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1 3 Int J Earth Sci (Geol Rundsch) (2014) 103:1433–1451 DOI 10.1007/s00531-014-1034-5 ORIGINAL PAPER U–Pb geochronology and petrology of the late Paleozoic Gil Marquez pluton: magmatism in the Variscan suture zone, southern Iberia, during continental collision and the amalgamation of Pangea Evan R. Gladney · James A. Braid · J. Brendan Murphy · Cecilio Quesada · Christopher R. M. McFarlane Received: 1 October 2013 / Accepted: 13 May 2014 / Published online: 31 May 2014 © Springer-Verlag Berlin Heidelberg 2014 older gabbro indicates that is was not proximal to a shear zone neither at the time of emplacement, nor during its subsequent history. The unfoliated porphyritic granite and unfoliated biotite granite cut the foliation of the intermedi- ate phase indicating emplacement during the waning stages of collision, while the ca. 335 Ma biotite granite intrudes the Santa Ira Flysch, thereby providing a tight constraint for the latest stage of deformation in the PDLZ. Keywords Gil Marquez pluton · Sierra Norte Batholith · Southern Iberia · U–Pb geochronology · Variscan suture zone · Collisional magmatism Introduction Although magmatism in collisional settings is well docu- mented (e.g., Dilek 2006; Gutiérrez-Alonso et al. 2011), the origin of plutonic complexes that stitch suture zones which developed during collision is not well understood. In ancient collisional belts, this task is particularly difficult because these zones are commonly destroyed or obscured by younger geologic events. However, in southern Iberia, the suture zone, which records the final amalgamation stages of Pangea in the Late Paleozoic, is exposed and crosscut by syn- to postcollisional intrusive igneous rocks (Eden 1991; Braid et al. 2010, 2012; Quesada 1991; Pereira et al. 2012). This suture zone, known as the Pulo do Lobo Zone (PDLZ), is the southernmost expression of the Variscan orogen in western Europe and formed during the closure of the Rheic Ocean and the resulting terminal collision between Laurussia and Gondwana (Matte 2001; Stamp- fli and Borel 2002; Gutiérrez-Alonso et al. 2008, 2011; Braid et al. 2011). The PDLZ is widely interpreted to be Abstract The origin of plutonic complexes that stitch suture zones developed during collision is not well under- stood. In southern Iberia, the Pulo du Lobo suture zone (PDLZ) is intruded by the syn- to postcollisional Gil Marquez pluton (GMP), thought to be part of the Sierra Norte Batholith. U–Pb (LA-ICPMS, zircon) data on vari- ous phases of the GMP yield from oldest to youngest: (1) a 354.4 ± 7.6 Ma unfoliated gabbro; (2) a 345.6 ± 2.5 Ma foliated intermediate phase; (3) a 346.5 ± 5.4 Ma unfoli- ated porphyritic granite; (4) a 335.1 ± 2.8 Ma unfoliated biotite granite. This sequence is consistent with cross-cut- ting relationships observed in the field. The range in ages is consistent with interpretations that the GMP is part of the composite (ca. 350–308 Ma) SNB. Inherited ages pre- served in the GMP intermediate and felsic phases indicate that its magmas traversed through South Portuguese Zone and PDLZ crust during emplacement. The ca. 345 Ma emplacement of the late kinematic foliated intermediate phase constrains the age of late-stage strike slip deforma- tion within the PDLZ, and the lack of a foliation in the Electronic supplementary material The online version of this article (doi:10.1007/s00531-014-1034-5) contains supplementary material, which is available to authorized users. E. R. Gladney · J. A. Braid · J. B. Murphy (*) Department of Earth Sciences, Saint Francis Xavier University, Antigonish, NS B2G 2W5, Canada e-mail: [email protected] C. Quesada Instituto Geológico y Minero de España, c/Rios Rosas 23, 28003 Madrid, Spain C. R. M. McFarlane Department of Earth Sciences, University of New Brunswick, PO Box 4400, Fredericton, NB E3B 5A3, Canada

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Int J Earth Sci (Geol Rundsch) (2014) 103:1433–1451DOI 10.1007/s00531-014-1034-5

ORIGInal PaPER

U–Pb geochronology and petrology of the late Paleozoic Gil Marquez pluton: magmatism in the Variscan suture zone, southern Iberia, during continental collision and the amalgamation of Pangea

Evan R. Gladney · James A. Braid · J. Brendan Murphy · Cecilio Quesada · Christopher R. M. McFarlane

Received: 1 October 2013 / accepted: 13 May 2014 / Published online: 31 May 2014 © Springer-Verlag Berlin Heidelberg 2014

older gabbro indicates that is was not proximal to a shear zone neither at the time of emplacement, nor during its subsequent history. The unfoliated porphyritic granite and unfoliated biotite granite cut the foliation of the intermedi-ate phase indicating emplacement during the waning stages of collision, while the ca. 335 Ma biotite granite intrudes the Santa Ira Flysch, thereby providing a tight constraint for the latest stage of deformation in the PDlZ.

Keywords Gil Marquez pluton · Sierra norte Batholith · Southern Iberia · U–Pb geochronology · Variscan suture zone · Collisional magmatism

Introduction

although magmatism in collisional settings is well docu-mented (e.g., Dilek 2006; Gutiérrez-alonso et al. 2011), the origin of plutonic complexes that stitch suture zones which developed during collision is not well understood. In ancient collisional belts, this task is particularly difficult because these zones are commonly destroyed or obscured by younger geologic events. However, in southern Iberia, the suture zone, which records the final amalgamation stages of Pangea in the late Paleozoic, is exposed and crosscut by syn- to postcollisional intrusive igneous rocks (Eden 1991; Braid et al. 2010, 2012; Quesada 1991; Pereira et al. 2012).

