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Precambrian Research 245 (2014) 186206

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Precambrian Research

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Mesoproterozoic crust in the San Lucas Range (Colombia): An insightinto the crustal evolution ofthe northern Andes

Federico A. Cuadros a,, Nilson F. Botelho a, Oswaldo Ordnez-Carmona b,Massimo Matteini a

a Instituto de Geocincias,Universidadede Braslia, CampusUniversitrio Darcy Ribeiro, AsaNorte, CEP 70910-900Braslia, DF, Brazilb Universidad Nacional de Colombia, Sede Medelln, AA 1027 Medelln, Colombia

a r t i c l e i n f o

Article history:

Received 24 November 2013Received in revised form 17 February 2014Accepted 18 February 2014Available online 28 February 2014

Keywords:

SanLucas RangePost-collisional graniteTransitional mafic rocksEarly MesoproterozoicGrenvillian/Sveconorwegian orogeny

a b s t r a c t

The San Lucas Range (SLR) is located atthe northernmost end ofthe Central Cordillera ofColombia andisconsidered part ofthe Chibcha Terrane, which is characterized by medium- to high-grade rocks with LateMesoproterozoicEarly Neoproterozoic metamorphic ages. Granite-gneiss and metamafic rocks, includ-ing metamonzogabbro, amphibolite and granulite, crop out in the northern portion ofthe SLR, with aLower Jurassic granodioritic batholith intruding all the above-mentioned units. The geochemical fea-tures, in terms ofmajor and trace element contents and UPb zircon geochronology, suggest protolithcrystallization ofboth felsic and mafic rocks in a post-collisional setting between 1.54 and 1.50 Ga. Inaddition, positive Ndvalues and initial87 Sr/86Sr ratios less than 0.7045 indicate a mantle origin for thisbimodal association, with TDMvalues between 1.7 and 1.5 Ga, suggesting a juvenile character. A cor-relation between the studied granitic rocks and the A-type Rio Uaups Granitic Suite in the Rio NegroProvince of the Amazonian Craton can be established, thus constraining a provenance from southernlatitudes for the Chibcha Terrane, as suggested by earlier models. Metamorphic rims ofzircons fromboth felsic and mafic rocks yielded ages between 1180 and 930 Ma, which are consistent with the agesofrelated metamorphic terranes in Ecuador, Venezuela, Per, Mxico and Central America. The latterterranes are regarded as having been part ofthe northwestern border ofAmazonia during its collision

with Baltica in the context ofthe Grenvillian/Sveconorwegian orogeny, which was related to the finalassembly ofRodinia.

2014 Elsevier B.V. All rights reserved.

1. Introduction

The San Lucas Range (SLR) is located at the northernmost endof the Central Cordillera of Colombia and bears special interestas one of its most promising regions for gold and silver miner-alization, both hydrothermal and alluvial in nature. It is also oneof the key places where the basement of the Chibcha (or East-ern Colombian) Terrane can be observed directly (Restrepo andToussaint, 1988; Toussaint and Restrepo, 1996; Ordnez-Carmonaet al. , 2006). Despite its economic and geoscientific importance,the SLR remains poorly studied mainly due to the difficulties ofaccessing the area and social issues.

Corresponding author. Tel.: +55 61 31077029.E-mail addresses: [email protected], [email protected]

(F.A. Cuadros), [email protected] (N.F. Botelho), [email protected](O. Ordnez-Carmona), [email protected] (M. Matteini).

For example, it has been recognized that the unit known as theSan Lucas Gneiss is far from being lithologically homogeneous, as itgroups several metamorphic rock types having different protoliths(e.g., Feininger et al., 1973). Nevertheless, this heterogeneous char-acter has not yet been represented cartographically, as the unitis currently mapped as a whole entity along its entire extension.Another example of the poor geological understanding of this areais the nature andoriginof its gold mineralizations, forwhich a clearclassification and a robust, quantitative data-supported metalloge-netic model have yet to be presented.

The research carried out in the SLR during the second half of the 20th century did not consider the geological knowledgeof the region beyond petrographic and stratigraphic descriptionsof its units, with few or no considerations about their genesisor the geodynamic settings of their formation. During the 1990sand the first decade of the 21st century, the first modern tec-tonic models for the Colombian Andes, particularly for the easternand northern flanks of the Central Cordillera, began to be pub-lished, stressing the importance of the Grenvillian orogeny on its

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F.A. Cuadros et al. / Precambrian Research 245 (2014) 186206 187

geological evolution (e.g., Toussaint and Restrepo, 1996; Restrepo-Pace et al., 1997; Ordnez-Carmona et al., 1999, 2002b, 2006;Cordani et al., 2005;Cardona et al., 2010a). This was a major break-through because it allowed that portion of the Colombian Andesto be geologically articulated in relation to the rest of the Andeanchain, for which a Grenvillian geochronological signature has beenrecognized in the blocks that constitute its basement (e.g., Fucket al., 2008; Ramos, 2009, 2010).

At present, it is difficult to unravel the pre-Neoproterozoicgeological history of the Colombian cordilleras, and this study con-stitutes one of the first attempts to assess the origin of some ofthe units that make up their basement. This work, although notintended to solve all the problems and unknowns related to theregional geology of the SLR, does provide insight into the natureand geodynamic evolution of one small part of the range, namely,its northernmost portion in the municipality of Barranco de Loba,south of the Bolvar department.

2. Geological setting

2.1. Regional framework

The geological evolution of the Colombian territory has beendescribed within the context of terrane accretion models, whichstate that the western portion of the country consists of a mosaicof allochthonous lithospheric blocks docked to the northwesternborder of the Amazonian Craton (INGEOMINAS, 1983; McCourtet al., 1984; Restrepo and Toussaint, 1988). One of those blockscorresponds to the Chibcha Terrane (Fig. 1), with its basementbeing almost entirely covered by Phanerozoic sedimentary rocksand displaying restricted exposures along the eastern flank ofthe Central Cordillera, the Quetame and Santander Massifs in theEastern Cordillera, the Perij Range, the Sierra Nevada de SantaMarta Massif and the Guajira Peninsula (Toussaint and Restrepo,1996; Ordnez-Carmona et al., 2006). A common characteristic

linking these provinces is the presence of amphibolite to granulite-facies rocks, which are paraderived metamorphic units that haveyielded UPb zircon metamorphic ages between 1200 and 890Ma.These rocks would be the result of the reworking of pre-existentcrustal materials as old as 1800 Ma (e.g., Restrepo-Pace et al., 1997;Ordnez-Carmona et al., 1999, 2002b, 2006; Cordani et al., 2005;Cardona et al., 2006, 2010a). Locally, this basement is uncon-formably overlain by low-grade metasedimentary rocks displayingOrdovician-Silurian faunas and palynomorphs and unmetamor-phosed sedimentary strata ranging from Devonian to Cenozoicin age (e.g., Harrison, 1929; Botero, 1940; Mojica et al., 1988;Grsser and Prssl, 1991; Ward et al., 1973), which allows con-straining a low-grade metamorphic event that took place duringLate SilurianEarly Devonian times.

Early to Middle Jurassic volcaniclastic rocks that are inter-mediate to felsic in composition were deposited mainly on thewestern margin of the Chibcha Terrane. These rocks, as well as thepre-Devonian metamorphic basement, were intruded by quartz-dioritictomonzograniticbatholithsthathaveyieldedagesbetween130 and 214Ma (Feininger et al., 1973; Vesga and Barrero, 1978;Sillitoe et al., 1982; McCourt et al., 1984; Jaramilloet al., 1980; Drret al., 1995; Cardona et al., 2006) and host important prospects ofAu, Ag, Cuand Mo mineralizations. The Chibcha Terrane is thoughtto be separatedfrom theAmazonian Craton to the east by the faultsof the Guaicramo system, while the western boundary with theTaham Terrane is marked by the Ot-Pericos Fault. The docking oftheChibchaTerrane onto itscurrentpositioncould have takenplaceduring the Late Paleozoic (Restrepo and Toussaint,1988; Toussaint

and Restrepo, 1996).

