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Precambrian Research 99 (2000) 1–32www.elsevier.com/locate/precamres

Tectono-thermal evolution, magma emplacement,and shear zone development in the Caruaru area

(Borborema Province, NE Brazil )

S.P. Neves a,*, A. Vauchez b, G. Feraud ca Departamento de Geologia, Universidade Federal de Pernambuco, Recife, Pernambuco 50740-530, Brazil

b Laboratoire de Tectonophysique, Universite des Sciences et Techniques de Languedoc, 34095, Montpellier Cedex 5, Francec Institut de Geodynamique, Universite de Nice, Parc Valrose, Nice, 06034, France

Received 30 November 1998; accepted 11 May 1999

Abstract

The Neoproterozoic Borborema Province (BP) in northeastern Brazil is characterized by a regional flat-lyingfoliation and by abundant magmatic rocks commonly spatially associated with large transcurrent shear zones.Combined field, micropetrographic, thermobarometric and magmatic fabric studies carried out on plutons, countryrocks, and shear zones in the eastern domain of the province (the Caruaru area) reveal that: (1) an early episode ofregional deformation, with a top-to-the-NE displacement, was followed by the development of conjugate strike–slipshear zones; (2) similar low-pressure (<600 MPa)/high-temperature (>650°C) metamorphic conditions occurredduring the two events; (3) plutons intruded the flat-lying foliation; (4) magma emplacement slightly predated strainlocalization in transcurrent shear zones; but (5) plutons underwent strike–slip deformation before complete crystalliza-tion. Therefore, regional deformation, pluton emplacement and shear zone development were successive eventsoccurring over a relatively short time span. 40Ar/39Ar laser dating of amphibole (584 Ma) and biotite (545–553 Ma)single grains supports slow cooling (#5°C/Ma) of country rocks through the argon closure temperatures of theseminerals. Complex, sometimes discordant, amphibole ages bracketed between 552 and 575 Ma for shear zones maybe attributed to episodic activity and/or dike swarm intrusions into them, which disturbed the Ar system. Biotite agesfor the shear zones (533–545 Ma) also imply a relatively low cooling rate. These data show that the massive injectionof magmas in the continental crust of the BP during the Brasiliano/Pan-African orogeny produced a long-livedthermal anomaly of regional extent. The 40Ar/39Ar ages in the Caruaru area are older than in other segments of theBP with similar geological characteristics. This indicates a tectono-thermal history for the BP that is more complexthan previously recognized, with either diachronous deformation, magmatism and associated low-P, high-T metamor-phism, or contrasting cooling of the different domains, possibly due to differential vertical movements. © 2000Published by Elsevier Science B.V. All rights reserved.

Keywords: Ar/Ar ages; Borborema Province; Magma emplacement; Neoproterozoic orogeny; Shear zones

* Corresponding author.E-mail address: [email protected] (S.P. Neves)

0301-9268/00/$ - see front matter © 2000 Published by Elsevier Science B.V. All rights reserved.PII: S0301-9268 ( 99 ) 00026-1

2 S.P. Neves et al. / Precambrian Research 99 (2000) 1–32

1. Introduction the plutons, and the P–T conditions duringregional and shear zone-related deformation.These data are combined with 40Ar/39Ar geochro-Granitic magmatism is the main mechanism

responsible for the internal differentiation of the nology in order to constrain the timing of mag-matic and tectonic events, to approximate thecontinental crust and may also contribute to conti-

nental growth if the magma contains a significant time–temperature evolution of the studied area,and to compare these results with the tectono-fraction of mantle component (e.g. Vielzeuf et al.,

1990; Miller and Wooden, 1994). Continental- thermal evolution of other domains in the BP.scale strike–slip faults, however, play a major rolein the reorganization of the continental lithosphere(e.g. Tapponnier et al., 1986; Daly, 1988). Thestudy of the relationships between these processes 2. Geological settingin the broader context of the tectonic evolution oforogenic belts is therefore a fundamental step to 2.1. The Borborema Provinceunderstand better how the continental crustevolves through time. The general geology of the BP is reviewed in

Caby et al. (1991), and shear zones and granitoidAbundant synkinematic plutons and largetranscurrent shear zones, which commonly occur magmatism are addressed, respectively, in recent

reviews by Vauchez et al. (1995), Sial (1993),in close spatial association, are prominent featuresof the Borborema Province (BP) of northeastern Neves and Mariano (1997), and Ferreira et al.

(1998). The BP consists of basement rocks ofBrazil, making it a key area to study the correlationbetween magmatic and tectonic events. The BP Paleoproterozoic age involving small Archean

nuclei (Pessoa et al., 1986; Hackspacher et al.,tectono-thermal evolution took place during theNeoproterozoic Brasiliano/Pan-African orogeny 1990; Legrand et al., 1991; Macedo et al., 1991;

Gaudette et al., 1993; Souza et al., 1993; Van(Almeida et al., 1981). It lies between the WestAfrican, Amazon and Sao Francisco/Congo Schmus et al., 1993, 1995; Dantas et al., 1998),

and volcano-sedimentary belts for which the agecratons, and has counterparts in the African conti-nent, especially the mobile belts of Togo–Ghana– of sedimentation and volcanism varies from the

Paleo- to the Neoproterozoic (Sa et al., 1991; BritoBenin, Nigeria and Cameron (Caby, 1989;Bertrand and Jardim de Sa, 1990; Caby et al., Neves et al., 1993; Van Schmus et al., 1995, 1996).

In addition to the Brasiliano orogeny, recent U–1991). The tectonic history of the BP thereforeplaces important constraints on the sequence of Pb zircon dating and whole-rock Sm–Nd data

have led some authors to postulate that a completeevents that culminated with the build-up of theWest Gondwana supercontinent. Wilson cycle (the so-called Cariris Velhos Cycle),

involving rifting at c. 1.1 Ga and final collision atIn this paper, we focus on the relationshipsbetween regional deformation, pluton emplace- c. 0.95 Ga, affected most of the BP south of the

Patos shear zone (Brito Neves et al., 1995; Vanment and shear-zone development in the Caruaruarea (State of Pernambuco), in the easternmost Schmus et al., 1995).

The BP is characterized by a low-angle foliationBP (Figs. 1 and 2). Field relationships betweenplutons and shear zones in this area are discussed of regional extent, observed in both supracrustal

and basement rocks (Caby et al., 1995), and by aby Neves and Vauchez (1995a) and the geochemis-try and petrology of plutons by Neves and Vauchez network of continental-scale, transcurrent ductile

shear zones (Vauchez et al., 1995). In the NW(1995b). In this paper, results of field, kinematic,microstructural, thermobarometric, and aniso- domain of the BP, the flat-lying foliation is associ-

ated with nappe emplacement towards the SSW,tropy of magnetic susceptibility (AMS) studies arepresented to compare the kinematics of country and with pressures up to 1 GPa reached during

prograde conditions, as in typical continental-stylerocks and shear zones, the internal magmatic fabricand the superimposed solid-state deformation in collision zones (Caby and Arthaud, 1986). In the

3S.P. Neves et al. / Precambrian Research 99 (2000) 1–32

Fig. 1. Main strike–slip shear zones and granitoid batholiths in the Borborema Province of Northeast Brazil. The location of thestudied area is also shown. SPSZ: Senador Pompeu shear zone; PASZ: Patos shear zone; CGSZ: Campina Grande shear zone; PESZ:Pernambuco shear zone.

Cachoeirinha/Salgueiro belt (Fig. 1), Al-in horn- with these two domains, pressure–temperatureconditions in the remainder of the province areblende barometry in synkinematic plutons and

mineral assemblages in their contact aureoles indi- typically in the range 550–750°C and 250–600 MPa (Lima, 1992; Vauchez and Egydio-Silva,cate that medium to high pressures (600–900 MPa)

were attained under temperatures typical of the 1992; Corsini et al., 1996).Major shear zones are NE-trending in the NWgreenschist facies (Sial et al., 1996). In contrast


Fig. 2. (a) Simplified geological map of the Caruaru area. FNSZ: Fazenda Nova shear zone; EPSZ: East Pernambuco shear zone;(b) lower hemisphere, equal area stereoplots of poles to foliation and stretching lineation from country rocks; (c) cross-sections alonglines shown in (a). Legend: 1: shear sense of main strike–slip faults; 2: shear sense of smaller strike–slip shear zones; 3: stretchinglineation in country rocks; 4: dip and strike of gneissic foliation in country rocks; 5: grey gneiss and migmatite; 6: schist and paragneiss;7: Taquaritinga orthogneiss; 8: banded metadiorite; 9: mylonite; 10: monzonite/gabbro; 11: coarse-grained granite; 12: porphyriticgranite/diorite; 13: syenite; 14: leucogranite; 15: other granitoids.

domain of the BP, whereas two dextral, Pernambuco shear zones systems, linked with nar-rower (typically #1 km) N- to NE-trending shearEW-trending, movement zones up to 30 km in

width, the Patos-Campina Grande and the zones, dominate in the eastern domain (Fig. 1).