This suture zone, known as the Pulo do lobo Zone (PDlZ), is the southernmost expression of the Variscan orogen in western Europe and formed during the closure of the Rheic Ocean and the resulting terminal collision between laurussia and Gondwana (Matte 2001; Stamp-fli and Borel 2002; Gutiérrez-alonso et al. 2008, 2011; Braid et al. 2011). The PDlZ is widely interpreted to be

Abstract The origin of plutonic complexes that stitch suture zones developed during collision is not well under-stood. In southern Iberia, the Pulo du lobo suture zone (PDlZ) is intruded by the syn- to postcollisional Gil Marquez pluton (GMP), thought to be part of the Sierra norte Batholith. U–Pb (la-ICPMS, zircon) data on vari-ous phases of the GMP yield from oldest to youngest: (1) a 354.4 ± 7.6 Ma unfoliated gabbro; (2) a 345.6 ± 2.5 Ma foliated intermediate phase; (3) a 346.5 ± 5.4 Ma unfoli-ated porphyritic granite; (4) a 335.1 ± 2.8 Ma unfoliated biotite granite. This sequence is consistent with cross-cut-ting relationships observed in the field. The range in ages is consistent with interpretations that the GMP is part of the composite (ca. 350–308 Ma) SnB. Inherited ages pre-served in the GMP intermediate and felsic phases indicate that its magmas traversed through South Portuguese Zone and PDlZ crust during emplacement. The ca. 345 Ma emplacement of the late kinematic foliated intermediate phase constrains the age of late-stage strike slip deforma-tion within the PDlZ, and the lack of a foliation in the

Electronic supplementary material The online version of this article (doi:10.1007/s00531-014-1034-5) contains supplementary material, which is available to authorized users.

E. R. Gladney · J. a. Braid · J. B. Murphy (*) Department of Earth Sciences, Saint Francis Xavier University, antigonish, nS B2G 2W5, Canadae-mail: [email protected]

C. Quesada Instituto Geológico y Minero de España, c/Rios Rosas 23, 28003 Madrid, Spain

C. R. M. McFarlane Department of Earth Sciences, University of new Brunswick, PO Box 4400, Fredericton, nB E3B 5a3, Canada

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Fig. 1 (Inset) Setting of the Rheic suture with respect to Variscan and Cadomian Massifs in southeastern Europe (adapted from nance et al. 2010). an overview of the geology of the South Portuguese and Pulo do lobo zones (adapted from Braid et al. 2012)

Fig. 2 a summary of the geology of the GMP and host Pulo do lobo zone, with sample locations shown

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a classic accretionary complex that records the collision between these continents (Fig. 1; Eden 1991; Quesada and Dallmeyer 1994; Onézime et al. 2003; Braid et al. 2011) and separates a portion of laurussia (South Portu-guese Zone, SPZ) from Gondwana (Ossa Morena Zone, OMZ). The SPZ and PDlZ are crosscut by the Sierra norte Batholith (Fig. 1), for which published data suggest intru-sion occurred between ca. 350 and 300 Ma (Dunning et al. 2002, de la Rosa 1992; Braid et al. 2010, 2012).

This study focuses on the composite Gil Marquez pluton (GMP), an important component of the Sierra norte Batho-lith, which intrudes the PDlZ suture zone. The GMP is com-prised primarily of gabbro, granite, and granodiorite. These rocks have the potential to elucidate the late-stage tectonic evolution of this collisional setting and may provide insights into processes responsible for the generation of magmas within suture zones, as well as temporal constraints for the emplacement of the host rocks within the suture zone.

The GMP contains both foliated and nonfoliated com-ponents suggesting their syn- to postcollision emplacement

(Castro et al. 1995; de la Rosa et al. 2002; Braid et al. 2012). However, (1) the timing of pluton emplacement, (2) the timing and origin of the foliation relative to regional defor-mation, and (3) the age relationships between the internal phases of the pluton remain poorly understood. In this paper, we provide new U–Pb laser ablation inductively coupled plasma mass spectrometry (la-ICP-MS) analyses of mag-matic, inherited, and xenocrystic zircons from different com-ponents of the GMP. Taken together with petrographic and field observations, these data provide insights into the age(s) and emplacement mechanism(s) of the GMP, its likely crus-tal sources, and the relationship between fabric development in both the pluton and the suture zone host rocks.

Geologic setting

The late Paleozoic collision of Gondwana and laurus-sia resulted from the closure of the Rheic Ocean and was a major event in the formation of the Variscan orogenic

Fig. 3 Field relationships within the GMP, and with sur-rounding host rocks. a Unfoli-ated gabbro intruded by foliated intermediate phase. b Unfoli-ated gabbro intruded by biotite granite. c Parallel foliation shared between granodiorite and quartz diorite, demonstrat-ing coeval emplacement and supporting magma mingling processes. d Biotite granite intruding foliated intermediate phase, and intersecting the folia-tion. e Biotite granite intrud-ing host Pulo do lobo schist and transecting the foliation. f Porphyritic granite intruding a quartz wacke of the Ribeira de limas Formation (high angle to the intrusive contact)

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belt and the amalgamation of Pangea (Matte and Ribeiro 1975; Franke 1989, 2000; Matte 2001; Stampfli and Borel 2002; Onézime et al. 2003; Van Der Voo 2004; Mur-phy and nance 2008; Braid et al. 2010, 2011; Weil et al. 2010; Gutiérrez-alonso et al. 2008, 2011; Martínez Cata-lán 2011). In southern Iberia, the Pangean suture zone is unusually well exposed and separates the parautochthonous Ossa Morena Zone (Gondwana) from the South Portuguese Zone (SPZ) (laurussia) (Onézime et al. 2002, 2003; nance et al. 2010; Braid et al. 2011, 2012). abundant 355–300 Ma calc-alkaline magmatism along the southern margin of the OMZ is interpreted to be genetically linked to subduction beneath the Gondwanan margin (Jesus et al. 2007; Cas-tro et al. 2002) marking the oblique closure of the Rheic Ocean and juxtaposition of the SPZ to the south (Quesada 1991). During this convergence, oceanic metasedimentary rocks, sedimentary mélange, mafic mélange, and flysch deposits of the PDlZ are thought to have accreted on the OMZ upper plate and deformed during continued sinistral convergence (Fig. 1; Eden 1991; de la Rosa et al. 2002;

Simancas et al. 2005; Braid et al. 2010; Dahn et al. 2014). However, recent detrital zircon data show that the meta-sedimentary rocks are not derived from either the SPZ or the OMZ, suggesting the PDlZ likely had a more complex tectonic history (Braid et al. 2011).

