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Massive generation of atypical ferrosilicic magmas along the Gondwanaactive margin: Implications for cold plumes and back-arc magma generation

C. Fernndez a,, R. Becchio b, A. Castro c, J.M. Viramonte b, I. Moreno-Ventas c, L.G. Corretg d

aDepartamento de Geodinmica y Paleontologa, Universidad de Huelva, Campus del Carmen, 21071 Huelva, Spainb CONICET, Instituto Geonorte, Facultad de Ciencias Naturales, Universidad Nacional de Salta, Buenos Aires 177, 4400-Salta, Argentina

c Departamento de Geologa, Universidad de Huelva, Campus del Carmen, 21071 Huelva, Spaind Departamento de Geologa, Universidad de Oviedo, Arias de Velasco s/n Oviedo, Spain

Received 22 December 2007; received in revised form 1 April 2008; accepted 7 April 2008Available 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 ofLate CambrianEarly Ordovician 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 thatferrosilicic magmas resulted from near-total melting (8090%) of crustal sources of metagreywacke and charnockite affinities, possibly derivedfrom Neoproterozoic volcanoclastic sediments and/or their granulite facies equivalents, under very high temperatures (1000 C1200 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 theferrosilicic 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 CambrianEarly Ordovician; Iberia; Puna

1. Introduction

It is broadly documented that an active margin at theGondwana supercontinent resulted in large-scale crustal rework-ing and net addition of continental crust during Late Cambrian toEarly Ordovician times (510460 Ma) (Bock et al., 2000;Lucassenet al.,2000; Ramos and Aleman, 2000; Zimmermanand

Bahlburg, 2003; Hongn and Riller, 2007). Geochemical featuresand radiometric age determinations of magmatic rocks, togetherwith detailed structural studies, are used in combination toidentifynew terrainsas derivedfrommagmaticactivity associatedwith a complex active margin along a large part of Gondwana. Inthe Variscan belt, these terranes represent in part ancientintraoceanic arcs and microcontinents that were separated fromGondwana during Late Cambrian to Early Ordovician times andattached again to its margin during Late Palaeozoic times

(Martnez Cataln et al., 1997; Matte, 2001; Pin et al., 2006).Among the most important igneous formations related tomagmatic activityat the Gondwanamargin during LateCambrianto Early Ordovician times, is a thick sequence of silicic magmaticrocks, widely represented in the Iberian peninsula by the so-calledOllo de Sapo sequence (Hernndez Sampelayo, 1922; PargaPondal et al., 1964; Martnez Cataln et al., 2004), and in the

South American continent by the Famatinian

Eastern Punamagmatic eruptive belts (Pankhurst et al 1998; Saavedra et al.,1998; Coira et al., 1999; Hongn andRiller, 2007; Viramonte et al.,2007). In both cases the magmatic sequence is a several km thickassociation of silicic rock of igneous origin in which plutonic,subvolcanic and eruptive facies can be identified. A magmatic

provenance is strike forward in the Puna eruptive belt, where noorogenic event has substantially modified the original relations ofthe magmatic rocks. In Iberia, these rocks were severely affected

by deformation and metamorphism during the Variscan orogenyand the textural relations were largely obliterated. The identity ingeochemical features and age of these igneous sequences, the

Available online at www.sciencedirect.com

Gondwana Research 14 (2008) 451473www.elsevier.com/locate/gr

Corresponding author. Fax: +34 959219440.E-mail address:[email protected](C. Fernndez).

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
mailto:[email protected]://dx.doi.org/10.1016/j.gr.2008.04.001http://dx.doi.org/10.1016/j.gr.2008.04.001mailto:[email protected]


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Eastern Puna eruptive belt in South America and the Ollo de Sapounit in Iberia, as well as the position of both zones at theGondwana continental margin indicate that they formed part ofthe same magmatic event. In South America, the Famatina belt, amagmatic lineament of more than 2000 km (Chew et al., 2007),represents a typical arc with calc-alkaline magmas and the

development of large batholiths (Pankhurst et al., 1998; Saavedraet al., 1998; Dahlquist et al., 2005). In Iberia, evidences of arcmagmatism for Early Ordovician times are found in the Ordenescomplex (Abati et al., 1999). Therefore, it has been proposed thatthese magmatic rocks were formed in relation to back-arcspreading associated with an active subduction (Coiraetal.,1999)identified by arc magmatism. In both cases, Iberia and SouthAmerica, the magmatic rocks of arc affinities and calc-alkaline

batholiths related to the active margin of Gondwana during

Cambrian to Ordovician times, occupied positions more externalthan the silicic magmatic belts (Ollo de Sapo and Eastern Punaeruptive belt). Consequently, the formation in a back-arc tectoniccontext seems to be the more plausible scenario for this silicicmagmatism.

Textural relations have been modified for the Ollo de Sapo

rocks. However, in the Eastern Puna eruptive belt the originalvolcanic provenance and derivation from silicic melts areevident. Given their atypical composition, the fact that thesesilicic magmas are derived by crystallisation from a melt sets animportant petrogenetic problem. Conditions for the generationof these atypical silicic magmas are discussed here. Theirimplications are considered in a plate-tectonic scenario relatingFe-rich, silicic magma production with active subduction and

back-arc spreading. In the first part of the paper we show the

Fig. 1. Geological map of northwest Iberia (modified fromRodrguez-Fernndez, 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 onFarias et al., 1987) and its location in the realmof the Variscan orogen during the Late Palaeozoic (left, modified fromMartnez Cataln et al., 1997).

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main geological and geochemical features of this particular Fe-rich, silicic magmatism, that we callferrosilicicin a broad sense,making emphasis in the similarities between igneous formationsseparated by thousands of km from South America to Iberia.Second, we analyse the petrogenesis of these magmatic rocks bymeans of their geochemical signatures and melting experiments.

PT conditions for magma generation will be constrasted incurrently accepted tectonic models based on geological andstructural relations (Coira, et al 1999; Lucassen et al. 2000;Stampfli and Borel, 2002; Gutirrez-Alonso et al., 2003; vonRaumer et al., 2003; Kleine et al., 2004; Kirschbaum et al., 2006;Hongn and Riller, 2007; Viramonte et al., 2007). We showfinally the implications that this ferrosilicic magmatism mayhave in understanding the thermal and petrogenetic processesrelated to subduction of crustal material and the formation ofcold plumes in the mantle wedge (Gerya and Yuen, 2003) andtheir magmatic consequences (Castro and Gerya, 2007).

2. General features of the Cambro-Ordovician ferrosilicicmagmatism in the Iberian Peninsula (Ollo de Sapo) and

South America (Puna eruptive belt)

2.1. Geological features of the Ollo de Sapo

The Ollo de Sapo formationis a complex and characteristic unitof theVariscan Iberian massif (HernndezSampelayo, 1922;PargaPondal et al., 1964). The exposure of the Ollo de Sapo formationdescribes a huge anticlynorium that outlines the Ibero-Armoricanarc at the northwestern Iberian massif (Fig. 1). This unit is mostlycomprised of gneissose rocks showing the appearance of augengneisses, typically showing blue quartz crystals, large (up to

15 cm) K-feldspar megacrysts (Fig. 2a and b), and Na-richplagioclase crystals.Fine-grained facies are commonlyobserved atthe top of the unit, locally interspersed with quartzites and slates.Variscan, low- to high-grade metamorphism and several deforma-tion phases affected the Ollo de Sapo formation (Martnez Catalnet al., 2004). Low-grade, scarcely deformed gneisses show

euhedral K-feldspar and plagioclase crystals, embayed, roundedquartz crystals and biotite clots within a recrystallised but fine-grained matrix with quartz, plagioclase, biotite, muscovite and K-feldspar. Eutaxitic textures with rare vesicles have been locallydescribed (Navidad et al., 1992), as well as glass shards and lithicfragments (Dez Montes et al., 2004; Dez Montes, 2007). The K-feldspar megacrysts often show albitic rims (rapakiwi texture).These textural features strongly point to a volcanic and volca-noclastic origin of the Ollo de Sapo formation (Parga Pondal et al.,1964; Navidad, 1978a;Ortegaet al., 1996). Accordingto this, mostof the Ollo de Sapo formation has been interpreted as due to thedeformation and metamorphism of welded ignimbrites, rhyolites

and volcanic tuffs, subvolcanic facies, and related volcanoclasticand volcano-sedimentary series. The Ollo de Sapo formationstratigraphically overlies the Neoproterozoic Schist and Grey-wacke complex or the Cambrian metasediments, and it underliesthe slates and quartzites of the Early Ordovician (Fig. 3).Radiometric absolute dating yielded ages ranging from 468 to495 Ma (Gebauer, 1993; Valverde-Vaquero and Dunning, 2000;Dez Montes, 2007; Montero et al., 2007; Bea et al., 2007).