Fig. 2. (continued)

Granitoid plutons are commonly spatially associ- 2.2. The Caruaru areaated with the shear zones, and synkinematic crys-tallization has been demonstrated for several Country rocks in the Caruaru area include

stromatitic migmatites, banded grey gneisses,plutons in the Serido (Archanjo et al., 1992, 1994)and Caruaru areas (Neves and Vauchez, 1995a). micaschists and paragneisses, and the Taquaritinga


orthogneiss, which represents the largest unit in consisting of a core of gabbros and gabbronoritesand a border of monzonites (Fig. 2). Xenoliths ofthe studied area (Fig. 2). The Taquaritinga

orthogneiss forms a high topography and is sepa- the metadiorite that marks the base of theTaquaritinga orthogneiss are occasionally foundrated from underlying micaschists and paragneisses

by a banded quartz metadiorite/tonalite (Fig. 2). in the Toritama pluton and in the Fazenda Novabatholith.The Taquaritinga orthogneiss has a dominantly

syenogranitic composition and contains large Two ductile strike–slip shear zones are observedin the studied area: the dextral, EW-trending, EastK-feldspar megacrysts up to 10 cm long in a matrix

of smaller K-feldspar, plagioclase, quartz, amphi- Pernambuco and the sinistral, NE-trending,Fazenda Nova shear zones (Fig. 2). The formerbole and biotite. Whereas the Taquaritinga

orthogneiss displays a penetrative high-temper- was previously considered as belonging to a con-tinuous, 700 km long lineament, but recent studiesature solid-state fabric, the metadiorite is less

strongly deformed and preserves a magmatic layer- (Vauchez and Egydio-Silva, 1992; Vauchez et al.,1995; Neves and Mariano, 1999) have shown thating defined by variations in color index and grain

size. Although apparently less deformed than the it is split into two branches separated by morethan 100 km. The eastern branch (the EPSZ)orthogneiss, which yielded a near concordant

zircon U–Pb age of 1521±7 Ma, the metadiorite extends westward over 200 km from the coast. TheFazenda Nova shear zone (FNSZ) usually followshas been dated at 1973±34 Ma (zircon U–Pb; Sa

et al., 1997), a result similar to other ages deter- the lithological boundary either between theFazenda Nova and Serra da Japeganga batholiths,mined for basement rocks elsewhere in the BP.

The 1.52 Ga age of the Taquaritinga orthogneiss or between the Toritama pluton and migmatiticcountry rocks. The EPSZ bounds the Serra dasuggests an anorogenic setting since no tectonic

event of this age is known in the BP. Japeganga batholith to the south. Smaller sinistralshear zones, parallel to the FNSZ, cross-cut theThe regional foliation usually trends NE–SW

and dips gently southeastward, except in the north- Serra da Japeganga batholith (Fig. 2). Tens ofcentimetres- to several metres wide transcurrenteastern portion of the Taquaritinga orthogneiss

where it dips gently to SW–WSW (Fig. 2). shear zones displaying both dextral and sinistralshear senses also cross-cut and clearly post-dateAlthough the Taquaritinga orthogneiss is strongly

deformed and may even display mylonitic fabrics, the flat-lying foliation of the Taquaritingaorthogneiss. Dike swarms are present within thestretching lineations are not common. Reliable

small-scale kinematic criteria are also scarce and two main shear zones. Individual dikes vary froma few centimetres to some hundreds metres insometimes indicate contradictory shear senses.

However, where observed, lineations are consis- width and exhibit a wide range of compositionsand deformation conditions, especially in thetently SW- to W-oriented (Fig. 2), and large-scale

asymmetrical structures (Fig. 3) indicate a dis- EPSZ.placement with top to east–northeast. Stretchinglineations are more common in paragneisses andgrey orthogneisses, where they are consistently 3. Plutons and dike swarmsNE–SW oriented (Fig. 2, and personal observa-tions north of the studied area). 3.1. Plutons

Three main comagmatic bodies occur in thestudy area: the Fazenda Nova and Serra da The Fazenda Nova batholith consists of an

association of coarsely porphyritic granites andJapeganga batholiths and the Toritama pluton(Fig. 2). They form a large granitoid massif diorites. A coarse-grained granite is the dominant

rock type in the Serra da Japeganga batholith.(#800 km2) intruded by late granitoids. To thenorthwest, the massif is intrusive into the The Toritama pluton is composed of quartz-bear-

ing syenites to monzonites that range in textureTaquaritinga orthogneiss, which is also intrudedby the Santa Cruz pluton, an inversely zoned stock from equigranular to porphyritic, the latter being


Fig. 3. Sigmoidal foliation in the mylonitic Taquaritinga orthogneiss indicating a northeastward displacement (NE to the right). Atthe bottom of the picture, a sill of undeformed to slightly deformed diorite intruding the flat-lying foliation of the orthogneisss maybe observed.

characterized by deep-brown K-feldspar meg- 1995a,b). Samples of the coarse-grained granite andporphyritic granite have overlapping compositionsacrysts mantled by oligoclase. In all rock types,

the main mineral phases are amphibole, biotite, and many major and trace elements display goodlinear correlations in Harker diagrams (Neves andK-feldspar, plagioclase, and quartz. Rare clinopyr-

oxene occurs as isolated grains and as cores in Vauchez, 1995b; Neves and Mariano, 1997), indicat-ing that these rocks are not only comagmatic butamphibole crystals in the diorites and is an essen-

tial phase in some samples of the Toritama pluton. also cogenetic. The contact of the equigranularsyenite with the diorite is also gradational, withSphene, magnetite, apatite, zircon, and allanite are

the accessory phases in the coarse-grained and monzonitic and monzodioritic facies forming atransition zone up to several kilometres wide,porphyritic granites and in the syenites; opaque

phases are scarce in the diorites. making it difficult to trace a limit between theToritama pluton and the Fazenda Nova batholith.Field relationships indicate that all rock types in

the granitoid massif have broadly the same age; A transition between the porphyritic syenite and theporphyritic granite is also observed: approachingtransitional facies between the different magma types

strongly support a comagmatic origin. Classical the Fazenda Nova batholith, the proportion ofquartz in the porphyritic syenite progressivelycomingling structures indicate that the porphyritic

granite and the diorites coexisted as contemporane- increases to the point that the rock looks like theporphyritic granite except that K-feldspar mega-ous melts (Fig. 4), and an original magmatic contact

has been preserved at the southwestern tip of the crysts remain mantled and deep brown. Syeniticdikes are locally observed intruding the porphyriticFazenda Nova batholith, where a gradual transition

from the porphyritic granite to the coarse-grained granite, but diffuse contacts are more common, andthe syenite is also locally back-veined by leucocraticgranite has been observed over distances up to

several hundred meters (Neves and Vauchez, dikes rooted in the porphyritic granite.


Fig. 4. Comingling structures at the boundary between the porphyritic granite and the diorite, indicating that they coexisted ascontemporaneous magmas.

The boundary of the Toritama pluton and the identical, within error, to the U/Pb zircon age ofthe Bom Jardim complex (592±7 Ma; GuimaraesFazenda Nova batholith with the country rock is

generally marked by a zone of partial melting, up and Silva Filho, 1998), an intrusion petrographi-cally and geochemically similar to the Toritamato several tens of metres wide, containing anatectic

leucogranites. Together with post-kinematic poiki- pluton occurring a few tens of kilometres east ofthe study area. A Rb–Sr isochron constructedlitic cordierite in micaschists, observed more than

1 km away from the northern contact of the using samples of the Bom Jardim complex andToritama pluton yielded an age of 585±35 MaFazenda Nova batholith, and the stability of

the original metamorphic assemblage away from (Guimaraes and Silva Filho, 1998), in agreementwith the U–Pb ages. Therefore, an age of approxi-the partial melting zone, this points to a high

degree of heat advection toward the country rock. mately 590 Ma is regarded as the best estimate forthe crystallization of the magmatic rocks in theThe contact of the massif with the country rocks

is usually discordant and locally concordant. Caruaru area.Where partial melting is less pronounced, it canclearly be seen that the gneissic foliation is cut by 3.2. Dike swarmsthe intrusions. The lack of quenched margins or acontact metamorphic aureole indicates that the Dike swarms in the FNSZ are mainly composed

of alternating decimetric to metric bands of dioriteshost rocks were at high temperatures during intru-sion, and thus that emplacement did not occur and leucogranites concordant with the foliation of

the mylonitic granitoids which they intrude.long after peak metamorphism.A zircon U/Pb age of 588±12 Ma was obtained Leucogranites are feldspar-rich and normally con-

tain less than 3 vol.% of either biotite or muscovite.by Guimaraes et al. (1998) for the porphyriticfacies of the Fazenda Nova batholith. This age is Hornblende in dioritic dikes shows local retrogres-


Fig. 5. Example of dike swarm in the EPSZ. Magmas of various composition have been emplaced in the mylonitic foliation. Theyusually display a well-developed magmatic foliation and may be free of solid-state deformation or slightly to moderately mylonitized.

sion to bluish green amphibole, epidote and alternating decimetric to metric bands of leuco-granite, porphyritic syenite and coarse-grainedsphene. The partial stability of amphibole and

biotite, and the presence of myrmekite and mech- monzonite, and equigranular diorite display evi-dence of several episodes of injection (Fig. 5).anical twinning in plagioclase indicate deformation

at medium-low amphibolite- to upper greenschist Porphyritic syenites and monzonites wereemplaced first as dikes up to several hundredfacies conditions.