The exposed geology of the SPZ contains mostly Devo-nian-Carboniferous sedimentary and bimodal volcanic sequences of the Iberian Pyrite Belt (IPB), which con-sists from oldest to youngest: (1) upper Devonian Phyllite Quartzite (PQ) Group (Braid et al. 2012); (2) late Famen-nian to middle Visean age Volcano Siliceous Complex (VSC), hosting the volcanogenic massive sulfide (VMS) mineralization (Dunning et al. 2002; Rosa et al. 2008); and (3) Upper Visean to the Serpukhovian turbiditic flysch group (Schermerhorn 1971; Oliveira et al. 1986). The PQ rocks were deposited in a sub-tidal environment in a sand bar and fan delta system on a shallow-marine continental platform. Detrital zircon data from PQ host rocks yield age populations dominated by ca. 1.8–2.3 and ca. 0.5–0.7 Ga, with minor 2.5–2.9 Ga zircons (Braid et al. 2011). The

Fig. 4 Enclaves, Xenoliths, and the magmatic aureole of the GMP. a Ribeira de limas quartz wacke Xenolith hosted in foli-ated granodiorite. The foliation of the granodiorite wraps the xenolith and displays a pressure shadow. b Mafic dyke intruding unfoliated gabbro and foliated granodiorite. The gabbro was previously intruded by the granodiorite, with a fine-grained chill margin present. c Mag-matic enclave hosted in coeval granodiorite and quartz diorite providing further evidence of magma mingling processes. d Enclave hosted in foliated granodiorite, stretched in the foliation direction. e Xenoliths of Ribeira de limas and Gil Marquez gabbro in a biotite granite. f Cordierite growth in the Ribeira de limas host rocks, within the northern magmatic aureole of the GMP

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Fig. 5 Petrography of gabbro (ERG66). a–d Photomicrographs dis-playing medium-grained unfoliated gabbro, comprised of hornblende (hb), plagioclase (pl), and augite (aug) with minor apatite, zircon (zr),

titanite, skeletal Ilmenite (op), and trace chlorite (chl) and calcite (cal); the plagioclase exhibits subophitic intergrowths with augite and hornblende. e Macroscopic field sample of gabbro

Fig. 6 Petrography of quartz diorite (ERG21). a–b Photomicro-graphs displaying strongly foliated, coarse-grained quartz diorite, comprised of andesine (pl), biotite (bt), and hornblende with minor quartz (~10–15 %, (qtz), orthopyroxene, apatite (ap), k-feldspar, zir-

con (zr), and opaque minerals. c–d Myrmekitic texture exhibited by an intergrowth of quartz and plagioclase. e Macroscopic field sample of quartz diorite

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VSC is comprised of mafic and felsic rocks, with tuffites, siltstones, purple shales, and minor limestones, with the latter dated by conodonts and cephalopods constraining its depositional age as upper Famennian-Tournaisian to upper Visean (Braid et al. 2012).

The PDlZ is comprised of four polydeformed fault-bounded lithotectonic units (Fig. 1; Braid et al. 2011): (1) quartz–mica schists and local quartzite mélange of the Pulo do lobo Formation; (2) quartz wackes and phyllites of the Ribeira de limas Formation (RDl), which con-tains palynomorphs interpreted as evidence for a Givetian-Frasnian depositional age (Oliveira et al. 1986; Giese et al. 1988); (3) hectometer- to meter-scale internally deformed olistostromal quartzites in a polydeformed phyllite–quartz-ite matrix (alajar mélange); (4) tectonically emplaced mafic blocks in a volcaniclastic and schistose matrix of the Peramora mélange (Eden 1991; Braid et al. 2011, 2012; Dahn et al. 2014). The age of the protolith of the imbricated sedimentary sequences in the Peramora mélange is con-strained by paleontological data (Eden and andrews 1990) that yield a Givetian to Famennian age (ca. 390–360 Ma). The Santa Ira Flysch (SIF) unconformably overlies all four PDlZ lithotectonic units and is comprised of relatively simply deformed graywackes, shales, and siltstones (Fig. 1; Braid et al. 2011). a late Devonian to early Carbonifer-ous depositional age has been inferred from the spores and

acritarchs (Eden 1991), although Dahn et al. (2014) suggest that these ages may be misleading.

The PDlZ is intruded by the GMP, which is a compo-nent of the ca. 350 and 300 Ma Sierra norte Batholith (Figs. 1, 2; de la Rosa 1992; Dunning et al. 2002; Braid et al. 2011). The Sierra norte Batholith is a voluminous composite batholith, comprised of granitic, tonalitic, gab-broic and dioritic compositions, which intruded the PDlZ and SPZ and is considered to be syn- to posttectonic with respect to the Variscan deformation in the region (Fig. 1; Simancas 1986; de la Rosa et al. 1993; Soriano and Casas 2002; Braid et al. 2010). The SnB has been interpreted to represent either: (1) the plutonic counterpart of coeval VSC exposed in the SPZ (Soler 1980; Schultz et al. 1987) or (2) late-orogenic intrusive complexes, unrelated to the VSC volcanism (Simancas 1986). Based on whole rock Rb–Sr isotopic data, de la Rosa et al. (1993, 2002) interpreted the SnB granitoids to be the products of mixing/mingling of magmas derived from melting of the lithospheric mantle and the lower crust in an active continental margin setting. Published age data suggest that the components of the GMP could range in age from 359 to 325 Ma (Kramm et al. 1991; Giese et al. 1993; de la Rosa et al. 2002; Braid et al. 2012).

according to de la Rosa (1992), emplacement of the GMP occurred after the main deformation phase of the Variscan orogeny. However, the intermediate phases

Fig. 7 Petrography of biotite granite (ERG11). a–b Photomicro-graphs displaying coarse-grained biotite granite, comprised of quartz (qtz), albite-oligoclase (pl), microcline, biotite (bt) with minor apatite

(ap), zircon, titanite, and opaque minerals. c Perthitic texture between albite (ab) and microcline (mc). d Oscillatory zoning in plagioclase. e Macroscopic field sample of biotite granite

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display a steep (70–90o) east–west foliation, which is par-allel to the main orogenic fabric in the host rocks and is interpreted to reflect late stages of regional deformation of the PDlZ during its emplacement and crystallization (Castro et al. 1995). In this context, intrusion and emplace-ment of the GMP have been interpreted to be synkinematic with respect to the final (transpressional) phase of regional deformation (Castro et al. 1995).

