Petrographical and geochemical signatures of the Granja paragneisses ( Médio Coreaú Domain , NW Ceará , Brasil )

The Granja Granulite Complex (GGC) exposed in the Médio Coreaú Domain (NW Ceará, Brasil) consists mainly of garnet and sillimanite migmatitic paragneisses enclosing discontinuous lenses of mafic granulites and enderbites. According to the published geochronological data, this high-grade metamorphic belt represents a segment of the Paleoproterozoic basement intensely reworked during the Brasiliano / Pan-African Orogeny in the Neoproterozoic (600 Ma). The Granja paragneisses are strongly foliated rocks characterized by the alternance of dark garnet-biotitesillimanite-rich layers and millimeter-thick leucocratic quartz-feldspathic bands, interpreted as indicative of incipient melting. As melt contents increase, layer-parallel leucosomes become thicker and a well-developed stromatic layering is defined. Both the gneissic and stromatic fabrics are strongly overprinted by a penetrative mylonitic foliation correlated to the last reactivation of the dextral NE-SW trending Granja Shear Zone (GCZ) that cuts across the studied area. Mineral assemblages and microstructures indicate that these rocks were affected by granulite-facies metamorphism and anatexis followed by decompression and cooling. In order to constrain the protolith composition of the Granja paragneisses, twelve whole-rock samples from the parts of the migmatitic paragneisses that appear to have undergone little or no melt extraction were analysed for major and trace elements. In the classification diagram of Herron (1988), the samples plot in the transition between the greywacke and the pelite fields, suggesting that the pre-metamorphic sequence was dominantly composed by shales and immature clastic sediments (greywackes). Their chondrite normalized REE patterns show a moderate LREE enrichment (LaN/YbN = 9.46–15.50), flat HREE profiles and negative Eu anomalies (Eu/Eu* = 0.63–0.82), closely resembling those of PAAS (Post-Archean average Australian Shale) and Early Proterozoic Greywackes. Geochemical data also suggest that the precursor sediments of the Granja paragneisses derived from source areas of felsic to intermediate composition and were deposited in a tectonically active continental margin / continental island arc setting.


Introduction
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 petrological 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.
Geologically, the area is included within the Borborema 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 Km 2 , 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).
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., 2008Santos et al., , 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
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 veintype 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 (D 2 and D 3 ).
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 D 4 .In the studied area, the last Brasiliano ductile deformation event (D 4 ) is related to the reactivation of the dextral Granja Shear Zone (GSZ) and produced a NE-SW striking foliation, steeply dipping towards  et al., 2003).
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.
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 ribbonlike 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 S 1 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 S 4 foliation associated with dextral shearing in highstrain zones.Intense dynamic recrystallization of the leucosomes suggests that much of the D 4 deformation was imposed while the migmatites were subsolidus.Prismatic sillimanite and elongated ribbon-like quartz aggregates define a strong sub-horizontal lineation on the S 4 foliation planes.

Metamorphic evolution
On the basis of microstructures and relations between mineral phases, it is possible to distinguish four main stages of metamorphism: a prograde metamorphic stage (M 1 ), a peak-metamorphic stage (M 2 ), a post-peak decompression stage (M 3 ) and a retrograde cooling stage (M 4 ).
The M 1 prograde assemblage is represented by inclusions of kyanite, sillimanite, biotite, quartz, plagioclase, ilmenite and rutile within M 2 garnet neoblasts (Grt 1 ).The occurrence of very thin leucosomes folded by D 2 suggests that partial melting conditions were probably reached during M 1 through the muscovite dehydration reaction (Pëto, 1976): Following the relict M 1 metamorphic event, a M 2 peak-metamorphic paragenesis composed of Grt 1 + Sil + Bt + Qz + Plg + Kfs was developed involving the fluid-absent incongruent melting reaction of biotite (Le Breton & Thompson, 1988): The lack of primary muscovite in all the analysed rocks and the presence of millimetre-to centimetrescale quartz-feldspar rich leucosomes parallel to S 2 foliation show that the biotite-dehydration melting reaction was crossed during this metamorphic event.The M 3 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): M 3 led to the production of additional melts and inclusion-free new garnet (Grt 2 ), preserved mainly in the leucosomes.
In the last metamorphic stage (M 4 ), the earlyformed foliations were reworked by shearing and a strong S 4 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 (Bt 2 ) + sillimanite (Sil 2 ).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.

Analytical methods
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 rockmaterial 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.
The samples from the Granja paragneisses show a relatively wide compositional range with SiO 2 between 66.27 and 72.50% and Al 2 O 3 between 11.91 and 15.94%.As illustrated in the P 2 O 5 /TiO 2 vs. MgO/CaO discrimination diagram proposed by Werner (1987), most of the analysed rocks display low P 2 O 5 /TiO 2 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 Fe 2 O 3 /K 2 O vs. log SiO 2 /Al 2 O 3 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 100TiO 2 /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 K 2 O/ Na 2 O ratios (>1), whilst the other six display K 2 O/ Na 2 O 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 siltclay 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.

Provenance and tectonic setting
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 K 2 O/Na 2 O vs. SiO 2 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  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).

Conclusions
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 (M 1 ), a peak-metamorphic stage (M 2 ), a post-peak decompression stage (M 3 ) and a retrograde cooling stage (M 4 ).
Partial melting conditions appear to have been reached during M 1 , continued during the metamorphic peak (M 2 ) and persisted during most of the post-peak decompression stage (M 3 ).The retrograde cooling path (M 4 ) is coeval with D 4 shearing and was accompanied by intense dynamic recrystallization of the leucosomes suggesting that melt crystallization was completed before the end of the D 4 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.

Fig. 2 .
Fig. 2.-Simplified geological map for the Granja region, showing the location of the analysed samples (modified from Cavalcanteet al., 2003).

Table 2 .-Trace element data for the Granja paragneisses. PAAS and EP GREY compositions are given in the last two columns
* PAAS -Post-Archean average Australian Shale