Pergamon Joumal of Sourh Anwican Earth Scimces. Vol. IO. No. 2, pp. 147-155. IYY7

8 IYY7 Published hv Elrvier Scimcc Lad

PII: so89~9811(97)ooo124 All rights -cd. Printed in Great Britain

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New Evidence on the Nature of the Frontal Cordillera Ophiolitic Belt - Argentina

DANIEL A. GREGORI and ERNEST0 A. BJERG

Departamento de Geologia, Universidad National de1 Sur-ONICET, San Juan 670,8COO Bahia Blanca, Argentina. usgregor@riba.edu.ar - usbjerg@riba.edu.ar

(Received November 1995; accepted 20 January 1997)

Absbvct - New petrographic and chemical data of volcanic rocks from the Metales Belt, Rio de las Tunas area, in the Frontal Cordillera of Argentina are presented. The rocks display a broadly basaltic composition. ‘PNO varieties of basaltic dikes can be dif- ferentiated. One group, whose mineralogy comprise essentially tremolite-actinolite, plagioclase, pyroxene and limited amounts of olivine as the main phases, display metamorphic textures. The other group show porphyritic textures consisting of zoned plagi- o&se and K-feldspar. Zoned orthopyroxenes are scarce. Chemically, the rocks are similar to the Caradocian basaltic rocks (the ofiolitas famatinianas) from the Precordillera, San Juan Province. REE chondrite-normal&d patterns and overall abundances, along with other petrologic and geochemical similarities, suggest a common mode of origin for both groups of basaltic rocks. Their geochemical and petrographical characateristics are consistent with a continental-scale extension environment during Eopalaeo- zoic times that could include an oceanic rift in an early stage of development, a transitional ridge segment or oceanic intraplate magmas, similar to those erupted in oceanic islands. Q 1997 Published by Elsevier Science Ltd

Resumen - Se presentan nuevos dates petrogrticos y qufmicos de rotas volc6nicas de la Faja Metales, aflorantes en el &rea del Rio de las Tunas, en la Cordillera Frontal de Argentina. Las rotas muestran una composicidn bastitica. DOS variedades de diques bastiticos pueden ser reconocidos. El primer grupo muestra 10s efectos del metamorfismo regional y se halla compuesto por tremolita-actinolita, plagioclasa, piroxeno y olivino. El otro grupo, que presenta texturas porfirfticas, se compone de plagioclasa zonada y feldespato potiico. principalmente. Se observan adem& escasas cantidades de ortopiroxeno zonado. Qufmicamente estas rotas son similares a las rotas bas5ltica.s Caradocianas (las oliolitas famatinianas) de la Pmcordillera de la provincia de San Juan. Los disefios de 10s diagramas de tierras rams normalizados a condritos y sus abundancias junta con otras caracterfsticas petrogr6ficas y geoqufmicas sugieren un modo corntin de origen para ambos grupos de row bas&kas. Los dates petrognlficos y geoquimicos de ambos grupos de row son coherentes con un ambiente extensional a escala continental durante el Eopaleozoico. Tal tipo de ambiente podrfa incluir un estadio temprano en la evolucidn de un rift oc&nico, un segment0 de cordillera oceAnica de1 tipo transitional o magmas de intraplaca coma aquellos eruptados en islas oceAnicas.

INTRODUCTION

The geological configuration of the Andean Cordillera documents the geological evolution of the south-western Gondwana margin from Precambrian to Quatemary times. In this geological context, the Precordillera and Frontal Cordillera of Argentina have been considered as two sepa- rate terranes each with its own geological evolution.

The first one was considered as a displaced terrane (Ramos, 1988) and recently as an accreted terrane during the Ordovician with Laurentian afinities (Astini et al., 1996), whereas the second, (the Chilenia terrane, Ramos et al. 1984) represents a microcontinental block accreted to the Gondwana margin during Late Devonian times (Ramos et al., 1986).

A disrupted mafic-ultramafic belt that extends from 29” S to 33” S represents a suture zone between Chilenia and Precordillera terranes (Ramos et al. 1986). The mafic- ultramafic rocks cropping out in the northern portion of the suture zone (28” to 33” S) were studied by Kay ef al.