Dikes are more abundant and compositionally metres wide that are cross-cut by dikes of dioriteand leucogranite. Leucogranites commonly intrudevaried in the EPSZ. Besides leucogranites and

diorites, the most common rock types are porphy- diorite, but dioritic dikes may also cross-cut leuco-granites. The mafic to intermediate rocks have aritic syenites to monzonites with large K-feldspar

megacrysts. Other petrographic types include distinctive greenish color due to extensive retro-gression of mafic minerals to fibrous amphibole,coarse-grained syenites and monzonites, scarcely

porphyritic monzonites to monzodiorites, and green biotite, epidote, sphene and sporadic chlo-rite. Additionally, feldspar crystals are frequentlyleuco-quartz diorites. Dike swarms consisting of


broken and partially sericitized and saussuritized. bution to the bulk susceptibility than do theiron-bearing silicates. Recent applications of theThese observations indicate deformation under

greenschist facies conditions. AMS technique to the study of magnetic fabricsin granitoids (e.g. Bouchez et al., 1990, 1997;Bouillin et al., 1993; Leblanc et al., 1994) haveshown that the K1>K2>K3 axes of the AMS4. Magmatic fabricsellipsoid have direct relationships to the magmaticfabric ( K1 is parallel to the magmatic lineation,4.1. Data acquisitionand K3 is perpendicular to the magmatic foliation),and that the shape of the magnetic anisotropyThe rocks in the Fazenda Nova/Serra da

Japeganga complex and in the Toritama pluton ellipsoid may be quantified using the Flinn parame-ter Pf=[((K1/K2)−1)/((K2/K3)−1)].display an usually well-defined magmatic foliation

marked by the planar disposition of tabular crys- The coarse-grained granite, porphyritic graniteand syenite are ferromagnetic (bulk susceptibilitytals of feldspar (Fig. 6), hornblende prisms and

biotite flakes. However, linear fabrics are rarely of the order of 10−3 SI ) whereas the diorite isparamagnetic (bulk susceptibility generally lessdiscernible in the field due to poor exposure of

foliation surfaces. A reconnaissance anisotropy of than 10−4 SI ). In ferromagnetic rocks, variationsin susceptibility and anisotropy magnitudes willmagnetic susceptibility (AMS) study was therefore

performed in order to complement the field data. depend on the magnetite content of the sampleand the scattering in size and shape of magnetiteFifty stations as regularly spaced as possible were

sampled for AMS measurements: 11 in the grains. All magnetite-bearing plutons studied todate in the BP, including those of the presentToritama pluton, 16 in the Serra da Japeganga

batholith SE of the FNSZ, and 23 in the Fazenda contribution, show a high variability of theseparameters, impeding any evaluation of strainNova batholith and Serra da Japeganga batholith

NW of the FNSZ. At each station, two oriented intensity. However, through detailed fabric studies,Archanjo et al. (1995) demonstrated that thisblock samples a few metres apart were collected

and drilled in the laboratory to obtain two cylin- variability does not affect the directional stabilityof the AMS ellipsoid and that magnetic and mag-ders, from which two specimens were cut. All

samples from the Serra da Japeganga batholith matic fabrics may be equated (Cruden andLauneau, 1994). In this paper, we assumed thatare from the coarse-grained granite, except for two

diorites. In the Fazenda Nova batholith, samples this also applies to the studied plutons because,although discrepancies between the magmatic foli-were taken from the dominant rock-type on the

outcrop, either porphyritic granite or diorite. All ation measured in the field and the magneticfoliation may be observed in some places (especi-samples from the Toritama pluton are from

syenites, either porphyritic or equigranular. In ally in the Toritama pluton), for most sites, theyagree quite well.collecting these samples, rocks exhibiting evidence

of solid-state deformation were avoided. Allsamples were analysed with a Kappabridge KLY2 4.3. Structure and magmatic fabric[see Bouchez et al. (1990) and Bouchez (1997) fora description of the methodology used ]. In the Toritama pluton, low- to moderate

inward dipping magmatic foliations are dominant(Fig. 7), and the magmatic lineation is mainly4.2. Significance of AMS fabricsNW-directed (Fig. 8). Most samples have oblate-type magnetic susceptibility ellipsoid shapesDepending on the susceptibility magnitude,

granitoid rocks may be divided into paramagnetic, (Pf<1; Fig. 8).The magmatic foliation in the Fazenda Novawhere iron is contained in paramagnetic minerals

(mostly biotite and hornblende), and ferromag- batholith and the northwestern portion of theSerra da Japeganga batholith is mainly southwestnetic, where magnetite gives a much higher contri-


Fig. 6. Magmatic foliation marked by the preferred orientation of tabular feldspars in the Toritama syenite. The pen is 15 cm long.

to southeast dipping (Fig. 9), implying that the vicinity of the subsidiary sinistral shear zones thatcross-cut the batholith; (2) ENE- to EW-trendingdiorites are geometrically below the porphyritic

granite, which, in turn, is overlain by the coarse- steeply dipping foliations dominate near the EPSZ;and (3) NW to NS-trending foliations prevailgrained granite (see cross-sections on Figs. 2 and

9). Close to the plutons’ boundaries, the magmatic away from the mylonitic belts. The magmaticlineation (Fig. 8) is rather dispersed, but a WNWfoliation is nearly parallel to the contact with the

country rock. It generally increases in dip and is to NW trend is dominant away from shear zones,and compares favourably with the orientationsprogressively rotated into parallelism with the

mylonitic fabric approaching the FNSZ. It also observed in the Toritama and Fazenda Nova plu-tons. Here, plane to prolate type AMS ellipsoidbecomes subparallel with the solid-state foliation

in the vicinity of the subsidiary shear zones that fabrics dominate close to the shear zones, whereasplane to oblate AMS type ellipsoid shapes aresplay off the southwest termination of the FNSZ.

The magmatic lineation (Fig. 8) consistently trends more common in the northeastern part of thebatholith (Fig. 8).parallel to the strike of the mylonitic belt near the

FNSZ, whereas away from the FNSZ, it is morescattered: NW-trending directions are morecommon in the diorite and in the southern portion 5. Shear zonesof the batholith, and locally, NE- to EW-trendinglineations are also observed. Most samples display 5.1. Geometry and kinematicsan oblate- to plane-type AMS ellipsoid (Fig. 8).

The magmatic foliation in the Serra da The FNSZ branches from the EPSZ and extendsabout 45 km northeastward to the northeasternJapeganga batholith SE of the FNSZ (Fig. 10)

shows considerable variation in both dip and end of the Toritama pluton (Fig. 2). The faultzone comprises a mylonitic belt up to 1 km widestrike, but: (1) strongly dipping, NE-trending,

magmatic foliations are more common in the in its central segment and narrows northeastward.


Fig. 7. Schematic map of the magmatic and magnetic foliations in the Toritama pluton and of the mylonitic foliation in the northeasttermination of the FNSZ.

