RENDICONTI DELLA SOCJETA ITAUANA DJ MINERALOGIA E PETROLOGIA, 1988, \bl. 4l-2, pp. 307·342

Brasiliano age peralkaline plutonic rocksof the Central structural domain, Northeast Brazil

ALCIDES N. SIAL, VALDEREZ P. FERREIRA

Depl. of Geology, Federal University of Pemambuco, Redfe, PE, 50.000, Brazil

ABsrRAcr. - The Precambrian SW·NE trending centralstructural domain of Northeast Brazil is comprised ofthr~ segments: the Serido Fold Belt (SFB); theCachoeirinha . Salgueiro Fold Belt (CSf); and the Riachodo Pontal Fold Belt (RFB). Peralkalioe plutonic rocksare widespuad in the CSF, where they werc emplacedbetween 450 and 510 Ma and around 660 Ma. Theyconstitute twO syenite groups. One group is soclic topotassic, quartz-nonnative syenites that form ring-dikes,dike sets, and small stocks. The second group is potassicto uhrapotassic, nepheline.normative syenites alignedalong the southern boundary of the CSF, forming thesyenitoid line, aoo twO dike swarms with about 50 dikeseach; they are sphene and magnetite.bearing andaegirine-augite and riebeckite . arfvedsonite . rich. Inits northern extension, the syenitoid line changes intoquartz monzonites with shoshonitic affinities.

The silica·saturated group in the CSF is very high inK, Sr and Ba, high in P and Ti and low in Zr and Nb.MORB·normalized spidergrams show Ce, Y and Srnpositive anomalies and Nb and Ti negative anomalies.The oversaturated group displays negative P and positiveZr anomalies. 1be p::lSsibility of an aoomalously enrichedmantle is suggested by high K, P, Ba, Sr and REEcontents of alkali.pyroxenite inclusions in the first group.Both groups show similar REE pallerns, LREE·enrichedand HREE-depleted, with discrete or absent Euanomalies, due either to the high f02 duringcrystallization or to a possible cumulate origin for theserocks. Alkali-pyroxenite inclusions display LREE­enriched and HREE-depleted patterns. MORB­normalized spidergram~ for rocks with shoshoniticaffinities show P, Ti and Nb negative anomalies, andZr and Rb positive anomalies. REE patterns are LREE­enriched and approximately flat in HREE.

W.R. olB() for the saturated group (+ 6 to + 8 permilSmow), suggest differentiation from a mafic magma withminor, if any, post.magmatic alteration, or straightmantle derivation with minor crustal assimilation. Theol8() values for the oversaturated group, ( + 8 to + 10permil Smow) suggest interaction with meteoric waterand more significant crustal contamination.

The regional geographic patterns displa}'ed by theperalkaline plutons in the central structural domain seemto follow major sigmoidal fault zones, and are perhapsrelated to pull-apart processes along these zones,associated with the Patos-Aurora and Pernambucotranscurrent lineaments.

Introduction

The Proterozoic central structural domain(CSO) in central Northeast Brazil extends for900 km and occupies about 90.000 km2 withSW-NE trending metamorphic rocks. Itcontains the largest number of granitoids inNortheast Brazil and is comprised of threefold belt segments: the Serido (SFB, 3a, inFig, 1), the Cachoeirinha-Salgueiro (CSF, 3b,in Fig. 1); and the Riacho do Pontal (RPF,3c, in Fig. 1).

Based on petrography and texturalrelationships, ALMEIDA et al. (1967)recognized four granite-types in the CSF andproposed a tectonic classification: a) Concei~oand ltaporanga-types, syntectonic at 650 ±30 Ma, represented respectively by epidote­bearing granodiorite and porphyriticgranodiorite to granite; b) Itapetim-type, latetectonic at 540 ± 25 Ma, and composed ofslightly porphyritic dike rocks of compositionsimilar to the Itaporanga-type; and c)Catingueira-type, post-tectonic at 460-510 Maand comprised of peralkaline aegirine-bearingquartz-syenite to granite_ Latet, it was notedthat the Itaporanga-type has potassic-calc-

308 SIAL AN., FERRElRA V.P.

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BRASIUANO AGE PERALXAUNE PWlONIC IlOCKS OF TIlE CENTRAL STRucruRAL DOMAIN. ETC. 309

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alkaline affinities, while the Concei~ao andltapetim-types are typically cale-a1kaline rocks(SIAL et al., 1981a, 1981h; SIAL, 1986). Inaddition, plutons with ttondhjemitic affinities(Serrita-rype) and a saturated peralkalineassocialion (Tnun/ootype) were recorded (SlALet al., 19813, 1981b; FERRElRA, 1986;FERRElRA and SlAL, 1986). Each group showsdistinctive oxygen isotope and REE patterns(SIAL, 1984a, 1984b), and each has a dearlydefined geographic distribution (Fig. 2).

The peralkaline group is widespread in theCSF and includes the oversaturatedCatingueira-type (At.MElDA et al., 1967). Thisis commonly associated with fault zones andintruded into Cachoeirinha rnetasa:l.iments(e.g. Quandu, CavaIos, Ikrnardo Vieira, CanaBrava), or constitutes a syeniroid line(Triunfo-type, FERRElRA and SIll, 1986;FERREIRA, 1986; StAL, 1986), defined byintrusions into the basement that roughlyfollow the southern boundary of the CSF, ordiscontinuously along the northern boundary(e.g. Ouricuri) of the CSF.

The knowled.ge of peralkaline bodies in theSFB is limited.. Dike rocks such as thoseforming the Santa Luzia hill, near Dona !nes,Paral'ba state, reported as aegirine-bearing(BARBOSA et al., 1974) are possibly equivalentto the Catingueira-type. Also, rocks withpossible peralkaline affinities were mentionedfor several other plact:s (e.g. Equador, RioGrande do None; SoUZA, 1987; Al;U, SANrOS,1968).

Little is known about granitoids in theRPF, located marginally to the Sao Franciscoeraton and south of the Pernambucolineament (Fig. 1). The geological history ofthese granitoids seems to be similar to thoseat the CSF, although no systematic study hasbeen carried out and only preliminarygeochemical data are available.

SANTOS and CALDASSO (1978) recognizedin this region syn- to late-tectonic graniticrocks of calc-alkaline to slightly alkalinenature. They also identified syn-kinematicgranodiorites of diffuse contacts intrudingschists and gneisses, as well as granites tosyenites which intruded phyllites. Thisgranite-syenite association is probablyequivaknt to the peralkaline rocks of the CSF.

GAVA et al. (1984) recognized a late to poSt­kinematic suite of alkaline granitoids namedSerra da Aldeia, geographically distributed inthree groups of plutons (Tigre, Sussuarana andSao Joao do Piau!, Piau! state), withoutapparent chemical distinction among them.

Occurrence and petrography

1. The Cachoeirinha·Salgueiro Fold Belt (CSF)

(a) THE OVERSATURATEDPERALKALlNE GROUP OF ROCKS

Rocks in this group form stocks of variablesize, dikes and ring-dikes, usually intopographic n=lief, intruding rather low ­grade metamocphics of the PrecambrianCachoeirinha Group in the CSF (e.g. Quandu,Cavalos, Cana Brava and Campo Grandestocks, Santa Antonio dike, all in Pernambucostate), metasediments of the SalgueiroGroup (ring-dikes at Serrita stock,Pernambuco), or are found near the contactof the Cachoeirinha and Serido Groupmetasediments (e.g. Catingueira dike set,Paral'ba state) (Fig. 2). Petrographically, theyare mostly aegirine-bearing quartz-syenites togranites.

The E-W trending Catingueira dike set,with five dikes, (the longest of which is 12km long), is composed of medium to coarse­grained leucocratic quartz syenite to granite.Modal mineralogy is essentially microcline,albite, quartz, with accessory aegirine, sphene,blue amphibole, minor biotite, magnetite,apatite and ferroaugite. Aegirine, up to 1.5%,is partially replaced by a blue amphibole,which, with sphene and magnetite, indicateshigh oxygen fugacity prevailing duringcrystallization. Minor fJuorite was detectedin two of these dikes.

PeralkaIine ring-dikes are found in twosmall stocks near Serrita town (Fig. 2).TopographicalIy, the cores of these stocks areat ground level, while the ring-dikes are inrelief (e.g. Vassouras, Macacos, Boqueirao andSerrita hills). The cores are composed ofbiotite tonalite to granodiorite withtrondhjemitic affinities (StAL, 1986; FERR.EIRA

BRASIUANO AGE PERALKAUNE PUrrONIC ROCKS Of THE CENTRAL SIRUCTURAL OOMAIN, ETt:. 311

and SIAL, 1986). Ring-dikes are mainlyaegirine-bearing quartz-alkali-feldspar syeniteto .IkaIi-feJdsP't' 8'atllte_ MineraIogicaIIy, theyare similar to the Catingueir-a-type, exceptthey do not contain fluorite. Instead, minor,zoned cassiterite associated with pyroxene ispresent. Albitization was locally importantand chess-board textures in microcline aresometimes present. Cumulate textures in thin,alternating pyroxene and microcIine layers(photo 1) (e.g. Boqueirao hill), sometimesoffset by microfractures, forming «enechelon» structures are recorded. Narrow

An E·W trending subvolcanic dike ser atabout 30 km north of Serrita, along SantoAntonio Creek, encompasses aegirine-bearingtrachytes. Mineralogically I they containmicrodine, aegirine, and magnetitephenocryslS in a quenched, blue amphibole­rich groundmass. Microdine, somerimesmantled by albite, forms agglomerates(glomeroporphyric textures) and includesaegirine and blue amphibole. Aegirine showsdiscontinuous zoning with a pale green core.A brown amphibole with blue margins is alsopresent.

