Probing the composition of unexposed basement, South Portuguese Zone, southern Iberia: implications for the connections between the Appalachian and Variscan orogens

Probing the composition of unexposed basement,South Portuguese Zone, southern Iberia:implications for the connections between theAppalachian and Variscan orogens

James A. Braid, J. Brendan Murphy, Cecilio Quesada, Luke Bickerton, andJames K. Mortensen

Abstract: Geochemistry and Sm–Nd and U–Pb (magmatic zircon) isotope data from a postcollisional batholith that cross-cuts the allochthonous South Portuguese Zone (SPZ) of southern Iberia suggest that the basement is compositionally morejuvenile than the exposed upper crust. The SPZ is an allochthonous terrane of the late Paleozoic Variscan orogen. The oldestexposed units in the SPZ are Late Devonian continental clastics, and as a result, the origins of the SPZ are unknown. Multi-faceted inherited zircon cores from a granitoid batholith (Sierra Norte Batholith, SNB) reveal Neoproterozoic (ca. 561–647 Ma) and Mesoproterozoic ages (ca. 1075 – ca. 1116 Ma). Granitoid samples are characterized by ɛNd values rangingfrom +1.4 to –9.6 and model ages ca. 0.76–1.8 Ga. Conversely, the exposed Late Devonian clastics of the SPZ are charac-terized by more negative ɛNd values (–7.5 to –10.4). Taken together, U–Pb and Sm–Nd data indicate the lower crust thatmelted to yield the SNB was (i) Neoproterozoic (ca. 560–650 Ma) to Mesoproterozoic (ca. 1.0–1.2 Ga) in age, (ii) was notcompositionally similar to the overlying Devono-Carboniferous continental detritus but was instead more juvenile, withmodel ages between ca. 0.9–1.2 Ga. This unusual relationship is similar to the relationship between the relatively juvenilebasement and ancient upper crust documented in the exposed portion of the Meguma terrane in the northern Appalachians,which paleogeographic reconstructions show was immediately outboard of southern Iberia in the Late Devonian.

Résumé : Selon la géochimie et des données isotopiques Sm–Nd et U–Pb (sur zircon magmatique) provenant d’un batholitepost-collision qui recoupe la zone allochtone South Portuguese (SPZ) de la péninsule ibérique, la composition du socle seraitplus jeune que la croûte supérieure qui affleure. La SPZ est un terrane allochtone de l’orogène varisque (Paléozoïque). Lesunités les plus anciennes qui affleurent dans la SPZ sont des roches clastiques datant du Dévonien tardif; les origines de laSPZ sont donc inconnues. Des noyaux multidimensionnels de zircons hérités d’un batholite granitoïde (le batholite SierraNorte, SNB) indiquent des âges néoprotérozoïques (∼561 à 647 Ma) et mésoprotérozoïques (∼1075 à ∼1116 Ma). Les échan-tillons du granitoïde sont caractérisés par des valeurs ɛNd variant de +1,4 à –9,6 et des âges modèles ∼0,76 à 1,8 Ga. Parcontre les roches clastiques affleurant dans la SPZ et datant du Dévonien tardif sont caractérisées par des valeurs 3Nd plus né-gatives (–7,5 à –10,4). Prises ensemble, les données U-Pb et Sm-Nd indiquent que la croûte inférieure qui a fondu pour don-ner la SNB (i) avait un âge néoprotérozoïque (∼560 à 650 Ma) à mésoprotérozoïque (∼1,0 à 1,2 Ga) et (ii) n’avait pas unecomposition semblable à celle du détritus continental Dévonien-Carbonifère sus-jacent, mais était plutôt plus jeune avec desâges modèles entre ∼0,9 à 1,2 Ga. Cette relation inhabituelle est semblable à la relation entre le socle relativement jeune etl’ancienne croûte documentée dans la partie qui affleure du terrane de Meguma dans les Appalaches du Nord et qui, selon lesreconstructions paléogéographiques, était immédiatement au large du sud de la péninsule ibérique au Dévonien tardif.

[Traduit par la Rédaction]

Introduction

Allochthonous terranes typically preserve evidence of theiroriginal tectonic setting as well as events relating to theiraccretion and subsequent dispersal (e.g., McWilliams and

Howell 1982; Beck 1989; Dallmeyer et al. 1991; Van derVoo 1993; Fernández-Suárez et al. 2002). Therefore, deter-mining the geologic history of orogenic belts typically re-quires an understanding of the tectonic evolution andpaleogeography of allochthonous terranes prior to accretion.

Received 26 August 2010. Accepted 25 June 2011. Published at www.nrcresearchpress.com/cjes on 14 March 2012.

Paper handled by Associate Editor Maurice Colpron.

J.A. Braid, J.B. Murphy, and L. Bickerton. Department of Earth Sciences, Saint Francis Xavier University, Antigonish, NS B2G 2W5,Canada.C. Quesada. Instituto Geológico y Minero de España, C/ Ríos Rosas 23, 28003 Madrid, Spain.J.K. Mortensen. Department of Earth and Ocean Sciences, The University of British Columbia, 339 Stores Road, Vancouver, BCV6T 1Z4, Canada.

Corresponding author: James A. Braid (e-mail: [email protected]).

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Can. J. Earth Sci. 49: 591–613 (2012) doi:10.1139/E11-071 Published by NRC Research Press

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Terrane accretion is commonly accompanied by the depo-sition of syn- to post-orogenic sedimentary sequences, whichoverstep terrane boundaries and cover the preorogenic geol-ogy. Such a scenario occurs in southern Iberia where an al-lochthonous terrane in the late Paleozoic Variscan orogen ofWestern Europe, known as the South Portuguese Zone (SPZ;Lotze 1945), exposes only late Paleozoic clastic rocks andgranitoid rocks, which intrude across the terrane boundary(Fig. 1). The SPZ is located outboard of a suture zone (Pulodo Lobo Zone, PDLZ) to the north, which separates SPZfrom the Iberian autochthon (Fig. 1). The suture zone is par-ticularly significant, as it is widely thought to have developedduring the closure of the late Paleozoic Rheic Ocean and ter-minal collision between Gondwana and Laurussia (Quesadaet al. 1994; Onézime et al. 2003), which is a major event inthe formation of the supercontinent Pangea.Lithologic (Onézime et al. 2003; Simancas et al. 2005) and

geochronological data (e.g., Braid et al. 2011) indicate theSPZ was outboard of the Gondwanan margin at least until theLate Devonian. However, the oldest exposed units in the SPZare Late Devonian continental clastic strata, and as a result,the composition of the SPZ basement cannot be directly deter-mined. Despite this limited geologic record, the pre-VariscanSPZ crust is thought to be a fragment of a peri-Gondwananterrane; either the Meguma terrane (Martínez Catalán et al.1997; de la Rosa et al. 2001) or Avalonia (e.g., Leistel et al.1998; Simancas et al. 2005). These interpretations, which im-ply a connection between the SPZ and peri-Gondwanan ter-ranes (i.e Avalonia, Meguma), are inferred from latePaleozoic reconstructions (e.g., McKerrow and Scotese1990; Scotese 2003; Woodcock et al. 2007), which placesouthern Iberia adjacent to Maritime Canada during the for-mation of Pangea. These terranes are currently exposed inthe northern Appalachians and in southern Britain (e.g.,Keppie 1985, 1993; Murphy et al. 2004; Hibbard et al.2006; Waldron et al. 2009; Nance et al. 2010).Although Late Devonian reconstructions provide a general