The SLR is located at the northern end of the Central Cordillera,east of the Ot-Pericos Fault (Fig. 2), thus forming part of thewestern border of the Chibcha Terrane. Orthoderived quartz-feldspar gneisses, amphibolites and retrograded mafic granulites(Ordnez-Carmona et al., 2008) grouped into the unit known asthe San Lucas Gneiss make up the basement of this region, forwhich an ID-TIMS 207Pb/206Pb age in zircon close to 1150 Mahas been obtained (INGEOMINAS-UIS, 2006), although Ordnez-Carmona et al. (2009) reported an LA-ICPMS UPb age in zirconof ca. 1500Ma. The San Lucas Gneiss is overlain on its northernportion by volcaniclastic rocks comprising tuffs, volcanic breccias,agglomerates, sandstones and siltstones of the Noren Formation,for which an Early to Middle Jurassic age has been assigned basedon stratigraphic relations to other fossil-bearing rocks farthersouth (Bogot and Aluja, 1981). Granodioritic rocks of the NorosBatholith, which arguably represents the northern extension ofthe Segovia Batholith in the department of Antioquia (Gonzlez,2001), intrude the former units and have yielded crystallizationages between 160 and 200Ma (INGEOMINAS-UIS, 2006; Mesz,2008; Ordnez-Carmona et al., 2009).

2.2. Local geology

Next, field and petrographic descriptions of the units foundwithin the area of study are presented. For geographical and geo-logical references, see Figs. 2 and 3.

2.2.1. Granite-gneissThis group makes up the bulk of the area of study and com-

prises medium- to coarse-grained rocks ranging from monzo- tosyenogranitic in composition. These rocks are leucocratic and dis-play a compositional banding/foliation that appears as alternatinglight and dark layers, the former being quartz- and feldspar-rich,while the latter are often enriched in biotite and/or amphibole(Fig. 4A). Locally, the rocks are enriched in potassic feldspar, whichimprints a characteristic pinkish color on the rock massifs. In thinsection, the mineral paragenesis consists of quartz, microcline, pla-

gioclase, biotiteand hornblende,with chlorite commonly replacingthe two latter minerals as a result of retrograde reactions in rimsand along cleavage planes. Masses of chlorite and/or biotite andhornblende relicts are arranged as narrow strips that give therock a banded appearance. Opaque minerals commonly consistof magnetite and ilmenite, with small grains of pyrite being rareand apparently derived from hydrothermal alteration along withsericite, clays and epidote. Zircon, allanite and apatite are the mostcommon accessory minerals in these rocks, with titanite associatedwithchloritizedbiotitecrystals.Thetextureismainlyrelictaphyric,xenoblastic and equigranular, with grain boundaries ranging fromlobate to curved. Metamorphic growth is evidenced by poikilo-blastic textures of feldspar crystals engulfing small rounded quartzgrains. The quartz commonly displays sutured boundaries, poly-

gonization textures, oriented stretched grains, undulose extinctionand local micro-graphic and myrmekitic intergrowths.

2.2.2. Metamafic rocksRocks from this group display fairly similar mineralogies, but

with variable modal proportions and degrees of recrystallization.These rocks are observed to intrude the granite-gneiss, cross-cutting its banded structure (Fig. 4B). Their occurrence is morerestricted than that of the granite-gneiss, and their recognitionis, in some instances, hindered by migmatization processes thatimprint a more leucocratic and, sometimes, even layered aspectthat makes it difficult to distinguish between the metamafic andthe quartz-feldspathic rocks (Fig. 4C and D).

In outcrop, one of the rock types (petrographically classified

as leuco-norite but referred to as metamonzogabbro hereafter;


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188 F.A. Cuadros et al. / Precambrian Research 245 (2014) 186206

Fig. 1. Geologicalsketch of the Chibcha Terrane (right). The frame on theleft displays thetectonic configuration of suspect terranes in Colombia. Note that country bordersand coastlines arenot meant to be actualterrane boundaries butrather aredrawnin such a fortuitous mannerfor simplicity. TheDubious Taham blockscan be related tothe Panzen Terrane ofOrdnez-Carmona and Pimentel (2002a). The Caribbean blocks in northern Colombia are drawn separately given their distinct geological historyrelated to Cretaceous Caribbean arcs (e.g., Cardona et al., 2010b; Weber et al., 2009). The limits of the Gorgona Terrane are those drawn by Cediel et al. (2003) and Serranoet al. (2011). Modified from Toussaint and Restrepo (1996), Ordnez-Carmona et al. (2006) and INGEOMINAS (2007). Abbreviations: GP: Guajira Peninsula. SNSM: SierraNevada de Santa Marta. PR: Perij Range. SM: Santander Massif. SLR: San Lucas Range. EC: Eastern Cordillera. CC: Central Cordillera. QM: Quetame Massif. OF: Oca Fault.

SMBF: Santa Marta-Bucaramanga Fault. OPF: Ot-Pericos Fault. GF: Guaicramo Fault.

see Section 4.1) displays dark tones that turn somewhat lighterwhen weathered. In thin section, however, the color index ofthese rocks barely reaches 30%. The mineral paragenesis consistsof plagioclase, hornblende, biotite, orthopyroxene, clinopyroxene,apatite and FeTi oxides. Hornblende and biotite are commonlysub- to euhedral, display intergrowth relationships and have atendency to occur at the interstitial spaces between plagioclasegrains. Locally, the ferromagnesian minerals display retrograda-tion to chlorite, with pyroxenes being more susceptible to thisprocess. Orthopyroxene is the most susceptible phase, alwaysappearing as internally fragmented crystals with the accumu-lation of fine-grained opaque masses along the cracks. Locally,

orthopyroxene displays a poikiloblastic texture engulfing smalllamellae of biotite. Hornblende, in turn, displays poikiloblastic andpartial corona textures engulfing both clino- and orthopyroxenecrystals. Potassic feldspar is found in low amounts, and quartz,which is generally absent, is sometimes found as tiny intersti-tial crystals or within myrmekitic intergrowths. The textures arexenoblastic and equigranular, preserving what could have beena magmatic cumulate texture, with plagioclase as the cumulusphase and clinopyroxene, hornblende, biotite, apatite and FeTioxides as the intercumulus phases, although some tiny euhedralcrystals of the latter minerals can also be found within largerplagioclase crystals. Despite the poikiloblastic texture shown by


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Fig. 2. Regional geological map ofthe SLR.Modified from INGEOMINAS (2007).

some orthopyroxene crystals, it is likely that the mineral had alsobeen part of the magmatic paragenesis, forming the preferred

nucleation sites for later metamorphic orthopyroxene. Recrystal-lization is also recognizedby local triplejunctions of mafic mineralsand anhedral crystals of plagioclase, which display sutured grainboundaries. Weak compositional zoning and wedge-shaped twin-ing lamellae are also observed in the latter mineral. Fine- tomedium-grained apatiteis fairly abundant, comprising 12% of themodal composition.

A second rock type is represented by amphibolites that havecolor indexes between 40 and 50%. One subtype of amphiboliteis basically composed of anhedral plagioclase and poikiloblastichornblende. Although the xenoblastic texture is widespread in thisrock, granoblastic texture relicts are locally preserved. There aresome instances in which small chlorite-rich grains are observed,which are most likely pyroxene relicts. Although not abundant,

symplectic intergrowths between biotite and quartz can be found

in this amphibolite subtype. Fine-grained apatite appears as themain accessory mineral in the rock.

A third subtype of amphibolite is characterized by a finer grainsize and plagioclase, hornblende and clinopyroxene as essentialminerals. The general texture of the rock is xenoblastic, withhornblende displaying a poikiloblastic texture defined by roundedclinopyroxeneinclusionsordefiningpartialcoronatexturesaroundthe latter. Some hornblende crystals also seem to be part of theengulfed phases, which possibly indicates the existence of a pre-metamorphicgenerationofhornblende,asalsodeducedfromsmallhornblende inclusions within poikiloblastic orthopyroxenes of themetamonzogabbro (see above). Chlorite hydrothermal veinlets arecommonly found cutting these rocks.

The third rock type consists of retrograded mafic granulitecomposed of plagioclase, hornblende, clinopyroxene and orthopy-roxene, with no or small amounts of FeTi oxides. The general

texture of the rock is xenoblastic with curved grain boundaries.


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Fig. 3. Geological map of the area of study displaying the sample locations. Position in the SLR as shown in Fig. 2. Some samples were taken outside this area (see Table 1and Appendix I).

Fig. 4. Photographs of outcrops of the studied rocks. (A) Banded granite-gneiss displaying alternating felsic and mafic layers. (B) Metamafic intrusive rock cross-cuttingbanding/foliation of thegneiss.(C) Ptygmaticfolding in migmatized (stromatic) mafic rocks.(D) Diatexitic migmatitederived from mafic rock. Some of themelanosome canbe seen as rafts of schollen withinmassive to vein-like leucosomeat the center of thepicture.