PhotO 1. - Cumuhlle textures in thin, alternate pyroxcne and microcline lIye", oUset by microfraetures in .enechelora Structure:. Boqucirio hiD, ring-dike at Scrritl nod.

peralkaline to ca1c-a1kaline dikes, up to one­meter wide, cut the tonalites and granodioritesof the core of the stock. The rounded Quanduand Cavalos quartz syenite stocks, Sitio dosMoreiras, Pernambuco, mineralogicaIlyresemble the Serra dos Macacos ring-dike atSerrita. Zoned microcline shows flake perthiteand includes early crystallized a1bite andpyroxene. Aegirine shows discontinuouszoning and, as in most peralkaline rocks inthe CSF, was partially converted to blueamphibole. Biotite, as in most of theseperalkaline bodies, associates with pyroxeneand includes zircon grains.

(bl THE SATURATED GROUPOF ROCKS AND THE SYENITOID LINE

Peralkaline rocks along syenitoid line areusually pink, aegirine·bearing, alkali-feldsparsyenites, with some clots of aegirine andlocally with dark, silica-enriched material(FERREIRA, 1986; FERREIRA and SIAL, 1986;SlAL, 1986)_

The syenitoid line extends for about 150km from Terra Nova, through Bom Nome toTriunfo towns. Next to Terra Nova, adiscontinuous quartz alkali-feldspar syenitedike, forms the Case, Livramento, Duas

312 SIAL A.N., FERREIRA V.P.

Irmb, and Paulo ridges, up to 4 km wide and600 m above sea-level. It intruded biotite·schists of the Salgueiro Group and theporphyritic hornblende syenite of the TerraNova batholith. At the Case ridge,hornblende syenite xenoliths are abundantand reacted with the host peralkaJine syeniteliquid, showing rounded to diffuse contacts.Microdine shows cross-hatched twinningoverprinting earlier Carlsbad twins andexhibits rows of elongate to needle-likepyroxene inclusions. Less often, itpoikilitically encloses aegirine grains, blueamphibole and apatite. Aegirine phenocrysrsshow discontinuous zoning, gradation intoblue amphibole and magnetite, and ofteninclude sphene and apatite. Biorite, usuallyassociated with aegirine and imerstitial quartzwith wavy extinction, is rare.

Dark, narrow peralkaline dikes, a fewmeters long, related to the Terra Nova dikeswarm, cut across the Case: dike, often with«en echelon» structure (photo 2) and trendingN-S or E-W. They are composed of aegirine­bearing alkali-feldspar syenite.

Next to Bom Nome town, Pernambuco, aperalkaline syenite dike about 10 km long and1.5 km wide intruded the contact zonebetween Cachoeirinha metamorphics andbasement rocks. It is partially covered byCretaceous sediments of the Sao Jose doBelmonte Basin. This dike is sheared andexhibits irregularly shaped, flintlike, darkportions which break conchoKlally, giving thedike a composite appearance. Th~ dark rocksare quanz-enriched, with aegirine weatheredand partially transformed into blue amphioole,and hematite filling parallel fractures.

The Triunfo batholith, the mostvoluminous peralkaline piu ton in NortheastBrazil (aLout 600 km2), is elongate in theSW-NE direction, parallels the regionalmetamorphic trend, and is in contact withmigmatites of the basement and in faultcontacts with Cachoeirinha and SalgueiroGroup metamorphics. In this nearly flat­topped batholith, biotite-schist xenoliths arewidespread, indicating that t~ present levelof exposure is not far from t~ original topof the piu ton. Three fauhs CUt across thebatholith, locally generating shear fabrics and

Photo 2. - Peralkaline syenitic dike related to the TerraNova dike swum, cuning across the peralkalinc CasCsycnilic dike (p.n of the sycnitoid line) with -en echdol'lltstNClUre.

stretching lineation. Flow textures areassociated with primary planar structure, andwhere aligned, flat, baked biotite-schist andpyroxenite xenoliths are found. Cumulatetextures are shown by alternating microclineand pyroxenite layers, which are also recordedin some autoliths. Sulphides are associatedwith late pyroxenite or syenite veins.

Oval-shaped, globular or streaky pyroxeniteinclusions (photo) throughout the batholith,varying from a few cm to one-meter long, areusually aligned with the flow foliation of thehost syenite. These inclusions are composedof aegirine (90%), sphene, blue amphibole,and microcline. Narrow pyroxenite dikes show«boudinu (photo 4) and are regarded as syn­plutonic dikes, according to PITcHER (1979),intruded during the last stages of emplacementof the Triunfo batholith.

Disk-like, biotite-schist xenoliths (hornfels),

BRASILlANO AGE PERALKALlNE PWTQNIC lOCKS OF THE CENTRAL STRUcruRAL OOMAIN, ETC. 313

Photo 3. - Clinopyroxenite indl,lsions of different shape and size in the Triunfo batholith.

Photo 4. - Srn·plutonic pyroxenite dike in the Triunfo batholith.

are oriented according to £low foliation of thehost syenite. Some small xenoliths showcoarse, rttrystallized biotite, sometimessurrounded by a thin layer of fine-grainedpyroxene.

Aegirine is typically oriented according tothe magmatic £Iow and shows discontinuouszoning and pale green cores (ferroaugite?)

(photo 5). Locally, it forms aggregates in atypical cumulate textUl(:. Aegirine oftencontains sphene and apatite, and whenpartially transformed into blue amphibole,magnetite grains are present in fractures andcleavage planes. Biotite is also seen associatedwith pyroxene, as in the other peralkalineplutons in the CSF. Sphene is the most

314 SIAL A,N., FERREIRA V.P.

Photo 5. - Euhedral aegirine showing discontinuous zoning and partially transformed into blue amphibole, inTtiunfo batholith.

Photo 6. - Breeda structure, with fragments of meta-mafic, gabbroic and metasedimentary rocks, in a peralkalinedike of the Manaira-Princesa lzabe1 dike swarm.

abundant accessory phase, often found inaegirine or microcline, and apatite is also seenin almost all other phases. Texturalrelationships suggest that sphene and apatitewere the first phases to crystallize, followedby aegirine and microc1ine. Magmatic

hydrothermal activity (autometasomatism) isindicated by albitization at the microclineborders, and by blue amphibole formed fromaegirine. Quartz, the last phase to crystallize,is preferentially found next to fault zones,suggesting that it did not crystallize primarly

BRASIUANO ACE PERALKAUNE PWTONIC ROCKS OF TIlE CENTRAL STRUCTURAL DOMAIN, ETC. } 15

from the magma.[n the Terra Nova region of Pernambuco,

over 50 peralkaline dikes, often 15-20 cm wideand up to 6 km long, intruded the Tern Novabatholith, biotite-schists of the SalgueiroGroup, and the western portion of theSalgueiro badlOlith. These dikes showmultiple trends, but are N-S where cutting theSalgueiro batholith. Another dike swarm ofthis kind is next to Manafra and Princesababel towns, Par31'ba, north of the Triunfob,tholith, with ",bpooel dikes intruding lowgrade metamorphics of the CSF and basementrocks. Narrow dikes, NE-SW-trending, 10 mwide and up to 2 km long, are in topographicrelief where intruding granitic rocks. Whenintruding metavolcanics of the CSF, theyshow chilled contacts and one-meter widehomfets aureole. Breccia structures withfragments of meta·mafic, gabbroic, andmetasedimentary rocks are present (photo 6).

The rocks in these two swarms are mostlysubvolcanic, varying from aegirine-bearingtrachytes to syenites. Less often, quartz·aIkali­fddspar syenites and alkali·feldspar granitesare present.

At Terra Nova, the dikes are grey to gr~nor brown, with predominantly aphanitictextures, and are essentially composed ofaegirine, blue amphibole, orthoclase, sphene,apatite, magnetite and quartz. Similarcomposition is observed in the dikes ofPrinttsa babd-Manaira, except they aremicroscopically more: similar to the Triunfosyenites.

Orthoclase is often zoned and pyroxeneneedle·inclusions grew according to its zonesor cleavages in epitaxial growth (photo 7).Flake perthites are common, and albite is alsoseen as isolated grains. Pyroxene isrepresented by aegirine or ferroaugite withaegirinic borders, partially transformed intoblue amphibole plus magnetite, formingglomeroporphyric textures. Ferroaugite,lessabundant, includes apatite and sphene. Greenbiotite is rare, usually associated with the palegreen pyroxene, and often has zirconinclusions.

In the northern boundary of the CSF, atits southwest extremity, 5 km west ofOuricuri town, Pernambuco, there: is a series

of aligned, elongate SW-NE trending aegirine­bearing syenite stocks. They form the AltOGrande, Bodes, Covado, Serrinha, Pdado,Tranqueira, Quixaba and Queimada hil1s (Fig.2), extending for about 30 km. These rocksexhibit a metamorphic banding, dipping }OO

east, overprinting an igneous layering(microcline and pyroxenite alternate layers,5 to 10 cm thick). These struaures are locallycut by orthoamphibolite dikes.

At the Serrinha hill, the meta-syenite isinterlayered with meta-gabbro,locally tightlyfolded (photo 8) and with «boudins~. Biotite­schist megaxenoliths partially baked aresometimes found. Small, angular pieces ofsuch baked rocks are found in pegmatite dikeswhich lack signs of deformation and whosevugs are filled with black to white calcite orshow a yellowish non-identified material.

In the syenite, aegirine grains showpreferential orientation forming bands withwhich recrystallized quartz may be associated,and forming sub-grains in poikiloblastictextures. Some aegirine was transformed intoblue amphibole along its cleavage planes andfractures, in which magnetite is associated.Biotite shows wavy extinction and kinkbands, and apatite and sphene are the mainaccessory phases.

