The belt of metagabbros of La Pampa: Lower Paleozoic back-arc magmatism in south-central Argentina

Journal of South American Earth Sciences 28 (2009) 383–397

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The belt of metagabbros of La Pampa: Lower Paleozoic back-arc magmatism in south-central Argentina Carlos J. Chernicoff a,b,*, Eduardo O. Zappettini b, Luisa M. Villar a,b, Farid Chemale Jr. c, Laura Hernández d a

Council for Scientific and Technical Research, Ave. Julio A. Roca 651, 1067 Buenos Aires, Argentina Geological and Mining Survey of Argentina, Ave. Julio A. Roca 651, 1067 Buenos Aires, Argentina c Ave. Bento Gonçalves, 9500-Porto Alegre, Brazil d Instituto GEA, University of Concepción, Casilla 160-C, Concepción, Chile b

a r t i c l e

i n f o

Keywords: Metagabbros La Pampa mafic belt Famatinian (Lower Paleozoic) back-arc South-central Argentina

a b s t r a c t Combined geological, geochronological, geochemical and geophysical studies have led to identification of a large (300 km long, 5 km wide) N–S trending belt of metagabbros in the province of La Pampa, south-central Argentina. This belt, though only poorly exposed in the localities of Valle Daza and Sierra de Lonco Vaca, stands out in the geophysical data (aeromagnetics and gravity). Modeling of the aeromagnetic data permits estimation of the geometry of the belt of metagabbros and surrounding rocks. The main rock type exposed is metagabbros with relict magmatic nucleii where layering is preserved. A counterclockwise P–T evolution affected these rocks, i.e., during the Middle Ordovician the protolith reached an initial granulite facies of metamorphism (M1), evolving to amphibolite facies (M2). During the Upper Devonian, a retrograde, greenschist facies metamorphism (M3) partially affected the metagabbros. The whole-rock Sm–Nd data suggest a juvenile source from a depleted mantle, with model ages ranging from 552 to 574 Ma, and positive Epsilon values of 6.51–6.82. A crystallization age of 480 Ma is based on geological considerations, i.e. geochronological data of the host rocks as well as comparisons with the Las Aguilas mafic–ultramafic belt of Sierra de San Luis (central Argentina). The geochemical studies indicate an enriched MORB and back-arc signature. The La Pampa metagabbros are interpreted to be originated as a result of the extension that took place in a back-arc setting coevally with the Famatinian magmatic arc (very poorly exposed in the western part of the study area). The extensional event was´aborted´ by the collision of the Cuyania terrane with Pampia-Gondwana in the Middle Ordovician, causing deformation and metamorphism throughout the arc– back-arc region. The similarities between the La Pampa metagabbros and the mafic–ultramafic Las Aguilas belt of the Sierra de San Luis are very conspicuous, for example, the age (Lower Paleozoic), geochemical signature and timing of metamorphism (dated at ca. 465 Ma in the study area), which allow definition of a single, mafic back-arc belt in central Argentina, from San Luis to La Pampa. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The basement of La Pampa province, south-central Argentina (Fig. 1), comprises the southern portion of the Pampia, Cuyania and Chilenia allochthonous to para-autochthonous terranes, amalgamated to the western margin of Gondwana during the Cambrian, Middle Ordovician and Upper Devonian, respectively (see e.g. Ramos, 1988, 1999, 2004; Astini et al., 1996; Pankhurst et al., 1998). Their precise boundaries were defined by Chernicoff and Zappettini (2003, 2004), who also indicate that the most eastern * Corresponding author. Address: Council for Scientific and Technical Research, CONICET, Argentina. E-mail address: [email protected] (C.J. Chernicoff). 0895-9811/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2009.04.011

portion of the basement of La Pampa province corresponds to the Río de La Plata craton. The Río de la Plata craton (Almeida et al., 1973) represents autochthonous Precambrian Gondwana basement. It is known to have undergone major granitic magmatism at ca. 2100 Ma, with Archean remnants dated at 3410 Ma (Hartmann et al., 2001; Cingolani et al., 2002; Pankhurst et al., 2003). The basement of the central-northern La Pampa province (Fig. 1) was interpreted as the southern extension of the Eastern Sierras Pampeanas (Stappenbeck, 1913; Linares et al., 1980). Ramos (1996) had suggested that it belonged partly to the Pampia terrane and partly to the Río de la Plata Craton, locating the boundary roughly west of 66° W. In their geological interpretation of the aeromagnetic data of central La Pampa, Chernicoff and Zappettini (2003, 2004) indicated

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eastern La Pampa province irrespective of terrane boundaries, was restricted to the exposures of metasediments on the Río de la Plata craton (Chernicoff and Zappettini, 2003, 2004). The fault zone that delineates the Pampia-Río de la Plata craton suture is interpreted as the southern continuation of the Transbrasiliano Lineament (see e.g. Fig. 6 in Leal et al., 2003). The boundary between Pampia and Cuyania is delineated by a large NNW-trending magnetic anomaly that coincides with the suture proposed by Ramos and Vujovich (1999), and terminates abruptly at the northern margin of the Patagonia terrane (Chernicoff and Zappettini, 2003, 2004). This anomaly arises from metaquartz-diorites and metatonalites that are poorly exposed at Paso del Bote (Villar et al., 2005), i.e. on the Pampia side of the Cuyania–Pampia suture. These rocks represent the southernmost Famatinian (Lower Paleozoic) magmatic arc, which was affected by metamorphism due to the accretion of the Cuyania terrane (Villar et al., 2005). A K/Ar age on amphibole from the metamorphic basement at Paso del Bote yielded 467.1 ± 13 Ma (Tickyj et al., 1999). To the east of the Cuyania–Pampia suture and within the Pampia terrane, a high-gradient, roughly N–S-trending magnetic anomaly was first identified by Chernicoff et al. (2005). This anomaly is originated by the metagabbros that scarcely crop out at Valle Daza and Sierra de Lonco Vaca, but are interpreted to form an extensive belt. Geochemical data of the metagabbros and their proposed back-arc setting were presented by Zappettini et al. (2005). On the Cuyania side of the Cuyania–Pampia terrane divide, the depocentre of a Late Ordovician–Devonian marine foreland basin generated after the Middle Ordovician collision of Cuyania – Curacó basin – is conspicuously delineated by a narrow NNW-oriented magnetic low. Its fill primarily comprises sandstones and shales of the La Horqueta Formation, representing the southern continuation of the same unit exposed in the San Rafael Block of the Cuyania terrane (Chernicoff and Zappettini, 2003, 2004, 2005a; Chernicoff et al., 2008). In the present study, the interpreted belt of metagabbros is reviewed on the basis of new field observations, chemical analyses of the mafic unit, geological/structural interpretation of aeromagnetic and gravimetric data, as well as Nd isotope geochemical data. Sm–Nd isotopic data of the Valle Daza metagabbros are presented for the first time, and their crystallization age is discussed on the basis of geological constraints such as geochronological data of the country rocks, as well as comparison with the Las Aguilas mafic–ultramafic belt of the Sierra de San Luis. 2. Geological setting

Fig. 1. (Upper): map of accreted terranes in the southern region of South America (after Chernicoff and Zappettini, 2004), with locations of the study area and greater region. (Lower): locality map of the study area and greater region. L.A.B.: Las Aguilas mafic–ultramafic belt.

that the boundary between the basements of Pampia and the Río de la Plata craton is marked by an abrupt change in the orientation of the magnetic fabric (parallel to schistosity/foliation) on either side of a prominent N–S to NNE trending lineament located approximately at 65° W. This structural break is accompanied by lithologic changes, as well as differences in the age of metamorphism, on either side of the regional structure. The term ‘‘Las Piedras Metamorphic Complex” of Tickyj et al. (1999), which had been defined for the whole metamorphic basement of the south-

The host rocks of the metagabbros comprise schists and gneisses of sedimentary protoliths, poorly exposed in the central portion of La Pampa province (Pampia terrane). The main exposures of these metasediments are: (a) quartz–plagioclase–biotite schists of the Sierra Lonco Vaca (Párica, 1986a,b; González et al., 2005; Chernicoff et al., 2007); the latter authors carried out the U–Pb SHRIMP dating of the schists, reporting that the youngest detrital zircon age is 515 ± 4.6 Ma; (b) quartz–plagioclase–biotite schists of the Green quarry, dated at ca. 500 Ma (Chernicoff et al., 2006, 2007); (c) biotite–garnet schists–gneisses of Paso del Bote (Tickyj et al., 1999) and (d) biotite–garnet schists to gneisses of Valle Daza, with metamorphic ages of ca. 461 Ma (Ar/Ar in biotite; Tickyj et al., 1999) and 464.7 ± 7.3 Ma (zircon U–Pb SHRIMP; unpublished data of the present authors). According to Chernicoff et al. (2006, 2007), these rocks belong to a Lower Paleozoic supracrustal sedimentary sequence originated in a foreland basin at the southwestern margin of Gondwana that was subject to metamorphism at ca. 461–465 Ma during the Famatinian orogeny related to the docking of the Cuyania terrane.

