Granulite-facies metamorphic rocks provide crucial insights into the tectonothermal evolution of ancient orogens and deep crustal processes. Due to complex polyphase metamorphic and deformation overprinting, the chemical and textural characteristics of the original sedimentary and igneous precursors of granulite sequences are no longer preserved. Unravelling their previous geological history is a major goal of petrolo-gical investigation that can only be achieved by careful and comprehensive geochemical and isotope studies.

The main objective of this work is to summarize new and available petrographical and geochemical data for the kinzigitic gneisses exposed in the Médio Coreaú Domain in NE Brasil and use whole-rock geochemistry to discriminate their original protolith signatures and identify potential sediment source areas and the tectonic setting prevailing at the time of sedimentation.

The Granja region is located in NW Ceará, NE Brasil, at latitudes between 02°47'S-03°22'S and longitudes between 40°20'W-41°24'W (Figs. 1 and 2). Geologically, the area is included within the Bor-borema Province (BP), which represents a segment of the Brasiliano / Pan-African orogenic belt developed by the collision of the Congo-São Francisco and São Luís-West African cratons during the amalgamation of West Gondwana, in the Late Neoproterozoic (ca. 600 Ma) (Almeida et al., 1981; Cordani et al., 2000). Covering an area of over 400,000 Km², the BP is divided into three main structural blocks: the Northern domain, the Transversal domain and the Southern domain, bounded by a system of sinuous and branched crustal scale shear zones (Caby, 1989).

Based on tectonostratigraphic and geochronological (U-Pb, Sm-Nd) evidence, the Northern block was further subdivided into the following major units, from north to south: the Médio Coreaú, the Ceará Central and the Rio Grande do Norte domains, bordered by the Patos, Senador Pompeu and Transbrasiliano shear zones (Santos et al., 2008; Fig. 1).

As shown in Figures 1 and 2, the study area is situated within the Médio Coreaú Domain (MCD). The oldest rocks exposed in the MCD are Early Paleoproterozoic and span a wide spectrum of lithological types, ranging from orthogneisses with tonalite–trondhjemite–granodiorite (TTG) affinities to hornblende gneisses, amphibolites, mafic granulites, enderbites, kinzigites and khondalites (Santos et al., 2009; Amaral et al., 2012). U–Pb zircon ages obtained for the TTG orthogneisses (2.36–2.29 Ga) provide reliable time constraints for the crystallization age of their igneous protoliths, whilst the positive ɛNd_i values displayed by this rock suite point to a mantle-derived origin in an arc-type geodynamic setting (Fetter et al., 2000; Nogueira Neto, 2000; Santos et al., 2008, 2009; Amaral et al., 2012; Praxedes et al., 2012). According to Fetter et al. (2000), the precursors of both the igneous-derived mafic granulites and enderbites and the granulitic paragneisses (kinzigites and khondalites) have also an Early Paleoproterozoic age.

During the Brasiliano / Pan-African Orogeny, all the MCD basement sequences were extensively affected by high-grade regional metamorphism and migmatization. Estimated peak metamorphic conditions vary between 7–10 kbar and 750–840°C for the mafic granulites and granulitic paragneisses and between 5–6 kbar and 600–700°C for the TTG orthogneisses (Nogueira Neto, 2000). The age of the Brasiliano migmatitic / granulitic metamorphic event is reasonably well constrained at ca. 600 Ma by U-Pb and Ar-Ar geochronological data (e.g. Monié et al., 1997; Fetter et al., 2000; Santos et al., 2008, 2009).

The MCD high-grade metamorphic complex is partially overlain by supracrustal volcanosedimentary and/or sedimentary successions belonging either to the Late Paleoproterozoic Saquinho sequence or to the Neoproterozoic Martinópole-Ubajara groups (e.g. Santos et al., 2008, 2009; Amaral et al., 2012). Both the infra- and the supracrustal rock suites were intruded by syn- to post-tectonic Brasiliano granitoids with ages ranging between 590–532 Ma (Santos et al., 2008). Finally, the Early Paleozoic sediments of the Jaibaras/Sairi troughs correspond to early molasse-type deposits post-dating the Brasiliano collage (Santos et al., 2008).

The Granja Granulite Complex (GGC) constitutes a NE–SW-trending belt occupying the western sector of the MCD (Fig. 2). It contacts with the TTG migmatitic gneisses to the SE through the Granja dextral Shear Zone and with the supracrustal Martinópole sequence to the NW through the Estreito dextral Shear Zone (Fig. 2). The GGC is mainly composed of garnet–sillimanite migmatitic paragneisses enclosing distended lenses of mafic granulites, enderbites and enderbitic gneisses.