Mylonites in the belt are mainly derived from strain zones are common. The heterogeneous flowand narrow width of the mylonitic belt in theigneous rocks. Schists and paragneisses on the

southwest termination of the shear zone preserve syenite may be related to the stiffer rheology ofthis quartz-poor lithology.a flat-lying foliation, and the migmatites on the

southeast side of the Toritama pluton show only The mylonitic foliation in the FNSZ dips steeplyto the vertical (Figs. 7 and 9), the stretchinglocal evidence of mylonitization. The strain inten-

sity decreases rapidly away from the mylonitic lineation plunges systematically 5–30° northeast-wards (Fig. 8), and kinematic indicators indicatebelt, such that within less than 1 km, the plutonic

wall rocks preserve their primary magmatic fabric. sinistral strike–slip. Assuming simple shear, theseobservations indicate sinistral oblique slip with aA strain gradient is particularly well recorded by

the coarse-grained granite on the southeastern side northwest-side-up dip–slip component during duc-tile flow. Dike swarms were intruded parallel toof the FNSZ where the undeformed (in the solid-

state) granite grades progressively into protomy- the mylonitic foliation in the central-northeasternportion of the FNSZ (Fig. 9). They are variablylonite and then into mylonite. Deformation is first

recorded by elongation of quartz grains, then by deformed, but generally display strong myloniticfabrics.spaced shear planes until a pervasive S/C fabric

dominates. The deformation is more heteroge- In contrast with the simple geometric patterndisplayed by the FNSZ, deformation in the EPSZneously partitioned in the Toritama pluton, where

large lenses up to 10 m long surrounded by higher was heterogeneously partitioned. Eastward, the


Fig. 8. Magmatic and magnetic lineations in the Toritama pluton and the Fazenda Nova/Serra da Japeganga complex. The stretchinglineation in the shear zones is also shown. The Flinn parameter [Pf=(K1/K2)−1/(K2/K3)−1] characterizes the shape of the AMSellipsoid.

mylonitic foliation trends N70° to N80°E and three main mylonitic belts (Fig. 8). The northernbranch is connected to the FNSZ and initiallycarries an eastward-plunging stretching lineation

(Figs. 8 and 10). The protoliths of the mylonites follows the lithological contact between the coarse-grained granite and its country rock. It continuesare dominantly rocks belonging to dike swarms or

large syenitic/monzonitic dikes, but the deforma- eastwards inside the Serra da Japeganga batholith,where it finally merges with a smaller sinistraltion also affected the coarse-grained granite of the

Serra da Japeganga batholith (Fig. 10). In this shear zone. The central branch is mainly concen-trated in the southernmost tip of the Serra dalatter, shear criteria consistently indicate dextral

sense (Fig. 11). Westward, the EPSZ is split into Japeganga pluton and in an EW-trending dike


Fig. 9. Schematic map of the magmatic and magnetic foliations in the Fazenda Nova batholith and northwestern Serra da Japegangabatholith, and of the mylonitic foliation in the FNSZ.

swarm. The southern branch cuts across ortho- 5.2. Mylonitic microstructures and quartz c-axisfabricsgneisses and metasedimentary screens (Fig. 12). A

low strain zone, where an original discordantcontact between the coarse-grained granite and the Mylonitic granitoids display an alignment of

feldspar crystals and microfractures filled by quartzcountry rock was preserved, is observed betweenthe northern and central branches. The mylonitic and feldspar in feldspar porphyroclasts, a texture

indicative of rupture due to stress concentrationfoliation in these belts strikes approximately EWand dips >70° (Fig. 13), the stretching lineation at contacts between grains during the submagmatic

stage (Bouchez et al., 1992). These observationsis generally eastward-plunging (Fig. 8), and shearcriteria indicate dextral shear (Fig. 12). At the support a synkinematic transition from magmatic

to solid-state deformation in the shear zonesjunction between the E-trending and the ENE-trending segments, dike swarms display a distinct (Neves and Vauchez, 1995a). High-temperature,

solid-state deformation in the FNSZ is indicatedNE-orientation, suggesting emplacement in anextensional domain related to a fault stepover zone by the stability of magmatic hornblende and biotite

and by evidence of diffusion-assisted deformation(Fig. 13).


Fig. 10. Schematic map of the magmatic and magnetic foliations in the central and eastern portion of the Serra da Japegangabatholith, and of the mylonitic foliation in shear zones.

processes, for instance, partial recrystallization of features may also be observed in samples of thecoarse-grained granite in the EPSZ. Initial defor-feldspars through formation of myrmekite, and

interpenetrating quartz grain boundaries. These mation in this shear zone under amphibolite facies


Fig. 11. Coarse-grained granite mylonitized in the EPSZ. The mylonite is characterized by the development of quartz ribbons andquartz tails over K-feldspar. Extensional shear-bands oblique on the vertical foliation, together with asymmetric pressure-shadows,indicate dextral shearing. The pencil is 10 cm long.

Fig. 12. Mylonitic to ultra-mylonitic metasediments from the EPSZ. Synkinematic sillimanite indicates initial mylonitization underhigh T conditions. The shear sense is dextral. The length of the pen is 15 cm.


Fig. 13. Schematic map showing a step-over between two segments of the EPSZ and associated dike swarms. 1: trace of myloniticfoliation; 2: dike swarms; 3: porphyritic syenite/monzonite; 4: granitoids; 5: attitude of mylonitic foliation. C: Caruaru town.

conditions is also attested by crystallization of basal- a� slip systems are observed in somesamples, the dominance of high-temperature fab-synkinematic sillimanite in mylonitic garnet-

bearing biotite schists (Fig. 12). However, retro- rics indicates that crystal-plastic flow in the FNSZmostly ended at temperatures above 500°C. Thegression of mafic minerals to a greenschist facies

mineralogy in the magmatic rocks (fibrous amphi- lower last three stereographic plots of Fig. 14illustrate, from left to right, the progressive domi-bole, green biotite, epidote, sphene±chlorite), and

the microstructure of mylonitic metasediments nance of the basal- a� slip system in samples fromthe EPSZ, suggesting that deformation continued(Fig. 12) indicate that deformation continued

under decreasing temperature conditions. In dike during cooling in this shear zone.swarms, there is no evidence of any previous high-temperature fabrics, suggesting that they intrudedrelatively low-temperature country rocks. 6. Preliminary thermobarometric estimates

Quartz in mylonitic granitoids has recrystallizedto equidimensional grains with straight to serrated 6.1. Temperature estimates during regional

deformationgrain boundaries, suggesting late- to postkinematicannealing. Quartz c-axis orientations were mea-sured for five samples from the FNSZ and three The most common metamorphic mineral assem-

blage in metapelites and paragneisses in the Caru-from the EPSZ (Fig. 14). The samples show asym-metric girdle fabrics indicative of non-coaxial flow aru area is quartz+plagioclase+K-feldspar+

biotite+garnet±muscovite. Synkinematic silli-(e.g. Bouchez et al., 1983) and are consistent withthe sinistral and dextral shear senses inferred from manite is rare. Cordierite is more common and

may form either large poikiloblasts, with inclusionsmesoscopic and microstructural observations.However, in some samples, small circle distribu- of biotite-quartz±muscovite±garnet, or elongate

grains parallel to the foliation. Because cordieritetions of c-axes around the pole to foliation mayindicate a component of coaxial flattening in addi- does not commonly coexist with muscovite but

may include grains of this mineral, it was probablytion to the dominant non-coaxial shear.Maximum concentration of c-axes close to the formed by the reaction muscovite+Mg-biotite+

Quartz=Mg-cordierite+K-feldspar+H2O, whichY structural axes for samples from the FNSZindicates activation of the high-temperature implies temperatures in excess of 600°C for pres-

sures greater than 250 MPa (Seifert, 1976). Highprismatic- a� glide system (e.g. Nicolas andPoirier, 1976). Although fabrics suggesting activa- temperatures during regional metamorphism are

also implied by local anatexis of metasedimentstion of the lower temperature rhombohedral- and


Fig. 14. Lower-hemisphere, equal-area stereographic projections of quartz c-axes fabrics from selected samples of the FNSZ andEPSZ. The numbers at the top and bottom right are contour intervals in per cent and maximum density, respectively. Two hundredpoints were measured for each plot except for TQ-356 where N=167.

and by the coexistence of garnet and amphibole during regional metamorphism was less than 500–600 MPa (Mukhopadhyay and Hodaway, 1994,in grey gneisses and in the Taquaritinga

orthogneiss. However, the presence of cordierite and references therein). Retrograde mineral assem-blages are restricted, typically consisting ofand sillimanite, together with the absence of kya-

nite, implies that the maximum pressure attained chlorite+quartz partly replacing garnet and/or


biotite in metapelites, and epidote+titanite grow- however, differ by only ±12°C from each other,with a mean value of 668°C. These data, in con-ing after amphibole in orthogneisses.

