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Gondwana Research 14 (2008) 451–473 www.elsevier.com/locate/gr

Massive generation of atypical ferrosilicic magmas along the active margin: Implications for cold plumes and back-arc magma generation ⁎ C. Fernández a, , R. Becchio b, A. Castro c, J.M. Viramonte b, I. Moreno-Ventas c, L.G. Corretgé d

a Departamento de Geodinámica y Paleontología, Universidad de Huelva, Campus del Carmen, 21071 Huelva, Spain b CONICET, Instituto Geonorte, Facultad de Ciencias Naturales, Universidad Nacional de Salta, Buenos Aires 177, 4400-Salta, c Departamento de Geología, Universidad de Huelva, Campus del Carmen, 21071 Huelva, Spain d Departamento de Geología, Universidad de Oviedo, Arias de Velasco s/n Oviedo, Spain Received 22 December 2007; received in revised form 1 April 2008; accepted 7 April 2008 Available online 15 April 2008

Abstract

One of the most intriguing characteristics of the northern (Iberia) and southern (Puna) Gondwana margins is the presence of large volumes of Late –Early magmatic rocks with ferrosilicic composition, i.e., rocks with high iron and silica contents (FeON4.0 wt.%, SiON63 wt.%) for very low contents in calcium (CaOb1.5 wt.%). Geological and geochemical features, as well as experimental results, show that ferrosilicic magmas resulted from near-total melting (80–90%) of crustal sources of metagreywacke and charnockite affinities, possibly derived from Neoproterozoic volcanoclastic sediments and/or their granulite facies equivalents, under very high temperatures (1000 °C–1200 °C) and at pressures of 1.0 to 2.0 GPa. A plausible tectonic setting for this peculiar magmatism is a back-arc region subjected to extension, with the ferrosilicic magmas ascending from a deep cold diapir or mantle wedge plume. Rifting in the back-arc progressed until the aperture of an ocean basin (the Rheic ocean) in the northern margin of Gondwana, but became aborted in Argentina. © 2008 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Keywords: Gondwana; Ferrosilicic magmatism; Late Cambrian–Early Ordovician; Iberia; Puna

1. Introduction (Martínez Catalán et al., 1997; Matte, 2001; Pin et al., 2006). Among the most important igneous formations related to It is broadly documented that an active margin at the magmatic activity at the Gondwana margin during Late Cambrian Gondwana supercontinent resulted in large-scale crustal rework- to Early Ordovician times, is a thick sequence of silicic magmatic ing and net addition of continental crust during Late Cambrian to rocks, widely represented in the Iberian peninsula by the so-called Early Ordovician times (510–460 Ma) (Bock et al., 2000; “Ollo de Sapo” sequence (Hernández Sampelayo, 1922; Parga Lucassen et al., 2000; Ramos and Aleman, 2000; Zimmerman and Pondal et al., 1964; Martínez Catalán et al., 2004), and in the Bahlburg, 2003; Hongn and Riller, 2007). Geochemical features South American continent by the Famatinian–Eastern Puna and radiometric age determinations of magmatic rocks, together magmatic eruptive belts (Pankhurst et al 1998; Saavedra et al., with detailed structural studies, are used in combination to 1998; Coira et al., 1999; Hongn and Riller, 2007; Viramonte et al., identify new terrains as derived from magmatic activity associated 2007). In both cases the magmatic sequence is a several km thick with a complex active margin along a large part of Gondwana. In association of silicic rock of igneous origin in which plutonic, the Variscan belt, these represent in part ancient subvolcanic and eruptive facies can be identified. A magmatic intraoceanic arcs and microcontinents that were separated from provenance is strike forward in the Puna eruptive belt, where no Gondwana during Late Cambrian to Early Ordovician times and orogenic event has substantially modified the original relations of attached again to its margin during Late Palaeozoic times the magmatic rocks. In Iberia, these rocks were severely affected by deformation and metamorphism during the Variscan orogeny ⁎ Corresponding author. Fax: +34 959219440. and the textural relations were largely obliterated. The identity in E-mail address: [email protected] (C. Fernández). geochemical features and age of these igneous sequences, the

1342-937X/$ - see front matter © 2008 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2008.04.001 452 C. Fernández et al. / Gondwana Research 14 (2008) 451–473

Eastern Puna eruptive belt in South America and the Ollo de Sapo Cambrian to Ordovician times, occupied positions more external unit in Iberia, as well as the position of both zones at the than the silicic magmatic belts (Ollo de Sapo and Eastern Puna Gondwana continental margin indicate that they formed part of eruptive belt). Consequently, the formation in a back-arc tectonic the same magmatic event. In South America, the Famatina belt, a context seems to be the more plausible scenario for this silicic magmatic lineament of more than 2000 km (Chew et al., 2007), magmatism. represents a typical arc with calc-alkaline magmas and the Textural relations have been modified for the Ollo de Sapo development of large batholiths (Pankhurst et al., 1998; Saavedra rocks. However, in the Eastern Puna eruptive belt the original et al., 1998; Dahlquist et al., 2005). In Iberia, evidences of arc volcanic provenance and derivation from silicic melts are magmatism for Early Ordovician times are found in the Ordenes evident. Given their atypical composition, the fact that these complex (Abati et al., 1999). Therefore, it has been proposed that silicic magmas are derived by crystallisation from a melt sets an these magmatic rocks were formed in relation to back-arc important petrogenetic problem. Conditions for the generation spreading associated with an active (Coira et al., 1999) of these atypical silicic magmas are discussed here. Their identified by arc magmatism. In both cases, Iberia and South implications are considered in a plate-tectonic scenario relating America, the magmatic rocks of arc affinities and calc-alkaline Fe-rich, silicic magma production with active subduction and batholiths related to the active margin of Gondwana during back-arc spreading. In the first part of the paper we show the

Fig. 1. Geological map of northwest Iberia (modified from Rodríguez-Fernández, 2004) showing the exposures of the main units studied in this work, the Ollo de Sapo formation and the Schist and Greywacke complex. Inset shows a simplified sketch of the Iberian Massif (right, based on Farias et al., 1987) and its location in the realm of the Variscan orogen during the Late Palaeozoic (left, modified from Martínez Catalán et al., 1997). C. Fernández et al. / Gondwana Research 14 (2008) 451–473 453 main geological and geochemical features of this particular Fe- 15 cm) K-feldspar megacrysts (Fig. 2a and b), and Na-rich rich, silicic magmatism, that we call ferrosilicic in a broad sense, plagioclase crystals. Fine-grained facies are commonly observed at making emphasis in the similarities between igneous formations the top of the unit, locally interspersed with quartzites and slates. separated by thousands of km from South America to Iberia. Variscan, low- to high-grade metamorphism and several deforma- Second, we analyse the petrogenesis of these magmatic rocks by tion phases affected the Ollo de Sapo formation (Martínez Catalán means of their geochemical signatures and melting experiments. et al., 2004). Low-grade, scarcely deformed gneisses show PT conditions for magma generation will be constrasted in euhedral K-feldspar and plagioclase crystals, embayed, rounded currently accepted tectonic models based on geological and quartz crystals and biotite clots within a recrystallised but fine- structural relations (Coira, et al 1999; Lucassen et al. 2000; grained matrix with quartz, plagioclase, biotite, muscovite and K- Stampfli and Borel, 2002; Gutiérrez-Alonso et al., 2003; von feldspar. Eutaxitic textures with rare vesicles have been locally Raumer et al., 2003; Kleine et al., 2004; Kirschbaum et al., 2006; described (Navidad et al., 1992), as well as glass shards and lithic Hongn and Riller, 2007; Viramonte et al., 2007). We show fragments (Díez Montes et al., 2004; Díez Montes, 2007). The K- finally the implications that this ferrosilicic magmatism may feldspar megacrysts often show albitic rims (rapakiwi texture). have in understanding the thermal and petrogenetic processes These textural features strongly point to a volcanic and volca- related to subduction of crustal material and the formation of noclastic origin of the Ollo de Sapo formation (Parga Pondal et al., cold plumes in the mantle wedge (Gerya and Yuen, 2003) and 1964; Navidad, 1978a; Ortega et al., 1996). According to this, most their magmatic consequences (Castro and Gerya, 2007). of the Ollo de Sapo formation has been interpreted as due to the deformation and metamorphism of welded ignimbrites, rhyolites 2. General features of the Cambro-Ordovician ferrosilicic and volcanic tuffs, subvolcanic facies, and related volcanoclastic magmatism in the Iberian Peninsula (Ollo de Sapo) and and volcano-sedimentary series. The Ollo de Sapo formation South America (Puna eruptive belt) stratigraphically overlies the Neoproterozoic Schist and Grey- wacke complex or the Cambrian metasediments, and it underlies 2.1. Geological features of the Ollo de Sapo the slates and quartzites of the Early Ordovician (Fig. 3). Radiometric absolute dating yielded ages ranging from 468 to The Ollo de Sapo formation is a complex and characteristic unit 495 Ma (Gebauer, 1993; Valverde-Vaquero and Dunning, 2000; of the Variscan Iberian massif (Hernández Sampelayo, 1922; Parga Díez Montes, 2007; Montero et al., 2007; Bea et al., 2007). Pondal et al., 1964). The exposure of the Ollo de Sapo formation describes a huge anticlynorium that outlines the Ibero-Armorican 2.2. Geological features of the Eastern Puna eruptive belt arc at the northwestern Iberian massif (Fig. 1). This unit is mostly comprised of gneissose rocks showing the appearance of augen In northwestern Argentina, the Cambro-Ordovician magma- gneisses, typically showing blue quartz crystals, large (up to tism (Famatinian Cycle; e.g., Rapela et al., 1992) is dominantly

Fig. 2. Field photographs of the Late Cambrian–Early Ordovician magmatic rocks. Ollo de Sapo (Iberian massif): (a) coarse-grained facies in an area of metamorphic low-grade, showing large K-feldspar crystals and blue quartz crystals in a fine-grained matrix, Sanabria region; (b) migmatised coarse-grained facies; some melt pockets are concentrated in pressure shadows around K-feldspar crystals, Sanabria region. Eastern Puna eruptive belt: (c) deformed volcanic facies with large K-felspar crystals; (d) typical undeformed coarse-grained volcanic facies with abundant blue quartz crystals and K-feldspar megacrysts with rapakiwi texture. 454 C. Fernández et al. / Gondwana Research 14 (2008) 451–473

silicic with mafic rocks comprising less than 1 vol.%. This magmatism is represented by two parallel, N–S-trending belts; the western belt is known as the Western Puna eruptive belt (Palma et al., 1986) and the eastern one as the Eastern Puna eruptive belt (Méndez et al., 1973). The first is mainly constituted by granitoids and volcano-sedimentary successions cropping out discontinuously from northeastern Chile to the northwest of La Rioja (Fig. 4). The Eastern Puna eruptive belt is represented by a ca. 600 km, N–S trending belt from near 17° S in to near 27° S in (eastern mag- matic belt in Fig. 4). This belt can be divided into two sectors by the Calama–Olacapato–Toro lineament (Fig. 4). The northern one (22°–24° S) is characterised by a dominant lavic and sub- volcanic bimodal volcanism associated with sedimentary sequences (Coira et al., 1999). Minor bodies of granitoids are also observed in this sector (Coira et al., 1999; Kirschbaum et al., 2006). Large volumes of plutonic rocks and volcano- Fig. 3. Synthetic columns of the Palaeozoic units of the Central Iberian Zone sedimentary sequences characterise the southern sector (24°– including the fine- and coarse-grained gneisses of the Ollo de Sapo formation 27° S) of the Eastern Puna eruptive belt (Viramonte et al., and related granitic gneisses (simplified from Díez Montes et al., 2004). See 2007). Locally, these rocks are deformed (Fig. 2 c) and affected Fig. 1 for location of Hiendelaencina and Sanabria. by medium-grade metamorphism.

