U-Pb evidence for Late Neoarchean crustal reworking in the southern São Francisco Craton ( Minas Gerais , Brazil )

The Passa Tempo Metamorphic Complex is one of several metamorphic complexes that form the Archean sialic crust of the southern São Francisco Craton. It encompasses hypersthene-bearing gneissic rocks, with subordinate NWor EW-trending mafic-ultramafic bodies and granodioritic to alkali-granitic, weakly foliated, and light-colored granitoids. These granitoids are the product of generalized migmatization that followed granulite-facies metamorphism. To determine the ages of the granulite-facies metamorphism and granitoid genesis, we obtained U-Pb ages on zircon extracted from the mesosome and leucosome of the migmatitic gneisses. For the mesosome, a discordia that intercepts Concordia at 2622 ± 18 Ma is interpreted as a minimum age for granulite-facies metamorphism. For the leucosome, the upper intercept of discordia at 2599 ± 45 Ma corresponds to migmatization and granitoid genesis. Contemporaneous metamorphism and magmatism have been documented elsewhere in the São Francisco Craton, especially in the southern portion, demonstrating vast and vigorous reworking of sialic crust by the end of the Neoarchean.


INTRODUCTION
The São Francisco Craton (Figure 1) is a Precambrian continental platform (Almeida 1977, 1981, Almeida et al. 1981, Alkmim et al. 1993) whose sialic crust consists predominantly of amphibolite facies TTG gneisses and minor granulite facies TTG gneisses that are accompanied by tonalitic-granitic plutons and ultramafic-mafic bodies of varied age and metamorphic grade.Some of these petrographic associations are named ''metamorphic complexes'', for example, the Bonfim Metamorphic Complex (around Bonfim village), Campo Belo Metamorphic Complex (around Campo Belo village), and Passa Tempo Metamorphic Complex (around Passa Tempo village).
All of these granitoids were emplaced during the Rio das Velhas III Event (RV3E, Endo 1997, Table I) that deformed the Rio das Velhas Supergroup, under a brittle-ductile, dextral transpressional regime with N-S-trending flow plan and tectonic transport from NE to SW (Endo 1997).Despite Bonfim and Campo Belo metamorphic complexes, the isotopic database (Fiumari et al. 1985, Teixeira 1985) for Passa Tempo Metamorphic Complex is very meager.The existing data being limited to a Rb-Sr isochron age of 2566 ± 53 Ma, and K-Ar apparent ages ranging from 2000 to 1750 Ma (Figure 2, Table II).Now this paper reports the first U-Pb data on zircon from the Passa Tempo Metamorphic Complex.

Geologic Setting
The Passa Tempo Metamorphic Complex (Figure 2) is situated between Campo Belo Metamorphic Complex to the southwest, and the Bonfim meta-morphic Complex to the northeast.Although the Passa Tempo Metamorphic Complex oval geomorphologic feature is well characterized in topographic maps (at a 1:250,000 scale), its geologic boundaries with neighboring metamorphic complexes are not perfectly depicted (Figure 1).The Passa Tempo Metamorphic Complex encompasses tonalitic to granitic regional gneisses with minor mafic and ultramafic rocks, migmatites, and granitoids (Figure 2).Two prominent trends (NNE and NNW) characterize the gneissic layering.ENEtrending shear planes are superimposed (Figure 3).

U-Pb GEOCHRONOLOGY
We extracted zircons from two Passa Tempo Metamorphic Complex lithotypes near the city of Desterro de Entre Rios (Figure 2).A quartz-monzonitic mesosome (OPU 1349) contains orthopyroxene, which guarantees that the rock crystallized in the granulite facies.A quartz-syenitic leucosome (OPU 1348) formed by migmatization that followed highgrade metamorphism event.Initial separation of zircon by standard procedures at the Laboratory of Sample Preparation for Geochronology (LOPAG) of the Department of Geology of the School of Mines of Ouro Preto Federal University was followed by sample chemistry and mass spectrometry, also by standard procedures at the Laboratory of U-Pb Geochronology of the Center of Geochronologic Research -CPGeo -of the Institute of Geosciences of São Paulo University.The least magnetic zircon fractions were chosen for mechanical abrasion.Transparent, euhedral, colorless zircon crystals (concentrates M3-E, M3-C, M4-D and M2-A), and a light brown (concentrate M3-B) from the mesosome (OPU 1349; Table III) exhibit prisms with axes ratios of the order of 2/1 to 2.5/1.5.Leucosome zircons (OPU-1348; Table III) are similarly euhedral, transparent, and colorless (concentrate M3-A) to light brown (concentrates M3-B, M4-AA and M3-C), a little fractured, with short prisms exhibiting axis ratios varying between 2.5/ 1.5 and 2.0/1.5.Zircons were mechanically abraded with pyrite for 15 minutes in a steel capsule, leached with hot HNO 3 , rinsed with water, and sonified.From sample OPU-1349, we selected abraded zircon fraction M3, and populations M3-AA(1) and M3-AA(3) for isotopic dilution analysis by standard procedures, employing a mixed 205 Pb-235 U spike (Krogh 1973 with minor modification by Basei et al. 1995), and performing calculations using the ISOPLOT program (Ludwig 1998).From sample OPU-1348, we chose population M4-AA for analysis.CPGeo uses a VG 354 thermal ionization mass spectrometer with 5-cup collectors and a Daly detector.