2.2. Geological features of the Eastern Puna eruptive belt

In northwestern Argentina, the Cambro-Ordovician magma-tism (Famatinian Cycle; e.g.,Rapela et al., 1992) is dominantly

Fig. 2. Field photographs of the Late CambrianEarly Ordovician magmatic rocks. Ollo de Sapo (Iberian massif): (a) coarse-grained facies in an area of metamorphiclow-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-felsparcrystals; (d) typical undeformed coarse-grained volcanic facies with abundant blue quartz crystals and K-feldspar megacrysts with rapakiwi texture.

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silicic with mafic rocks comprising less than 1 vol.%. Thismagmatism is represented by two parallel, NS-trending belts;the western belt is known as the Western Puna eruptive belt(Palma et al., 1986) and the eastern one as the Eastern Punaeruptive belt (Mndez et al., 1973). The first is mainlyconstituted by granitoids and volcano-sedimentary successions

cropping out discontinuously from northeastern Chile to thenorthwest of La Rioja (Fig. 4). The Eastern Puna eruptive belt isrepresented by a ca. 600 km, NS trending belt from near 17 Sin Bolivia to near 27 S in Catamarca province (eastern mag-matic belt inFig. 4). This belt can be divided into two sectors bythe CalamaOlacapatoToro lineament (Fig. 4). The northernone (2224 S) is characterised by a dominant lavic and sub-volcanic bimodal volcanism associated with sedimentarysequences (Coira et al., 1999). Minor bodies of granitoids arealso observed in this sector (Coira et al., 1999; Kirschbaumet al., 2006). Large volumes of plutonic rocks and volcano-sedimentary sequences characterise the southern sector (24

27 S) of the Eastern Puna eruptive belt (Viramonte et al.,2007). Locally, these rocks are deformed (Fig. 2c) and affectedby medium-grade metamorphism.

Fig. 3. Synthetic columns of the Palaeozoic units of the Central Iberian Zone

including the fine- and coarse-grained gneisses of the Ollo de Sapo formationand related granitic gneisses (simplified from Dez Montes et al., 2004). SeeFig. 1for location of Hiendelaencina and Sanabria.

Fig. 4. Simplified geological map of northwestern Argentina showing the distribution of the Palaeozoic magmatic and metamorphic rocks, the Puncoviscanaformation, and the principal Palaeozoic mountain ranges.



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The features of the silicic volcanic rocks of the Eastern Punaeruptive belt are strikingly similar to those of the Ollo de Sapoformation. They are composed of alkali feldspar, plagioclase(An10), and quartz phenocrysts, the latter with commoncorrosion bays, included in recrystallised, fine-grained matrixof quartz, alkali feldspar, biotite, muscovite, and sericite. Blue

quartz crystals and K-feldspar megacrysts (up to 10 cm inlength) are distinctive features (Fig. 2d). K-feldspar megacrystsare euhedral with common inclusions of biotite and quartz.Most of these megacrysts show perthitic, graphic and rapakiwi-like textures (Fig. 2d). To the South of 26 S, the Eastern Punaeruptive belt overlies a medium- to high-grade metamorphic

basement with Sm/Nd and UPb metamorphic ages of 515500 Ma (Becchio et al., 1999; Lucassen et al., 2000; Lucassenand Becchio, 2003). Radiometric absolute dating of the rocks ofthe Eastern Puna eruptive belt yielded ages ranging from 460 to490 Ma (Omarini et al., 1984; Lork and Bahlburg, 1993;Viramonte et al., 2007).

2.3. PunaIberia correlation during Late Cambrian and Early

Ordovician: Plate reconstructions

Recent plate-tectonic reconstructions of Gondwana duringthe Late Cambrian and Early Ordovician (e.g., Stampfli andBorel, 2002; von Raumer et al., 2003) locate this continent atthe Southern Hemisphere, extending from the Equator to theSouth Pole (Fig. 5). The proto-Tethys ocean subducted beneaththe Western Gondwana margin along most of the present-day

northern Africa and western South America. The Famatinianbelt of South America (including the Puna region) was a typicalEarly-Palaeozoic subduction orogen. In the northern Africa andEuropean counterparts (including Iberia), a magmatic arc wasdeveloped at the Avalonia and Cadomia terranes that weredisrupted from Gondwana after the inception of the Rheic ocean

as a back-arc basin. It is important to realise that the Famatinianand the AfricanEuropean margins of Gondwana probably didnot constitute a continuous subduction belt as shown in theidealized sketch ofFig. 5. Recent studies of the Variscan orogenin southern Mexico suggest that a transform boundary separatedOaxaquia and Avalonia during the CambrianEarly Ordovician,therefore connecting the European margin of Gondwana withthe South American Famatinian belt (e.g., Keppie et al., in

press). In any case, plate reconstruction indicates that Puna andIberia share a similar tectonic setting at the western margin ofGondwana during Cambrian and Early Ordovician times.

2.4. Common geochemical features for Iberia and Puna

Although the magmatic provenance of the Ollo de Saporocks is well documented, the correlation with the non-modifiedvolcanic rocks of the Eastern Puna eruptive belt makes thisinterpretation more solid and raises an interesting problem onthe petrogenesis of these unusual magmatic rocks. The maingeochemical distinctive feature with respect to normal silicicmelts is the high iron content (FeO N4.0 wt.%) for very lowcontents in calcium (CaOb1.5 wt.%). That is, these silicic melts

Fig. 5. Late CambrianEarly 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 fromAstini (1998),Pankhurst et al. (1998),Rapela et al. (1998),Stampfli and Borel(2002),von Raumer et al. (2003),Gutirrez-Alonso et al. (2003),Cawood (2005),Rapalini (2005),Chew et al. (2007).



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are extremely rich in iron compared to calcium and for thisreason we have used the term ferrosilicic to refer these rocks inthis study. They are richer in Fe than normal rhyolites and

poorer in Ca than normal dacites. They have marked chemicalaffinities with charnockite rocks, and are also close to felsicgranulite xenoliths, sampled by basalts and lamprophyres, and

widely represented in the studied regions, Iberia and Puna(Lucassen et al., 1999; Villaseca et al., 1999).Table 1shows theaverage compositions for major elements of Cambro-Ordovi-cian ferrosilicic magmatic rocks from Iberia and Argentina,together with average compositions of regionally related

Neoproterozoic turbidites and felsic granulite xenoliths.The comparison between the chemistry of the ferrosilicic

magmatic rocks in Iberia and Puna yields a very closesimilarity inmajor and trace elements. The most outstanding geochemicalfeatures that define this atypical magmatism are: (1) The highsilica content, typical of silicic magmas of dacite to rhyolitecomposition. (2) The slightly peraluminous character with values

of alumina saturation index, ASIN1.2. Ollo de Sapo rocks haveaverage ASI values slightly higher than those of the Puna eruptivebelt (Table 1) due to the Ca depletion observed in Ollo de Sapocompared to its American equivalent. (3) The high contents in Feand Mg, more than twice the normal values of silicic magmas,namely calc-alkaline dacites and rhyolites. (4) The very lowcontents in Ca, less than half the magmatic rocks with equivalentFe and Mg contents. (5) The high alkali contents, particularly K.Minor geochemical differences can be observed between the twostudied domains. Interestingly, these differences mirror similardifferences between the corresponding Neoproterozoic turbiditicformations, a feature that will be addressed below.

The compositional particularity of these magmatic rocks canbe seen in the multicomponental diagrams ofFig. 6. In the AB

diagram (Debon and Le Fort, 1983, 1988), the ferrosilicicmagmatic rocks of Eastern Puna eruptive belt and Ollo de Sapo

plot in the peraluminous field (AN0) and with values of B(ferromagnesian components) between 50 and 150, very close tothe field of turbiditic sediments and felsic xenoliths of charnockiteaffinity, and not related to typical calc-alkaline fractionation

trends. Similar relations are observed at the KB diagram.There are intrusive granitoid bodies, associated with the Ollo

de Sapo volcanic formation, with an age close to that of thehosting volcanic rocks (Montero et al., 2007). So, these are partof a common magmatic event. One of these granites is theAntoita gneiss (Navidad and Peinado, 1981) related to the Ollode Sapo unit in the Hiendelaencina area (Fig. 1). This granitealso has an atypical composition compared with other

peraluminous granites derived by partial melting of metagrey-wackes. This granite is plotted in the diagrams (AB and KB)ofFig. 6 in an intermediate position between the ferrosilicicmelts (Ollo de Sapo and volcanic rocks of the Eastern Puna

eruptive belt) and the partial melts obtained experimentally atlow-melt fraction from metagreywackes (Montel and Vielzeuf,1997; Patio Douce and Beard, 1996; Castro et al., 1999).

