Late Cambrian magmatic arc activity in peri-Gondwana: geochemical evidence from metagranitoid rocks of the Basal Allochthonous Units of NW Iberia

The geochemical characterization of igneous rocks is the main source of information about the origin and evolution of magmas, and it is widely used to investigate the geodynamic setting of ancient terranes. The basic principles underlying this methodology assumes that magmas generated by similar geologic processes share common geochemical and isotopic features, making possible the comparison through tectonic setting discrimination diagrams (e.g.Pearce and Cann, 1973; Floyd and Winchester, 1975; Pearce et al., 1984; Winchester et al., 1995). Although the complexities of magmatic processes and the interaction of magmas with the wall rocks can sometimes make the interpretation of geochemical data not straightforward (e.g.Frost and Mahood, 1987; Castro et al., 1991; Neves and Vauchez, 1995; Chappell, 1996), their use in combination with other independent geological evidences are still the best tool to interpret magmatism and its geological setting.

The northern margin of Gondwana was active during the Neoproterozoic (e.g.Nance et al., 1991). This is evidenced in some sections of the Iberian Massif where Neoproterozoic deformation was accompanied by magmatic arc activity (Fernández-Suárez et al., 1998; Nance et al., 2002; Bandrés et al., 2004; Castiñeiras et al., 2008; Pereira et al., 2011; Henriques et al., 2015; Orejana et al., 2015; Rubio-Ordóñez et al., 2015). The long-lived tectonic activity along this margin, related to the dynamics of a coupled subduction-arc system, generated a complex ensemble of peripheral terranes, the peri-Gondwanan terranes (Avalonian–Cadomian orogeny; Quesada, 1990; Nance and Murphy, 1994; Eguíluz et al., 2000; Linnemann et al., 2000; Murphy et al., 2004). Subduction polarity during the Neoproterozoic is unanimously considered as directed towards Gondwana.

It is generally accepted that the system of peripheral magmatic arcs flanking northern Gondwana by the late Neoproterozoic gradually gave way to a passive margin, through the development of a rifting and the corresponding back-arc that gave way to the opening of the Rheic Ocean during the Lower Paleozoic (Sánchez-García et al., 2003; Linnemann et al., 2007; Nance et al., 2010; Fuenlabrada et al., 2016). Most models propose that the back-arc rifting that gave birth to the Rheic Ocean was fully active during Cambro-Ordovician times (ca. 540–450Ma; Crowley et al., 2000; Linnemann et al., 2004, 2008; Murphy et al., 2006; Díez Fernández et al., 2015), and that extension was likely dominated by Iapetus slab pull (e.g.Nance et al., 2010 and references therein). However, other models consider that subduction beneath peri-Gondwana remained during the Cambrian and Ordovician (Abati et al., 1999, 2010; Santos et al., 2002; Arenas et al., 2009; Díez Fernández et al., 2010; Rubio-Ordóñez et al., 2012; Andonaegui et al., 2012, 2016a; Dias da Silva et al., 2016), and that the Lower Paleozoic rifting resulted from extension in a broad back-arc region (Arenas et al., 2007; Abati et al., 2010; Díez Fernández et al., 2012a). The late Cambrian-Upper Ordovician magmatism in the Upper Parautochthon of the Galicia-Trás-os-Montes in Morais (NE Portugal) and in the Basal Units was originated in this back-arc setting and probably in the Gondwana side of the basin (extended continental crust evolving to a passive margin setting, Dias da Silva et al., 2016).

Whether subduction existed or not during Lower Paleozoic at the margin of Gondwana, as well as the position of passive margin are still a matter of debate. In this work, we present the main geochemical characteristics of the oldest (Late Cambrian) group of magmatic rocks from the Malpica-Tui Complex (NW Iberia), in order to determine their geodynamic context of formation. We build up on previous models and further discuss about Late Cambrian paleogeographic reconstructions of the perigondwanan domain that is preserved in the NW Iberian Massif of the Variscan orogen. Our data support that a Late Cambrian subduction-related magmatic activity existed in the perigondwanan realm located next to north Africa thus strengthening the idea of a long-lived active margin for this region, which caused back-arc extension, detachment and drifting of perigondwanan terranes (and magmatic arcs), oceanization and establishment of a passive margin in N-Gondwana.