Fig. 4. Simplified geological map of northwestern Argentina showing the distribution of the Palaeozoic magmatic and metamorphic rocks, the Puncoviscana formation, and the principal Palaeozoic mountain ranges. C. Fernández et al. / Gondwana Research 14 (2008) 451–473 455

The features of the silicic volcanic rocks of the Eastern Puna northern Africa and western South America. The Famatinian eruptive belt are strikingly similar to those of the Ollo de Sapo belt of South America (including the Puna region) was a typical formation. They are composed of alkali feldspar, plagioclase Early-Palaeozoic subduction orogen. In the northern Africa and (An10), and quartz phenocrysts, the latter with common European counterparts (including Iberia), a magmatic arc was corrosion bays, included in recrystallised, fine-grained matrix developed at the Avalonia and Cadomia terranes that were of quartz, alkali feldspar, biotite, muscovite, and sericite. Blue disrupted from Gondwana after the inception of the Rheic ocean quartz crystals and K-feldspar megacrysts (up to 10 cm in as a back-arc basin. It is important to realise that the Famatinian length) are distinctive features (Fig. 2 d). K-feldspar megacrysts and the African–European margins of Gondwana probably did are euhedral with common inclusions of biotite and quartz. not constitute a continuous subduction belt as shown in the Most of these megacrysts show perthitic, graphic and rapakiwi- idealized sketch of Fig. 5. Recent studies of the Variscan orogen like textures (Fig. 2 d). To the South of 26° S, the Eastern Puna in southern Mexico suggest that a transform boundary separated eruptive belt overlies a medium- to high-grade metamorphic Oaxaquia and Avalonia during the Cambrian–Early Ordovician, basement with Sm/Nd and U–Pb metamorphic ages of 515– therefore connecting the European margin of Gondwana with 500 Ma (Becchio et al., 1999; Lucassen et al., 2000; Lucassen the South American Famatinian belt (e.g., Keppie et al., in and Becchio, 2003). Radiometric absolute dating of the rocks of press). In any case, plate reconstruction indicates that Puna and the Eastern Puna eruptive belt yielded ages ranging from 460 to Iberia share a similar tectonic setting at the western margin of 490 Ma (Omarini et al., 1984; Lork and Bahlburg, 1993; Gondwana during Cambrian and Early Ordovician times. Viramonte et al., 2007). 2.4. Common geochemical features for Iberia and Puna 2.3. Puna–Iberia correlation during Late Cambrian and Early Ordovician: Plate reconstructions Although the magmatic provenance of the Ollo de Sapo rocks is well documented, the correlation with the non-modified Recent plate-tectonic reconstructions of Gondwana during volcanic rocks of the Eastern Puna eruptive belt makes this the Late Cambrian and Early Ordovician (e.g., Stampfli and interpretation more solid and raises an interesting problem on Borel, 2002; von Raumer et al., 2003) locate this continent at the petrogenesis of these unusual magmatic rocks. The main the Southern Hemisphere, extending from the Equator to the geochemical distinctive feature with respect to normal silicic South Pole (Fig. 5). The proto-Tethys ocean subducted beneath melts is the high iron content (FeON4.0 wt.%) for very low the Western Gondwana margin along most of the present-day contents in calcium (CaOb1.5 wt.%). That is, these silicic melts

Fig. 5. Late Cambrian–Early Ordovician tectonic reconstruction of Gondwana and adjacent terranes centred on the South Pole. The two short straight segments indicate the approximate location of the profiles shown in Fig. 15. Modified from Astini (1998), Pankhurst et al. (1998), Rapela et al. (1998), Stampfli and Borel (2002), von Raumer et al. (2003), Gutiérrez-Alonso et al. (2003), Cawood (2005), Rapalini (2005), Chew et al. (2007). 456 C. Fernández et al. / Gondwana Research 14 (2008) 451–473

Table 1 Average major element compositions of ferrosilicic magmatic rocks and plausible source materials from Puna and Iberia Ollo Iberia (1) Puna erupt. belt (2). SGC, Iberia (1) Puncoviscana (2) HR-40 (3) Felsic xenoliths Felsic xenoliths Iberia (4) Puna (5) Average S.D. Average S.D. Average S.D. Average S.D. Average S.D. Average S.D. (n=95) (n=59) (n=68) (n=13) (n=13) (n=3)

SiO2 67.53 2.81 69.12 2.51 64.16 7.16 70.40 5.17 64.78 62.31 3.85 62.85 2.76 TiO2 0.55 0.18 0.60 0.21 0.84 0.23 0.71 0.11 0.87 0.99 0.2 1.01 0.16 Al2O3 15.79 1.21 14.64 0.85 17.64 3.79 13.22 2.70 16.39 16.57 1.15 15.34 0.29 FeOt 3.77 1.03 3.84 1.05 6.08 1.57 4.54 1.03 5.74 7.08 1.39 7.35 1.27 MgO 1.56 0.57 1.62 0.55 2.12 0.62 1.93 0.43 2.17 3.71 0.96 4.64 0.75 MnO 0.04 0.02 0.07 0.02 0.04 0.03 0.08 0.02 0.04 0.1 0.03 0.12 0.02 CaO 1.19 0.51 1.63 0.84 0.27 0.23 0.93 0.43 0.48 1.59 0.62 1.80 0.73 Na2O 2.86 0.58 2.81 0.50 1.55 0.77 2.38 0.46 2.24 2.63 0.72 3.17 1.11 K2O 4.18 0.73 4.02 0.86 3.43 1.12 2.86 1.07 2.77 3.35 1.1 1.00 0.41 P2O5 0.17 0.07 0.19 0.06 0.15 0.09 0.17 0.05 0.22 0.14 0.07 0.05 0.02 L.O.I. 1.90 0.74 1.07 0.46 1.63 1.98 2.46 0.70 3.34 1.47 0.67 1.53 0.85 Total 99.80 98.88 98.03 96.89 99.75 99.95 98.87 Mg# 0.41 0.42 0.38 0.43 0.40 0.48 0.53 TA 7.04 6.82 4.98 5.23 5.01 5.98 4.17 A/(CNK) 1.41 1.25 2.79 1.54 2.17 1.53 1.60 B 93 96 139 114 137 105 128.67 A 86 53 213 88 173 112 113.14 K/K+Ca 0.81 0.75 0.94 0.78 0.87 0.71 0.40 Fe/Fe+Ca 0.72 0.66 0.95 0.79 0.90 0.78 0.76 (1), (2) According to data compilation in Fig. 6, (3) Greywacke from Ugidos (1997a). (4) Data from Villaseca et al. (1999). (5) Data from Lucassen et al. (1999); n is number of samples and S.D. the standard deviation; SGC is the Schist and Greywacke complex. A/(CNK): Alumina saturation index (ASI). A, B: see Fig. 6. are extremely rich in iron compared to calcium and for this diagram (Debon and Le Fort, 1983, 1988), the ferrosilicic reason we have used the term ferrosilicic to refer these rocks in magmatic rocks of Eastern Puna eruptive belt and Ollo de Sapo this study. They are richer in Fe than normal rhyolites and plot in the peraluminous field (AN0) and with values of B poorer in Ca than normal dacites. They have marked chemical (ferromagnesian components) between 50 and 150, very close to affinities with charnockite rocks, and are also close to felsic the field of turbiditic sediments and felsic xenoliths of charnockite granulite xenoliths, sampled by basalts and lamprophyres, and affinity, and not related to typical calc-alkaline fractionation widely represented in the studied regions, Iberia and Puna trends. Similar relations are observed at the K–Bdiagram. (Lucassen et al., 1999; Villaseca et al., 1999). Table 1 shows the There are intrusive granitoid bodies, associated with the Ollo average compositions for major elements of Cambro-Ordovi- de Sapo volcanic formation, with an age close to that of the cian ferrosilicic magmatic rocks from Iberia and Argentina, hosting volcanic rocks (Montero et al., 2007). So, these are part together with average compositions of regionally related of a common magmatic event. One of these granites is the Neoproterozoic turbidites and felsic granulite xenoliths. Antoñita gneiss (Navidad and Peinado, 1981) related to the Ollo The comparison between the chemistry of the ferrosilicic de Sapo unit in the Hiendelaencina area (Fig. 1). This granite magmatic rocks in Iberia and Puna yields a very close similarity in also has an atypical composition compared with other major and trace elements. The most outstanding geochemical peraluminous granites derived by partial melting of metagrey- features that define this atypical magmatism are: (1) The high wackes. This granite is plotted in the diagrams (A–B and K–B) silica content, typical of silicic magmas of dacite to rhyolite of Fig. 6 in an intermediate position between the ferrosilicic composition. (2) The slightly peraluminous character with values melts (Ollo de Sapo and volcanic rocks of the Eastern Puna of alumina saturation index, ASIN1.2. Ollo de Sapo rocks have eruptive belt) and the partial melts obtained experimentally at average ASI values slightly higher than those of the Puna eruptive low-melt fraction from metagreywackes (Montel and Vielzeuf, belt (Table 1) due to the Ca depletion observed in Ollo de Sapo 1997; Patiño Douce and Beard, 1996; Castro et al., 1999). compared to its American equivalent. (3) The high contents in Fe and Mg, more than twice the normal values of silicic magmas, 3. Petrogenetic constraints on ferrosilicic magma generation namely calc-alkaline dacites and rhyolites. (4) The very low contents in Ca, less than half the magmatic rocks with equivalent The petrogenetic interest of this Cambro-Ordovician ferrosi- Fe and Mg contents. (5) The high alkali contents, particularly K. licic magmatism, observed along the Gondwana margin for Minor geochemical differences can be observed between the two thousands of km, lies on its rarity. In particular, the identification of studied domains. Interestingly, these differences mirror similar the source and the conditions for melting are key features that may differences between the corresponding Neoproterozoic turbiditic have implications on large-scale tectonic processes. The slightly formations, a feature that will be addressed below. peraluminous character is among the most relevant geochemical The compositional particularity of these magmatic rocks can features. This feature may be used to rule out any relation with be seen in the multicomponental diagrams of Fig. 6.IntheA–B alkaline or metaluminous magmatism (A-type). This is C. Fernández et al. / Gondwana Research 14 (2008) 451–473 457