(1984), Haller & Ramos (1984; 1993), Ramos et al. (1986), Haller (1995) and Davis et al. (1995). These rocks are interbedded with distal limestone turbidites, pelagic lime-

stones and cherts. Although problems in interpretation still remains, the investigation led to the conclusion that these rocks were generated in back arc basins or transitional mid- ocean ridges, Kay et al. (1984).

In the belt portion extending from 32”30’ S to 33”30’ S detailed studies on the ultramafic rocks were carried out among others by Bjerg et al. (1991, 1993); Villar, (1969); Villar & Donari, (1989), Villar et al. (1991) and Ruviiios & Gregori, (1996). Detailed mapping of the Rio de las Tunas area, located in the western side of the Cuchilla de Guar- guaraz, Frontal Cordillera, Bjerg et al. (1990, 1993, 1994 a-b), revealed the presence of low-grade metasedi- mentary rocks and a mafic-ultramafic sequence composed of serpentinized ultramafic rocks, altered gabbros, metaba- sites, basic dykes and pillow lavas. Based on geological and geochemical criteria these authors assigned these rocks to an ophiolitic sequence.

Similar ideas were proposed by Villar & Donari (1989) in their the study of the Los Gateados Complex, cropping out on the eastern side of Cuchilla de Guarguaraz. How- ever, Villar et al. (1991) in a re-evaluation of the Los Gat- eados Complex and based on platinum group chemistry

Address all correspondence and reprint request to D.A. Gregori. Departamento de Geologia, Universidad National de1 Sur-CONICET, San Juan 670,800O Bahia Blanca, Argentina. Telephone [54] 91 25196 ext. 360; Fax [54] 91 880614. E-mail: usgregor@eriba.edu.ar

147

148 D. A. GREGORI and E. A. BJERG

concluded that this complex is related to komatitic rocks rather than ophiolitic rocks.

A better understanding of these mafic-ultramafic rocks is important, not only from the perspective of the evolution of the Frontal Cordillera basement but also from a more regional perspective, such as the relationships between the Chilenia and the Precordillera terrane basement since these rocks could represent the boundary between these two adjacent terranes.

In this paper we present new petrological and geo- chemical data of the mafic rocks exposed on the western side of the Cuchilla de Guarguaraz, Frontal Cordillera, in an attempt to establish their origin and possible tectonic setting and to contribute to the elucidation of the role played by these rocks during the evolution of the Frontal Cordillera. Preliminary comparisons are also made with rocks exposed in the Precordilleran area.

REGIONAL GEOLOGY

Numerous serpentinized ultramafic bodies crop out in the Frontal Cordillera of Mendoza province, central west- ern Argentina (Fig. 1). They are hosted by folded NE- strik- ing mica and amphibole schists and marbles, with mineral paragenesis typical of low grade metamorphism, Bjerg et al. (1990), belonging to the Complejo Metamdrfico (Cam- brian, 500+50 Ma, Caminos et al. 1982).

The ultramafic rocks, which are of uncertain age (between 500 and 300 Ma), were considered to represent part of an Alpine-type belt, Villar (1985) and occur over a distance of 60 km (Fig. l), as elongated NE-striking bodies

with associated sulphide and talc mineralizations: Brodtkorb (1970, 1971); Bjerg (1984, 1985); Gregori (1985); Gregori & Bjerg (1992) and Bjerg et al. (1993). Based on the data of mafic and ultramatic rocks in the west- ern Precordillera and northern Frontal Cordillera, Haller & Ramos (1984, 1993) considered these rocks to constitute an ophiolitic belt. Villar & Donnari (1989) also proposed that the serpentinized ultramafic rocks in the Salamanca district represent the basal section of an ophiolite complex.

Recent field observations have shown that in the Las Tunas area the mafic-ultramafic sequence is composed of serpentinized ultramafic rocks, altered gabbros, basaltic dykes and lava flows showing pillow structures. The whole sequence has been affected by intense folding, faulting and low grade metamorphism, which has partly obscured the original nature of the rocks.

