U – Pb zircon SHRIMP evidence for Cambrian volcanism in the Schistose Domain within the Galicia-Trás-os-Montes Zone ( Variscan Orogen , NW Iberian Peninsula )

P. F a r i a s e t a l . G e o l o g i c a A c t a , 1 2 ( 3 ) , 2 0 9 2 1 8 ( 2 0 1 4 ) D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 1 4 . 1 2 . 3 . 3 U-Pb dating of Cambrian volcanics in the Variscan Orogen 210 Silurian in age either non-concordant with the Lower Palaeozoic series of de Central–Iberian Zone (ZCI) (Ferragne, 1972; Fernández Pompa and Monteserín López, 1976) or concordant and forming the upper part of the same series (Matte, 1968; Romariz, 1969; Iglesias and Robardet, 1980; Bastida et al., 1984). However, all the recognized Silurian faunas (Graptolites) occur right below the Schistose Domain basal thrust (González Clavijo, 1997; Marcos and Farias, 1999; Marcos et al., 2002; Marcos and Llana-Fúnez, 2002) in rocks that forms part of the autochtonous unit, represented by the Palaeozoic sequence of the Ollo de Sapo antiform (Central–Iberian Zone, Fig. 1). Palaeontological findings in the Trás-Os-Montes (Pereira et al., 2000) and Cabo Ortegal areas (Rodríguez et al., 2004), as well as U–Pb dating of felsic volcanic rocks near the Cabo Ortegal Complex (ValverdeVaquero et al., 2005) and in the Verín synform (ValverdeVaquero et al., 2007), establish an Ordovician and Silurian age for some of the rocks of this domain. Furthermore, according to U–Pb detrital zircon analysis of this sequence the maximum sedimentation age is latest Neoproterozoic (c. 560Ma) (Díez Fernández et al., 2012). In spite of this data, some authors (Piçarra et al., 2006) still consider the Schistose Domain to be Silurian in age, defending its “parautochthonous” character. In this contribution we present U–Pb zircon-ages for rhyolitic and dacitic meta-volcanites interbeded in the lower part of the parautochthonous series in the Northeast limb of the Verín-Bragança-Alcañices synform. The cathodoluminescence (CL) studies on multiphase rocks show (e.g. Ordóñez-Casado, 1999, Ordóñez-Casado et al., 2001) that the internal structure of zircons is frequently independent of its morphology. The microanalysis of the different domains within one zircon by means of Sensitive High Resolution Ion MicroProbe (SHRIMP) is considered among the most trustworthy methods to obtain radiometric data of magmatic and high-grade metamorphic rocks. This method allows us to avoid the inherited component of the inner cores, obtaining more reliable ages.


IntroductIon
The Schistose Domain, also named Para-autochthonous Thrust Complex (Ribeiro et al., 1990), forms the lowermost tectonic unit of the Galicia-Trás-os-Montes Zone (GTMZ) (Farias et al., 1987), the most internal one of the Variscan Belt in the NW of the Iberian Peninsula.It is constituted by a metasedimentary sequence more than 4000m thick interpreted as the most external sediments of the continental margin of Gondwana (Farias et al., 1987;Ribeiro et al., 1990, Martínez Catalán et al., 1999;Marcos et al., 2002;Arenas et al., 2004b;Murphy et al., 2008;Díez Fernández et al., 2012).The extension, geometrical relationships and stratigraphic differences with the neighboring regions allows the Schistose Domain to be considered as a major allochtonous Variscan unit with a displacement of over several tens of kilometres (Ribeiro, 1974;Farias et al., 1987;Barrera et al., 1989;Farias, 1990).
The age of the Schistose Domain is a subject of controversy.For a long time, these rocks were considered Silurian in age either non-concordant with the Lower Palaeozoic series of de Central-Iberian Zone (ZCI) (Ferragne, 1972;Fernández Pompa and Monteserín López, 1976) or concordant and forming the upper part of the same series (Matte, 1968;Romariz, 1969;Iglesias and Robardet, 1980;Bastida et al., 1984).However, all the recognized Silurian faunas (Graptolites) occur right below the Schistose Domain basal thrust (González Clavijo, 1997;Marcos and Farias, 1999;Marcos et al., 2002;Marcos and Llana-Fúnez, 2002) in rocks that forms part of the autochtonous unit, represented by the Palaeozoic sequence of the Ollo de Sapo antiform (Central-Iberian Zone, Fig. 1).Palaeontological findings in the Trás-Os-Montes (Pereira et al., 2000) and Cabo Ortegal areas (Rodríguez et al., 2004), as well as U-Pb dating of felsic volcanic rocks near the Cabo Ortegal Complex (Valverde-Vaquero et al., 2005) and in the Verín synform (Valverde-Vaquero et al., 2007), establish an Ordovician and Silurian age for some of the rocks of this domain.Furthermore, according to U-Pb detrital zircon analysis of this sequence the maximum sedimentation age is latest Neoproterozoic (c.560Ma) (Díez Fernández et al., 2012).In spite of this data, some authors (Piçarra et al., 2006) still consider the Schistose Domain to be Silurian in age, defending its "parautochthonous" character.
In this contribution we present U-Pb zircon-ages for rhyolitic and dacitic meta-volcanites interbeded in the lower part of the parautochthonous series in the Northeast limb of the Verín-Bragança-Alcañices synform.The cathodoluminescence (CL) studies on multiphase rocks show (e.g.Ordóñez-Casado, 1999, Ordóñez-Casado et al., 2001) that the internal structure of zircons is frequently independent of its morphology.The microanalysis of the different domains within one zircon by means of Sensitive High Resolution Ion MicroProbe (SHRIMP) is considered among the most trustworthy methods to obtain radiometric data of magmatic and high-grade metamorphic rocks.This method allows us to avoid the inherited component of the inner cores, obtaining more reliable ages.

