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:

Biotite ± garnet-bearing felsic granulites display a prevalent inequigranular granoblastic fabric. This lithotype comprises coarse-grained porphyroclasts embedded in a fine- to medium-grained matrix. It is characterized by the mineral assemblage orthoclase + plagioclase + quartz + biotite ± garnet. Zircon, titanite, apatite, allanite, monazite and opaque minerals are the common accessory minerals, plus secondary minerals like chlorite, scapolite, sericite, epidote and carbonate.

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_69,1prp_23,0grs_4,6sps_3,3 and contains quartz and feldspars inclusions. The late garnet forms symplectitic intergrowth with opaque minerals.

Figure 4 Photomicrographs of the granulite-facies rocks. A-C - Felsic granultes. a) Antiperthitic porphyroclastic plagioclase with garnet inclusion, besides perthitic orthoclase of biotite ± garnet-bearing felsic granulite; b) Mineral assemblage of the orthopyroxene-bearing felsic granulite; c) Coronitic texture defined by symplectitic garnet around plagioclase in orthopyroxene-bearing felsic granulite; D-F - Mafic granulites. d) Granoblastic fabric. e) Cummingtonite pseudomorph after orthopyroxene. f) Corona of garnet around plagioclase; G-I - Aluminous granulites. g) Biotite and hercinite spinel inclusions in porphyroblastic garnet; h) Corona of symplectitic garnet around earlier garnet and ilmenite; i) Kink band in orthopyroxene. Abbreviations: Kfs-alkali-feldspar; Pl-plagioclase; Or-orthoclase; Grt-garnet; Qz-quartz; Ilm-ilmenite; Hbl-hornblende; Bt-biotite; Opx-orthopyroxene; Cpx-clinopyroxene; Cum-cummingtonite; Hc-hercynite. 

Orthopyroxene-bearing felsic granulites display inequigranular granoblastic fabric (Figure 4b). This lithotype comprises coarse-grained feldspar porphyroclasts surrounded by a fine to medium-grained matrix. It's characterized by the mineral assemblage plagioclase + orthoclase + quartz + orthopyroxene + biotite ± clinopyroxene ± garnet ± hornblende. Zircon, titanite, apatite, monazite and opaque minerals are the main accessory minerals, plus secondary minerals like chlorite, cummingtonite, epidote and sericite.

Plagioclase (25 - 50% vol), orthoclase (<1 - 45%) and quartz (15 - 35%) are microstructurally similar to those described above. Plagioclase is andesine (An_42-43) and orthoclase has a composition of An₁Ab₁₀Or₈₉. Orthopyroxene (1 - 18%) is Fe-hypersthene (En_46-48) and shows an anhedral to subhedral crystal shape and evidences of intracrystalline deformation, e.g. undulose extinction. However, most grains are partially or totally replaced by fine-grained aggregates of biotite ± cummingtonite ± quartz ± opaque minerals. Biotite (<1 - 20%) is reddish-brown (X_Mg = 0.52 and 0.55-0.58 apfu of Ti) and displays undulose extinction. A secondary generation is greenish-brown and replaces orthopyroxene. Clinopyroxene (0 - 12%) is usually better preserved than orthopyroxene but also exhibits replacement by symplectitic intergrowths of quartz + amphiboles. Garnet (0-10%) is alm_65,4grs_18,5prp_12,0sps_4,0uv_0,1 and may compose symplectitic intergrowth with ilmenite or forms coronas around plagioclase (Figure 4c). Hornblende (0 - 8%) occurs both as individual brownish-green crystals of ferropargasite (15eNK estimate, ^{Leake et al., 1997}) as in aggregates replacing pyroxenes.

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.

Plagioclase (15 - 40% vol) is andesine (An_41-43) and occurs as anhedral to subhedral antiperthitic grains. The main deformation microstructures are undulose extinction, deformation twins, subgrains and core-and-mantle structures. Orthopyroxene (0 - 35%) is hypersthene (En_56-57) and shows anhedral to subhedral crystal shape and is partially or totally replaced by fibrous aggregates of biotite and/or cummingtonite (Figure 4e). Clinopyroxene (0 - 30%) is diopside (Wo₄₇En₃₉Fe₁₄) and occurs both in granular aggregates with orthopyroxene or as coronas around this mineral. Commonly clinopyroxene exhibits replacement by actinolite and/or hornblende. Both biotite (<1 - 30%) and hornblende (0 - 45%) occur in two generations. The primary consists of large reddish-brown crystals of biotite (X_Mg = 0.60-0.62 and 0.44-0.45 apfu of Ti) and brownish-green crystals of edenite (15eNK estimate, ^{Leake et al., 1997}). The secondary generation occurs as fine-grained aggregates replacing pyroxenes. Quartz (<1 - 15%) usually forms monomineralic aggregates and displays deformation microstructures like undulose extinction and chessboard subgrains. Garnet (0 - 8%) has a composition alm_58,5prp_19,5grs_18,5sps_3,4uv_0,1 and may constitute symplectitic intergrowth with ilmenite or coronitic textures at pyroxene-plagioclase contacts (Figure 4f).