The Complejo Metamorfico (Fig. 1) and the ultramahc bodies were intruded and covered by Late Palaeozoic- Triassic granitic and rhyolitic rocks (Polanski, 1972). The intrusive rocks constitute the Frontal Cordillera Composite Batholith, which extends for over 130 km from N to S. Several plutons and stocks were mapped and described by Caminos (1965) and Polanski (1972). K-Ar and Rb-Sr radiometric ages have been reported by Dessanti & Cami- nos (1967) and Caminos et al. (1982).

New investigations, Gregori er al. (1996), have shown that the Frontal Cordillera granitoids may have originated by partial melting of relatively mafic, high-K talc alkaline lower crust during a period of Late Paleozoic extension. The ultramafic rocks are unconformably covered by Car- boniferous sedimentary rocks.

+ + + Late Paleeozoic I Lde Pdaeozoic + + + Gcsntbids sedimsntery RX&S 8.

Swpetiinimd MelemorphiC lAbb88ib lenses Comprex

111 Fault

Fig. 1: Simplified geological map of the Frontal Cordiliera between 33” 15’ S and 33’ 27’ S showing location of the Rio de Las Tunas area.

New Evidence on the Nature of the Frontal Cordillera Ophiolitic Belt 149

LOCAL GEOLOGY

West of Cuchilla de Guarguaraz, in the Rio de las Tunas area, a 1.5-2 km thick westerly dipping mafic-ultramafic belt, hereafter referred as the Metales belt, was recognised. (Fig. 2). The belt is dismembered by several east-vergent shear zones. A shear zone running parallel to the Rio de las Tunas overthrusts the Metales belt on the metamorphic rocks. The metamorphic rocks can be referred to the Com- plejo Metam&lico, Polanski (1972), and correlated with rocks of similar characteristics cropping out east of the Cuchilla de Guarguaraz, Bjerg et al., (1990). They display predominantly NE foliation.

During field work several different units were recog- nised in the Metales belt. They include serpentinized ultra- mafic rocks, altered gabbros, basaltic dykes and pillow lavas. The ultramafic rocks appear as elongated sleeves running nearly parallel to the Rio de las Tunas. They are constituted by massive serpentinites where few relicts of the original mineralogy can be recognised. The lower con- tact between the ultramafic and the metamorphic rocks is represented by an ultramafic basal tectonite extending from Arroyo Pabell6n to Arroyo Yesera. (Fig. 2).

The gabbroid rocks are scarce and were recognised only in the area of Arroyo Seco, whereas the basaltic rocks are represented mainly as dykes in the Arroyo Negro, Arroyo Las Picazas and Arroyo Pabellon.

Two types of basic dykes were recognised: a), metaba- salts affected by greenschist facies metamorphism and deformation and b), slightly metamorphosed basal&

The first ones intrude both the ultramafic and metamor- phic rocks. The contact between the dykes and the host rocks is sharp. Dyke thickness range from 1 m to 20 m. Predominant strike is roughly coincident with the regional structure. Some of them show intense folding and fractur- ing as a result of severe deformational episodes.

The second group of baste dykes have been recognised mainly in the area west of the Metales belt and few out- crops in the east. They intrude both the schistose and the ultramafic rocks but their field relationship with the meta- basites has not been recognised. The dykes thickness vary between a few centimetres up to 50 meters. Although most dykes are fairly regular some are complicated by apophysis that leave the parent dyke and undulate through the nearby

LEGEND 69” i3’ W

l-l Quaternary deposits

El Alto Tunuyln Formation

PI Metabasites and basaltic dikes

3

% Basaltic pillow lava and lava flows

gj

2

Serpentinites Ik d

bed Marbles and y calcareous schists

I Greenschists

Fig. 2: Geological sketch map of the western side of the Guarguaraz Range showing the Metales Belt.

150 D. A. GREGORI and E. A. BJERG

metamorphic rocks. Some dykes show chilled margins with only few showing one way chilling.