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Although a poikiloblastic texture can be found in some orthopy-roxenes, poikiloblastic and partial corona textures of hornblendeengulfing both clino- and orthopyroxene can also be observed.Relicts of granoblastic texture are widespread, and plagioclase iscommonly equigranular and anhedral, displaying wedge-shapedtwining lamellae.

When migmatized, the metamafic rocks mentioned above dis-playmelanosomesverysimilartotheoriginalhigh-graderocks(i.e.,coinciding with the paleosome). In one of the samples, a stromatic-phlebitic structure can be observed in a migmatized granulite,which in thin section consists of coarse-grained layers composedof quartz, plagioclase and biotite (leucosome) cutting the origi-nal mafic paleosome of generally smaller grain size. Rafts of theoriginal minerals belonging to the granulite are in some instancespreserved within the leucosome. In the latter case, these mineralsdisplay distinctive alteration and corrosion; this is more evident inthe orthopyroxene, which transforms into chlorite and Fe oxidesalong grain boundaries and internal cracks.

2.2.3. Leucogranite-gneissThis type of rock is rather rare in the studied area, having

been found in one single outcrop; therefore, this unit was notstudied in detail. This rock displays a foliation defined by a par-allel arrangement of biotite strips and conspicuous folding. Localhydrothermal alteration is evidenced by coarse-grained epidoteand potassic feldspar around siliceous veinlets. This gneiss seemsto be intruded by the metamonzogabbro described above. In thinsection, the rock displays a xenoblastic texture and is composed ofquartz, plagioclase, potassic feldspar and biotite, with widespreadepidote formed by hydrothermal alteration. Mineralogically, therock displays a granodioritic-tonalitic composition, with a colorindex no greater than 5%. Recrystallization is evidenced by a poik-iloblastic texture of some potassic feldspar and plagioclase grainsand sutured grain boundaries in quartz and feldspar. Myrmekitictextures are common, and the quartz displays polygonizationand undulose extinction. Biotite displays irregular to lamel-lar habits and moderate to strong chloritization along cleavage

planes.

2.2.4. Volcanic rocksBecause the Mesozoic volcanic processes of the SLR were

not part of the main focus of this work, these rocks were notcharacterized in detail, and only a brief mention is given here.These rocks, which are grouped within the Noren Formation(Clavijo, 1996), range from andesitic to rhyolitic in compositionand are represented mainly by tuffs, breccias, agglomerates andignimbrites indicative of explosive activity. Lavic rocks, althoughpresent, seem to be subordinate to the former group. The tex-ture is commonly porphyritic, with the coarse-grained populationcomposed of phenocrysts and crystal and rock fragments. Thegroundmass is commonly hyalocrystalline, and in some instances

very small oriented feldspar laths define a trachytoid texture.Phenocrysts are composed of clinopyroxene and plagioclase inthe more mafic members, while plagioclase and quartz dom-inate in the more silicic rocks. Fragments commonly includeplagioclase, quartz and pumice, although zircon and chloritizedmafic minerals are sometimes observed. The volcanic rocks aredeposited on the metamorphic basement described above, anddacitic dykes can also be found cutting these rocks at somelocations.

2.2.5. GranodioriteThis unit, which is referred to as the Noros Batholith (Bogot

and Aluja, 1981), intrudes all the metamorphic and volcanic rocksdescribed above and commonly displays mafic enclaves at hand

specimen scale that are indicative of magma mingling. The unit

is coarse-grained and undeformed and has quartz, plagioclase,potassic feldspar, biotite and hornblende as essential minerals.Zircon and apatite constitute the accessory mineral assemblage.The general texture is aphyric, subidiomorphic and equigranular,although some porphyritic-like textures can be locally observed,with medium-grained quartz composing the groundmass betweenlarger subhedral to euhedral crystals of feldspars, biotite and horn-blende. Plagioclase commonly occurs as large crystals displayingoscillating zoning and moderate to strong argillization and sericiti-zation.Potassic feldspar crystals tendto be subhedral andare easilyrecognized by their perthitic blebs and weaker alteration. Horn-blende can be subhedral or skeletal with inclusions of opaques,quartz and apatite. The hornblende often displays chloritizationand colorless cores of clinopyroxene with fairly irregular bound-aries, indicating an unequilibrated reaction rim. Biotite is generallyskeletal and poikilitic, engulfing smaller crystals of plagioclase,opaques and apatite. The biotite displays variable chloritization;as a result of this alteration, numerous small opaque grains (mostlikelyilmenite) accumulate internally withinthe chlorite. It is com-mon to observe intergrowths between hornblende and biotite,which indicate equilibrium crystallization between these miner-als. Local hydrothermal alteration is recognized by the formationofepidote and muscovite associated with the biotite. The quartz con-stitutes a late phase characterized by small anhedral grains growninterstitially between larger crystals of other minerals. Myrmekiticintergrowths with plagioclase are quite common. Opaque min-erals constitute 12% of the modal composition and consist ofanhedral grains of magnetite displaying exsolution lamellae ofilmenite.

2.2.6. Gold-bearing quartz veinsGold-bearing quartz veins are widespread within the studied

area and consist of variable proportions of pyrite, galena, chal-copyrite and sphalerite accompanied by quartz as a main ganguemineral andsubordinatedfeldspar,chlorite andcalcite.At theCulo-Alzao site (farther south of the study area), chalcopyrite masses

within veins engulf hessite (Ag2Te) cores rimmed by tetradymite(Bi2Te2S) coronas. The latter telluride can also be seen filling thincracks in the chalcopyrite. The texture of the ore ranges from mas-sive to brecciated, and the vein thickness varies between a fewcentimeters up to 1 m. Small gold grains a few micrometers in sizewith a finenessbetween 590and 670(with silver as themaincom-panion) canbe found withinthe sulfides (mainly pyrite). Associatedhydrothermal alterations correspond to sericitization and silicifi-cation within the proximal zones near the veins (typically a fewcentimeters or meters wide), while chloritization and epidotiza-tion dominate the distal alteration assemblages. The veins displayremarkable structural control, following both regional and localstructures trending NNE-SSW and ESE-WNW, which correspondin several instances to shear-zone fillings and are found cut-

ting all the above mentioned metamorphic, volcanic and plutonicunits.

3. Analyticalmethods

3.1. Major and trace element geochemistry

Chemical analyses of rock powder were performed by Acme-Labs Ltd. following 4A and 4B routines. The former involvedICP-AES analysis of major and minor elements after lithium metab-orate/tetraborate fusion and dilute HNO3digestion. In routine 4B,traceelementswereanalyzedbyICP-MSfollowingthesameprepa-ration as in 4A and digestion in aqua regia of an additional separatesplit.


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3.2. UPb geochronology

The methodology and equipment set-up used for the UPbgeochronology closely followed that presented by Bhn et al.(2009), to whom the reader is referred for further details. Geo-chronological UPbanalyseswere carried outat the geochronologylaboratory of the University of Braslia, using a Thermo FinniganNeptune multicollector inductively coupled plasma mass spec-trometer. The input of mineral substance into the spectrometerwas achieved by means of the laser ablation technique using a NewWave 213m NdYAG solid state laser. The sampling conditionsvaried according to the sample characteristics to optimize isotopesignals, with a beam diameter of 1030m and a laser energy ran-gingfrom0.5to1.2J/cm2 at a frequency of10 Hz. The samples wereinserted into a He-flushed laser chamber, with the gas flux main-tained between 0.35 and 0.45l/min. The removal of 204Hg fromthe He flux was achieved by passing the gas through glass tubescontaining gold-coated quartz particles intended to minimize theisobaric interference with 204 Pb, thus allowing the application ofcommon lead corrections. For standard and sample analysis, thesignals were collected in a single block with 40 cycles of 1.049seach, withthe reading of the signals starting after they hadattainedtheir maximum following the onset of ablation.

The standard-sample bracketing technique was applied by ana-lyzing one standard point and one blank every four or eightsample points, thus accounting for instrumental drift. The inter-national standard used here was the GJ-1 zircon, provided bythe ARC National Key Centre for Geochemical Evolution andMetallogeny of Continents (GEMOC), Australia. The standards ref-erence ages, afterJackson et al. (2004), are as follows:207Pb/206Pbage=608.61.1Ma, 206Pb/238U age= 600.41.8Ma, 207Pb/235Uage=602.13.0Ma. Data reduction was achieved using a spread-sheet set up at the geochronology laboratory of the BrasliaUniversity; this spreadsheet allowed evaluation of the isotoperatios of the 40 cycles on a 2 rejection basis. The corrected ratiosand associated calculated ages were then displayed using Isoplot 3(Ludwig, 2009).