3. Petrogenetic constraints on ferrosilicic magma generation

The petrogenetic interest of this Cambro-Ordovician ferrosi-licic magmatism, observed along the Gondwana margin forthousandsof km,lies on its rarity. In particular, the identification ofthesourceandthe conditions for melting are key features that mayhave implications on large-scale tectonic processes. The slightly

peraluminous character is among the most relevant geochemicalfeatures. This feature may be used to rule out any relation withalkaline or metaluminous magmatism (A-type). This is

Table 1Average 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 xenolithsIberia (4)

Felsic xenolithsPuna (5)

Average(n =95)

S.D. Average(n =59)

S.D. Average(n =68)

S.D. Average(n =13)

S.D. Average(n =13)

S.D. Average(n = 3)

S.D.

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 2.76TiO2 0.55 0.18 0.60 0.21 0.84 0.23 0.71 0.11 0.87 0.99 0.2 1.01 0.16Al2O3 15.79 1.21 14.64 0.85 17.64 3.79 13.22 2.70 16.39 16.57 1.15 15.34 0.29FeOt 3.77 1.03 3.84 1.05 6.08 1.57 4.54 1.03 5.74 7.08 1.39 7.35 1.27MgO 1.56 0.57 1.62 0.55 2.12 0.62 1.93 0.43 2.17 3.71 0.96 4.64 0.75MnO 0.04 0.02 0.07 0.02 0.04 0.03 0.08 0.02 0.04 0.1 0.03 0.12 0.02CaO 1.19 0.51 1.63 0.84 0.27 0.23 0.93 0.43 0.48 1.59 0.62 1.80 0.73Na2O 2.86 0.58 2.81 0.50 1.55 0.77 2.38 0.46 2.24 2.63 0.72 3.17 1.11K2O 4.18 0.73 4.02 0.86 3.43 1.12 2.86 1.07 2.77 3.35 1.1 1.00 0.41P2O5 0.17 0.07 0.19 0.06 0.15 0.09 0.17 0.05 0.22 0.14 0.07 0.05 0.02L.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 0.85Total 99.80 98.88 98.03 96.89 99.75 99.95 98.87Mg# 0.41 0.42 0.38 0.43 0.40 0.48 0.53TA 7.04 6.82 4.98 5.23 5.01 5.98 4.17A/(CNK) 1.41 1.25 2.79 1.54 2.17 1.53 1.60

B 93 96 139 114 137 105 128.67A 86 53 213 88 173 112 113.14K/K+Ca 0.81 0.75 0.94 0.78 0.87 0.71 0.40Fe/Fe+Ca 0.72 0.66 0.95 0.79 0.90 0.78 0.76

(1), (2) According to data compilation inFig. 6, (3) Greywacke fromUgidos (1997a). (4) Data fromVillaseca et al. (1999). (5) Data fromLucassen et al. (1999);nisnumber of samples and S.D. the standard deviation; SGC is the Schist and Greywacke complex. A/(CNK): Alumina saturation index (ASI). A, B: seeFig. 6.



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independent of the extensional, anorogenic or not, tectonicenvironment in which the Cambro-Ordovician magmatism wasdeveloped. However, some kind of relation to charnockite rocks,

characterised by a metaluminous to slightly peraluminouscharacter (Frost and Frost, 2007) cannot be discarded (seediscussion below).

Fractionation from a calc-alkaline trend and melting from acrustal source of appropriate composition are the two plausiblehypotheses that will be considered here.

3.1. Hypothesis 1: Fractionation from a calc-alkaline trend

It is apparent from the major elementchemistry of the Cambro-Ordovician ferrosilicic magmatism that derivation from a calc-alkaline trend is unlikely. This is mainly based on the low Cacontents for high Fe and Mg contents. Magmatic fractionationimplies that Ca and Mg+Fe are jointly fractionated by cotectic

separation of pyroxene/amphibole plus plagioclase from silicatemagma. This is clearly observed in the AB diagram ofFig. 6.However, the nature of the fractionating phases is determined byintensive 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(Ghiorsoand Sack, 1995; Asimow andGhiorso, 1998)allowustosimulate any fractionation trend. Fig. 7 shows two theoreticaltrends calculated by equilibrium crystallisation of Hb-diorite ofcalc-alkaline composition from the Gredos batholith (Moreno-Ventas et al., 1995) with an initial water content of 6.0 wt.%. CaOand MgO contents decrease with crystallisation at 1.0 GPa. At2.0 GPa, CaO remains constant at high values (N6 wt.%) until theend of crystallisation. Even at 1.0 GPa the CaO content is 2.5times higher than the maximum values of the ferrosilicic rocks.Also the natural trends displayed by calc-alkaline batholiths plotat 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 togenerate ferrosilic magmas.

Fig. 6. Multicationic diagrams showing the relations between the Late CambrianEarly Ordovician ferrosilicic magmatic rocks of Iberia (Ollo de Sapo) and SouthAmerica (Eastern Puna eruptive belt). These volcanic rocks plot in the field of

Neoproterozoic sediments and out of the normal calc-alkalinetrends. Experimentalpartial melts from metasediments and leucogranites are also plotted forcomparison. (a) AB diagram byDebon and Le Fort (1983, 1988). (b) KBdiagram.Data sources for Iberia: Garca de Figuerola(1966), Capdevila (1969), GilIbarguchi (1978),Navidad (1978b),Holtz (1987),Beetsma (1995),Briggs (1995),Ugidos et al. (1997a,b, 2001),Valladares et al. (2000),Castro et al. (2000, 2003),Corretg et al. (2001), Beaet al.(2003). Data sources forPuna: Damm et al.(1990),Coira et al. (1999), Lucassen et al. (2001), Do Campo and Guevara (2005),Zimmermann (2005),Kirschbaum et al. (2006),Viramonte et al. (2007).

Fig. 7. MgO vs. CaO (wt.% oxides) diagram plotting the Ordovician ferrosilicicmagmatic rocks of Puna and Iberia together with experimental melts obtained fromnear-total melting of Neoproterozoic turbidites from Iberia (Castro et al., submittedfor publication) attemperatures from1000 to1200 Cand 1.0 to2.0 GPa.Note thatexperimental melts at 1000 C plot in the area of leucogranites. Some of the mostgranitic facies of the Ollo de Sapo (e.g. the Antoita gneiss) plot in this area. Alsotraced are the fields of Neoproterozoic turbidites from both geological domains: thePuncoviscana formation in Argentina and the Schist and Greywacke complex inIberia. These are very close in composition, the Schist and Greywacke complexbeing poorer in Ca comparedwith the Argentinian equivalent. Ugidos et al. (1997a,b) reportedthe low contentin Ca of the IberianNeoproterozoicturbidites as a uniquefeature of these metasedimentary rocks. This feature is transmitted to the magmasderived by near-total melting and,consequently, the ferrosilicic magmas of the OllodeSapo areslightly poorer in Cacompared with the Puna eruptivebeltin Argentina,in parallel with the slightly richer Ca content of the Puncoviscana source rocks. Itcan be noted that the compositions of the ferrosilicic magmas plot outside the calc-alkaline trends depicted here by the South Patagonian batholith in Chile (Hervet al., 2007) andthe Gredos batholithin CentralSpain (Moreno-Ventas et al.,1995).Fractionation trends from a water-rich basic magma (6 wt.% water) at 1.0 and2.0 GPa are traced according to model predictions by MELTS algorithm. These areveryfar fromthe field of theferrosilicic magmas. These cannot be derivedby neitherpartial melting from a calc-alkaline igneous source (tonalite) as analysed below inFig. 8, nor by any kind of fractionation process related to a calc-alkaline magmaticseries. The field of the lower crust xenoliths (felsic) from the Central System inIberia (Villaseca et al.,1999) and from the Puna region (Lucassen et al.,1999)isalso

traced in thisdiagram. These are enrichedin Mg withrespect to the metagreywackesindicating the residualcharacter of the lowercrustin thearea (Villaseca et al. 1999).