Characteristically they are only slightly metamorphosed and preserve their igneous mineralogy and texture. This feature allow their differentiation from the metabasites in the field.

Pillow lava flows are the last recognised unit. They are in tectonic contact with the ultramatic bodies as observed in the Arroyo Pabellbn. Pillow structures range in size between 40 and 80 cm in diameter and they are highly altered. Neither sedimentary nor pyroclastic rocks associ- ated with pillow lavas were recognised.

Due to the strong deformation, the main structural fea- ture of this area is one of folded and fault delimited blocks. Major faults in this area are displayed in Fig. 2. The Gua- nacos fault which dips to the NW delimits the northwest- ernmost outcrops of the serpentinised rocks. It extends from the Arroyo Pabell& in the SW to the Quebrada de las Picazas in the NE. A major shear zone closely parallel to the Guanacos Fault follows the Rio de las Tunas Valley, extending from the Arroyo Pabelldn in the SW to the Santa Clara River in the N. Minor WNW-ESE and NW-SE strik- ing faults cut the dominant regional structure, leading to displacements in the Metales belt.

Folding and faulting are not always easy to recognise in the field. Minor asymmetrical fold structures have been observed in the Arroyo Pabellon and Quebrada La Yesera, whose axial planes dips 70-80” SE and strikes 40-60” NE.

These observations allow us to interpret the Metales belt as a NE running fault delimited structure, dismembered by east-vergent thrust faults.

Analytical Methods

Fifty three samples of metabasites and basic dykes from the Rio de las Tunas area were analysed. The samples were reduced to powder using a jaw crusher and an agate mortar. Major and some trace elements were analysed by X ray fluorescense at the Mining University of Leoben, Austria; and at the Jaume Almera Institute, Barcelona, Spain. Rare earth elements, Hf, Ta and Th were analysed by INAA at ACTLABS, Canada and at the University of Munich.

Since the aim of this contribution is to identify the origi- nal tectonic setting of the metabasites and basic dykes, we concentrate on the so-called immobile elements, the rare- earth elements (REE) and the high-field strength elements, whose abundances in basalts are thought not to be greatly affected by alteration and low-grade metamorphism (e.g. Winchester & Floyd, 1976, Wood, 1980).

PETROGRAPHICAL AND CHEMICAL CHARACTERISTICS OF THE METABASITES AND BASIC DIKES OF THE METALES BELT

Due the strong alteration and serpentinization of the ultramafic and pillow lavas, this study concentrates on the petrography and chemistry of the metabasites and basic

dykes. Detailed petrographic and chemical data of the metamorphic rocks can be found in Bjerg er al. (1990) and Gregori & Bjerg (1992).

According to these authors typical metamorphic para- genesis are: quartz-muscovite-biotite-chlorite and quartz- muscovite-biotite-garnet-chlorite, which are indicative of low grade metamorphic conditions, Winkler (1979). ChemicaI analyses indicated that these rocks were derived from sedimentary rocks, including sandstones, grey- wackes, mudstones and marls.

Metabasites

This group of rocks is composed mainly of metamor- phic minerals. Some primary minerals and textures can still be recognized. Originally the rocks appear to have been composed of plagioclase, pyroxene and a limitated amount of olivine, with a porphyritic texture.

Their present mineral assemblage is that of greenschist facies and they show nematoblastic textures essentially composed of amphiboles (tremolite-actinolite), quartz, biotite, feldspar, plagioclase and epidote. Garnet occurs rarely. Accessory phases are chlorite, calcite and titanite.

Selected analyses of metabasites are presented in Table 1. Most of them are basal& (Le Maitre, 1989) having 43.3- 5 1.8 SiO, wt.%. Some samples are basaltic andesites, hav- ing 53.2 to 56.5 SiO, wt.% (Le Ma&e, 1989). MgO ranges from 4.3 to 11.7 wt%. Variation of MgO and CaO with SiOl erratic, perhaps as a result of alteration. In any case, the degree of alteration/metamorphism of the rocks is such that abundances of mobile elements are unlikely to have been changed from the original magmatic values. In the Zr/TiOZ Si02 wt% diagram (Winchester & Floyd, 1977) samples group in the fields of alkaline basalts and sub-alkaline basalts.