C.J. Chernicoff et al. / Journal of South American Earth Sciences 28 (2009) 383–397

These metasediments exhibit a roughly N–S trending schistosity/foliation, both as measured directly in the scarce outcrops, and as revealed by the magnetic fabric, i.e. at regional scale (Chernicoff and Zappettini, 2003, 2004; see also aeromagnetic data, in Fig. 2). They are divided into two blocks (Western and Eastern Blocks, in Fig. 2b) by a major N–S trending, west-vergent, steeplydipping thrust. This structure has been interpreted by Chernicoff et al. (2005) to represent the same fault first identified by Stappenbeck (1913) between the localities of Telén and Victorica (Fig. 1) on the basis of drilled hole information – in that location, a vertical displacement of more than 624 m was reported. The metagabbros are aligned in the eastern side (hangingwall) of this regional fault system, where they give rise to a prominent magnetic anomaly identified by Chernicoff and Zappettini (2003, 2004) and Chernicoff et al. (2005). This anomaly coincides with the gravity highs indicated by Kostadinoff et al. (2001).

385

Chernicoff and Zappettini (2003, 2004) and Chernicoff et al. (2005) referred to the studied metagabbros as forming a N–S trending belt with small exposures at Valle Daza and Lonco Vaca, but in physical continuity between the two localities, as inferred from the combined aeromagnetic and gravimetric data. At Lonco Vaca, Párica (1986a,b) and González et al. (2005) referred to these mafic rocks as amphibolites, without interpreting their protolith. In both areas, the metagabbros are intruded by Devonian granites and small Permian pegmatitic bodies. The intrusive relationships were first established by Párica (1986a,b) at the Lonco Vaca quarry, and are herein confirmed for the Valle Daza area. 3. The metagabbros The Valle Daza exposures were first referred to by Stappenbeck (1913) as amphibolites, and briefly described by Linares et al.

Fig. 2. (a) (Left): geophysical data of the study region (1: gravimetric data; 2: aeromagnetic data). L45222: locality of modeled section (see Fig. 18); (b) (right): basement geology of the study region, based on limited exposures, drilled hole information and geophysical data (modified from Chernicoff et al., 2005 and Zappettini et al., 2005). The segment of the mafic belt from Victorica to El Durazno is supported by ground gravimetric data of Kostadinoff et al. (2001). See location in Fig. 1.

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(1980) as pyroxenic granoblastites. Outcrops in this low-relief region are extremely scarce. The type area of the Valle Daza metagabbros is located 4 km south of the Valle Daza estate, at the southern shore of the Valle Daza salt pan, where they are partly exposed for about 3–4 km (depending on the pan level). At this site, the metagabbros exhibit a conspicuous NNE trending foliation. The gabbros intruded the protolith of the quartz–plagioclase–biotite paraschists and were, in turn, intruded by Devonian granites and pegmatites. Smaller, scattered exposures of metagabbros are located a few kilometers to the south of the type area. Also, numerous samples have been recovered from shallow water wells at various sectors of the Valle Daza estate. Furthermore, about 3 km to the west of the estate, a subtle hump on the dirt road N°. Eighteen indicates the location of an additional exposure of metagabbros, coincident with the (geophysically interpreted) fault that marks the western boundary of the mafic belt (see Fig. 2b). Zappettini et al. (2005) and Delpino et al. (2005) described the main lithologies of the Valle Daza type exposures.

In the Lonco Vaca hillocks, the mafic rocks were also first referred to as amphibolites by Pastore (1932), Linares et al. (1980) and Párica (1986a,b). At this locality the outcrops are very limited, though a quarry helps with the rock-type identification. Detailed mapping of the quarry is given by Párica (1986a), who indicates a number of NNE trending amphibolite lenses hosted by micaschists and paragneisses. In turn they are intruded by Devonian granitic and pegmatitic dykes. Párica (1986a,b) described relictic mineral assemblages of high-grade metamorphism in the country rocks (sillimanite–orthoclase–garnet–biotite) as well as in the amphibolite lenses (labradorite-pyroxene). He also recognized two events of deformation that affected both the mafics and country rocks. 3.1. Mineral chemistry For the present work, microprobe analyses of clinopyroxene, amphibole and plagioclase from the metagabbros were carried out at the Institute of Applied Economic Geology of the University

Table 1 Chemical composition of clinopyroxenes. Numbers of ions on the basis of 6 oxygens (Fe + 3 content estimated by charge balance). Classification of Morimoto (1989). Sample Analysis

M-37 1

M-37 2

M-37 3

M-37 4

M-37 5

M-37 6

M-37 7

M-37 8

M-41 1

M-41 2

M-41 3

VD-1 1

VD-1 2

VD-1 3

28-D 1

28-D 2

Origin SiO2 TiO2 Al2O3 FeO Cr2O3 MnO MgO CaO Na2O

M 51.01 0.30 2.78 10.39 0.06 0.32 12.12 22.66 0.53

I 52.45 0.03 0.64 10.05 0.03 0.34 12.57 23.86 0.29

I 52.41 0.02 0.59 10.08 0.01 0.64 12.24 23.99 0.30

I 52.44 0.01 0.75 9.04 0.03 0.40 13.03 24.92 0.19

I 51.63 0.06 0.89 10.41 0.04 0.35 12.19 23.62 0.36

I 52.25 0.08 1.11 9.33 0.03 0.32 13.01 23.85 0.33

I 52.39 0.02 0.61 9.80 0.03 0.60 12.15 24.85 0.28

I 52.09 0.08 1.12 8.92 0.04 0.35 13.20 23.52 0.39

M 51.45 0.34 2.66 7.34 0.05 0.21 13.72 23.66 0.62

M 50.95 0.38 2.94 7.59 0.04 0.25 13.38 24.11 0.68

M 51.25 0.45 3.21 7.77 0.04 0.25 13.27 23.67 0.70

M 50.48 0.33 2.99 8.87 0.01 0.33 12.48 23.62 0.66

M 51.13 0.31 2.72 8.55 0.02 0.30 12.94 22.96 0.68

I 52.69 0.03 0.98 7.36 0.02 0.37 13.80 24.90 0.31

M 50.01 0.27 3.69 8.58 0.00 0.52 12.56 23.85 0.69

M 50.48 0.21 3.21 8.35 0.00 0.46 13.11 24.00 0.67

Total

100.18

100.26

100.28

100.81

99.55

100.31

100.74

99.71

100.05

100.32

100.61

99.77

99.61

100.46

100.18

100.48

TSi TAl TFe3 ST M1Al M1Ti M1Fe3 M1Fe2 M1Cr M1Mg M2Mg M2Fe2 M2Mn M2Ca M2Na

1.909 0.091 0.000 2.000 0.031 0.008 0.080 0.203 0.002 0.676 0.000 0.043 0.010 0.908 0.039

1.964 0.028 0.008 2.000 0.000 0.001 0.054 0.242 0.001 0.702 0.000 0.011 0.011 0.957 0.021

1.966 0.026 0.008 2.000 0.000 0.001 0.054 0.254 0.000 0.685 0.000 0.000 0.020 0.964 0.022

1.947 0.033 0.020 2.000 0.000 0.000 0.065 0.195 0.001 0.721 0.000 0.000 0.012 0.991 0.013

1.950 0.040 0.011 2.001 0.000 0.002 0.072 0.239 0.001 0.686 0.000 0.007 0.011 0.956 0.026

1.948 0.049 0.004 2.001 0.000 0.002 0.071 0.203 0.001 0.723 0.000 0.013 0.010 0.953 0.024

1.956 0.027 0.017 2.000 0.000 0.001 0.062 0.226 0.001 0.676 0.000 0.000 0.019 0.994 0.020

1.949 0.049 0.002 2.000 0.000 0.002 0.073 0.187 0.001 0.736 0.000 0.017 0.011 0.943 0.028

1.902 0.098 0.000 2.000 0.018 0.009 0.104 0.111 0.001 0.756 0.000 0.012 0.007 0.937 0.044