paleogeographic framework for the Paleozoic evolution of theSPZ, the potential connections between the SPZ and thenorthern Appalachians remain poorly documented. Therefore,determining the original affinity of the SPZ has profound im-plications on our understanding of the processes affecting theAppalachian and Variscan orogens as well as the timing andgeometry of the formation of Pangea.As the basement to the Upper Devonian clastic successions

is unexposed in the SPZ, its age and composition must bedetermined by indirect methods, such as U–Pb geochronol-ogy of detrital zircons in the clastic successions and of xeno-crystic cores of zircons in plutonic rocks. In addition, Sm–Ndisotopic analyses of clastic rocks can provide information onthe provenance and tectonic processes, such as uplift or ter-rane accretion that accompanied deposition (e.g., Thorogood1990; Murphy et al. 1996; Murphy and Nance 2002); andSm–Nd isotopic analyses of crustally derived plutonic andvolcanic rocks can provide constraints on isotopic composi-tion of basement sources (e.g., DePaolo 1981, 1988).To constrain the composition and origin of the SPZ base-

ment, we present new laser-ablation – inductively coupledplasma – mass spectrometry (LA–ICP–MS) zircon geochro-nological data, geochemical, and Sm–Nd isotopic data fromrepresentative samples from a granite batholith (Sierra Norte

Batholith, SNB; de la Rosa 1992), which crosscuts both thePDLZ and the SPZ. We also present new lithogeochemicaland Sm–Nd isotopic data from exposed sedimentary sequen-ces in the SPZ and PDLZ. These data facilitate a comparisonbetween the SPZ and various tectonostratigraphic zones inthe northern Appalachians (e.g., Meguma and Avalonia),which may have been connected to SPZ in the late Paleozoic.Finally, we attempt to evaluate the significance of these con-nections in interpreting the geometry and timing surroundingthe closure of the Rheic Ocean and the formation of Pangea.

GeologyThe SPZ of the Iberian Massif forms part of the Variscan

orogenic belt in Western Europe (Leistel et al. 1998;Carvalho et al. 1999; Franke 2000). The SPZ is in faultedcontact to the north with the PDLZ, which contains a se-quence of mafic rocks known as the Beja–Acebuches ophio-lite complex. The PDLZ, in turn, is in faulted contact to thenortheast with the Ossa Morena Zone (Fig. 1). The OssaMorena Zone has faunal affinities with Gondwana through-out the Paleozoic (e.g., Robardet 2003) and is generallythought to have accreted obliquely to Gondwana (Iberian au-tochthon) in the Neoproterozoic along an orogen-scale trans-current shear zone (Tomar–Badajoz–Córdoba shear zone).This shear zone was reactivated in the Carboniferous duringthe collision between Laurussia and Gondwana (Quesadaand Dallmeyer 1994). During the Late Devonian, the conti-nental margin of the SPZ is widely held to have driftednorthward by subduction beneath the Ossa Morena Zonemargin of Gondwana until ca. 330 Ma (Jesus et al. 2007).The PDLZ crops out between the SPZ and Gondwana and isclassically interpreted as an accretionary complex betweenSPZ and Ossa Morena Zone that developed during the clo-sure of the Rheic Ocean (e.g., Eden 1991) in the Late Devon-ian – Early Carboniferous.

SPZThe exposed geology of the SPZ is dominated by the

Upper Devonian – lower Carboniferous sedimentary and bi-modal volcanic sequences of the Iberian Pyrite Belt (Fig. 1;e.g., Schermerhorn 1971; Onézime et al. 2003). Three lithos-tratigraphic formations are recognized in the Iberian PyriteBelt (Schermerhorn 1971) from the oldest to the youngest:(i) the Upper Devonian detrital Phyllite Quartzite Group,which are continental clastic strata; (ii) the Volcano SiliceousComplex, hosting the volcanogenic massive sulphide (VMS)mineralization of late Famennian to middle Visean age (Dun-ning et al. 2002; Rosa et al. 2008); and (iii) an upper Viseanto the Serpukhovian turbiditic flysch group (Schermerhorn1971; Oliveira 1990).The Phyllite Quartzite Group is composed of siliciclastic

rocks deposited in a subtidal environment from fan delta andsand bar systems on a shallow-marine continental platform(Moreno and Saez 1990; Oliveira 1990; Moreno et al. 1996).The base of this unit is not exposed and has a minimumthickness of 300–400 m (Soriano and Martí 1999). The dep-ositional age is constrained by the presence of Fammenianconodonts in limestone lenses interbedded with the clasticstrata (Boogaard and Schermerhorn 1980, 1981). Detrital zir-con U–Pb age data from the Phyllite Quartzite Group display

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age populations dominated by Paleoproterozoic (ca. 1.8–2.3 Ga) and Neoproterozoic (ca. 0.5–0.7 Ga) zircons withminor Archean zircons (ca. 2.5–2.9 Ga) (Braid et al. 2011).Volcano Siliceous Complex volcanic and sedimentary

strata accumulated conformably on the Phyllite QuartziteGroup shelf facies basement (Oliveira 1990). The VolcanoSiliceous Complex is composed of mafic and felsic rocks, in-terfingered with purple shales, siltstones, tuffites, and minorlimestones. These rocks are further subdivided into threesedimentary and igneous successions (termed V1, V2, andV3, from lowermost to uppermost). A unit of purple shaleoccurs along the contact between the V2 and V3, and is gen-erally referred to as the purple shale horizon. Limestonerocks of the intermediate succession (V2) are dated by cono-donts and cephalopods as upper Famennian–Tournaisian tolower upper Visean (Boogaard 1963; Oliveira 1983; Oliveiraet al. 1986.). The large variation in the geochemistry of thevolcanic rocks compared with that of the sedimentary stratasuggests that the sedimentary rocks were not derived fromthe coeval mafic and felsic volcanics (Boulter et al. 2004).

PDLZThe PDLZ is characterized by a series of tectonically im-

bricated polydeformed metasedimentary rocks, olistostromalmélange, and tectonic mélange deposits overlain by a rela-tively simply deformed flysch sequence (e.g., Eden 1991;Braid et al. 2010) (Fig. 1). Although nomenclature and lithol-

ogies vary between Spanish and Portuguese sections, there isa general consensus that the lowermost unit is a tectonicmafic mélange (e.g., Eden 1991), with local occurrences ofamphibolite blocks in a tectonically imbricated volcaniclasticand schistose matrix. These mélange deposits are alsocrosscut by mafic dykes and are in fault contact with a poly-deformed sedimentary mélange deposit composed of olistos-tromal phacoidal quartzites in a phyllite–quartzite matrix,which in turn are in fault contact with a sequence of polyde-formed quartzwackes and phyllites. Collectively, these unitsare unconformably overlain by a Visean (ca. 330 – ca.347 Ma; Braid et al. 2011) simply deformed sequence of silt-stone and greywacke known as the Santa Iria Flysch (SIF).Detailed descriptions of the mélange, metasedimentary, andflysch deposits are given in Braid et al. (2010).The PDLZ is classically interpreted as an accretionary

complex developed along the margin of the Ossa MorenaZone during the collision between Gondwana and Laurussia(e.g., Eden 1991; Onézime et al. 1999, 2003). However, re-cent detrital zircon U–Pb age data (Braid et al. 2011) reveala more complex history. Olistostromal quartzite clasts andmatrix from the polydeformed PDLZ have detrital zirconpopulations that cannot be derived from either the SPZ orGondwana. These rocks are both characterized by an abun-dance of Mesoproterozoic zircons (ca. 1.0–1.5 Ga), a subor-dinate Paleoproterozoic (ca. 1.6–1.9 Ga) population, as wellas minor Archean and euhedral ca. 440 Ma zircons. PDLZ

Fig. 1. Summary of the geology of the South Portuguese and Pulo do Lobo zones in the study area (adapted from Oliveira 1990). For detailedPulo do Lobo Zone geology, see Braid et al. (2010). Location of sedimentary samples for lithogeochemistry and Sm–Nd isotopes shown.