3.3. Nd and Sr isotope geochemistry

All the Sm, Nd and Sr isotope analyses were performed at theGeochronology Laboratoryof the University of Braslia.The analyti-calprocedures applied in thisstudy formeasuring the 147Sm/144Ndand143Nd/144Nd isotope ratios were those described by Gioia andPimentel (2000). Rock samples were ground down to powder,and 149Sm and 150Nd spike solutions were added. The separa-tion of Sm from Nd was accomplished by using cation exchangecolumns, after which the obtained fractions were evaporated withtwo drops of 0.025N H3PO4. The residue was dissolved in 1l of5% distilled HNO3 and loaded onto a double Re filament assem-

bly. The mass spectrometer used was a Finnigan MAT 262 with7 collectors with analyses made in static mode. Uncertaintiesfor 147Sm/144Nd and 143Nd/144Nd ratios were better than 0.2%(2) and 0.0045% (2), respectively, based on an analysis of theBHVO-1 international rock standard. The 143Nd/144Nd ratio wasnormalized using 146Nd/144Nd= 0.7219, and the employed decayconstant was 6.541012 y1 (Lugmair and Marti, 1978).

For the Sr isotope analyses, the methodology employed wasthat ofGioia et al. (1999). After acid digestion and separation incation exchange columns, the Sr-bearing fractions were depositedalong with 1l o f H3PO4onto a Ta filament of a Finnigan MAT 262mass spectrometer. Based on an analysis of the NBS987 interna-tional standard, the87Sr/86Sr ratio uncertainties were better than0.0036% (2). The87Rb/86Sr ratio was calculated based on the Rb

and Sr concentrations reported in the trace element whole-rock

analysis following the procedure indicated by Faure and Mensing(2005).

4. Results

4.1. Geochemistry

The results of the major and trace element analyses are summa-rized in Table 1. Granite-gneiss displays high SiO2 values rangingbetween 65 and 73wt.% and total alkalis (Na2O + K2O) between7.2 and 8.7 wt.%. When plotted in the PQ diagram ofDebon andLe Fort (1982), these rocks show compositions between adamel-lite (or monzogranite, as preferred by Le Maitre et al. , 2002) andmore alkaline granite, reflecting enrichments in Na2O and K2Othat grant an alkali-calcic (or nearly so) character to this unit(Fig. 5B). Samples 021-03 and JS-020 have a metaluminous char-acter, as evidenced by Shands A/NK values >1 and A/CNK


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Table 1

Major andtraceelementwhole-rockdata.Samples JS-020and LC-004are from Mesz(2008) and wereincludedhere for comparative purposes.These samples wereanalyzedby X-ray fluorescence. Major oxide valuesare in weightpercent,while trace elements are reported in ppm.

Element Granite-gneiss Amphibolite Metamonzogabbro

021-01 021-02 021-03 JS-020 005-01 020-02 020-03 LC-004

SiO2 72.31 73.19 64.90 68.58 50.49 56.19 53.77 48.56TiO2 0.50 0.28 0.83 0.55 1.69 1.34 1.53 1.50Al2O3 12.36 12.76 14.11 14.15 14.00 19.33 19.05 14.35Fe2O3 3.30 2.99 5.87 4.29 10.63 5.85 7.17 15.33MnO 0.03 0.02 0.07 0.06 0.23 0.08 0.11 0.27MgO 0.76 0.26 1.45 0.92 7.27 2.57 2.80 3.91CaO 0.70 0.35 3.58 2.50 7.84 6.64 7.40 7.23Na2O 2.34 2.56 3.18 3.21 4.06 4.85 5.02 3.41K2O 5.73 6.09 4.03 4.45 0.90 1.79 1.31 1.10P2O5 0.09 0.03 0.20 0.16 0.60 0.42 0.50 0.50Cr2O3 0.00 0.00 0.01 0.00 0.04 0.00 0.00 0.00LOI 1.60 1.10 1.50 1.90 0.70 1.10TOT/C 0.12 0.04 0.19 0.11 0.02TOT/S 0.05

Sum 99.73 99.65 99.70 98.86 99.69 99.74 99.72 96.14

A/CNK 1.09 1.12 0.88 0.97FeO/(FeOt+MgO) 0.80 0.91 0.78 0.81 0.57 0.67 0.70 0.78La 111.40 96.20 60.10 27.20 19.10 22.50Ce 210.60 197.00 125.40 169.30 66.00 42.50 53.60 40.30Pr 22.87 23.50 15.33 8.93 5.48 7.46

Nd 76.60 82.40 58.50 72.99 36.30 21.90 31.70 28.33Sm 13.86 16.44 11.93 13.17 7.55 4.42 6.77 6.22Eu 1.19 2.02 1.95 2.18 1.38 1.66Gd 13.17 14.84 11.66 7.21 3.98 6.27Tb 2.22 2.44 1.87 1.03 0.57 0.95Dy 13.90 13.79 11.34 5.66 3.08 5.17Ho 2.88 2.89 2.28 1.11 0.57 0.97Er 8.87 8.28 6.30 3.26 1.68 2.94Tm 1.41 1.22 0.94 0.43 0.23 0.43Yb 9.00 7.76 5.80 2.89 1.53 2.64Lu 1.35 1.14 0.88 0.42 0.22 0.39Ba 473.00 798.00 782.00 1084.70 315.00 511.00 374.00 424.50Be 1.00 4.00 5.00 3.00Cs 0.50 1.10 0.70 0.20 0.60 0.20Ga 17.30 21.40 20.50 20.80 20.30 20.80 24.00 19.80Hf 11.40 15.60 11.00 4.70 3.90 2.40Nb 21.10 18.10 18.80 25.20 17.60 10.60 15.00 13.10

Rb 237.40 221.40 113.90 133.80 7.80 48.40 16.40 12.11Sc 6.00 4.00 13.00 24.00 11.00 18.00Sn 10.00 6.00 2.00 1.00 1.00Sr 76.70 93.00 200.90 231.30 401.40 657.50 646.60 367.40Ta 2.10 1.50 1.40 0.90 0.90 1.30Th 70.00 26.90 7.70 8.90 0.60 1.60 1.40 2.30U 10.30 3.70 3.40 0.40 0.60 0.50V 27.00 56.00 37.10 171.00 116.00 264.00 197.20Y 84.00 76.70 60.80 57.40 31.40 17.00 29.00 32.90Zr 319.40 524.10 393.60 340.70 187.50 147.90 92.20 79.40Ni 5.30 1.60 12.60 5.40 58.10 7.40 3.10 4.90Pb 5.20 4.50 1.90 18.80 0.60 2.90 2.20 11.30Zn 56.00 45.00 62.00 69.00 66.00 38.00 59.00 98.00

Sample locations:021-01: 83853.6791 N;740537.7262 W.021-02: 83853.6791 N;740537.7262 W.021-03: 83853.6791 N;740537.7262 W.

JS-020: 85814.4091 N;74022.9000 W.020-02: 84025.2415 N;740548.8510 W.020-03: 84025.2415 N;740548.8510 W.LC-004: 83919.1760 N;740552.3556 W.005-01: 83547.1669 N;740558.1928 W.

slightly negative anomalies of Nb, Ta, Zr and Hf (Fig. 7A). Thesepatterns are similar to those of within-plate granites of attenuatedcontinental lithosphere from Skaergaard and Mull (Pearce et al.,1984).