GeoloGIcAl SettInG
The GTMZ is the most internal zone of the Variscan Belt in the NW of the Iberian Peninsula (Fig. 1).It is thrusted over Palaeozoic rocks of the neighbouring FIGURe 1. Geological sketch of the NW Iberian Península, showing zones established by Lotze (1945), Julivert et al. (1972) and Farias et al. (1987).CIZ.Two domains have been distinguished within the GTMZ: i) the Schistose Domain, composed by more than 4000m thick metasedimentary sequence with interbeded metavolcanics; and ii) the Allochthonous Complexes, tectonically emplaced over the former and composed by mafic-ultramafic and quartz-feldspathic rocks with different origin, grouped into different tectonic units (e.g.oceanic, ophiolitic and continental) (Farias et al., 1987;see Arenas et al., 2004a and references therein).

Variscan Belt of Western Europe
The Schistose Domain forms a thin sheet thrusted between the Bragança Allochthonous Complex and the underlying Lower Palaeozoic metasediments of the Ollo de Sapo antiform (Farias, 1990;González Clavijo, 1997;González Clavijo and Martínez Catalán, 2002).Two stratigraphic units, the Nogueira and Paraño groups, have been established in the Schistose Domain.The former is mainly composed by black shales, lidites and scarce tobaceous-cineritic volcanites, with a minimum thickness of 500m.The Paraño Group constitutes a more than 3000m thick succession of siliciclastic rocks (phyllites, psamites, greywackes, quartzites and microconglomerates) with interbeded layers of metavolcanites, mainly rhyolites and some felsic trachytes (Fig. 2).
The rocks of the Schistose Domain have been deformed under greenschist facies conditions and affected by three main Variscan deformation episodes.They show a pervasive S1 cleavage related with tight folds that are visible only when affecting the quartzite beds.S2 phyllonite fabrics developed in association to the Galicia-Trás-os-Montes basal thrust and open right D3 folds at all scales, including crenulation cleavage, are folding the previous structures (Farias, 1990;Dallmeyer et al., 1997;Marcos and Farias, 1999).

SAmPle deScrIPtIon
Two samples (COS-7 and COS-8) were collected in metavolcanic levels outcropping close to the Spanish-Portuguese border in the Bragança-Alcañices area (Fig. 3).Both levels are interbeded in the lowermost part of the Paraño Group, beneath the quartzite and the volcanic layers located more to the NW, in the Verín Synform, and dated as c. 439Ma by Valverde-Vaquero et al. (2007).They are lenticular bodies of felsic metavolcanites with variable thickness (up to 150m) and some kilometers in lateral extent.
COS-7: Located in the village of Soutelo (Portugal,41º 53' 18.39''N / 6º 48' 23.71''W), it is a rhyolitic to rhyodacitic crystal-rich tuffaceous sandstone (nomenclature based on McPhie et al., 1993), with porphyritic features and a crystal content up to 70%.The rock is constituted by quartz (up to 3,5mm in size and an abundance of 30-35%), K-feldspar (Kfs, up to 3mm in size and an abundance of 15%), plagioclase (Pl, 5%) and biotite (3%), with minor sedimentary clasts (slates and fine to very fine grained sandstone) and vitreous clasts (totally replaced by sericite + hematite), in a very fine grained matrix composed by sericite and quartz that obliterates any previous vitreous texture of the matrix (Fig. 4A).All components are slightly rounded.Neither internal orientation of crystals nor grain classification can be observed (Fig. 4D).The rock is affected by a hydrothermal and/or metamorphic event, with development of a sericitic alteration.Pl and Kfs are partially replaced by sericite + quartz, and biotite is replaced by prenhite, sericite, hematite and titanite (Fig. 4E).