Neoproterozoic and Paleozoic rock series that formed in the margin of Gondwana are currently exposed as juxtaposed geotectonic units along the so-called Variscan belt (Quesada et al., 1994; Martínez Catalán et al., 1997; Franke, 2000; von Raumer et al., 2003; Ballèvre et al., 2009; Faure et al., 2009; Rossi et al., 2009; Schulmann et al., 2009; Arenas et al., 2016a). Juxtaposition resulted from the progressive collision of Gondwana and Laurussia during the Late Paleozoic (Matte, 1991; Ribeiro et al., 2007; Martínez Catalán et al., 2009; Kroner and Romer, 2013; Díez Fernández et al., 2016).

One of the salient features of the Variscan orogen in NW Iberia (Fig. 1) is the presence of a Variscan tectonic pile of far-travelled units resting on top of sedimentary and igneous sequences (Ries and Shackleton, 1971) with Gondwana derivation (Martínez Catalán et al., 2004). The overriding tectonic units are divided according to their continental or oceanic affinity, structural position, and tectonothermal evolution (Arenas et al., 2016, and references therein). These units occur in tectonic klippen (Martínez Catalán et al., 2007, 2009) that can be extended to SW Iberia (Díez Fernández and Arenas, 2015), and are collectively referred to as the Allochthonous Complexes (Arenas et al., 1986).

FIGURE 1
Map and composite cross-section of the NW part of the Iberian Massif. Note the structural position of the Basal Units of the Allochthonous Complexes (after Martínez Catalán et al., 2007). The location of the inset shown in Figure 2A is indicated with a thick dashed line.

In the upper structural position of the Allochthonous Complexes of NW Iberia (Fig. 1), the Upper Units show continental affinity and Gondwanan provenance (Fernández-Suárez et al., 2003; Albert et al., 2015a, 2015b). On the grounds of metamorphic, geochronological and geochemical data, these units are considered as tectonic slices of a Cambrian perigondwanan arc (Abati et al., 1999; Santos et al., 2002; Andonaegui et al., 2002, 2012, 2016b; Castiñeiras, 2005; Fuenlabrada et al., 2010; Albert et al., 2015a). A group of ophiolitic units lies in an intermediate structural position (Fig. 1) and accounts for tectonic slices of Cambrian–Ordovician and Devonian oceanic to transitional crust, variably metamorphosed and deformed during the Variscan orogeny (Arenas and Sánchez Martínez, 2015). In the lower structural position (Fig. 1), the Basal Units show continental affinity (Arenas et al., 1986; Rodríguez Aller, 2005) and Gondwanan provenance (Díez Fernández et al., 2010, 2013; Fuenlabrada et al., 2012). The Malpica-Tui Complex is the best exposure of these lower units (Fig. 2A).

FIGURE 2
A) Geological map of the northern section of the Malpica-Tui Complex and its relative autochthon (Díez Fernández, 2011; Díez Fernández et al., 2011). See regional location in Figure 1. The location of samples selected for geochemical analysis is included. B) Idealized lithostratigraphic column of the lower sequence of the Basal Units (Díez Fernández et al., 2010). C) Main foliation of the metatonalites (eclogitic orthogneisses) defined by quartz, white mica, omphacite, garnet and zoisite (plane polarized light). D) Non-deformed metagranodiorite with hypidiomorphic inequigranular texture defined by plagioclase, quartz, biotite and K-feldspar. Note the growth of garnet coronas around biotite and the partial replacement of plagioclase by albite, clinozoisite, and phengite (cross polarized light).

The metasedimentary rocks of the lower sequence of the Basal Units are intruded by granitoids, later transformed into orthogneisses, with compositions ranging between quartz-syenites, high- K granites, granites, granodiorites and tonalites (Fig. 2A). This sequence also contains less abundant alkaline mafic rocks (Rodríguez Aller, 2005), which now occur as amphibolites and variably retrogressed eclogites. The granitic rocks define two compositional suites (Floor, 1966; Arps, 1970; Montero, 1993; Montero et al., 1998, 2009; Rodríguez Aller, 2005): a dominant suite with calc-alkaline composition and intruded by the mafic rocks (ca. 493–480Ma; Montero et al., 2009; Abati et al., 2010; Díez Fernández et al., 2012a), and a younger suite with alkaline-peralkaline affinity (ca. 480–470 Ma; Rodríguez et al., 2007; Montero et al., 2009; Díez Fernández et al., 2012a). The alkaline-peralkaline granitoids are not intruded by the mafic rocks, which thence can be interpreted as a dyke swarm emplaced in between the two granitic suites. Dating of the alkaline-peralkaline granites places the age of the alkaline mafic dykes that intruded the calc-alkaline granites at ~480–475Ma.