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polydeformed samples lack the Neoproterozoic (ca. 0.6–0.9 Ga) and Paleoproterozoic (ca. 2.0–2.5 Ga) detrital zirconsthat are typical of late Paleozoic sedimentary rocks derivedfrom either Gondwana (Ossa Morena Zone), peri-Gondwananterranes (e.g., Meguma terrane), or the Middle Devoniancontinental clastics (Phyllite Quartzite Group) of the SPZ.These data suggest the PDLZ mélange and metasedimentaryrocks were derived from neither the upper plate (Ossa Mor-ena Zone) nor the lower plate (SPZ) (LA–ICP–MS; Braid etal. 2011). To the north, the PDLZ is in fault contact with theBeja–Acebuches ophiolite (ca. 334 ± 2 Ma; Azor et al.2008), which has ophiolitic affinities (Silva et al. 1990;Fonseca and Ribeiro 1993; Quesada et al. 1994) and delin-eates the northern contact between the PDLZ and OssaMorena Zone.

SNBThe ca. 347 Ma (Dunning et al. 2002) SNB is a composite

batholith (Fig. 2) that intrudes the PDLZ and the SPZ (de laRosa 1992; Castro et al. 1995). One of its components, theGil Márquez granodiorites (ca. 330 Ma, LA–ICP–MS; de laRosa et al. 2001) locally intrudes the polydeformed mélange,metasedimentary rocks, and the flysch of the PDLZ. The GilMárquez granodiorites are typically foliated parallel to theeast–west orogenic grain and are interpreted to have beenemplaced during the latest stages of deformation in thePDLZ (e.g., Castro et al. 1995). The ca. 330 Ma age datewas obtained exclusively from these foliated granodiorites.The remainder of the SNB is composed of nonfoliated gab-bro, diorite, tonalite, monzogranite, and granite and cropsout in the northeast part of the Iberian Pyrite Belt (Soler1980; Schütz et al. 1987) (Fig. 2).This batholith is interpreted to represent either the deep

equivalents of the Volcano Siliceous Complex volcanics ofthe Iberian Pyrite Belt in the SPZ (Soler 1980; Schütz et al.1987), or late-orogenic intrusions, which are unrelated to therocks of the Volcano Siliceous Complex (Simancas 1983).Rb–Sr whole rock isotopic data are interpreted to indicatethat the SNB calc-alkaline granitoids are the product of inter-action the magmas derived from the lithospheric mantle withmagmas developed by partial melting of a deep mafic andpelitic granulitic crust in an active continental margin (de laRosa et al. 1993, 2001).

Sample selection and analytical methodsTo further constrain the composition of the SPZ basement,

representative samples were collected from granitoid rocks ofthe SNB for LA–ICP–MS U–Pb dating, lithogeochemical,and Sm–Nd isotope analysis (Fig. 2). As the granitoids wereemplaced following collision of the SPZ and the Ossa Mor-ena Zone and are generally considered a product of meltingthe deep continental crust, these samples may preserve an in-herited signature of the SPZ basement. The foliated Gil Már-quez pluton, which locally crosscuts the PDLZ suture, hasbeen dated at ca. 330 Ma (de la Rosa et al. 2001) and thenonfoliated SNB batholith at ca. 354 Ma (Dunning et al.2002). To better assess the potential contribution of SPZbasement to the granitoid melt away from the suture zone aswell as the range in age of magmatism across the batholith,

samples were selected from both the foliated Gil Márquezpluton and nonfoliated rocks of the SNB outboard of thePDLZ (Fig. 2). Furthermore, to assess (i) the relative contri-bution of the exposed sedimentary units in the SPZ andPDLZ to the granitoid melts and (ii) potential differences incomposition of the lower and upper crust, we also presentlithogeochemistry and Sm–Nd isotopic data from representa-tive samples collected from sedimentary units in both theSPZ and the PDLZ, which are crosscut by the SNB (Fig. 1).The locations of all analyzed samples during this study aregiven in Table SD-1.1

Lithogeochemistry and Sm–Nd isotopesThe major and selected trace elements were analyzed by

X-ray fluorescence at the Nova Scotia Regional GeochemicalCentre at St. Mary’s University, Halifax, Nova Scotia. Detailsof the analytical methods are given in Dostal et al. (1994).Rare-earth element (REE) analyses were also determined atMemorial University, St. John’s, Newfoundland, by induc-tively coupled plasma – mass spectrometry (ICP–MS) ac-cording to methods described in Jenner et al. (1990).Samples were analyzed for Sm–Nd compositions at the At-lantic Universities Regional Isotopic Facility (AURIF) at Me-morial University. Analytical procedures for Sm–Nd analysesare described in Kerr et al. (1995). The Nd isotope signaturein clastic sedimentary rocks is interpreted to represent theweighted average of values for detrital contributions from thevarious source areas (see Arndt and Goldstein 1987; Thoro-good 1990; Murphy and Nance 2002). The geochemical (Ta-ble SD-2) and Sm–Nd (Table SD-3) data are available assupplementary data.1

U–Pb LA–ICP–MSApproximately 16–30 zircon grains from each of four rep-

resentative granitoid samples taken from both the Gil Már-quez granodiorite (two samples) and across the SNB (twosamples) were mounted, polished, and imaged by electronbackscatter (samples JAB-09; JAB-28) and cathode lumines-cence (ACR-04; JB-26B). Of these samples, two were foli-ated (JAB-09; ACR-04) and two were nonfoliated (JB–26B;JAB-28). Scanning electron microscopy (SEM) imaging wasdone at the Pacific Centre for Isotopic and Geochemical Re-search (PCIGR) at The University of British Columbia, Van-couver, British Columbia. Cathode luminescence imageswere obtained at the Zircon and Accessory Phase Laboratory(ZAPLab) at The University of Western Ontario, London,Ontario. Zircon grains were analyzed for their U and Pb iso-topic composition using a thermo-element 2 high-resolutioninductively coupled plasma – mass spectrometer coupled toa new wave research 213 nm Nd–YAG laser. Detailed de-scription of analytical instrumentation, analytical protocoland methodology, data reduction and age calculation at thePCIGR at The University of British Columbia are describedin Mortensen et al. (1995, 2007). All zircons were analyzedusing line scans, with a laser spot diameter of 20 µm alongeither a zircon core or rim. For each line scan (one analysis)that crossed both a core and a rim, data were selected fromeither the core or rim section. In some cases, where the rims

1Supplementary data are available with the article through the journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/e11-071.