The meta·gabbros are composed of albiteand amphibole which form agg!omerntes, orare disposed in paralld bands where theamphibole is preferentially euhedral.

(c) ROCKS WITH SHOSHONlTICAFFINITIES

Among the rocks in this group, those whichform the Solidao and Teixeira batholiths, inthe prolongation of the syenitoid line (Fig. 2),are the most important ones. At Solidao, therocks are essentially quartz-monzonites, whileat Teixeira they tend to be mostly quartz·syenites. Besides these two plutons, theeastern portion of the Salgueiro batholith,Pernambuco, bears some shoshonitictendencies.

The Solidao batholith, next to Solidaovillage, is elongate NE-SW, oocupies 140 km,forms the Solidao and Queimada hills andintrudes gneisses and migmatites of the

}16 SIAL A.N., FERREIRA V.P.

PhoIO 7. - Zoned onhoclase showing pyroxene needle-inclusions, grown according to its zones or ckavages. in• dike of the Manaira-Pril'll;e5a babe! dike swarm.

?hoIO 8. - Met...yenitc inu:rlayeraJ wilh mela-gabbro, lightly folded, Dear Ouricuri town, Pernamburo.

basement. The Teixdra batholith is elongatein the E-W direction, 60 km long and about10 km wide, and forms the Borborema andTeixeira hills. This pluton intruded quartzitesand biotite-schisu to the north and gneissesand quartzites to the south.

Both batholiths are predominantlyequigranular,leucocr:atic rocks. In the Solidaobatholith, a1bite and microcline predominateover quartz, which often shows wavyextinction and forms sub-grains. Bluish-greenamphibole (probably ferroedenite) is the main

BRASlUANO AGE PERALKAUNE PWTON1C lOCKS OF THE CEN1ltAL STRUCTURAL DOMAIN, ElC. } 17

mafic mineral, with pale green pyroxene cores.Minor biotite, associated with amphibole,partially altered to chlorite, includes zonedzircon grains and apatite. Ferroaugite isrelatively rare in the Solidao batholith andusually exhibits borders transformed intoamphibole. Sphene is the main accessoryphase, included in all major phases.

In the Teixeira batholith, perthiticmicrocline (flake and veined perthite)predominates over plagioclase and quartz.Plagioclase in contact with microcline, usuallyexhibits drop-like quartz inclusions,mymerkites, and is often deformed with benttwinning planes and wavy extinction. Quartzforms subgrains and equiJibriwn textures withgrains boundaries meeting at 120 0 angles.

Among the maHc phases, bluish-greenamphibole (probably ferroedenite) is the mainphase, sometimes forming agglomerates. Itshows simple twinning and inclusions ofzircon, apatite and sphene. Biotite is foundincluded in amphibole and also showsinclusions of zircon. Primary epidote is foundas euhedral to subeuhedral grains, includedin biotite or the border of the amphibole, intextural relationships similar to thosedescribed by ZEN and HAMMARSTROM (1984).Some epidote grains have allanite cores.

The eastern portion of the compositeSalgueiro bathoJith, one of the largest pluronsin the west of Pernambuco, next to Salgueirotown, petrographically resembles the Solidaobatholith. It is composed of ratherhomogeneous, coarse-grained porphyriticquartz-monzonite to quartz-syenite. ThispIuton was late to posttectonic emplaced andpierced the Precambrian biotite-schists of theSalgueiro Group.

Mafic minerals usually occupy less that 10percent of the volume of the rocks. Pyroxene(ferrosalite to ferroaugite), as dongate grains,was partially transformed into a bluish-greenamphibole. Ferroedenite often forms clots,and biotite £lakes, strongly pleochroic withinamphibole or replacing this mineral, are alsopresent. Flame perthite and, less often, patchperthite, seems to have partially replacedplagioclase, which in turn appears to assimilateand corrode microcline. Oscillatory zonedmicrocline, although not common, is also

present. Quartz grains are sub-plates and ofteninclude pyroxene or amphiboles. Zonedzircons are in almost all phases. Zoned allaniteis surrounded by epidote and apatite needlesare often fourx:l within amphibole and biotite.Iron oxide minerals are usually absent.

Sughcly pc"phyrit;, fine·gNined dikes, d"kgreen to black, one meter-wide at most, cutthe Salgueiro batholith. In a few places, whiteor pink feldspar phenocrysts of microcline, upto 0.5 cm long are observed. 1beir mineralogyincludes microcline, amphibole, magnetite,chIorite, quartz and epidote. Microcline oftenhas magnetite inclusions, oscillatory zoning,fractures filled by quartz, and plagioclasemantles. Quartz in small amounts, usuallyinterstitial, shows wavy extinction, and alkali­amphibole exhibits strong pleochroism fromblue to violet.

2. The Smdo Fold Belt

At this belt, BARBOSA et al. (1974) referredto the Santa Luzia pluton near Dona rnestown, Paraiba, and to elongate plutonstrending SW-NE and roughly paralleling theCacerengo Fault zone, as hedenbergite oraegirine-bearing granitoids.

Between PiOJf and Barra de Santa Rosa,Paraiba, a large elongate pluton, slightlyporphyritic, with brownish fddspar, exhibitsquartz·monzonitic composition. Similar to therocks in the Teixeira batholith, it perhapsrepresents its northern prolongation, beyondthe Pates lineament. Among the three groupsof granitoids fttognized by SANTQS (1968) inAr;u region, Rio Grande do None, herecognized a late-orogenic one, composed ofmonzonite-adamellite and alkali-syenite·granite associations. the latter being probablyequivalent to the Catingueira-type of AuiElDAet al. (1967).

Next to Equador, not far horn the Patoslineament, Rio Grande do Norte, sheets ofbiotite·monzosyenitic to clinopyroxene·monzosyenitic gneisses are interpreted ashaving shoshonitic character, and havingintruded rocks of the Archean basement(Caico complex) and Jucurutu and EquadorProterozoic supracrustal (SoUZA, 1987). Theydo not pierce, however, metamorphics of the

318 SlAL A.N., FERRElRA V.P.

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younger Serido Group. Although SOUZA(1987) proposed that those gneisses wereanorogenic plulons equivalent to alkalineorthogneisses of the west Pan-African chaindescribed by CABY (1984), their relative timeof emplacement is not dear. They arecomposed of ferrosalite to ferrohedenbergite,ferrohastingsite, magnetite, sphene,mierocIine and plagiodase.

3. The Riacho do Pontal Fold BeltAt the northwest portion of this belt, Piaui

state, a number of late to posttectonicgranitoids, the so-called «5erra da Aldeiasuite», forms round to elongate stocks anddikes which pierce low-grade metamorphics,the Phanerozoic Paraiba sedimentary basin.They are in topographic relief, as in Aldeiahill, or partially eroded to ground level, as near

BRASlUANO AGE PERALXAUNE PWIONIC IQCKS OF TIlE CENTRAl. STRUCTURAl. OOMAlN. ETC. 321

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Tigre and Sussuarana villages or SaoJoao doPiaui town. The pIu tons at Tigrepetrographically resemble the rocks at thering-structure in Semta and locally crosscutgranitoids similar to the calc-alkalineConceic;ao-type granitoids in the eSF.

GAVA et a1. (1984) used texturalrelationships and WRJGHT'S plot (1969) toidentify several types of granitoids bearingalkaline to peralkaline affinities. Among them

are f1uorite-beadng alkali-pyroxenegranodiorites. granites and syenites, andporphyritic. am(iUoole quartz IIxmzonites andbiotite granite. Hastingsite granites withbiotite and ilmenite form plutons, diapiricallyemplaced, with incipient foliation next 10

their margins.

Geochronology

ALMElOA et al., (1967) suggested that the

322 SlAL A.N., FERRElRA V.P.

synorogenic gran<Xiiorites, tonalites and calc­alkaline granites in the CSF were emplacedbetween 650-520 Ma, and that the peraIk~granites and syenites ~ younger (Cambro­Orclovician).

HURLEY et al. (1967) determined a K-Arage of 530 ± 30 Ma for the Serrita stock,which is probably its cooling age. Rh-Se datingof the stock was not successful because of itshigh Se and low Rb (SIAL and FERREIRA,

1985), similar to the Salgueiro batholith. Alow initial Se ratio of around 0.7040, however,was identified.

Recently, 40Arp9 ~ (R.D. DaUmeyer.unpublished data) and Rh-Se (L.E. Long,unpublished data) determinations have beencarried ou[. 40ArJ'9 AI ages I1Ulge from 625Ma (hb) to 605 Ma (hi) for Concei~ao·type

calc·alkaline plurons, and a value 620 Ma wasfound by Rh-Se for the Itaporanga plutons.As one of che peralkaline dikes in theCatingueira dike set cuts the calc-alkalineEmas batholith of the Concei~ao-type, ayounger age for the Catingueira dike set isexpected. For the Triunfo batholith in thesyenitoid line, W. Teixeira (in BRITO NEVES,1982) determined a Rb-Sr isochron around660 ± 60 Ma with an initial ratio of 0.7076.Crosscut relationships indicate two Brasiliano( = Pan-African) age peralkaline magmatismsmth< CSF and rnnoonding «gions. The olderone is represented by syenites in the syenitoidline that are crosSCUt by the Terra Nova dikeswarm, which also cuts the Salgueirobatholith. Possibly the Ouricuri peralkalinesyenite represents an older event, consideringits gneissie fabric.

Geoc.hemislty

1. The Cachoeirinha-Sa/gueiro Fold Belt

Major and trQ£e elements

(a) PERALKALINE ROCKS AND THESYENITOID LINE

Representative chemical analyses of themain peraJkallne rocks and those withshoshonitic affinities in the syenitoid line andwithin the CSF are in Tables 1 and 2,

including major, trace, rare-earth elements(REEl and CIPW norms.