AnAlytIcAl technIqueS
Zircons for isotope analyses were separated, after crushing and sieving, using an electromagnetic drum separator, heavy liquids as well as a Frantz isodynamic separator.The zircons were subsequently studied using a scanning electron microscope (SEM) equipped with secondary-electron and cathodoluminescence (CL) detectors at the Australian National University(ANU), Canberra, Australia.
The isotopic analysis of these domains was made at the Research School of Earth Sciences (RSES) of theANU, by means of a SHRIMP-II providing determinations of age based on isotopic analysis of U-Th-Pb in the CL-identified.For a full description of the ion-microprobe technique and data acquisition see Compston et al. (1984Compston et al. ( , 1986)).
The U-Pb data are presented in Table 1 (errors are given as 1σ, the individual ages were calculated from the radiogenic 206 Pb/ 238 U and plotted into Tera-Wasserburg diagram (Tera and Wasserburg, 1972).
Error boxes and single spot ages are 1σ.Average 206 Pb/ 238 U ages are given as weighted means (WM) and errors are expressed at the 95% confidence level, unless otherwise stated.'n' are numbers of spots analysed / number of zircon crystals.

Soutelo rhyolite (Sample coS-7)
The examined zircons are transparent, mainly elongated and vary from euhedral or subhedral to rounded.In terms of CL-images, the occurrence of inherited cores prevails (95% of the zircons examined contain cores)., 1 2 ( 3 ) , 2 0 9 -2 1 8 ( 2 0 1 4  They are often rounded with planar oscillatory pattern and, sometimes, resorbed areas.Surrounding the cores, there is new crystallization of euhedral rims with either oscillatory pattern (e.g.zircons 11 and 7; Fig. 5).The described rims evidence overgrowth during a magmatic stage.In terms of SHRIMP-analyses the inner cores reveal the presence of distinct provenance ages (although this was not the purpose of this work).
Data are shown in Table 1 and the TW-diagrams in Figure 5.An inherited zircon yielded a 206 Pb/ 238 U age of 1.8Ga.Inherited components due to mixing of cores and rim in the analysis is evidenced by spots 15.1, 16.1 and 8.1.Zircon 2.1 shows some partial Pb loss.The rest of the data define a quasi-Gaussian distribution yielding an average age of 499.8 ± 3.7Ma obtained on 12 spots located in rims within 12 crystals.The zircon domains analysed have U contents of 252 to 713ppm, and Th contents of 24 to 149ppm and the Th/U ratios range from 0.05 to 0.21.This age is interpreted as the age of the protolith formation.

Alcañices dacite (Sample coS-8)
Zircons are idiomorphic, euhedral with elongated tips.The CL-images show inner cores, often rounded (e.g.zircon 7; Fig. 6) with an oscillatory pattern surrounded by planar oscillatory domains, that show elongated tips with oscillatory pattern (e.g.zircons 7 and 13; Fig. 6).The CL-patterns allow us to assume a magmatic character of the zircons.They are euhedral elongated tips, planar oscillatory domains.Cores were rounded and sometimes resorbed before new crystallization occurred.Cores are very frequent (74% of the zircons examined contain cores).
Data are shown in Table 1 and the TW-diagram in Figure 6.It shows a 206 Pb/ 238 U age of 1.9Ga for an inherited zircon (spot 16.1) and the presence of Pb-loss (spot 11.1, 17.1).The rest of the data define a quasi-Gaussian distribution yielding an average age of 488.7 ± 3.7Ma obtained on 13 spots located in rims within 13 crystals.This age is interpreted as the age of the protolith formation.Inherited component due to mixing of cores and rim in the analysis is evidenced by spots 3.1 and 3.2.The domains that define the main cluster have U contents of 304-809ppm, Th contents of 30-65ppm and Th/U ratios that vary from 0.05 to 0.19.