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3.2. Hypothesis 2: Melting from a crustal source: Igneous

(tonalite) vs. sedimentary (greywacke) source composition

Partial melting of a calc-alkaline igneous source is ofteninvoked as a mechanismable to producesilica-rich melts of graniteto granodiorite composition. For instance,Patio Douce (1997)

produced experimentally metaluminous A-type granites by partialmelting of calc-alkaline sources. The possibility for producing theferrosilicic magmas at conditions different from those studied byPatio Douce (1997)must be taken into account. On the otherhand, the behaviour of metasedimentary sources under partialmelting is very well known as it has been reported in severalrelevant experimental studies (e.g.Vielzeuf and Holloway, 1988;Vielzeuf and Clemens, 1992; Patio Douce and Beard, 1996;Montel and Vielzeuf, 1997). However, all these studies were

performed at conditions constrained by natural observation inmigmatites, that is, at crustal pressures and temperatures up to1000 C. In all these experiments, the resulting melts have the

composition of peraluminous leucogranite, with low contents inFe,Mg and Ca. This is so even for moderate meltfractions of about50 vol.%, obtained in experiments at 1000 C. The reason is thatgreywackes have a high proportionof minimum melt composition,and melts are controlled by the system minimum until theircomponents are exhausted in the adequate proportions. Experi-mental results indicate that it is necessary to increase the meltfraction at values of more than 50 vol.% to produce compositionsthat leave the system minimum and, consequently, are enriched inFe and Mg. In terms of phase relations, this is equivalent to saythattemperatures higher than the system minimum are required for Feand Mg to be dissolved in the melt. However, not only theabundances of Fe and Mg are increased in the melt with increasing

temperature, Ca is also one of the non-minimum melt elements(minimum melt elements are Si, Al, Na, K) that will be enriched inthe melt with temperature. If available in the source, Ca will beincorporated to the melt together with Fe and Mg. It is interestingto note that the Cambro-Ordovician ferrosilicic magmas arecharacterised by very low Ca contents (CaOb1.5 wt.%), thisfeature being distinctive for this atypical magmatism. Theexplanation is that the source is equally poor in Ca. Greywackesare the only geochemical reservoir that undergone an enrichmentin Fe and Mg, bounded in clay minerals, and a depletion in Ca atthe same time (Taylor and McLennan, 1985), resulting from aweathering fractionation process in which Fe and Mg are

concentrated and Ca lixiviated from the geochemical reservoir.Calc-alkaline igneous protoliths can be ruled out because theydo not match the requirement of high Fe and Mg and low Ca. Low-melt fractions from calc-alkaline tonalites give melts close to theminimum granite melt. They can be more or less rich in Kdepending on the presence or not of free water in the meltingreaction (El-Biad, 2000) at moderate temperatures (b800 C).However, these melts become richer in Ca, together with Fe andMg, for slightly higher temperatures of about 950 C (El-Biad,2000). It is expected that the three non-minimum elements, Ca, Feand Mg, will be dissolved in the melt at higher temperaturesaccording to thermodynamic model predictions (Fig.8). Therefore,it follows that a Ca-rich, calc-alkaline protolith cannot producemelt with the composition of the ferrosilicic rocks studied here.

Greywackes and their high-grade granulite equivalents are theonly plausible alternative protolith due to their peculiar composi-tion. Lower crust felsic xenoliths, hosted basalts and lampro-

phyres, have been reported in central Iberia (Villaseca et al., 1999)and La Puna (Lucassen et al., 1999). These have charnockiticaffinities and are very close in composition to metagreywackes or

low-melt fraction residues (Fig. 6). According toVillaseca et al.(1999) these xenoliths may represent the average lower crustcomposition in Iberia, and they may be the high-pressureequivalent of Neoproterozoic metagreywackes involved in thegeneration of the Cambro-Ordovician ferrosilicic magmas. Arecent study of zirconsin felsic xenoliths from the SpanishCentralSystem (Fernndez-Surez et al., 2006) revealed some inheritedages coincident with the age of the Neoproterozoic sediments andalso with the age of the Ordovician ferrosilicic magmas. Theselithologies, greywackes, their low-melt fraction residues and thelower crust charnockitic xenoliths, have very close geochemicalsignatures (Table 1) and are potential sources for the Cambro-

Ordovician ferrosilicic magmatism. We refer these collectively asmetagreywackes in this paper. These metagreywackes havea highproportion of the minimum haplogranite melt with the conse-quence that moderate melt fractions of about 50 vol.% produceleucogranitecompositions.Melt fractions higher than 50 vol.% arerequired to produce compositions that depart from the minimumand are enriched in Fe and Mg. This enrichment is effective onceallthe available Ca is exhausted and the melt evolves to an atypicalcomposition rich in Fe and Mg. This can only happen attemperatures higher that 1000 C (Fig. 8). In both areas of theGondwana margin, Puna in Argentina and Iberia in Europe, thereis a thick sequence of turbiditic sediments of Neoproterozoic age,namely the Schist and Greywacke complex in Iberia and the

Puncoviscana formation in Argentina, that can be considered asthe most plausible source areas for the Lower Ordovicianmagmatism studied here. These turbiditic rocks are consideredhere as a geochemical reservoir with independence of themetamorphic grade.

3.3. Greywackes and equivalent reservoirs (charnockites) as

the source of ferrosilicic magmas: Geochemical constraints

The coincidence in composition of the Late CambrianEarlyOrdovician ferrosilicic magmas (Ollo de Sapo and Eastern Punaeruptive belt) and the Neoproterozoic turbiditic sedimentary

sequences that underlie these magmatic rocks, strongly suggestthat this particular magmatism, that cannot be related to magmaticfractionation nor partial melting processes, may be derived bynear-total melting of a crustal source of metagreywacke and/orcharnockite-like composition. As explained above, near-totalmelting at ultra-high temperature is required to account for thehigh Fe and Mg contents dissolved in a silicate melt, unless theferrosilicic magma represents a restite-melt system developed atmoderate temperatures within the range 750850 C (e.g.Castroet al., 2000). Melts fractions of the order of 0.1 to 0.2 are obtainedfrom similar protoliths for the latter temperature range and at

pressures of the lower to medium crust (e.g.Patio Douce andBeard, 1996; Montel and Vielzeuf, 1997; Castro et al. 1999). Theabsence of restitic material in these ferrosilicic magmatic rocks



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and the high viscosity of a melt-restite system with only 20 vol.%melt (below the rheological threshold ofFernandez and Barbarin,1990), make this interpretation very unlike. The most plausiblehypothesis is that theferrosilicicmelts werenear-liquid systemsinwhich most of the Fe and Mg were dissolved in the melt. Meltfractions of about 0.8 to 0.9 are required and temperaturesexceeding 1000C according to our previousexperimentalresults(see below). Consequently, we have explored the hypothesis ofnear-total melting of metagreywacke as the mechanism to pro-ducethe atypical ferrosilicic magmas. The geological information

of the two studied areas (Iberia and South America), shows thatthe available sources are (1) the thick turbiditic sequence ofNeoproterozoic age deposited in the Gondwana continentalmargin (Ugidos et al., 2003a,b), namely the Puncoviscanaformation in the Puna region of South America (Turner, 1960)and the Schist and Greywacke complex of the Iberian massif; and(2) a granulite source represented by the felsic charnockiticxenoliths (Lucassen et al., 1999; Villaseca et al., 1999). Bothsources share geochemical signatures in major and trace elementsas well as in isotopes (see below). Consequently, the results fromthe experimental study with the metagreywacke are valid for thegranulites as well.

Before following with the compositional comparisons thatmayhelpto constrain the source-magmarelationships,we go with

a short description of these Neoproterozoic sequences in Iberiaand Argentina. The Schist and Greywacke complex (Carringtonda Costa, 1950; Valladares et al., 2002; Rodrguez Alonso et al.,2004) is a metasedimentary unit that crops out extensively alongmost of the Central Iberian Zone (Fig. 1). It constitutes a rathermonotonous series of metapelites and metasandstones dated asLate Vendian to Early Cambrianaccording to its fossil content andUPb determinations in detrital zircons (Vidal et al., 1994;Gutirrez-Alonso et al., 2003; Ugidos et al., 2003a). Small bodiesof mafic magma intruded this unit (Lpez-Plaza et al., 2007). The

lower boundary of this unit is not exposed, and a minimumthickness of 8000 to 11,000 m has been locally described(Rodrguez Alonso et al., 2004). Two subunits have beenclassically distinguished in the Schist and Greywacke complex(Fig. 9a). The lower unit is mainly composed of lutites andsandstones, with minor contents of microconglomerates and vol-canoclastic layers (Rodrguez Alonso, 1985; lvarez Nava et al.,1988; SanJos et al., 1990; Valladares et al., 1998). The upper unitis predominantly pelitic, with subordinate amounts of sandstones,limestones and volcanic and volcanoclastic layers (Dez Balda,1980; San Jos, 1983; Rodrguez Alonso, 1985; lvarez Navaet al., 1988; Pieren, 2000). Pelites and sandstones are geochemi-cally homogeneous across the Central Iberian Zone (e.g.,Valladares et al, 2002; Ugidos et al., 2003a). Tectonically, the