At outcrop scale, the high-grade garnet–sillimanite migmatitic paragneisses are well-foliated fine- to medium-grained grey rocks characterized by the alternance of dark garnet-biotite-sillimanite-rich layers and millimeter-thick leucocratic quartz-feldspathic bands, interpreted as indicative of incipient melting (Fig. 3a). For increasing melt fractions, the leucosomes grade into thicker concordant vein-type leucosomes producing a conspicuous NE-SW trending stromatic layering, plunging 50–60° to SE (Fig. 3b). Field evidence reveals that the migmatitic fabrics were developed during the second and third Brasiliano deformation phases (D₂ and D₃).

The metric to decametric bodies of mafic granulites and enderbites enclosed in the migmatitic paragneisses from the GGC consist of dark grey to dark greenish-grey coloured rocks with textures ranging from fine- to medium-grained granoblastic to slightly banded. A weak NE-SW trending S_{2 + 3} foliation can also be observed in these lithologies.

The S_{2 + 3} tectonic fabrics recorded in both sequences (paragneisses / mafic granulites + enderbites) were heterogeneously folded and overprinted by D₄. In the studied area, the last Brasiliano ductile deformation event (D₄) is related to the reactivation of the dextral Granja Shear Zone (GSZ) and produced a NE-SW striking foliation, steeply dipping towards the SE (>70°). Mineral stretching lineations plunge gently towards northeast or southwest. In high-strain domains, a strong mylonitic / blastomylonitic fabric can be developed and the leucosomes are frequently boudinaged. Asymmetric S/C structures, when present, indicate a dominant dextral sense of movement.

Petrographically, the garnet–sillimanite paragneisses display significant grain size reduction and strong S₄ mylonitic fabrics, obscuring their previous metamorphic textures. The high-grade mineral assemblage is dominated by garnet (Grt), sillimanite (Sil), biotite (Bt), quartz (Qz) and plagioclase (Plg) (Figs. 4a–b). K-feldspar (Kfs) and cordierite (Crd) are occasionally present. Accessory minerals include opaques (Opq), apatite (Ap), zircon (Zrn) and rutile (Rt).

In the paragneisses affected by incipient melting, the gneissic fabric is defined by the alternance of millimeter-thick biotite-sillimanite-rich and highly recrystallized quartz-feldspar layers. Garnet is abundant and may constitute large poikiloblasts (up to 2,5 cm) wrapped by the granolepidoblastic matrix (Fig. 4c) or smaller, rounded to subhedral, inclusion-free porphyroblasts. Cordierite is anhedral to subhedral and occurs in close spatial association with biotite and sillimanite but can also be found in contact with quartz and feldspars.

As melt contents increase, layer-parallel leucosomes become thicker (>5 cm up to 20 cm) and the migmatitic paragneisses grade into stromatic metatexites. The coarser stromatic leucosomes show also fine-grained mylonitic or blastomylonitic fabrics and are predominantly composed of quartz (30–40 vol.%), plagioclase (20 vol.%), K-feldspar (10–20 vol.%), garnet (5 vol.%) and minor proportions of biotite and fibrolitic sillimanite. Zircon, monazite, apatite and opaques are common accessory phases. Quartz is generally present as finely recrystallized elongate grains with undulose extinction, deformation bands and lobated edges or as ribbon-like aggregates. Plagioclase dominates over alkali feldspar and exhibits wedged and flexured twins (Fig. 4d). K-feldspar is perthitic orthoclase and may occur as small augen mantled by rims of fine grained quartz-feldspar aggregates (Fig. 4e) or as anhedral interstitial crystals in the matrix. Myrmekites are occasionally present. In these leucosomes, the garnet porphyroblasts are generally rounded to subhedral, highly fractured and devoid of inclusions ( Fig. 4f). Biotite has a yellow-reddish to reddish-brown pleochroism and is frequently intergrown with fibrolitic sillimanite in very thin biotite-sillimanite selvages wrapping around the garnet neoblasts (Fig. 4f).

Evidence for the earlier S₁ fabric is only preserved as a discordant internal foliation (S_i) defined by inclusion trails of kyanite (rare), sillimanite, biotite, quartz, plagioclase, ilmenite and rutile within some garnet poikiloblasts. In low-strain domains, the S_{2 + 3} gneissic and stromatic banding constitute the main regional fabric observed in these rocks, being strongly transposed by a pervasive anastomosed S₄ foliation associated with dextral shearing in high-strain zones. Intense dynamic recrystallization of the leucosomes suggests that much of the D₄ deformation was imposed while the migmatites were subsolidus. Prismatic sillimanite and elongated ribbon-like quartz aggregates define a strong sub-horizontal lineation on the S₄ foliation planes.

On the basis of microstructures and relations between mineral phases, it is possible to distinguish four main stages of metamorphism: a prograde metamorphic stage (M₁), a peak-metamorphic stage (M₂), a post-peak decompression stage (M₃) and a retrograde cooling stage (M₄).