The syenites in the Livramento, Duas Irmiisand Case ridges are quartz normative. whilethe Triunfo, the largest pluton, is nepheline­normative. Except for the Case and BomNome bodies, all are acmite and Na­metasilicate-normative rocks. With oneexception, all of them are very K-enriched(K~O up to 12.80%), which lends to theserocks a potassic to ultrapotassic character,with K20/Na20 > 3.0.

The oversatw-ated peralkaline rocks showa broader Si02 variation than those at thesyenitoid line (60 to 7196), and K20/Na~O

varies from 0.54 to 2.84, lower than in thesatunlted peralkaline group of rocks. They aresocIic to potassic according to the DEBON andWORT (1983) classification, and also quartz,acmite and Na-metasilicate-normative rocks.

In the QAP diagram (Fig. 3), modalcompositions of the saturated peralkalinegroup follow the aJkaline-peralkaline trend ofLAMEYRE and BOWDEN (1982), while theoversaturated peralkaline group and rockswith shoshonitic affinities follow themonzonite (high K) trend.

The type of granitic magmatism can beidentified using the RI-R2 diagram (DE 1..AROCHE et al., 1980), where the milicationicparameter RI = 4Si-l1 (Na + K)-2 (Fe + Ti)opposes quartz to alkali feldspars and Fe-Timinerals, and R2 = AI + 2Mg + Ca expressesanorthite and mafic minerals. In Fig. 4a thesyenites in the syenitoid line show a positivetrend, which seems to foUow the source trendvisualized by BATCHELOR and BoWOEN (1985),mostly, however, in the undersaturated fieldof the diagram. This trend reflects theconstant increase in alkalis, especially K, andthe decrease in Ca + Mg + AI, generating theultrapotassic character of these rocks. Twosamples lie in the late-otogenic field of thediagram, in agrttment with fidd observations.Two pyroxenite samples also plot in theundersaturated field, correspondingapproximately to an aIkali-gabbro compositionin the DE L\ RocHE et al. classification(1980), at the extension of the saturatedperalkaline trend.

The oversatucated plutons exhibit a trend

BRASlUANO AGE PERALKAUNE Pll.noNIC ROCKS OF TIlE CENTRAL S1ll.ucruRAL DOMAIN. ETC. 323

distinct from the saturated ~ralkaline trend,with R2 approximately constan[ and a broadrange of RI, a result of major variation of totalalkalis.

In the AFM plot (Fig . .5c), the sawratedperalkaline group shows a trend whichsuggests little enrichment in Fe duringdifferentiation, compatible with a high oxygenfugacity during crystallization. Thiscontention is supported by the presence ofsphene and magnetite in most rocks of thisgroup.

Rocks of the saturated ~ralkaline groupcluster in the alkaline field in the alkalis versusSi02 plot (Fig. 6a), while those in theoversaturated group cluster along theshoshonitic group of granitoids. Besides thetemarkable K-enrichment in the saturatedgroup, it is also extremely enriched in Sr andBa, low in P, Ti, Zr and Nb. MORE­normalized spidergrams (Fig. 7) enhance thestrong enrichment in Sr, K and Rb relativeto Yb, and show slightly positive anomaliesin Srn and Y, and marked troughs in P andTL When normalized to primordial mantle(Fig. 8), the strong enrichment in Rh, Ba, Kand Sr besides Nd and Srn, is confirmed. Theoversaturated peralkaline group in MORB­normalized spidergrams (Fig. 10) shows astrong enrichment in K, Rh and Sr in relationto Yb, but has less Rb, P and Ti than thesaturated group, showing pronounced troughsin these elements.

(b) THE SHOSHONITIC ASSOCIATION

Since shoshonites were first described(mOINGS, 189.5) and their concept broadenedand modified by]oPuN (968) who proposeda shoshonitic series, there have beenimprecisions in their characterization. Themain reason is that this series shares somecharacteristics with the calc-alkaline andalkaline series. The most complete summaryon shoshonites was presented by MORRlSON

(1980) and after that, a number of workersmentioned shoshonitic granitoids, known alsoas the monwnitic series (NARDl, 1986), whoseknowledge is still limited. Some of the maincharacteristics of shoshonitic granitoids,compiled from several authors with few

,

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Fig. 3. - Q.A-P ternary plot for the peralkaline andshoshonitic granitoids of the Cachoeirinh•.SalgueiroFold Belt. Trends (1) calc·aIkaline trondhjcmitic (low K),(2) caJc.a1kaline monzonitic (high K) and (3) alkaline andperalkaline, arc from LAMEYRE and BoWllEN (l982}.

modifications, are shown in Table 3, as wellas the main characteristics of the Solidao,Teixeira and Salgueiro batholiths.

The rocks in the CSF which are probablerepresentative of the shoshonitic ormonzorutic series are all quartz-normative andthose from SoIidao batholith show acmite andNa·metasilicate in their norms. Although theyare sodic rocks according to the DEBON andLE FORT (1983) classification, with K/K(K + Na) > 004.5, this does not eliminate thepossibility that they belong to the shoshonitictrend. Riebeckite·bearing sodic rocksbelonging to the more evolved stages of theshoshonitic series were observed in Papua bySMITI-t (1972). In addition, arfvedsonite­bearing peralkaline rhyolites are associatedwith shoshonitic volcanics in andesitic arcs ofthe southwest Pacific (SMITH et al., 1977).

Modal compositions in the QAP diagram(Fig. 3) plot along the monwnite (high K)trend of LAMEYRE and BOWDEN (982), and

324 SlAL A.N., FERIl.E1RA V.P.

CACHOEIR1NHA-5ALGU£IRO FOLO 8ELT

",------------7"-----,o S~£M'TU 0' THE U(N'TOI(l[ LINE

......~~....••••••1.

a_••• ,

~•......,.. \

, \~f .... 1

.' \ \ ""'''''!l:'/ CA \ ....._,..

.. /" 2 \~I>~'" " "".·...t. "

.0 ." "-..." ••111.,.. \

ePll..~ f -.... J ~......." \

+<:1 / .... "".,,1,.... ...... \.... 9' A --.I' I,ll .... ,00 .... ~ ........ "-<9 ~.. • -~......•........\

..d'. - ~.".,. ···~,•.<."1.1..) \X -:-iC-__ ...:;::.... ...._.•. ~... \

---'N

•

•••>N· ,~ou

•

o '000

" -Rl'4Si-ll (NQ' K)· 2(F,.T;) aAIACHO PONTAL FOLO BELT

",-------------,.-------,

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0

•N

•

•

•,.,- .~>".. .." \

..+''''' \00 ~.J' \ I

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ol ..X", 2'~ .." ", .....,. '

...." .' " .."..~. "..0 .'" "'" J"'~ \." -' SA~-"",,,,,,,", \...... .... ...._ ...i," \. ~~. ·~"""h \.." .." 0

~ >~ "'.~ ,"1"" _ n. ' 05,•.0" 0 \

___ _ ". '. 1ti.~".. \--- \..••••..<-=A_ ' -:-)

..P...,···.....011<-

.. SUSSV •• o"o

o 'AO JO!O 00 .'.',11

R 1 • 4 Si - 11 {NO" K I • 2 (F, .. T i I bFig. 4. - Rl·R2 diagram (DE LA R()(J-JE el al., 1980; BATCUELOll and BoWDEN, 198') for the peralkaline groupand shoshonitic gTanilOids of the Cachoeirinha-Salgueiro Fold Bell (a) and for lIranitoids of the Riacho do PonlalFold Bdt (h). CA. Calc.alkaline, SA. suhalkaline and A. alkaline (trends from MAREVVOl. el al., 1987).

oversaturated peralkaline bodies plot next tothe minimum of the granite system, suggestingthat they differentiated from shoshoniticliquids.

These granitoids do not show a t~nd in theRl·R2 plot (Fig. 4a) probably due to thelimited data. They plot in the late orogenicfield, in consonance with field observationswhich indicate late to posttectonicemplacement. The trend of the mainmagmatic associations are shown in that

Figure: calc-alkaline (CA), subalkaline (SA)and alkaline (A), with the term subalkalinedefining plutonic rocks with chemicalcomposition lying between the fields foralkaline and calc-alkaline rocks (Frenchnomenclature). Teixeira and Solidiio samplesin the Rl·R2 diagram lie between the SA andA trends, interpreted here as an indication ofa transition between shoshonitic to peralka1ineaffinities.

Compared with the shoshonitic granitoids

BRASILlANO AGE PERALKAUNE PUrroNIC IQCKS OF TIlE CENTRAL STRucruRAL DOMAIN, ETC. 325

TABLE 3Main charac/eristiJ:s o/01choeirinha-Salgueiro Fold Bel/ grani/oids wilh shoshonitic affinities ascompared 10 shoshonitic grani/oids (compiled from TAUSON, 1983; NARDI, 1986; TSVETKOV, 1984;

PEARCE, 1983, wilh some modifications)

J~HO~ITlC SElltS cMIIltol1>S

I. 110101, _ .... it.. , ........{~.I...."" _""....1,...oo••••,..,u.. t. 1-'". •• __<I .. &....h~U.

>. • _.ha••_hi. _ • I , rill .. I 4iU••,••<1.,.,. _.a, 1 L~ ._ ••1 NI... I.~.