DISCUSSION OF U-Pb DATA
Data from the seven zircon populations of sample OPU 1349 (Table III) define a discordia line that intercept Concordia at 2622 ± 18 Ma (Figure 6), and for sample OPU-1348 (Table III), discordia-Concordia intercept is at 2599 ± 45 Ma (Figure 6).In both data sets the analytical points diverge from Concordia and, as discussed by Krogh and Davis (1975), this results mainly from ubiquitous although not severe fractures in zircon crystals.Populations of abraded zircons yield analytical points that are closer to Concordia (Figure 6).Dodson (1973) developed the theory of a mineral isotope closure during cooling, which occurs at least 800˚C for the U-Pb system in zircon (Heaman and Parrish 1991).The presence of orthopyroxene in the mesosome (sample OPU-1349) and elsewhere in Passa Tempo Metamorphic Complex indicates that regional metamorphism had reached granulite facies that, according petrographic evidence, formed under anhydrous conditions.We interpreted the 2622 ± 18 Ma U-Pb age as a minimum for the granulite-facies metamorphism, reflecting the peak regional metamorphism.In one of the analyzed fractions, zircon crystals with slightly rounded pyramidal faces contain small, apparently oriented globular fluid inclusions.Pupin (1976) states that such zircons typically have crystallized at high pressure, in eclogite or granulite facies host rock.The alkaligranitic leucosome (OPU 1348), containing microcline, plagioclase, quartz, biotite, mesoperthite, and hornblende, besides secondary epidote, opaque minerals and zircon, represents regional migmatization proper.The presence of amphiboles and micro-cline (the latter probably from potassic feldspar) that clearly formed at the expense of orthopyroxene demonstrates that felsic products were generated from later melting in the presence of aqueous fluids at shallower crustal levels.Some leucosome zircons (not analyzed) display conspicuous overgrowths.Pupin (1976) noted that aqueous fluids play an important role in making zircon overgrowths at middle lithosphere level.
As observed by Ashworth (1985), reactions related to the formation of ''minimum granites'' in laboratory experiments have always had potassic feldspar as one of the final products.In one of these reactions, biotite + garnet + albite + quartz, yielding potassic feldspar + garnet + orthopyroxene + H 2 O, at increasing temperature and pressure conditions, abundant formation of potassic feldspar can occur.The replacement of potassic feldspar by myrmekite (plagioclase + quartz), muscovite, and quartz, common in many migmatites, are interpreted as retrograde metamorphic reactions (Ashworth 1985).
Retrograde metamorphic features such as re- An Acad Bras Cienc (2003) 75 (4)  placement of pyroxene by hornblende and biotite, presence of myrmekite at the contact between plagioclase and potassic feldspar, and exsolution lamellae of orthopyroxene in clinopyroxene, to-gether relict texture and paragenesis, are observed in several Passa Tempo Metamorphic Complex samples.Thus it is likely that a quick crustal uplift led to the crystallization of microcline in the meso- some during the migmatization processes, and preserved anhydrous minerals from being totally replaced.In our study, the ages obtained from mesosome (2622 ± 18 Ma) and leucosome (2599 ± 45 Ma) are the same within error, suggesting that termination of granulite facies metamorphism and subsequent migmatization occurred in prompt succession.

CONCLUDING REMARKS
The U-  (Machado et al. 1996).Then, granulite-facies metamorphism and migmatization/granite genesis must have preceded deposition of this lower unit of the Minas Supergroup.
Our new U-Pb data support the hypothesis of a Neoarchean RV3E between 2.61 and 2.55 Ga (Endo 1997), under a ductile-brittle, dextral transpressional regime, with a N-S flow plane and tectonic transport from NE toward the SW.This event must have closed a large Neoarchean orogenic cycle, where the magmatism, metamorphism and deformation processes are well recorded in the São Francisco Craton and throughout South America (Almeida 1981).In this case, the tectonic environment for the RV3E would imply a continent-continent collision and is temporally in agreement with the continental superagglutination model proposed by Yale and Carpenter (1998), which states that a large continental mass, also envisaged by Condie (1998), formed between 2850 and 2500 Ma.Such agglutination would correspond to countless continental collisions related to the Jequié cycle.In this model, the duration of these supercontinents would be marked by the episodic occurrence of large juvenile igneous provinces and large mafic dike suites, products of mantle plumes formed due to buffering of the mantle under large continental masses, modifying its convective activity.Lagarde et al. (1992), classify the emplacement of granitoids in collisional orogens in three stages: 1 -syn-crustal thickening (granites with crustal petrographic and geochemical signatures); 2 -post-crustal thickening (granites with very different geochemical and petrographic signatures as a function of the variable involvement of crustal and mantle material coming from active magmatic sources); 3 -late-orogenic (calc-alkaline to potassic calc-alkaline granites).The Passa Tempo Meta-morphic Complex geologic data here presented, especially those related to the compositional characteristics of the granitoids, seem to indicate that the reworking in question must have occurred in a latetectonic stage.

An
Fig. 3 -Stereograms showing poles of the gneissic banding (a), shearing planes (b), with respective maximum concentration planes.

TABLE III Isotopic data obtained from selected zircon fractions of mesosome (OPU 1349) and leucosome (OPU 1348) samples.
: laboratory number; magnetic fractions: numbers in parentheses indicated the tilt used on FRANTZ separator at 1.5A current; # Pb radiogenic corrected for blank and initial Pb; U corrected for blank;* not corrected for blank or non-radiogenic Pb; total U and Pb concentrations corrected for analytical blank; ages given in Ma using Ludwig Isoplot/Ex program (1998), decay constants recommended bySteiger and Jäger (1977). SPU