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 orthopyroxene 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.

Garnet (9 - 40% vol) occurs in two generations. The primary generation (alm₆₃prp₃₀grs₅sps₂) consists of poikiloblastic anhedral porphyroblasts of up to 2 cm and contains fine-grained rounded or amoeboid inclusions of all other main minerals, besides acicular sillimanite and anhedral spinel (Figure 4g). The secondary generation (alm₆₇prp₂₁grs₈sps₃uv₁) forms symplectitic coronas around the primary garnet and also around ilmenite (Figure 4h). Plagioclase (15 - 25%) is oligoclase-labradorite (An_24-51) and occurs mainly as antiperthitic porphyroclasts, which exhibit interlobate or ameboid grain boundaries and evidences for intracrystalline deformation, like undulose extinction and deformation twins. Orthoclase (0 - 40%) has a composition An₁Ab₈Or₉₁ and often occurs as perthitic porphyroclasts. Quartz (10 - 30%) constitutes the matrix or occurs in monomineralic ribbons and shows the same deformation microstructures already described for others granulites. Biotite (2 - 25%) is reddish-brown (X_Mg = 0.60-0.74 and 0.43-0.61 apfu of Ti) and defines the penetrative foliation which wraps around porphyroblasts and porphyroclasts. The grains show deformation features like undulose extinction and kink bands. A secondary generation occurs filling fractures in orthopyroxene or surrounding this mineral. Orthopyroxene (0 - 30%) is hypersthene (En_57-66). It occurs as anhedral porphyroblasts and shows the same deformation features as biotite (Figure 4i). Green spinel (<1%) has an average chemical formula (Mg_0,3Fe_0,5Zn_0,2)(Al_1,9Cr_0,1)O₄. Due to the prevalence of Fe content, this spinel was classificated as hercynite (Figure 4g).

Felsic granulites

Felsic granulites are comprised of intermediate to acid rocks (58.32 - 76.14% SiO₂) (Table 1). In the TAS diagram (^{Cox et al., 1979} modified 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).

Figure 5 Chemical classification diagrams: a) TAS (Cox et al., 1979); b) AFM [(Na₂O+K₂O) - FeOt - MgO] (Irvine & Baragar, 1971); c) Aluminium saturation index (^{Shand, 1943}); d) log(SiO₂/Al₂O₃) versus log(Fe₂O₃/K₂O) (Herron, 1988); Tectonic discrination diagrams: e) Y x Nb (Pearce et al., 1984); f) R1-R2 (Batchelor & Bowden, 1985); g) Ti/Y x Zr/Y (^{Pearce & Gale 1977}); h) Zr-Nb-Y(Meschede, 1986). i) SiO₂ x log(K₂O/Na₂O) diagram for sandstones and argilites (Roser & Korsch, 1986). 

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 (Na₂O + K₂O), 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). The R₁-R₂ diagram of ^{De La Roche et al. (1980)} with geotectonic implications after ^{Batchelor & Bowden (1985)} suggests that a majority of the felsic granulites have been derived from Pre-collision granitoids, although some samples plot in the Syn-collision and Mantle fractionates fields (Figure 5f). According to the tectonic discrimination scheme of ^{Maniar & Piccoli (1989)} (not shown in this paper), the felsic granulites could have been derived from continental arc granitoids (CAG), island arc granitoids (IAG) or continental collison granitoids (CCG).

Mafic granulites

Mafic granulites comprise basic to intermediate rocks (46.76 - 56.20% SiO₂) (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.

Aluminous granulites

Aluminous granulites comprise basic, intermediate and acid rocks (49.83 - 73.62% SiO₂) (Table 1). The Al content ranges from 12.95 to 16.57% and the Shand's diagram shows that all the samples are peraluminous rocks, with ACNK between 1.14 and 2.02 (Figure 5c).

The protoliths were characterized based on the discriminant diagram of ^{Herron (1988)}, which relates log(Fe₂O₃^(t)/K₂O) vs. log(SiO₂/Al₂O₃) (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 K₂O, Na₂O and SiO₂ 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).