"10. CoO, .~.. I r AI ~ ....... ..-itll dlff.....<o.'otl... l.>u r.o ,j Jhb '.~"'ZOl > 0.$.,....h..,_f.$. H(it' Lit! (iIO, sd, In d Ill. J~..p .-« .... ,. h .... le i. dlll " d ...~or.(SIO,' no. Oi.d'i"••h.d f ,h• • 01.· .'lL&Uu..cl • _, (to hi",.. la .0<1 J IT........i'••••f~"- ... ~'u_,_rplo".'" I .. ""'or ZT, ........ t. IT 10 .boaloool '.', -...... , a .......11 I 10 ,"" .luH......!••.

TO.. • MIdo ,. • fT .

1_ h_ "" _ .........1 ._ ...••oeid••• ",H.I• .-b .f <.I.. _<l,i ...o.I ........._n-.l' 'TW•.

SIIOSI/OIiltlC ClAIUtolDS IN TIlE Cli', iiOi'ri!iilf liAZlL

I. ~........1... '. _.« ......It••J. co "rlil~,h ••: G-.- 500I. (l.n.........i<..."IiPo(F. '11&' o.n) ,I,•. Po.. , •....u ..ltll hl.1< .,..- ~I 1•• Soli...loo....litll I. 1 11, _t..-- 'I .

J.•• Q-"-' l{..._ pi" .,_ ."" _;1,....,~.IG"11_ ...I...f ~, ....... _ (lHZI.

S. d", ,~ un_I, htit' IIa (yp t. JJoo:p:piV, ST (yp t.1290 PJ>OO)' .,..... l:\o (yP •• "0 P.-): I. ~P ,. UO P"".':lO ..o .

• _.... .. •• lJO , h .... _ ....

(¥..... It.'.).,. rTOi>Oil, _.10.. ""hoM_ Oil., _ .......1 .....1­101.>-.

• • • •

(0 I (b I ( cl (d I

CIlCHO£IIl'NHIl'SIlLGU£IIlO fOLD SE LT (O,D,e)

t O•••• Oh ..... f ,. ....1'"'''' 'De"

RIIlCHO 00 PONTIlL fOLD eUT If)

6 T', ••

I) SaD ~OOO <ID Pi ... l

o SDII'DI.d ,..'DI',,"u .Dch 1"'~"O'f lift.)

Sft"'."ftltlc ,'D.ilD'dl IrD", Le••o. d~ SUI,Ilio G'Dftd. dD Sui, 8.D.11 • SUIIUO'OOO

Fig. ~. - Alkalis - FeD(I)· MgO ternary plot for peralkaline and shO$honitic rocks of the Cadloeirinha·SalgueiroFold Belt (a·cl and granilOids of the Riacho do Pontal Fold Bdl (d).

326 SIAL ....N.. FERRElRA V.P.

1Il0RRlSON 1980

SEItIES

Fig. 6. - Alkalis versus SiOl plot for pertikalinc andmosoonitic rtXlu cl the Cachoeirinh.Salgueiro FoldBelt (a) and granitoids of the Riacho do Pontal Fold Bdt(b).

".-.~ • ..1 CALC·ilLM.t.lINE $[lllES

S~OSHOItITIC .SSOClolTION

"r-----------------,

........-,/ \

/ \,\I,

"-----'"./ "

" " " " bSiO l %

"

•

•

a&0 70

SiO: %"

""0•

" • ,• ,

--),~"~0 ,

• " /.0

"~"Z , //~.....-• "~I ,

/"~, ",'-l(" /"

CACHOEIRINHA-SALGUEIRO FOLD BELT RIACHO 00 PONTAL FOLD BELT

o SoturoUd pt.alkalin, rocks (s.,.nitoid lin,) o Sao Jooo do Pioui

• Sholhonitic oroniloids

, Sussuorono

from Lavl1lS do Sul, Rio Grande do Sul, Brazil(NARDt, 1986) shown in the AFM diagram(Fig. 5a), they seem to represent more evolvedrocks. In the total alkali versus Si02 diagram(Fig. Gb), the shoshonitic group makes acluster distinct from the peralkaline saturatedone, plotting along the oversaturatedperalkaline rocks. In Fig. 9a, where logCaO/(Na2;0 + KzO) is plotted against Si02•they fall betw~n the area of the diagramreservo:l for the caIc·aIkaline rocks (BROWN,1982) and the peralkaline rocks underconsideration. In other words, they seem tobelong to the alkali-calcic series, acharacteristic of the shoshonitic series.

These granitoids in MORE-normalizedspidergrams (Fig. 10) show strong enrichmentin K, Rb, Sr and Srn and marked troughs in

P and Ti. They are slightly more enriched inZr and Ce than the saturated group ofperalkaline rocks. In primordial mantle­normalized spidergrams (Fig. 11), these rocksdiffer from the saturated peralkaline group ofrocks in relation to Zr which is slightly moreabundant in the shoshonitic granitoids and inCe which is slightly less abundant in thisgroup.

Mineral ch~istry

No systematic mineral chemistry study wascarried out on the rocks under consideration.There are, however, a few mineral analysesavailable on the Salgueiro batholith (SIAL etal., 1983), on the Serra dos Macaces ring-dikeand on the Catingueira dike. These analyses,obtained through electron microprobe at the

BRASIUANO AGE PERAlKAUNE PWTONIC ROCKS OF mE CENTRAL SlRucnJRAl DOMAIN, ETC.. 327

TABLE 4Representfltive mineralchemicalanalyses for peraJlta/ine rocks and/or those with shoshonitic affinities

of bodies within lhe Cachoeirinha-Salgueiro Fold Bell, Northeast Brazil~, ' .......n..

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University of Georgia. Athens, USA. and atSao Paulo University. Brazil, are in Table 4.A pyroxene analysis performed at theGEOSOL Laboratory. Belo Horizonte.Brazil. from the Triunfo bathol.ith. is includedfor comparison.

The pyroxene compositions in these plutonsdiffer slightly. At Salgueiro, SIAL et al. (1983)recognized a smooth chemical zoning withrespect to Fe which increases toward the edgeof grains, and atomic ratios Ca:Mg:Fe indicatea trend varying from ferrosalite to ferroaugite(Fig. 12). This trend might result fromextensive fractionation of this mineral duringmagma ascent. an assumption confirmed bytextural relationships.

The pyroxenes in the oversaturatedperalkaline group (Catingueira and Serra dosMacacos) are predominantly ferroaugite (Fig.12), with much less Ca contents than thepyroxene at Salgueiro. The Na of thepyroxenes at Catingueira is slightly lower than

at Serra dos Macacos, with Ca-Mg·Fe trendsat an angle to that of aegirine-augite at theMorutu district (GOMES et al., 1970). In theSerra dos Macacos. the Mg/Fe ratios arehigher than in the Catingueira pyroxenes.

The fractionation of pyroxmes in a graniticliquid with shoshonitic affinities (e.g.Salgueiro) could cause enrichment in alkalis,especially Na, in the residual liquid. Perhapssuch a fractionation, associated with that ofbiotite and/or Ca.amphibole, partiallyprovides a mechanism to generateoversaturated peralkaline rocks fromshoshonitic liquids.

The pyroxme at Triunfo. from a pyroxeniteenclave, shows Wo. En. Fe contents similarto pyroxene from the Salgueiro batholith.However, it shows lower Si, Fe, Ca contents.much higher Al, Na. Ti and P, while Mg isequivalent to the amounts in Salgueiropyroxenes. Its chemistry approaches the Serrados Macacos pyroxenes. Its high P content

328 SlAL A.N., FERRElRA V.P.

''''''

1111111

11

0 Perolkoline saturated rocksof the ayenitaid line (CSF)

~\ 11

11

1111 UltropotOSlic rocks fromthe west.rn Alp.

'00

\ I1I

\ 11

1111111111\~)Ij~~ I I11\I

'0 \

11111111

\

I1II,

/1m , .~

, ,I ,

0 , I ,

11111111

,. " ,"- ,"-

" 'f-w I '~

, I. '0. , I,. ,.. I,'" ,

0.' L~_~--:C-Oc-----CC--7---=-~---,~-;"",-!!-~---c!.Sr It Rb Ba rh Nb C. P Z. HIS'" T I Y Yb

Fig. 7. - MORB·mrmaliud spidergr:uns for rhe saturated peralkaLine roclu of the Cachoeirinha-Sa.lguciro FoldBelt. Shaded pliItlCI"n is from VENl'U1t:EU...I cl al. (1984), adda:l foe comparison.

(2.2% P20)) is compatible with a highphosphorus tugacity during crystallizalion andsuggests a P-enriched magma. Trace elementsin this pyroxene show a signature whichapproaches that of shoshonitic rocks, withhigh Ba, Sr, low Nb and intermediate Zr.ltshigh Ti content (Ti02 = 3.10%) suggests ahigher Ti02-activity, which coupled with a

high oxygen fugacity, favours formation ofMg-enriched aegirine (NIELSEN, 1979).

Amphiboles from the Salgueiro batholithand the Serrita stock are similar,corresponding to ferroedenite according toLEAxE's classification (l978). The amphibolesanalysed in the~ two plutons are ratheruniform in composition; those from the

BRASlUANO AGE PERALKAUNE PWIONIC lOCKS OF TIiE CENlRAL STRUCTURAL DOMAIN, ETC. 329

­",z•z"•

'. 0•o>

••,•oo•

_c.~~_~, ­•'. '~\'. ,

'l ' ,, ',,'' ,, ,",'"W\\"

SATURATED PERALKALlNE ROCKS

Of THE SYENlTOID LINE IN

THE CSf

/,-.';

/-::.:::::.---- ,

/,,!,,,

Eg. 8. - Primadial mamk·nonnaIiz.ed: spK\ergrams for the saturt.tcd peraIJcaline rocks of. the CdvJeirinha-SaIguciroFold Bdt. Wooo el al. (1979) nonnali:ting values have been used.