T estimates during regional metamorphism were junction with the observed mineral assemblages,indicate that country rocks in the Caruaruobtained on amphibole–plagioclase pairs from the

Taquaritinga orthogneiss and metadiorite using area suffered a high-temperature–low-pressuremetamorphism.the edenite–richterite thermometer of Holland and

Blundy (1994), a modified version of the Blundyand Holland (1990) thermometer (Table 1, 6.2. P–T conditions during strike–slip deformationFig. 15). This thermometer was used because itprovided more consistent results than the edenite– Mylonitic sillimanite schists in the EPSZ con-

taining both garnet and plagioclase allowed deter-tremolite thermometer. Temperatures calculatedfrom five amphibole–plagioclase pairs from the mination of pressure and temperature conditions

during strike–slip shearing. Representative micro-diorite (among which three are shown in Table 1)span the range 630–760°C (mean value=705°C), probe analyses, together with pressure and temper-

ature estimates, are given in Table 2. Garnets inwhich, together with the wide compositional varia-tion of the amphiboles, indicates that equilibrium the analysed sample have a very narrow composi-

tional range (XFe=0.32–0.33, XMg=0.03–0.04,was not completely achieved during deformation.Temperature estimates from five amphibole– XMn=0.04–0.06, XCa=0.01) and display no notice-

able chemical zoning. Plagioclase compositions areplagioclase pairs of the Taquaritinga orthogneiss,

Table 1Representative mineral analyses used for temperature estimates in country rocks of the Caruaru areaa

Orthogneiss Metadiorite

Plagioclase Amphibole Plagioclase Amphibole Plagioclase Amphibole Plagioclase Amphibole

SiO2 64.51 37.85 60.24 42.88 61.12 40.13 61.17 38.34TiO2 0 0.77 0 1.00 0 1.23 0 1.11Al2O3 22.23 10.93 23.85 9.41 24.00 12.00 24.14 11.67FeO 0.02 31.50 0 20.50 0.05 21.56 0.09 21.43MnO 0 0.66 0 0.45 0 0.57 0 0.51MgO 0 0.58 0 7.60 0 6.47 0 7.24CaO 3.50 10.97 5.45 11.91 5.43 11.74 5.94 11.85Na2O 9.70 1.35 8.40 1.13 8.91 1.37 8.26 1.35K2O 0.13 1.94 0.13 1.17 0.15 1.92 0.14 1.88Total 100.09 96.56 98.07 96.11 99.65 97.00 99.73 95.38Si 11.99 6.16 11.43 6.60 11.46 6.19 11.41 5.99Ti 0 0.09 0 0.11 0 0.14 0 0.13Al IV 4.87 1.84 5.34 1.40 5.31 1.81 5.31 2.01AlVI – 0.26 – 0.30 – 0.37 – 0.14Fe 0.00 4.29 0 2.64 0.01 2.78 0.01 2.80Mn 0.00 0.09 0 0.06 0 0.08 0 0.06Mg 0.00 0.14 0 1.74 0 1.49 0 1.52Ca 0.70 1.91 1.11 1.96 1.09 1.94 1.19 2.00Na 3.49 0.21 3.09 0.17 3.24 0.20 2.99 0.18K 0.03 0.42 0.03 0.23 0.04 0.38 0.03 0.40Total 21.09 15.64 21.00 15.40 21.14 15.58 20.94 15.58Xab 0.83 0.73 0.74 0.71Xan 0.17 0.26 0.25 0.28T (BH) 668°C (5) 632°C 668°C 757°C

a Temperature estimates using the edenite–richterite thermometer (Holland and Blundy, 1994) assuming a mean pressure value of500 MPa. The values in parentheses indicate the number of temperature determinations in the orthogneiss.


Fig. 15. Map showing samples location and results of thermobarometric calculations and 40Ar/39Ar geochronological dating. A andB are amphibole and biotite Ar ages, respectively (in Ma).


Table 2Representative mineral analyses and P–T estimates in shear zonesa

EPESZ FNSZ

Sample TQ-570 TQ-485 TQ-57 TQ-52B

Garnet Biotite Plag Amph Plag Amph Plag Amph Plag

SiO2 36.68 34.09 60.50 39.45 64.19 39.40 64.37 44.55 61.96TiO2 0 2.31 0 1.75 0 0.91 0 0.98 0Al2O3 20.93 18.73 24.73 9.68 22.37 9.79 22.08 7.75 25.94FeO 32.68 21.08 0.01 24.72 0.03 28.05 0.04 17.89 0.08MnO 5.57 0.18 0 0.52 0 0.55 – 0.37 0MgO 3.16 7.75 – 5.31 0 3.89 0 11.01 0CaO 1.33 0 6.45 11.38 4.03 11.31 3.61 11.78 5.29Na2O – 0.07 7.91 1.64 8.93 1.44 9.48 1.30 8.32K2O – 10.60 0.24 1.73 0.08 1.85 0.12 0.99 0.10Total 100.34 94.76 99.83 96.18 99.63 97.17 99.71 96.69 101.69Si 2.96 5.32 11.27 6.23 11.92 6.23 11.99 6.68 11.29Ti 0 0.27 0 0.21 0 0.11 0 0.11 0Al IV 0.04 2.68 5.43 1.77 4.90 1.77 4.85 1.32 5.57AlVI 1.94 0.76 – 0.03 – 0.05 – 0.05 –Fe 2.20 2.75 0 3.26 0 3.71 0.01 1.40 0.01Mn 0.38 0.02 0 0.07 0 0.07 0 0.05 0Mg 0.38 1.80 0 1.25 0 0.92 0 2.46 0Ca 0.11 0 1.29 1.92 0.80 1.92 0.72 1.89 1.03Na – 0.02 2.85 0.50 3.21 0.22 3.43 0.34 2.94K – 2.11 0.06 0.35 0.02 0.37 0.03 0.19 0.02Total 8.01 15.75 20.90 15.60 20.85 15.59 21.02 15.38 20.86XMg 0.15 0.40 0.35 0.20 0.64Xalm 0.71Xpy 0.13Xsps 0.13Xgrs 0.02Xab – 0.68 0.82Xan – 0.31 0.17T (GS) 706°C (5)T (IM) 696°C (5)T (HB) 733°C (5) 715°C (5) 686°C (6)T (Thermo) 672°C (5)P (Thermo) 574 MPa (5)

a GS: Ganguly and Saxena (1984); IM: Indares and Martignole (1985); HB: Holland and Blundy (1994); Thermo: THERMOCALC(Powell and Holland, 1988; Holland and Powell, 1990). The values in parentheses indicate the number of determinations realized inP–T estimates for each sample. The temperature calculated using HB assumed a mean pressure value of 500 MPa.

also quite uniform (An30–31). Biotite is relatively were used for the temperature estimates. The con-cordance between the two thermometers allows usAl- and Ti-rich (#19 and 3 wt.%, respectively)

and has Mg/(Mg+Fe) ratios in the range 0.31– to infer peak metamorphic temperatures of 700°C(Table 2, Fig. 15). Pressure and temperature esti-0.43, with lower values typical of grains included

in garnet. mates using the THERMOCALC program(Powell and Holland, 1988; Holland and Powell,Due to the relatively Mn-rich nature of garnet

(Xsps=0.09–0.13), the calibrations of Ganguly and 1990) yielded mean values of 575 MPa and 670°C,respectively. The temperature estimates by thisSaxena (1984) and Indares and Martignole (1985)


method are slightly lower than those obtained by the brittle–ductile transition, in disagreement withfield evidence indicating emplacement of thethe above calibrations, but, taking the uncertainties

into account, all results agree quite well. Toritama pluton in a ductile crust. This may haveresulted from a chemical composition differing tooThe edenite–richterite thermometer of Holland

and Blundy (1994) was also used to estimate much from those used in the experimental calibra-tion of the geobarometer. Another probable sourcetemperatures for two samples of mylonitic granit-

oid rocks from the FNSZ, where conventional of error is that generally, the amphiboles showsome degree of subsolidus alteration, suggestinggeothermobarometric techniques cannot be used

due to the absence of mylonites with metapelitic interaction with late magmatic fluids which mayhave disturbed an original melt/amphibole equilib-protoliths and for one sample of the coarse-grained

granite in the EPSZ. The EPSZ sample gave a rium composition. Therefore, we consider that apressure of 500±100 MPa probably better reflectsmean value of 733°C (Table 2, Fig. 15), slightly

higher than the temperatures calculated by the the pressure of emplacement for the plutons in thestudied area, which is similar to the pressuregarnet–biotite thermometer but still in conformity

with them, taking into account the uncertainty of estimated during strike–slip shearing and consis-tent with the presence of cordierite in the country±40°C of the amphibole–plagioclase thermometer

(Holland and Blundy, 1994). In two samples from rocks.the FNSZ, the edenite–richterite thermometeryielded mean values of 686 and 715°C (Table 2,Fig. 15). In all samples, the temperatures calcu- 7. 40Ar/39Ar geochronologylated from the different plagioclase–amphibolepairs vary within 10–35°C, suggesting attainment 7.1. Samples and experimental procedureof chemical equilibrium.