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were thick (i.e., >20 µm) two lines scans (two analyses) wereperformed on an individual zircon grain.Age uncertainties are reported at 2s, and either the Pb207/

Pb206 or the Pb206/U238 age is reported, depending on whichvalue gives the lower uncertainty (Ludwig 1998). A total of90 analyses were obtained in the four samples. Six analysesrevealed >10% discordance and were discarded. U–Pb con-cordia distribution plots for the remaining 84 concordantgrains are shown in Figs. 7 and 8 and compared in a relativeprobability plot (Fig. 11).

Results

SNB

LithogeochemistryNine representative samples were selected from the SNB

(Fig. 2). Sample JAB-24 was taken from foliated granitoidbodies within the SNB that crosscut the SIF within thePDLZ. Two SNB samples were selected from foliated grano-diorites near the town of Gil Márquez (JAB-09, JAB-02) andthe remaining six samples (JAB-10, JAB-25, JB-26B, JAB-26, JAB-27, JAB-28) were selected from various nonfoliatedSNB intrusive bodies. All samples display sericitization of

feldspars in thin section indicative of postemplacement alter-ation and (or) weathering. The major- and trace-elementchemistry (Tables SD-2, SD-3) for the SNB samples areavailable as supplementary data.1Most of the SNB samples are felsic (SiO2 >67 wt.%, on a

volatile-free basis), but one sample (JB-26B) has a SiO2 con-tent of ∼60 wt.%. TiO2, Fe2O3, Al2O3 (with exception of JB-26B), and CaO display negative linear correlations with SiO2,whereas Na2O and K2O display more complex patterns(Fig. 3a). Trace-element correlations with SiO2 display morecomplex patterns than the major elements, although Zr dis-plays a clear negative correlation (Fig. 3b).As all samples from the SNB display petrographic evi-

dence of alteration and (or) weathering, ratios involvingHFSE (high field-strength elements) and REEs are generallyconsidered more reliable indicators of the rock’s original geo-chemistry because they are relatively unaffected by thoseprocesses (e.g., McLennan et al. 1980; Bhatia and Crook1986). HFSE and REE abundances are emphasized, as theyare more reliable geochemical indicators of SNB magmaticaffinity and tectonic setting.Chondrite and primitive mantle normalized plots of REEs

are shown in Fig. 4a. A moderate enrichment in light rare-earth elements (LREE) in most SNB samples is reflected the

Fig. 2. Summary of the geology of the Sierra Norte Batholith (adapted from de la Rosa et al. 1993). Granitoid samples (sample numbershown on figure) for lithogeochemistry, Sm–Nd isotopes, and LA–ICP–MS zircon age dating. SPZ, South Portuguese Zone; SISZ, SouthIberian Shear Zone (Crespo-Blanc and Orozco 1991); OMZ, Ossa Morena Zone.

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range in (La/Sm)n ratio from 2.3 to 5.7. Sample JAB-27 dis-plays a slightly positive Sm anomaly and a relatively flatLREE profile reflected by a (La/Sm)n ratio of 1.6. Abundan-ces of heavy rare-earth elements (HREE) generally show flatprofiles, which are reflected in the range in (Gd/Lu)n ratio of0.59–1.5. All samples are characterized by a negative euro-pium anomaly, with [(Eu*/Eu) –1], where Eu*/Eu representsEu sample/interpolated Eu, varying between 0.2 (JAB-28) and4.6 (JAB-24), with a mean value of 1.41 (see inset Fig. 4a).

Sm–Nd isotopesThe Sm–Nd data for nine SNB samples (Table SD-3) are

available as supplementary data.1 To facilitate comparisonbetween formations, ɛNd values (relative to chondritic uni-form reservoir (CHUR)) given in the text are for an averageintrusive age (T = 330 Ma) of the SNB. Following Stern(2002), we report depleted mantle model ages, TDM, only forsamples with 147Sm/144Nd < 0.165. Of the nine samples, onesample (JAB-27) displays a 147Sm/144Nd > 0.165; therefore,

Fig. 3. (a) Selected major elements in Harker plots for the Sierra Norte Batholith (SNB) granitoid samples. (b) Selected trace elements inHarker plots for the SNB granitoid samples.



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its TDM age is not reported. Of the remaining eight samples,TDM ages vary from ca. 0.76 to 1.80 Ga. Taken together, allsamples in the batholith are characterized by a wide range inɛNd values from +1.4 to –9.6 (Fig. 5). Two samples (JAB-28and JAB-27) display comparatively more negative ɛNd valuesat T = 330 Ma (–9.6 and –7.5, respectively). The remainderof samples display a more limited range in ɛNd (–3.0 to 1.4)at T = 330 Ma, with model ages ca. 0.9–1.2 Ga.

U–Pb isotopic data, foliated rocksSample JAB-09 is a strongly foliated granodiorite collected

from a large outcrop exposed along a roadcut south of thevillage of Gil Márquez. The sample is coarse-grained (5–10 mm), and is dominated by quartz, plagioclase, K-feldspar,biotite, and amphibole, with subordinate apatite, sphene, zir-con, and opaque minerals. Zircons from sample JAB-09 typi-cally show multifaceted terminations, with either a stubby

Fig. 3 (concluded).

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euhedral or elongate morphology. SEM backscatter imagesreveal (i) zircon cores that are either multifaceted or roundedwith a thin rim or (ii) zircons without cores that are zonedand multifaceted (Fig. 6a). To determine both the age ofigneous crystallization and the ages of potential inheritedcores, line scans were performed on either the core or rim of30 representative grains. The results are listed in Table SD-41and plotted in Fig. 7. Of the 30 analyses, three are discordantand are not considered further. Twenty-four analyses wereperformed on either multifaceted zircons with no rims orrims of zircons with cores. These analyses yielded ages rang-ing from ca. 327 to ca. 386 Ma. Two analyses were also per-formed on rounded zircon cores and revealed concordantages of ca. 2040 and ca. 2075 Ma. One analysis was alsoperformed on a multifaceted zircon core and revealed a con-cordant age of ca. 1180 Ma.Sample ACR-04 was collected south of the Almonaster la

Real village from a granite body that locally crosscuts themetasedimentary rocks and the Santa Iría Flysch of thePDLZ (Fig. 2). The sample was collected from a penetra-tively foliated granitoid and is composed of predominantlyquartz, plagioclase, K-feldspar, biotite, and amphibole. Mostof the zircon grains (∼70%) are medium to large (100–200 µm in length), elongate subeuhedral prisms. The grainsrange from translucent yellow–brown to turbid, and generallyhave inclusions and fractures. The remainder of the grains(∼30%) are medium (∼100 µm in diameter) stubby and sub-

euhedral to euhedral. Stubby grains generally have more in-clusions and fractures. Cathodoluminescence (CL) imagingreveals that most zircons are characterized by complex zon-ing patterns. Inherited cores are irregular in shape but appearmultifaceted and zoned (Fig. 6b). To determine both the ageof igneous crystallization and the ages of potential inheritedcores, line scans were performed across 16 representativegrains. The U–Pb isotopic data (Table SD-41) are plotted inFig. 7. Of the 16 analyses, 15 yielded concordant or nearlyconcordant ages (i.e., <10% discordance). Of these 15 analy-ses, 12 yielded concordant ages ranging from ca. 331 to ca.357 Ma. Three analyses of multifaceted zircon cores yieldedconcordant ages of ca. 1075 – ca. 1116 Ma.

