The Costeiro Complex of southern Espírito Santo (Figs 2 and 4) is comprised of stromatic garnet-cordierite-sillimanite kinzigitic gneiss frequently interlayered with calc-silicate granulite (Ubu series) and less frequent leptinite (garnet-feldspar granulitic gneiss). Local changes in the original composition of the sedimentary pile account for gradual changes into more metaluminous layers of biotite-amphibole-garnet gneiss. This predominantly metasedimentary rock pile is partially melted and intruded by metaigneous high-amphibolite to granulitic gneisses, with prevailing hypersthene granodioritic to tonalitic compositions (G2 enderbites) and restricted quartz-monzonitic to granitic compositions (G2 charno-enderbites), locally with garnet. Contacts are mostly gradational but occasionally sharp, suggesting a tectonic control. The G2 enderbites form then green-colored discontinuous patches with diffuse boundaries within G2 grey amphibolitic gneisses (Figure 5 and 6) and kinzigite of the Costeiro Complex (Figure 4). The strongly foliated and variably migmatitic green to grey G2 gneisses are then cross cut by younger non-deformed granitic and pegmatite veins [Sluitner and Weber-Diefenbach, 1989; Féboli, 1993; Wiedemann et al., 1997; Teixeira, 1998].
Figure 4. Geological map of the Costeiro Complex
Geological map of the Costeiro Complex (modified from Sluitner & Weber-Diefenbach, 1989). Legend: 1 Alluvial and Quaternary deposits; 2 Barreiras Formation (Tertiary); 3 Cambro-Ordovician intrusive rocks; 4 Iriri-series, alternation of enderbitic gneisses with porphyroblastic feldspar-gneisses and calc-silicates; 5 migmatitic garnet-cordierite-gneisses; 6 amphibolitic gneiss in alternation with granulites (predominantly enderbitic); 7 same as 6, mostly migmatic; 8 garnet-bearing granulites (predominantly enderbitic); 9 sillimanite-quartzite; 10 alternation of quartzite and feldspar-gneisses; 11 garnet-biotite gneisses, locally migmatic; 12 amphibolitic gneisses; 13 sillimanite-garnet-cordierite-gneisses (kinzigite); 14 lineaments (faults and shear zones); 15 foliation attitude; 16 rivers; 17 main roads; 18 secondary roads.
Figure 5. Detailed map from the Siribeira region
Detailed map from the Siribeira region, southeast of Guarapari (modified after Teixeira, 1998). Legend: 1 quaternary deposits; 2 granitic veins; 3 pegmatite; 4 amphibolite-biotite-gneiss grading locally into enderbite; 5 leptinite; 6 domains dominated by enderbite; 7 house; 8 garden; 9 road; 10 water. Structural symbols as in Fig. 6.
Figure 6. Gradational contact between amphibolite-biotite-gneiss and enderbite in the Siribeira region
Gradational contact between amphibolite-biotite-gneiss and enderbite in the Siribeira region. Geological map in Fig. 5. Note greenish color on the first plane of the photograph contrasting with grayish hues at the back. Blow up of a typical enderbite.
The region around the Perocão Beach, north of Guarapari town, (Figure 7; see appendix 1 Field Guide) depicts, in a smaller scale, part of the complexity of the geology in the area. The predominant lithology consists of a foliated G2 metagranitoid (augen amphibolitic gneiss; 575±12 Ma U-Pb in zircons from Söllner et al., 1989) in contact with less foliated G3 enderbites. The age for G2 intrusive rocks corresponds to the first dehydration partial melting of the metasedimentary pile in the coastal region (anatexis I) and to the coeval intrusion of smaller amounts of less evolved, mafic melts. Remnants from the metasedimentary pile, consisting of kinzigite and calc-silicate rocks, crop out less than one kilometre southwards, along the Pescador Peninsula (Figure 8). Around 558 ± 2 Ma, a second dehydration melting episode culminated in the production of massive enderbitic domains trending along N-S directions, but partially preserving an older NE-SW foliation. This defines the G3 suite that resulted from the remelting of G2, anatexis II (Figure 7). Pegmatite and granitic veins in this Outcrop cross-cut all the other lithologies and are the result of a third anatetic episode in the region around 500 Ma, anatexis III (Wiedemann et al., 2002). This last episode seems to be related to an extensional phase which originated NE-SW to N-S striking dykes and shear zones.
Figure 7. Detailed map from the Perocão region, northwest of Guarapari
Detailed map from the Perocão region, northwest of Guarapari. The interfingered pattern between enderbite and amphibolite-biotite-gneiss represents the gradational and irregular contacts between the two.
