Two main supracrustal sequences overlie the basement complex of the Granja massif: the Ubajara and Martinópole Groups. Although the Ubajara Group may be the older supracrustal sequence (possibly late Paleoproterozoic, as discussed below), it is structurally higher than the Martinópole Group, which is late Neoproterozoic (Fetter, 1999; Fetter et al., 2003). Here, we present the supracrustal units in their structural order.
The Martinópole Group (Figure 4) is subdivided, from base to top, into four formations: Goiabeira, São Joaquim, Covão and Santa Terezinha. These formations are comprised mainly of fine-grained sedimentary rocks: pelitic, metagraywackes and quartzites with minor intercalations of calc-silicates, carbonates and felsic volcanic rocks, which are suggestive of sedimentation in a low-energy environment. While the tectonic environment responsible for this deposition remains unclear, Fetter et al. (2003) suggested that the Martinópole Group might represent a forearc basin that developed during the late Neoproterozoic growth of the Santa Quitéria continental arc complex located to the southeast in the Ceará Central Domain. Regardless of the tectonic setting, the four formations of the Martinópole Group were subjected to varying degrees of metamorphism and deformation during the Brasiliano orogeny. This is demonstrated by the different mineral assemblages and structures present in each one: the two lower formations attained middle amphibolite facies conditions, whereas the two upper formations only reached greenschist to prehnite-pumpelleyite facies.
The lowermost Goiabeira Formation is in tectonic contact (thrust fault) with basement rocks (Figure 6) and is characterized by garnet-chlorite schist, staurolite schist, muscovite-chlorite schist, biotite schist, kyanite schist, garnet-biotite schist and quartz-feldspar paragneiss that represent metamorphosed pelitic protoliths. These mineral assemblages characterize typical Barrovian amphibolite facies metamorphism.
Figure 6. Field photographs and microtextural features of the basal Goiabeira Formation of the Martinópole Group
Field photographs and microtextural features of the basal Goiabeira Formation of the Martinópole Group. a) Biotite schist showing low-dip foliation near contact with Chaval granite; b) refolded biotite schist from Chaval region; c-d) plane polarized and cross-polarized light photomicrographs showing asymmetric pressure shadows around staurolite crystal, formed by chlorite and muscovite indicate top-to-northwest thrust related to D2 deformation (top part of the micrograph moved to the left in relation to the lower part); e-f) two cross-polarized light photomicrographs showing kinks of twinning planes in kyanite resulting from compressional deformation D2. Photomicrographs width of view 5mm.
The São Joaquim Formation is comprised primarily of quartzite, with minor intercalations of schists, calcsilicates and metavolvanic rocks (metarhyolite). Metamorphic minerals present in this formation include kyanite, sillimanite, muscovite and staurolite, indicating intermediate pressure amphibolite facies metamorphism, below K-feldspar isograd. The metamorphic peak is recorded by sillimanite, which resulted from the destabilization of polymorphic kyanite. Retrometamorphism to greenschist facies conditions, during the final stage of Neoproterozoic deformation, is indicated by chlorite crystallization and brittle behavior, with fracturing of staurolite and sillimanite grains perpendicular to regional mineral stretching lineation NE-SW to E-W (Figure 7). Metarhyolite is interlayered with quartzite with have elongated muscovite, forming boudins up to 10 cm in length, with axis perpendicular to stretching lineation. The metarhyolites are strongly deformed and show ultramylonitic and mylonitic textures.
Figure 7. Photomicrograph of sillimanite that characterize the progressive deformation at different crustal level
Photomicrograph of sillimanite that characterize the progressive deformation at different crustal level, width of view 5mm, and define a well-developed regional mineral lineation.
The Covão Formation is a sequence comprised of muscovite-quartz-sericite-chlorite schists and minor quartzite layers. These clastic metasedimentary rocks show typical Bouma sequence features, and are interpreted as marine, continental slope turbidite deposits. The mineral paragenesis (chlorite-quartz-white mica) and microtectonic quartz features (lamellaes and deformation bands, subgrain boundary) in quartzite suggest prehnite-pumpelleyite to greenschist facies conditions.