1.880 0.120 0.000 2.000 0.008 0.010 0.138 0.097 0.001 0.736 0.000 0.000 0.008 0.953 0.049

1.887 0.113 0.000 2.000 0.027 0.012 0.109 0.122 0.001 0.729 0.000 0.008 0.008 0.934 0.050

1.884 0.116 0.000 2.000 0.016 0.009 0.129 0.148 0.000 0.695 0.000 0.000 0.010 0.945 0.048

1.908 0.092 0.000 2.000 0.027 0.009 0.095 0.148 0.001 0.720 0.000 0.023 0.009 0.918 0.049

1.948 0.043 0.010 2.001 0.000 0.001 0.072 0.146 0.001 0.761 0.000 0.000 0.012 0.986 0.022

1.856 0.144 0.000 2.000 0.017 0.008 0.000 0.266 0.000 0.695 0.000 0.000 0.016 0.948 0.050

1.864 0.136 0.000 2.000 0.003 0.006 0.000 0.258 0.000 0.722 0.000 0.000 0.014 0.949 0.048

Sum_cat

4.000

4.000

4.000

4.000

4.000

4.000

4.000

4.000

4.000

4.000

4.000

4.000

4.000

4.000

4.000

4.000

Ca Mg Fe2_Mn JD1 AE1 CFTS1 CTTS1 CATS1 WO1 EN1 FS1 Q J WO EN FS WEF JD AE

49.368 36.740 13.893 1.677 0.391 3.967 0.451 0.000 44.198 36.180 13.136 1.830 0.077 47.319 35.215 17.466 95.968 1.138 2.894

49.798 36.503 13.699 0.000 1.104 1.761 0.045 0.000 47.721 36.304 13.064 1.912 0.042 48.237 35.358 16.405 97.839 0.000 2.161

50.145 35.598 14.256 0.000 1.139 1.702 0.028 0.000 48.372 35.568 13.190 1.903 0.044 48.569 34.480 16.951 97.773 0.000 2.227

51.620 37.554 10.826 0.000 0.700 2.735 0.012 0.000 48.845 37.534 10.173 1.908 0.027 49.426 35.959 14.615 98.618 0.000 1.382

50.309 36.126 13.565 0.000 1.360 2.438 0.096 0.000 47.387 35.847 12.872 1.888 0.052 48.219 34.625 17.156 97.336 0.000 2.664

50.080 38.010 11.910 0.000 1.256 2.480 0.122 0.000 47.110 37.731 11.300 1.892 0.048 48.196 36.581 15.223 97.532 0.000 2.468

51.891 35.301 12.807 0.000 1.070 2.235 0.031 0.000 49.585 35.274 11.804 1.896 0.041 49.819 33.892 16.290 97.910 0.000 2.090

49.771 38.865 11.364 0.000 1.492 2.401 0.124 0.000 46.789 38.508 10.685 1.884 0.057 47.880 37.388 14.732 97.089 0.000 2.911

51.410 41.480 7.110 0.966 1.421 4.244 0.502 0.000 45.620 40.637 6.611 1.816 0.089 48.638 39.243 12.120 95.353 0.685 3.961

53.145 41.036 5.819 0.452 2.198 5.367 0.568 0.000 46.024 40.120 5.271 1.786 0.097 49.360 38.114 12.526 94.857 0.293 4.851

51.878 40.467 7.655 1.440 1.272 4.736 0.674 0.000 45.284 39.544 7.050 1.792 0.100 48.906 38.149 12.945 94.750 1.024 4.226

52.552 38.634 8.813 0.858 1.770 5.259 0.508 0.000 45.701 37.837 8.067 1.787 0.096 49.038 36.051 14.912 94.928 0.552 4.520

50.476 39.582 9.942 1.477 1.171 4.000 0.468 0.000 44.929 38.736 9.219 1.809 0.098 47.958 37.607 14.435 94.867 1.146 3.987

51.789 39.936 8.275 0.000 1.160 2.631 0.044 0.000 48.827 39.714 7.624 1.893 0.044 49.662 38.296 12.041 97.720 0.000 2.280

49.242 36.082 14.676 0.872 1.662 0.000 0.385 0.000 48.020 35.468 13.592 1.909 0.099 49.242 36.082 14.676 95.096 4.904 0.000

48.858 37.134 14.008 0.161 2.266 0.000 0.295 0.000 47.733 36.503 13.043 1.929 0.096 48.858 37.134 14.008 95.295 4.705 0.000

387

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Fig. 4. Plot of amphibole compositions in terms of Mg/(Mg + Fe2) (atomic proportions) for the calcic group where AnA + AK < 0.5; Ti < 0.5. Triangles: Lonco Vaca samples; circles: Valle Daza samples. Classification of Hawthorne (1981). Fig. 3. Plot of clinopyroxene compositions in terms of Wo–En–Fs (atomic proportions). Triangles: Lonco Vaca samples; circles: Valle Daza samples. Classification of Morimoto (1989).

of 5 mm. Data were reduced using ZAF corrections. Natural and synthetic silicates and oxides were used as standards. All analysed clinopyroxenes (magmatic and metamorphic) are classified as diopside (Table 1 and Fig. 3). The metamorphic diopsides are richer in Al, Na and Ti than the (relictic) igneous diopsides, containing up to 3.69% Al2O3, 0.45% TiO2 and 0.69% Na2O.

of Concepción, Chile. The analyses were performed on a JEOL JXA8600M electron microprobe using wavelength-dispersion spectrometry (WDS). Operating conditions included an accelerating voltage of 15 kV, a beam current of 20 nA and a beam diameter

Table 2 Chemical composition of amphiboles. Numbers of ions on the basis of 23 oxygens (Fe + 3 content estimated by charge balance). Classification of Hawthorne (1981). Sample Analysis

M-37 1

M-37 2

M-37 3

M-37 4

M-41 1

M-41 2

M-41 3

M-41 4

VD-1 1

VD-1 2

VD-1 3

28D 1

28D 2

28D 3

28D 4

28D 5

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O

43.32 1.46 10.84 17.75 0.29 10.60 11.68 1.38 0.73

43.75 1.18 10.20 17.32 0.28 10.97 12.03 1.32 0.67

43.35 1.26 10.56 17.35 0.28 10.63 11.64 1.38 0.69

44.13 1.21 9.91 17.57 0.30 10.73 12.07 1.31 0.64

41.31 1.98 13.00 14.96 0.23 11.41 12.10 2.19 0.81

41.45 1.93 12.92 14.82 0.25 11.47 12.14 2.30 0.77

41.12 1.94 12.95 14.86 0.23 11.50 12.18 2.27 0.81

40.98 1.98 13.06 14.84 0.27 11.38 11.93 2.25 0.79

40.98 1.65 12.33 16.45 0.30 10.63 11.63 1.85 1.19

41.19 1.76 12.42 16.63 0.28 10.70 11.84 1.84 1.24

41.13 1.49 12.45 16.24 0.28 10.58 11.89 1.79 1.23

42.990 0.636 11.600 15.890 0.381 11.530 11.910 1.402 0.850

45.210 0.508 9.430 16.220 0.408 11.770 12.010 1.176 0.607

42.710 0.606 11.050 16.900 0.338 11.180 12.310 1.277 0.869

42.980 0.608 11.420 16.280 0.409 11.300 12.200 1.363 0.821

43.720 0.634 11.630 15.260 0.394 11.680 12.150 1.298 0.730

Total

98.05

97.72

97.14

97.87

97.99

98.05

97.86

97.48

97.01

97.90

97.08

97.190

97.340

97.230

97.380

97.490

TSi TAl TFe3 TTi Sum_T CAl CFe3 CTi CMg CFe2 CMn CCa Sum_C BMg BFe2 BMn BCa BNa Sum_B ACa ANa AK Sum_A Sum_cat

6.394 1.606 0.000 0.000 8.000 0.278 0.777 0.162 2.332 1.414 0.036 0.000 5.000 0.000 0.000 0.000 1.847 0.153 2.000 0.000 0.242 0.137 0.380 15.380

6.483 1.517 0.000 0.000 8.000 0.263 0.664 0.132 2.423 1.482 0.035 0.000 5.000 0.000 0.000 0.000 1.910 0.090 2.000 0.000 0.289 0.127 0.416 15.416

6.453 1.547 0.000 0.000 8.000 0.305 0.717 0.141 2.359 1.442 0.035 0.000 5.000 0.000 0.000 0.000 1.857 0.143 2.000 0.000 0.255 0.131 0.386 15.386