separation of pyroxene/amphibole plus plagioclase from silicate magma. This is clearly observed in the A–BdiagramofFig. 6. However, the nature of the fractionating phases is determined by intensive variables and the water content of the parental magma. So, we have investigated the particular conditions at which a Ca- poor and Fe-rich melt could be derived by magmatic fractiona- tion. Calculations with the MELTS thermodynamic algorithm (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) allow us to simulate any fractionation trend. Fig. 7 shows two theoretical trends calculated by equilibrium crystallisation of Hb-diorite of calc-alkaline composition from the Gredos batholith (Moreno- Ventas et al., 1995) with an initial water content of 6.0 wt.%. CaO and MgO contents decrease with crystallisation at 1.0 GPa. At 2.0 GPa, CaO remains constant at high values (N6 wt.%) until the end of crystallisation. Even at 1.0 GPa the CaO content is 2.5 times higher than the maximum values of the ferrosilicic rocks. Also the natural trends displayed by calc-alkaline batholiths plot at higher values of Ca compared with these ferrosilicic magmas (Fig. 7). The conclusion is that fractionation from a parental calc- alkaline mafic magma can be discarded as a mechanism to generate ferrosilic magmas.

Fig. 6. Multicationic diagrams showing the relations between the Late Cambrian– Early Ordovician ferrosilicic magmatic rocks of Iberia (Ollo de Sapo) and South America (Eastern Puna eruptive belt). These volcanic rocks plot in the field of Neoproterozoic sediments and out of the normal calc-alkaline trends. Experimental partial melts from metasediments and leucogranites are also plotted for comparison. (a) A–BdiagrambyDebon and Le Fort (1983, 1988). (b) K–B diagram. Data sources for Iberia: García de Figuerola (1966), Capdevila (1969), Gil Ibarguchi (1978), Navidad (1978b), Holtz (1987), Beetsma (1995), Briggs (1995), Fig. 7. MgO vs. CaO (wt.% oxides) diagram plotting the Ordovician ferrosilicic Ugidos et al. (1997a,b, 2001), Valladares et al. (2000), Castro et al. (2000, 2003), magmatic rocks of Puna and Iberia together with experimental melts obtained from Corretgé et al. (2001), Bea et al. (2003). Data sources for Puna: Damm et al. (1990), near-total melting of Neoproterozoic turbidites from Iberia (Castro et al., submitted Coira et al. (1999), Lucassen et al. (2001), Do Campo and Guevara (2005), for publication) at temperatures from 1000 to 1200 °C and 1.0 to 2.0 GPa. Note that Zimmermann (2005), Kirschbaum et al. (2006), Viramonte et al. (2007). experimental melts at 1000 °C plot in the area of leucogranites. Some of the most granitic facies of the Ollo de Sapo (e.g. the Antoñita gneiss) plot in this area. Also traced are the fields of Neoproterozoic turbidites from both geological domains: the independent of the extensional, anorogenic or not, tectonic Puncoviscana formation in Argentina and the Schist and Greywacke complex in environment in which the Cambro-Ordovician magmatism was Iberia. These are very close in composition, the Schist and Greywacke complex developed. However, some kind of relation to charnockite rocks, being poorer in Ca compared with the Argentinian equivalent. Ugidos et al. (1997a, characterised by a metaluminous to slightly peraluminous b) reported the low content in Ca of the Iberian Neoproterozoic turbidites as a unique feature of these metasedimentary rocks. This feature is transmitted to the magmas character (Frost and Frost, 2007) cannot be discarded (see derived by near-total melting and, consequently, the ferrosilicic magmas of the Ollo discussion below). de Sapo are slightly poorer in Ca compared with the Puna eruptive belt in Argentina, Fractionation from a calc-alkaline trend and melting from a in parallel with the slightly richer Ca content of the Puncoviscana source rocks. It crustal source of appropriate composition are the two plausible can be noted that the compositions of the ferrosilicic magmas plot outside the calc- hypotheses that will be considered here. alkaline trends depicted here by the South Patagonian batholith in Chile (Hervé et al., 2007) and the Gredos batholith in Central Spain (Moreno-Ventas et al., 1995). Fractionation trends from a water-rich basic magma (6 wt.% water) at 1.0 and 3.1. Hypothesis 1: Fractionation from a calc-alkaline trend 2.0 GPa are traced according to model predictions by MELTS algorithm. These are very far from the field of the ferrosilicic magmas. These cannot be derived by neither It is apparent from the major element chemistry of the Cambro- partial melting from a calc-alkaline igneous source (tonalite) as analysed below in Ordovician ferrosilicic magmatism that derivation from a calc- Fig. 8, nor by any kind of fractionation process related to a calc-alkaline magmatic series. The field of the lower crust xenoliths (felsic) from the Central System in alkaline trend is unlikely. This is mainly based on the low Ca Iberia (Villaseca et al., 1999) and from the Puna region (Lucassen et al., 1999)isalso contents for high Fe and Mg contents. Magmatic fractionation traced in this diagram. These are enriched in Mg with respect to the metagreywackes implies that Ca and Mg+Fe are jointly fractionated by cotectic indicating the residual character of the lower crust in the area (Villaseca et al. 1999). 458 C. Fernández et al. / Gondwana Research 14 (2008) 451–473

3.2. Hypothesis 2: Melting from a crustal source: Igneous Greywackes and their high-grade granulite equivalents are the (tonalite) vs. sedimentary (greywacke) source composition only plausible alternative protolith due to their peculiar composi- tion. Lower crust felsic xenoliths, hosted basalts and lampro- Partial melting of a calc-alkaline igneous source is often phyres, have been reported in central Iberia (Villaseca et al., 1999) invoked as a mechanism able to produce silica-rich melts of granite and La Puna (Lucassen et al., 1999). These have charnockitic to granodiorite composition. For instance, Patiño Douce (1997) affinities and are very close in composition to metagreywackes or produced experimentally metaluminous A-type granites by partial low-melt fraction residues (Fig. 6). According to Villaseca et al. melting of calc-alkaline sources. The possibility for producing the (1999) these xenoliths may represent the average lower crust ferrosilicic magmas at conditions different from those studied by composition in Iberia, and they may be the high-pressure Patiño Douce (1997) must be taken into account. On the other equivalent of Neoproterozoic metagreywackes involved in the hand, the behaviour of metasedimentary sources under partial generation of the Cambro-Ordovician ferrosilicic magmas. A melting is very well known as it has been reported in several recent study of zircons in felsic xenoliths from the Spanish Central relevant experimental studies (e.g. Vielzeuf and Holloway, 1988; System (Fernández-Suárez et al., 2006) revealed some inherited Vielzeuf and Clemens, 1992; Patiño Douce and Beard, 1996; ages coincident with the age of the Neoproterozoic sediments and Montel and Vielzeuf, 1997). However, all these studies were also with the age of the Ordovician ferrosilicic magmas. These performed at conditions constrained by natural observation in lithologies, greywackes, their low-melt fraction residues and the migmatites, that is, at crustal pressures and temperatures up to lower crust charnockitic xenoliths, have very close geochemical 1000 °C. In all these experiments, the resulting melts have the signatures (Table 1) and are potential sources for the Cambro- composition of peraluminous leucogranite, with low contents in Ordovician ferrosilicic magmatism. We refer these collectively as Fe, Mg and Ca. This is so even for moderate melt fractions of about metagreywackes in this paper. These metagreywackes have a high 50 vol.%, obtained in experiments at 1000 °C. The reason is that proportion of the minimum haplogranite melt with the conse- greywackes have a high proportion of minimum melt composition, quence that moderate melt fractions of about 50 vol.% produce and melts are controlled by the system minimum until their leucogranite compositions. Melt fractions higher than 50 vol.% are components are exhausted in the adequate proportions. Experi- required to produce compositions that depart from the minimum mental results indicate that it is necessary to increase the melt and are enriched in Fe and Mg. This enrichment is effective once fraction at values of more than 50 vol.% to produce compositions all the available Ca is exhausted and the melt evolves to an atypical that leave the system minimum and, consequently, are enriched in composition rich in Fe and Mg. This can only happen at Fe and Mg. In terms of phase relations, this is equivalent to say that temperatures higher that 1000 °C (Fig. 8). In both areas of the temperatures higher than the system minimum are required for Fe Gondwana margin, Puna in Argentina and Iberia in Europe, there and Mg to be dissolved in the melt. However, not only the is a thick sequence of turbiditic sediments of Neoproterozoic age, abundances of Fe and Mg are increased in the melt with increasing namely the Schist and Greywacke complex in Iberia and the temperature, Ca is also one of the non-minimum melt elements Puncoviscana formation in Argentina, that can be considered as (minimum melt elements are Si, Al, Na, K) that will be enriched in the most plausible source areas for the Lower Ordovician the melt with temperature. If available in the source, Ca will be magmatism studied here. These turbiditic rocks are considered incorporated to the melt together with Fe and Mg. It is interesting here as a geochemical reservoir with independence of the to note that the Cambro-Ordovician ferrosilicic magmas are metamorphic grade. characterised by very low Ca contents (CaOb1.5 wt.%), this feature being distinctive for this atypical magmatism. The 3.3. Greywackes and equivalent reservoirs (charnockites) as explanation is that the source is equally poor in Ca. Greywackes the source of ferrosilicic magmas: Geochemical constraints are the only geochemical reservoir that undergone an enrichment in Fe and Mg, bounded in clay minerals, and a depletion in Ca at The coincidence in composition of the Late Cambrian–Early the same time (Taylor and McLennan, 1985), resulting from a Ordovician ferrosilicic magmas (Ollo de Sapo and Eastern Puna weathering fractionation process in which Fe and Mg are eruptive belt) and the Neoproterozoic turbiditic sedimentary concentrated and Ca lixiviated from the geochemical reservoir. sequences that underlie these magmatic rocks, strongly suggest Calc-alkaline igneous protoliths can be ruled out because they that this particular magmatism, that cannot be related to magmatic do not match the requirement of high Fe and Mg and low Ca. Low- fractionation nor partial melting processes, may be derived by melt fractions from calc-alkaline tonalites give melts close to the near-total melting of a crustal source of metagreywacke and/or minimum granite melt. They can be more or less rich in K charnockite-like composition. As explained above, near-total depending on the presence or not of free water in the melting melting at ultra-high temperature is required to account for the reaction (El-Biad, 2000) at moderate temperatures (b800 °C). high Fe and Mg contents dissolved in a silicate melt, unless the However, these melts become richer in Ca, together with Fe and ferrosilicic magma represents a restite-melt system developed at Mg, for slightly higher temperatures of about 950 °C (El-Biad, moderate temperatures within the range 750–850 °C (e.g. Castro 2000). It is expected that the three non-minimum elements, Ca, Fe et al., 2000). Melts fractions of the order of 0.1 to 0.2 are obtained and Mg, will be dissolved in the melt at higher temperatures from similar protoliths for the latter temperature range and at according to thermodynamic model predictions (Fig. 8). Therefore, pressures of the lower to medium crust (e.g. Patiño Douce and it follows that a Ca-rich, calc-alkaline protolith cannot produce Beard, 1996; Montel and Vielzeuf, 1997; Castro et al. 1999). The melt with the composition of the ferrosilicic rocks studied here. absence of restitic material in these ferrosilicic magmatic rocks C. Fernández et al. / Gondwana Research 14 (2008) 451–473 459