The temperature estimates during strike–slip A 40Ar/39Ar study on amphibole and biotitewas performed to constrain the chronology ofdeformation are reasonable if we consider that

solid-state deformation of magmatic rocks in the magmatic and tectonic events and gain insights onthe thermal history in the Caruaru area. Fiveshear zones started at near solidus conditions, as

discussed in the preceding section, and indicate samples were used for 40Ar/39Ar dating (Figs. 15and 16): two from the country rocks (TQ-4A andthat high temperatures were maintained after

regional deformation. TQ-25), one from an undeformed syenite(TQ-321), and two from granitoids in the FNSZand EPSZ (TQ-57 and TQ-485, respectively).6.3. Depth of pluton emplacement

Samples TQ-4A and TQ-25 are, respectively,from the Taquaritinga orthogneiss and the metadi-The depth of emplacement of the Fazenda

Nova/Serra da Japeganga complex was estimated orite at its base. Amphibole in the first is anunusually low-Mg (<1 wt.%) hastingsite contain-from pressures yielded by the Al-in-hornblende

geobarometer of Schmidt (1992) (Table 3, ing 1.9 to 2.3 wt.% K2O, whereas amphibole com-position in the latter varies from magnesianFig. 15). The calculated pressures range from 370

to 590 MPa, corresponding to depths between 11 hastingsite to hornblende, with K2O contentsbetween 1.2 and 2.0 wt.%.and 17 km. Guimaraes’s data from the Toritama

pluton (Guimaraes, 1989) recalculated using TQ-321 is a porphyritic quartz-syenite from theToritama pluton. This sample was chosen forSchmidt’s equation and a new microprobe analysis

show a pressure range of 200–290 MPa. Although analyses because, in contrast with most othersyenitic samples, the amphibole (magnesio-horn-the Toritama syenite contains the critical mineral

assemblage required for the application of the blende, K2O: 0.6–1.0 wt.%) does not contain clino-pyroxene cores and appears to be a primary, earlyAl-in-amphibole barometry, these are unrealisti-

cally low pressures. Even considering a high geo- magmatic phase, as suggested by its euhedral tosubhedral shape.thermal gradient, they correspond to depths above


Table 3Representative mineral analyses of amphibole rims used for pressure estimates in magmatic rocks of the Caruaru areaa

Sample Fazenda Nova Serra da Japeganga complex Toritama pluton

TQ-131b TQ-59b TQ-177b TQ-337c TQ-294c TQ-321c TO-2d TO-6d

SiO2 45.76 42.86 40.30 40.20 47.61 46.14 47.19 46.77TiO2 0.88 0.56 2.05 1.59 0.58 1.19 0.92 1.42Al2O3 8.79 10.10 9.74 8.79 6.64 6.68 6.20 7.12FeO 16.38 19.57 26.83 29.12 11.99 14.36 13.21 14.34MnO 0.23 0.38 0.49 0.60 0.22 0.26 0.32 0.00MgO 11.88 9.55 3.65 2.81 13.65 12.81 14.11 13.33CaO 11.90 11.96 10.96 10.93 12.27 11.67 11.40 11.41Na2O 1.36 1.22 1.67 1.69 1.37 1.64 1.97 1.67K2O 0.88 0.91 1.33 1.45 0.76 0.86 0.69 0.89Total 97.85 97.10 97.03 97.19 95.19 95.68 96.58 96.98Si 6.69 6.74 6.17 6.40 7.06 6.88 6.97 6.92Ti 0.16 0.10 0.18 0.19 0.06 0.13 0.10 0.16Al IV 1.31 1.26 1.83 1.60 0.94 1.11 1.03 1.08AlVI 0.14 0.40 0.04 0.05 0.22 0.06 0.05 0.16Fe 1.91 2.31 3.55 3.88 1.48 1.79 1.70 1.73Mn 0.02 0.10 0.08 0.08 0.03 0.03 0.04 0.00Mg 2.62 2.34 0.86 0.67 3.02 2.85 3.11 2.94Ca 1.87 1.92 1.87 1.87 1.95 1.87 1.80 1.81Na 0.09 0.33 0.43 0.52 0.39 0.48 0.50 0.45K 0.13 0.17 0.27 0.29 0.14 0.16 0.20 0.19Total 15.27 15.66 15.29 15.56 15.31 15.38 15.50 15.44P (MPa) 374 (7) 522 (4) 587 (5) 486 (7) 251 (8) 260 (3) 200 (3) 291 (3)

a Pressure estimates were obtained using the Al-in-amphibole barometer of Schmidt (1992). Values in parentheses indicates thenumber of analyses for each sample.b Original data set from Neves and Vauchez (1995b).c This study.d Original data set from Guimaraes (1989).

The protolith of samples TQ-57 and TQ-485 is trometer. The criteria used to define a plateau agewere: (1) the plateau region of the age spectrumthe coarse-grained granite from the Serra da

Japeganga batholith. TQ-57 is an S/C mylonite should include at least 70% of the 39Ar released;(2) there should be at least three steps in thefrom the FNSZ, and TQ-485 is a protomylonite

at the margin of the EPSZ. In both cases, large plateau; and (3) the individual fraction ages shouldagree within 2 s with the ‘integrated’ age of theamphibole grains (1–3 mm in length) appear to be

primary, whereas smaller grains (0.1–1 mm) prob- plateau segment.ably resulted from recrystallization during strike–slip deformation. Amphibole compositions plotbetween the fields for hastingsite, hastingsitic horn- 7.2. Resultsblende, magnesian hastingsite and magnesian has-tingsitic hornblende ( K2O in the order of Age spectra for amphibole and biotite grains

together with 37ArCa/39ArK ratios for amphibole1.7 wt.%).Samples were irradiated in the McMaster grains versus cumulative percentage 39Ar released

are presented in Fig. 16. Careful petrographicReactor (Hamilton, Canada), and step heatingexperiments were performed at the Universite de examination when selecting the single grains was

carried out in order to avoid grains showing anyNice (France) with a Coherent Innova 70-4 con-tinuous argon-ion laser and a VG3600 mass spec- signs of alteration and/or presence of inclusions.


(e)

(d)

(a)

(c)

(b)


In spite of this, the results show that some analysedgrains were affected by subsolidus alteration.

The apparent age spectra from the two analysedamphibole grains from sample TQ-4A are stronglydiscordant. Anomalously low 37ArCa/39ArK ratiosin the low temperature and in the last steps indicatethe presence of a potassium-rich contaminantphase, which was not detected through petro-graphic examination. One of the three analysedamphibole grains from sample TQ-25 also dis-played a very disturbed age spectrum. However, Fig. 17. Temperature–time diagram showing inferred coolingone grain gave a plateau age of 584.2±3.7 Ma for paths for country rocks (solid line) and shear zones (dotted line)

in the Caruaru area. See text for discussion.99% of the total 39Ar released, and another graindisplayed similar ages, within error bars, for morethan 70% of the total 39Ar released. The similar batholith do not necessarily indicate rapid cooling

following magma emplacement.ages obtained on these two grains indicate thatregional cooling to around 550–500°C (the Among the three analysed amphibole grains

from sample TQ-57, one grain displayed a veryaccepted range of closure temperature for Ardiffusion in amphibole; Harrison, 1981) occurred flat age spectrum, but not a plateau age, for 97%

of the 39Ar released, with a weighted mean age ofat about 584 Ma. The analysed biotite grains fromsamples TQ-4A and TQ-25 displayed plateau ages 552.8±1.1 Ma, whereas the other two grains dis-

played disturbed spectra. In the less disturbed one,of, respectively, 553.4±1.1 and 545.3±1.1 Ma formore than 85% of the 39Ar released. Considering a weighted mean age of 574.0±1.1 Ma was

obtained for 80% of the 39Ar released. In the other,the very flat biotite age spectrum for sample TQ-25,the 545 Ma age is considered as a better estimate a mean age of 575.3±1.1 Ma (30% 39Ar released)

calculated for the intermediate steps may be inter-of the time through which regional temperaturesfell to around 300°C (the approximate closure preted as a maximum estimate of the argon closure

age of this amphibole. Disturbed age spectra weretemperature for Ar diffusion in biotite; Harrisonet al., 1985). The amphibole and biotite ages for also obtained for the two amphibole grains from

sample TQ-485. One yielded an age between 555the metadioritic sample therefore indicate slowcooling, in the order of 5°C/Ma, through the and 576 Ma for the higher temperature steps,

whereas the other displayed an apparent saddle-amphibole–biotite closure temperature interval(Fig. 17). shaped age spectra, with an age of 577 Ma for the

lowest step in the saddle. From the two analysedAmphibole TQ-321 displayed a saddle-shapedage spectrum, indicating that it was probably biotite grains from sample TQ-57, one grain dis-

played a plateau age of 533.0±1.1 Ma for moreaffected by excess argon. A mean age of589.5±1.2 Ma (39% of the 39Ar released) calcu- than 85% of the 39Ar released. The other grain

displayed a disturbed age spectrum but yielded alated for the concordant apparent ages of thelowest part of the saddle represents a maximum weighted mean age of 543.8±1.1 Ma (amounting

to 50% of the 39Ar released) for the higher temper-estimate of the argon closure age of this amphibole.Therefore, the similar Ar age of the Toritama ature steps except the last two. Sample TQ-485

did not give a plateau according to our criteriapluton and the U/Pb zircon age (588±12 Ma;Guimaraes et al., 1998) of the Fazenda Nova (probably because of slight chloritization and

Fig. 16. 40Ar/39Ar age spectra for amphibole and biotite from samples of country rocks (a: TQ-25, metadiorite; b: TQ-4A, Taquaritingaorthogneiss), Toritama pluton (c: TQ-321), and shear zones (d: TQ-57, FNSZ; e: TQ-485, EPSZ) of the Caruaru area.37ArCa/39ArK plots for amphiboles are also shown. (P): plateau age.