The metamafic rocksyielded fairly uniform geochemicalresults,withSiO2varying between49 and 56wt.%andNa2O + K2O between4.5 and 6.6 wt.%. When the R1-R2 cationic classification schemeof De La Roche et al. (1980) is implemented, the samples dis-play a tendency to cluster within the monzogabbro field, halfway

between the alkalic and subalkalic domains. This moderate enrich-ment in alkalis is manifested in the presence of some quantitiesof potassic feldspar and an andesine composition of the plagio-clase. Because of these enrichments in Na2O, K2O and Al2O3 (thelatter being the case of the leucocratic plagioclase-rich samples),series classification diagrams (e.g., Kuno, 1966; Irvine and Baragar,1971; Jensen, 1976; Peccerillo and Taylor, 1976) turned out tobe misleading and were therefore not used. It is worth notingthat the analyzed samples have relatively high P2O5values (close


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Fig. 5. Major and traceelementgeochemical characteristicsof the granite-gneiss. (A) Diagram of Shands A/NK vs. A/CNK (Manniar and Piccoli, 1989). All the samples displaya positive feldspathoid-silica saturationindex (FSSI; Frost andFrost, 2008). (B)Modifiedalkali-lime index (MALI= Na2O + K2O-CaO)vs. SiO2diagram ofFrost et al.(2001). (C)Diagram ofFe*= FeOt/(FeOt+ MgO)vs.SiO2ofFrost et al.(2001). (D) Nbvs. Y tectonic discrimination diagram ofPearceet al.(1984). Notice that post-collisional granites alsoplot near thetriple junctionbetweenthe WPG, VAG +Syn-COLGand ORG fields (Pearce, 1996). (E) FeO*/MgOvs.Zr +Nb+ Ce+Y diagram ofWhalen et al. (1987). (F) Samplesare further grouped into theA 2 subtype, accordingto theYNb3Ga ternary diagram ofEby (1992).

to 0.50.6 wt.%), manifested as abundant apatite in the accessoryassemblage. Enriched REE patterns for these rocks (normalized Laand Lu values of 78111 and 9-17, respectively) are characterizedby moderate degrees of fractionation ([La/Lu]Nbetween 6 and 9),subtle to almost null Eu anomalies (Eu/Eu* between 0.77 and 1)and a rather flat HREE distribution (Fig.7C). The incompatible traceelement patterns normalized to primitive mantle (Fig. 7E) indicatea general affinity with an OIB composition, with some elementsabove and below the mean value. In general, negative anomaliesof Rb, Th, Ta, Nb, Zr and Hf are outstanding. For the amphibolite(sample 005-01), radiogenic elements such as Th and Pb displaymarkedly low normalized values similar to those of E-MORB. The

normalization to MORB ofPearce (1982) indicates no substantial

anomalies of Nb and Ta and slight to moderate anomalies of Zrand Hf. All samples are depleted in Cr, except for sample 005-01,whichdisplayscontentssimilartothoseofMORB(Fig.7D).Thispat-tern resembles that of transitional to alkalic within-plate basaltsfrom Gregory Rift and the Azores (Pearce, 1982) and doleritesand gabbros associated with A-type granites from Corsica (Bonin,2007).

4.2. UPb geochronology

The results of LA-ICPMS UPb analyses of zircons from twosamples of granodiorite, three samples of granite-gneiss and two

samples of metamonzogabbros aresummarized in appendix I. Both


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Fig. 6. Major and traceelement geochemical characteristicsof the granite-gneiss. (A) ASI vs. MgOdiagram. Upperdiscontinuous-linebounded field: Calc-alkaline differenti-ation trendsof Hercynian orogenic suitesfrom France andI- andS-type granite suitesfrom theLachland Fold Belt, Australia; Lower solid-line bounded field: differentiationtrends of alkaline anorogenic suites from Niger and the Kerguelen Islands. Fields drawn after Bilal and Giret (1999). (B) TiO2vs. MgO diagram ofBilal and Giret (1999). (C)BaRbSr ternary diagram ofEl Bouseily and El Sokkary (1975).

core and rim ofgrainswereanalyzed where possible with theaid ofcathodoluminescence(CL)andbackscatteredelectron(BSE)images(appendix II).

4.2.1. GranodioriteAlthoughthegranodioriteunitisnotpartofthebasementunder

consideration, knowing its crystallization age is essential for aproper interpretation of the UPb data of the hosting metamor-phic rocks because the intrusion of the Noros Batholith representsthe last extensive and most intense thermal event that took placein the region.

Zircons from the granodiorite are relatively small (70100m)and commonly display euhedral to subhedral habits with well-developed prismatic and bi-pyramidal forms. These zircons aregenerally colorless, with small black-colored opaque inclusionsbeing common. CL imaging reveals that, although not conspic-uous, the grains display oscillatory zoning (OZ) and are quite

rich in inclusions that sometimes display elongated shapes par-allel to crystal faces defined by the OZ. Light-colored, irregularlyshaped patches within which OZ can be observed are a com-mon feature occurring mainly at the apexes (pyramidal areas)of these zircons (Fig. 8). This texture is similar to that relatedto the complex growth zoning presented by Corfu et al. (2003).When the isotope data obtained from these zircons are plottedin Tera-Wasserburg diagrams (Fig. 9A and B), the grains exhibita tendency to fall near the concordia, although a variation in238U/206Pb ratios exists. A date close to 173 Ma (the youngestage range for both samples) can be assigned to the granodiorite,which is indicative of the approximate time of crystallization ofthe magma. Grains yielding older ages, although texturally andmorphologically similar to the rest of the grains (some of them,

perhaps, displaying a weaker CL), could represent zircons inherited

from earlier magmatic pulses. This assumption makes sense whentaking into account that the granodiorites locally display maficenclaves in outcrops, indicative of magma mingling (see Section

2.2.5).

4.2.2. Granite-gneissZircons from the granite-gneiss are typically large (between

130 and >150m) and elongated (length:width ratios between2:1 and 3:1), with round borders, dark-brown tones and a frag-ile character. Internally, CL images reveal cores displaying igneousOZ and bright, isotropic metamorphic rims that can be found insome grains and are too thin or virtually inexistent in others. Itis common to observe that the igneous OZ has been locally dis-rupted to various extents by reequilibration processes, as reflectedby recrystallization fronts,blurred and convolute zoning and thick-ened primary zones (Fig. 10AD), structures that are consideredindicative of high grade metamorphism (Hoskin and Black, 2000;

Corfu et al., 2003). The Pb/U ratios show a remarkable disper-sion reflected in ages varying between 1542 and 932Ma anddiscordant patterns suggesting conspicuous Pb losses. Few agesbetween 1900 and 1560Ma were obtained and are interpretedas belonging to inherited grains and cores. For the estimation ofages from discordant data, Pb-loss models calculating upper andlower intercepts of discordia lines were applied along with mean207Pb/206Pb ages to show that the two calculation methods gen-erally yield similar age intervals. Moreover, the lower interceptwas kept anchored at 1733Ma, which is the mean estimatedage of intrusion of the granodiorite (see above) and represents thelast regional disrupting thermal event undergone by the metamor-phic rocks. Using this method, a crystallization age for the igneousprotolith is estimated between 1544 and 1484Ma (Fig. 11AD).

Some bright rims and one core yielded dates between

1180 and


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Fig. 7. REEs and trace element patterns for both granitic and mafic rocks. (A) REE patterns of granite-gneiss samples. Chondrite normalization values from Evensen et al.(1978). (B) Traceelementpatternsof granite-gneisssamples. Normalization to the OceanRidge Granite ofPearceet al.(1984). (C) REE patterns of metamafic rocks. Chondritenormalizationvaluesfrom Evensenet al. (1978). (D) Traceelement patternsof metamafic rocksnormalizedto MORB ofPearce (1982). (E) Traceelementpatterns of metamaficrocks. Normalization values and mean ocean-island basalt (OIB), enriched mid-oceanridge basalt (E-MORB) and normal mid-ocean ridgebasalt (N-MORB) patterns fromSunand McDonough (1989). Dark gray points: samples of granite-gneiss. Light gray points: samples of metamafic rocks.

970Ma (Fig. 12AF), which are interpreted as the time of meta-morphism. Intermediate ages between 1490 and 1200Ma areregarded as artifacts caused by Pb losses(see the discussion sectionbelow).

