclusions.The late garnet forms symplectitic intergrowth with opaque minerals.

Mafic granulites
Mafic granulites are the second most abundant granulite-facies rock in the study area.They have a gabbroic composition and display inequigranular granoblastic fabric.This lithotype is composed by the mineral assemblage plagioclase + biotite ± orthopyroxene ± clinopyroxene ± hornblende ± quartz ± garnet (Figure 4d).Zircon, titanite, apatite, allanite and opaque minerals are the common accessory minerals, plus secondary minerals like actinolite, cummingtonite, epidote, scapolite and sericite.

Aluminous granulites
Aluminous granulites are characterized by the abundance of Al-rich minerals like garnet and biotite and by the presence of sillimanite and spinel, which occur only enclosed in garnet.The rock displays an inequigranular granoblastic to lepidoblastic fabric, characterized by coarse-grained garnet and orthopy-roxene porphyroblasts, which are embedded in a fine-to medium-grained matrix.This granulite is composed by the main mineral assemblage garnet + plagioclase + quartz + biotite ± orthoclase ± orthopyroxene.Zircon, spinel, sillimanite, apatite and opaque minerals are the common accessory minerals, plus secondary minerals like chlorite, epidote and sericite.

Felsic granulites
Felsic granulites are comprised of intermediate to acid rocks (58.32 -76.14% SiO 2 ) (Table 1).In the TAS diagram (Cox et al., 1979modified by Wilson, 1989), most of the biotite ± garnet-bearing felsic granulites plot within the granite field, with a few in the granodiorite field, while most of the orthopyroxene-bearing felsic granulites fall in the granodiorite field, with some of them in the diorite and granite field.According the same diagram, the felsic granulites belong to the subalkaline/ tholeiitic series (Figure 5a).
In the AFM plot of Irvine & Baragar (1971), the felsic granulites scatter around a calc-alkaline trend (Figure 5b).The most differentiated terms, i.e., those that plot near the vertex A (Na 2 O + K 2 O), correspond to biotite ± garnet-bearing felsic granulites.Regarding the aluminum saturation, the biotite ± garnet-bearing felsic granulites are peraluminous rocks, with Shand's index between 1.02 and 1.24.The orthopyroxene-bearing felsic granulites, in turn, plot between the fields of metaluminous and peraluminous rocks, with Shand's index range from 0.87 to 1.08 (Figure 5c).
According to the geochemical classification of Frost et al. (2001) for granitic rocks (not shown in this paper) the felsic granulites are magnesian granitoids and belong to the calcic and calc-alkaline series.The same authors have associated these chemical signatures with Cordilheran-type batholiths, island arcs plutons and plagiogranites.
The tectonic setting of the felsic granulites was characterized based on tectonic discrimination diagrams for granitoids.In the Nb vs. Y diagram of Pearce et al. (1984), most of the felsic granulites plot within the Volcanic arc + Syn-collisional granitoids field (Figure 5e).

Mafic granulites
Mafic granulites comprise basic to intermediate rocks (46.76 -56.20%SiO 2 ) (Table 1).In the TAS diagram, most of these granulites plot within the gabbro field and few of them in the diorite field.According the same diagram, they belong to the subalkaline/tholeiitic series (Figure 5a).
The AFM diagram shows a tholeiitic signature for most mafic granulites, except for two samples that fall within the calc-alkaline field (Figure 5b).According the Shand's diagram, all these granulites are metaluminous rocks, with ACNK values between 0.48 and 0.85 (Figure 5c).
The tectonic environment of the mafic granulites was characterized based on tectonic discrimination diagrams for basaltic rocks.According the Ti/Y vs. Zr/Y diagram of Pearce & Gale (1977), which separates Within-plate basalts and Plate margin basalts, all the mafic granulites show a geochemical signature consistent with Plate margin basalts (Figure 5g).In the ternary diagram of Meschede (1986), the mafic samples disperse in the fields of the E-type MORB -enriched in incompatible trace elements, Within-plate tholeiites + Island arc basalts, and N-type MORB -depleted in incompatible trace elements + Island arc basalts (Figure 5h).Again, no sample presents an exclusive intraplate basalt signature.
The protoliths were character-ized based on the discriminant diagram of Herron (1988), which relates log(Fe 2 O 3 (t) /K 2 O) vs. log(SiO 2 /Al 2 O 3 ) (Figure 5d).According to this diagram, three samples have geochemical characteristics of Fe-shale and one of them has characteristics of wacke.
The tectonic environment of the aluminous granulites was characterized according to the tectonic discrimination diagram for sandstones and argillites of Roser & Korsch (1986), which is based on the K 2 O, Na 2 O and SiO 2 content.This scheme suggests an oceanic island arc margin setting for three pelitic rocks and an active continental margin setting for the wacke sample (Figure 5i).