Fig. 8. Compositional curves for Fe, Mg and Ca calculated with the MELTS algorithm for melts derived from two source compositions: (a) and (c), tonalite; (b) and (d), metagreywacke, with 2 wt.% water and excess water, at 1.0 GPa. The chemical composition of melts at 1000 and 1100 °C, labelled as isotherms 1 and 2 respectively, are shown at each diagram. According to these model compositions, a calc-alkaline (tonalite) composition can be ruled out as the source of the ferrosilicic magmas. Ca is always the most abundant element in the melts derived from the tonalite source. Only a source poor in Ca and rich in Fe+Mg (greywacke) may produce these atypical magma compositions. Comparisons with the compositions of the Cambro-Ordovician ferrosilicic magmas indicate that the most favourable conditions are 1000 °C in excess water and 1100 °C with the water (2 wt.%) supplied by the hydrous minerals (micas) present in the source before melting. Compare composition 1 in (d) with composition 2 in (b), and these with the average composition for the Late Cambrian–Early Ordovician magmatic rocks shown in Table 1. In both cases (tonalite and greywacke source), the melt fraction is close to 90 vol.% (numbers beside the isotherms 1 and 2 give melt fractions). These theoretical predictions are coincident with the experimental study carried out with the same calc-alkaline tonalite by El-Biad (2000), and also with greywacke sources from the Iberian massif (Castro et al., submitted for publication). Oxides were recast to an anhydrous base. Curves represent saturation in the elements of reference for equilibrium batch melting simulations. They were traced by interpolation of 2 degrees interval determinations. Oxygen fugacity was set at QFM conditions. The composition of the metagreywacke is the HR-40 synthetic glass used in the experiments. It can be considered also representative for the felsic granulite xenoliths reported by Villaseca et al. (1999) (see Table 1). and the high viscosity of a melt-restite system with only 20 vol.% a short description of these Neoproterozoic sequences in Iberia melt (below the rheological threshold of Fernandez and Barbarin, and Argentina. The Schist and Greywacke complex (Carrington 1990), make this interpretation very unlike. The most plausible da Costa, 1950; Valladares et al., 2002; Rodríguez Alonso et al., hypothesis is that the ferrosilicic melts were near-liquid systems in 2004) is a metasedimentary unit that crops out extensively along which most of the Fe and Mg were dissolved in the melt. Melt most of the Central Iberian Zone (Fig. 1). It constitutes a rather fractions of about 0.8 to 0.9 are required and temperatures monotonous series of metapelites and metasandstones dated as exceeding 1000 °C according to our previous experimental results Late Vendian to Early Cambrian according to its fossil content and (see below). Consequently, we have explored the hypothesis of U–Pb determinations in detrital zircons (Vidaletal.,1994; near-total melting of metagreywacke as the mechanism to pro- Gutiérrez-Alonso et al., 2003; Ugidos et al., 2003a). Small bodies duce the atypical ferrosilicic magmas. The geological information of mafic magma intruded this unit (López-Plaza et al., 2007). The of the two studied areas (Iberia and South America), shows that lower boundary of this unit is not exposed, and a minimum the available sources are (1) the thick turbiditic sequence of thickness of 8000 to 11,000 m has been locally described Neoproterozoic age deposited in the Gondwana continental (Rodríguez Alonso et al., 2004). Two subunits have been margin (Ugidos et al., 2003a,b), namely the Puncoviscana classically distinguished in the Schist and Greywacke complex formation in the Puna region of South America (Turner, 1960) (Fig. 9a). The lower unit is mainly composed of lutites and and the Schist and Greywacke complex of the Iberian massif; and , with minor contents of microconglomerates and vol- (2) a granulite source represented by the felsic charnockitic canoclastic layers (Rodríguez Alonso, 1985; Álvarez Nava et al., xenoliths (Lucassen et al., 1999; Villaseca et al., 1999). Both 1988; San José et al., 1990; Valladares et al., 1998). The upper unit sources share geochemical signatures in major and trace elements is predominantly pelitic, with subordinate amounts of sandstones, as well as in isotopes (see below). Consequently, the results from and volcanic and volcanoclastic layers (Díez Balda, the experimental study with the metagreywacke are valid for the 1980; San José, 1983; Rodríguez Alonso, 1985; Álvarez Nava granulites as well. et al., 1988; Pieren, 2000). Pelites and sandstones are geochemi- Before following with the compositional comparisons that cally homogeneous across the Central Iberian Zone (e.g., may help to constrain the source-magma relationships, we go with Valladares et al, 2002; Ugidos et al., 2003a). Tectonically, the 460 C. Fernández et al. / Gondwana Research 14 (2008) 451–473

Fig. 9. Integrated and synthetic columns of Neoproterozoic metasedimentary units of Iberia and Puna. (a) The Schist and Greywacke complex (based on Valladares et al., 2002) of central Iberia. (b) The Puncoviscana formation of northwestern Argentina. The lower boundary of this unit is not exposed (modified from Omarini et al., 1999; Hongn et al., 2001; Buatois and Mángano, 2003; and Piñan-Llamas and Simpson, 2006).

Schist and Greywacke complex is interpreted as a predominantly eruptive belt, the volcanic and plutonic rocks have been nor- turbiditic and siliciclastic platform tectofacies associated with the malised with respect to the average composition of the Pun- evolution of an active continental margin during the late stages of coviscana Neoproterozoic sediments. In both cases, Puna and the Cadomian Orogeny (Rodríguez Alonso et al., 2004). Iberia, the average compositions of the Ollo de Sapo volcanic The rocks of the Eastern Puna eruptive belt overly a Neopro- formation and the Eastern Puna eruptive belt approach the terozoic turbiditic sequence known as Puncoviscana formation average compositions of their respective Neoproterozoic sedi- (Turner, 1960) and the Cambrian quartzite of the Meson Group ments. Ca and K are slightly enriched with respect to the source (Turner, 1960)(Fig. 9b). The Puncoviscana formation is a meta- (Fig. 10). However, the enrichment in Ca is very low compared sedimentary unit that crops out extensively along most of the with the peak of Ca displayed by calc-alkaline rocks of the northwestern and central Argentina (Fig. 4). This unit was inter- Western Puna eruptive belt (West Puna Magmatic Arc in preted as deposited during Neoproterozoic–EarlyCambrianbased Fig. 10b). It is interesting to note the negative anomaly in Fe on geochronological data of granitic rocks emplaced in this unit and Mg displayed by the calc-alkaline rocks. This is the most (Bachmann et al., 1987), detrital zircon analyses (Adams et al., prominent feature of the ferrosilicic magmas. They are richer in Fe 1990; Adams and Miller, 2007) and ichnofossils (Durand and and Mg than typical calc-alkaline compositions. Anatectic Aceñolaza, 1990; Buatois et al., 2000; Aceñolaza and Tortello, granites and low-fraction experimental melts (Fig. 10), developed 2003; Buatois and Mángano, 2003). This unit constitutes a rather from the same Neoproterozoic sediments or equivalent composi- monotonous series of metapelites and metasandstones and was tions, display a pattern departing considerably from the affected by a polyphase deformation and a very low-grade meta- ferrosilicic magmas. The pattern of the Antoñita gneiss is in the morphism (Omarini, 1983; Jezek, 1990). The lower boundary of middle between granite magmas developed by low degrees of this unit is not exposed. The Puncoviscana formation is interpreted partial melting (Fig. 10), and the Ollo de Sapo volcanics, possibly as a turbiditic sequence of passive continental margin (e.g., Jezek developed by a near-total melting of the same turbiditic source. et al., 1985; Rapela et al., 1998; Omarini et al., 1999; Do Campo The comparison for trace elements gives results similar to and Guevara, 2005; Piñan-Llamas and Simpson, 2006) developed major elements. Fig. 11 shows a Thompson plot for incompatible in a peripheral foreland-basin position, fed from an eastern fold and elements normalised to chondrite (Thompson, 1982). The thrust belt during (Zimmermann, 2005; Ramos patterns for average Neoproterozoic sediments and Late Cam- 2008). brian–Early Ordovician volcanics (Ollo de Sapo) are nearly Fig. 10 shows several comparative diagrams for major ele- coincident (Fig. 11a). Only small differences for Zr and the heavy ments using the mean composition of Neoproterozoic turbidites REE are observed. Leucogranites, derived by partial melting of from Iberia and Puna. For the case of the Iberian Neoproterozoic these sedimentary sources show marked differences due to the turbidites the average composition for these normalizations has fractionating effect of partial melting for most trace elements. This been calculated from the data by Ugidos et al. (1997a) using a effect is more marked if melting takes place by peritectic -to- ratio of 1:3. For the case of the Eastern Puna breakdown of biotite at pressures of more than 0.6 GPa due to C. Fernández et al. / Gondwana Research 14 (2008) 451–473 461

