40Ar/39Ar ages (600-570 Ma) of the Serra do Azeite transtensional shear zone: evidence for syncontractional extension in the Cajati area, southern Ribeira belt

This paper presents 40Ar/39Ar ages of the rocks from the Serra doAzeite transtensional shear zone in the southern part of the Ribeira belt, between the States of São Paulo and Paraná, and also discusses the regional correlations and the tectonic implications for other parts of the belt. The geochronological data suggest that transtensional deformation was active between 600 and 580 Ma (hornblende and muscovite apparent ages, respectively). This time span is considerably older than previous proposals for the period of activity of these structures (520-480Ma) in the northern segment of this belt and in the Araçuaí belt. Kinematic analysis of the dated mylonites shows extensional structures with top-downmovement to ESE compatible with structures found in other tectonic segments in the eastern portion of the Quadrilátero Ferrífero and in the Rio Doce Valley region. Our ages are situated in the same time interval defined for the alkaline magmatism of the Serra do Mar suite. We suggest that the regional tectonic framework was developed during continental-scale extension. This process has been coeval with convergent strain in the adjacent Neoproterozoic shear zones of the Apiaí/Ribeira and Araçuaí belts, which make up significant segments of these belts. The available data show that these structures may not be simply related to post-orogenic gravitational collapse, but must involve a more complex process probably related to dynamic balance between crustal thickening and thinning during tectonic convergence, basin formation and exhumation processes.


INTRODUCTION
Extensional structures have been described in several regions closely associated, temporally and/or spatially, with compressive tectonics (Coney and Harms 1984, Royden and Burchfield 1987, Ratschbacher et al. 1991a, b, Wheeler and Butler 1993, Braathen et al. 2002. These structures have been commonly interpreted as derived from gravitational instabilities generated by crustal overthickening and progressive uplift that accompany mountain building (e.g. Malavieille 1993). Such a frame-work may result from complex tectonic processes that involve dynamic plate interactions, in which extension may be contemporaneous with, later than, or decoupled from crustal shortening and deformation, either due to the steep downward slope of the oceanic slab in a subduction zone, in under the back-arc region, or from uplift of the astenospheric mantle wedge (Doglioni 1996). Other processes, such as changes in the kinematic vectors between plates and post-collisional rifting, may also explain the occurrence of extensional regimes (see also Glazner andBartley 1985, Dewey 1988).
The tectonic framework of southeastern Brazil, also called the Mantiqueira Province, has been attributed mainly to the Brasiliano/Pan-African orogeny developed between 1.0 and 0.5 Ga (Almeida et al. 1973, Brito Neves andCordani 1991). Marginal to this province a stable Neoproterozoic nucleus, the São Francisco craton, has been recognized (Almeida 1977), surrounding by Brasiliano fold-thrust belts such as the Araçuaí and Ribeira belts. Several well-preserved examples of extensional structures have been described in different segments of the São Francisco craton, presenting east/southeast-verging extension (Fig. 1), such as in the eastern portion of the Quadrilátero Ferrífero (Endo 1997), in the Araçuaí fold belt (Nalini 1997, Machado et al. 2001, Alkmim et al. 2002 and in the Costeiro Granite Belt (Basei et al. 2000), which crops out as a distinct unit within the Ribeira belt.
Records of the extensional settings in the Ribeira and Araçuaí belts are also interpreted as result from orogenic collapse postdating the regional contractional strain (Pedrosa Soares et al. 2000, Trouw et al. 2000, Heilbron et al. 2004).
In the southern segment of the Ribeira belt, extension is thought to have occurred between 610 and 570 Ma on the basis of indirect evidence (i.e. ages of alkaline granitic magmatism and sedimentary tectonic basin sedimentation (Siga Jr. et al. 1999, Basei et al. 2000. In the central and northern parts of this belt, extension is believed to have occurred between 520 and 480 Ma, although detailed geochronological and structural data are still not available (Trouw et al. 2000, Heilbron et al. 2004).
In the Cajati region, kinematic indicators related to ductile deformation, such as S-C foliation planes, asymmetric porphyroclasts and boudins, and asymmetrical extensional shear bands are quite consistent with oblique, E to SE extension towards east-to south, with important sinistral strike-slip motion (Dehler et al. 2000). These structures are related to a mid-to deep-crustal transtensive shear zone which developed after the main crustal thickening event under compressive tectonic regime of the Ribeira belt.
In this paper we present 40 Ar/ 39 Ar radiometric data for the mylonitic rocks related to the Serra do Azeite shear zone, and describe the extensional structures in the southern segment of the Ribeira belt (see Fig. 1). We also discuss regional structural correlations and tectonic implications for the Precambrian between São Paulo and Paraná states.