The Santa Terezinha Formation, at the top, consists of quartz-rich schists, metapelites, carbonates (dolomites, limestones and marlstones) with greywacke associations, rythmites, quartzite and inter-layered metarhyolites. These units crop out between the Maravilha Fault and Uruoca-Tucunduba Fault (Figure 4). The metarhyolite units contain phenocrysts of blue quartz and crop out as narrow (1–2 meters) semi-continuous beds near the Paulista Fault. They are inter-layered with a quartzite and muscovite schist sequence and are locally boudinaged. U-Pb zircon dating of the rhyolite indicate that volcanism and sedimentation occurred at 775 ± 11 Ma (Fetter et al., 1997).
The model ages of Goiabeira Formation between 1.24 and 1.32 Ga and Santa Terezinha Formation between 1.61 and 2.69 Ga (Fetter, 1999; Santos, 1999) suggest that the basal Goiabeira Formation of the Martinópole Group have a strong contribution of Neoproterozoic juvenile volcanic material, while the Santa Terezinha Formation represent a mixture between juvenile material and Paleoproterozoic basement gneisses (Figure 8). Neoproterozoic juvenile material occurs to the southeast of Transbrasiliano Lineament, in the Santa Quitéria magmatic arc (Figure 3; Fetter et al., 2003).
Figure 8. Nd evolution diagram showing data from the Goiabeira Formation
Nd evolution diagram showing data from the Goiabeira Formation (green lines) and Santa Terezinha Formation (brown lines). Dashed lines perpendicular to x-axis are age reference lines for Neoproterozoic rifting (800 Ma) and Brasiliano Orogen (600 Ma). The rocks of the Santa Terezinha Formation at the top of the Martinópole Group show a wide mixture between Neoproterozoic juvenile sources and Paleoproterozoic basement gneiss. Notation such as TJ-32 and Br-07 correspond to sample number.
The Ubajara Group is a greeschist facies, proximal, platformal volcano-sedimentary sequence comprised of three formations, from base to top: Caiçaras (slate, siltstone and sandstone), Trapiá-Frecheirinha (sandstone and carbonate) and Coreaú (sandstone and greywacke) Formations (Figure 4). The "Saquinho Volcanic Sequence", crops out between the Caiçaras and Trapiá-Frecheirinha Formation and is comprised of felsic to intermediate volcanic rocks, including traqui-andesite, rhyodacite, rhyolite and volcanic breccias and tuffs (Figure 9; Santos, 1999). U-Pb zircon data from a metarhyolite in this unit yielded a concordant age of 1785 ± 2 Ma, interpreted to be the crystallization age of the volcanic rock (Fetter, 1999). This age corresponds to a widespread 1.8-1.7 Ga extensional event recognized throughout South America (Brito Neves et al., 1995) that is also recorded by the Orós and Jaguaribeano volcano-sedimentary sequences in the eastern part of Ceará State (Sá et al., 1995). Consequently, the Saquinho Volcanic Body may reflect a manifestation of this 1.8-1.7 Ga extensional event in the MCD.
Figure 9. Textural features of the rocks of Saquinho Volcanic Sequence
Textural features of the rocks of Saquinho Volcanic Sequence. a) Volcanic breccias showing incipient deformation; b) metarhyolite with mylonite to ultramylonite texture resulting from D2 deformation; c) volcaniclastic rock preserved from deformation; d) photomicrograph of c); e) volcaniclastic rock showing weak plastic deformation, mainly in the matrix; and f) photomicrograph of e) shows a mylonitic texture. Photomicrograph width of view 2 mm.
Sial et al. (2000) inferred a late Neoproterozoic age for rocks of the uppermost Trapiá-Frecheirinha Formatiion based on stable carbon isotopes. Given the uncertainty inherent in this method, and the lack of more solid data, we assume a Paleoproterozoic age for the entire sequence, although the alternative that the Saquinho Volcanic Sequence was emplaced tectonically is plausible.