Fig. 10. Major element comparisons of Late Cambrian–Early Ordovician volcanics and related magmatic rocks normalised to their respective average of Neoproterozoic turbidites. Data sources as in Fig. 6. (a) Ollo de Sapo, Iberian Massif. (b) Puna eruptive belt and magmatic arc. See text for explanations. formation of garnet (Castro et al., 2000). The same comparison is Puncoviscana formation (Fig. 11b). The patterns for average made for the South America Cambro-Ordovician ferrosilicic Eastern Puna eruptive belt rocks and Puncoviscana metasedi- volcanic and plutonic rocks (Eastern Puna eruptive belt), ments are very similar. The Puncoviscana metasediments are leucogranites and the Neoproterozoic metasediments of the more enriched in Zr than the Eastern Puna eruptive belt. Felsic

Fig. 11. Chondrite normalised Thompson plot (Thompson, 1982) showing the patterns of the Late Cambrian–Early Ordovician volcanics, Neoproterozoic sediments and lower crust, felsic xenoliths of Iberia (a) and South America (b). Data sources for ferrosilicic magmatic rocks and Neoproterozoic sediments in Fig. 6. Data sources for xenoliths: Villaseca et al. (1999) and Lucassen et al. (1999). Explanations in the text. 462 C. Fernández et al. / Gondwana Research 14 (2008) 451–473

elements in granitic systems. These elements are trapped in accessory minerals (zircon, monazite) and they remain in the source (Bea and Montero, 1999). Also interesting is the com- parison with the felsic granulite xenoliths representing the lower crust in Puna and Iberia. These show REE patterns almost coincident with those of the metagreywackes and the ferrosilicic magmas (Fig. 12). Finally, it is also interesting to compare the Late Cambrian– Early Ordovician magmatic rocks and Neoproterozoic sediments in terms of the Sr–Nd isotope ratios. Fig. 13 shows the plot of representative samples form these formations in Iberia and South America. Cambro-Ordovician magmas show a narrow variation in Sr–Nd isotopes compared with the Neoproterozoic sediments. Lack of isotope re-equilibration at low temperatures over a hete- rogeneous turbiditic material with varied source components explains the variability in Sr–Nd isotopes of the Neoproterozoic sediments. It is expected that magmas derived by melting of these heterogeneous sources are more homogeneous due to the “averaging” effect of melting. This effect will be more effective at high degrees of melting at high temperatures. On one hand, high temperature favours isotope equilibrium and, on the other hand, high melt fractions favour melt homogenisation and segregation. Lower crust xenoliths from Iberia and Puna show similar Sr–Nd isotope ratios than the corresponding Neoproter- ozoic sediments (Fig. 13). Fig. 12. Chondrite normalised (Nakamura, 1974) plot showing the comparison between Late Cambrian–Early Ordovician volcanic rocks, Neoproterozoic 3.4. The contribution from experimental studies sediments and lower crust, felsic xenoliths in Iberia and South America. Data sources for ferrosilicic magmatic rocks and Neoproterozoic sediments in Fig. 6, 3.4.1. Previous results and PT constraints excepting Ollo de Sapo (Díez Montes, 2007). Data sources for xenoliths: The hypothesis that these ferrosilicic magmas were derived by Villaseca et al. (1999) and Lucassen et al. (1999). near-total melting of the underlying Neoproterozoic sedimentary rocks receives support not only from major element chemistry but granulite xenoliths of Iberia and Puna show trace element patterns also from trace elements and radiogenic isotopes as mentioned similar to those of Neoproterozoic sediments and Late Cambrian– Early Ordovician volcanic rocks (Fig. 11). Leucogranites, probably derived by partial melting of Puncoviscana sediments, have similar patterns in incompatible elements, whereas clear differences arise in compatible elements. The comparison for REE yields similar results (Fig. 12). The observation of some HREE fractionation in the ferrosilicic melts compared to the greywacke source, particularly in the case of the Iberian Ollo de Sapo rocks, indicates that garnet (Grt) was present in the source region with the implication that pressures of more than 1.0 GPa are required. Conditions of about 900 °C and 1.0 to 1.1 GPa have been reported for the felsic xenoliths mentioned above in Puna and Iberia. Consequently, these conditions, 1000 °C and 1.0 GPa were the minimum used in our melting experiments aimed to reproduce the atypical composi- tions of the ferrosilicic magmas as derived by near-total melting of a metagreywacke source. The patterns between Neoproter- ozoic metasediments and Eastern Puna eruptive belt rocks are coincident, suggesting again that the Eastern Puna eruptive belt rocks derived by near-total melting of the Neoproterozoic Puncoviscana formation. On the other hand, leucogranites show a similar pattern but depleted in total REE and with a greater Eu Fig. 13. Sr–Nd isotope ratios of Late Cambrian–Early Ordovician ferrosilicic anomaly. Depletion in total REE of leucogranite with respect to magmatic rocks, Neoproterozoic turbiditic sediments, and lower crust, felsic its source is a consequence of the refractory behaviour of REE xenoliths from Iberia and South America. Explanations in the text. C. Fernández et al. / Gondwana Research 14 (2008) 451–473 463 above. For this, however, a high temperature regime is required. reservoir with the average composition of metagreywackes. The Experimental studies, previously developed on these turbiditic high-grade equivalents of these compositions, e.g. charnockite rocks of general greywacke composition, gave rise to melt granulites, are also represented by these starting materials. fractions of 30 to 50 vol.%, for temperatures in the range 900– Experiments were carried out in end-loaded, Boyd-England 1000 °C, and melts of leucogranite (low Fe, low Mg) composition piston-cylinder apparatus at the University of Huelva. Metal (Patiño Douce and Beard, 1996; Montel and Vielzeuf, 1997; capsules containing 10 mg of sample (either rock powder or Castro et al. 1999). Consequently, temperatures higher than synthetic glass) are embedded in a pressure container of crushable 900 °C, even in the presence of water, are required to produce magnesia. The reported pressures are oil pressures measured with higher melt fractions and higher concentrations in Mg and Fe in electronic DRUCK PTX 1400 pressure transmitters, feeding the melt compared to leucogranites. If the source for this OMRON E5CK controllers, multiplied by ratio of ram-to-piston magmatism is the underlying turbiditic sequence and/or their areas, and were manually maintained within±5 bar of oil pressure granulite facies equivalents (charnockites), melt fractions of 80 to (ca. 250 bar on the sample). Temperatures were measured and 90 vol.% are required to account for the observed similarity in controlled with Pt100–Pt87Rh13 thermocouples feeding Euro- major and trace elements. The correlation between melt- therm 808 controllers with internal ice point compensators. temperature and Fe+Mg content (Johannes and Holtz, 1996, Temperature stability during all runs was ±5 °C. A NaCl sleeve and references herein) may be used as a first approach to with an inner glass protector is used for insulation. The resulting determine the temperature of generation of the ferrosilicic 1/2-inch diameter assembly is introduced into the CW pressure magmatic rocks. Saturation of Fe and Mg for a silicic melt with vessel and submitted to the desired run conditions. It has been this composition was calculated by equilibrium crystallisation at demonstrated (Patiño Douce and Beard 1994, 1995) that the constant pressure of 1.5 GPa using the MELTS algorithm graphite-based cell assemblies used in these experiments limit the (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998). The fO2 in the samples to a well-defined interval below the QFM results indicate that the liquidus phase of this system is always buffer (between QFM and QFM-2). The stability and composi- Opx at this pressure. This phase, Opx, is always present during the tions of ferromagnesian phases are not affected by fO2 variations crystallisation of the system with the implication that the reaction within the range imposed by these cell assemblies (Patiño Douce melt1→Opx+melt2 maintains the melt on saturation for the and Beard 1994, 1995), and these fO2 conditions are reasonable components Fe and Mg. A relevant result is that even in the case for deep crustal processes (see Patiño Douce and Beard 1996). that the ferrosilicic magmas were produced in the presence of Gold capsules were used for experiments at temperatures up water and were saturated in the source region at the pressure of to 1050 °C and AuPd capsules for higher temperatures. Run 1.5 GPa, the minimum temperature required for the lowest value duration has been set within critical values between the of Fe dissolution in the melt (FeO=3.4 wt.%) is of about 1000 °C. minimum time required to reach equilibrium and a maximum However, the general observation on the pre-eruptive phenocryst time to avoid Fe loss to the capsules or water loss in the case of assemblage of the ferrosilicic rocks, dominated by Qtz and Fsp, Au capsules. We have checked that duration on the order of 20 h strongly suggests that the system was not saturated in water. is sufficient to get equilibrium and to avoid Fe loss in runs at Taking into account these constraints, and those imposed by the 1200 °C. Shorter duration is needed to reach equilibrium in observed HREE depletion in some ferrosilicic magmatic rocks water-added runs at temperatures of 900 to 1100 °C. In this that suggests the presence of Grt in the source area, we have study we consider that equilibrium was reached if melt established the experimental conditions within the range of 1.0 to composition is homogeneous, with standard deviations within 2.0 GPa and 1000 to 1200 °C (Castro et al., submitted for WDS analytical errors of about 5 % relative. Also the lack of publication). significant compositional zoning in new-formed minerals is considered as a criteria of equilibrium. After the run, the 3.4.2. Starting materials and experimental procedures experiments were quenched by cutting-off the power. A fast A set of natural and synthetic materials have been used to dropping of temperature with rates of about 100 K/s is obtained, model source compositions. Average geochemical composi- minimising the formation of quenching minerals. Al-silicate tions of natural occurring rocks were used in both cases. needles formed by quenching were observed in few cases. A Synthetic glasses are used to model the turbidite source material fast heating ramp of 100 K/min (the maximum allowed by the (HR-40 in Table 1). Pure oxides were mixed at room Eurotherm 808 controller) was applied to all experiments in temperature with Na and K silicates in the desired proportions. order to avoid the formation of metastable phases during Iron is added in the form of ferric oxide. After mixing for half an heating. Capsules were checked for tears, mounted in epoxy and hour in an acetone medium, the homogeneous mixture is half cut with a diamond disk, for examination with the SEM and introduced in small portions in a graphite crucible inside a EPMA. Mineral and melt proportions were determined by vertical-loading furnace previously set at 1500 °C. Natural image analyses with the ImageJ software over back-scattered rocks were also used in some experiments. However, the use of electron (BSE) images (Z-contrast). Chemical compositions for synthetic glasses is preferred in this study, particularly if a minerals and melts (glasses) were determined by WDS with a completely dry system is required. Natural rocks contain a JEOL JXA 8200 Superprobe equipped with four WDS channels significant amount of water in micas and chlorite (see Table 1). at the University of Huelva. A combination of silicates and The use of these natural systems is preferred for experiments oxides were used as standards for calibration. Operating with a low water content. Both systems represent a geochemical conditions were 15 kV accelerating voltage and 15 nA probe 464