Field relationships

Four main intrusive phases of the GMP were documented and are from oldest to youngest (Fig. 2): (1) an unfoliated gabbro that comprises a significant portion of the GMP; (2) a foliated intermediate phase comprised of quartz dior-ite, tonalite, and granodiorite, which is the most abundant phase in the GMP; (3) an unfoliated porphyritic granite; and (4) an unfoliated biotite granite.

The unfoliated gabbro intrudes the RDl unit of the PDlZ, and the intrusive contact is exposed in numerous

localities throughout the pluton (Dahn et al. 2014). The gabbro is intruded by all other phases (Figs. 2, 3), and xenoliths of gabbro are common in all phases of the GMP, especially in the biotite granite (Fig. 4e), indicating that the gabbro is the oldest component of the GMP. The foli-ated intermediate phases intrude the gabbro, but are cross-cut by the unfoliated biotite granite (Figs. 2, 3) and by the porphyritic granite, indicating that these intermediate phases are older than both granite phases. The composi-tionally intermediate phases display mingling textures between mafic and felsic magmas and show evidence of hybridization, ranging in composition from quartz diorite to granodiorite. The foliated granodiorite, biotite granite, and porphyritic granite also intrude the lithologies of the PDlZ to the north, east, south, west (Figs. 2, 3), and con-tain abundant xenoliths and enclaves of the PDlZ meta-sedimentary rocks (Fig. 4a, d). Dahn et al. (2014) define the contact aureole in further detail. Some, but not all, enclaves and xenoliths are elongate parallel to the folia-tion (Fig. 4d). The tectonic foliations are commonly con-tinuous with the regional foliation in the PDlZ wall rock,

Fig. 8 Concordia plots and SEM photomicrographs of gabbro (ERG66). a Concordia plot of magmatic age of gabbro. b Macro-scopic field sample of gabbro. c Concordia plot of inherited zircon population. d SEM image of a subhedral stubby inherited zircon with

heterogeneous overgrowth zoning, and spot analyses location e SEM image of an oval ‘soccer ball’ inherited zircon with heterogeneous zoning, and spot analyses location

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although local occurrences of the foliation at a high angle to the wall rock also occur (Fig. 3f, e). Porphyritic granite intrudes both the gabbro and foliated granodiorite (Figs. 2, 3) and is intruded by the biotite granite phase (Figs. 2, 3), providing evidence that the biotite granite is the young-est phase in the GMP. additionally, alkali granite (Monte Chico Pluton) intrudes the Peramora Mélange (Fig. 2) (PDlZ) to the southwest of the town of aroche (Dahn et al. 2014). although an intrusive contact between alkali granite pluton and the PDlZ is exposed, its contact with the main GMP pluton is not exposed and the relationship is poorly understood (Dahn et al. 2014).

Sampling

Samples were selected from the GMP for U–Pb (zir-con) age dating in order to enhance the understanding of the intrusive age of different plutonic phases, the age of

foliation development, and the regional significance of its emplacement into the PDlZ suture zone. Ten samples (Fig. 2) were selected: (1) two biotite granites (ERG11, ERG57a) and one porphyritic granite (ERG65) from the unfoliated phases; (2) one quartz diorite (ERG21) and one tonalite (ERG29) from the intermediate foliated phases; (3) four unfoliated gabbro samples (ERG66, ERG81a, ERG81C, and ERG89); and (4) one unfoliated alkali gran-ite (ERG62) from the Monte Chico intrusion to the nW of the main GMP body.

Sample descriptions

Gabbro

The unfoliated gabbro is medium grained and comprised of hornblende, plagioclase, and augite with minor apatite, zircon, titanite, skeletal ilmenite, and trace chlorite and cal-cite. The plagioclase displays sub-ophitic intergrowths with

Fig. 9 Concordia plots and SEM photomicrographs of gabbro (ERG81a). a Concordia plot of magmatic age of gabbro. b SEM image of euhedral magmatic zircon with oscillatory zoning, and spot

analyses location. c Concordia plot of inherited zircon population. d SEM image of morphology’s of zircons present in the gabbro. e Mac-roscopic field sample of gabbro

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augite and hornblende (Fig. 5). ERG81a, 81C, and 89 are fine to medium grained and have the same mineralogy and textures as ERG66 (with additional minor biotite) and are interpreted to belong to the same consolidated gabbroic body. The presence of minor calcite and epidote (Fig. 5) shows the original gabbro has limited secondary alteration and/or metamorphism.

Intermediate foliated components

The quartz diorite (ERG21) is strongly foliated, coarse grained, and comprised of plagioclase (andesine), biotite, and hornblende, with minor quartz (~15 %), orthopyroxene, apatite, K-feldspar, zircon, and opaque minerals (Fig. 6). ERG 21 exhibits myrmekitic texture, pericline polysyn-thetic twinning, and oscillatory zoning in the plagioclase (Fig. 6b). The tonalite (ERG29) shows a moderate tectonic foliation, is coarse grained, and is comprised of plagioclase, biotite, and quartz, with minor hornblende, apatite, zircon, and opaque minerals. The foliation is defined by the pre-ferred orientation of euhedral plagioclase (which exhibits

oscillatory zoning) and K-feldspar megacrysts, along with interstitial biotite and hornblende.