Discussion
The field work showed that the granulite-facies rocks exhibit evidence for medium-to high-grade deformation, as manifested by folded banding and mylonitic fabric.At the microscope scale, high-grade deformation is characterized by lobates and amoeboid boundaries in quartzfeldspar grains and also chessboard subgrains in quartz.Medium-grade evidence is represented by coreand-mantle structures in feldspars wrapped by quartz ribbons.
The biotite ± garnet-bearing felsic granulites, despite the absence of orthopyroxene, were interpreted as belonging to the granulite-facies due the predominance of orthoclase instead microcline, the antiperthitic plagioclase, the Ti-rich biotite, the absolute absence of primary moscovite and, finally, the associated occurrence in some outcrops with typical mafic granulites.The orthopyroxenebearing felsic granulites show the same features as the former, besides the orthopyroxene content.The main reaction texture observed in this rock is the symplectitic coronas of garnet and ilmenite around plagioclase.The origin of coronal garnet has been a subject of debate.Sen & Battacharya (1993) opine that a garnet-forming reaction is triggered during retrograde metamorphism as a consequence of near-isobaric cooling (IBC-path).On the other hand, Maji et al. (2008) advocate a prograde growth of garnet at the expense of plagioclase + ilmenite ± biotite ± hornblende ± quartz.
On the mafic rocks, the granulite-facies metamorphism is characterized by the paragenesis plagioclase + orthopyroxene + clinopyroxene, which is typical of medium-P granulite-facies metabasites (Green & Ringwood, 1967in Yardley, 2004).However, in some mafic granulites only orthopyroxene occurs, while in others only clinopyroxene is found.According to Best (2003), at higher P, orthopyroxene is consumed in garnet-forming reactions as orthopyroxene + plagioclase → garnet + quartz and orthopyroxene + plagioclase → garnet + clinopyroxene + quartz.The former reaction may have generated the coronitic garnet on orthopyroxene-plagioclase contacts, while the latter may have originated the clinopyroxene coronas on orthopyroxene.Harley (1989) associates both reaction textures to retrograde IBC-paths.
The chemical classification dia-grams suggest that the felsic granulites protoliths are granodiorites, granites and diorites.They are predominantly peraluminous rocks belonging to calc-alkaline series.The tectonic discrimination diagrams point to convergent margin tectonic settings to the felsic granulites.
The mafic granulites protholits are gabbros and subordinate diorites.They are metaluminous rocks and have a tholeiitic character.The discriminant diagrams suggest that the mafic pro-tholits were associated to plate boundary environments.However, these plots could not defined if they were convergent or divergent tectonic settings, as the samples show both island arcs and MORB signatures.
The aluminous granulites are peraluminous rocks with protholits that show compositional similarities with modern shales and wackes.According to the tectonic discrimination diagram, they have signatures analogous to convergent setting sediments.

Conclusions
The mineral assemblages indicate that the Pedra Dourada Granulite was metamorphosed under medium-P granulitefacies conditions and retrometamorphosed under greenschist-to amphibolite-facies conditions, as indicated by the secondary mineral assemblages.The chemical composition of the studied samples shows that the protoliths are acid, intermediary, and basic igneous rocks of calcalkaline or tholeiitic signature, besides the peraluminous sedimentary rocks.The tectonic discrimination diagrams suggest that these lithologies could be associated with convergent tectonic settings, wherein the mafic granulites may also be linked to extensional environments.The coronitic garnet textures present in the granulite-facies rocks suggest a near-isobaric cooling (IBC-path) after the metamorphic peak (Ellis, 1987;Harley, 1989).Based on the tectonic setting of the Pedra Dourada Granulite, this P-T path could be related to crustal thickening during the Transamazonian tectonothermal events (2,1-2,0 Ga) or the Brasiliano tectonothermal events (590-574 Ma) (Noce et al., 2007).Further petrological studies and geochronological data may help to decipher the role of each event on the genesis and exhumation of the granulite terrains.

Figure 1
Figure 1 Geotectonic setting of the PDG.a) The Araçuaí Orogen and adjacents units (São Francisco Craton and Ribeira Orogen).The hatched rectangle corresponds to Figure 1b.Modified from Pedrosa-Soares et al. (2007); b) Regional geologic map of the Araçuaí Belt showing the location of the PDG.The dotted rectangle corresponds to Figure 2. Modified from Peres et al. (2004).

Table 1 -
Part I -Chemical analyses of selected samples of the Pedra Dourada Granulite.

Table 1 -
Part I I -Chemical analyses of selected samples of the Pedra Dourada Granulite.