Fig. 8. Compositional curves for Fe, Mg and Ca calculated with the MELTS algorithm for melts derived fromtwo source compositions: (a) and (c), tonalite; (b) and (d),metagreywacke, with 2 wt.% waterand excesswater,at 1.0 GPa. The chemical composition of melts at 1000 and1100 C, labelledas isotherms 1 and 2 respectively, areshown 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 isalways 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 atypicalmagma compositions. Comparisons with the compositions of the Cambro-Ordovician ferrosilicic magmas indicate that the most favourable conditions are 1000 C inexcess 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) withcomposition 2 in (b), and these with the average composition for the Late CambrianEarly Ordovician magmatic rocks shown inTable 1. In both cases (tonalite andgreywacke source),the melt fractionis close to 90 vol.% (numbersbesidethe isotherms 1 and 2 givemelt fractions). These theoretical predictionsare coincident withtheexperimental study carried out with the same calc-alkaline tonalite byEl-Biad (2000), and also with greywacke sources from the Iberian massif (Castro et al., submittedfor publication). Oxides wererecast to an anhydrous base. Curves represent saturation in the elementsof reference for equilibriumbatch meltingsimulations. Theyweretraced by interpolation of 2 degrees intervaldeterminations. Oxygen fugacitywas set at QFM conditions.The compositionof the metagreywacke is the HR-40 syntheticglass used in the experiments. It can be considered also representative for the felsic granulite xenoliths reported byVillaseca et al. (1999)(seeTable 1).



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formation of garnet (Castro et al., 2000). The same comparison ismade for the South America Cambro-Ordovician ferrosilicicvolcanic and plutonic rocks (Eastern Puna eruptive belt),leucogranites and the Neoproterozoic metasediments of the

Puncoviscana formation (Fig. 11b). The patterns for averageEastern Puna eruptive belt rocks and Puncoviscana metasedi-ments are very similar. The Puncoviscana metasediments aremore enriched in Zr than the Eastern Puna eruptive belt. Felsic

Fig. 11. Chondrite normalised Thompson plot (Thompson, 1982) showing the patterns of the Late CambrianEarly 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 sourcesfor xenoliths:Villaseca et al. (1999)andLucassen et al. (1999). Explanations in the text.

Fig. 10. Major element comparisons of Late CambrianEarly Ordovician volcanics and related magmatic rocks normalised to their respective average ofNeoproterozoic turbidites. Data sources as inFig. 6. (a) Ollo de Sapo, Iberian Massif. (b) Puna eruptive belt and magmatic arc. See text for explanations.



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above. For this, however, a high temperature regime is required.Experimental studies, previously developed on these turbiditicrocks of general greywacke composition, gave rise to meltfractions of 30 to 50 vol.%, for temperatures in the range 9001000 C, and melts of leucogranite (low Fe,low Mg)composition(Patio Douce and Beard, 1996; Montel and Vielzeuf, 1997;

Castro et al. 1999). Consequently, temperatures higher than900 C, even in the presence of water, are required to producehigher melt fractions and higher concentrations in Mg and Fe inthe melt compared to leucogranites. If the source for thismagmatism is the underlying turbiditic sequence and/or theirgranulite facies equivalents (charnockites), melt fractions of 80 to90 vol.% are required to account for the observed similarity inmajor and trace elements. The correlation between melt-temperature and Fe+Mg content (Johannes and Holtz, 1996,and references herein) may be used as a first approach todetermine the temperature of generation of the ferrosilicicmagmatic rocks. Saturation of Fe and Mg for a silicic melt with

this composition was calculated by equilibrium crystallisation atconstant pressure of 1.5 GPa using the MELTS algorithm(Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998). Theresults indicate that the liquidus phase of this system is alwaysOpx at this pressure.This phase, Opx, is always present during thecrystallisation of the system with the implication that the reactionmelt1Opx+melt2 maintains the melt on saturation for thecomponents Fe and Mg. A relevant result is that even in the casethat the ferrosilicic magmas were produced in the presence ofwater and were saturated in the source region at the pressure of1.5 GPa, the minimum temperature required for the lowest valueof Fe dissolution in the melt (FeO= 3.4 wt.%) is of about 1000 C.However, the general observation on the pre-eruptive phenocryst

assemblage of the ferrosilicic rocks, dominated by Qtz and Fsp,strongly suggests that the system was not saturated in water.Taking into account these constraints, and those imposed by theobserved HREE depletion in some ferrosilicic magmatic rocksthat suggests the presence of Grt in the source area, we haveestablished the experimental conditions within the range of 1.0 to2.0 GPa and 1000 to 1200 C (Castro et al., submitted for

publication).

3.4.2. Starting materials and experimental procedures

A set of natural and synthetic materials have been used tomodel source compositions. Average geochemical composi-

tions of natural occurring rocks were used in both cases.Synthetic glasses are used to model the turbidite source material(HR-40 in Table 1). Pure oxides were mixed at roomtemperature with Na and K silicates in the desired proportions.Iron is added in the form of ferric oxide. After mixing for half anhour in an acetone medium, the homogeneous mixture isintroduced in small portions in a graphite crucible inside avertical-loading furnace previously set at 1500 C. Naturalrocks were also used in some experiments. However, the use ofsynthetic glasses is preferred in this study, particularly if acompletely dry system is required. Natural rocks contain asignificant amount of water in micas and chlorite (seeTable 1).The use of these natural systems is preferred for experimentswith a low water content. Both systems represent a geochemical

reservoir with the average composition of metagreywackes. Thehigh-grade equivalents of these compositions, e.g. charnockitegranulites, are also represented by these starting materials.

Experiments were carried out in end-loaded, Boyd-Englandpiston-cylinder apparatus at the University of Huelva. Metalcapsules containing 10 mg of sample (either rock powder or

synthetic glass) are embeddedin a pressure container of crushablemagnesia. The reported pressures are oil pressures measured withelectronic DRUCK PTX 1400 pressure transmitters, feedingOMRON E5CK controllers, multiplied by ratio of ram-to-pistonareas, and were manually maintained within 5 bar of oil pressure(ca. 250 bar on the sample). Temperatures were measured andcontrolled with Pt100Pt87Rh13 thermocouples feeding Euro-therm 808 controllers with internal ice point compensators.Temperature stability during all runs was 5 C. A NaCl sleevewith an inner glass protector is used for insulation. The resulting1/2-inch diameter assembly is introduced into the CW pressurevessel and submitted to the desired run conditions. It has been

demonstrated (Patio Douce and Beard 1994, 1995) that thegraphite-based cell assemblies used in these experiments limit thefO2 in the samples to a well-defined interval below the QFMbuffer (between QFM and QFM-2). The stability and composi-tions of ferromagnesian phases are not affected byfO2variationswithin the range imposed by these cell assemblies (Patio Douceand Beard 1994, 1995), and these fO2conditions are reasonablefor deep crustal processes (seePatio Douce and Beard 1996).

Gold capsules were used for experiments at temperatures upto 1050 C and AuPd capsules for higher temperatures. Runduration has been set within critical values between theminimum time required to reach equilibrium and a maximumtime to avoid Fe loss to the capsules or water loss in the case of

Au capsules. We have checked that duration on the order of 20 his sufficient to get equilibrium and to avoid Fe loss in runs at1200 C. Shorter duration is needed to reach equilibrium inwater-added runs at temperatures of 900 to 1100 C. In thisstudy we consider that equilibrium was reached if meltcomposition is homogeneous, with standard deviations withinWDS analytical errors of about 5 % relative. Also the lack ofsignificant compositional zoning in new-formed minerals isconsidered as a criteria of equilibrium. After the run, theexperiments were quenched by cutting-off the power. A fastdropping of temperature with rates of about 100 K/s is obtained,minimising the formation of quenching minerals. Al-silicate

needles formed by quenching were observed in few cases. Afast heating ramp of 100 K/min (the maximum allowed by theEurotherm 808 controller) was applied to all experiments inorder to avoid the formation of metastable phases duringheating. Capsules were checked for tears, mounted in epoxy andhalf cut with a diamond disk, for examination with the SEM andEPMA. Mineral and melt proportions were determined byimage analyses with the ImageJ software over back-scatteredelectron (BSE) images (Z-contrast). Chemical compositions forminerals and melts (glasses) were determined by WDS with aJEOL JXA 8200 Superprobe equipped with four WDS channelsat the University of Huelva. A combination of silicates andoxides were used as standards for calibration. Operatingconditions were 15 kV accelerating voltage and 15 nA probe



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Table 2

Experimental conditions and melt compositionsRun Reference Sample P (GPa) T (C) Water

(wt.%)duration(h)

Assemblage(vol.%)

Vol.%Melt

n SiO2 TiO2 Al2O3 F eO (t) MgO MnO

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. Cor (b1) 0.14 0.04 0.09 0.06 0.03

9 IK07-1 CXG3:1 1.9 1000 2 88 Glass (36). Grt (39). Qtz (16)Fsp (9)

36 4 73.69 0.31 16.49 0.97 0.29 0.01 0.54 0.04 0.25 0.05 0.02 0.02

10 AC07-17 HR40 1.5 1000 10 15 Glass (95). Grt (4)AlSil+Ru (b1)

95 4 71.68 0.51 16.59 3.42 1.26 n.d.