Salgueiro batholith are slighty lower in Mgand Fe (Fe/(Fe + Mg) < 0.75), and higher inTi than the Serrita amphibole. This behaviourperhaps indicates that they crystallized atrelatively high temperature, according toRuSE'S (1974) suggestion. SlAL et al., (1983)observe that AJIV predominates over AtVl atthe Salgueiro amphiboles, a behaviour typicalfor minerals formed at high temperatures(THOMPSON, 1947).

Recently, HAMMARSTRON and ZEN (1986)proposed an empirical igneous geobarometerbased on aluminum in hornblende. Theyassumed that At content of hornblende is anindicator of pressures for crystallization ofplutonic rocks of appropriate bulk

composition, where the relation betwttn AIin hornblende and pressures is expressed byp", -3.92 + 5.03 AlT. Preliminary estimatesof pressure using this method for the Serritaand Salgueiro amphiboles (Table 5) indicatethat the first one crystallized around 3.6 kbarand the second around 4.0 khar pressure.

The Santo Antonio Creek amphibole ishigher in Si arx:I Na, and lower in Ca, Ti andAl. The problem of high Si in minetalformulae, with values greater than 8.0, canbe explained by incomplete transformation ofpyroxene into amphibole. Even so, theyapproach magnesian-arfvedsonite. accordingto LEAu's classification (l978).

HO SlAL A.N.• FERRElRA V.P.

••

•..

""""

••

.1----"'~~"',,"'---___1

oo"

.... _0'o·u

,••,••

•

00X •••

• •

,·......'"9...",.,",1'.

..

..

.... _0.'o·u

,·•

,•

OHo ""OH'.".' C•• I ,.CO

•

...o oio ... .i....., •• '

.•'--_--;.,_~__••\_-~--~,~.--~-J

'10,% ba

••.•'--_--;.,-~--••\_-~--.,~.~_..J

SIO, .,.

. ,. .. ,.. I:lo ., •••

• L nu' A ••••••••••

Fig. 9. - C"c-alkali ratio-silica trends for peTll.lkaline and shoshonitic rocks of the Cachoeirinha-Salgueiro FoldBelt (a) and Riacho do Pomal Fold Belt (h). The trend for several volcanic magmatic arcs are from BROWN (1982).

TABLE 5Pressure estimate for the crystallization 0/ Qmphiboles from the Serrita and Salgueiro plu/ons

S_JI",·,ro

AS-In

f'nl!nOCfllsts 1 1* ~. ..rr.-ss"... ".b 3.b ::I.b '3.1ouu,r I

1-1* ~-1

J.1j 'I.\!

Rare~Qrlh elements

10 the central structural domain, only rocksfrom the CSF we~ analysed for REE. Somewere analysed by XRF in the Departementof Geology, Memorial University,Newfoundland, Canada and others byinduced coupled plasma at the GEOSOLLaboratory, Bdo Horizonte, Brazil.

The peralkaline saturated group (e.g.Triunfo batholith and Momalno·Prin=a Iubeldike swarm, Fig. 13a, b) has REE signaturescharacterized by strong enrichment of theLREE and depletion of HREE (FERREIRA andSIAL, 1987). The patterns for this group havediscrete negative Eu anomalies (Eu/Eu'" variesfrom 0.62 to 0.76) with the total REE from89 to 437 ppm. There is an increase of the

BRASIUANO AGE PERALKAUNE PWTONIC ROCKS OF THE CENTRAL STRUCTIJRAL DOMAIN, ETC. JJ 1

","",-------------------------,

rocks from the

"

• Shoshonitic Qronitoidl CSF

C Oversoturated perolkalinerocks CSF

I1I11 Ulf<, p,t, ..I,

westun Alps

Shashonitic rocks from thewestern Alps

I

Sf K Rb 110 T~ Nb C. P Zr HI Sill Tl , "Fig. 10. - MORS-normalized spidergrams for oversaturated peralkaline rodu and shoshonitic graniloids of theCachoeirinha.Salgueiro Fold Belt.

total REE with the decrease of Si02 and~O, with caO showing opposite behaviour.The patterns for the oversaturated peralkalinegroup (Fig. 1x) depart from those usuaUyobserved for this kind of rock, lacking thetypical negative Eu anomaly. In contrast, aEu anomaly is either missing or slightlypositive (Eu/Eu· varies from 0.86 to 1.89),

with total REE from 67 to 309 ppm.The missing Eu anomaly perhaps reflects

rather high oxygen fugacity duringcrysta.l.lization (I)lwcE, 1975; HANSON, 1980).Alternatively, the Eu anomaly could havebeen inhibited by Ba and Sr contents in themagmas, in the way foreseen by Bnuc et al.(1979).

332 SIAL A.N., FERREIRA V.P.

·='n--------------------------,

• Sho.honitie roek. of 11" CfS

o OVluoluroad plrolltolin••oets.ill"" CSF

•""z l" •> ,," " ,• t'"0 Id• t,,'0

'"> I"- t"• 11' /(• ",,1/1/ ' .

~ • ,~

" '" 00

:~Ii,

• '.f.f/

Fig. 11. - Primordial mantle normaliud $pidergrtmS for O\'ersatunllcd penllkaline rocks and shosOOnitic graniloidiof the Cachocirinha·Salgueiro Fold Bell.

The pyroxenite enclaves are REE-enriched,with LREE approximately 240-900 timeschondritic abundances (Fig. Dd). The patternfor aegirine (Fig. 13d) resembles those forferrohedenbergite (MAHOOD and HILDRETH,1983). LREE are 100 times chondriticabundances and HREE are about 20 times,HREE showing upward concavity (PERREIRAand SIAL, 1987).

The REE patterns for the oversaruratedpualkaline rocks from ttt ring dikes at Serritastocks (Fig. DO an: LREE-enriched andHREE-depleted with no Eu anomaly. One ofthe patterns, however, shows REE muchlower than in~ other samples, HREE below

the chondrite values, and a posltlve Euanomaly. Similar to feldspar patterns, this oneperhaps represents a feldspathic cumulate.

The granitoids with shoshonitic affinities(Solidao and Teixeira batholiths, Fig. 13f)display LREE-enriched patterns with HREEapproximately flat. They show no Euanomaly. Similar patterns were found byNARDI (1986) for the Lavras do Sui and SantaRita shoshonitic: granitoids in Rio Grande doSul state, Brazil.

Stable iJoto~

All oxygm isotope analysesW~ performed

BRASlLlANQ AGE PERALKAUNE PLUlON1C ROCKS OF THE CENlRAL STRUCTURAL OOMAIN, ETC. 333

by reaction with fluorine at the Departmentof Geology, University of Georgia, Athens,USA. Standard analytical procedure is foundin WENNER (1981). Only rocks from the CSFwere analyzed and results are in Table 6,where values for the basement andsupracrustal rocks are included.

Most of the bodies analyzed exhibit anoverall variation of about 2 permil and thedata define three groups of peralkaIine rocks.The Triunfo batholith, in the syenitoid line,shows 0180 values rather low (+ 6 < 0180< + 8), suggesting either differentiation frommafic magma without post-magmaticalteration, or mantle derivation, with minorcrustal contamination. Values for theoversaturated peralkaIine Catingueira dike,the eastern portion of the Salgueiro batholithwhich bears shoshonitic affinities, and the Jingdike of Serrita, are higher (+ 8 < 0180

< + 10). These divergences can be ascribedto differences in the isotope compositions ofthe protoliths, degree of differentiation,interaction with meteoric water, or morelikely, to the nature and degree of assimilationof crustal rocks by mantle-derived magmas.Because .6. (Q-KF) for igneous rocks in isotopeequilibrium ranges from 0.8 to 2.0 (TAYLORand EpSTEIN, 1962; O'NEIL and TAYLOR,1966), and numbers outside this range wererecorded for the Catingueira dike and theSalgueiro batholith, there is slight isotopedisequilibrium in these rocks, probably causedby exchange between feldspar and meteoricwater at sub-solidus temperature.

The Santo Antonio dikes have the highestvalues, 0180 > + 10. They penetrate high0180, low-grade metamorphic rocks of theCSF and perhaps the high values observedresulted from the interaction between those

TABLE 6Oxygen isotope analyses 0/ peralkaline rocks in the Cachoeirinha·Salgueiro Fold Belt, Northeast

Brazil

SER-43

SER-44

ROCK TiPE W.R. QUARTZ n:UlSPAIl q-p~"O .,. '''0

(SHOlI) 0 O(SKOl/> " (SIiW")

.ubvolc.nio fine-grained + 12.21

Hg-ufved.oni<o-bcoring > 10.60

trochytu

S.oto Antooio Cruk. 30 b north

at Serrit•• Puoaiobuco

SER-63 aegirino-bcoring

CAT-5

CAT-2

CAT-l

CAT-6

TRF-13

TRF-14

TRF-~

TRF-I

S-12

S-20

'·W,.,

grani tU

ugidne-buring

q~.ru-.H-_ali­

fdd.par sy..ni'u

ugirine-buring

elhli-feldspar

syen; tu

torroaugite

q"srtt ""'ntoniu

of sho.hon;,ic

.ffiniti ..

•

•

•

•

9.66 Sura ••• Il.ocacos, Suriu, Per-

nalllbuco (ring-dih)

9.81 · 8.67 • 10. J7 · I. 70 c.tingueira. Parafba (diko)

9.35

8.69 · ..., · 8.45 • 0.35

8.12

7.95 Triunto, Pern....uco (batholith;

7.79 syenitoid line)

6.79

6.79

9.95 "astern portion of ". Salgueiro

9.93 · 12.22 • 9.92 • 2.30 batholich. Pern.",b"co

9.35

8.92 • 11. )'5 · 8.92 • 2.83

~ rock.