U–Pb isotopic data, nonfoliated rocksSample JB-26B is from an outcrop of nonfoliated grano-

diorites exposed along a roadcut near the town of Castillo delas Guardas (Fig. 2). The granodiorite is composed primarilyof quartz, plagioclase, K-feldspar, biotite, and amphibole,with minor apatite, monazite, titanite, zircon, and opaques.Zircons from sample JB-26B show a broad range of mor-phologies including (i) pale yellow slightly elongate, pris-matic, and multifaceted (∼40%) and (ii) clear, stubby,prismatic, and multifacted (∼30%) to (iii) magmatically re-sorbed (∼15%) and (iv) multifaceted and fractured (∼15%).CL imaging reveals most zircons contain complex zoningpatterns with no inherited cores (Fig. 6c). Fourteen represen-

Fig. 4. Chondrite-normalized rare-earth element (REE) profiles for (a) Sierra Norte Batholith granitoid samples, (b) Volcano Siliceous Com-plex and Phyllite Quartzite Group of the South Portuguese Zone (SPZ), and (c) olistostromal mélange of the Pulo do Lobo Zone (PDLZ).Inset figures showing variations in light rare-earth elements (LREE) and heavy rare-earth elements (HREE) enrichment or depletion andeuropium anomalies. Normalizing values from Sun and McDonough (1989). Eu*/Eu, Eu sample/interpolated Eu.



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tative grains were analyzed from sample JB-26B. All yieldedconcordant or near concordant ages (<10% discordance),with a mean age of 330.8 ± 1.8 Ma. The U–Pb isotopic data(Table SD-4)1 are plotted in Fig. 7.Sample JAB-28 was collected from a granitoid that crops

out near the town of Zufre (Fig. 2). The sample is nonfoliatedand composed primarily of quartz, plagioclase, K-feldspar, bio-tite, and muscovite. Most of the zircon grains (∼70%) fromsample JAB-28 are medium to large (100–200 µm inlength), euhedral to subeuhedral prisms. The grains rangefrom translucent yellow–brown to turbid, and generallyhave inclusions and fractures. The remainders of the grains(∼30%) are medium-sized (∼100 µm in diameter), stubby torounded. BSE imaging shows that the majority of zircons(∼70%) show no zonation, and the remainder (20%) havemultifaceted elongate cores rimmed by thick overgrowths orare rounded unzoned zircons rimmed by thin overgrowths(Fig. 6d). To determine both the age of igneous crystalliza-tion and the ages of potential inherited cores, line scanswere performed across 30 representative grains (one analy-sis per grain). The U–Pb isotopic data (Table SD-4)1 are

Fig. 4 (continued).

Fig. 4 (concluded).

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plotted in Fig. 8. Of these 30 analyses, all yielded concord-ant ages. Analysis of multifaceted prismatic zircons and zir-con rims (15 analyses) reveal concordant ages ranging fromca. 309 to ca. 361 Ma. Ten analyses of multifaceted zirconcores reveal concordant ages ranging from ca. 561 to ca.647 Ma and five analyses of rounded zircon cores withthin overgrowths yield concordant ages ranging from ca.1989 to ca. 2910 Ma (Fig. 8).

Sedimentary units: lithogeochemistry and Sm–Nd

SPZTwelve samples were selected from clastic rocks in the V2,

V3, and purple shale (PS) horizon of the Volcano SiliceousComplex, and three samples were selected from the LateDevonian Phyllite Quartzite Group (JB-17, JB-24, JB-25) inthe SPZ. These sedimentary units are all crosscut by theSNB. The major-and trace-element chemistry for the VolcanoSiliceous Complex and Phyllite Quartzite Group rocks isgiven in Table S11 and Sm–Nd data in Table S31.Overall, major oxide concentrations show large ranges in

concentrations. One sample from the V2 contains nearly100% SiO2. In the other V2 samples, SiO2 ranges from ∼57to 86 wt.%, Al2O3 from trace amounts to ∼20 wt.%, TiO2 fromtrace amounts to 1.07 wt.%, CaO from 0.03 to 0.24 wt.%, andFe2O3 from ∼4.2 to 10 wt.%. By comparison, major-elementabundances in the V3 samples show a more restricted rangein concentration with SiO2, ranging from ∼72 to 77 wt.%,Al2O3 from ∼10.8 to 13.7 wt.%, TiO2 from ∼0.3 to

0.9 wt.%, CaO from 0.05 to 0.3 wt.%, and Fe2O3 from∼4.6 to 7.2 wt.%. Major-element geochemistry of the PSsamples reveal SiO2 ranging from ∼64 to 65 wt.%, Al2O3from ∼18.3 to 21.2 wt.%, TiO2 from ∼0.7 to 0.8 wt.%,CaO from 0.09 to 0.18 wt.%, and Fe2O3 from ∼7.3 to7.7 wt.%. Phyllite Quartzite Group samples reveal SiO2ranging from ∼54 to 74 wt.%, Al2O3 from ∼2 to 25 wt.%,TiO2 from ∼0.23 to 0.96 wt.%, CaO from 0.04 to 0.12 wt.%,and Fe2O3 from ∼6% to 36 wt.%.To more fully document the geochemistry of the SPZ,

published major-element (40 samples) and REE (five sam-ples) data of the Volcano Siliceous Complex (Boulter et al.2004) are included on geochemical plots (Figs. 4b, 9). Com-plex patterns on plots such as log(SiO2/Al2O3) versus log(Na2O/K2O) Volcano Siliceous Complex rocks (Fig. 9a)likely reflect alkali mobility during sedimentary processessuch as weathering and diagenesis. On a Fe2O3 + MgO ver-sus Al2O3/SiO2 diagram (after Bhatia 1983), the samples dis-play a linear, positive correlation between Fe2O3 + MgO andAl2O3/SiO2 (Fig. 9d). The significance of this trend is un-clear, and its apparent arc to active-margin to passive-marginsignature may reflect the tectonic setting either of the strataor of the source rocks.On a plot of Zr/Nb versus Ti/Nb (Fig. 10a), Volcano Sili-

ceous Complex samples show a general trend of increasingTi/Nb with increasing Zr/Nb, and Phyllite Quartzite Groupsamples display little variation in Ti/Nb. On a Zr/Y versusTi/Y diagram (Fig. 10b), the Volcano Siliceous Complexsamples show a positive correlation between Zr/Y and Ti/Y.

Fig. 5. ɛNd (T) versus time (Ga) diagram (at T = 350 Ma), comparing Sm–Nd isotope data for the Pulo do Lobo Zone (PDLZ), SouthPortuguese Zone (SPZ), and Sierra Norte Batholith (SNB) with typical Sm–Nd isotope compositions of Avalonian crust (Murphy et al. 1996,2000), 1.0–1.2 Ga (Samson et al. 2000) for rocks in the North American Grenville Province, Meguma metasedimentary rocks (MMS), andMeguma granitoid rocks (MGR) (Clarke et al. 1997). Depleted mantle evolution curve is from the model of DePaolo (1981). PQ, phyllitequartzite; VSC, volcano siliceous complex.