Porphyritic granite

Porphyritic granite (ERG65) intrudes both the gabbro and intermediate foliated phases (Fig. 2). The sample has a por-phyritic texture with phenocrysts of k-feldspar and consists of quartz, plagioclase, and amphibole with minor apatite, zircon, titanite, and opaque minerals. Micrographic inter-growths of quartz and k-feldspar occur in ERG65, along with albite twinning and oscillatory zoning in the plagioclase.

Biotite granite

The biotite granite (ERG11, 57a) is the youngest phase and is coarse grained and comprised of quartz, plagioclase (albite-oligoclase), K-feldspar (microcline), biotite, with minor apatite, zircon, titanite, and opaque minerals. Per-thitic texture and oscillatory zoning are common in plagio-clase (Fig. 7).

Fig. 10 Concordia plots and SEM photomicrographs of gabbro (ERG81C). a Concordia plot of magmatic age of gabbro. b Macro-scopic field sample of gabbro. c Concordia plot of inherited zircon

population. d SEM image of subhedral fractured inherited zircon with no zoning, and spot analyses location. e SEM image of rounded sub-hedral inherited zircon with no zoning, and spot analyses location

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Alkali granite

The alkali granite (ERG62) from the Monte Chico plu-ton, which intrudes the Peramora Mélange (PDlZ), is medium-coarse grained and comprised of quartz, plagio-clase (albite), K-feldspar (microcline), biotite, with minor apatite, zircon, titanite, hornblende, and opaque minerals. abundant zoning and twinning is present in the plagioclase.

U–Pb geochronology

Methods

Heavy minerals were separated from samples ERG11, 62, 65, 21, 29, 66, 81a, and 89 by Overburden Drilling Man-agement limited (ODM). Samples ERG57a and 81C as well as duplicate samples ERG 11 and 62 were processed and separated at the Earth Sciences Mineral Separation facility at Dalhousie University. Samples sent to ODM were crushed to <2 mm fraction by activation labs in lan-caster, On, and returned to ODM. a cleaner quartz sample

was inserted between each sample during crushing to pre-vent carryover and contamination of the samples. ODM screened the crushed material to 1 mm and processed the <1 mm fractions on a shaking table. The concentrates from the shaking table were further refined by hand panning. a rare earth pencil magnet was then used to separate the magnetic and paramagnetic minerals. Zircon grains were hand-picked from the nonmagnetic fraction, isolated from the rest of the sample, and encapsulated in a separate vial. Samples sent to Dalhousie University were crushed and sieved on a shaking table into different fractions, with the finest fraction being <250 micrometers, separating light and heavy minerals (density separation). Separated heavy mineral fractions were further subjected to the heavy liquid (sodium polytungstate) separation, as an additional means of concentrating Zircons. The remaining concentrate was separated using a Frantz Isodynamic Magnetic Separator, to extract remaining heavy minerals (rutile, magnetite, gar-net). Separation was conducted at 0.4 and 0.8 a with result-ant fractions encapsulated in separate vials based on mag-netic and nonmagnetic properties.

Fig. 11 Concordia plot, relative probability plot, and SEM pho-tomicrographs of gabbro (ERG89). a Concordia plot of magmatic age of gabbro. b SEM image of fractured euhedral magmatic zircon with heterogeneous zoning, and spot analyses location. c Zircon his-

togram/relative probability plot. d SEM image of oval ‘soccer ball’ inherited zircon with heterogeneous zoning, and spot analyses. e Macroscopic field sample of gabbro

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Analysis

Zircon dating (U–Pb), by la-ICP-MS, was performed at the University of new Brunswick, Department of Earth Sciences, using a Resonetics M-50 193 nm excimer laser system connected, via nylon tubing, to an agilent 7700× quadrupole ICP-MS equipped with dual external rotary pumps (see archibald et al. 2013 for details). Zircons were ablated using 24-μm craters for U–Pb geochronol-ogy and 33-μm craters for Trace/REE. a 4.5-Hz rep-etition with the laser energy at the target (fluence) regu-lated at 3.5 J/cm2 was used for all analysis. an analysis comprised 20 s of background gas collection followed by 25 s of ablation. a total of 278 analyses were done on the cores and rims of zircons in lithologies ranging from gabbroic to granitic in composition; 52 spot analyses of zircons from gabbroic samples (ERG66, 12; ERG81a, 21; ERG81C, 11; and ERG89, 8), 40 spot analyses from foliated phases with intermediate composition (ERG 21,

18; ERG 29, 22), and 186 spot analyses of zircons from granite samples (ERG11, 105; ERG57a, 20; ERG65, 26; and ERG62, 35). all ages used and uncertainties are reported at 2σ, or 95 % confidence. Data were reduced using Iolite 2.15 software, and plots generated using Isop-lot 3.71 (ludwig 2003). Data were organized by sample and rock type then sorted by percent discordance. Dis-cordance was calculated by the following formula: % Discordance = 100 % * [(207Pb/206Pb age − 206Pb/238U age)/207Pb/206Pb age] (Gehrels et al. 2006). a discordance level of <10 % was chosen as the cutoff for acceptable error, and such analyses are deemed to be concordant. The data from all spot analyses are given in the Data Reposi-tory (Tables EG1-12), but only concordant analyses are discussed in the following sections. Our interpretations are largely based on concordant magmatic and inherited age populations. a population is considered robust if it con-sists of a cluster of three or more zircons with similar ages (Gehrels et al. 2006).