Cachoeirinhe phyllit..

.upracrusul

rock.

Solgueiro .i.c..ehie..

Group

Ua". Gro"p gnei.....

..ig1llolltitu

> IS.65

• 13.72

+ 12.63

334 SIAL A.N., FERREIRA VP.

rocks.The rocks with shoshonitic affinities display

values equivalent to the Catingueira type.Probably they resulted from mantle derivedmagmas with some continental crustcontribution.

2. TM Riacho do Ponkl/ Fold &/t

Fifty analyses of major and some traceelements for granitoids in the RPF are

available in GAV" et al. (1984). Someo£ th~analyses are iD. Table 7 with their respectiveCIPW norms. No stable isotopes or REEanalyses are available for this segment of theCSD.

These rocks have an alkalinity index(K/<Na + K» in the range of socIic to sodic·potassic, approaching a potassic composition,according to the DEBON and LE FORT (1983)classification, hut no ulrrapotassic rock wasfound. Although previously regarded as an

TABLE 7Representative partial chemical analyses 0/granitaids in the Riacho do Pontlll (GAV" et ai" 1984)

and 5erid6 (SOUZA, 1987) Fold Belts

.1.~HQ .. _ONIAL , OlU ~~L , ~LUUO "'LU nL!

d. .10;;'0 •• ~ ,"., h"".".n. ! ""....on..."

, ...ft,n, .. ''"ft"",,, "'ft,n'4. ",.,nOlft. f.ou,. ...... ...d • orlno,'".. ....... • Iou",, , , • , , , , •~.02 •V.•• 11 ••• .1."" ..... H." ...:.. '''.SI U.~. ..... lI.a• .~.~. 5' .a• 15." 5.... 11 ...!'O~ •• U ..~ •• :J<! '.1 • •• U •• I!> '.n ..~. '.1. •• N •• 11 3." '.21 '.'2 ....

"'2<>J •~.n n ..... ,..".. ".,.. ,.... .~ .... 'J." ..... ..... 'J.V' ... ~' ...... ... J' ..... ".2'• .2CI~ ..~ ..~ ,.... ..:>• .... ..... • .•a ..." .. ~. 1.2. ,." ..... .. '" 1.1' ....'" .. " ..~ I."" .... I. '2 .... ..U '."a ..~ I." '.H .... '.n 5 ... ..~.-. .... .... •••• .... .... .... .... ..... ...~ ..U .... .." ..." •. '2

._.H' '.Ja •. J. .... ".2' •• aa .... '.J" •.2 • .... •. H 2 ... ,." .... '.11 c"... .. "" ,.", ~.~~ '.'2 •• H .....a I."'" 1.7~ ~... '.N 2 ... '.N '.H '.H '.H•• :/1 ~..~ .... .... ~.U , .•a 5.15 •• N ~... '."" ~... ,... 1.2• '.13 3." '.11'N .... '.19 :I... :>.30 '." '.H .... '.'" .... .... 2 .... .... .... ,... ...."H .... .... .... .... .... .... .... .... ..... .... ..... •• W' .... •. n .....""'....."'" '.H •. n ' ••2 .... .... '.H '.,,' '.19 .... '.H '.11 ••U H.H '.H '.H' ... ...~ ..H •.la ..." .." '.H .... .... ...' ....'"

..~1t.........

, , , , , ,• .... ~. ~•• U ..... ~..' :k••5 2 •. :11 ..... '5." a .... :<2. , •• .... .... .... .... '.H 1.11 .... •• '11 ..H ....• ...... 11 2'.13 ~'.IT ,,"J.'a ...... 2. 2'.5. ..... 2'.22 IT."" ...h.. .a.~3 3'. " ".1' ~, ... 2•••• '~.I' ».n ':>.2' ,...... 2'.231.•• • .12 ..... ~.U a.N '.11 .." '.13 •. N a."" .... .... .." .... .... .... '.N •••• .... ...... .... .... .... .... .." .... 1."6 .... ..... ...... .... .... .... '.2a .... .... •••• .... 2.~ • ..a3.. .... .... .... .... .... .... .. " .... .... • ••S.. ..". '."'" ..n '.'" a.~. .... .... .."..... '.".. .... •••2 ....~ , .11 .... .... .... .... .... '.31-. •••• .... .... 2.a. a.a. ..'" .... ..... ..... ....., .... • • '2 2." ...~ 3... '.U ·... 1.5' •. J' ,.,." ..". ...., ..., '.a. ..~. ..... .... ...a" ..a• ..~".. .... .... .. ... .... 2."5 .... .... .... ,.~, ...... '.2. ..,. .... •••• .... ..~ .. ••Z' .... ..12 ••••

< 11..... <l.....~

, , , , , , , , • ,.. 1/'111 "'h. '''2. ~~.. ... 2... 22•• 211. .na nil ... ... m '" ,~... 122 ." '" ". '" ". '" ". "'9. ~. ... .. ... H' -•• "" ,~ .... 13•• " ,,~. ... • 03• m ". ,.. .,'" ... ...

'" .. .. .. .. .. "'" N .. .. H .. .. .. ..•• '"H .. .. ," .. " .. " N .. " .. .. .. .., .. .. .. .. .. .. ..

" ... • H ," ,.. ... ," ~. 'H

" H H .. " " " " "..rll'''," .. It. • -" H' ...<~.u .........." ...." •-"~........ ,< ". _ ....n".-.ul •• ,......_' .. '. ,- ........._' ........ ,. ........... .. <........... - ....... ,' C w.. " " .• u ........ ', • -" ". ....... ,.".

BRASIUANO AGE PERALKAUNE PWlONIC ROCKS Of THE CENTRAL snmcruRAL DOMAIN, ETC. 335

Co

t

"

• SHOSHONlTlC GRANITOID (SALGU[IRO "ATHOLITH I

x OYEItSATURATED ROCKS (CATINGUEIRA DII(E. SERItADOS ".ACACOS RING DIKE 1

'RACTION.o.TION TREND 'OR PYROI(ENES FRO'"MDRUTU (Y.o.GI, 19531

Fill, 12, - CompositionaJ data for clinopyroxenes from the oversaturated peralkaline Catinllueir. dike, Serra dosM.cacos tinll·dike and Sa11lueiro batholith ploued in the pyroxene quadrilateral. Morutu trend (VAGI, 19.B) isadded for comparison.

alkaline: provincc, moS[ of these rocks bearsome: corundum in the: norms and only one:sample: among the: fifty anal)"Ud has acmite:.Howe:ver, a few analyses show peralkalinityinde:x around 1.0.

The: RI-Rl plot (Fig. 4b) ddines twosubparallel trends. One: of the:m (sample: fromSussuarana granitoids) sc=c=ms to fonow theboundary between late-orogenic andanorogenic granitoids, a contention whichmatches field observations. The second onc(samples from Tigre and Sao Joao do Piau{granitoids) follows the boundary betwc=c=n thefields reserved for late:-orogenic and post­collision uplift granitoids and penetrates intothe syn-collision granitoid fidd.

In the AFM triangular plot (Fig. 5d) thesegranitoids show two trends, which suggestthat magmas evolved independe:ntly, unde:rdifferent oxygm fugacity conditions.

The: analyzcd granitoids of this belt clusterin the shoshonitic field of the alkalis vs. silicadiagram (F>g_ 6b) ilighcly groding into the c,"c­alkaline and alkaline fields.

In Fig. 9b, the granitoids from the RPF liein part in the area reserved for the: calc·alkaline granitoids, and like the granitoids ofthe eSF, they grade into the area of alkalinerocks (Fig. 9a). MORB-normalizedspidergrams (Fig. 15) show a markedenrichment in K, Rh, Ba, and Th, and adepletion in Ti, with Zr equal or twice as highas the: corresporxling MORB value. Strontiumvaries from below MORB values up to values7 times greater. Early plagioclase fractionationperhaps explains the depletion in Sr in someof these rocks. Their high Ba and Sr conte:ntsassociated with low Nb, Ti and Zr in additionto their MORB-normalized trace elementsignatures suggest that they belong to the:shoshonitic association.

Perrogenesis: a Lll.E-enriched mantle source?

Hypotheses on ulrrapotassic magmageneration require (THOMPSON, 1985; NELSONet al., 1986); (a) partial melting in theIithosphe:ric mantle, previously enriched in

H6 SIAL A.N., FERRElRA V.P.

-- So•••••• d p.,qlhIoU- 'OC" 100U"0;d h •• )

-'-•C•0• »0• »u .-,• •U0

, -• • .-• .-•••• ,,'u••o ........c••• -....... 00". DIU

-: ~"

»..,• So""O"O ",",,,1_01,..

""'., (_i,o_Pr,oco,o'._1 al.........)

-- o........,a'.o DO.Olkoh..- .CKloI .,_ '0' CSf

-"'-• ....... " ...» ' "'-. '.

.~,

... "

• ,~, ' ....., ,w.

" -,-' -• "",.""'.......,.....u -.OC ............,.. O,U ,. Hu.· .........___Ino·.IttIOU_ ...-...•

... Co..... ,..... TO ..... £< , •••

o... 0:0 1 1•••

blA Go , ..

c

... Co ........... n ......~T.......