In the Zr versus ZrN diagram (Pearce & Norry, 1979) the majority of samples plot (Fig. 3) in the field of within plate basalt (WPB). Few samples plot in the Mid-Ocean

20

10

ZrlY

I A: Within date basal*, B: Island arc basalts C: Midocetan ridae basalts I

Fig. 3: Zrpl versus Zr plot (after Pearce & Norry, 1979) of the Metaies Belt Metabasites. Note the oredominance of WPR.

New Evidence on the Nature of the Frontal Cordillera Ophiolitic Belt

Table 1 Rqresentative anaIyses of the Me&s Belt Met&&es

151

Sample Name SiO2

Ti02

Al203

Fe0

MllO

Mgo CaO

Na20

K20

P205

LO1 TOTAL

11280289 01010394 03 100293 05 100293 04120293 09 140293 05270294

41.45 47.74 49.48 49.35 42.08 52.52 47.00 1.57 1.93 1.50 1.43 1.30 1.62 3.09

12.69 13.30 14.71 15.49 15.88 15.57 14.60 8.72 12.70 8.43 9.19 13.04 9.66 15.40 0.13 0.22 0.17 0.16 0.22 0.17 0.31

10.84 6.44 6.42 6.12 9.44 6.36 3.81 16.06 11.34 11.10 12.82 8.80 8.30 7.87

1.56 1.78 3.30 2.56 1.17 3.86 3.14 0.61 0.25 0.35 0.42 2.23 0.69 0.55 0.25 0.19 0.16 0.15 0.11 0.21 0.34 6.11 4.11 4.35 2.29 5.64 1.02 3.85

99.99 100.00 99.97 99.98 99.91 99.98 99.96

Cr 1% 110 175 209 236 193 15 Ni 92 108 104 126 192 91 61 co 64 45 30 33 51 33 34 SC 32 42 33 33 37 34 36 V 211 287 161 153 117 156 129 Rb 19 5 8 7 56 8 37 CS 0.25 0.17 0.11 0.15 3.70 0.07 b. d Ba 149 24 145 304 3206 299 76 Sr n.d 222 296 376 261 349 207 Ga 19 18 18 19 23 12 16 Ta 2.90 0.80 0.09 1.00 0.80 0.50 b.d Nb 7.0 19.0 12.0 20.0 16.0 15.0 8.0 Hf 2.40 3.00 3.10 1.90 1.90 2.80 3.80 Zr 104 124 121 89 62 100 209 Y 21 35 33 25 29 27 49 Th 3.99 0.80 0.90 0.50 0.08 0.90 1.00 U 0.93 0.09 0.50 0.10 0.07 0.06 b.d

La 11.90 8.40 9.90 6.80 4.20 10.10 10.80 Ce 26.10 21.00 22.00 17.00 11.00 23.00 24.00 Nd 14.60 14.00 14.00 11.00 8.00 14.00 14.00 Sm 3.80 4.33 3.81 3.25 2.65 3.74 4.80 Etl 1.40 1.48 1.27 1.23 1.02 1.34 1.70 Tb b.d 0.90 0.80 0.70 0.80 0.80 b.d Yb b.d 3.19 3.26 2.63 3.30 3.07 3.40 LU 0.29 0.42 0.44 0.37 0.43 0.41 0.47

n.d: Not determined. b.l.:Below limit of detection.

ridge basalt field. In the Cr versus Ti plot (Pearce, 1975) most samples plot in the field of ocean floor basal& In the Ti/lOO-a-Y*3 diagram (Pearce & Cann, 1973) the sam- ples plot (Fig. 4) in the fields of within plate basalt and ocean floor basalt.

