The Apiaí, São Roque and Embú terranes

These terranes occupy a central position in the region considered in this paper (Figure 18, 19 and 20). Their relation to the adjacent orogens is at present not fully clear. Some authors consider the Apiaí and São Roque Terranes as the margin of the Paranapanema paleocontinent and hence as related to the Brasília Orogen. However, these units have been considered in most of the literature as parts of the Ribeira Belt and the name Ribeira is derived from the Ribeira Valley, located in this area.

Apiaí and São Roque Terranes

The limit between the Apiaí and São Roque Terranes on one side and the Socorro lobe of the Socorro-Guaxupé Nappe on the other side is marked by the Jundiuvira dextral shear zone, accompanied by an important metamorphic jump from high grade rocks in the nappe to low grade rocks in the terranes. The southeastern limit of these two terranes is also defined by dextral shear zones; the Lançinha shear zone forms the contact with the Curitiba Terrane and the Caucaia-Rio Jaguari shear zone is the contact with the Embu Terrane (Figure 18, 19 and 20).

Paleoproterozoic Basement

Exposures of Paleoproterozoic basement in the Apiaí Terrane are rare and in the São Roque Terrane they have not been described until present. Elongated bodies of syenogranitic porphyroclastic orthogneisses with Statherian ages (ca. 1.75 Ga) are associated with Rhyacian (ca. 2.1 Ga) monzogranitic to granodioritic gneisses (Kaulfuss, 2001; Cury et al., 2002), forming elongated cores (e.g., Betara and Tigre) of Mesoproterozoic sequences (Figure 19). Although outcrops of basement rocks are scarce, their regional importance is indicated by Sm-Nd model ages of the metasedimentary cover units, that systematically show Paleoproterozoic values (Reis Neto, 1994; Basei et al., 1997; Prazeres Filho, 2000; Prazeres Filho et al., 2003; Ragatky et al., 2003).

Mesoproteozoic Metasedimentary Units

The metasedimentary rocks that crop out in the southwestern portion of the Apiaí Terrane are classically referred to as Açungui Group (Almeida, 1956; Marini et al., 1967; Campanha et al., 1987; Fiori, 1992; Campanha and Sadowsky, 1999). More recent studies suggest that this group is actually composed of three megasequences. The metasedimentary cover, exposed in elongated NE-SW belts limited by steep shear zones (Figure 17 and 18), will be described below, initiating with the Mesoproterozoic units (ca. 1.4 Ga; Juliani et al., 2000; Basei et al., 2003; Weber et al., 2003; Oliveira et al., 2003), (Figure 18 and 19).

Figure 17. Metamorphic map of the zone of interference

Metamorphic map of the zone of interference

Metamorphic map of the zone of interference between the southern Brasília Orogen and the central part of the Ribeira Orogen (after Trouw et al., 2000, 2003; Ribeiro et al., 2003; Valeriano et al., 2004 and Heilbron et al, 2004). Legend: 1- basement rocks (no metamorphic grade indicated); 2- greenschist facies cover rocks; 3- Ky zone; 4- Ky+Sill zone; 5- Sill zone; 6- Ky-Kf zone (high-P granulites); 7- high-T granulites of the Guaxupé Nappe; 8- local occurrence of retrogressed eclogites; 9- tectonic transport of Brasilia Orogen; 10- tectonic transport of Ribeira Orogen; 11- location relative to SFC (São Francisco Craton). Cities: Pa- Passos, Al- Alpinópolis, BE- Boa Esperança, La- Lavras, SJD- São João del Rei, Ba- Barbacena, Va- Varginha, Ja- Jacuí, Alf- Alfenas, TC- Três Corações, Ca- Carvalhos, An- Andrelândia, Li- Liberdade, LD- Lima Duarte, PA- Pouso Alto.

Figure 18. Tectonic map of the Apiaí

Tectonic map of the Apiaí

Tectonic map of the Apiaí, Guaxupé, Curitiba, Embú and Luis Alves Terranes (after Campos Neto, unpublished).