, •• f[,U,U ""'OC".-.' 10LOO.0 u,"oc'"

,_r---------,'.----

P.rolOollno 'inGdj ... 01 S."i,o

•

•

'...·r---------,

d

•• '- -0.__.. 1"_• ..• ..........._ ...VOOU_

-,.--• -u -•0 -•0 •• »u,»• --.u »

0 =.•

Fig. 13. - Chondrit...·l'KXmalizcd REE.pallerl1$ for saUlrlltec! pcralkalinc rocks of the syeniroid line (I), M.nafr.­Princesa lubd dike swarm (b), ovttSaturated pcralkalinc rocks (e), pyroxenite and aegirinc of the Triunfo b.t:hoI.ith(d), oversltUnllcd peralkalinc ring.dikc It Semla slock (e), shoshonilic graniloids (0. (FEtaElllA and SIAL, 1986;NEYES, 1986; FEJUI£JllA and StAL, 1987; StAL, 1987).

incompatible elements; (b) generation in theasthenosphere with contamination in thelithospheric mantle during magma ascent; or(c) enrichment through crusta! sediments ormegaliths recycled by subduction.

The petrological and geochemicalcharacteristics of the shoshonitic series suggesta generation by partial fusion of a previouslylarge-ion Iithophile elements a..ILE)-enrichedmantle (GIROD, 1978; BARKER, 1978; CARRand FARDY, 1984). Alternatively a genesis bymantle melting and crustal contamination hasbeen proposed.

Alkali enrichment in the mantle has beensuggested by different processes and theinfluence of metasomatizing fluids has been~phasized by several authors (ERU.NK, 1976;MENZIES and MUllTIfY, 1980, ERLANK et al.,

1982; MENZIES and WASS, 1983; RODEN etal., 1984; ROGERS et al., 1985; COMIN­CHlARAMONTI et al., 1986; FOOOR andMcKEE, 1986; DUDAS et al., 1987).

Shoshonitic associations characterize thetransition of orogenic to alkaline magmatism.They are usually associated with processesleading to the formation of rifts in areas ofcrustal thickening, or are rdated to sulxl.uctionof oceanic crust or ensialic orogenesis(MARTIN, 1983).

The geochemical patterns observed in theperalkaline and shoshonitic granitoids in theCentral Structural Domain lead to theassumption of possible mantle LILE­enrichment prior to or during the LateBrasiliano cycle, or alternativdy, to systematiccrustal contamination with rocks of

BRASILlANO AGE PERALKAUNE PLUTONIC ROCKS OF THE CEN1'RAL STRUCTURAL OOMAIN, ETC. 337

c,G,

Hb0,

Ol.. Pl

"c,"

" c,"Vb, '" Vb"

00 '" '"

0 '" '"0 0'0 0 '0

"''''

Vb,',:--~,:--"~O--C,~O':"J,,O

ao SATURATED PEIlAlKALlNE ROCKS

Yb,

":--~':--"~O--CW.,-,:,~b

6 OVERSJ,TURATED PERAlKALINEROCKS

Fig. 14. - CeJYbN versus YbN plot for saturated (a) and oversaturated (b) peralkaJine group of rocks. At thetop, the vectors indicate the frllCtional crystallization paths for the indicated minerals: GR, garnet; CP, dinopyroxene;HB, horblende, OP, orthopyroxene; OL, olivine and PL, plagioclase (VENTUIlELLI et al., 1984).

appropriate composition.Metamorphic rocks of the Seridb Fold Belt

(Table 3), including orthogneisses of theArchean Caica complex and supracrustals(biotite-schist, quartzite and cordieritegneisses), show high Ba and Sr and low Ti andNb, conforming to geochemical patternsobserved for the ultrapotassic and shoshoniticgroups under consideration. However, as allgroups of granitoids recognized in the CSFand surroundings by ALMEIDA et al. (1967)and SlAL (l984a, 1984b, 1986) systematicallyexhibit high to extremely high K, Ba, and Sr,and LREE-enrichment, it is difficult to acceptassimilation as the major process leading toultrapotassic or shoshonitic compositions.

There is no doubt that crustal assimilationtook place, as rec:orded by schist xenoIiths andby oxygen isotopes, but this would notsystematically raise the LILE to values as highas those observed in the peralkaline orshoshonitic rocks in the CSF. Theseconclusions imply in that magmas wereprobably generated in a REE- and LILE­enriched mantle and underwent some crustalcontamination in variable amounts. Thosewhich filled the fractures in the syenitoidline (Triun£o-type syenites) were lesscontaminated, as indicated by lower 6180values.

A metasomatized mantle beneath NortheastBrazil has been proposed before. Areas of

338 SIAL A.N., FERREIRA V.P.

o

(:,

•

500 Jado do Plou;

Tigre

Sussuarano

fromrocksAlps

Shoshoniticthe west.rn- I

a\7''')"\' ,",

,\I

" -'\'\ -

\ \ '-- ::.\ ':t\ ::\ '\~ =.\~ -=\ \\: -

\ I\'\\\1 \\1 ,

\. \,, ,, \" ,

,\ \.'\~O::<~","\\ \'

\\\'~,\~~\ \

\\\\ \\~\ \,,\, \ -\~\\\ ,

'\ 'b--'"

000

0.'L~-~_~~ ~~~~_~_~~_4-_~---'

Sr I( Rb 80 Th Nb C. P Z. HI Sm TI Y Vb

Fig. l:i. - MORB.normalized spidergrams for granitoids o( the Riacho do Pontal Fold Beh.

veined or metasomatized mantle like thoseproposed for South Atlantic Ocean (e.g.WaIvis. ridge, Bouvet) have bttn recognizedin the mantle lx=neath the Mesozoic POliguarbasin, Rio Grande do Norte state (FODOR andMCKEE. 1986). Mesozoic tholeiitic diabasesand Tertiary alkali basalts in Rio Grande doNorte and Para1'ba states shOW" very high K,Ba, Sr, P (StAL, 1974) which reflect a LILE-

enriched mantle-source. These observationsimply that the mantle beneath NonheastBrazil has undergone successive enrichmentevents or a major late Precambrianenrichment, or the existence of ananomalously ULE~nriched area in the mantlesince Archean or primordial times.

The syenitoid line of probable Brasiliano( = Pan-African) age may find in the

BRASWANQ AGE PERALKAUNE PWlONIC ROCKS OF THE CENTRAL Sl1U.ICI1JRAL DOMAIN, E1C. 339

~ralkaline, lineary Itiuba syenite dike (120km long, 12 km wide in northern Bahia, whichseems to meet the syenitoid line at 1200 angle)a Transamazonian ( = Eburnean) to Archeananalogue. This syenite also exhibits high Baand Sr (CONCEI~AO et al., 1987) and REEpatterns (FIGUEIREDO, 1985) similar to thoseshown here, and this leads to the conclusionthat the LlLE-enrichment happenedwhenever magmas bearing some alkaline orshoshonitk affinities were formed, or that themantle was enriched since Archean times.

The chemistry of pyroxenite enclaves(LREE-enricho:::f) in the peralkalinc: syenite (ofthe syenitoid line) regarded as earlyfractionated material reinforces the conceptof mantle derived magma already LILE­enriched. Because the pyroxene contains ahigh amount of phosphorus (around 2%), itsfractionadon caused the depletion of Pobserved in the spidergrams.

More structural and Sr, Ph and Nd isoto~

data are necessary to precisely describe thenature of the mantle source, the crustalcontribution, and the time of LILE­enrichment in the mantle.

Probably subalkaline or moderate alkalinemagmas formed in the upper mantle throughpartial mehing of a garnet-peridotite source.These magmas intruded well-developedftactures to form the peralkaline saturatedgroup of rocks of the syenitoid line, orinteracted int~sely with the crust to produceshoshonitic granitoids and peralkalioeoversaturated rocks. The mechanism throughwhich the shashonitic liquid passed intooversaturated peralkalioe ones is 001 clear.Biotite in amphibole in the shoshoniticgranitoids suggests its early fractionation andthis could explain the decrease in Mg, Fe, Caand subsequent enrichment in Na in Iheresidual liquid.

The lower 611lQ for the saturated group ofperalkaline rocks (e.g. Triunfo batholith) iscompatible with mantle derivation, followedby minor crustal contamination. Theoversaturated peralkaline group shows higher0180, not much different from theshoshonitic gr:anitoids. In a preliminary 618Qversus 87Srj86Sr plot for the saturatedperalkaline group and rocks with shoshonitic

affinities, FERR£IRA (1986) found the formerto have undergone a source contaminationwhile the latter suffered crustalcontamination.

The regional geographic patterns displayedby the peralkaline plutons in the CSF, inwhich they seem to follow major sigmoidalfault zones (e.g. the syenitoid line and thehorn-shaped Catingueira dike) are perhapsrelated to pull-apart processes along thesezones, associated with the Patos-Aurora andPernambuco transcurrent lineaments. Theshoshonitic granitoids represent latetectonically emplaced plutons grading intopost-tectonically emplaced, oversaturatedperalkaline piu tons. On the other hand, theemplacement of several dike swarms withperalkaline affinities probably followed theestablishment of thermal anomaliesresponsible for doming and crustal fracturingduring Late Brasiliano cycle in the CSF andelsewhere in Northeast Brazil.

Ackflowkdgements. _ We are indebted to Ihe FINEP(Financiadon de Enudos e Projelos) Agency whichthrough the PADCT (Programa de Apoio aoDesenvolvimento CiendHoo e TCCD016gico) progl"1lmpartially supported this study. One of us (ANS) wantsto express his gratinne to the CNPq (ConseIho NlltklnaIde Desenvolvimento Ciemiffco e Tecnol6gicol ~ncyfor a grant.in-aid which covered the fint expe~withthe field work. We wish also 10 Ihank Dr. D.B. Wcnnerwho allowed ANS to use his stable isotope Laboratoryin UGA and R.V. Fodor for helpful comments on anell.!"ly version of lhe manuscript. Finally, tan word ofthanks goes to G. Mariano who analyz.ed oxygm isotopesin a few sampJes from the Triunfo batholith and 10 H.MPortO who helped prqw-ing some of the diagrams.

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