4.2.3. MetamonzogabbroZircons of this unit are commonly 120 to >150m in size with

length:width ratiosbetween 2:1 and 1:1. They aretranslucent with

apinkishhueanddisplayroundedborders,whichinafewinstances

yield prismatic and pyramidal crystal faces. BSE imaging revealsinternal patchy zoning defining broad light and dark bands thatare usually parallel but not concentric (Fig. 10EH). This type oftexture is similar to that found in zircons from intermediate tomaficigneousrocks(Corfuetal.,2003). Metamorphicisotropicrimsoccasionally occur in these grains, and disequilibrium textures aremoderately abundant in the form of recrystallization fronts, convo-lute zoning and blurred primary structures (Fig. 10EH). As in the

case of the granite-gneiss, the Pb/U ratios indicate Pb-losses and


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Fig. 8. CL images of zircons from granodiorite. OZ, inclusions and complex growthzoning (lighter areas at crystal apexes) arecommon features of zircons from this unit. (A)zircon from sample 028-01. (B)zircon from sample 030-01.

considerable data dispersion, leading to calculated dates between1542 and 964 Ma. An age for the protolith crystallization can beinferred at 15421501 Ma (Fig. 11E and F). Metamorphic rims andone core yielded ages between 1180 and 964 Ma. These ages aresimilar to those obtained from zircons of the granite-gneiss and arethus interpreted as resulting from the same metamorphic event

that affected those rocks (Fig. 13). As in the case of the granite-gneiss, intermediate ages between >1450 and 1200Ma are notregarded as representing real geological events but rather incom-plete reequilibration due to Pb losses.

From the above data, it can be concluded that the oldest agesfor the granite-gneiss and the metamonzogabbro (representing thetime of crystallizationof their protoliths) fall withinthe same range(15401500 Ma), implying that these units arenearly coeval. Thisbehavior can also be recognized from the concordia diagram whenthe Pb/U ratios of all samples are plotted together (Fig. 14A) andfrom the probability density plot of zircon core ages of both rocktypes (Fig. 14C). A tendency can also be observed for metamorphicrims to cluster at ages between 1190 and 1000Ma (Fig. 14B).

4.3. Nd and Sr isotope geochemistry

The results of the SmNd and RbSr isotope geochemistryare summarized in Table 2. For the latter system, only 87Sr/86Srratios were measured in the laboratory, while 87Rb/86Sr ratioswere calculated based on the Rb and Sr concentrations presentedin Section 4.1. Unfortunately, these calculations led to anoma-lous initial 87Sr/86Sr ratios for the granite-gneiss, which will notbe considered in further interpretations. These anomalies couldbe due to Rb remobilizations induced by the metamorphism or

metasomatism of these rocks. From the geochronological data ofthe previous section, the preferred age for the granite-gneiss, themetamonzogabbro and the amphibolite was taken as 1520Ma. Thegranite-gneiss yielded Nd values between +3.0 and +4.7, whichsuggests an origin from a depleted mantle reservoir. A similar sit-uation is observed for the metamonzogabbro and the amphibolite,

whichdisplayNd valuesbetween+2.3and+4.6andinitial87Sr/86Srratios ranging from 0.7020 to 0.7037. The TDMages for this groupof rocks roughly fall within the same interval between 1509 and1733Ma. The similarity between the lower boundary of this inter-val and the presumed crystallization age indicates a nearly juvenilecharacter for these mantle-derived rocks, although the participa-tion of sources that differentiated earlier also seems possible, assuggested by the older TDMages. The latter possibility is supportedby the presence of inherited zircons yielding 207Pb/206Pb agesbetween 1600 and 1900Ma within the granite-gneiss (see Section4.2.2). Altogether, this bimodal set of rocks defines a homogeneouspath in the Ndvs. age diagram ofFig. 15.

5. Discussion

The granite-gneiss displays high amounts of incompatible traceelements and Ga/Al ratios, which allow classification of its pro-tolith as A-type granite according to the geochemical criteria ofPearce et al. (1984) and Whalen et al. (1987). The moderately fer-roan nature of these rocks also supports this conclusion becausethisisafeaturecommontothatgroupofgranites( Frostetal.,2001).Moreover, the analyzed rocks can be assigned to the A2subgroupcharacterized by Y/Nb ratios greater than 1.2, which is related togranites withsourceschemically similar to islandarc or continental

Fig. 9. Results of UPb analyses of granodiorite samples suggesting a crystallization age of173Ma. (A) Tera-Wasserburg diagram of data from sample 028-01. (B) Tera-

Wasserburg diagramof data from sample 030-01.


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Fig. 10. CL images of zircons from thegranite-gneiss andBSE images of zircons from metamonzogabbro. (A)Grain displaying internal OZand a bright metamorphic rimwitha recrystallization front at the center-left end. (B) Grain displaying a dark, convolute-zoned inherited core. (C) zircon with a thin metamorphic rim and convolutezoning atthe core. (D) Grain displaying bright, thickenedprimary zones.Some convolute zoningis observed at thelower portion of thecrystal. (E)Graindisplaying patchyzoning. (F)Zircon showing patchyzoning anda recrystallizationfrontat itslower-leftborder.(G) Grain with internal patchyzoning anda metamorphic rim. (H)Crystalwith an internalconvolute structure. Note that thebright areas in theBSE imagesappear dark under CL andvice versa.Round black spots in thezircons of themetamonzogabbro correspondto ablated areas.

marginbasalts (Eby,1990,1992). Theconstraint on a mantlesourcefor the magmas that later became the protolith of the granite-gneiss is further supported by positive values ofNd between+3.0 and +4.7. In addition, it is interesting to note that withinthe broad realm of A-type rocks, the granite-gneiss is classifiedinto the post-orogenic/post-collisional category when, for exam-ple, considerations about major element contents are taken into

account by means of the discriminant functions ofAgrawal (1995)or when the trace element-based tectonic discrimination diagramsofPearce et al. (1984) are used. In this case, samples tend to clus-ter in the WPG field, very close to the boundaries with the otherVAG, ORG and Syn-COLG groups, where post-collisional granitesare commonly plotted (Pearce, 1996). Normally, post-collisionalgranites display heterogeneous chemical compositions that


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Fig. 11. Results of UPbanalysesof themetamorphic rocks.Samples 017-05(A),022-01 (B)and PGG-18(C andD) arefromthe granite-gneiss,whilesamples020-02 (E)and020-03 (F) are from the metamonzogabbro. Solid-line ellipsoids represent analyses of grain cores.

suggestinteractions between themantleand a thickenedcontinen-tal crust, leading to mixed signatures (Pearce, 1996; Bonin, 2007).That the obtained Nd values still retain a mantle identity wouldindicate that the crust where the protolith of the granite-gneisswas emplaced did not have such a significant thickness, which isconsistent with their multi-element patterns, similar to those ofwithin-plate granites emplaced in an attenuated continental litho-sphere,aspresentedbyPearceetal.(1984). Features indicating thata subduction zone-like mantle associated with a previous orogenic

phase might have contributed to sourcing the precursor magmas

of the granite-gneiss include the subdued, though still noticeable,negative anomalies of Nb and Ta. These characteristics, along withMgO/TiO2 ratios varying between values typical of granites fromanorogenic and subduction zone settings (Bilal and Giret, 1999),indicate a transitional character for the source and conditions ofgeneration of the precursor magmas of the granite-gneiss, midwaybetween orogenic/collisional and anorogenic environments. Thisfeature has been observed by Bonin et al. (1998) and Bonin (2007),whodescribe post-orogenic suitesat their initial stagesas being the

result of processes still influenced by crustal materials subducted


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Fig. 12. Results of UPb analyses of themetamorphic rocks.Samples 017-05(A, B andC), 022-01(D) andPGG-18 (E andF) arefrom thegranite-gneiss. Solid-line ellipsoids:grain cores; discontinuous-line ellipsoids: grain rims.

below the orogenic subcontinental lithospheric mantle, with thedominantlymetaluminouscalc-alkalinesuitesshowingashiftfromnormalto high to very high K associationsand crystallization undernearly water-saturated conditions, leading to subsolvus mineralo-gies and textures. It is for this reason that post-orogenic suitescommonly plot withinthe A2field anddisplay an alkali-calcic char-acter (Bonin et al., 1998; Bonin, 2007), such as the granite-gneissconsidered in this study.