GEOLOGICAL SETTING AND STRUCTURE OF THE SERRA DO AZEITE TRANSTENSIONAL SHEAR ZONE
The studied area is located in the southern part of the Ribeira belt -northern edge of a high grade terrain called of Costeiro Granite belt by Basei et al. 2000 -where the Serra do Azeite shear zone is one of the main structures together with other subparallel shear zones (Figs. 1A and B).
The second sequence is characterized by banded and laminated orthogneisses that vary in composition from granite to tonalite in the felsic parts, and amphibolite, diorite and ultramafic in the dark bands or inclusions. This unit was correlated by Vasconcelos et al. (1999) with the Atuba Complex, yielding U-Pb zircon ages between 2100 to 1800 Ma, and was reworked during the Brasiliano Orogeny (Siga Jr. 1995, Picanço et al. 1998. The third sequence is composed by coarse-grained and mylonitic gabbro, and anorthosites that are correlated with the Serra Negra Complex (Vasconcelos et al. 1999).
The Serra do Azeite shear zone was previously described as having recumbent to isoclinal folds, refolded by inclined folds, oriented with E-W to ESE trending axes with NNW or N to NNE vergences, respectively Granite belt, and localization of the study area (modified from Machado et al. 2001, Dehler et al. 2000, Basei et al. 2000. (Campos Neto 1983). The author recognized that the orthogneissic units overthrust the metasedimentary rocks, with top-to-northwest sense of motion.
Recent kinematic studies, however, have described ductile sinistral transtensional tectonics with top-downto ESE for the same region (Dehler et al. 2000) associated with the Serra do Azeite shear zone. These authors described mylonites with foliation dipping to southeast, associated with a well-developed SE-SSE plunging stretching lineation. The foliation exhibits shallow to moderate dips (in the first sequence and part of the second) in the northern part of the studied area, and steeper dips in the third sequence on top of mylonites in the southern part (see Figs. 2 and 3). The geometry shown by the disposition of the mylonitic foliation resembles an incomplete fan-shape ( Fig. 3: A-A'), usually reported as a structure associated with transpressive/transtensive regimes.
Petrographic and microtectonic data for the peraluminous mylonitic association mainly show kyanite reequilibration to sillimanite (fibrolite) in the presence of primary muscovite. This suggests that mylonitization developed under medium-grade metamorphic conditions, at medium pressure (Silva andAlgarte 1981a, Vasconcelos et al. 1999). The unstable kyanite was replaced by sillimanite which marks the transtensional stretching lineation, suggesting a higher pressure, pri-  An Acad Bras Cienc (2007) 79 (4) mary metamorphic environment, followed by lower pressure and/or higher temperature. This microtectonic evidence suggests a previously shortened crust affected by transtensional deformation along a clockwise P-T-t path (Dehler et al. 2000).
Microstructures also show evidence of dynamic recrystallization, with features such as core-mantle and deformation bands in quartz porphyroclasts. Polygonized quartz grains with irregular boundaries occur in the matrix. These features suggest that ductile deformation probably occurred both during and after the metamorphic peak, contemporary with the retrograde metamorphic path (Dehler et al. 2006).
These features show that the deformation was generated under amphibolite facies metamorphic conditions, and that its development continued during the progressive uplift and the regional crustal cooling. This situation produced extensional ductile to brittle structures under the same regional stress field. A K/Ar investigation of mylonites in this area yielded cooling ages of 565 ± 39 Ma in hornblende, 521 ± 26 Ma in biotite, and 587 ± 21 Ma in phlogopite (Campagnoli 1996). 40 Ar/ 39 Ar isotopic measurements were carried out in the MAP-215-50 mass spectrometer of the Geochronological Laboratory of the Institute of Geosciences of São Paulo University. The mass spectrometer is linked online to a stainless steel extraction system which operates under ultra-high vacuum conditions. For technical details see Vasconcelos et al. (2002). 39 Ar/ 40 Ar analyses were performed using a laser source, and a routine step heating procedure. Other details about the analytical procedures, including mineral separation, irradiation, reliable calibration tests, standards, accuracy and precision are given by Vasconcelos et al. (2002). 40 Ar/ 39 Ar dating was made on crystals from two samples of Costeiro Granite belt (Figs. 4A, B and C). One of the samples is a late kinematic pegmatite (NM-200B and C), while the other is a mylonitic orthogneiss (NM-0200-A). The pegmatite cross-cut mylonitic foliation of the orthogneiss (Fig. 5A). Two isotopic analyses were per-formed in muscovite (pegmatite) and one in hornblende (orthogneiss) (see Fig. 4). The two muscovite crystals from the pegmatite yielded plateau ages of 579.2 ± 1.2 Ma (NM-200C) and 574.5 ± 1.3 Ma (NM-200B), while the hornblende from the orthogneiss yielded a plateau age of 594.7 ± 1.3 Ma (NM-200A) (Figs. 4A, B and C; Table I). An ideogram plotted for muscovite two crystals (crystals NM-200C + NM-200B) yields a high probability peak at 579.1 and a weighted mean age of 577 ± 3 Ma (Fig. 4D).