Rare earth element (REE) variation diagrams normal- ised to chondrite (Sun & McDonough, 1989) are presented in Fig. 5. Most of the samples plot close to the E-MORB composition of Sun & McDonough (1989). The abundance of the elements increases progressively in E-MORB rela- tive to N-MORB in relationship to their incompatibility in the mantle source, resulting in smooth negative slopes (La&u, between 1.04 and 4.39) in Fig. 5. They display

patterns similar to those shown by Type 1 Early Tertiary basalts (Kerr, 1995) of Mull, Skye and the Faeroe Islands. According to Kerr (1994) the incompatible trace element contents can be explained in terms of a progressively thin- ning lithosphere and extensive astbenospheric melting at successively shallower depths.

E-MORB forms at spreading centres in the proximity of mantle plumes such as in Iceland, where OIB-source man- tle convected by the plume may he a partial magma source, and where mantle temperatures are higher than normal, McKenzie & O’Nions (1991). Enriched MORB is also erupted on some seamounts close to major spreading cen- tres characterised by normal MORB.

152 D. A. GREGORI and E. A. BJERG

WPB:D

cAR0.c

zr Y3

Fig. 4: TiilOO-ZrY*3 plot (after Pearce & Cann 1973) of the Met-

ales Belt Metabasites.

1000 A Metales Belt Metabasites

x Metales Belt Basic Dikes

I .

Le Ce Nd Sm Eu Tb Yb Lu

Fig. 5: REE patterns of selected Metales Belt Metabasites and

Basic Dykes

Basaltic Dikes

This group of rocks display porphyritic textures, almost devoid of any metamorphic overprint. Phenocrystals of zoned plagioclase, K-feldspar and zoned orthopyroxenes

are scarce. The groundmass display an intersertal texture

constituted by twinned laths of plagioclase (An,,,Ab,,) oli- vine, clinopyroxene and orthopyroxene crystals. Chlorite

and epidote occur in the groundmass and are the result of alteration processes.

A common feature of these dykes is their chilled mar- gins which can be clearly seen in thin sections.

Several coarse grained xenoliths are included in these dikes. They are composed of K-feldspar, clinopyroxene and orthopyroxene, hornblende, Fe-Ti oxides and minor amounts of calcite. Their size range between 2 and 40 cm. These dykes also carry pyroxene and K-feldspar xenoc- lysts.

Representative analyses are presented in Table 1. The

dykes are basalts (Le Maitre, 1989) having 41.5-52.1 SiO, wt.%. MgO ranges from 4.5 to 12.7 wt%. Variation of

MgO versus SiO, is erratic. CaO-SiO, plot in the range of

46-50 SiO, wt.%. and 5.610 CaO wt.%. In the Zr/TiOZ

SiO, wt% diagram (Winchester & Floyd, 1977) samples

group in the fields of alkaline basalts and sub-alkaline

basalts.

In the Zr versus ZrN diagram (Pearce & Norry, 1979)

the majority of samples plot in the field (Fig. 6) of within

plate basalt (WPB). In the Cr versus Ti diagram (Pearce,

1975) all samples plot in the held of ocean floor basalts (Fig. 7). In the Ti/lOO-Zr-Y*3 diagram (Pearce & Cann,

1973) the samples plot mainly (Fig. 8) in the field of within

plate basalt.

Rare earth element (REE) variation diagrams normal-

ised to chondrite (Sun & McDonough, 1989) are presented

in Fig. 5, with La&u, between 6.22 and 13.11.

Spider diagrams are similar to those of the oceanic

island basalts (OIB). Our data set of OIB basalts display

chondrite normalised REE variation diagrams that resem-

ble those of the Snaefell volcanic centre, Iceland (Hards er

al., 1995) and type 3 basalts of Kerr (1995), found in the

Early Tertiary basalts of Skye, the Faeroe Islands, SE and West Greenland and in the Voering Plateau. All these

basalts are believed to have originated by decompressional melting of a hot mantle plume head (the Iceland Plume),

White (1988).

REGIONAL COMPARISON

Rocks of similar characteristics cropping out in the

western margin of the Precordillera were studied by Stap-

penbeck (1910) Borrello (1969) Haller & Ramos (1984,

1993) and Kay ef al. (1984). Haller & Ramos (1984, 1993)

assigned them to an ophiolitic sequence (ofiolitas famatini-

anas) composed of basaltic pillow lavas, turbiditic and

pelagic sediments, gabbros and hypabyssal mafic to ultra-

mafic bodies interbedded in the Caradocian sedimentary

rocks of the Yerba Loca Formation. These rocks display a west-vergent imbrication and the sequence extends from

La Rioja province in the north to Las Tunas river (Mendoza

province) in the south.