0.32 0.06 0.23 0.11 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 12 AC07-31c H R40 1.5 1100 0 42 Glass (68). Qtz (14). Grt (12)

Ru+AlSil+Cor+Sp (6)68 8 71.21 0.62 15.74 2.89 0.99 0.01

0.33 0.02 0.43 0.12 0.02 0.01

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

0.27 0.05 0.16 0.27 0.08 0.02 14 AC07-32c H R40 1.9 1100 0 42 Glass (30). Fsp (30). Opx (14)

Cor+ Sp+Ru+Il (4)30 2 68.48 0.91 15.89 3.34 0.44 0.00

15 AC07-31a CXG1:1 1.5 1100 2 42 Glass (66). Qtz (20)Grt (9). Fsp (4). AlSil (1)

66 6 69.26 0.56 16.89 2.13 0.80 0.02 0.48 0.03 0.61 0.03 0.03 0.02

16 AC07-32a CXG1:1 1.9 1100 2 42 Glass (60). Grt (15)AlSil+Ru+Cor (10)

60 10 71.88 0.66 15.69 1.97 0.96 0.03 0.17 0.06 0.14 0.08 0.04 0.02

17 AC07-20 HR40 1.5 1200 0 12 Glass (89). Qtz (10)Sp+Cor (1)

89 6 66.36 1.03 17.23 6.15 2.30 0.02 0.37 0.04 0.21 0.18 0.08 0.01

18 AC07-25 HR40 2.0 1200 0 13 Glass (81). Qtz (10). Opx (2)Sp+ Cor (6). St (1)

81 7 68.67 1.14 16.69 3.24 2.11 0.03 0.85 0.16 0.25 0.17 0.12 0.02

19 AC07-13 CXG1:1 1.5 1200 2 14 Glass (83). Qtz (16)Sp+AlSil (1)

83 4 70.36 0.86 15.88 3.91 2.01 n.d. 0.49 0.02 0.19 0.28 0.06

20 AC07-26 CXG1:1 2.0 1200 N10 13 Glass (76). Qtz (19)Opx (4). Sp +Cor (1)

76 7 69.86 0.84 16.07 3.13 2.14 0.04 0.22 0.08 0.08 0.11 0.08 0.01

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.


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current. A defocused beam of 30 m was used for glass analysesin order to minimise Na loss. Only in cases of small melt poolsin low-melt fraction experiments, a normal smaller beam of 3 to10 m was used to avoid contamination with X-rays from thesurroundings. In these cases, a significant Na loss of about 30 to50% relative was observed.

3.4.3. Experimental results

Table 2shows the compositions of melts from this study. Allthe melts are peraluminous and rich in Fe and most of them fairlymatch the composition of the ferrosilicic magmas (Figs.7and14).The mineral assemblages and melt fractions for these melting

experiments indicate that a combination of temperatures andwater contents may account for the compositional variability ofthese magmatic rocks. It has been determinedin this experimentalstudy that water has a strong control on the melt fraction. This can

be observed in experiments at 1.5 GPa and 1000 C in excesswater. These have in common a high melt fraction (85to 95 vol.%

melt). However, these melts are relatively poor in Fe and Mgcompared with the ferrosilicic magmas. The reason is the strongfractionating effect of Grt (25 wt.% FeO and 12 wt.% MgO) onthe Fe content of the system. Nevertheless, some Grt must begenerated in the source region, to account for the slight depletionin the HREE of some ferrosilicic rocks (particularly the Ollo deSapo) with respect to the Neoproterozoic sediments and theirgranulitic equivalents. Many experimental melts produced at1000, 1100 and 1200 C, at pressures of 1.4 to 2.0 GPa, fall withinthe area of the ferrosilicic magmas (Figs. 7 and 14). Combiningthese compositional requirements with the need for Grt in thesource region, the conditions may be more constrained. At

1200 C, Grt is systematically absent. Pressures higher than2.0 GPa are requiredto stabilise Grt at this temperature. However,the melts developed at 1200 C and 1.5 to 2.0 GPa fall within thefieldof the ferrosilicic magmas in the uppermost region of theFeMg compositional area (Fig. 7).

The observed HREE patterns, indicative in some cases of Grtinthe residue, suggest that, at least a part of the Fe+ Mg rich faciesof the Ollo de Sapo magmatic rocks, those with MgON2.0 wt.%and FeON3.0 wt.%, may have been developed at conditions ofabout1200CintheabsenceofGrt.Itmustberemarkedthatthereis no general rule on this correlation and that some mafic faciesmay have been developed in a Grt-present assemblage. The pointof interest here is that some mafic facies have no HREE depletion

and these may have been formed at temperatures of about1200 C. These conditions are possibly more extendedin the caseof the Puna eruptive belt as denoted by the absence of any HREEdepletion compared with the Neoproterozoic turbidites of thePuncoviscana formation (Fig. 12), that is, the hypothetical sourceof the Cambro-Ordovician magmatism in the Famatinianmagmatic 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 thegranitic bodies associated with the Cambro-Ordovician magma-tism (Antoita gneiss in Iberia). It is concluded from thisexperimental study (Castro et al., submitted) that temperatures of1000 C (excess water) and 1100 C (dry) at pressures of 1.4 to

2.0 GPa are the most plausible conditions for the generation offerrosilicic melts with the composition of the Cambro-Ordovicianmagmatic rocks (Ollo de Sapo)of Iberia. Some of the facies richerin Fe+ Mg possibly require higher temperature conditions,

perhaps of about 1200 C. Interestingly, an exhaustive study ofzircon inheritance in the Ollo de Sapo and related rocks by Beaet al. (2007), has shown that the lower limit for the maximumtemperature reached by these magmas was of 900 C. Thesemagmatic rocks are characterised by a wide variation in the Fe+Mg content and a narrow variation in silica (6276 wt.% SiO2)and peraluminosity (Alumina saturation index, ASIN1). Accord-ing to the experimental results these variations can be attributed tovariations in intensive variables, P, T, water activity, morethantheheterogeneous composition of the source (Fig. 14). At the high

Fig. 14. (a) Multicationic AB diagram (Debon and Le Fort, 1983, 1988) plottingtheexperimental meltsat 1000,1100and 1200 Cand atpressures ranging from 1.4to 1.9 GPa, and the field of the ferrosilicic magmatic rocks. CXG1:1, CXG3:1 andHR40 are the starting materials. Also shown is the field of low-melt fractionexperiments from the same source materials. Note the compositional similarity ofexperiments and natural magmas and how these compositions deviate from thetypical cafemic (calc-alkaline) trend, traced here afterDebon and Le Fort (1983).Also note the similarities with the granulite (felsic) xenoliths from the two studiedregions, Iberia and Puna. (b) Close-up view of the relevant part of diagram AB,showing the vector linking sources and melts in the experiments. These show thatthe compositional variability found in the Cambrian-Ordovician ferrosilicicmagmatic rocks may be accounted for by differences in temperature in the sourceregionmorethan thedifferences in thecomposition of thesource.This is importantbecause 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,

averaging the heterogeneous source. So, variations in melt composition can bemore likely the result of changes in temperature.



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melt fractions required, melt pools will be connected allowingcomplete mixing and homogenisation at the time of segregation.The implication is that temperature was not homogeneous at thesource region with important gradients of more than 200 Cdistributed in a relatively small volume of source material. Theseobservations are discussed below together with the heat source

and heating mechanisms.

4. Discussion

Ferrosilicic magmatic rocks from the Eastern Puna eruptivebelt and Iberian massif show an unusual chemical compositionin major elements. As indicated above, they are characterised byhigh FeO contents of about 4 wt.% and Mg contents of 1.5 wt.%,for high silica values ranging from 65 to 70 wt.% SiO2and verylow CaO (b1,5 wt.% CaO). They are more than twice richer inFe and Mg compared to anatectic leucogranites. The lavic natureof these rocks and the absence of restitic materials dragged from

the source indicate that these high contents in Fe and Mg weredissolved in the melts. The identification of such a kind of highFe, silicic magmas in Cambro-Ordovician series of theGondwana margin entails important tectonic implications.These magmas do not form part of the typical calc-alkalinetrends that characterise magmatic arcs. The hypothesis that theseferrosilicic magmas were derived by near-total melting of theunderlying Neoproterozoic sedimentary rocks or high-grademetamorphic equivalents receives support not only from majorelement chemistry but also from trace elements, radiogenicisotopes and experimental petrology, as mentioned above. Forthis, however, a high temperature regime is required. Theimplications of this unusual thermal regime and its tectonic

implications are discussed here.