GEOCHRONOLOGICAL DATA AND KINEMATIC ANALYSIS
A previous kinematic analysis carried out in the Serra do Azeite shear zone suggested a transtensional tectonic regime in the northeast-trending sinistral shear zones (Dehler et al. 2000). The mylonites exhibit heterogeneous deformation, zones with constrictional (L-tectonites), flattening, and simple shear strains occur, with a well marked stretching lineation plunging to SE-E. Kinematic indicators such as S-C fabrics, σ -porphyroclasts, asymmetric boudins and shear bands, are quite consistent with an oblique east-to southeast-directed extension, with a significant sinistral slip motion (Dehler et al. 2000).
The 40 Ar/ 39 Ar data obtained here for the hornblende of the mylonitic orthogneiss show a plateau age of 594 ± 1.3 Ma (NM-200A; Fig. 4C). The muscovite grains from the late kinematic pegmatite yielded middle age of 577 ± 3 Ma (NM-200C, Fig. 4D). This muscovite age constrains the time of mylonitic foliation of the orthogneisses in the Serra do Azeite shear zone, because the pegmatite cross cuts one to this structure (Fig. 5). By extrapolation, we consider this age as a lower limit for the development of transtentional structures in Cajati region.
The slightly younger plateau age of 574.5 ± 1.3 Ma (NM-200 B) (Fig. 4B) is interpreted to mark the regional metamorphic cooling. Allowing for the larger errors the previously reported K/Ar ages for phlogopite (587 Ma) and hornblende (565 Ma) are consistent with the 40 Ar/ 39 Ar ages obtained here, except for the biotite age (see Campagnoli 1996). This age range is older than the upper limit of previous estimates for the collision in the Ribeira and Araçuaí orogens, considered between 580 and 520 Ma (Heilbron et al. 2004). The 580 Ma age may correspond to the peak of metamorphism and deformation. Our 40 Ar/ 39 Ar geochronological data suggest that ductile extension took place between 594 to 574 Ma.
In the studied area, field relations suggest that the ductile extensional shearing predates the emplacement of Guaraú granites, and may be associated to previous crustal thinning and melting which generated the magmas. This model also accounts for the ages of the alkaline granites of the Serra do Mar suite (600 and 570 Ma: Siga Jr. 1995) in the region. The model apparently explains the coexistence in the same orogenic space of extensional and compressional structures that overprinted the orogenic belts during convergence against the São Francisco Craton.

TRANSTENSIONAL STRAIN AND THERMAL HISTORY
The hornblende cooling age data presented here (584 Ma) is probably close to that of the regional medium-grade metamorphic peak of the Ribeira belt (between 500 e 600 • C: Winkler 1967). On the other hand, taking into account the critical blocking temperature of muscovite (350 • C ± 50 • C: Berger and York 1981), the 40 Ar/ 39 Ar ages obtained (between 574 and 579 Ma: middle age of 577±3 Ma) represent the time at which the metamorphic conditions reached to incipient grade of Winkler (1967) (between 200 and 350 • C). The 40 Ar/ 39 Ar ages obtained here suggest a cooling rate at around 10 • C/Ma for this time interval (∼ 20 Ma), being this value compatible with a crustal exhumation up to 6 Km. Extensional structures have been also described in different marginal domains of the southern São Francisco Craton (Endo 1997, Nalini 1997, Alkmim and Marshak 1998, Machado et al. 2001, Alkmim et al. 2002. They have been described in the Quadrilátero Ferrífero area and adjacent regions of Furquim and Dom Silvério toward the east (Chemale et al. 1994, Endo and Nalini 1992, Hippertt et al. 1992, Endo 1997. The age of these extensional structures is still uncertain, but in the Quadrilátero Ferrífero (i.e. Moeda syncline) they have been assigned to the Transamazonian (2100 Ma), linked with the "orogenic collapse" of the Minas Orogeny (Chemale et al. 1994, Marshak et al. 1997, Alkmin and Marshak 1998. Other workers, however, have recently identified extensional structures in the eastern region of the Ferrífero Quadrilátero as Brasiliano (Endo 1997), supported by geologic arguments. In the southern portion of the Ribeira belt, the temporal relationship between magmatism and the extensional structures suggests the emplacement of Neoproterozoic alkaline plutons during this extensional tectonic event.
In the Furquim region, an extensive and thickened mylonitic quartzite with extensional fabric, including S/ C foliation and shear bands, crops out (Endo 1997). This mylonite zone has a N-S trend with a mineral stretching lineation plunging down-dip to east. In the Dom Silvério region, east of Furquim, discontinuous mylonite zones have top-to east extension (Endo 1997). Although some of these extensional shear zones, especially those in the western portion of the Quadrilátero Ferrífero (Moeda syncline), apparently are Transamazonian in age (Marshak et al. 1997), Endo (1997 considered that the extensional structures of the eastern border of the Quadrilátero Ferrífero were developed after the compressional peak of the Brasiliano orogeny (∼ 600 Ma).
In the southern portion of the Ribeira belt (Costeiro Granite Belt and Luis Alves terrane), the alkaline magmatism of the Serra do Mar Suite has ages between 594 and 570 Ma (Siga Jr. 1995). This magmatism has been considered as recording an extensional tectonic event (Siga Jr. 1995). Recently, conventional U/Pb ages of 582 ± 4 Ma and 582 ± 9 Ma were obtained for the Serra do Cordeiro and Votupoca granites, respectively, which are correlated with this suite (Passarelli et al. 2004).