6.545 1.455 0.000 0.000 8.000 0.276 0.576 0.135 2.372 1.604 0.038 0.000 5.000 0.000 0.000 0.000 1.918 0.082 2.000 0.000 0.295 0.121 0.416 15.416

6.115 1.885 0.000 0.000 8.000 0.381 0.444 0.220 2.518 1.408 0.029 0.000 5.000 0.000 0.000 0.000 1.919 0.081 2.000 0.000 0.548 0.153 0.701 15.701

6.136 1.864 0.000 0.000 8.000 0.388 0.390 0.215 2.531 1.444 0.031 0.000 5.000 0.000 0.000 0.000 1.925 0.075 2.000 0.000 0.586 0.145 0.731 15.731

6.103 1.897 0.000 0.000 8.000 0.366 0.418 0.217 2.544 1.426 0.029 0.000 5.000 0.000 0.000 0.000 1.937 0.063 2.000 0.000 0.590 0.153 0.743 15.743

6.092 1.908 0.000 0.000 8.000 0.378 0.489 0.221 2.522 1.356 0.034 0.000 5.000 0.000 0.000 0.000 1.900 0.100 2.000 0.000 0.549 0.150 0.698 15.698

6.152 1.848 0.000 0.000 8.000 0.332 0.637 0.186 2.379 1.429 0.038 0.000 5.000 0.000 0.000 0.000 1.871 0.129 2.000 0.000 0.409 0.228 0.637 15.637

6.138 1.862 0.000 0.000 8.000 0.318 0.601 0.197 2.377 1.471 0.035 0.000 5.000 0.000 0.000 0.000 1.890 0.110 2.000 0.000 0.422 0.236 0.658 15.658

6.181 1.819 0.000 0.000 8.000 0.385 0.511 0.168 2.370 1.530 0.036 0.000 5.000 0.000 0.000 0.000 1.914 0.086 2.000 0.000 0.436 0.236 0.672 15.672

6.355 1.645 0.000 0.000 8.000 0.376 0.792 0.071 2.541 1.172 0.048 0.000 5.000 0.000 0.000 0.000 1.886 0.114 2.000 0.000 0.288 0.160 0.448 15.448

6.667 1.333 0.000 0.000 8.000 0.305 0.670 0.056 2.587 1.330 0.051 0.000 5.000 0.000 0.000 0.000 1.897 0.103 2.000 0.000 0.234 0.114 0.348 15.348

6.362 1.638 0.000 0.000 8.000 0.302 0.738 0.068 2.483 1.367 0.043 0.000 5.000 0.000 0.000 0.000 1.964 0.036 2.000 0.000 0.333 0.165 0.498 15.498

6.371 1.629 0.000 0.000 8.000 0.366 0.706 0.068 2.497 1.312 0.051 0.000 5.000 0.000 0.000 0.000 1.937 0.063 2.000 0.000 0.329 0.155 0.484 15.484

6.429 1.571 0.000 0.000 8.000 0.444 0.652 0.070 2.560 1.224 0.049 0.000 5.000 0.000 0.000 0.000 1.914 0.086 2.000 0.000 0.284 0.137 0.421 15.421

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Fig. 5. Plot of amphibole compositions of the Valle Daza samples in terms of Mg/ (Mg + Fe2) (atomic proportions) for the calcic group where AnA + AK > 0.5; Ti < 0.5; Fe3 > Alvi.

Amphiboles (Table 2) are magnesian hastingsite and tschermakitic hornblende partly overlapping the limit with magnesiohonblendes (Figs. 4 and 5). Composition of plagioclases (Table 3; Fig. 6) ranges as follows: An60.01 Ab39.36 to An60.27 Ab39.22 in the relictic gabbros; An43.3 Ab55.8 to An51.5 Ab47.1 in the plagioclasite; An50.6 Ab48.8 to An56.7 Ab42.8 in the granulite facies; An45.06 Ab54.06 to An45.87 Ab53.69 in the amphibolite facies. The orthoclase component is slightly higher in the plagioclase of plagioclasites (up to 2.3%) than in the gabbros, granulites and amphibolites (up to 0.8%).

Fig. 6. Composition of plagioclases from La Pampa belt: in the relictic gabbros (diamonds), plagioclasite (squares) and metagabbros – granulite facies (circles), amphibolite facies (crosses).

side-rich and labradorite-rich layers preserved from the effect of metamorphism; Fig. 7a). Scarce plagioclasite layers up to 5 cm thick are recognized, with their magmatic texture partly preserved. Their mineralogical composition consists of andesine slightly altered to epidote, and up to 5% of idiomorphic apatite; their transition to granulite is clearly shown by the development of granoblastic texture (Fig. 7b).

3.2. Relictic igneous lithologies

3.3. Metamorphic conditions

Relics of medium (1–3 mm) to fine grained (<1 mm) gabbros were identified. They comprise magmatic diopside (Table 1, analyses M37-2 to 8, and VD1-3), fresh plagioclase (labradorite), sphene, interstitial magnetite and ilmenite. Diopside is locally deformed and replaced by hornblende showing sieve texture; hornblende also forms replacement rims. There is evidence of magmatic layering both on a mesoscopic and microscopic scale (alternating diop-

3.3.1. Granulite facies This facies is preserved in minor lenses of diopside and labradorite, with ilmenite showing no rims of sphene (Fig. 7c). Very scarce orthopyroxene has been identified in the Lonco Vaca mafic rocks and is thought to have been extensively ‘consumed’ during subsequent retrograde metamorphism. Hornblende of this facies is lighter in colour and less pleochroic than hornblende from the

Table 3 Chemical composition of plagioclases. Numbers of ions on the basis of 32 oxygens. Sample Analysis

VD-1 1

VD-1 2 (rim)