Table 2 Experimental conditions and melt compositions

Run Reference Sample P (GPa) T (°C) Water duration Assemblage Vol.% n SiO2 TiO2 Al2O3 FeO (t) MgO MnO CaO Na2OK2OP2O5 Total 100-total (wt.%) (h) (vol.%) Melt 8 AC07-14 CXG3:1 1.5 1000 20 15 Glass (85). Grt (10). St (1) 85 3 74.67 0.36 14.92 2.05 0.99 n.d. 0.46 2.98 3.58 n.d. 100 0.24 .Frádze l odaaRsac 4(08 451 (2008) 14 Research Gondwana / al. et Fernández C. Cor (b1) 0.14 0.04 0.09 0.06 0.03 0.02 0.02 0.02 9 IK07-1 CXG3:1 1.9 1000 2 88 Glass (36). Grt (39). Qtz (16) 36 4 73.69 0.31 16.49 0.97 0.29 0.01 0.21 3.32 4.21 0.50 100 11.53 Fsp (9) 0.54 0.04 0.25 0.05 0.02 0.02 0.01 0.13 0.05 0.21 10 AC07-17 HR40 1.5 1000 10 15 Glass (95). Grt (4) 95 4 71.68 0.51 16.59 3.42 1.26 n.d. 0.77 2.71 3.05 n.d. 100 16.68 AlSil+Ru (b1) 0.32 0.06 0.23 0.11 0.05 0.02 0.05 0.05 11 AC07-23 CXG1:1 1.5 1000 N10 40 Glass (85). Grt (10) Qtz (2) 85 4 73.96 0.46 14.76 2.61 1.15 0.04 0.50 2.86 3.42 0.25 100 11.75 12 AC07-31c HR40 1.5 1100 0 42 Glass (68). Qtz (14). Grt (12) 68 8 71.21 0.62 15.74 2.89 0.99 0.01 0.56 3.59 4.16 0.22 100 Ru+AlSil+Cor+Sp (6) 0.33 0.02 0.43 0.12 0.02 0.01 0.01 0.11 0.05 0.02 13 AC07-33 HR-40 1.5 1100 N10 24 Glass (95). Sp+Cor (5) 95 6 71.37 0.94 18.73 5.39 2.26 0.01 0.65 0.14 0.49 0.02 100 16.31 0.27 0.05 0.16 0.27 0.08 0.02 0.01 0.06 0.07 0.02 14 AC07-32c HR40 1.9 1100 0 42 Glass (30). Fsp (30). Opx (14) 30 2 68.48 0.91 15.89 3.34 0.44 0.00 0.84 4.40 4.91 0.03 99 4.21 Cor+Sp+Ru+Il (4) 15 AC07-31a CXG1:1 1.5 1100 2 42 Glass (66). Qtz (20) 66 6 69.26 0.56 16.89 2.13 0.80 0.02 1.38 3.40 5.32 0.24 100 4.48 Grt (9). Fsp (4). AlSil (1) 0.48 0.03 0.61 0.03 0.03 0.02 0.02 0.06 0.10 0.03 16 AC07-32a CXG1:1 1.9 1100 2 42 Glass (60). Grt (15) 60 10 71.88 0.66 15.69 1.97 0.96 0.03 0.51 3.82 4.26 0.22 100 7.31 AlSil+Ru+Cor (10) 0.17 0.06 0.14 0.08 0.04 0.02 0.02 0.08 0.04 0.04 17 AC07-20 HR40 1.5 1200 0 12 Glass (89). Qtz (10) 89 6 66.36 1.03 17.23 6.15 2.30 0.02 0.74 3.36 2.77 0.04 100 4.15