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Similarly, on a Zr/V versus Ti/V diagram (Fig. 10c), VolcanoSiliceous Complex samples lie on a trend of higher Zr/Vwith higher Ti/V.Chondrite-normalized REE patterns for all SPZ samples

(Fig. 4b) are moderately sloping and display moderate andrelatively restricted LREE enrichment reflected in (La/Sm)nof 2.6–4.3. HREE generally display a flat profile, with (Gd/Lu)n ranging from 0.94 to 1.72. All samples are characterizedby a slight negative europium anomaly, with [(Eu*/Eu) – 1]of 0.17 (JAB-12, V3) and 0.84 (JAB-11, V2) (see insetFig. 4b). Sample JAB-19 displays a negative Ce anomaly.The Sm–Nd isotopic data for Volcano Siliceous Complex

and Phyllite Quartzite Group samples are listed in Table S2.1To facilitate comparison between formations, ɛNd values (rel-ative to CHUR) given in the text are for the same depositio-nal age (T = 350 Ma). Taken together, the Volcano SiliceousComplex samples are characterized by a wide range of ɛNdvalues. V2 samples range from –11.2 to –0.7, purple shalesamples from –7.1 to –8.4, and V3 samples from –5.9 to –1.6(Fig. 5). On the other hand, Phyllite Quartzite Group samplesshow a relatively restricted range (from –10.4 to –7.5).

PDLZSamples were selected from the polydeformed mélange,

matrix, and metasedimentary rocks of the PDLZ (RSA-01,quartzite phacoid; RSA-02, matrix; and AC-03; quartzite)

and siltstones from the overlying SIF (samples JAB-01, JAB-03, JAB-08). The major-element chemistry (Table S11) forsamples from the polydeformed PDLZ rocks displays highSiO2 >85 wt.%, whereas SIF samples display SiO2 between70 and 80 wt.%. TiO2 in the PDLZ samples varies from0.21 to 0.58 wt.%, CaO from 0.04 to 0.13 wt.%, Fe2O3 from∼2.5 to 3.5 wt.%. Na2O typically occurs in very low concen-trations. By comparison, SIF samples are higher in TiO2(from ∼0.8 to 0.9 wt.%) and Fe2O3 (from ∼4.6 to 6.7 wt.%)and lower in CaO (from 0.02 to 0.04 wt.%). On plots such aslog SiO2/Al2O3 versus log Na2O/K2O, PDLZ polydeformedrocks display extremely low Na2O/K2O (owing to the veryminor abundance of Na2O in the samples) and a slightly neg-ative correlation (Fig. 9a).On the Fe2O3 + MgO versus Al2O3/(CaO+Na2O) and the

Fe2O3 + MgO versus Al2O3/SiO2 diagrams (Figs. 9b, 9c), therocks display more complex patterns, suggesting alkali mobi-lity during weathering. However, on the Al2O3/SiO2 versusFe2O3 + MgO diagram (Fig. 9d), the polydeformed PDLZand SIF samples display increasing Al2O3/SiO2 with increas-ing Fe2O3 + MgO. The relatively low Fe2O3 + MgO andAl2O3/SiO2 in the PDLZ samples are typical of upper crustalrocks either deposited in, or derived from, a passive marginsetting (after Bhatia 1983).On a plot of Zr/Nb versus Ti/Nb (Fig. 10a), PDLZ polyde-

formed samples (RSA-01–RSA-03) display a general trend of

Fig. 6. (a) Scanning electron microscopy (SEM) backscatter images of representative zircons in sample JAB-09. (b) Representative cathodo-luminescence (CL) images of zircons in sample ACR-04. (c) Representative CL images of zircons in sample JB-26B. (d) SEM backscatterimages of representative zircons in sample JAB-28. Dark lines in SEM images are the raster ion probe analysis.

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Fig. 6 (continued).



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increasing Ti/Nb ratio with increasing Zr/Nb, whereas SIFsamples display a limited range in Ti/Nb and Zr/Nb. Alterna-tively, on a plot of Zr/Y versus Ti/Y, SIF samples exhibit apositive relationship between Zr/Y and Ti/Y (Fig. 10b). On aplot of Ti/V versus Zr/V, both SIF and PDLZ polydeformedsamples together display an increasing trend between Ti/Vand Zr/V (Fig. 9c)Chondrite-normalized REE patterns for the polydeformed

PDLZ are gently sloping (Fig. 4c) and display moderateLREE enrichment, with a relatively restricted range (La/Sm = 3.5–3.8). HREE display flat profiles as reflected in(Gd/Lu)n, which ranges from 0.88 to 1.2. All polydeformedPDLZ samples also display a negative europium anomaly([(Eu*/Eu) – 1] = 0.41–1.27).Sm–Nd compositions of the olistostromal mélange and the

metasedimentary rocks (RDL) are characterized by negativeɛNd values (calculated at T = 350 Ma) (–0.9, metasedi-ments), (–8.4, phacoidal quartzite), and (–6.8, phyllitic ma-trix) (Fig. 5).

Interpretation

Magmatic age of the SNBThe youngest ages yielded by prismatic zircon crystals and

rims indicate a concordant age, ranging from 325.5 ±1.74 Ma (sample ACR-04) to 333 ± 4.78 Ma (sample JAB-09) for the foliated Gil Márquez pluton, which within our un-certainty is indistinguishable from the U–Pb age on a late

leucogranite dated by de la Rosa et al. 2001 (ca. 330 Ma).For the nonfoliated rocks of the SNB, magmatic ages rangefrom 308.3 ± 3.03 Ma (sample JB-26B) to 309 ± 4.43 Ma(sample JAB-28) (Figs. 7, 8). This range in age suggests thenonfoliated rocks are younger than the foliated rocks (GilMárquez pluton), and the batholith as a whole is composite,recording multiple stages of magmatism.

Inherited ages of the SNBThe zircon cores in SNB samples, which reveal ages rang-

ing from ca. 561 to ca. 647 Ma in sample JAB-28, from ca.1075 to ca. 1116 Ma in sample ACR-04, and ca. 1185 Ma insample JAB-09, all display a multifaceted inherited core mor-phology (Fig. 6). On the other hand, older Paleoproterozoicinherited cores with concordant ages of ca. 2005 Ma (sampleJAB-28) and ca. 2073 Ma (sample JAB-09) as well as Ar-chean inherited cores with concordant ages of ca. 2759, ca.2828, and ca. 2910 Ma (sample JAB-28) all display a well-rounded morphology (Fig. 6).Multifaceted zircons are generally indicative of a magmatic

protolith (e.g., Timmermann et al. 2000). Furthermore, detri-tal zircons from the Phyllite Quartzite Group and the PDLZare consistently well rounded (Braid 2011; Braid et al.2011). Most likely, the protolith for the multifaceted cores isthe unexposed basement to the Phyllite Quartzite Group,whereas the rounded cores probably originated from sedi-mentary rocks containing late Paleoproterozoic and Archeandetrital zircons. As a result, the magmatic inherited cores in

Fig. 6 (concluded).