4.1. Implications of ferrosilicic melts in the thermal regime at

the Gondwana margin

To transfer these geochemical features to the magmas, itrequires a near-total melting of a turbiditic sediment or its high-grade equivalent, andfor thatto occur an ultra-high temperature oftheorderof10001200Cisrequiredinthesource.ThishighTisalso required by Fe and Mg dissolution in silicic magmas,normally restricted to very low values (0.51.8 wt.% FeO, 0.10.8 wt.% MgO) for the normal temperatures required for granite

magma generation by low-fraction partial melting of the sameturbiditicsources (Montel and Vielzeuf, 1997; Castro et al.,1999).Three major problems arise to account for this near-total

melting process. First, the need fora largethermal anomaly able toheat the metasedimentary source at temperatures of the order of1000 to 1200 C. Second, a closed system is needed to preventthat partial melts are segregated, in particular when they reachmelt fractions of 30 to 40 vol.%. At these melt fractions therheological threshold is passed, the melt has continuity in thesystem and the segregation is likely to occur (e.g.Sawyer, 1996).However, the process invoked according to our experimentalresults requires that melt remains in the source until a high meltfraction of 70 to 90 vol.% is reached. It is well known that this isoften not the case of anatectic areas in the continental crust,where

melts are segregated in relation to any tectonic activity acting atthe time of partial melting. Melt segregation at 510 vol.% melt iseven possible at actively deforming belts (e.g., Brown andRushmer, 1997). Consequently, partial melting with low-meltfractions (5 to 30 vol. % melt) is typically a crustal processimposed by the rheology of the crustal materials with contrasted

compositions and strong rheological discontinuities. Possibleexceptions to this are mid-ocean ridges where active deformationof partially molten peridotite can result in melt migration undervery low-melt fractions (5 vol.%) (e.g., Daines, 1997).Furthermore, partial melting of crustal protoliths is alwaysassociated to orogenic processes, either extensional or compres-sional, that favour the segregation of melts via shear bands or anytype of local extensional regime in response to deviatoric stressesacting on a rheologically discontinuous crustal environment. Itseems very unlike that a melting fraction of about 70 to 90 vol.%can be produced in the continental crust, unless protected fromlarge-scale deforming zones. The high temperatures needed for

this melting process are also very unlike in the continental crust.And finally, the third petrogenetic problem is the high degree ofzircon inheritance shown by the Cambro-Ordovician magmaticrocks of the Iberian massif, which is indicative of extremely rapidheating rates (Bea et al., 2007). Fast melt transport suggests anextensional settingcorrelative to the riftingprocess that took placeduring the Early Palaeozoic in the northern margin of Gondwana(Bea et al., 2007).

A possible solution for thehigh temperatures(1000 to 1200 Cas shown here) and the fast melting rates is that melting occurredat the core of a cold diapir formed by subducted sedimentsascending into the hot mantle wedge. The possibility for thermalrheological unstabilities that give rise to these mantle wedge

megastructures was analysed in detail by Gerya and Yuen (2003).These structures haverecentlyattired theinterest of geologists andgeophysicists, as they constitute a part of the plate-driven uppermantle circulation (e.g.,Ernst, 2007), together with the hydrous

plumes described by Maruyama et al. (2007). Gerya andStoeckhert (2005)have investigated numerical models appliedto analyse the evolution of continental active margins. In thesemodels large portions of the upper continental crust (e.g.sediments) may be subducted and transported by cold diapirs tothe mantle wedge, giving rise to large silicic magma chambers.Melt segregation at low melting rates is very unlike at the core ofthese cold diapirs as they move ascending through the mantle

wedgewithin a nearly continuous medium. Also reaction with themantle rocks may help to preservethesemegastructures due to theformation of a reactive aureole rich in pyroxene and amphibole(Castro and Gerya, 2007). Numerical experiments simulating theevolution of cold diapirs (e.g., Gerya and Stoeckhert, 2005)inthemantle wedge show that the isotherms are folded around theintruding diapir, denoting that the ascending rate of the relativelycold body is faster than thermal conduction rate in the mantle.Temperatures in excess of 1000 C are easily attained in the coreof the diapir at the final stages of its evolution (Gerya andStoeckhert, 2005), as well as strong thermal gradients areestablished from the core to the periphery of the diapir. Thismodel also explains the location of sedimentary sources at largedepths (2 GPa), and the absence of a cortege of basic rocks



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accompanying the ferrosilicic magmatism. In what follows, thecold-diapir model will be used with preference to describe thesuggested evolution of the continental margin of Gondwana fromIberia to La Puna during the Late Cambrian and Early Ordovician.

4.2. Evolution of the Late CambrianEarly Ordovician margin

of Gondwana and generation of the ferrosilicic magmatism

Deposition of huge turbiditic successions at Iberia (Schist andGreywacke complex) and Puna (Puncoviscana formation)finished the major Neoproterozoic changes that deeply affectedthe solid Earth, the biosphere and the climate (Stern, in press;Meert and Lieberman, in press; Komiya et al., in press; Maruyamaand Santosh, 2008; Rinoet al., 2008). Afterwards, an ancientringof fireencircled most of the continental margin of Gondwanaduring the Late CambrianEarly Ordovician (Fig. 5), according to

plate-tectonic reconstructions based on geological (Stampfli andBorel, 2002; Gutirrez-Alonso et al., 2003; Cawood, 2005; Chew

et al., 2007; Vaughan and Pankhurst, 2007) and paleomagneticinformation (Van der Voo, 1993; Torsvik, 1998; Pharaoh, 1999;Rapalini, 2005). Near the location of the Ordovician South Pole,

the Proto-Tethys and Tornquist oceans were subducting along theGondwana margin, and a CambrianEarly Ordovician magmaticarc was active in the Avalonia and Cadomia terranes (Figs. 5and15a). Proofs of this magmatic activity arepresently exposed atthe remnants of these terranes along the Appalachian and Variscanorogenic belts (Winchester and Van Staal, 1995; O'Brien et al.,

1997; Abati et al., 1999). Behind the magmatic arc, extensionaltectonics, rifting and seafloor spreading marked the back-arcregion during the Late CambrianEarly Ordovician, heralding theopening of the Rheic ocean at the northern margin of Gondwana.Intense slab roll-back (Stampfli and Borel, 2002) and ridgesubduction (Gutirrez-Alonso et al., 2003) are some putativecauses of this extensional episode in the back-arc region.Extension due to slab roll-back is a well-documented mechanismin modern tectonic settings like the Andes (e.g.Ramos, 1999).Contemporary volcanism at the extending margin of Gondwanawas characterised by the emission of ferrosilicic magmatismgiving place to the Ollo de Sapo formation (Iberia, Fig. 15a,

middle panels) and the Eastern Puna eruptive belt (Puna,Fig. 15b). The rest of the Ordovician, the Silurian and Devonianpre-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 marginsof Gondwana. The rectangles of the third-row panels indicatethe enlarged areashown in the fourth-row panels. For locationof cross-sections see Fig.5.Seetext for further explanation. The subducting margin in the case of the Puna cross-section (b, upper panel) immediately followed a previous stage of passive margin.

http://dx.doi.org/doi:10.1016/j.gr.2007.08.006http://dx.doi.org/doi:10.1016/j.gr.2007.08.006http://dx.doi.org/doi:10.1016/j.gr.2007.08.006http://dx.doi.org/doi:10.1016/j.gr.2007.08.006http://dx.doi.org/doi:10.1016/j.gr.2007.08.006http://dx.doi.org/doi:10.1016/j.gr.2007.08.006


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show sedimentary features corresponding to a siliciclasticplatform compatible with a passive continental margin(Fig. 15a, lower panel, left), as explained before. It must bestressed that in the model illustrated inFig. 15, no causality exists

betweenasthenosphericuplift, whichis a consequence of back-arcbasin opening, and wholesale melting of the subducted sediments

within the cold diapir, which is due to thermal equilibration of thesedimentary plume with the mantle wedge. Nevertheless, theextensional lithospheric rupture favoured the rapid ascent of themolten sediments towards Earth's surface.