Sp+Cor (1) 0.37 0.04 0.21 0.18 0.08 0.01 0.03 0.05 0.02 0.02 – 473 18 AC07-25 HR40 2.0 1200 0 13 Glass (81). Qtz (10). Opx (2) 81 7 68.67 1.14 16.69 3.24 2.11 0.03 0.82 4.15 3.14 0.02 100 7.62 Sp+Cor (6). St (1) 0.85 0.16 0.25 0.17 0.12 0.02 0.05 0.73 0.13 0.01 19 AC07-13 CXG1:1 1.5 1200 2 14 Glass (83). Qtz (16) 83 4 70.36 0.86 15.88 3.91 2.01 n.d. 0.54 3.31 3.14 n.d. 100 3.14 Sp+AlSil (1) 0.49 0.02 0.19 0.28 0.06 0.04 0.06 0.01 20 AC07-26 CXG1:1 2.0 1200 N10 13 Glass (76). Qtz (19) 76 7 69.86 0.84 16.07 3.13 2.14 0.04 0.47 4.03 3.26 0.18 100 8.32 Opx (4). Sp+Cor (1) 0.22 0.08 0.08 0.11 0.08 0.01 0.02 0.03 0.08 0.03 Glass compositions recasted to 100. n is the number of analyses; numbers in italics are standard deviation; n.d., not detected. Mineral abbreviations: Qtz: quartz, Fsp: feldspar, Grt: garnet, Opx: orthpyroxene, Sp: spinel, Cor: corundum, AlSil: Al-silicate, Ru: rutile, Il: ilmenite, St: staurolite. C. Fernández et al. / Gondwana Research 14 (2008) 451–473 465 current. A defocused beam of 30 µm was used for glass analyses experiments indicate that a combination of temperatures and in order to minimise Na loss. Only in cases of small melt pools water contents may account for the compositional variability of in low-melt fraction experiments, a normal smaller beam of 3 to these magmatic rocks. It has been determined in this experimental 10 µm was used to avoid contamination with X-rays from the study that water has a strong control on the melt fraction. This can surroundings. In these cases, a significant Na loss of about 30 to be observed in experiments at 1.5 GPa and 1000 °C in excess 50% relative was observed. water. These have in common a high melt fraction (85 to 95 vol.% melt). However, these melts are relatively poor in Fe and Mg 3.4.3. Experimental results compared with the ferrosilicic magmas. The reason is the strong Table 2 shows the compositions of melts from this study. All fractionating effect of Grt (25 wt.% FeO and 12 wt.% MgO) on the melts are peraluminous and rich in Fe and most of them fairly the Fe content of the system. Nevertheless, some Grt must be match the composition of the ferrosilicic magmas (Figs. 7 and 14). generated in the source region, to account for the slight depletion The mineral assemblages and melt fractions for these melting in the HREE of some ferrosilicic rocks (particularly the Ollo de Sapo) with respect to the Neoproterozoic sediments and their granulitic equivalents. Many experimental melts produced at 1000, 1100 and 1200 °C, at pressures of 1.4 to 2.0 GPa, fall within the area of the ferrosilicic magmas (Figs. 7 and 14). Combining these compositional requirements with the need for Grt in the source region, the conditions may be more constrained. At 1200 °C, Grt is systematically absent. Pressures higher than 2.0 GPa are required to stabilise Grt at this temperature. However, the melts developed at 1200 °C and 1.5 to 2.0 GPa fall within the field of the ferrosilicic magmas in the uppermost region of the Fe– Mg compositional area (Fig. 7). The observed HREE patterns, indicative in some cases of Grt in the residue, suggest that, at least a part of the Fe+Mg rich facies of the Ollo de Sapo magmatic rocks, those with MgON2.0 wt.% and FeON3.0 wt.%, may have been developed at conditions of about 1200 °C in the absence of Grt. It must be remarked that there is no general rule on this correlation and that some mafic facies may have been developed in a Grt-present assemblage. The point of interest here is that some mafic facies have no HREE depletion and these may have been formed at temperatures of about 1200 °C. These conditions are possibly more extended in the case of the Puna eruptive belt as denoted by the absence of any HREE depletion compared with the Neoproterozoic turbidites of the Puncoviscana formation (Fig. 12), that is, the hypothetical source of the Cambro-Ordovician magmatism in the Famatinian magmatic belt. Low water runs at 1100 °C and 1.5 to 2.0 GPa produce melts poor in Fe and Mg, coincident with the field of the granitic bodies associated with the Cambro-Ordovician magma- tism (Antoñita gneiss in Iberia). It is concluded from this experimental study (Castro et al., submitted) that temperatures of 1000 °C (excess water) and 1100 °C (dry) at pressures of 1.4 to – Fig. 14. (a) Multicationic A B diagram (Debon and Le Fort, 1983, 1988) plotting 2.0 GPa are the most plausible conditions for the generation of the experimental melts at 1000, 1100 and 1200 °C and at pressures ranging from 1.4 to 1.9 GPa, and the field of the ferrosilicic magmatic rocks. CXG1:1, CXG3:1 and ferrosilicic melts with the composition of the Cambro-Ordovician HR40 are the starting materials. Also shown is the field of low-melt fraction magmatic rocks (Ollo de Sapo) of Iberia. Some of the facies richer experiments from the same source materials. Note the compositional similarity of in Fe+Mg possibly require higher temperature conditions, experiments and natural magmas and how these compositions deviate from the perhaps of about 1200 °C. Interestingly, an exhaustive study of typical cafemic (calc-alkaline) trend, traced here after Debon and Le Fort (1983). zircon inheritance in the Ollo de Sapo and related rocks by Bea Also note the similarities with the granulite (felsic) xenoliths from the two studied regions, Iberia and Puna. (b) Close-up view of the relevant part of diagram A–B, et al. (2007), has shown that the lower limit for the maximum showing the vector linking sources and melts in the experiments. These show that temperature reached by these magmas was of 900 °C. These the compositional variability found in the Cambrian-Ordovician ferrosilicic magmatic rocks are characterised by a wide variation in the Fe+ magmatic rocks may be accounted for by differences in temperature in the source Mg content and a narrow variation in silica (62–76 wt.% SiO2) region more than the differences in the composition of the source. This is important and peraluminosity (Alumina saturation index, ASIN1). Accord- because the heterogeneity found in the metagreywackes is at the scale of cm to m. Melts developed at high melt fractions will tend to become homogenised, ing to the experimental results these variations can be attributed to averaging the heterogeneous source. So, variations in melt composition can be variations in intensive variables, P, T, water activity, more than the more likely the result of changes in temperature. heterogeneous composition of the source (Fig. 14). At the high 466 C. Fernández et al. / Gondwana Research 14 (2008) 451–473 melt fractions required, melt pools will be connected allowing melts are segregated in relation to any tectonic activity acting at complete mixing and homogenisation at the time of segregation. the time of partial melting. Melt segregation at 5–10 vol.% melt is The implication is that temperature was not homogeneous at the even possible at actively deforming belts (e.g., Brown and source region with important gradients of more than 200 °C Rushmer, 1997). Consequently, partial melting with low-melt distributed in a relatively small volume of source material. These fractions (5 to 30 vol. % melt) is typically a crustal process observations are discussed below together with the heat source imposed by the rheology of the crustal materials with contrasted and heating mechanisms. compositions and strong rheological discontinuities. Possible exceptions to this are mid-ocean ridges where active deformation 4. Discussion of partially molten peridotite can result in melt migration under very low-melt fractions (≤5 vol.%) (e.g., Daines, 1997). Ferrosilicic magmatic rocks from the Eastern Puna eruptive Furthermore, partial melting of crustal protoliths is always belt and Iberian massif show an unusual chemical composition associated to orogenic processes, either extensional or compres- in major elements. As indicated above, they are characterised by sional, that favour the segregation of melts via shear bands or any high FeO contents of about 4 wt.% and Mg contents of 1.5 wt.%, type of local extensional regime in response to deviatoric stresses for high silica values ranging from 65 to 70 wt.% SiO2 and very acting on a rheologically discontinuous crustal environment. It low CaO (b1,5 wt.% CaO). They are more than twice richer in seems very unlike that a melting fraction of about 70 to 90 vol.% Fe and Mg compared to anatectic leucogranites. The lavic nature can be produced in the continental crust, unless protected from of these rocks and the absence of restitic materials dragged from large-scale deforming zones. The high temperatures needed for the source indicate that these high contents in Fe and Mg were this melting process are also very unlike in the continental crust. dissolved in the melts. The identification of such a kind of high And finally, the third petrogenetic problem is the high degree of Fe, silicic magmas in Cambro-Ordovician series of the zircon inheritance shown by the Cambro-Ordovician magmatic Gondwana margin entails important tectonic implications. rocks of the Iberian massif, which is indicative of extremely rapid These magmas do not form part of the typical calc-alkaline heating rates (Bea et al., 2007). Fast melt transport suggests an trends that characterise magmatic arcs. The hypothesis that these extensional setting correlative to the rifting process that took place ferrosilicic magmas were derived by near-total melting of the during the Early Palaeozoic in the northern margin of Gondwana underlying Neoproterozoic sedimentary rocks or high-grade (Bea et al., 2007). metamorphic equivalents receives support not only from major A possible solution for the high temperatures (1000 to 1200 °C element chemistry but also from trace elements, radiogenic as shown here) and the fast melting rates is that melting occurred isotopes and experimental petrology, as mentioned above. For at the core of a cold diapir formed by subducted sediments this, however, a high temperature regime is required. The ascending into the hot mantle wedge. The possibility for thermal– implications of this unusual thermal regime and its tectonic rheological unstabilities that give rise to these mantle wedge implications are discussed here. megastructures was analysed in detail by Gerya and Yuen (2003). These structures have recently attired the interest of geologists and 4.1. Implications of ferrosilicic melts in the thermal regime at geophysicists, as they constitute a part of the plate-driven upper the Gondwana margin mantle circulation (e.g., Ernst, 2007), together with the hydrous plumes described by Maruyama et al. (2007). Gerya and To transfer these geochemical features to the magmas, it Stoeckhert (2005) have investigated numerical models applied requires a near-total melting of a turbiditic sediment or its high- to analyse the evolution of continental active margins. In these grade equivalent, and for that to occur an ultra-high temperature of models large portions of the upper continental crust (e.g. the order of 1000–1200 °C is required in the source. This high T is sediments) may be subducted and transported by cold diapirs to also required by Fe and Mg dissolution in silicic magmas, the mantle wedge, giving rise to large silicic magma chambers. normally restricted to very low values (0.5–1.8 wt.% FeO, 0.1– Melt segregation at low melting rates is very unlike at the core of 0.8 wt.% MgO) for the normal temperatures required for granite these cold diapirs as they move ascending through the mantle magma generation by low-fraction partial melting of the same wedge within a nearly continuous medium. Also reaction with the turbiditic sources (Montel and Vielzeuf, 1997; Castro et al., 1999). mantle rocks may help to preserve these megastructures due to the Three major problems arise to account for this near-total formation of a reactive aureole rich in pyroxene and amphibole melting process. First, the need for a large thermal anomaly able to (Castro and Gerya, 2007). Numerical experiments simulating the heat the metasedimentary source at temperatures of the order of evolution of cold diapirs (e.g., Gerya and Stoeckhert, 2005)inthe 1000 to 1200 °C. Second, a closed system is needed to prevent mantle wedge show that the isotherms are folded around the that partial melts are segregated, in particular when they reach intruding diapir, denoting that the ascending rate of the relatively melt fractions of 30 to 40 vol.%. At these melt fractions the cold body is faster than thermal conduction rate in the mantle. rheological threshold is passed, the melt has continuity in the Temperatures in excess of 1000 °C are easily attained in the core system and the segregation is likely to occur (e.g. Sawyer, 1996). of the diapir at the final stages of its evolution (Gerya and However, the process invoked according to our experimental Stoeckhert, 2005), as well as strong thermal gradients are results requires that melt remains in the source until a high melt established from the core to the periphery of the diapir. This fraction of 70 to 90 vol.% is reached. It is well known that this is model also explains the location of sedimentary sources at large often not the case of anatectic areas in the continental crust, where depths (≈2 GPa), and the absence of a cortege of basic rocks C. Fernández et al. / Gondwana Research 14 (2008) 451–473 467 accompanying the ferrosilicic magmatism. In what follows, the the Proto-Tethys and Tornquist oceans were subducting along the cold-diapir model will be used with preference to describe the Gondwana margin, and a Cambrian–Early Ordovician magmatic suggested evolution of the continental margin of Gondwana from arc was active in the Avalonia and Cadomia terranes (Figs. 5 Iberia to La Puna during the Late Cambrian and Early Ordovician. and 15a). Proofs of this magmatic activity are presently exposed at the remnants of these terranes along the Appalachian and Variscan 4.2. Evolution of the Late Cambrian–Early Ordovician margin orogenic belts (Winchester and Van Staal, 1995; O'Brien et al., of Gondwana and generation of the ferrosilicic magmatism 1997; Abati et al., 1999). Behind the magmatic arc, extensional tectonics, rifting and seafloor spreading marked the back-arc Deposition of huge turbiditic successions at Iberia (Schist and region during the Late Cambrian–Early Ordovician, heralding the Greywacke complex) and Puna (Puncoviscana formation) opening of the Rheic ocean at the northern margin of Gondwana. finished the major Neoproterozoic changes that deeply affected Intense slab roll-back (Stampfli and Borel, 2002) and ridge the solid Earth, the biosphere and the climate (Stern, in press; subduction (Gutiérrez-Alonso et al., 2003) are some putative Meert and Lieberman, in press; Komiya et al., in press; Maruyama causes of this extensional episode in the back-arc region. and Santosh, 2008; Rino et al., 2008). Afterwards, an ancient “ring Extension due to slab roll-back is a well-documented mechanism of fire” encircled most of the continental margin of Gondwana in modern tectonic settings like the Andes (e.g. Ramos, 1999). during the Late Cambrian–Early Ordovician (Fig. 5), according to Contemporary volcanism at the extending margin of Gondwana plate-tectonic reconstructions based on geological (Stampfli and was characterised by the emission of ferrosilicic magmatism Borel, 2002; Gutiérrez-Alonso et al., 2003; Cawood, 2005; Chew giving place to the Ollo de Sapo formation (Iberia, Fig. 15 a, et al., 2007; Vaughan and Pankhurst, 2007) and paleomagnetic middle panels) and the Eastern Puna eruptive belt (Puna, information (Van der Voo, 1993; Torsvik, 1998; Pharaoh, 1999; Fig. 15 b). The rest of the Ordovician, the and Rapalini, 2005). Near the location of the Ordovician South Pole, pre-orogenic stratigraphic sequence overlying the Ollo de Sapo