Figure 19. Geological map of the Apiaí Terrane

Geological map of the Apiaí Terrane

Geological map of the Apiaí Terrane and adjacent Curitiba and Luis Alves Terranes (data from Campos Neto, 2000; Campanha, 1991). Legend: Apiaí Terrane (1-12): 1- Castro Group; 2- A-type granite; 3- Itu suite; 4- Camarinha Formation; 5- mylonitic rocks; 6- calc-alkaline granite; 7- Iporanga (IP) Formation, Córrego dos Marques Sequence (CM), Saivá association (SV), Apiaí Gabbro (ap); 8- Abapã/Bairro da Estiva Formation; 9- Itaiacóca Group (IT), Lageado Supergroup (LG), Capirú Formation (CP); 10- Águas Claras Formation (AC), Votuverava Formation (VT), Serra das Andorinhas Formation (SA); 11- Perau Formation (PE), Betara nucleus (BT), Piririca Sequence (PI); 12- peralkaline orthogneisses. Curitiba, Luis Alves and Paranaguá terranes (13-19): 13- extensional basins and peralkaline granites; 14- calc-alkaline granites; 15- granitoids; 16- proto-mylonitic metasediments; 17- Turvo-Cajati sequence; 18- Atuba Complex (AT)/Juréia granulite (JU); 19- Orthogneisses and granulites of the Luis Alves Terrane.

The basal unit, of volcano-sedimentary nature (Perau Formation), is composed of quartzites, few marbles and calc-silicate rocks, graphite schists, calc-phyllites and banded iron formation with common intercalations of subalkaline tholeiitic metabasalts with chemical affinity to ocean floor transitional to island-arc tholeiites (Daitx, 1996). These units were correlated with the Betara Sequence (Piekarz, 1981) and with the Piririca Sequence (Perrotta, 1996). The upper part of this metavolcano-sedimentary unit is made up of a thick and extensive turbiditic unit, the Votuverava Formation (Figure 19), composed of metarhythmites, metasiltites, metasandstones and graphitic phyllites, with graded bedding, parallel lamination and climbing ripple lamination. Metaconglomerates, amphibolites, tourmalinites, banded iron formation and Mn-rich metachert are still present but less common as in the lower part, suggesting that the tholeiitic volcanism continued to be active, but in a more restricted way. Within the Votuverava Formation there are also marbles and quartzites interbedded with micaschists (Serra das Andorinhas Sequence). The central belt is dominantly carbonatic (Aguas Claras Formation, Figure 19), with marbles, calc-silicate rocks, calc-phyllites, quartzites and metacherts, associated with amphibole schists, garnet-chlorite-biotite schists, mafic to intermediate metatuffs and amphibolites. The metavolcanic-sedimentary Serra do Itaberaba Group occurs as a disconnected extension towards the northeast, in the São Roque Terrane. At the base it is predominantly composed of metabasic rocks with tholeiitic ocean floor affinity, grading towards the top to andesitic and rhyolitic metavolcanics with island arc affinity, interbedded with metatuffs, chemical clastic metasedimentary rocks, pelites and immature psammites (Juliani, 1993; Juliani and Beljavskis, 1995).

Neoproterozoic metasedimentary units

The Neoproterozoic sequences (Figure 18 and 19) are carbonatic shelf sequences, now limited by steep lateral shear zones. The Itaiacoca Group (Almeida, 1956; Souza, 1990), limited by the Paraná Basin and by the Cunhaporanga and Três Córregos batholiths, is constituted from bottom to top by metasiltites, metarhythmites and phyllites, fine grained orthoquartzites and metadolomites with stromatolites, algal mats, oolites and tepee structures (Fairchild, 1997, 1982; Theodorovicz et al., 1988). Lenticular bodies of amphibolite are common. The Lageado Subgroup (Campanha et al., 1987) was defined as a regressive progradational sequence, ranging from shallow shelf to carbonatic ramp (Pires 1991). Its detailed stratigraphy is described in Petri and Suguio (1969), Campos Neto (1983), Campanha et al. (1987), Pires (1991) and Campanha (1991).