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the SNB samples likely preserve zircons from the melting ofSPZ basement, whereas the rounded zircon cores are xeno-crysts probably derived from the Devono-Carboniferous sedi-mentary rocks of the Phyllite Quartzite Group or PDLZ(which both contain late Paleoproterozoic and Archean detri-tal zircons; Braid 2011; Braid et al. 2011). Furthermore, mostrounded inherited cores contain thin magmatic rims, in con-trast with thicker well-zoned rims that occur on prismaticcores of Neoproterozoic age. These different morphologicalcharacteristics suggest that the rounded inherited cores arexenocrysts, which were likely incorporated into the meltfrom the adjacent host rock, late in the evolution of the plu-ton. The numerous Paleoproterozoic and Archean inheritedcores in sample JAB-28 suggest that this sample was derivedfrom a more evolved granite, which was contaminated by theupper crust of the Phyllite Quartzite Group and (or) the Vol-cano Siliceous Complex.

Sm–Nd isotopes and geochemistry of the SNB

SNB sourceThe SNB samples display ɛNd values ranging from +1.4

to –9.6 and model ages ca. 0.76–1.8 Ga. One sample (JAB-27) with high 147Sm/144Nd (>0.165) also has a flat LREE pro-file, consistent with fractionation of accessory phase duringcrystallization of the granite melt, thereby invalidating TDMcalculations (Arndt and Goldstein 1987). On average, the bulkof the SNB samples have less negative ɛNd values (–3.0 to1.4) than the Phyllite Quartzite Group detritus (ɛNd ∼ –7.5to –10.4), the Volcano Siliceous Complex detritus (ɛNd ∼11.2to –0.7), and the PDLZ detritus (ɛNd ∼ –6.8 to –9.0). How-ever, two samples (JAB-28 and JAB-27) display more negativeɛNd (–9.6 and –7.5, respectively) compared with the otherSNB samples, suggesting either melting of a relatively ancientbasement or significant contamination from the upper crust.

Fig. 7. U–Pb concordia plot of samples (A) ACR-04; (B) JB-26B; (C) JAB-09. (D) Enlargement of concordia plot for Paleozoic zircons ofsample JAB-09 (ellipses are 2s).



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Samples JAB-28 and JAB-27 have ɛNd signatures similarto the Phyllite Quartzite Group (which they crosscut), sug-gesting potential contamination from Phyllite QuartziteGroup detritus. These data support the interpretation thatrounded Paleoproterozoic (ca. 2.0 Ga) and Archean (ca. 2.7–2.9 Ga) inherited cores in sample JAB-28 are indicative of alate-stage crustal contamination. This interpretation is furthersupported by the presence of these zircon populations (ca.1.8–2.3 and ca. 2.5–2.9 Ga) in Phyllite Quartzite Group de-trital zircons samples Braid et al. (2011).Taken together, Sm–Nd and U–Pb data suggest that (i) the

granitoid melts of samples JAB-28 and JAB-27 were conta-minated by the overlying SPZ strata, and (ii) the source forthe SNB was on average more juvenile than the sedimentaryunits that they crosscut. This relationship indicates the lowercrust that melted to yield the SNB was not compositionallysimilar to the Devono-Carboniferous continental detritus(Phyllite Quartzite Group, Volcano Siliceous Complex, andPDLZ), but was instead derived from a more juvenile lowercrustal source. The inferred age of this lower crustal sourceis ca. 561 to ca. 1116 Ma (based on inherited core ages).

Relationship between the PDLZ and SPZGeochemical comparison of rocks involving interelement

ratios of high field-strength elements are especially signifi-cant because these plots are relatively insensitive to sedimen-tary processes that affect the modal abundance of theaccessory phases in which these elements reside (McLennanet al. 1980). The geochemical signatures of the all SPZ rockscompared with PDLZ rocks show fundamental geochemicaldifferences, suggesting that derivation from a similar sourceis unlikely. On interelement ratio plots, PDLZ samples gener-ally display a more restricted range in Ti/Nb, Zr/Y, and Ti/Nbthan detritus from the SPZ (Fig. 9). Chondrite-normalizedREE patterns in the SPZ rocks compared with PDLZ mél-ange samples are characterized by higher REEs, especiallyCe to Tb, and gentler HREE profiles. To a first order, differ-ences in REE profiles suggest the PDLZ and SPZ detrituswere not derived from the same source and support the exoticorigin of the quartzite mélange and associated metasedimen-tary rocks suggested by Braid et al. (2011).Within the SPZ, Phyllite Quartzite Group samples gener-

ally display more restricted Ti/Nb, Zr/Y, and Ti/Nb ratios

Fig. 8. (A) U–Pb concordia plot of sample JAB-28. (B) Enlargement of concordia plot for Paleozoic zircons. (C) Enlargement of Neoproter-ozoic to Cambrian zircons. (D) Enlargement of concordia plot for Paleoproterozoic to Archean zircons.

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than the Volcano Siliceous Complex sedimentary rocks(Fig. 10). Volcano Siliceous Complex sedimentary rocks alsocontain a wider range in ɛNd (–11 to –0.7; Fig. 5), suggest-ing a variable contribution from more juvenile sources prob-ably associated to coeval bimodal magmatic activity. On the

other hand, the Phyllite Quartzite Group samples show a rel-atively restricted range (ɛNd ∼ –6.8 to –9.0), suggesting de-tritus was derived from (on average) a much older sourcethan in the Volcano Siliceous Complex. In this case, thewide range in ɛNd values for the Volcano Siliceous Complex

Fig. 9. South Portuguese Zone (SPZ) and Pulo do Lobo Zone (PDLZ) sedimentary rocks plotted on selected published discrimination diagrams,emphasizing major-element chemical variations: (A) log Na2O/K2O versus log SiO2/Al2O3; (B) K2O/Na2O versus SiO2 (after Roser and Korsch1986); (C) Al2O3/(CaO + Na2O) versus Fe2O3 + MgO (after Bhatia 1983); (D) Al2O3/SiO2 versus Fe2O3 + MgO (after Bhatia 1983); (E) K2O/Na2Oversus Fe2O3 + MgO (after Bhatia 1983). (C–E) 1, oceanic island arc; 2, continental island arc; 3, active continental margin; 4, passive margin.ACM, active continental margin; PM, passive margin; PQ, phyllite quartzite; SIF, Santa Iria Flysch; VSC, volcano siliceous complex.



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detritus may be attributed to the variable isotopic characteris-tics of the source rocks (e.g., juvenile volcanics and more an-cient sedimentary rocks).

Discussion

Taken together, geochemistry, Sm–Nd isotopes, and zircongeochronology and morphology suggest that the SNB gran-

itoids (i) are part of a composite batholith displaying a broadrange of crystallization ages, (ii) are the product of melting aNeoproterozoic to Mesoproterozoic (ca. 0.56–1.1 Ga) base-ment source, (iii) were in some cases contaminated whenthey incorporated older detrital zircon xenocrysts from therocks they intruded, and (iv) are a product of melting asource that was isotopically more juvenile compared with theDevono-Carboniferous SPZ and PDLZ strata that they intrude.

Fig. 10. Plots using interelement ratios of high field-strength elements: (A) Zr/Nb versus Ti/Nb; (B) Zr/V versus Ti/V; (C) Zr/Y versus Ti/Y.MMS, Meguma Group metasedimentary rocks. Acronyms as in Fig. 9.