The M₁ prograde assemblage is represented by inclusions of kyanite, sillimanite, biotite, quartz, plagioclase, ilmenite and rutile within M₂ garnet neoblasts (Grt₁). The occurrence of very thin leucosomes folded by D₂ suggests that partial melting conditions were probably reached during M₁ through the muscovite dehydration reaction (Pëto, 1976): 1 Ms + Plg + Qz ⇒ A l 2 Si O 5 ( Sil ) + Kfs + melt

Following the relict M₁ metamorphic event, a M₂ peak-metamorphic paragenesis composed of Grt₁ + Sil + Bt + Qz + Plg + Kfs was developed involving the fluid-absent incongruent melting reaction of biotite (Le Breton & Thompson, 1988): 2 Bt + Sil + Plg + Qz ⇒ Gr t 1 + Kfs + melt

The lack of primary muscovite in all the analysed rocks and the presence of millimetre- to centimetre-scale quartz-feldspar rich leucosomes parallel to S₂ foliation show that the biotite-dehydration melting reaction was crossed during this metamorphic event.

The M₃ metamorphic decompression stage is marked by the first appearance of cordierite, which forms relatively large grains of different shape in the rock matrix. This episode occurred in the sillimanite stability field and was controlled by the reaction (Spear et al., 1999): Bt + Al 2 SiO 5 ( Sil ) ⇒ Grt 2 + Crd 1 + melt

M₃ led to the production of additional melts and inclusion-free new garnet (Grt₂), preserved mainly in the leucosomes.

In the last metamorphic stage (M₄), the early-formed foliations were reworked by shearing and a strong S₄ mylonitic fabric was developed. Reaction textures appear to reflect a cooling history marked by the reversal of reactions 2 and 3 and the crystallization of biotite (Bt₂) + sillimanite (Sil₂). The presence of biotite and sillimanite along the rims of garnet neoblasts provides a strong argument in favour of retrogression by garnet consumption. Simultaneously, the melts crystallized and the water released during their solidification contributed to the retrograde reactions.

Twelve representative samples of the Granja paragneisses were selected for purposes of geochemical characterization. Sampling strategy involved the collection of specimens that showed no evidence of any significant loss, or gain, of anatectic melts and could therefore provide the closest estimate of the original composition of the protolith. Only the paragneisses containing very thin leucosomes and a relatively homogeneous appearance were selected for chemical analysis. From these, 5–10 kg of rock-material was collected, crushed and split to obtain representative samples of ∼250 g. Each sample was finally pulverized in an agate mill. Sample locations are shown in Figure 2.

Sample GR70 was recently collected and analysed for major (ICP-ES) and trace elements (ICP-MS) at ACME Labs (Vancouver, Canada), whilst the geochemical data for the remaining eleven samples were previously obtained by Gama Jr. (1992), Santos (1993) and Nogueira Neto (2000), using X-Ray Fluorescence spectrometry and ICP-MS at GEOSOL (Belo Horizonte, Brasil). Major and trace element compositions are presented in Tables 1 and 2.

As previously discussed, the Granja paragneisses experienced granulite facies metamorphism. Their compositions may therefore have been strongly modified by metamorphic overprint and anatexis. However, bulk-rock major and trace element geochemistry provide, in some cases, major constraints on both the nature and the provenance of the putative protoliths for high-grade metamorphic rocks (e.g. Bhatia & Crook, 1986; Roser & Korsch 1986, 1988; Otamendi & Patiño Douce, 2001; Augustsson & Bahlburg, 2003; Abu El-Enen, 2008, 2011).

The samples from the Granja paragneisses show a relatively wide compositional range with SiO₂ between 66.27 and 72.50% and Al₂O₃ between 11.91 and 15.94%. As illustrated in the P₂O₅/TiO₂ vs. MgO/CaO discrimination diagram proposed by Werner (1987), most of the analysed rocks display low P₂O₅/TiO₂ ratios coupled with slightly variable MgO/CaO values pointing to a sedimentary origin (Fig. 5). This strongly suggests that the migmatite samples selected for chemical analysis have undergone little or no melt extraction and may therefore have bulk major element compositions similar to those of their original protoliths.

In the log Fe₂O₃/K₂O vs. log SiO₂/Al₂O₃ classification diagram of Herron (1988), the investigated migmatitic gneisses plot mainly within the greywacke field or straddle the boundary between the greywacke and shale domains (Fig. 6). Conformable results are obtained using the Al/3-K vs. Al/3-Na (La Roche, 1968) and the Fe + Al + Ti vs. Ca + Mg (Moine & La Roche, 1968) millicationic diagrams (Figs. 7a–b). The low Ca and Mg contents observed in these rocks are consistent with the absence of a carbonate component in their precursor sediments.