The Capiru Group occurs south of the Lancinha shear zone (Figure 18 and 19) overlying the gneisses of the Curitiba Terrane through a basal shear zone of thick mylonites with tectonic movement to E-SE. Fiori (1992) recognized three lithologic associations: 1) phyllites and quartzites, 2) marbles with stromatolitic structures and intraformational breccias, with subordinate phyllites and quartzites, 3) alternating phyllites, quartzites and marbles. These units are interpreted as the Neoproterozoic passive margin of the Curitiba Terrane.

A second group of Neoproterozoic sequences, apparently overlying the first unconformably, is characterised by immature terrigenous metasedimentary rocks with polymictic conglomerates and/or "microconglomeratic phyllites". The pebbles and granules are composed of phyllites and schists showing deformed schistosity from the source area. The oldest formation, the Abapã-Bairro da Estiva Formation (Figure 19; Souza, 1990; Reis Neto, 1994) is dominated by meta-arkoses and alkaline metavolcanic rocks, and was dated at ca. 630-640 Ma (Siga Jr. et al., 1995, 2003). The Iporanga Formation is a flysch-type orogenic unit with matrix supported metaconglomerates and metabreccias intercalated with microconglomeratic phyllitic rhythmites. Metabasic rocks are of calc-alkaline affinity (Perrotta, 1996) and were dated (Apiaí Gabbro) at ca. 615 Ma (Hackspacher et al., 2000). The low grade psammo-pelitic units of the Corrego dos Marques Formation (Bistrich et al., 1985) and the metacalcareous rocks with rudaceous intercalations of the Saivá Unit (Fiori, 1992) were tentatively correlated with these sequences.

The extension towards the northeast of these Neoproterozoic units corresponds to the São Roque Group that overlies the Serra do Itaberaba Group unconformably. It is subdivided in four units (Hasui, 1975, Bergmann, 1988): a basal metavolcanic-sedimentary unit with lenses of marble and stromatolitic metadolomite (Pirapora do Bom Jesus Formation), followed by a thick psammitic succession (Pirajibu Formation), metarhythmites and phyllites (Estrada dos Romeiros Formation) and arkosic metasandstones on top (Boturuna Formation). Intrusive metarhyolites were dated at ca. 610 Ma, considered as a minimum age for the deposition of the São Roque Group (Hackspacher et al., 2000).

Orogenic stages

The present understanding of the regional metamorphism of the Apiaí-São Roque Domain is still fragmentary. It seems probable that the Mesoproterozoic sequences contain a metamorphic and deformational evolution previous to the formation of the Neoproterozoic basins, as was proposed for the Serra do Itaberaba Group (medium amphibolite facies metamorphism with kyanite related to a pre-São Roque schistosity; Juliani, 1993). In general, predominates rocks of greenschist facies metamorphism of low pressure, with chloritoid and andalusite in metapelites. Kyanite is present in the Capiru Formation, related to low temperatures. Upper greenschist and amphibolite facies are present in the Aguas Claras Formation and locally in the Votuverava Formation that further to the northeast attains anatectic conditions. The thermal aureole of the Cunhaporanga Granite is superposed on the schistosity of the Itaiacoca Group and the Abapã Formation and its age (ca. 600 Ma, PrazeresFilho, 2000) can therefore be considered as the minimum age for the deformation.