The metamafic rocks display a major element chemistry sug-

gesting a transitional character, with the analyzed samples being

classified as monzogabbros according to the scheme ofDe La Rocheet al. (1980). Their enrichment in LREEs precludes an origin in aMORB setting and suggests instead an OIB-like source for the pre-cursor magmas. Similarly, the multi-element diagram patterns ofthe metamafic rocks resemble that of the mean OIB ofSun andMcDonough (1989). Some departures from this mean pattern areobserved on the analyzed rocks; according to Sun and McDonough(1989), the influence of arc-related or sedimentary materials onthe source of the magmas could be responsible for the depletions

in Nb, Ta, Sr, P, Eu and Ti, as well as for the enrichment in Ba,


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Fig. 13. Results of UPb analyses of the metamorphic rocks. Samples 020-02 (A and B) and 020-03 (C and D) are from the metamonzogabbro. Solid-line ellipsoids: graincores; discontinuous-line ellipsoids: grain rims.

while the lower contents of Rb and K could account for residualphlogopite holding back these elements at the mantle. Melting athigh pressures would cause the observed depletions in Zr and Hfas well as, again, Sr, P and Ti. Mantle rocks at depths greater than30 km are unlikely to have plagioclase as an essential constituent(Wilson, 1989); thus, high-pressure melting could also explain thesubdued to almost null negative europium anomaly observed inthe REE patterns of the metamafic rocks. In addition, high wateramounts evidenced by relatively abundant hornblende and biotite(the latter observed in the lower-grade lithologies) also point tosubduction zone mantle as participating in the generation of themagmas. Altogether, the source of the metamafic rocks seems tobe complex in nature, involving more than one type of material;

this is also reflected by their variable Nd TDM between 1.51 and1.73Ga, which could imply the involvement of older underplatemafic rocks.

The UPb ages in zircons obtained from the granite-gneissand the metamafic rocks range between 1540 and 930 Ma, withnoticeable Pb losses and intermediary ages ranging from 1200 to>1400Ma. As shown in Fig. 14B, most of the ages determined in themetamorphic rims of zircons from both units fall within the rangebetween 1180 and 930Ma, which is interpreted as the result of ahigh-grade Grenvillian metamorphic event related to the assemblyof Rodinia at the end of the Mesoproterozoic (e.g., Li et al., 2008;Fuck et al., 2008). This is not a new finding for the region com-prising the northern portion of the Central Cordillera of Colombia,

Table 2

SmNd and RbSr isotope data of the studied samples. Lithologies and locations are given in Table 1. Numbers in parentheses next to143 Nd/144Nd and 87 Sr/86Sr ratioscorrespond to 2errorsin thelast digits. Uncertainties in TDMvalues were determined from variations produced by.

Sample Sm ( ppm) Nd ( ppm) 143Nd/144Nd 147Sm/144Nd Nd(T) TDM(Ga) Rba (ppm) Sra (ppm) 87Sr/86Sr 87Rb/86Srb 87Sr/86Sric T (Ma)

021-01 13.529 72.888 0.512033 (22) 0.1122 +4.7 1.509 (0.033) 237.4 76.7 0.8588 (3) 9.1127 0.6638 1520021-02 16.018 83.933 0.512027 (14) 0.1154 +3.9 1.567 (0.022) 221.4 93 0.8361 (3) 6.9937 0.6864 1520021-03 12.731 61.901 0.512071 (15) 0.1243 +3.0 1.648 (0.025) 113.9 200.9 0.7437 (2) 1.6507 0.7084 1520005-01 74.243 363.492 0.512049 (20) 0.1235 +2.8 1.670 (0.034) 7.8 401.4 0.7049 (2) 0.0564 0.7037 1520020-02 4.735 23.265 0.512139 (16) 0.123 +4.6 1.511 (0.027) 48.4 657.5 0.7065 (2) 0.2136 0.7020 1520020-03 7.142 32.087 0.512137 (20) 0.1345 +2.3 1.733 (0.038) 16.4 646.6 0.7053 (1) 0.0736 0.7037 1520

a Data taken from Table 1.b Calculated ratios.c Anomalousvalues obtained for thegranite gneiss will notbe considered forinterpretations below.

Present-day143

Nd/144

NdCHUR and147

Sm/144

NdCHUR valuesused in calculationswere 0.512638 and 0.1966, respectively (Jacobsen and Wasserburg, 1980, 1984).


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Fig. 14. Summary of UPb data of thegranite-gneiss and themetamonzogabbro.(A) Concordia diagram displaying Pb/U ratios of all samples. (B)Probability density plot ofthe ages obtained in the zircon metamorphic rims of all samples. (C) Probability density plot of the ages obtained in the zircon cores of all samples. Black ellipsoids: graincores; light-gray ellipsoids: grain rims; dark-gray filled ellipsoids: inherited grains.

as previous works have already yielded similar results for this and

other areas of the Colombian Andes (Restrepo-Pace et al., 1997;Ordnez-Carmona et al., 1999, 2002b; Cordani et al., 2005; Cardonaet al., 2010a; Ibnez-Meja et al., 2011). By contrast, the oldest agesobtained from zircon cores (except those from inherited grains)range between1500 and 1540 Ma and are interpreted as the timeof crystallization of the protoliths of both the granite-gneiss andthe metamafic group, thus being essentially coeval. Several agesbetween1200and>1400Ma canbeestimatedusingPb-lossmodelsand207Pb/206Pb means. From a geological point of view, however,

Fig. 15. Evolution Ndvs.age diagram of theanalyzedsamples. Dark gray patterns:

samples of thegranite-gneiss. Light gray patterns:samples of themetamafic rocks.

no rocks recording events of those ages have been reported to date

in this part of the Central Cordillera. Therefore, these results wouldrepresent artifacts induced by multi-stage Pb lossescausedinitiallyby the Grenvillian metamorphism and later by the thermal dis-turbance associated with the intrusion of the Noros Batholith at173Ma. Furthermore, if a thermal event had indeed taken placeclose to 1300Ma, then it should be equally represented in boththe granitic and mafic rocks, which is not the case for the granite-gneiss. Concordant ages of zircon cores and rims falling within thisinterval are thus interpreted as protolith memory effects causedby an incomplete reequilibration of Pb/U ratios within zircon. Thishas been shown to be a common feature in zircons of rocks fromhigh-grade terranes (Hoskin and Black, 2000).

Ndvalues between +2.3 and +4.6 for the metamafic rocks aresimilar to those of the granite-gneiss, indicating a mantle source

common to both rock types. In addition, their TDMages are similarand define a homogeneous domain in Fig. 15, with protolith agesbeing close to this range, suggesting that those magmas are nearly

juvenile in nature and constitute a bimodal association, which,according to the characteristics described above, would have beenemplaced in a post-collisional regime at 1.52Ga.

Bimodal A-type associations are a rather common feature ofpost-collisional settings and are frequently linked to transcurrentshear tectonic contexts (Bonin, 2007). There are several relatedexamples in the literature describing A-type granites and associ-ated rocks that were likely emplaced in compressive-transpressiveregimes, such as the Estrela Granite Complex of Carajs in Brazil(Barros et al., 2001, 2009), the granitic rocks west of the EastEuropean Craton (Skridlaite et al., 2003) and the Stenshuvud andKarlshamn granites of southern Sweden (Cecys, 2004). At such


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settings, the granitic magmas are prone to develop anisotropic fab-rics consistent with syntectonic deformation. Some granites evendisplay chemical characteristics midway between I- and A-types(Cecys, 2004). In this sense, the anorogenic nature of those rocks isquestionable, as they reflect a relation more compatible with oro-genic contexts. It is suggested that the banded structure definedby alternating felsic and mafic layers observed in the granite-gneiss of the SLR could have originated in a similar fashion, thusexplaining their apparent truncation by the nearly coeval and vir-tually isotropicmetamafic rocks(Fig.4B).Accordingto NaslundandMcBirney (1996), the deformation of partially crystallized magmascan generate a layering resulting from the segregation of liquids,yielding sharply defined dark and light layers that can, in extremecases, be monomineralic, although magmatic flow is also capableof generating this type of structure. However, no firm statementcan be made about the nature and origin of the banded structureof the granite-gneiss, as detailed structural and kinematic analy-ses were not conducted. This issue remains to be resolved duringfuturestudiesinvolvingmorefieldobservations.A TDM ageof0.6Gafor a deformed metagabbro dyke found near Pueblito Meja wouldsuggest that the truncation of the banding of the granite-gneissis caused by younger mafic intrusions (INGEOMINAS-UIS, 2006).These data, however, are considered unreliable because no rocksof that age have been found in this sector of the Colombian Andes,andno metamorphism youngerthanthe Grenvillian event hasbeendocumented within the region.