The empirical discrimination ratio 100TiO₂/Zr (wt.%/ppm) based on transition metals (Ti) and HFSE (Zr), which are assumed to have an immobile behaviour during metamorphism, is lower than 0.4 in all the analysed paragneisses and points to a significant input of psammitic material to the original clastic sequence (Garcia et al., 1991; Abu El-Enen, 2011).

Six of the paragneiss samples have high K₂O/Na₂O ratios (>1), whilst the other six display K₂O/Na₂O values lower than 1, suggesting some diversity of protolith rock types (pelitic to semipelitic). In the A-CN-K ternary plot of Nesbitt & Young (1984), the samples define a trend between silt-clay and clay sediments consistent with a mixed greywacke-shale composition (Fig. 8).

Calculation of the chemical index of alteration (CIA; Nesbitt & Young 1982) gives CIA values of 53 to 65 indicative of low to moderate degrees of chemical weathering for the sediment parent rocks, supporting a provenance from igneous felsic to intermediate crustal sources.

Figure 9 illustrates the REE chondrite normalized patterns for six representative samples of the paragneiss suite. Overall, these patterns are characterized by: (a) moderate LREE / HREE fractionation (La_N/Yb_N = 9.46–15.50), (b) flat HREE profiles (Gd_N/Yb_N = 1.37–1.93) and (c) negative Eu anomalies of small amplitude (Eu/Eu* = 0.63–0.82). Their close similarities to Post-Archean average Australian Shale (PAAS, Nance & Taylor, 1976) and Early Proterozoic Greywackes (EP GREY, Condie, 1993) provide additional evidence for a pelitic to semipelitic derivation and reinforce the hypothesis that melt loss from these rocks has been negligible. The Eu/Eu* and Gd_N/Yb_N ratios are also within the range of values (Eu/Eu* < 0.85; Gd_N/Yb_N<2.0) found in Post-Archean metasediments by McLennan & Taylor ( 1991) and McLennan et al. (1995) (Fig. 10).

Several attempts have been made to use major and trace elements as provenance indicators. In the F1-F2 diagram of Roser & Korsch (1988), the analysed samples plot within the fields of primary felsic or intermediate source rocks pointing to a major contribution of this type of igneous materials for the genesis of their precursor sediments (Fig. 11). Their Th/Sc and Zr/Sc ratios are similar to the upper continental crust (McLennan et al., 1993) confirming their provenance from a felsic to intermediate source area (Fig. 12).

According to the K₂O/Na₂O vs. SiO₂ diagram of Roser & Korsch (1986), sediment deposition would have occurred at an active continental margin (Fig. 13). As shown in the ternary diagrams of Bhatia & Crook (1986), a continental island arc setting is indicated by the highly incompatible trace elements (La, Sc, Th, Zr) for the investigated paragneisses (Figs. 14a–b).

Finally, regional constraints suggest that the sedimentary protoliths of the Granja granulitic paragneisses could have resulted from erosion of intermediate to felsic igneous rocks similar to the TTG orthogneisses exposed in adjacent areas of the MCD (Fig. 2). This is further supported by the Early Proterozoic U-Pb ages obtained in detrital zircons from the paragneisses and the island arc tectonic setting inferred for the TTG sequence (Fetter et al., 2000).

Based on the preliminary petrographical and geochemical data presented in this study, it is possible to draw the following conclusions:

The garnet-sillimanite migmatitic paragneisses exposed in the MCD experienced intense deformation and granulite facies metamorphism during the Brasiliano Orogeny. Microstructures and mineral assemblages in the Granja paragneisses reveal a metamorphic history involving four main stages: a prograde metamorphic stage (M₁), a peak-metamorphic stage (M₂), a post-peak decompression stage (M₃) and a retrograde cooling stage (M₄).

Partial melting conditions appear to have been reached during M₁, continued during the metamorphic peak (M₂) and persisted during most of the post-peak decompression stage (M₃). The retrograde cooling path (M₄) is coeval with D₄ shearing and was accompanied by intense dynamic recrystallization of the leucosomes suggesting that melt crystallization was completed before the end of the D₄ tectonic event.

Some of the migmatitic paragneisses from the Granja Granulite Complex appear to have undergone little or no melt extraction and provide the closest estimate for the composition of the original protoliths. Whole-rock geochemical data for these rocks suggest that the precursor sediments had mixed greywacke-shale compositions and could have resulted from erosion of intermediate to felsic igneous rocks similar to the TTG orthogneisses. Sediment deposition would have occurred at an active continental marginal / continental island arc setting.

Given the evidence for the presence of melt in the samples analysed, the results obtained must be interpreted with caution and any inferences on the provenance of the Granja paragneisses require further support from isotopic and detrital zircon studies.