Figure 8. Dehydration melting of the sillimanite-garnet-cordierite gneiss
Dehydration melting of the sillimanite-garnet-cordierite gneiss (kinzigite) from the Pescador Beach, Guarapari. Garnet-bearing melts give rise to a stromatic fabric.
Figure 9. R1-R2-multicationic diagram
R1-R2-multicationic diagram (petrographic divisions after de la Roche et al., 1980); 1 mantle plagiogranite; 2 destructive active plate margin (pre-plate collision); 3 Caledonian permited plutons (late orogenic); 4 sub-alkaline plutons (late orogenic); 5 alkaline / peralkaline magmatism (post-orogenic); 6 anatetic magmatism (syn-orogenic) (from Batchelor and Bowden, 1985). A: G1 tonalite to granodiorite gneisses (data from Geiger, 1993); mafic enclaves, amphibolites (abbreviated as amphib. in figure), Vectors from field 6 towards A and B indicate the change in melt composition under progressive equilibrium partial melting of rocks of monzodioritic (A) and monzonitic (B) bulk compositions; partial melting vectors (towards A and B) and metasedimentary sources (greywacke, pelites, schists and field 6 from Batchelor and Bowden, 1985). B: G2 and G3 enderbites and enderbitic-gneisses (data from Sluitner, 1987). C: G5 high-K [Santa Angélica and Castelo data from Horn, 1987; Iconha data from Offman, 1990; Pedra Azul data from Platzer, 1997; G5 tholeiitic suite (Jacutinga) from Ludka, 1997]. D: G5 very high-K suite (Mimoso do Sul data from Ludka, 1991; Venda Nova data from Horn, 1987). Individual plutons are not discriminated on this diagram. Discussion in text.
In order to evaluate the suggestion of a dominant sedimentary source for the generation of this early magmatic suites, geochemical data from G1, G2 and G3 suites from Sluitner, 1989, Offman, 1990 and Geiger, 1993 have been plotted on the multicationic diagram from de la Roche et al. (1980) and Batchelor & Bowden (1985) (Figure 9a,b). All these suites fall mainly in the second discrimination field, which accounts for an origin under predominantly destructive plate conditions. G1, with its mafic enclaves and metabasic intrusions plots towards field 1 (for mantle derivates). G2 samples by contrast are dispersed at the prolongation of pelites and greywacke fields, corroborating the significant role of a sedimentary source in their origin inferred from field relationships. When compared with G1, most G2 samples plot in the same tectonic field 2, however, as an expanded group regarding the R1-axis towards field 1 and confirming the strong metasedimentary component. G3 compositions are restricted to the neighbourhood of field 6, for crustal melts, reinforcing field observations suggesting an origin from the partial remelting from G2. While G1 consists mostly of metaluminous tonalitic and granodioritic compositions, significant contribution of sediments in G2 and G3 drives their composition towards peraluminous granitic melts (Figure 10a). Nevertheless the presence of less evolved melts in the G2 suite in comparison to G3 suggests additional sources in the origin of G2.
Figure 10. A/CNK vs. A/NK
A/CNK vs. A/NK (alumina saturation diagram from Shand, 1943). A: G1 tonalite to granodiorite gneisses (data from Geiger, 1993); G2 and G3 enderbites and enderbitic-gneisses. B: G5 high-K suuite. Data sources as in Fig. 9. Discussion in text.
The metaigneous assemblage corresponding to suites G2 and G3 in the Costeiro Complex consists of continuous compositions from granodiorite to quartz-monzonite (Sluitner, 1989) with relatively uniform values of K2O (Figure 11a), Rb and Sr. The K2O-contents of the intermediate rocks are moderate to high. For the acidic rocks, like enderbite (hypersthene tonalite) and charnockite (hypersthene granite) anomalously high-K values could be the result of accumulation of K-feldspars through differentiation and/or tectonic processes, rather than represent a shoshonitic component in this magmatism.
Figure 11. SiO2 x K2O diagram
SiO2 x K2O diagram. A: G1 tonalite to granodiorite gneisses; G2 and G3 enderbites and enderbitic-gneisses. B: Fields from Le Maitre, 1989; G5 high-K and very high-K suite. Data sources as in Fig. 9. Discussion in text.
Ba- and Sr- contents in G1, G2 and G3 suites are high (typically 1000 ppm Ba for G2 and G3 and slightly less for G1; Figure 12a), even when compared to other igneous sequences of Pan-African ages (Rajesh, 2004; Asrat et al, 2004). G1 rocks are considerably richer in Sr than G2 and G3, particularly at high Ba contents. Chondrite normalized REE-diagrams are particularly useful for comparison between suites and their possible sources. Fractionated REE-patterns (La/Yb < 16) with moderate total values and very similar REE-patterns for samples from different but adjacent layers of enderbites (G2) and amphibolitic gneisses (G2) point towards similar sources.