VD-1 3

VD-1 4

VD-1 5

VD-3 1

VD-3 2

VD-3 3

VD-3 4

VD-3 5

VD-3 6

28-D 1

28-D 2

28-D 3

28-D 4

28-D 5

28-D 6

SiO2 Al2O3 Fe2O3 CaO Na2O K2O

54.87 29.24 0.11 11.09 5.36 0.13

55.03 28.84 0.11 10.59 5.63 0.11

54.39 29.61 0.11 11.10 5.49 0.04

54.01 29.71 0.17 11.37 5.27 0.14

53.51 30.21 0.19 11.77 4.91 0.09

55.30 28.50 0.15 10.35 5.67 0.41

56.22 27.84 0.11 9.22 6.57 0.16

55.18 29.23 0.13 10.48 5.60 0.27

54.90 29.11 0.14 10.82 5.46 0.25

58.02 24.88 0.17 10.64 5.35 0.26

55.53 28.29 0.18 9.59 6.08 0.22

53.49 29.86 0.09 11.52 5.18 0.06

52.75 30.52 0.18 12.28 4.45 0.11

55.62 28.61 0.15 9.43 6.25 0.16

53.68 30.38 0.19 11.74 4.90 0.05

55.64 28.44 0.15 9.54 6.17 0.08

52.24 30.26 0.17 12.40 4.46 0.09

Total

100.81

100.31

100.74

100.67

100.68

100.38

100.12

100.88

100.68

99.33

99.89

100.20

100.28

100.23

100.93

100.02

99.62

Si Al Fe Ca Na K

9.824 6.169 0.015 2.128 1.862 0.030

9.891 6.108 0.015 2.039 1.963 0.024

9.751 6.256 0.015 2.132 1.909 0.010

9.704 6.290 0.023 2.189 1.835 0.032

9.617 6.398 0.026 2.266 1.709 0.022

9.942 6.039 0.020 1.993 1.975 0.093

10.100 5.894 0.015 1.775 2.287 0.036

9.864 6.158 0.018 2.008 1.940 0.061

9.844 6.151 0.019 2.079 1.900 0.056

10.491 5.302 0.024 2.061 1.877 0.060

10.006 6.007 0.025 1.852 2.124 0.051

9.657 6.354 0.014 2.228 1.813 0.014

9.528 6.499 0.027 2.377 1.559 0.025

9.987 6.055 0.023 1.815 2.177 0.036

9.619 6.416 0.028 2.254 1.701 0.011

10.007 6.028 0.023 1.838 2.151 0.018

9.510 6.492 0.026 2.419 1.574 0.021

CatTot

20.029

20.041

20.073

20.073

20.037

20.063

20.107

20.048

20.049

19.815

20.065

20.080

20.014

20.092

20.029

20.064

20.042

An % Ab % Or %

52.9 46.3 0.7

50.6 48.8 0.6

52.6 47.1 0.3

54.0 45.3 0.8

56.7 42.8 0.5

49.1 48.6 2.3

43.3 55.8 0.9

50.1 48.4 1.5

51.5 47.1 1.4

51.5 47.0 1.5

46.0 52.7 1.3

55.0 44.7 0.3

60.0 39.4 0.6

45.1 54.1 0.9

56.8 42.9 0.3

45.9 53.7 0.4

60.3 39.2 0.5

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389

Fig. 7. Photomicrographs illustrating relictic igneous lithologies and metamorphic assemblages of the La Pampa metagabbros. (A) Magmatic layering (alternating diopsiderich and labradorite-rich layers) in the relict gabbro. (B) Plagioclasite with evidence of transition to granulite facies (development of granoblastic texture: upper-left corner). (C) Diopside–labradorite–ilmenite association of the granulite facies. (D) Diopside-tschermakitic hornblende-recrystallized plagioclase assemblage of the amphibolite facies. E. Ilmenite with rim of sphene, associated with magnesian hastingsite, developed in the amphibolite facies. (F) Prehnite–chlorite association derived from hornblende, developed during the greenschist-facies retrograde metamorphism. Reference: Diop: diopside; And: andesine; Lab: labradorite; Ilm: ilmenite; Tscherm: tschermakitic hornblende; Sph: sphene; Hast: magnesian hastingsite; Preh: prehnite; Chl: chlorite; Hnb: hornblende. A, B and F: crossed polarizers; C, D and E: plane-polarized light. Scale bars are 1.5 mm long.

amphibolite facies. The high Al content (up to 3.69% Al2O3) in diopside (Table 1) is typical of granulite facies (e.g. Tsujimori and Liou, 2004) where clinopyroxene recrystallization occurs at ca. 8–9 kbar. 3.3.2. Amphibolite facies Granulites recrystallized under the amphibolite facies metamorphism, showing granonematoblastic textures. The mineralogy of this facies comprises variable amounts of hornblende, recrystallized plagioclase and relictic diopside partly replaced by green hornblende (Fig. 7d). The plagioclase is andesine; ilmenite developed rims of sphene (Fig. 7e). Amphiboles of this facies are magnesian hastingsite and tschermakitic hornblende (Table 2), both being indicative of high grade conditions in this facies (Gilbert et al., 1981).

3.3.3. Greenschist facies This superimposed, retrograde metamorphism is evidenced by the transition of hornblende to epidote and to pale-green actinolite, and by the formation of albite and chlorite at the expense of calcic plagioclases. At Lonco Vaca this retrograde metamorphism locally originated greenschist bands characterized by a tremolite–zoicite–biotite–chlorite association derived from hornblende (Fig. 7f). Very locally, at Valle Daza, the retrograde metamorphism reached the prehnite–pumpellyite and zeolite facies. The prehnite–pumpellyite facies is characterized by the pumpellyite–epidote–albite–quartz association, prehnite veinlets, and scarce prehnitization of plagioclase. The zeolite facies comprises very scarce fibrous zeolite.

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Evidence from regional geology indicates that: (i) the high temperature event of metamorphism and deformation affected the metagabbros (but not the small granitic bodies of Valle Daza and Lonco Vaca, dated at 405 Ma, i.e. Lower Devonian; Chernicoff et al., 2008). This high temperature event of metamorphism has also been identified in the basement of the Sierra de San Luis (e.g. Hauzenberger et al., 2001), where it was dated at 460– 450 Ma (Sims et al., 1998), and (ii): the low temperature event, associated with shearing at various scales, superimposed a retrograde metamorphism on the metagabbros and on the Lower Devonian granites. This low temperature event has also been recognized and dated in the Sierras de San Luis and Córdoba e.g. by Sims et al.

(1998), who assigned it to the Achalian Orogeny that occurred at 370–360 Ma (Upper Devonian). This succession of metamorphic events defines a counterclockwise P–T evolution that affected the metagabbros, i.e. during the Middle Ordovician the protolith reached an initial granulite facies of metamorphism (M1), evolving to amphibolite facies (M2). During the Upper Devonian, a retrograde metamorphism that reached the greenschist facies (M3) locally affected the metagabbros. A similar tectonometamorphic evolution has recently been established by Delpino et al. (2007) for the basement of the Sierra de San Luis. 3.4. Geochemistry Chemical analyses of the metagabbros were made in order to determine their magmatic affinities and to trace their petrogenesis (Table 4). The data were compared with the Las Aguilas mafic–

Table 4 Chemical analyses of the La Pampa metagabbros. Wt%

LV 28d

LV 29b

VD 1

VD 2

VD 3

VD 37

VD 39a

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI

45.76 1.431 17.32 11.95 0.169 5.24 12.93 2.54 0.44 0.1 1.46

49.01 1.342 14.3 11.36 0.21 8.49 8.82 3.25 0.42 0.11 2.42

45.62 1.39 14.01 12.79 0.21 7.87 13.07 2.46 0.70 0.16 0.90

45.90 1.00 16.72 10.03 0.16 6.98 14.39 2.41 0.44 0.08 0.95

51.53 0.05 26.05 0.72 0.02 0.22 12.08 5.54 0.44 2.71 0.59

49.62 1.21 14.39 9.36 0.20 5.79 15.19 2.63 0.13 0.09 0.78

45.99 1.88 13.13 13.54 0.19 6.75 10.31 2.90 0.51 0.19 4.41

Totals

99.33

99.74

99.18

99.06

99.94

99.39

99.78

V Cr Co Ni Cu Zn Ga Ge As Rb Sr Y Zr Nb Mo Ag In Sn Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Tl Pb Bi Th U

313 260 46 80 <10 70 19 2 <5 8 126 31 77 2 <2 <0.5 <0.2 <1 <0.5 2.4 47 2.7 8.6 1.47 8.8 3 1.2 4.3 0.8 5.2 1.1 3.3 0.48 3 0.45 2.4 <0.1 <1 <0.1 <5 <0.4 <0.1 <0.1

290 230 45 60 160 120 17 2 <5 14 125 33 73 2 <2 <0.5 <0.2 <1 <0.5 0.7 19 2.7 8.8 1.56 9 3.1 1.22 4.4 0.8 5.4 1.2 3.5 0.52 3.3 0.5 2.3 0.1 1 <0.1 10 0.7 <0.1 0.2

265 488 92 262 56 148 23 3 <5 10 148 32 85 9 <2 <0.5 <0.2 8 <0.5 <0.5 403 9.0 24.6 3.60 18.3 5.4 1.83 6.3 1.1 6.4 1.3 4.1 0.58 3.4 0.49 2.6 0.5 <1 0.3 <5 25.6 0.4 0.2

183 440 78 276 10 85 20 2 <5 8 229 20 69 4 <2 <0.5 <0.2 8 <0.5 <0.5 418 4.0 10.6 1.66 9.1 2.8 1.12 3.6 0.6 3.8 0.8 2.5 0.37 2.3 0.34 2.1 0.2 1 0.2 <5 71.5 0.5 0.3

16 26 2 <20 32 <30 22 1 <5 7 487 23 <5 17 <2 <0.5 <0.2 <1 <0.5 <0.5 354 24.0 43.8 5.27 20.9 4.8 1.38 4.8 0.8 4.3 0.8 2.1 0.28 1.6 0.20 <0.2 <0.1 <1 <0.1 5 23.7 1.7 0.9

249 350 35 60 50 40 15 2 <5 3 179 27 71 2 <2 <0.5 <0.2 <1 <0.5 <0.5 207 4.3 11.2 1.79 9.7 3 1.15 3.9 0.7 4.4 0.9 2.8 0.4 2.5 0.38 2.2 0.1 <1 <0.1 <5 <0.4 0.2 0.1

354 230 47 70 50 100 19 2 <5 13 139 43 118 5 <2 <0.5 <0.2 1 <0.5 <0.5 70 5.2 16 2.73 14.8 4.6 1.65 6.1 1.1 7 1.5 4.5 0.66 4.1 0.62 3.4 0.3 <1 0.2 <5 <0.4 <0.1 0.3

Fig. 8. Chemical classification of metagabbros in the TAS diagram adapted by Wilson (1989). Triangles: Lonco Vaca samples. Circles: Valle Daza samples.