Fig. 12 Concordia plot, relative probability plot, and SEM photomi-crographs of quartz diorite (ERG21). a Concordia plot of magmatic age of quartz diorite. b SEM image of euhedral magmatic zircon with oscillatory zoning, and spot analyses location. c Zircon histogram/

relative probability plot. d SEM image of oval ‘soccer ball’ inherited zircon with heterogeneous zoning, and spot analyses. e Equivalent Cathodoluminescence image of zircon grain in figure d, where zoning mimics that of the Backscattered Electron image

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Results

Gabbro (ERG66, 81a, 81C, 89)

Zircons from the gabbroic samples commonly show mul-tifaceted terminations, and the majority of the grains have a euhedral–subhedral stubby morphology, with a minor amount of zircons having an oval ‘soccer ball’ habit (Figs. 8, 9, 10, 11). SEM-backscattered electron imaging reveals (1) zircon cores, which are multifaceted to rounded and contain heterogeneous overgrowth, oscillatory zoning overgrowth, or homogenous overgrowth rims or (2) zircons without cores, which show heterogeneous zoning, oscilla-tory zoning, or no zoning (Figs. 8, 9, 10, 11).

Sample ERG66 (Figs. 5, 8, Table EG-1): Of 12 spot analyses (Fig. 8), six are concordant. Of these, two yield an age of ca. 346.3 ± 7.7 Ma and two yield an older age of 2,668 ± 75 Ma. The remaining two concordant grains yield ages of 399.5 ± 37 and 2,553 ± 46 Ma. Sample ERG81a (Fig. 9, Table EG-2): Of 21 spot analyses (Fig. 9a), fifteen

are concordant. Two yielded an age of 355.9 ± 15 Ma, and four define an older population at 452.7 ± 4.5 Ma. The remaining nine concordant grains yield ages ranging from 996.3 ± 13 to 1,963 ± 26 Ma. Sample ERG81C (Fig. 10, Table EG-3): Of 11 spot analyses (Fig. 10), four are concord-ant. These yield ages from 368.7 ± 9.6 to 522.6 ± 12 Ma. Sample ERG89 (Fig. 11, Table EG-4): Of 8 spot analyses, only one is concordant and yields an age of 2,673 ± 35 Ma.

Intermediate foliated components (ERG21, 29)

Zircons from the intermediate phases commonly show mul-tifaceted terminations with the majority of the grains hav-ing a euhedral to subhedral stubby morphology, and some zircons with oval ‘soccer ball’ habits are present (Figs. 12, 13). SEM-backscattered electron imaging reveals (1) zircon cores, which are multifaceted to rounded and contain heter-ogeneous overgrowth or oscillatory zoning overgrowth rims (Fig. 12) or (2) zircons without cores, which show hetero-geneous or homogeneous zoning, or no zoning (Fig. 13c).

Fig. 13 Concordia plots and SEM photomicrographs of tonalite (ERG29). a Concordia plot of magmatic age of the tonalite. b SEM image of stubby euhedral magmatic zircon with faint zoning, and

spot analyses location. c Concordia plot of inherited zircon popula-tion. d SEM image displaying morphology’s of zircons present in the tonalite. e Macroscopic field sample of tonalite

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Sample ERG21 (Figs. 6, 12, Table EG-5): Of 17 spot analyses, nine are concordant. Of these, a cluster of four zircons define a population and yield an age of 348.5 ± 4.0 Ma. The remaining five concordant grains range in age from 627.9 ± 10 to 2,717 ± 27 Ma. Sam-ple ERG29 (Fig. 13, Table EG-6): Of 23 spot analy-ses, eight are concordant. Of these, three yield an age of 338.2 ± 4.5 Ma and four a slightly older age of 346.3 ± 3.2 Ma (Fig. 13a). The remaining concordant grain gives an age of 365 ± 6.5 Ma. Exposed field relationships of the GMP show that the intermediate phases are coeval, and therefore, both samples were combined giving an age of 345.6 ± 2.5 Ma based on eleven concordant grains.

Granites (ERG11, 57a, 65, 62)

Zircons from granite samples dominantly show multifac-eted terminations with the majority of the grains having

a euhedral to subhedral elongate morphology, although zircons with oval ‘soccer ball’ and euhedral to subhedral stubby habits are also present (Figs. 16e, 17e). SEM-back-scattered electron imaging reveals (1) zircon cores, which are multifaceted to rounded and contain heterogeneous overgrowth or oscillatory zoning overgrowth rims or (2) zircons without cores, which show heterogeneous zoning, oscillatory zoning, and are homogeneous (e.g., Fig. 17e).

Sample ERG11 (Figs. 7, 14, Table EG-7, 7.1): Of 104 spot analyses, fifty-four are concordant. Several popula-tions can be defined. a cluster of four zircons define the youngest population with an age of 336.8 ± 4.0 Ma, with older populations of 355.4 ± 1.7 Ma (seven zircons), 513.2 ± 2.8 Ma (six zircons), and 2,699 ± 35 Ma (three zircons). The remaining thirty-three concordant grains range in age from 433.9 ± 6.5 to 3,533 ± 22 Ma.

Sample ERG57a (Fig. 15, Table EG-8). Of 21 spot analyses, nine are concordant. Of these, one yields an

Fig. 14 Concordia plots and SEM photomicrographs of biotite gran-ite (ERG11). a Concordia plot of magmatic age of the biotite granite. b SEM image of elongate euhedral magmatic zircon with oscillatory zoning, and spot analyses location. c Concordia plot of inherited zir-

con population age. d SEM image of rounded subhedral inherited zir-con with heterogeneous zoning, and spot analyses location. e SEM image of fractured euhedral inherited zircon with oscillatory over-growth, and spot analyses location

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age of 336.8 ± 7.3 Ma and the youngest population is a cluster of three zircons, which collectively yield an age of 494.1 ± 6.7 Ma, while remaining five concordant grains range in age from 1,400 ± 20 to 1,838 ± 36 Ma. Both bio-tite granite samples (give sample numbers) are from the same pluton. Therefore, the data from both samples were combined giving a population of 335.1 ± 2.8 Ma defined by a cluster of five concordant grains.

Sample ERG65 (Fig. 16, Table EG-9). Of 26 spot anal-yses, thirteen are concordant. Of these, a cluster of three zircons yield an age of 346.5 ± 5.4 Ma and the remaining ten concordant grains range in age from 361.4 ± 9.0 Ma to 1,890 ± 19 Ma. Sample ERG62 (Monte Chico Gran-ite, Fig. 17, Table EG10). Of 35 spot analyses, eight are concordant. Of these, a cluster of six zircons yield an age population of 344.8 ± 3.0 Ma, and the other two concord-ant grains give ages of 341.9 ± 6.6 and 349.2 ± 5.4 Ma (Fig. 17b).