NW vergent recumbent folding with S1 schistosity along the axial plane was described in the São Roque Domain in the northeastern part of the Votuverava Formation (Campos Neto and Basei, 1983; Campos Neto et al., 1990) and in the Itaiacoca Group (Siga Jr. et al., 2003). These structures have a divergent regional pattern, changing towards the southeast to a duplex system of thrust sheets with tectonic transport to E-SE limited by the allochthonous front of the Capiru Formation (Fiori, 1992). A second metamorphic and deformational pulse is related to steep shear zones (Campanha, 2002) that acted at low temperatures (Faleiros, 2003), generating transpressive zones with SE vergent folding associated to a regional crenulation cleavage (S2). These shear zones define most limits between the stratigraphic units and also the terrane limits. Synkinematic granites of the Rio Jaguari Mylonitic Belt with amphibolite facies mylonites yielded ages of ca. 590 Ma (Janasi et al., 2003), compatible with Ar-Ar ages of the Serra do Itaberaba Group (Oliveira et al., 2003).

The granitic magmatism shows some similarity in terms of type, age and geologic evolution with the one of the Socorro-Guaxupé Nappe. This suggests the common evolution for the two marginal domains of the Paranapanema Plate. Potassic calc-alkaline granites are predominant; they constitute the main portion of three extensive NE-SW elongated batholiths (Cunhaporanga, Três Córregos e Agudos Grandes). Numerous isolated plutons, including the ones that are intrusive in the São Roque Domain, are probably satellites of these large granitic masses, considering the similarities in composition and age. The granitic magmatism is less diverse as compared to the Socorro-Guaxupé Nappe; this may reflect a more shallow and homogeneous crustal level of exposure without the para-autochthonous granites and the charnockites. U-Pb ages indicate that the Três Corregos and Agudos Grandes batholiths crystallised between ca. 615 and 605 Ma (hornblende-biotite granites of high colour index) and at ca. 600 Ma (late isolated intrusions). The late intrusions correspond to strongly contaminated and differentiated parts of the same association (the late orogenic granites of the Piedade region; Janasi et al., 2001). Older crystallisation ages (ca. 630-620 Ma) were reported for some plutons of the São Roque Domain (Töpfner, 1996) and for the Três Corregos batholith (PrazeresFilho et al., 2003) but these ages should be considered with caution because of the presence of inherited zircons.

The potassic calc-alkaline granites are interpreted as of hybrid origin, including a basic component derived from the mantle and variable but important components from pre-existing continental crust. The available geochemical, isotopic and geochronological data indicate that different levels of continental crust were involved in the generation of these magmas. Negative Nd(t) values of -17 to -19, predominant in the Três Corregos batholith, combined with the Pb isotopic signature and the age of inherited zircon crystals (PrazeresFilho et al., 2003) point to a component of old lower crust, possibly granulitic basement, in the source of the magma. The middle crust, non granulitic and possibly younger, is an important component of other bodies (Cunhaporanga Batholith, with Nd(t) from -11 to -13, PrazeresFilho et al., 2003).

Peraluminous leucogranites of exclusively crustal origin are rare and form small isolated intrusions (Turvo and Tico-Tico granites, dated at ca. 610 and ca. 620 Ma, respectively, Janasi et al., 2001; Töpfner, 1996). The increased contamination observed in the younger portions (ca. 600 Ma) of the Agudos Grandes Batholith is probably related to the local heating of this portion of the crust after the passage of important volumes of magma.

Post-Orogenic Stage

The post-orogenic period is marked by the intrusion of shallow granitic plutons dated at ca. 590-580 Ma, localised in the Itu Granitic Province (Figure 19, Capão Bonito, São Miguel Arcanjo, Sguario). These are pink granites with A-type geochemical signature of the aluminous series (Leite, 2003). Some other elongated plutons, marginal to the Agudos Grandes and Três Corregos batholiths, are also formed by A-type granites, but have ages of ca. 565 Ma (Janasi et al., 2001; PrazeresFilho, 2001). This somewhat younger age could reflect a distinct event, possibly related to tectonic activity in the terranes located to the east.