Table 5 CIPW volatile-free norm for the La Pampa metagabbros. In computing C.I.P.W. norms, Fe2O3/(Fe2O3 + FeO) was set equal to 0.20 for the metagabbros, and to 0.35 for the plagioclasite, following Middlemost (1989). Wt%

LV 28d

LV 29b

VD 1

VD 2

VD 3

VD 37

VD 39a

Q (S) or (KAS6) ab (NAS6) an (CAS2) lc(KAS4) ne(NAS2) C(A) ac(NFS4) ns(NS) Di wo(CS) Di en(MS) Di fs(FS) Hy en(MS) Hy fs(FS) Ol fo(M2S) Ol fa(F2S) mt(FF) he(F) il(FT) ap(CP) Totals

0 2.69 18.03 35.61 0 2.23 0 0 0 12.52 6.2 6.06 0 0 5.12 5.52 2.99 0 2.8 0.23 100

0 2.56 28.33 23.89 0 0 0 0 0 8.6 5.06 3.11 6.42 3.94 7.29 4.94 2.99 0 2.63 0.25 100

0 4.25 12.33 25.73 0 4.87 0 0 0 16.63 9.14 6.87 0 0 7.73 6.41 2.97 0 2.71 0.36 100

0 2.73 13.45 27.34 0 4.28 0 0 0 19.64 11.24 7.51 0 0 4.94 3.65 3.04 0 1.99 0.18 100

0 2.62 43.52 44.34 0 1.96 0.3 0 0 0 0 0 0 0 0.39 0.48 0.36 0 0.09 5.95 100

0 0.79 22.83 27.81 0 0.01 0 0 0 20.52 11.47 8.22 0 0 2.39 1.89 1.49 0 2.37 0.2 100

0 3.21 26.11 22.67 0 0 0 0 0 12.8 6.79 5.6 1.02 0.84 7.12 6.48 3.09 0 3.83 0.44 100

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391

TiO2 (slightly depleted in La Pampa). The latter differences are regarded as local variations related to the metamorphic processes. Basic magmas can be generated in various geotectonic environments. The REE pattern of the La Pampa metagabbros (Fig. 10) has an E-MORB signature, coincident with that of the Sierra de San Luis mafic–ultramafic rocks. The trace elements spider diagram (Fig. 11) shows a flat profile and E-MORB affinities for both the La Pampa and Sierra de San Luis mafic rocks. It also reveals variable Th content (Sierra de San Luis) and a negative anomaly of Ti (La Pampa). In the log Ti/Y–Nb/Y diagram (Fig. 12) of Pearce (1982), mafic samples from La Pampa and Sierra de San Luis plot in the MORB field. The La Pampa metagabbros also plot in the MORB + BAB (back-arc basalts) field of the Ti/V diagram of Shervais (1982) (Fig. 13) that discriminates arc tholeiites, MORB and alkali basalts.

Fig. 9. AFM diagram after Irving and Baragar (1971) showing the La Pampa metagabbros compositions and trend. Symbols: see Fig. 8.

Fig. 11. E-MORB normalized incompatible-element distribution diagrams for La Pampa metagabbros (vertical hatching) and San Luis mafic–ultramafic belt (horizontal hatching). Data for San Luis belt from Brogioni (2001) and Brogioni and Ribot (1994).

Fig. 10. E-MORB normalized REE-patterns of La Pampa metagabbros (vertical hatching) and San Luis mafic–ultramafic belt (horizontal hatching). Data for San Luis belt from Brogioni (2001) and Brogioni and Ribot (1994).

ultramafic belt of the Sierra de San Luis, where geochemical data are available for a number of individual bodies (Virorco, El Fierro, La Melada and La Gruta; Brogioni and Ribot, 1994; Brogioni, 2001). In the SiO2/Na2O + K2O diagram of Wilson (1989), the La Pampa metagabbros plot in the field of gabbros, in the limit of subalkaline to alkaline series (Fig. 8). This affinity is also reflected in the normative compositions, where all samples are olivine normative and most of them show normative nepheline (Table 5). In the AFM diagram all samples plot in the tholeiitic field (Fig. 9). Concerning the major elements, TiO2, Al2O3 and Na2O are similar to MORB average contents, but they are enriched in K2O and depleted in SiO2. Both the major element content of the La Pampa and Sierra de San Luis mafic rocks are similar, with minor differences in MgO and Fe2O3tot (slightly enriched in La Pampa) and in SiO2 and

Fig. 12. Ti/Y–Nb/Y discrimination diagram of Pearce (1982). Symbols for La Pampa metagabbros: see Fig. 8. Mafics from the Sierra de San Luis (squares) for comparison. San Luis data from Brogioni (2001) and Brogioni and Ribot (1994).

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In both the Ti–Zr (Fig. 14) and TiO2–FeOT/MgO diagrams (Fig. 15), the samples from both areas show a similar distribution trend (with some dispersion of data for the Sierra de San Luis mafic rocks), plotting in the MORB (Fig. 14) and MORB + BAB (back-arc basalts) (Fig. 15) fields. The La–Y–Nb diagram (Fig. 16) allows discrimination between oceanic, continental and volcanic-arc basalts. The La Pampa metagabbros plot in the 2B field that corresponds to back-arc basic magmatism and also slightly straddle the boundary between the 2B, 3C (E-MORB) and 3D (N-MORB) fields. The back-arc rifting envisaged for the mafic rocks of La Pampa (Zappettini et al., 2005) developed on continental crust, as is also the case for the Las Aguilas mafic–ultramafic belt of the Sierra de San Luis by Brogioni and Ribot (1994). The absence of exposed ultramafic rocks in La Pampa does not preclude its existence; in fact, the very high magnetic signature identified in La Pampa (Fig. 2a) may partly arise from buried ultramafic rocks.

Fig. 15. TiO2 versus FeT/MgO abundances of the La Pampa and San Luis belts. MORB + Back arc basalts (BAB), Intra-arc tholeiites (IAT), IA, CA and B, from Hawkins (1980). Symbols for La Pampa metagabbros: see Fig. 8. San Luis mafic–ultramafic belt (solid squares) for comparison. San Luis data from Brogioni (2001) and Brogioni and Ribot (1994).

Fig. 13. V versus Ti/1000 diagram of Shervais (1982) applied to the La Pampa metagabbros. Symbols: see Fig. 8.

Fig. 16. La/10–Y/15–Nb/8 discrimination diagram for La Pampa metagabbros. Fields subdivision from Cabanis and Lecolle (1989). (1A) Calk-alkali basalts, (1C) volcanic-arc basalts, (1B) overlapping area between 1A and 1C, (2A) continental basalts, (2B) back-arc basin basalts, (3A) alkali basalts, (3B and 3C) E-type MORB, (3D) N-type MORB. Symbols: see Fig. 8.

Fig. 14. Ti versus Zr abundances of the La Pampa and San Luis belts. A, B, C and D fields from Shervais (1982). Symbols for La Pampa metagabbros: see Fig. 8. San Luis mafic–ultramafic belt (squares) for comparison. San Luis data from Brogioni (2001) and Brogioni and Ribot (1994).

From a metallogenetic point of view, and consistent with the geotectonic setting and age of the mafic rocks, the metallotect (Ni–Cu–Co magmatic mineralizations) defined in the Sierra de San Luis can be extended south to La Pampa. The Valle Daza metagabbros have a high Ni–Cu–Co content and, when compared to the Virorco mineral deposit (Las Aguilas belt), are enriched in Ni, which can be an indication of their mining potential (Zappettini et al., 2005).

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C.J. Chernicoff et al. / Journal of South American Earth Sciences 28 (2009) 383–397 Table 6 Sm–Nd isotopic data for the WR metagabbros. Sample

Sm (ppm)

Nd (ppm)

14

7Sm/144Nd

MM 241 MM 242

2.13 5.20

6.86 18.62

0.1874 0.1689

143

Nd/144Nd (t = 0)

0.51296 0.51288

Error (ppm)

Epsilon Nd (0)*

Epsilon Nd (t)**

TDM***

30 13

6.25 4.81

6.82 6.51

552 574

Epsilon Nd(0) = ((143Nd/144Nd[sample, now]/0.512638) I)  104. * Calculated assuming143Nd/144Nd today = 0.512638 with data normalized to 146Nd/144Nd = 0.72190. ** End(t) = ((143Nd/144Nd[sample, t]/143Nd/144Nd[CHUR, t]) I)  104, where t = 480 Ma (see text for explanation). *** Calculated following model of DePaolo (198 I).