Interpretation

Magmatic ages of the GMP

The magmatic age for the various phases of the GMP was determined from euhedral zircon crystals, using spot analy-ses on rims. The youngest concordant population of three or more euhedral zircon crystals was used to determine magmatic age (e.g., Gehrels et al. 2006). Older concordant populations and individual grains are interpreted as inher-ited ages and represent either xenocrysts from wall rock, or residue from the source rock. For a more detailed descrip-tion on the criteria of how individual zircon grains were selected to avoid zoning, inclusions, lead loss, etc., see Corfu et al. (2003).

Field data suggest that unfoliated gabbroic phase is the oldest component of the GMP. no single sample of gab-bro contains a cluster of three or more concordant zircons.

Fig. 15 Concordia plots and SEM photomicrographs of biotite gran-ite (ERG57a). a Concordia plot of magmatic age of the biotite gran-ite. b SEM image of euhedral magmatic zircon with oscillatory zon-ing, and spot analyses location. c Concordia plot of inherited zircon

population age. d SEM image of euhedral inherited zircon with het-erogeneous zoning, and spot analyses location. e SEM image of oval ‘soccer ball’ inherited zircon with faint rim growth, and spot analyses location

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However, all gabbro samples were selected from the same gabbroic body, and the results of all samples were com-bined to define a population and give a more robust age. The youngest population is defined by a cluster of four con-cordant grains and yields an age of 354.4 ± 7.6 Ma. Based on exposed field relationships, which indicate that the foli-ated intermediate phases (granodiorite, tonalite) are coeval, we combined the data from samples ERG21 and ERG29. a cluster of eleven concordant zircons defines a population of 345.6 ± 2.5 Ma, which is the interpreted as the age of crystallization of the various components of the intermedi-ate phase. In the unfoliated porphyritic granite (ERG65), the youngest population (348.2 ± 5.4 Ma, three euhedral zircons) is interpreted as magmatic age (Fig. 16).

The unfoliated biotite granite (ERG11, ERG57a) contains euhedral zircon grains ranging in age from 336.8 ± 4.0 Ma (Fig. 14) to 342.9 ± 3.5 Ma (Fig. 15). Both granite samples (ERG11 and ERG57a) show coeval field relationships; consequently, their crystallization age

is interpreted to be 335.1 ± 2.8 Ma based on a concord-ant zircon population of 5 grains. The youngest age popu-lation in the Monte Chico alkali granite is 342.9 ± 3.1 Ma (Fig. 17), which is interpreted as the intrusive age of the pluton, based on 6 concordant grains. although the genetic link between Monte Chico Pluton and the GMP cannot be deduced from field observations, these age data suggest a coeval relationship.

Inherited ages of the GMP

Zircon cores from the GMP samples show multifaceted or rounded inherited core morphology (Figs. 8b, 17b) and record numerous inherited age populations, based on three or more grains (Table 1). The ca. 360–3,533 Ma inherited ages come from sub-rounded to rounded core analyses (e.g., Figs. 8d, 11d, 12d, 16e). Multifaceted zircon crystals typically imply a magmatic protolith (Timmermann et al. 2000; Braid et al. 2012), whereas detrital zircons are well

Fig. 16 Concordia plot, relative probability plot, and SEM photomi-crographs of porphyritic granite (ERG65). a Concordia plot of mag-matic age of porphyritic granite. b SEM image of euhedral magmatic zircon with oscillatory zoning, and spot analyses location. c Zircon

histogram/relative probability plot. d SEM image of subhedral inher-ited zircon with heterogeneous zoning, and spot analyses. e SEM image of oval ‘soccer ball’ inherited zircon with no zoning, and spot analyses location

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rounded and represent inherited grains from a sedimentary protolith (Braid et al. 2011, 2012).

ERG66 has a population at 2,668 ± 75 Ma. ERG81a has a population at 452.7 ± 4.5 Ma, and individual grains ranging in age from 996 to 1,891 Ma. ERG81C has a population at 374.2 ± 7.7, and individual grains rang-ing in age from 399 to 523 Ma. ERG89 has no distinct populations (Table EG-4). ERG21 has individual zircons ranging 382–628 Ma and 2,309–2,717 Ma. ERG29 has a population at 346.3 ± 3.2 Ma, with individual grains ranging from 353 to 365 Ma. ERG11 has populations of 355.4 ± 1.7 Ma, 513.2 ± 2.8 Ma, and 2,699 ± 35 Ma, with individual zircons ranging from 434 to 497 Ma, 543 to 547 Ma, and 1,034 to 1,815 Ma. ERG57a has a popu-lation of 494.1 ± 6.7 Ma, with individual zircons rang-ing from 1,223 to 1,838 Ma. ERG65 has individual zir-cons ranging from 361 to 467 Ma and 1,849 to 1,890 Ma. ERG62 has no distinct inherited populations (Table EG-10).

Discussion

The U–Pb (zircon) crystallization ages reported herein are consistent with field relationships, which indicate that the gabbro is the oldest component and the unfoliated bio-tite granite is the youngest component of the GMP. These data also indicate that the Monte Chico pluton is coeval with the emplacement of the GMP plutonic complex. Taken together, these data imply that the GMP was part of an active magmatic system for ca. 20–30 Ma, during which time mafic to felsic rocks intruded the PDlZ. The range in ages (354.4 ± 7.6 to 335.1 ± 2.8 Ma) is consist-ent with interpretations that the GMP is part of the com-posite ca. 350–308 Ma SnB (de la Rosa et al. 2002; Dun-ning et al. 2002; Braid et al. 2011, 2012). Other possible correlatives include an upper Devonian bimodal volcanic series (IPB) of the SPZ and sub-volcanic rocks of the SnB (Soler 1980). The northern volcanic rocks of the IPB and the Campo Frio suite of the SPZ contain zircons dated at