The Rio Negro Province (RNP) of the Amazonian Craton, com-prising northwestern Brazil, southeastern Colombia and southernVenezuela,has a basement composed of calc-alkalic monzograniticto tonalitic gneisses yielding ages between 1.86 and 1.78Ga(Tassinariand Macambira, 1999; Santoset al., 2000). This basementis intruded, among other units, by granitic rocks of the Rio UaupsSuite, including titanite-biotite monzogranite, biotite granodio-rite, leuco-monzogranite and leuco-syenogranite (DallAgnol andMacambira, 1992). These granites have yielded a RbSr whole-rock age of1459Ma (DallAgnol and Macambira, 1992), althoughSantos et al. (2000) reported older UPb ages in zircon close to

1520Ma, as well as Ndvalues between 5.0 and 1.22 with TDMranging from 1.99 to 2.12Ga. Based on an initial87 Sr/86Sr ratio of0.7063 obtained for this suite, DallAgnol and Macambira (1992)proposedaprobableorigininacollisionalsettingfortheRioUaupsGranites. Geochemically, however, these granites display within-plate and A-type characteristics, falling within the A2group ofEby(1992). Further geochemical data presented by DallAgnol (1992)showed that the rocks of this suite display features similar to thoseof both orogenic and anorogenic granites, without clearly fittingwithin any of these two classes. Thus, DallAgnol (1992) recognizedthat the RioUaups granites would not represent typical collisionalgranites, leaving the question open about a possible relation toa post-collisional/anorogenic setting. Given the above character-istics, it might be stated, in principle, that there is a petrological

and geochronological correlation between the granite-gneiss ofthe SLR and the Rio Uaups Suite, thus establishing a linkage tothe RNP of the Amazonian Craton. Some important differencesshould, however, be noted, such as the apparently crustal ori-gin of the Rio Uaups Suite, as well as their consequently olderTDM ages and the absence of associated mafic rocks. Moreover,the Uaups River Suite is associated with the coeval Rio IcanaSuite,composedofperaluminousS-typegranites( DallAgnol,1992;Santos et al., 2000). However, DallAgnol (1992) does not dis-card a mantle source with a substantial later crustal contributionfor the origin of the Rio Uaups granites. Inherited zircons withages between 1900 and 1600 Ma found within the granite-gneisswould support a genetic relationship with the RNP of the Amazo-nian Craton. A similar correlation had been proposed by Cardona et

al. (2010a)on geochronologicalgrounds for paraderivedunits from

the Chibcha Terrane in Colombia and other similar lithosphericblocks in Ecuador, Per and Venezuela, which were interpretedas generating from the metamorphism of sediments coming fromthe Amazonian Craton and being deposited in basins along anactive margin of the craton. Moreover, paleomagnetic data fromBayonaet al.(2006,2010) suggest that the Colombian massifs werebrought from latitudes farther south, similar to the present daypositions of northern Per and Ecuador, thus favoring the corre-lation with the RNP stated here for the Mesoproterozoic rocks ofthe SLR. Ages between 1180 and 930 Ma in metamorphic rimsof zircons of the granite-gneiss and the metamafic rocks supportthe participation of the SLR in a Grenvillian event along with theother massifs of the Chibcha Terrane in the Colombian Andes andotherblocksinEcuador,Per,Venezuela,MexicoandCentralAmer-ica, following recent models in which this group of fragmentswould have been located at or near the northwestern border ofAmazonia interacting with Baltica during the end of the Meso-proterozoic and Early Neoproterozoic (Keppie and Dostal, 2007;Bogdanova et al., 2008;Li et al., 2008; Johansson, 2009;Keppie andOrtega-Gutierrez, 2010; Cardona et al., 2010a; Ibnez-Meja et al.,2011).

It is worth noting that this work is the first to offer a detaileddescription of Early Mesoproterozoic (Calymmian) orthoderivedrocks in the Chibcha Terrane because materials of this age (i.e., zir-cons) had only been found as inherited components in paraderivedmetamorphic rocks reworked during Grenvillian events (e.g.,Restrepo-Pace et al., 1997; Cordani et al., 2005; Ordnez-Carmonaet al.,1999, 2002b; Cardona et al.,2010a;Ibnez-Meja et al., 2011).Ordnez-Carmona et al. (2009) had already reported a UPb agein zircon of 1501Ma for the granite-gneiss, but its meaning andgeotectonic context were poorly constrained. The San Lucas Gneissand metamafic rocks, as part of the more broadly extended post-collisional to anorogenic domain of the RNP of the AmazonianCraton, would represent the source of zircons with ages close to1.51.54Ga found within the Precambrian units of the ColombianAndes. Thus, the rocks of the SLR constitute the true basement ofthose Precambrian terranes in the region, with the paraderived

rocks reported by earlier works representing supracrustal covers.Moreover, the metaigneous units of the SLR can be regarded as thesecond oldest rocks currently known in the Andean chain, with theoldest rocks being those of the Arequipa-Antofalla Block for whichan early tectono-magmatic history between 1.79 and 2.0Ga is con-strained (Wrner et al. , 2000; Loewy et al., 2004; Casquet et al.,2010).

6. Conclusions

The work carried out during this study allows the followingconclusions to be made:

- The basement of the northern portion of the SLR is composedof a bimodal association of monzogranitic and monzogabbroicrocks that were metamorphosed under granulite facies and sub-sequently retrograded to amphibolite facies. Thus, the classicalconception that regarded these units as a metasedimentarysequence similar to those found in the eastern flank of the Cen-tral Cordillera, the Sierra Nevada de Santa Marta and SantanderMassifs and the Guajira Peninsula is challenged.

- TheUPb geochronology ofzirconcores indicatesa crystallizationage between 1.54 and 1.50Ga for the protoliths of the granite-gneiss and the metamafic rocks, while metamorphic rims yieldedages between 1180 and 930 Ma. These data, along with positiveNdvalues and a TDMbetween 1.5 and 1.7Ga, indicate a mantle-derived origin and a virtually juvenile character for those coeval

units.


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- Both groups of metamorphosed felsic and mafic rocks displaychemical features indicating that the mantle source that gaveorigin to their precursor magmas had characteristics that weretransitional between enriched (indicative of a within-plate set-ting) and both depleted and hydrated (more compatible withan arc setting), which suggests that these magmas formed asa bimodal association in a post-collisional context. Moreover,their occurrence in a transpressive environment (as exemplifiedby other suites from Northeastern Europe and Northern Brazil)might account for the rather syntectonic character reflected inthe banded structure of the granite-gneiss, which seems to becrosscut by the metamafic rocks. Nevertheless, further structuraland petrologic studies should be conducted to verify the validityof this statement.

- The orthoderived rocks of the northern part of the SLR con-stitute the first reported occurrence of Early Mesoproterozoic(Calymmian) rocks in the Colombian Andes, as similar (and evenolder) ages had only been found by previous studies in detritalzircons of younger Late MesoproterozoicEarly Neoproterozoicparaderived metamorphic rocks. The orthoderived units of theSLR can thus be regarded as the second oldest rocks found so farin the Andes.

- The post-collisional granitic rocks described in this study canbe correlated with the A-type Rio Uaups Suite in the RNP ofthe Amazonian Craton, for which an intrusion age has beenconstrained at 1520Ma. This places the SLR within the sametectonic context of other massifs in the Colombian Andes aswell as similar lithospheric blocks in Ecuador, Per, Venezuela,Mexico and Central America, all of which are regarded as hav-ing been located at or near the northwestern border of theAmazonian Craton during the Late Mesoproterozoic. The SLR,however, would represent a part of the basement of the Ama-zonia paleocontinent, while the other blocks mainly correspondto sedimentary infillings of marginal basins or peripheral arcsand/or microcontinents. According to recent models, this groupof blocks might have been brought from southern latitudes afterbeing involved in the collision between Amazonia and Baltica

during the Grenvillian/Sveconorwegian orogeny from 1140 to970Ma.

Acknowledgements

This work was possible thanks to financial support from theCoordenaco de Aperfeicoamento de Pessoal de Nvel Superior(CAPES) and the Conselho Nacional de Desenvolvimento Cient-fico e Tecnolgico (CNPq, grant 305833/2010-3) of Brazil. We alsothank the Facultad de Minas Gemma Group of the UniversidadNacional de ColombiaSede Medellnfor its financial support dur-ingthefieldwork.Valuablecommentsongeochronological,isotopegeochemistry and regional geology of Brazil given by Prof. MoacirMacambira (Universidade Federal de Par) are specially acknowl-edged, as well as enlightening observations on petrology of maficrocks from Prof. Csar Fonseca Ferreira Filho (Universidade deBraslia) and Senior Geologist Reinaldo Santana Correia de Britofrom the Brazilian Geological Survey (CPRM). Assistance from thetechnical staff of the Geochronology and Petrography laboratoriesof the Universidade de Braslia is also gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/

j.precamres.2014.02.010.

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