The late to post-collisional basins (Basei et al., 1998) correspond to the ruditic to psammitic deposits of the Camarinha Formation, interpreted as a foreland basin related to the last phases of the transpressive shear zones. The Castro Group represents a post-orogenic extensional basin with extensive acid to intermediate volcanism, associated to immature psammitic rocks intercalated with distal pelitic terms. The age of this volcanism is lower Cambrian according to Cordani et al. (1999).

The Embú Terrane

The E-W trending Embú Terrane is limited towards north and south by thick dextral shear zones (Caucaia-Rio Jaguari and Cubatão, respectively); along strike it wedges out against the Juiz de Fora Terrane towards the east and towards the Apiaí Terrane to the west, both with tectonic contacts still poorly known (Figure 20). It differs from the surrounding terranes by the nature and age of its infrastructure, by its granitic magmatism and by older Neoproterozoic ages for its metamorphism.

Figure 20. Geological map of the Embú Terrane

Geological map of the Embú Terrane

Geological map of the Embú Terrane and adjacent Apiaí Terrane (modified from Campos Neto, 2000). Legend: 1- A-type granites; 2- Itu Province granites; 3- Bt- and Bt-Ms-granites; 4- porphyritic to mylonitic Hbl-Bt granitoids; 5- Bt- and Ms-Bt higher-level crustal granites and undifferentiated types; 6- lower-level crustal Bt granites; 7- Quebra Cangalha-type Ms-granites and Lagoinha-type Bt-granites; 8- S-type Bt-tonalitic-granitic orthogneisses (Australian-type); 9- Rio Jaguari mylonites; Embu Complex (10-13): 10- Rio Una Unit with schists, quartzites and calc-silicate rocks (amphibolite facies); 11- Rio Una Unit with schists and quartzites (greenschist facies); 12- Rio Paraibuna Unit with quartzitic gneisses, Bt-gneisses, Sill-gneisses and calc-silicate rocks; 13- Redenção da Serra Unit with Sill gneisses, amphibolites, Bt-gneisses, calc-silicate rocks and marbles, migmatitic textures are frequent; 14-rhytmic phyllites and quartzites (Votuverava Formation); 15- mica-schists to gneisses and migmatites; 16- tonalitic gneisses and migmatites (Rio Capivari Complex); 17- enderbitic gneisses of the Juiz de Fora Complex.

The elongated Paleoproterozoic infrastructure (Complexo Rio Capivari; Fernandes et al., 1990), is composed of migmatitic orthogneisses with granitic to tonalitic composition of ca. 2.0 Ga (Babinski et al., 2001). The migmatitic orthogneisses of the Cubatão Range were tentatively correlated with these rocks (Sadowski, 1977).

The metasedimentary superstructure of unknown age corresponds to the Embú Complex (Hasui, 1975) with three litho-stratigraphic units (Fernandes 1990): the Rio Una Unit, on top, dominated by mica-schists and immature quartzites; the Rio Paraibuna Unit, of clastic to chemical nature, with abundant quartzites and calc-silicate rocks and with intercalations of fine grained biotite gneisses and amphibolites; and the Redenção da Serra Unit, the most important of the three, composed of peraluminous gneisses, plagioclase-biotite gneisses/schists, amphibolites calc-silicate gneisses and restricted marbles.

The principal metamorphism is of amphibolite facies in the zone of sillimanite-muscovite, grading laterally to sillimanite-K-feldspar, associated with anatexis (605-770 ºC to 5-6 kbar; Vieira, 1996). Rocks of greenschist facies are present along the Cubatão Shear Zone and also in the Rio Una Unit, southeast of the São Paulo Basin, where they show the transition, from NW to SE, from greenschist to amphibolite facies, with garnet, kyanite and sillimanite. The age of this metamorphism is ca. 790 Ma (Vlach, 2001) consistent with ages obtained by Vieira and Tassinari (1988) and by Cordani et al. (2002). The geodynamic context of this event, that is unknown in the adjacent terranes, is at present not well understood.