Figure 12. Log Ba vs Sr diagram
Log Ba vs Sr diagram. A: G1 tonalite to granodiorite gneisses; G2 and G3 enderbites and enderbitic-gneisses. B: Fields from Le Maitre, 1989; G5 high-K and very high-K suite. Data sources as in Fig. 9. Discussion in text.
Figure 13a shows REE patterns from the most basic portions of the G1 suite: amphibolite to amphibolite-gneisses and metagabbros. With the exception of one metagabbro sample, showing lower total REE values, therefore a more depleted behaviour, all other metabasites depict a rather flat pattern with slight enrichments of light REE and about 10 times the chondrite amount for heavy REE. The metadiorite sample of G1 (Figure 13b) is very similar to the enriched metagabbroic ones, with no Eu-anomaly. The granodiorite and tonalitic to granitic gneisses, on the other hand, depict clear enrichment in light REE-patterns, with increasing negative Eu-anomalies for increasing differentiation. This could be explained through partial melting of a rock where feldspar is retained in the source. G2 and G3 suites show similar REE-patterns, which may be a further support for their consanguinity (Fig 13c,e). REE-patterns are similar to those of metasedimentary gneisses and kinzigite (Figure 13d), which suggests its involvement in the origin of G2 and G3 suites.
Figure 13. Rare Earth Elements abundances normalized to chondrite
Rare Earth Elements abundances normalized to chondrite (after McDonough and Sun, 1995) plotted against REE atomic numbers. A: G1 amphibole-gneisses and metagabbros B: G1 gneisses of granite, granodiorite, tonalite- and dioritic composition (from Geiger, 1993). C: G2 and G3 enderbitic-gneisses; kinzigite (from Geiger, 1993). D: G2 garnet-gneisses and kinzigite (from Geiger, 1993). E: G3 enderbites and associated enclave (from Sluitner, 1987). F: Amphibolite from Costeiro Complex (from Sluitner, 1987). Discussion in text.
Similarities between preliminary Sm/Nd, U/Pb and Rb/Sr isotopic data, in minerals and whole rock, further suggests a common affiliation between amphibolitic, enderbitic and kinzigitic gneisses (Table 1 and Figure 14). Very high Sr87/Sr86 ratios and very negative Nd values are evidence of a predominat sedimentary origin for the magmatic suites. The side-by-side formation of enderbitic and amphibolitic rocks was probably dependent on changes in the CO2/H2O pressure conditions. This process should be studied, in more detail, in order to decypher the precise formation mechanism for hypersthene-bearing rocks during the Neoproterozoic.
Geothermometry using orthopyroxene-garnet and orthopyroxene-clinopyroxene pairs and geobarometry using orthopyroxene-garnet-plagioclase-quartz, yield metamorphic conditions up to 750ºC and pressures between 3-4 kb to 6.9 kb for G2 enderbitic rocks (Sluitner, 1989).
In summary, the whole package of sediment-derived gneisses of the Paraíba do Sul unit underwent successive partial dehydration melting episodes, probably according to the following reaction: biotite + plagioclase + quartz = K-feldspar + orthopyroxene + garnet + cordierite + spinel + melt (Montel & Vielzeuf, 1997; Brown, 2001; Rushmer, 2001). In an early stage, anatexis I originated G2 granitoids at 575 ±2 Ma, which included contribution of lower crustal melts (Sluitner & Weber-Diefenbach, 1989). This was followed by anatexis II from 565 to 558 Ma, when the rock package crossed once more the solidus curve (ptigmatic folding) under mostly dry granulitic and moderate to low pressure conditions (from P ~3-4 to ~ 6.5 kb; Sluitner, 1989). This phase gave rise to G3 augen gneisses, granitoid gneisses and enderbitic gneisses. The presence of heterogeneous metamorphic banding, frequent interbedded calc-silicate lenses, geochemical and isotopic data, point towards a common primary supracrustal origin for both G2 and G3 suites (Söllner et al, 2000; Wiedeman et al., 2002). P-T differences between different partial melting episodes and the contribution of other source materials may explain chemical variations between G2 and G3. Back reaction between the granulitic package and H2O-rich fluids, still present in the crust, produced local retrometamorphism. This induced the replacement of pyroxenes by biotite and the development of symplectitic and coronitic textures in granulitic/enderbitic rocks (Sluitner, 1989; Wiedemann et al., 1997). The final thermal event in the region took place in the interval between 500±15 Ma (U-Pb in zircon) and 492±15 Ma (U-Pb in titanite). These youngest ages are related to the granitic plutonism in this area (Alfredo Chaves-Iconha pluton; Fig. 4).