The calc-alkaline suite comprises granitic compositions that range between high- K granites and granites (alkali granites), granodiorites and tonalites. However, the tonalites and granodiorites are consistently the oldest rocks among the suite and intruded, compared to the alkali granites, in a rather narrow time interval (ca. 493–489Ma; Abati et al., 2010; Díez Fernández et al., 2012a). The ages of the alkali granites spread towards the age of the alkalineperalkaline granitoids, suggesting a transition between the two main suites.

Previous geochemical and geochronological studies proposed different interpretations for the origin of the two magmatic suites. Rodríguez Aller (2005) proposed a model in which both (calc-alkaline and alkaline-peralkaline) would be cogenetic post-collisional suites, related to a single magmatic evolution. In contrast, Abati et al. (2010) suggested two separate events. The Cambrian calc-alkaline magmatism would be related to the waning stages of arc activity associated with subduction beneath the northern margin of Gondwana. Then, a switch from subduction to rifting would be marked by the intrusion of alkaline magmas during the Ordovician (see Díez Fernández et al., 2012a for further refinement).

Our contribution focuses on the oldest group of metagranitoids (metatonalites and metagranodiorites) intruding the lower sequence of the Basal Units. We have paid particular attention to this group because previous background suggests that it represents one of the compositional poles of the magmatism that affected these units. Therefore, characterizing its geochemical features and tectonic setting may contribute to a better understanding of the evolving paleogeography of the perigondwanan realm in Lower Paleozoic times. What follows is a short description of the two lithologies analysed.

Seven samples of metatonalites and 14 samples of metagranodiorites were selected for whole-rock chemical analysis. Sampling locations are shown in Figure 2A. The metatonalites were collected from sites located as far as possible from mafic dykes. The less deformed and metamorphosed facies of metagranodiorites were preferentially selected for analysis.

The samples were crushed with a steel jaw crusher and powdered to 200 mesh grain size in an agate mill at the Universidad Complutense de Madrid. Chemical analyses were carried out at Activation Laboratories Ltd. (Actlabs) in Canada using the 4Lithoresearch procedure. Chemical digestion with lithium metaborate/tetraborate was followed by measurement of elemental concentrations via inductively coupled plasma mass spectrometry (ICPMS). Chemical data are included in Table 1.

The study of the oldest granitoids that intruded the Basal Units of the Allochthonous Complexes of NW Iberia provides a reference point to discuss about the evolving paleogeography and geodynamic setting of the perigondwanan realm in the Lower Paleozoic. From a magmatic point of view, the onset of igneous activity intruding this section of Gondwana seems to have occurred in the late Cambrian (ca. 493–489Ma; Abati et al., 2010; Díez Fernández et al., 2012a). This event is represented by the intrusion of the tonalitic and granodioritic magmas analysed in this work. The geochemical analyses of these magmas (Figs. 3, 4 and 5) suggest that such activity was related to the subduction of oceanic crust (i.e. suprasubduction setting). In both types of magma, this is typified by: i) slight enrichment in incompatible elements (Rb, Ba, Th, U), ii) negative anomalies in Nb, Ta, P, and Ti, iii) negative anomalies in Eu, and iv) chemical discrimination carried out by tectonic setting diagrams. Additionally, positive Pb anomalies suggest a crustal component in the genesis of both types of magma.