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Late Devonian paleogeographic reconstructions (e.g.,Martínez Catalán et al. 1997; Scotese 2003; Simancas et al.2005; Nance et al. 2010) provide constraints for the SPZprovenance and show Gondwana and Laurussia, both flank-ing the Rheic Ocean and in relative proximity to the Iberianautochthon. Therefore, based on these reconstructions,potential candidates for the basement of the SPZ in the LateDevonian are (i) Gondwana (Iberian autochthon) or (ii) peri-Gondwanan terranes such as Avalonia or Meguma, whichlay along the outer Laurussian margin at that time.The Gondwanan autochthon (Iberia) is an unlikely candi-

date, as SNB model ages from this study are younger thanTDM Nd model ages and inherited zircon cores (de la Rosaet al. 2001) of granitic rocks of Ossa Morena Zone and fromthe Central Iberian Zone, where most rocks are between1.5 Ga (e.g., Castro et al. 1995; Bea et al. 1999) and 2.0 Ga(Barbero et al. 1995; Linnemann et al. 2004). Therefore,based on the comparatively juvenile basement signature ofthe SPZ basement, our data support the conclusion of de laRosa et al. (2001), who suggest this signature is indicativeof an exotic origin of the SPZ relative to the rest of IberianMassif (e.g., Simancas et al. 2005). An exotic origin of the

Fig. 11. Relative probability plots of U–Pb data from Sierra Norte Batholith (SNB) granitoid samples compared with U–Pb detrital zirconrelative probability distribution plots for samples from the Pulo do Lobo Zone (PDLZ; suture) and the South Portuguese Zone (SPZ; Laurus-sia?) from this study compared with samples (highlighted in red in online version) from early Silurian Kirkcolm Formation of the SouthernUplands Terrane of the British Caledonides (after Waldron et al. 2008), the Cambro-Ordovician Meguma terrane from the Northern Appala-chians (after Waldron et al. 2009), and the Devono-Carboniferous Horton Group from the St. Mary’s Basin of the Northern Appalachians(after Murphy and Hamilton 2000) (in red in online version). Plots were generated by ISOPLOT (Ludwig 2003).



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SPZ is also consistent with detrital zircon data (Braid et al.2011) from the PDLZ and the Phyllite Quartzite Group(Fig. 11).Several correlations have been proposed between the SPZ

and the northern Appalachians based on lithostratigraphic,structural, and geophysical and geochemical evidence (e.g.,Lefort 1988; Martínez Catalán et al. 1997; de la Rosa et al.2001). Although the ages found in the uncontaminated inher-ited zircon cores from the SPZ are younger than the TDM Ndmodel ages (ca. 1.7 Ga; Clarke and Halliday 1985) and U–Pb zircon and monazite ages of the Meguma Group (ca. 2–3Ga; Krogh and Keppie 1990), the ages are broadly similar tothe TDM Nd model ages of granulite facies xenoliths derivedfrom the lower crust beneath the Meguma Zone (ca. 1.0–1.1 Ga; Eberz et al. 1991), ɛNd values from Meguma Gran-itoid rocks (Figs. 5, 12), and volcanics (Whiterock Forma-tion) (Fig. 12). Together, these signatures are all consistentwith the average range of Avalonian igneous rocks (ca. 0.8–

1.1 Ga; Keppie et al. 1997; Fig. 12). These observations arealso consistent with geochemical data (TDM ∼0.95) for theGil Márquez pluton (Castro et al. 1995) and inherited zirconcores in the Gil Márquez pluton (<0.95 Ga; de la Rosa et al.2001). Taken together, these data support the suggestion thatthe SPZ is related to Avalonia as exposed in the northern Ap-palachians.However, (i) the abundant Paleoproterozoic detrital zircons

(ca. 1.8–2.3 Ga) (Braid et al. 2011), (ii) the lack of ca 1.0 Gazircons, and (iii) older model ages (ca. 2.0 Ga) from thePhyllite Quartzite Group strata contrast sharply with signa-tures typical of strata deposited on Avalonia or Avalonian de-rived rocks (ɛNd ∼+1 to –4 at T = 350) (Murphy and Nance2002). Therefore, although the SPZ basement displays anAvalonian-type signature, the basement and Phyllite Quartz-ite Group clastics together are not consistent with derivationfrom one of Avalonia sensu stricto. On the other hand, whencompared with the Meguma terrane metasedimentary rocks

Fig. 12. (A) Early Carboniferous reconstruction of Pangea around Iberia and the northern Appalachians, based largely on Martínez Catalán etal. (1997) and Lefort (1988). (B) Insets showing simplified crustal columns of the Meguma terrane and the South Portuguese Zone (SPZ) and(C) comparative Sm–Nd isotopic data.

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(Goldenville Group), the Phyllite Quartzite Group rocks arebroadly similar in Ti/Nb, Zr/Y, and Ti/Nb ratios, but VolcanoSiliceous Complex rocks show a somewhat wider range.These data support the detrital zircon data (Braid et al.2011), which indicate the Phyllite Quartzite Group has a de-trital zircon signature similar to Meguma metasedimentaryrocks. Taken together, geochemical and detrital zircon datasuggest the Phyllite Quartzite Group displays signatures con-sistent with derivation from Meguma terrane metasedimen-tary rocks (i.e., Goldenville Formation; Fig. 11), whereas theVolcano Siliceous Complex sedimentary rocks were likelyderived from a more heterogeneous source. This heterogene-ity is likely a reflection of the contribution of the coevalmafic and felsic volcanics to Volcano Siliceous Complexsediments. Taken together, these data imply either (i) thePhyllite Quartzite Group was not derived from the SPZ base-ment or (ii) the SPZ prior to the Late Devonian containedAvalonian-type basement beneath strata derived from a moreancient cratonic source.Deposition of the Phyllite Quartzite Group and the Vol-

cano Siliceous Complex sedimentary rocks in the Late Dev-onian is generally considered the result of local extension inan intracontinental rift basin (e.g., Mullane 1998; Quesada1998; Rosa et al. 2010). Therefore, the local dominant sourcefor the relatively immature sediments of the Phyllite Quartz-ite Group was likely the SPZ itself (Mullane 1998). As a re-sult, the latter scenario is more likely, where the SPZ iscomposed of Paleoproterozoic rocks (ɛNd = 6.8 to –9) abovea relatively juvenile basement (ɛNd = +1.4 to –3.0; Fig. 5).These more ancient rocks were in turn the local source forDevonian – Carboniferous Phyllite Quartzite Group clastics.This relationship is similar to the relationship between the

relatively juvenile basement and ancient upper crust docu-mented in the exposed portion of the Meguma terrane in thenorthern Appalachians (Owen et al. 1988) and therefore sup-ports the suggestion that the SPZ has pre-Variscan affinitiesto the northern Appalachians and potentially the Meguma ter-rane (e.g., Lefort 1988; Martínez Catalán et al. 1997; de laRosa et al. 2001; Fig. 12).

AcknowledgementsThis work is part of a Ph.D. dissertation by J.A.B. The sup-

port of the Natural Sciences and Engineering Research Councilof Canada (NSERC) through the PGS-D grant to J.A.B., dis-covery grants to J.K.M. and J.B.M., and research capacitygrants to J.B.M are acknowledged. J.A.B. also acknowl-edges the support of the Dalhousie Killam predoctoralscholarship program and St. Francis Xavier UniversityCouncil for research grants to J.B.M. We thank Nick Cul-shaw, Rebecca Jamieson, Jim Hibbard, and Gabriel Gutier-rez Alonso for their comments and discussions as well asDes Mosher (The University of Western Ontario, London,Ontario) for help with CL imaging.

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