The S1 schistosity is only preserved as relicts and seems to be related to the high temperature mineral associations. S2, with low to medium dips to SE, is associated with a strong mineral and stretching lineation that plunges weakly to NE and shows evidence of tectonic transport with top to SW. However, thrusting to the north was also reported (Santarem da Silva, 1992). The presence of an incipient foliation that was correlated to S2 because of its orientation, observed in the peraluminous Quebra-Cangalha Batholith with crystallization age of ca. 655 Ma [Janasi et al., 2003], provides a maximum age for S2.

Later tight folding, verging to NW, deforms S2 and is associated with a crenulation cleavage S3. The transcurrent dextral shear zones that define the limits of the terrane and that divide it internally into an anastomosing pattern are composed of thick subvertical mylonitic rocks. They control the shape and intrusion of several granites, dated at ca. 590 Ma (Filipov and Janasi, 2001; Janasi et al., 2003).

The Neoproterozoic magmatism is characterized by the absence of metaluminous granites, like the porphyritic hornblende-biotite granites that are important in the adjacent terranes (Janasi and Ulbrich, 1991). This fact suggests a significant tectonic importance for the Caucaia-Rio Jaguari shear zone. The predominant petrographic types are porphyritic to inequigranular biotite granites (Mauá, Itapeti, Santa Catarina and dominant facies in the Lagoinha Batholith) and equigranular muscovite-biotite monzogranites (Guacuri, Santa Branca, Mogi das Cruzes and dominant facies of the Quebra-Cangalha Batholith). The more differentiated parts are muscovite-garnet-tourmaline granites, aplites and pegmatites that form small isolated bodies.

Radiometric age determinations are scarce, but recent data reveal that the peraluminous granites were generated during at least 200 million years in successive episodes of recycling of continental crust (Janasi et al., 2003). However, most granites of the Embú Terrane were generated in a short time interval, between ca. 600 and 590 Ma, after the metamorphic and magmatic peak in the Apiaí Terrane, but preceding this peak in the Juiz de Fora and Costeiro Domains. In the central portion (Mogi das Cruzes region, south of the Guararema Fault) important changes in the character of the granitic magmatism seem to have taken place within this short time interval. The granites formed around 600 Ma (Santa Branca and possibly Santa Catarina) contain a foliation induced in the solid state and enclaves of dominantly metamorphic rocks; they also have more fractionated rare earth patterns, indicating contributions from varied sources including the lower continental crust. The younger granites (Mauá and Mogi das Cruzes, ca. 590 Ma; Filipov and Janasi, 2001;) have characteristics of shallow intrusions with magmatic foliation and abundant microgranitic enclaves. Their rare earth patterns are less fractionated, with expressive negative anomalies of Eu indicative of sources within the stability field of feldspar, possibly the middle or upper crust. The granites that crop out in the eastern extreme of the terrane (São José do Barreiro region) are of similar age (600-580 Ma; Valladares, 1996; Pereira et al., 2001) and chemistry (Funil Granite) as the Mauá and Mogi das Cruzes granites. However, the Quebra-Cangalha Batholith has a significantly older age (ca. 655 Ma; Janasi et al., 2003). The tectonic significance of this batholith and of the neighbouring Lagoinha Batholith is at present uncertain.

Older Neoproterozoic granites (ca. 790-780 Ma) were identified as elongated bodies of mylonitic orthogneisses. Several occurrences with these characteristics were described (Fernandes et al., 1990), but only two of them were dated, one to the southwest of the city of São Paulo (Cordani et al., 2002) and the other one from São José dos Campos (Vlach, 2001). This same age was obtained from monazite in metapelites (Vlach, 2001), showing the importance of this magmatic/metamorphic event in the Embú Terrane. Petrographically these rocks are comparable to the S-type granites from Australia (peraluminous biotite tonalites). They could therefore reflect the incorporation of residues derived from sedimentary protoliths but it is also possible that they originated from magmas with a basic component strongly contaminated by pelites.