4. Nd isotope geochemistry 4.1. Geochronological constraints The geochronological data available for the country rocks of the metagabbros impose a number of geological constraints on their crystallization age, the following data being herein considered: (1) Zircon U–Pb SHRIMP dating of the Green quarry schists yielded 500 ± 3 Ma (Upper Cambrian) for the youngest detrital zircon, indicating the maximum age for the onset of sedimentation (Chernicoff et al., 2006, 2007). The latter authors also report a U–Pb SHRIMP age of 515 ± 4.6 Ma for the youngest detrital zircon of the equivalent Lonco Vaca schists. (2) U–Pb SHRIMP dating of fully recrystallized metamorphic zircons from the Paso del Bote schists yielded 464.7 ± 7.3 Ma (unpublished data of the authors), interpreted as the age of metamorphism related to the docking of the Cuyania terrane. This age is consistent with the Ar/Ar metamorphic age of 461 ± 2 Ma indicated by Tickyj et al. (1999) for the basement rocks of Valle Daza. Therefore, the crystallization age of the metagabbros should be younger than ca. 500 Ma (because the gabbros intruded the country sequence) and older than ca. 465 Ma (because the gabbros and their host rocks were affected by metamorphism coevally). It should also be mentioned that Sims et al. (1998) obtained an U–Pb zircon age of 478 ± 6 Ma from a felsic segregation in the Las Aguilas mafic–ultramafic belt of Sierra de San Luis – herein regarded as equivalent to the La Pampa metagabbros (see below) – and that we have recently obtained a U–Pb SHRIMP zircon age of crystallization of 475.7 ± 2.3 Ma (unpublished data) for the magmatic arc rocks (metaquartz-diorites–metatonalites) of Paso del Bote indicated by Villar et al. (2005). 4.2. Sm/Nd data Whole-rock Sm/Nd isotopic analysis has been carried for the La Pampa metagabbros. A brief description of the methodology is given first. Methodology. Whole-rock powders were spiked with mixed 149 Sm–150Nd tracer and dissolved in a Teflon vial using a HF– HNO3 mixture and 6 N HCl until complete material dissolution. Column procedures used cationic AG-50W-X8 (200–400 mesh) resin in order to separate Rb, Sr and REE, followed by Sm and Nd separation using anionic politeflon HDEHP LN-B50-A (100–200 m) resin according to Patchett and Ruiz (1987). Each sample was dried to a solid and then loaded with 0.25 N H3PO4 on the appropriate filament (single Ta for Sm and triple Ta–Re–Ta for Nd). Isotopic ratios were measured in static mode with a VG Sector 54 multi-collector mass spectrometer at the Laboratório de Geologia Isotópica of Universidade Federal do Rio Grande do Sul (Brazil). We normally collected 100–120 ratios with a 1-volt 88Sr beam and a 0.5–1-volt 144 Nd beam. Nd ratios were normalized to 146Nd/144Nd = 0.7219, respectively. All analyses were adjusted for variations from instru-

Fig. 17. Epsilon Nd diagram for the La Pampa metagabbro samples showing its juvenile signature at 480 Ma.

mental bias due to periodic adjustment of collector positions as monitored by measurements of the internal standards of the laboratory. Measurements for the Spex 143Nd/144Nd = 0.511130 ± 0.000010. Total blanks average was <150 pg for Sm and <500 pg for Nd. Correction for blank was insignificant for Nd isotopic compositions and generally insignificant for Sm/Nd ratios. Neodymium crustal residence ages (TDM) were calculated following the depleted mantle model of De Paolo (1981). The whole-rock Sm–Nd data of the metagabbros suggest a juvenile source for these rocks, with model ages of 552 and 574 Ma, and positive Epsilon values of 6.51 and 6.82, respectively (t = 480 Ma) (see Table 6 and Fig. 17). The assumed t value (480 Ma = crystallization age) was based on the geological considerations presented above, i.e. (a) oldest possible age of the protolith of the host schists, (b) age of metamorphism of the host schists, and (c) crystallization ages of the magmatic back-arc rocks of Sierra de San Luis and magmatic arc rocks of Paso del Bote. The depleted mantle source of the La Pampa metagabbros is consistent with their enriched MORB and back-arc signature. 5. 3-D magnetic modeling of the belt of metagabbros For the purpose of modeling, the available aeromagnetic data were preferred over the available gravimetric data due to their much higher resolution. Despite their lower resolution, gravimetric data have proved to be consistent with the aeromagnetic data of

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central La Pampa. Fig. 2a shows the geophysical coverage of the study region and the location of the modeled transect. Modeling of the aeromagnetic data is presented in Fig. 18A. It is based on a 15 km long segment of an E–W line located immediately south of Valle Daza, taken from the aeromagnetic survey of the province of La Pampa (line 45222; SEGEMAR, 2004). A survey altitude of 100 m and sampling rate of 8 m provide a high resolution for the magnetic profile. The selected segment traverses both the metagabbros and the flanking paraschists and paragneisses (Fig. 2b). The model establishes that the mafic rocks in this profile are 5 km wide, also picking out ´ magnetic layering´ at this scale, i.e. metagabbroic horizons with different magnetic susceptibilities, always within a range of high susceptibility values (which could well indicate the existence of an ultramafic component). These layers would correspond to either lithological variations of the protolith or mineralogical changes produced by the superimposed metamorphism. The actual magnetic susceptibility values (layer E1: 0.02– 0.03 SI; layer E2: 0.055 SI) contrast with the much lower values of the surrounding host rocks (0.005 SI). The alternating nature of the E1 and E2 layers may be due to faulting. The occurrence of tectonic repetition by thrusting of slivers of metagabbro is suggested in the interpretation of the modeling (Fig. 18B). This faulting may be as old as Famatinian and/or younger (up to Cenozoic). Immediately west of the mafic belt there is a drastic drop in the calculated magnetic susceptibility (down to 10 6 SI), reflected by a

Fig. 18. (A) Modeling of an aeromagnetic profile (segment of survey line 45222; SEGEMAR, 2004) (see location on Fig. 2). References: Solid line: observed anomaly; dashed line: calculated anomaly; E1 and E2: layers of metagabbro; absissa: xcoordinate of Gauss–Kruger projection. NB: values of the profile correspond to magnetic anomalies plus a constant = 24.784 nT (IGRF of central point of aeromagnetic survey). (B) Geological interpretation of the magnetic model.

low magnetic anomaly. This signature is interpreted to correspond to the location of a shear zone, herein referred to as Valle Daza shear zone, where severe demagnetization must have taken place. The Valle Daza shear zone would be equivalent to the Las Aguilas shear zone that delineates the western boundary of the Las Aguilas mafic–ultramafic belt in the Sierra de San Luis (Chernicoff and Ramos, 2003).

6. Geotectonic evolution The integration of all available geological, geochemical and geophysical data of the study region permits us to establish a geotectonic model summarized in two schematic profile sketches (Fig. 19). The first sketch (Fig. 19A) represents the geotectonic configuration during the Lower Paleozoic (>475 Ma). It involves the subduction of oceanic crust at the southwestern Gondwana margin, with the concomitant development of a magmatic arc, and extension in the back-arc region leading to the intrusion of gabbros at ca. 480 Ma (this article). Concerning the magmatic arc, which in the Sierra de San Luis is known to have been active since ca. 507 Ma (von Gosen et al., 2002), in La Pampa is represented by the metaquartz-diorites and metatonalites of Paso del Bote (Villar et al., 2005) dated at 475.7 ± 2.3 Ma (U–Pb SHRIMP zircon age of crystallization, unpublished data of the authors). The second sketch (Fig. 19B) represents the geotectonic configuration at present, which involves four stages, i.e.: 1. Middle Ordovician. Collision of Cuyania. This event caused compression and metamorphism throughout the arc–back-arc region, affecting the magmatic units and their host rocks at 464.7 ± 7.3 Ma (U–Pb SHRIMP age of fully recrystallized metamorphic zircons borne by the host paraschists and paragneisses, unpublished data of the authors, consistent with Ar/Ar metamorphic age of 461 ± 2 Ma indicated by Tickyj et al. (1999), for the basement rocks of Valle Daza). As a result, metaquartz-diorites and metatonalites were developed in the arc region (Villar et al., 2005), and metagabbros in the back-arc area (Chernicoff et al., 2005; Zappettini et al., 2005; this article). Later in this stage, the deformation was concentrated in shear zones bounding the arc- and back-arc derived metamorphic rocks. A Late Ordovician–Devonian marine foreland basin (Curacó basin) developed after the Cuyania accretion, with the depocentre located on Cuyanian crust, immediately to the west of the Cuyania–Pampia terrane divide (Chernicoff and Zappettini, 2003, 2004, 2005a; Chernicoff et al., 2008). 2. Devonian. During the Early Devonian there were intrusions of granite (e.g. at ca. 405 Ma in the study area; Chernicoff et al., 2008). The collision of Chilenia in the Late Devonian caused: (i) reactivation of shear zones, e.g. Valle Daza shear zone; some shear zones affected Early Devonian granites; and (ii) large strike-slip displacements along major structures (e.g. Cuyania–Pampia suture, Valle Daza shear zone) that controlled the formation of transtensional pull-apart basins. Coevally, in northern Argentina the reactivation of terrane boundaries and other major structures occurred (see e.g. Fernández Seveso and Tankard, 1995). 3. Upper Paleozoic. During this stage, red beds were deposited in small pull-apart basins, e.g. Arizona basin, southern San Luis province, and smaller depocenters at Telén and Valle Daza (Chernicoff and Zappettini, 2005b, 2005c, 2007). 4. Cenozoic. At this stage, the final reactivation of the Valle Daza shear zone (and other major structures, e.g. the Cuyania–Pampia suture) occurred in La Pampa. The Valle Daza shear zone was reactivated as a west-vergent, high-angle reverse fault. This