dIScuSSIon And concluSIonS
The Schistose Domain of the Galicia-Trás-os-Montes Zone has been interpreted as the outboard edge of the Iberian terrane.The Soutelo rhyolite and the Alcañices dacite correspond to the lowermost levels of the Paraño Group in the northeast limb of the Verín-Bragança synform and according to the data presented we can propose a protolith age around 499.8 ± 3.7Ma (Upper Cambrian) and 488.7 ± 3.7Ma (Upper Cambrian-Lower Ordovician), respectively.These ages are older than the ones obtained by U-Pb zircon conventional techniques of 439.6 ± 5Ma (Silurian) for a felsic trachyte in the middle part of the Paraño Group in the Verin Synform (Valverde-Vaquero et al., 2007), and an age of 475 ± 2Ma (Lower Ordovician) for a rhyolite in equivalent series near Cabo Ortegal (Valverde-Vaquero et al., 2005).According to the overall data, the age of the Paraño Group ranges from the uppermost Cambrian to, at least, lower Silurian.
Series of lower-Middle Ordovician felsic magmatism described in the autochthon of the Schistose Domain, the Ollo de Sapo Antiform of the CIZ (Valverde-Vaquero and Dunning, 2000), include the Ollo de Sapo Formation (Fm.), formed by hundreds of meters of metavolcanic, volcanosedimentary and igneous rocks with similar chemical signature to the acidic volcanites of the Paraño Group (Gallastegui et al., 1987;Montero et al., 2009).Apart from their siliciclastic and volcanic character, the Lower Paleozoic formations of the Ollo de Sapo Domain (CIZ) and the Paraño Group (GTMSD) are quite different.Nonetheless, the presence of few Pan-African detrital zircons within the Alcañices and Soutelo volcanics with ages around 600-500Ma indicates their derivation from Gondwana.Moreover, both include inherited zircons with provenance ages of ca.1.8Ga.Similar ages have been Figure 3

FIGURe 2 .
FIGURe 2. Synthetic stratigraphic column of the Schistose Domain in the NE limb of the Verín-Bragança Synform, showing the stratigraphic location of the collected samples and the samples dated by Valverde-Vaquero et al. (2007).
) D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 1 4 . 1 2 .3 .3 U-Pb dating of Cambrian volcanics in the Variscan Orogen 212

FIGURe 3 .
FIGURe 3. Geological map of the area located between Puebla Sanabria and Bragança (see Fig.1) along the Spanish-Portuguese border, showing the location of the collected samples.

FIGURe 4 .
FIGURe 4. Several features of the analyzed samples.Main phases are pinpointed (abbreviation names of mineral based on Whitney and Evans, 2010, except LFg: lithic fragment).A) Macroscopic photograph of COS-07 sample.An orange weathering rim can be observed, but no sorting or orientation can be detected in rock fragment.B) Photomicrograph of COS-07 sample (base of image are 4 cm length).The abundance of main phases can be observed (Qz and Kfs) and the typical volcanic features of quartz.C) Photomicrograph of COS-08 sample (base of image are 4cm length).It is possible to observe the presence of quartz veins with the same orientation and an irregular distribution.A slightly orientation of rock can be deduced based on main mineral phases.D) Microscopical feature of COS-07 sample with polarized light.Alteration of feldspars and porphydic features can be observed as well as the edges slightly rounded of quartz crystals.E) Detailed aspect of a totally altered biotite in COS-07 sample.Biotite is replaced by prenhite + sericite (with colour into the exfoliation planes), hematite (Hem) and titanite (Ttn) (leucoxene).F) Microscopical aspect of COS-08 sample under cross polarized light.As in COS-07, quartz and plagioclase can be observed, with typical features of volcanic crystals and slightly rounded in their edges.G) Detailed aspect of another zone of COS-08 sample with a recrystallized vitreous matrix that preserves a previous spherulitic and perlitic hydration textures.

FIGURe 5 .
FIGURe 5. Cathodoluminiscence images of zircons from the Soutelo ryolite (COS-7).Zircons: nº 7.1, 10.1 and 11.1.A) U-Pb Tera-Wasserburg diagram for zircons from the Soutelo ryolite (COS-7) showing all the data points; B) Enlargement of A. Error boxes are plotted as 1σ.Average 206Pb/238U ages are given as weighted means and errors are expressed at the 95% confidence level.The small numbers denote the spot number.Filled symbols in the TW-diagrams show the population that lie on the mixing lines.'n' denotes 12 spots on 12 zircon crystals for the protolith population.

FIGURe 6 .
FIGURe 6. Cathodoluminiscence images of zircons from the Alcañices dacite (COS-8).Zircons: nº 7.1 and 13.1.A) U-Pb Tera-Wasserburg diagram for zircons from the Alcañices dacite (COS-8) showing all the data points; B) Enlargement of A. Error boxes are plotted as 1σ.Average 206Pb/238U ages are given as weighted means and errors are expressed at the 95% confidence level.The small numbers denote the spot number.Filled symbols in the TW-diagrams show the population that lie on the mixing lines.'n' denotes 13 spots on 13 zircon crystals for the magmatic population.