According to detrital zircon provenance studies, the sedimentary sequences of the Basal Units were deposited in the periphery of the north Gondwanan continental margin, receiving the major input of sediments from the West African Craton (Díez Fernández et al., 2010). Isotopic analyses of the metasedimentary rocks of these sequences are in agreement with this interpretation (Fuenlabrada et al., 2012). Moreover, both approaches suggest that the lower sedimentary sequence, in which the Cambrian magmatism intruded, was deposited in close relation to a magmatic arc. For this sequence, the maximum age of sedimentation was constrained at ~560Ma using detrital zircon grains, although a comparison with other equivalent, well-dated sequences of the Iberian Massif and from other parts of the Variscan belt favours a latest Neoproterozoic age of deposit (~560– 545Ma; Díez Fernández et al., 2010, 2017). It not possible to constrain the precise position in the arc system based solely in geochemistry, but according to the probable polarity of the neoproterozoic subduction towards Gondwana, and with the fact that the granitoid rocks intrudes an Ediacaran synorogenic flysch basin, we suggest that the late Cambrian metatonalites and metagranodiorites intruded an incipient back arc basin. Furthermore, the upper sequence of the Basal Units point for a more oceanic realm in the N-Gondwana shelf than the lower sequence, which, together its present day structural position makes more probable a setting in the Gondwana side of the back-arc basin (Fig. 6).

FIGURE 6
Synthetic paleogeographic model for the perigondwanan realm during the late Cambrian. The intrusion of the tonalites and granodiorites localized in the Basal Units took place in an arc-related setting, along with the felsic and mafic magmatism of similar age that is observed in the Ophiolitic and upper units of the Allochthonous Complexes of NW Iberia. Approximate paleogeographic locations were extracted from Fernández-Suárez et al. (2003), Gómez Barreiro et al. (2007), Díez Fernández et al. (2010), Bea et al. (2010), and Albert et al. (2015a).

There seems to exist a gap between the latest Neoproterozoic arc activity registered in the Basal Units and the late Cambrian magmatism that intruded later. Taking all these data together, the section of Gondwana represented in the Basal Units of the Allochthonous Complexes of NW Iberia was the site of (pulsed?) Andean type magmatic arc activity from the late Neoproterozoic up to at least late Cambrian times. Interestingly, a convergent margin with similar duration to that of the Basal Units has been also inferred for the Upper Units of the allocthonous Complexes of NW Iberia via integration of multi-proxy analyses (see compilation in Andonaegui et al., 2016a).

Palinspastic restoration of major Variscan thrusts in NW Iberia provides an approach to the pre-collisional paleogeography across the perigondwanan realm (Martínez Catalán et al., 2009; Díez Fernández et al., 2016). In Lower Paleozoic times, the Upper Units of the Allochthonous Complexes would occupy the most external part of the margin, with some of the Ophiolitic and then the Basal Units located inboard. The relative autochthon to the Allochthonous Complexes would be located farther inwards, next to mainland Gondwana. The location of the Vila de Cruces ophiolite in between the Basal and Upper units suggests that these two latter sections of the margin would be separated by oceanic/transitional crust, dated at ca. 497Ma (Arenas et al., 2007). The geochemical composition of this crust is typical for a supra-subduction setting (Sánchez Martínez et al., 2009), which leads us to the conclusion that the Upper, Ophiolitic, and Basal units were members of a single magmatic arc system in the Lower Paleozoic (Fig. 6). Localized extension within this system gave way to back-arc basins in late Cambrian times, such as those represented by the Vila de Cruces ophiolite (Arenas et al., 2007) or by the upper sequence of the Basal Units (Díez Fernández et al., 2010). Further extensional activity across the margin heralded the onset of intra-continental rifting, which favoured the arrival of alkaline-peralkaline magmas during the Ordovician.

New geochemical data obtained from late Cambrian metatonalites and metagranodiorites of the Basal Units of the Allochthonous Complexes of NW Iberia suggest that those granitoids were formed in a subduction-related setting. The section of the margin of Gondwana where they intruded (next to current North Africa) was part of an Andean-type margin during the Neoproterozoic. Our new data supports that ongoing subduction surpassed the Neoproterozoic-Cambrian transition, what confers larger persistence to the peripheral orogens of Gondwana before the onset of rift-related activity.

This work has been funded the Ministerio de Economía y Competitividad through research grant CGL2016-76438-P. We thank Icaro Dias da Silva, Ricardo Arenas and an anonymous referee for detailed and constructive reviews which did much improve the manuscript.