Fig. 17 Concordia plot, relative probability plot, and SEM photomi-crographs of alkali granite (ERG62). a Concordia plot of magmatic age of alkali granite. b SEM image of euhedral magmatic zircon with heterogeneous zoning, and spot analyses location. c Zircon histo-

gram/relative probability plot. d SEM image displaying zircon mor-phology’s present in the alkali granite. e Macroscopic field sample of alkali granite

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349.8 ± 0.90 and 346.3 ± 0.8 Ma, respectively (Barrie et al. 2002). The volcanic rocks of the IPB are similar in age to the GMP gabbro. The Campo Frio suite is similar in age to the intermediate phase and porphyritic granite of the GMP. The different phases of the GMP contain a range of inherited age populations spanning from 355.4 ± 1.7 to 2,699 ± 35 Ma (Table 1), including an important Mesopro-terozoic inherited component.

These inherited ages are inconsistent with those from the OMZ crust, which lacks Mesoproterozoic zircons (Braid et al. 2011). Instead, our data suggest that GMP traversed through SPZ and PDlZ crust during emplace-ment. a quartzite from the PDlZ bordering the GMP has zircon populations of 1.0–1.5 and 1.6–1.9 Ga (Braid et al. 2011), which are comparable with inherited populations found in the GMP granites (Table 1, EG7-9), suggesting the PDlZ quartzites may be the source of these popula-tions in the GMP. a nonfoliated granite sample from the SnB contains neoproterozoic (ca. 561–647 Ma) and Mes-oproterozoic ages (ca. 1.08–1.12 Ga (Braid et al. 2012). Both these inherited ages populations also occur within the GMP (Table 1, EG1-12), suggesting the GMP and the composite SnB intruded similar crustal sources. In both bodies, the younger zircons are pristine and multifaceted,

whereas the older zircons have an oval shaped morphol-ogy (Braid et al. 2011). The ages of the inherited zircons are consistent with derivation from the SPZ and the PDlZ. The oldest known exposed unit of the SPZ is the Phyllite Quartzite Group (PQ), which has a max depositional age of 438.7 ± 4.38 Ma, and contains inherited zircons popula-tions of archean age ca. 2.5–3.0 Ga (Braid et al. 2011).

The 355–335 Ma intrusion of the GMP provides a mini-mum age for the components of the suture zone. The ca. 345 Ma emplacement of the late kinematic foliated phases constrains the age of late-stage strike slip deformation within the PDlZ. The steep foliation in the intermediate phases is parallel to the main orogenic fabric in the host rocks and is interpreted to reflect late stages of regional deformation of the PDlZ during its emplacement. The foli-ation probably reflects northward-directed oblique conver-gence and associated regional sinistral shear (Castro et al. 1995). The emplacement of the intermediate phases along a shear zone attests to the heterogeneous style of deforma-tion. The lack of a foliation in the older gabbro indicates that is was not proximal to a shear zone neither at the time of emplacement, nor during its subsequent history. The GMP is a magmatic plumbing system that was active for ca. 20 Ma, during which time it intruded the Variscan

Table 1 Summary of the results of the geochronological data from this study

For details on each sample, see electronic files Tables EG-1 to EG-13a ages reported in Ma; 206/238 for Zircons <1 Ga and 207/206 for Zircons >1 Ga (Gehrels et al. 2006)b ages in italics represent a population of ≥2 grains. Individual Zircon ranges in normal font

Samples Rock type latitude longitude Interpreted agea,b Inherited age(s)a,b

ERG11 bt-Granite 37.853954 −6.810555 339 ± 6.6 355.4 ± 1.7, 513.2 ± 2.8, 2,699 ± 35ca.434–497, ca.543–547, ca.1,034–1,815, ca.2,485–3,533

ERG57a bt-Granite 37.853013 −6.812671 342.9 ± 3.5 494.1 ± 6.7ca.1,223–1,838

ERG11/57a bt-Granite n/a n/a 335.1 ± 2.8

ERG65 Porphyritic granite 37.856512 −6.803243 346.5 ± 5.4

ca.361–467, ca. 1,849–1,890

ERG62 alkali granite 37.916659 −7.066271 342.9 ± 3.1

ERG21 qtz-Diorite 37.849355 −6.8338 348.5 ± 4.0

ca.382–628, ca. 2,309–2,717

ERG29 Tonalite 37.850384 −6.833535 338.2 ± 4.5 346.3 ± 3.2ca.353–365

ERG21/29 Intermediate phases n/a n/a 345.6 ± 2.5

ERG66 Gabbro 37.854138 −6.805176 346.3 ± 7.7 2,668 ± 75ca. 399.5

ERG81a Gabbro 37.855515 −6.836935 355.9 ± 15 452.7 ± 4.5ca.492, ca.996–1,891, ca.2,702

ERG81C Gabbro 37.855515 −6.836935 351.8 ± 2.9 374.2 ± 7.7ca.399–523

ERG89 Gabbro 37.854077 −6.835998 2,673 ± 35

ERG66,81a,81C,89

Gabbro n/a n/a 354.4 ± 7.6

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suture zone throughout the final stages of continental colli-sion between the OMZ (Gondwana) and SPZ (laurussia).

The unfoliated porphyritic granite and unfoliated biotite granite cut the foliation of the intermediate phases, during the waning stages of collision. The biotite granite intrudes the porphyritic granite and the SIF (Fig. 2), which uncon-formably overlies all other units of the PDlZ and has a maximum depositional age of 345–350 Ma (Braid et al. 2011, 2012). The relatively simple style of deformation exhibited by the SIF probably reflects the latest stages of deformation within the PDlZ, and the ca. 335 Ma age of the granite provides a tight constraint for the age of that deformation.

Acknowledgments We thank Ricardo arenas and Vaclav Kachlik for their constructive and insightful reviews and Jarda Dostal for edi-torial handling. This research was funded by the natural Sciences and Engineering Research Council of Canada (nSERC) Discovery grants to JaB and JBM.

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