SUMMARY AND CONCLUSIONS
The extensional structures described in the Cajati region show similar kinematic features similar to those of the extensional structures described in the Araçuaí belt in the eastern portion of the Quadrilátero Ferrífero, in the Furquim and Dom Silvério regions (Endo 1997). In spite of the uncertainty concerning the timing of these structures, they are clearly younger than the extensional ones found in the western portion of the Quadrilátero Ferrífero (Chemale et al. 1994, Marshak et al. 1997, Alkmim and Marshak 1998. A previous kinematic analysis carried out in the Serra do Azeite shear zone, southeastern Brazil, suggests sinistral transtension for the main movement phase of an E-NE trending ductile shear zone (Dehler et al. 2000). The deformation of the mylonites is heterogeneous. The domains present constriction (L-tectonites), flattening, simple shear strains. A well marked stretching lineation plunging to SE-E is observed. Kinematic indicators such as S-C fabrics, σ -porphyroclast, asymmetric boudins and shear bands are quite consistent with an oblique east-to southeast-directed extension, and significant sinistral slip motion (Dehler et al. 2000). As a whole, the oblique extensional tectonics was responsible for the generalized hanging-wall collapse, followed by lateral rock-mass extrusion and uplift/exhumation of the northern Apiaí belt (Dehler et al. 2000), as suggested by the overall kinematics of the Serra do Azeite shear zone. 40 Ar/ 39 Ar ages in hornblende and muscovite confirm the relatively fast cooling after the metamorphic peak in agreement with the previous microstructural and metamorphic studies (Dehler et al. 2000). The scarcity of geochronological data does not permit us to define the exact time of formation of these structures. The development of alkaline magmatism (Suite Serra do Mar) mainly in the southern region of the Costeiro Granite Belt, dated between 600 and 570 Ma (Siga Jr. 1995), suggests that a tectonic relationship between them may exist.
In conclusion, we suggest for the Cajati region, and probably for a large part of the Costeiro Granite Belt in southern São Paulo and northern Paraná, a diachronic tectonic evolution in the orogenic space at least between ∼ 620 to 570 Ma. During this interval, while in the Cajati domain an extensional tectonic regime operated, in the adjacent ones (i.e. Apiaí and Araçuaí belts) a compressive regime occurred, suggesting that distinct tectonic regimes may have been active at the same time interval as a function of the orogenic dynamics rather than a function time. In the first setting, the tectonic regime was dominantly transtensional and may have been accompanied by crustal thinning, while the second was dominantly transpressive, and may have been accompanied by crustal thickening.
Finally, we consider that extension related with the emplacement of post-collisional plutons found in the Araçuaí and Paraíba do Sul belts with age between 520 and 500 Ma (Alkmim et al. 2002, Pedrosa-Soares et al. 2003, Heilbron et al. 2004) is the youngest extensional event.

ACKNOWLEDGMENTS
The authors are grateful to Thelma Samara for drafting the figures, to Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (Grants 93/1830-1; 95/01204-9) for supporting field work, to Dr. Ian McReath and Tariana Brocardo Machado for reviews of the manuscript, to two anonymous reviewers of the ABC for their constructive comments and suggestions. RM acknowledges CNPq research fellowship (30.0423/82-9).