39Ar recoil during irradiation) but displayed a flat able to determine whether this event resulted fromthrusting or extension. Metamorphic mineralage spectrum for 99% of the 39Ar released, with a

weighted mean age of 545±1.1 Ma, which is con- assemblages and geothermometric estimates indi-cate that the accompanying metamorphismsidered a reliable age.

An explanation that can account for the com- occurred under low-P (<500–600 MPa), high-Tconditions (>600–650°C). The lower strainplexity of the 40Ar/39Ar data for samples TQ-57

and TQ-485 involves the episodic injection of dike recorded in the metadiorite relative to theTaquaritinga orthogneiss can be attributed toswarms in the shear zones. The youngest ages are

interpreted as having been obtained in amphibole strain partitioning, with localization of the defor-mation in the quartz-rich, and thus a weaker,and biotite neoblasts that remained open to argon

re-equilibration during the last increments of overlying unit. There is no evidence of deformationduring cooling to temperatures corresponding todeformation in the shear zones. However, the older

ages might correspond to the time when argon the greenschist facies. Therefore, the thermochro-nologic data represent only post-kinematic cooling,would definitely be trapped in the core of large

amphibole and biotite grains. Rapid temperature implying that regional tectonic activity had ceasedby 580–590 Ma.increases caused by the intrusion of dike swarms

could lead to incomplete argon diffusive loss in Shear zone development and magma emplace-ment cannot have been delayed much relative tothese grains, thus producing the disturbed spectra.

Fig. 17 shows the inferred cooling path resulting the regional deformation. The lack of quenchtextures at the margins of plutons and partialfrom this process.melting in pluton’s aureoles suggest that magmaemplacement occurred while country rocks werestill at a high temperature. A similar conclusion8. Discussionholds for shear zone development because temper-ature estimates are close during regional and8.1. Tectonic and magmatic evolution of the

Caruaru area strike–slip deformation (Fig. 15). Pluton emplace-ment may have slightly predated shear zone devel-opment, as suggested by the following lines ofThe combination of field, AMS, micropetrofa-

bric, thermobarometric and geochronologic studies evidence (Neves and Vauchez, 1995a; Neves et al.,1996): (1) truncation of internal magmatic con-indicates that regional deformation and metamor-

phism, pluton emplacement, and shear zone devel- tacts by the FNSZ; (2) absence of mylonitic fabricsin xenoliths in the coarse-grained granite close toopment in the Caruaru area were broadly coeval,

implying that tectonism and magmatism in this the EPSZ, suggesting they were incorporated intothe granitic magma before shear zone initiation;area are Brasiliano age events. Neither polymeta-

morphic mineral assemblages nor overprinting (3) dominance of low- to moderate-dipping mag-matic foliations in the Toritama and Fazendarelationships have been identified, which could

suggest that the Caruaru area was affected by a Nova plutons and NW–SE to NS-striking folia-tions in the central and eastern part of the Serraprevious orogenic event of late Mesoproterozoic

to early Neoproterozoic age, as suggested for other da Japeganga batholith; (4) preservation in theFazenda Nova/Serra da Japeganga complex of andomains of the BP south of the Patos shear zone

(Brito Neves et al., 1995; Van Schmus et al., 1995). early magmatic stratigraphy which is related tothe pattern of the magmatic foliation; and (5) theThe regional-scale consistency in lineation trend

in the Taquaritinga orthogneiss, in schists and rather small volume of country rock deformed inthe FNSZ and EPSZ, which is more consistentparagneisses, and in grey gneisses (Fig. 2), together

with rare but unambiguous shear criteria (Fig. 3), with strain localization in the magmatic bodiesthan with the almost complete obliteration ofindicates that country rocks were deformed by

non-coaxial shear, with a top-to-the-NE sense of preexisting shear zones.Emplacement of dike swarms followed plutons’displacement. However, not enough data are avail-


crystallization and solid-state motion of the shear emplacement or only during the transcurrentregime. In the first possibility, magmatic lineationszones at high temperature. Dike swarms in the

FNSZ preserve medium- to high-temperature oriented NW–SE could be attributed to NW–SEextension of the magma body. In the second case,solid-state microstructures, suggesting that their

emplacement occurred before dike swarms in the magmatic lineations parallel to the mylonitic fabricclose to shear zones combined with dispersed butEPSZ. The latter show extensive retrogression of

magmatic minerals to a greenschist facies mineral- dominantly NW–SE directed magmatic lineationsaway from them (Fig. 15) could be linked to theogy, indicating that emplacement and deformation

mostly occurred after cooling to the greenschist development of a magmatic positive flowerstructure.facies.

The chronology of events in the Caruaru area The younger Ar amphibole ages in the shearzones relative to the country rocks is a consequencemay thus be explained by a tectonic model involv-

ing an early episode of non-coaxial shear, with a of the long time that large intrusions take to cool(the Ar amphibole age of 589.5±1.2 Ma obtainedtop-to-the-NE sense of displacement, followed by

pluton emplacement, then by shear zone nucleation for the Toritama pluton represents a maximumestimate and therefore does not necessarily implyand development, and, finally, by dike swarm

emplacement into shear zones. The large volume rapid cooling of the plutons, as seen in the preced-ing section). For example, assuming that the plu-of magmatic rocks implies that at least local sig-

nificant extension must have occurred to create tons were still at a high temperature when thecountry rocks cooled to around 500°C at 584 Ma,room for their emplacement. Given the proximity

of the U/Pb zircon age of the Fazenda Nova a time span of 8–14 Ma is necessary to reachthermal equilibrium if initial temperatures of,batholith (588 Ma; Guimaraes et al., 1998) and

the Ar amphibole age of the country rocks respectively, 600 and 650°C are considered [calcu-lated using the Peacock (1990) package and assum-(584 Ma), and the evidence for intrusion at high

temperature, it may be deduced that the country ing a pluton thickness of 5 km]. These crudeestimates agree well with the maximum Ar amphi-rocks experienced a period of relatively fast cooling

after magma emplacement (Fig. 17). This suggests bole ages (#575 Ma) obtained in the shear zones.The slightly higher pressure calculated for thethe occurrence of an extensional phase of a

regional, rather than a local, extent, possibly EPSZ (575 MPa) when compared with the esti-mated pressure of regional metamorphism (<500–reflecting the fluctuating regional strain field during

the transition from the flat-lying foliation-forming 600 MPa) suggests that development of the shearzones occurred in a contractional setting. Theevent to the transcurrent regime. Afterwards, heat

conduction from the plutons, and also possibly opposed shear senses of the EPSZ and FNSZindicate that they are conjugate shear zones.from intrusions at depth, gave away to a longer

period in which regional temperatures dropped Because, for a ductile material, the direction ofbulk shortening bisects the obtuse angle formedslowly. It is only for this latter period that the

cooling rate of 5°C/Ma deduced from the Ar by the shear zones (Ramsey and Huber, 1987), anapproximately NW–SE orientation of the bulkamphibole and biotite ages is valid (Fig. 17). As

the cooling rate deduced for the 500–300°C shortening direction may be inferred in the presentcase (Neves and Mariano, 1999).interval cannot be extrapolated to high temper-

atures, it places no constraints on the time of thepeak of metamorphism. 8.2. Implications

The transcurrent regime that followed the devel-opment of the regional foliation was strongly The comparison of the tectonic, magmatic and

thermal evolution of the Caruaru area with otherinfluenced by pluton emplacement, with strainlocalizing into the granitoids or the hotter regions segments of the BP and equivalent terrains in

western Africa places important constraints on theclose to them. Development of the magmatic fabricin the plutons might have started either during geodynamic evolution of the BP during the


Brasiliano orogeny. The Caruaru area, in the east- also suggest that they experienced synchronousstrike–slip deformation. However, development ofern domain of the BP, displays both similarities

and differences with the northeastern (Serido the flat-lying foliation in the eastern domainoccurred under low-P, high-T metamorphic condi-region) and northwestern (north of the Senador

Pompeu shear zone) domains of the BP (see tions, and the younger 40Ar/39Ar biotite ages (552–533 Ma) in the Caruaru area show that a largeFig. 1). These three domains have been correlated,

respectively, with the Cameroon province, the period of time was necessary for denudation ofthe eastern domain to occur.Nigerian segment of the Trans-Saharan belt, and

the external part of the Pan-African belt east of Low-P, high-T metamorphic conditions arerecorded in the northeastern domain (Lima, 1992;the Pan-African suture in Togo, Ghana and Benin