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Fig. 19. Geotectonic evolution of the study region, depicted schematically in two sections: (A) geotectonic configuration during the Lower Paleozoic; (B) geotectonic configuration at present (see text for explanation of four stages involved in this sketch; window indicates location of modeled profile, Fig. 18).

fault was first identified by Stappenbeck (1913) between Telén and Victorica, and interpreted by Chernicoff et al. (2005) as the structure that marks the western boundary of the metagabbros at Valle Daza. In the present study, on the basis of gravimetric data (Fig. 2a), it has been interpreted to extend approximately 130 km to the north of Valle Daza, where it forms the western boundary of the Lonco Vaca hillocks. It is also interpreted that deflections of this major structure off its N–S trend, e.g. that at about 36°S, gave rise to roughly E–W transcurrent movements (Fig. 2b). It is considered that this fault corresponds to the same reverse fault system that further north, in central Argentina, caused the uplift of the Eastern Sierras Pampeanas (e.g. Sierra de San Luis). An example of this fault system in the Sierra de San Luis is the Las Aguilas thrust (Chernicoff and Ramos, 2003) that delineates the western boundary of the Las Aguilas mafic–ultramafic belt. The Valle Daza and Las Aguilas thrusts are considered to be responsible for the tectonic emplacement of the mafic belt in La Pampa and in the Sierra de San Luis, respectively. Either mafic belt and the adjacent thrusts are regarded to be in physical continuity under modern cover sediments.

7. Conclusions The province of La Pampa in south-central Argentina is a region of very low relief mostly covered by modern sediments. The use of geophysical data in the study region has been critical in outlining important geological boundaries because of the scarcity of exposures; aeromagnetic data have the property of being able to see through the thin Quaternary cover. The 300 km long and 5 km wide N–S trending belt of metagabbros is only poorly exposed in the localities of Valle Daza and Sierra de Lonco Vaca, but stands out in the geophysical data (aeromagnetics and gravimetry) due to its high magnetic intensity and gradient, high magnetic susceptibility and high gravimetric gradient.

Concerning the age of the metagabbros, the geological constraints indicate that they are younger than ca. 500 Ma and older than ca. 465 Ma (ages of the protolith of the country paraschists and the metamorphism of the country schists, respectively). The whole-rock Sm–Nd data of the metagabbros yielded model ages of 552 and 574 Ma. Assuming a crystallization age of 480 Ma based on the above geological considerations, positive Epsilon values of 6.51 and 6.82 were calculated, suggesting a juvenile source from a depleted mantle for the gabbros. Such a source is consistent with the enriched MORB and back-arc geochemical signature obtained for these rocks. The main rock type of the belt are metagabbros with relictic magmatic nucleii where layering is preserved. A counterclockwise P–T evolution affected the mafic belt, i.e. during the Middle Ordovician the protolith reached an initial granulite facies of metamorphism (M1), evolving to amphibolite facies (M2). The presence of diopside with high Al2O3 content is indicative of high-pressure conditions, and its partial transformation to hornblende is consistent with a retrogressive evolution. Both M1 and M2 are related to the collision of Cuyania. During the Upper Devonian, a retrograde, greenschist facies metamorphism (M3) partially affected the metagabbros. M3 would be related to the collision of Chilenia. The similarities between the La Pampa belt of metagabbros and the mafic–ultramafic Las Aguilas belt of Sierra de San Luis (central Argentina, i.e. approximately 200 km to the north of the study area) are most conspicuous, though the intervening region is covered by Quaternary sediments. A number of such similarities as the following stand out, i.e.: i. Geochronological data constrain the age of both belts, as well as the magmatic arc in Sierra de San Luis and in La Pampa, to the Lower Paleozoic. Notably, in La Pampa province the Lower Paleozoic (Famatinian) arc is very poorly exposed, e.g. at Paso del Bote (Villar et al., 2005), though its full extent is inferred from the geophysical data (Fig. 2). We have recently obtained a U–Pb SHRIMP zircon age of crystallization of 475.7 ± 2.3 Ma (unpublished data) for the

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ii. iii.

iv.

v.

vi.

vii.

metaquartz-diorites–metatonalites of Paso del Bote, comparable to the 478 ± 6 Ma U–Pb zircon age obtained by Sims et al. (1998) from a felsic segregation in the Las Aguilas mafic–ultramafic belt of Sierra de San Luis. The REE pattern of both belts indicate a MORB signature. Both belts are located to the east of the Famatinian magmatic arc. Also, both magmatic arc–back-arc systems developed on the Pampia side of the Cuyania–Pampia suture. Admittedly, the arc–back-arc distance is larger in La Pampa than in the Sierra de San Luis. In fact, the Las Aguilas belt is in tectonic contact with the arc rocks, and is emplaced in the footwall – i.e. to the east – of the Arroyo de las Aguilas thrust (Chernicoff and Ramos, 2003). But this tectonic contact between the Famatinian arc and back-arc in the Sierra de San Luis is due to the severe shortening related to the location of the Sierra de San Luis within the modern Pampean flat subduction segment – latitudes 27°S to 33°S – (Ramos et al., 2002), which is not the case of the La Pampa region. Both belts are approximately N–S trending, have similar widths and are located within a longitude range of one degree. Both are aligned, and are roughly parallel to the magmatic arc. The accretion of Cuyania against Pampia caused deformation and regional metamorphism throughout the arc–back-arc region both in La Pampa and in the Sierra de San Luis (see e.g. von Gosen, 1998; von Gosen et al., 2002; Tickyj et al., 1999; Chernicoff and Ramos, 2003; Thomas and Astini, 2003; Villar et al., 2005; Chernicoff et al., 2008). Timing of this event in the study area was established at ca. 461– 465 Ma. Both belts are affected by ductile shear zones that developed during the collision of Cuyania and were reactivated in the Upper Devonian during the collision of the Chilenia terrane. A final reactivation of these shear zones occurred during the Cenozoic as west-vergent, steeply-dipping thrusts, causing the tectonic emplacement of the belts. The La Pampa belt of metagabbros underwent a counterclockwise tectonometamorphic evolution, similar to that reported by Delpino et al. (2007) for the Lower Paleozoic metamorphic basement of the Sierra de San Luis.

In both the Sierra de San Luis (e.g. Chernicoff and Ramos, 2003, and references therein) and La Pampa province, the mafic rocks are regarded to be originated as a result of the extension that took place in a back-arc setting coevally with the Famatinian magmatic arc, an event that was later´aborted´ by the collision of the Cuyania terrane against Pampia-Gondwana in the Middle Ordovician. The overall considerations referred to above allow definition of a single, Famatinian back-arc belt in central Argentina, from San Luis to La Pampa. Acknowledgements This work received financial support from Research Grants PIP5008 and PIP0424 of CONICET (Council for Scientific and Technical Research of Argentina) and from the Geological Survey of Argentina (SEGEMAR). SEGEMAR also provided logistical support and geophysical data. We thank the Smithsonian National Museum of Natural History (USA) for providing standards for probe analyses (USNM 137041, USNM 117733, USNM 85276, USNM 143966). References Almeida, F.F.M., Amaral, G., Cordani, U.G., Kawashita, K., 1973. The Precambrian evolution of the South American cratonic margin, south of the Amazon River. In:

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