Fig. 15. Schematic cross-sections of Iberia (a) and Puna between 22° S and 26° S (b) showing the proposed Early Palaeozoic tectonic evolution along the northern and western margins of Gondwana. The rectangles of the third-row panels indicate the enlarged area shown in the fourth-row panels. For location of cross-sections see Fig. 5.See text for further explanation. The subducting margin in the case of the Puna cross-section (b, upper panel) immediately followed a previous stage of . 468 C. Fernández et al. / Gondwana Research 14 (2008) 451–473 show sedimentary features corresponding to a siliciclastic Both, the magmatic arc (Western Puna eruptive belt) and the platform compatible with a passive continental margin back-arc basin with the ferrosilicic magmas (Eastern Puna (Fig. 15a, lower panel, left), as explained before. It must be eruptive belt) are presently adjacent to the Famatinian belt of stressed that in the model illustrated in Fig. 15, no causality exists South America, which recalls the Cambro-Ordovician paleogeo- between asthenospheric uplift, which is a consequence of back-arc graphy, only slightly modified by younger subduction orogenies basin opening, and wholesale melting of the subducted sediments (Fig. 15b). The evolution of the southwestern margin of within the cold diapir, which is due to thermal equilibration of the Gondwana during the Neoproterozoic–Early Palaeozoic has sedimentary plume with the mantle wedge. Nevertheless, the been interpreted considering allochthonous and paraauthochto- extensional lithospheric rupture favoured the rapid ascent of the nous terranes (Coira et al., 1982, Allmendinger et al., 1983; molten sediments towards Earth's surface. Aceñolaza and Toselli, 1984; Ramos et al., 1986; Dalziel and The geochemical and experimental constraints exposed in this Forsythe, 1985; Ramos, 1988; Bahlburg, 1990, Damm et al., paper strongly support that the ferrosilicic magmatism was derived 1990; Bahlburg and Breitkreuz, 1991; Rapela et al., 1992; Conti from a metagreywacke source or its granulite, residual or not, et al., 1996; Coira et al., 1999) or an authochtonous origin equivalent (charnockites), at temperatures higher than those (Lucassen et al., 2000; Do Campo and Guevara, 2005; Lucassen normally found in the continental crust. The information supplied and Franz, 2005; Miller and Sollner, 2005; Zimmermann, 2005). by the study of lower crust xenoliths in basalts and lamprophyres in Fig. 15b shows a schematic evolution of the Gondwana margin of Puna and Iberia, respectively, is of great value. In both regions, the the Central Andes. The Puncoviscana formation is interpreted as a felsic xenoliths represent a lower crust equilibrated at 1.0 GPa. In turbiditic sequence of a passive margin (Rapela et al., 1998; Do South America, this atypical lower crust is the dominant in large Campo and Guevara, 2005; Piñan-Llamas and Simpson, 2006) areas of the Andean basement (Lucassen et al., 1999). Interestingly, deposited during Neoproterozoic–Early Cambrian. After deposi- these two regions, Iberia and South America, are exceptional for the tion of the Puncoviscana formation, an eastward subduction of dominance of felsic granulites in the lower crust (Rudnick and Gao, oceanic crust started during the Early–Middle Cambrian 2003). Also, as it has been stated in this study, these two regions are (~530 Ma; Mulcahy et al., 2007)(Fig. 15b, upper panel), where exceptional for the presence of large volumes of ferrosilicic a quiescent period of ~80–70 Ma (520–440 Ma) generated a magmas developed during the Late Cambrian–Early Ordovician. widespread high temperature–low pressure metamorphism The slight differences found between the Neoproterozoic grey- (Lucassen et al., 2000; Lucassen and Becchio, 2003; Hongn wackes, the xenoliths and the ferrosilicic magmas in terms of and Riller, 2007). During Late Cambrian–Early Ordovician HREE may be informative about the role played by this atypical times, two subparallel magmatic belts were developed, i) the felsic lower crust in the generation of the ferrosilicic magmas. There Western Puna eruptive belt that is represented by calk-alcaline is a slight depletion in HREE in the Ollo de Sapo compared to the magmatism with arc-like petrologic, geochemical and isotopic lower crust xenoliths. These may represent low-melt fraction characteristics, ii) the Eastern Puna eruptive belt that is constituted residues developed in equilibrium with Grt (cf. Villaseca et al., by a widespread ferrosilicic magmatism associated with lesser 1999). This residual material may be either the source of the mafic magmatism emplaced in an extensional back-arc setting ferrosilicic magmas or the deep-seated equivalents of these that (Fig. 15b, middle panels). Alonso et al. (2007) have described an were previously depleted in an early stage of the melting process Ordovician extensional episode in the nearby Precordillera and that retain the residual solid material (e.g., Grt). In both cases, . Finally, at Devonian times, (ca. 400 Ma) both, the compositionally they are residual metagreywackes. The high Neoproterozoic and lower Palaeozoic units were exhumed due to temperatures required for the development of this ferrosilicic the Pampean and Famatinian orogenies (Fig. 15b, lower panel). magmatism are more suitable in the context of cold diapirs quickly Astini and Davila (2004) and Kleine et al. (2004), suggested that ascending through the mantle wedge. Therefore, the possibility the Famatinian system could be a southward continuation of the exists that this anomalous and large silicic crust was developed by Western Puna eruptive belt. According to these authors and based attachment to the lower crust of silicic diapirs coming from on petrological and geochemical data, the Famatinian System subducted materials. In this case, the silicic lower crust, represented could be correlated with the Western Puna eruptive belt (calc- by the felsic xenoliths in Iberia and Puna, would be the deep alkaline arc signature) and the Eastern Puna eruptive belt re- equivalent of the ferrosilicic magmas and it would have been presents a back-arc setting where ferrosilicic magmas were gene- developed during the Late Cambrian–Early Ordovician by rated. Recently, Chew et al. (2007) proposed that the Famatinian amalgamation of cold diapirs. Alternatively, and following the metamorphism and subduction-related magmatism were contin- most classical view, this lower crust can be the source of the uous from Patagonia (Pankhurst et al., 2006) through northern ferrosilicic magmas, produced for instance by metamorphism and Argentina and Chile to as far north as Colombia and Venezuela, a slight depletion of a greywacke protolith, that underwent near-total distance of nearly 7000 km. Successive episodes of subduction melting during Late Cambrian–Early Ordovicianbylithosphere and dominated the southwestern margin of Gondwana underplating. In both cases, cold diapirs and underplating, the during the Early Palaeozoic (Vaughan and Pankhurst, 2007). process took place in an extensional regime. With independence of Distinctly from the Famatinian belt, where the Cambro- the genetic model, the relation between ferrosilicic magmas, Ordovician subduction orogen is well preserved, the geological metagreywackes and the felsic lower crust appears evident from history at the European margin of Gondwana included the this study. These areas may be, consequently, of great interest to test opening of the Rheic ocean separating the magmatic arc from the these two contrasted models. passive margin of Iberia (compare Fig. 15aandb).Afterward,the C. Fernández et al. / Gondwana Research 14 (2008) 451–473 469

Variscan orogeny tectonically disrupted and juxtaposed these Victor A. Ramos, Shoji Arai and an anonymous reviewer. We terranes (Fig. 15a, lower panel, right). However, geological, are grateful to M. Santosh for his editorial assistance. geochemical and geochronological data guarantee the correlation of this zone with the Famatinian orogen. The result of this References correlation shows a huge subduction orogen whose unusual geochemical imprint (ferrosilic magmatism) needs a particular Aceñolaza, G.F., Tortello, M.F., 2003. El Alisal: a new locality with trace fossils petrogenetic model to be explained. of the Puncoviscana formation (late –early Cambrian) in , Argentina. Geologica Acta 1, 95–102. 5. Conclusions Aceñolaza, F.G., Toselli, A., 1984. Lower Ordovician volcanism in North West Argentina. In: Bruton, D.L. (Ed.), Aspects of the Ordovician System. Paleontological Contributions of the University of Oslo, vol. 295, pp. 203–209. Large volumes of Late Cambrian–Early Ordovician ferrosi- Abati, J., Dunning, G.R., Arenas, R., Díaz García, F., González Cuadra, P., licic magmatic rocks appear at the West Gondwana margin. Martínez Catalán, J.R., Andonaegui, P., 1999. Early Ordovician orogenic Ferrosilicic magmatism is characterised by high iron content event in Galicia (NW Spain): evidence from U–Pb ages in the uppermost unit – (FeON4.0 wt.%) and low contents in calcium (CaOb1.5 wt.%). of the Órdenes Complex. Earth and Planetary Science Letters 165, 213 228. Adams, C., Miller, H., 2007. Detrital zircon ages of the Puncoviscana formation of The previous discussion about the geological context of the NW Argentina, and their bearing on stratigraphic age and provenance. Late Cambrian–Early Ordovician ferrosilic magmatism of the Abstracts 20th Colloquium on Latin American Earth Sciences, Kiel, pp. 68–69. West Gondwana margin is pointing to a common origin for Adams, C., Miller, H., Toselli, J., 1990. Nuevas edades de metamorfismo por el these rocks, with strong petrogenetic and tectonic implications. método K/Ar de la Formación Puncoviscana y equivalentes. Serie de – These magmatic rocks show geochemical signatures that depart Correlación Geológica 4, 209 219. Allmendinger, R., Ramos, V., Jordan, T., Palma, M., Isacks, B., 1983. from those explained by current models of magma generation Paleogeography and Andean structural geometry, Northwest Argentina. at active plate margins and intracontinental rifting, including Tectonics 2 (1), 1–16. normal fractionation of calc-alkaline magmas and partial melt- Alonso, J.L., Gallastegui, J., García-Sansegundo, J., Farias, P., Rodríguez- ing of common protoliths as sediments, calc-alkaline igneous Fernández, L.R., Ramos, V.A., 2007. Extensional tectonics and gravitational rocks or MORB-derived metabasites involved in convergent collapse in an Ordovician passive margin: the Western Argentine Precor- dillera. Gondwana Research 13 (2), 204–215. doi:10.1016/j.gr.2007.05.014. active margins. Geological, geochemical and experimental data Álvarez Nava, H., García Casquero, J.L., Gil Toja, A., Hernández Urroz, J., Lorenzo show that generation of ferrosilicic magmas took place by near- Álvarez, S., López Díaz, F., Mira López, M., Monteserín, V.,Nozal, F., Pardo, M.V., total melting (80–90%) of crustal sources of metagreywacke Picart, J., Robles, R., Santamaría, J., Sole, F.J., 1988. Unidades litoestrati- or charnockite affinities, subjected to very high temperatures gráficas de los materiales precámbricos-cámbricos en la mitad suroriental de la – (1000 °C–1200 °C) and at pressures of 1.0 to 2.0 Gpa. Fur- Zona Centro-Ibérica. II Congreso Geológico de España 1, 19 22. Asimow, P.D., Ghiorso, M.S., 1998. Algorithmic modifications extending thermore, this particular magmatism, far from being local, MELTS to calculate subsolidus phase relations. 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Science and Education and partially from Projects: PIP- Bea, F., Montero, P., González-Lodeiro, F., Talavera, C., 2007. Zircon N°6103 CONICET, grant N°1350/1 CIUNSA and PICT inheritance reveals exceptionally fast crustal magma generation processes in Central Iberia during the Cambro-Ordovician. Journal of Petrology 48, N°07-38131 ANPCyT. We thank J.M. Ugidos (Salamanca) 2327–2339. for comments on the geochemistry of Iberian Neoproterozoic Becchio, R., Lucassen, F., Kasemann, S., Franz, G., Viramonte, J.G., 1999. turbidites. This paper benefited from thoughtful reviews by Geoquímica y sistematica isotópica de rocas metamórficas del Paleozoico 470 C. Fernández et al. / Gondwana Research 14 (2008) 451–473

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