(Caby, 1989; Caby et al., 1991), and Trompette Corsini et al., 1996; this study), and slow coolingrates (3–4°C/Ma), similar to that estimated for(1997) suggested that in the late Neoproterozoic,

the BP and its counterparts in west Africa were the Caruaru area (5°C/Ma), are found in theCampina Grande shear zone, the eastern part ofpart of the Northeast Brazil–Central West Africa

continent. the Patos shear zone, and in basement and coverrocks in the Serido region (Corsini et al., 1998).In the northwestern domain of the BP, the flat-

lying foliation is associated with nappe emplace- However, in these places, the amphibole (540–545 Ma) and muscovite and biotite ages (500–ment towards the SSW, and with pressures up to

1 GPa reached during prograde conditions (Caby 505 Ma) are considerably younger than in theCaruaru area. U–Pb zircon ages of magmatic rocksand Arthaud, 1986). In Ghana and Togo, 40Ar/

39Ar muscovite ages indicate that nappe stacking also differ in the two domains. The Fazenda Novabatholith and the Toritama pluton crystallized atoccurred between 634 and 578 Ma (Attoh et al.,

1997). Because 40Ar/39Ar amphibole ages from around 590 Ma (Guimaraes and Silva Filho, 1998;Guimaraes et al., 1998), whereas the oldest agesgranulites in the Granja and Senador Pompeu

shear zones have ages in the range 570–575 Ma obtained in the Serido region are for dioritesemplaced at 579±7 Ma (Leterrier et al., 1994).(Monie et al., 1997), this suggests that shear zone

development was partly coeval with the end of Whereas normal tectonic processes may accountfor the geological evolution of the northwesternnappe emplacement. The proximity of the amphi-

bole and biotite ages (559–569 Ma) implies a fast domain, conventional plate tectonic scenarios arenot very satisfactory to explain the evolution ofcooling rate of #20°C/Ma (Monie et al., 1997)

for this segment of the BP. The geological charac- the eastern and northeastern domains. Any modelfor these domains must account for: (1) the low-teristics of the northwestern sector of the BP are

thus typical of those in modern orogenic belts, P, high-T metamorphism; (2) the abundant mag-matism; (3) the slow cooling rate; and (4) thewhere crustal thickening is followed by shear zone

development and then by relatively rapid exhuma- diachronous evolution indicated by the Ar/Ar andU–Pb data.tion of the deep parts of the orogen.

In Cameroon, U/Pb ages of 630±5 Ma (Toteu Areas of low-P, high-T metamorphism andabundant magmatism are usually ascribed to lith-et al., 1990), and 616 +7/−5 Ma and 620±10 Ma

(Toteu et al., 1986; Toteu et al., 1994) were ospheric delamination (Bird, 1979) or convectivethinning of the lithosphere (Houseman et al., 1981;obtained, respectively, in re-crystallized zircons

from micashists and in magmatic zircons from Platt and England, 1994) following lithosphericthickening in convergent orogens. Successivemetadiorites emplaced into the flat-lying foliation.

If the correlation with the geology of Cameroon events of delamination have been proposed toexplain magmatic and metamorphic pulses in someis valid, these data suggest that development of

the regional foliation in the eastern domain of the orogenic belts (e.g. Collins and Vernon, 1994), butapplication of this model to the BP faces problems.BP was approximately concomitant with that in

the northwestern domain. Similar 40Ar/39Ar Delamination events are of short duration andresult in rapid uplift and exhumation (e.g. Kayamphibole ages for shear zones of these two areas


and Kay, 1993), and thus in relatively fast cooling, cratons. It is premature to offer a comprehensivemodel, but a working hypothesis is to postulatein contrast with the situation observed in thethat a non-plate tectonic factor, namely an upwell-eastern and northeastern domains. Also, there ising mantle plume (Neves and Mariano, 1997),no evidence for either substantial thickening orprovided the force necessary to temporarily over-post-orogenic extension in these domains. Recentcome the compressive stress. A mantle plumenumerical experiments considering viscous strainimpinging on the base of the continental litho-heating as an energy source in orogenic processessphere must spread out horizontally, transmittingsuggest that heat production may be episodic, withlarge stress to, and at the same time heating up,frequencies in the range of 20–50 Ma ( Kincaidthe overlying lithosphere. As a plate moves overand Silver, 1996). However, diachronous meta-an approximately stagnant plume, it is expectedmorphic events resulting from this mechanism oncethat these effects will, with time, progressivelyagain face theoretical and observational problemsmigrate from the site of the initial impact andbecause: (1) high stress levels (>200 MPa) aredecrease in intensity. The greater volume and therequired, and (2) it demands substantialold ages of magmatic rocks in the eastern domainthickening.as compared with the Serido region is consistentCompression of a previously thinned continen-with these expectations if southward displacementtal crust is another process that can account forof BP is assumed, a conjecture that may be eval-low-P, high-T metamorphism and voluminousuated through a paleomagnetic approach.magmatism (e.g. Thompson, 1989). The occur-

Irrespective of the specific mechanism responsi-rence of two successive extension–compression epi-ble for the triggering of magmatic events, the largesodes is an attractive model to explain the ageamounts of magmas emplaced in the easterndifferences between the eastern and northeasterndomain and, to a lesser extent, in the northeasterndomains of the BP because it offers a realisticdomain, may have influenced the subsequent evo-scenario where: (1) the development of the flat-lution of the BP. The geochemical characteristics

lying foliation and most of magma production and of plutons in the Caruaru area and elsewhere inemplacement occur during the extension event; (2) the BP indicate upward migration and emplace-development of transpressive shear zones takes ment in the middle crust of magmas produced byplace during the contractional phase; and (3) the melting of deep seated crustal protoliths, intraplat-final thickness of the crust is not much greater ing of mantle-derived magmas, and extensivethan the original thickness, and so reestablishment mixing between mafic and felsic magmas (e.g.to a normal thickness may occur without any Neves and Vauchez, 1995b; Neves and Mariano,extensional collapse. 1997; Ferreira et al., 1998), implying that signifi-

Younger ages of plutons and deformation have cant internal differentiation and crustal growthbeen observed away from the trench in some may have occurred during the Brasiliano orogeny.continental magmatic arcs. Tobisch et al. (1995) After cessation of the igneous activity and restora-attributed this to contraction, resulting from slab tion of a normal geothermal gradient, the mechan-flattening, of regions previously subjected to a ical structure of the crust was probablyneutral or extensional regime consequent from considerably modified, leaving a lower crust domi-asthenospheric mantle corner flow. However, the nated by depleted granulites and a middle crustlack of evidence for Brasiliano-age obducted oce- containing significant proportions of stiff I-typeanic crust in the interior domains of the BP and hybrid igneous rocks, which may have contrib-indicates that the orogen is ensialic. The main uted to the final cratonization of the BP.problem with the application of the extension–compression hypothesis to the BP is thus to finda physical mechanism that could produce extension Acknowledgementsin an intracontinental setting subjected to compres-sive forces resulting from the convergence of the This research was partially funded by European

Community Project CI 1-0320-F-CD. Much of theSao Francisco/Congo, Amazon and West African


Bouchez, J.-L., Delas, C., Gleizes, G., Nedelec, A., 1992. Sub-data presented in the paper were acquired whenmagmatic microfractures in granites. Geology 20, 35–38.S.P.N. was a doctoral fellow at the Universite

Bouchez, J.L., Hutton, D.H.W., Stephens, W.E. (Eds.), Gran-Montpellier II. The Conselho Nacional de ite: From Segregation of Melt to Emplacement Fabrics 1997.Desenvolvimento Cientıfico e Tecnologico (CNPq- Kluwer Academic, Dordrecht.Brazil ) is thanked for Ph.D. fellowship and subse- Bouillin, J.P., Bouchez, J.L., Lespinasse, P., Pecher, A., 1993.

Granite emplacement in an extensional setting: an AMSquent financial support through grant 523448/96-1.study of the magmatic structures of Monte Carpanne (Elba,AMS studies were carried out at the LaboratoireItaly). Earth Planet. Sci. Lett. 118, 263–279.de Petrophysique et Tectonique of the Universite

Brito Neves, B.B., Van Schmus, W.R., Babinski, M., Sabin, T.,Paul-Sabatier (Toulouse, France). We thank J.-L. 1993. O evento de magmatismo de 1,0 Ga nas faixas moveisBouchez, P. Lespinasse and Ph. Olivier for their ao norte do craton do Sao Francisco. Resumos Expandidos,

II Simposio do Craton do Sao Francisco, Salvador,help in the acquisition and interpretation of these243–245.data. Corrections of the English and constructive

Brito Neves, B.B., van Schmus, W.R., Santos, E.J., Camposcomments by the journal reviewers, especiallyNeto, M.C., Kozuch, M., 1995. O evento Cariris Velhos na

those of J. Percival, helped to improve the Provıncia Borborema: integracao de dados, implicacoes emanuscript. perspectivas. Rev. Bras. Geoc. 25, 279–296.

Caby, R., Arthaud, M.H., 1986. Major precambrian nappes ofthe Brazilian belt, Ceara, northeast Brazil. Geology 14,871–874.

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