The geochemical and experimental constraints exposed in thispaper strongly support that the ferrosilicic magmatism was derivedfrom a metagreywacke source or its granulite, residual or not,equivalent (charnockites), at temperatures higher than thosenormally found in the continental crust. The information supplied

by the study of lower crust xenoliths in basalts and lamprophyres inPuna and Iberia, respectively, is of great value. In both regions, thefelsic xenoliths represent a lower crust equilibrated at 1.0 GPa. In

South America, this atypical lower crust is the dominant in largeareas of the Andean basement (Lucassen et al., 1999). Interestingly,these two regions, IberiaandSouth America,areexceptionalfor thedominance of felsicgranulites in the lowercrust (Rudnick and Gao,2003). Also, as it has been stated in this study, these two regions areexceptional for the presence of large volumes of ferrosilicicmagmas developed during the Late CambrianEarly Ordovician.The slight differences found between the Neoproterozoic grey-wackes, the xenoliths and the ferrosilicic magmas in terms ofHREE may be informative about the role played by this atypicalfelsic lower crust in the generation of the ferrosilicic magmas.Thereis a slight depletion in HREE in the Ollo de Sapo compared to thelower crust xenoliths. These may represent low-melt fraction

residues developed in equilibrium with Grt (cf. Villaseca et al.,1999). This residual material may be either the source of theferrosilicic magmas or the deep-seated equivalents of these thatwere previously depleted in an early stage of the melting processand that retain the residual solid material (e.g., Grt). In both cases,compositionally they are residual metagreywackes. The hightemperatures required for the development of this ferrosilicicmagmatism are more suitable in the context of cold diapirs quicklyascending through the mantle wedge. Therefore, the possibilityexists that this anomalous and large silicic crust was developed byattachment to the lower crust of silicic diapirs coming fromsubducted materials. In this case, the silicic lower crust, represented

by the felsic xenoliths in Iberia and Puna, would be the deepequivalent of the ferrosilicic magmas and it would have beendeveloped during the Late CambrianEarly Ordovician byamalgamation of cold diapirs. Alternatively, and following themost classical view, this lower crust can be the source of theferrosilicic magmas, produced for instance by metamorphism andslight depletion of a greywacke protolith, that underwent near-totalmelting during Late CambrianEarly Ordovician by lithosphereunderplating. In both cases, cold diapirs and underplating, the

process took place in an extensional regime. With independence ofthe genetic model, the relation between ferrosilicic magmas,metagreywackes and the felsic lower crust appears evident fromthis study. Theseareas may be,consequently, of great interest to testthese two contrasted models.

Both, the magmatic arc (Western Puna eruptive belt) and theback-arc basin with the ferrosilicic magmas (Eastern Punaeruptive belt) are presently adjacent to the Famatinian belt ofSouth America, which recalls the Cambro-Ordovician paleogeo-graphy, only slightly modified by younger subduction orogenies(Fig. 15b). The evolution of the southwestern margin of

Gondwana during the NeoproterozoicEarly Palaeozoic hasbeen interpreted considering allochthonous and paraauthochto-nous terranes (Coira et al., 1982, Allmendinger et al., 1983;Aceolaza and Toselli, 1984; Ramos et al., 1986; Dalziel andForsythe, 1985; Ramos, 1988; Bahlburg, 1990, Damm et al.,1990; Bahlburg and Breitkreuz, 1991; Rapela et al., 1992; Contiet al., 1996; Coira et al., 1999) or an authochtonous origin(Lucassen et al., 2000; Do Campo and Guevara, 2005; Lucassenand Franz, 2005; Miller and Sollner, 2005; Zimmermann, 2005).Fig. 15b shows a schematic evolution of the Gondwana margin ofthe CentralAndes. The Puncoviscana formation is interpreted as aturbiditic sequence of a passive margin (Rapela et al., 1998; Do

Campo and Guevara, 2005; Pian-Llamas and Simpson, 2006)deposited during NeoproterozoicEarly Cambrian. After deposi-tion of the Puncoviscana formation, an eastward subduction ofoceanic crust started during the EarlyMiddle Cambrian(~530 Ma;Mulcahy et al., 2007)(Fig. 15b, upper panel), wherea quiescent period of ~8070 Ma (520440 Ma) generated awidespread high temperaturelow pressure metamorphism(Lucassen et al., 2000; Lucassen and Becchio, 2003; Hongnand Riller, 2007). During Late CambrianEarly Ordoviciantimes, two subparallel magmatic belts were developed, i) theWestern Puna eruptive belt that is represented by calk-alcalinemagmatism with arc-like petrologic, geochemical and isotopiccharacteristics, ii) the Eastern Punaeruptive belt that is constituted

by a widespread ferrosilicic magmatism associated with lessermafic magmatism emplaced in an extensional back-arc setting(Fig. 15b, middle panels).Alonso et al. (2007)have described anOrdovician extensional episode in the nearby Precordilleraterrane. Finally, at Devonian times, (ca. 400 Ma) both, the

Neoproterozoic and lower Palaeozoic units were exhumed due tothe Pampean and Famatinian orogenies (Fig. 15b, lower panel).Astini and Davila (2004)andKleine et al. (2004), suggested thatthe Famatinian system could be a southward continuation of theWestern Puna eruptive belt. According to these authors and basedon petrological and geochemical data, the Famatinian Systemcould be correlated with the Western Puna eruptive belt (calc-

alkaline arc signature) and the Eastern Puna eruptive belt re-presents a back-arc setting where ferrosilicic magmas were gene-rated. Recently,Chew et al. (2007)proposed that the Famatinianmetamorphism and subduction-related magmatism were contin-uous from Patagonia (Pankhurst et al., 2006) through northernArgentina and Chile to as far north as Colombia and Venezuela, adistance of nearly 7000 km. Successive episodes of subductionand accretion dominated the southwestern margin of Gondwanaduring the Early Palaeozoic (Vaughan and Pankhurst, 2007).

Distinctly from the Famatinian belt, where the Cambro-Ordovician subduction orogen is well preserved, the geologicalhistory at the European margin of Gondwana included theopening of the Rheic ocean separating the magmatic arc from the

passive margin of Iberia (compare Fig. 15a and b). Afterward, the



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Variscan orogeny tectonically disrupted and juxtaposed theseterranes (Fig. 15a, lower panel, right). However, geological,geochemical and geochronological data guarantee the correlationof this zone with the Famatinian orogen. The result of thiscorrelation shows a huge subduction orogen whose unusualgeochemical imprint (ferrosilic magmatism) needs a particular

petrogenetic model to be explained.

5. Conclusions

Large volumes of Late CambrianEarly Ordovician ferrosi-licic magmatic rocks appear at the West Gondwana margin.Ferrosilicic magmatism is characterised by high iron content(FeON4.0 wt.%) and low contents in calcium (CaOb1.5 wt.%).The previous discussion about the geological context of theLate CambrianEarly Ordovician ferrosilic magmatism of theWest Gondwana margin is pointing to a common origin forthese rocks, with strong petrogenetic and tectonic implications.

These magmatic rocks show geochemical signatures that departfrom those explained by current models of magma generationat active plate margins and intracontinental rifting, includingnormal fractionation of calc-alkaline magmas and partial melt-ing of common protoliths as sediments, calc-alkaline igneousrocks or MORB-derived metabasites involved in convergentactive margins. Geological, geochemical and experimental datashow that generation of ferrosilicic magmas took place by near-total melting (8090%) of crustal sources of metagreywackeor charnockite affinities, subjected to very high temperatures(1000 C1200 C) and at pressures of 1.0 to 2.0 Gpa. Fur-thermore, this particular magmatism, far from being local,appears widely represented during Late CambrianEarly

Ordovician times in places separated by thousands of km,and always associated with an active continental margin at theWestern edge of the Gondwana supercontinent. The petro-genetical constraints discussed in this work for the LateCambrianEarly Ordovician ferrosilicic magmatism point to ascenario that includes sediment subduction, back-arc rifting,and possible evolution of cold diapirs that ascended through themantle wedge, became accreted near the lithosphere base, andwere rapidly and strongly heated and molten generating mag-mas that were finally transported to upper crustal levels in anextensional setting.

The fact that a magmatism with features suggesting such a

particular petrogenetic process is worldwide represented onlyduring a given time span from 490 to 450 Ma, may have strongimplications on plate reconstructions and paleoenvironmentalconditions for this time period.

Acknowledgements

We gratefully acknowledge the financial support fromProject CGL2004-06808-CO4 of the Spanish Ministry ofScience and Education and partially from Projects: PIP-

N6103 CONICET, grant N1350/1 CIUNSA and PICTN07-38131 ANPCyT. We thank J.M. Ugidos (Salamanca)for comments on the geochemistry of Iberian Neoproterozoicturbidites. This paper benefited from thoughtful reviews by

Victor A. Ramos, Shoji Arai and an anonymous reviewer. Weare grateful to M. Santosh for his editorial assistance.

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