20

A: Wlthin plats basatts B: Island arc basatts

10 , C: Wdeean ridge basatts

zrlv

LLP 1

I I

10 100 1000

Zrtppm)

Fig. 6: Zr/Y versus Zr plot (after Pearce & Norry, 1979) of the Metales Belt Basaltic Dykes. Note the predominance of WPB.

New Evidence on the Nature of the Frontal Cordillera Ophiolitic Belt 153

LKT: Low Potassium Tholeiii

OFB: Ocean Floor BassIts

LKT

100

Cr (wm) 1000

Fig. 7: Ti versus Cr plot (after Pearce, 19759) of the Metales Belt Basaltic Dykes. Nearly all samples plot in the Ocean Floor Basalts field.

zr Y-3

Fig. 8: Ti/lOO-Zr-Y*3 plot (after Pearce & Cann 1973) of the Metales Belt Basaltic Dykes.

The Metales belt E-MORB metabasites (Fig. 5) have similar REE patterns (La&u, between 1.04 and 4.39, La,, values varying between 17.7 and 50.2) to those Ordovician basalts studied by Kay et al. (1984) whose La&u, vary between 1.82 and 3. land La,, values varying between 27.7 and 56.5. The geochemical similarities of both group of rocks strongly support the idea that both group of basal& where probably derived from a similar original source rock.

The Metales belt basic dykes show a LREE enrichment (La, varying between 81.01 to 162.4) which has no coun-

terpart in the so far available data from the ofiolitas famat- inianas and La&u, relationships varying between 6.24 and 7.27.

Ramos & Kay (1991) reported for the Triassic basalts of the Cuyo rift basin La&u, relationships varying between 6.88 and 7.85 and La, varying between 65.8 - 73.8, as can be seen in Fig. 9. The above mentioned relationships pre-

elude, with the available data, any assumption regarding a common origin or time relationship.

The ultramafic rocks from the Metales belt show a slight LREE enrichment which is also reported by Kay et al. (1984) for wherlites from Rodeo in the San Juan province indicating that both group of ultramafic rocks were probably derived from a similar enriched source region.

The main difference between the Metales belt metaba- sites and the Precodilleran basalts is the metamorphic proc- ess that affected the Metales belt metabasalts. The latter are related to metamorphic rocks dating 500 * 50 Ma (Cami- nos, 1979) whereas those from the Precordillera are unmetamorphosed and related to sedimentary rocks with Caradocian age. The Precordilleran basalts and the metaba- sites from the Metales belt were probably erupted in a simi- lar tectonic environment although new isotopic data are necessary in order to establish more precisely the age of the Metales belt metabasites.

DISCUSSION AND CONCLUSIONS

The Metales belt metabasites were intruded before the metamorphic peak as shown by their mineralogy which contrast with that of the basic dykes which are only slightly metamorphosed. Unfortunately the exact timing of these two magmatic events is unknown due to the lack of radio- metric dating and the fact that the basic dykes intrude the ultramafic rocks and schists but their relationship with the metabasites has not been recognised so far.

The effect of the regional metamorphism on these rocks preclude a comparison based only on major elements. It is more reasonable to elucidate their possible cogenetic rela- tionships based mainly on trace elements and REE. Several trace elements relationships (ZriY-Zr/Nb; Y/Cr and Cr/Ni) show linear correlations suggesting that both groups of basaltic rocks share a common evolution trend.

The new geochemical and petrographic data of metaba- sites and basic dykes from the Metales belt presented in

1000

100

10

1

l Ksy at al. (MM) m Ramos & Kay (1Wl)

La Ce Nd Sm Eu Tb Yb Lu

Fig. 9: REE patterns of samples from Kay ef al. (1984) and Ramos & Kay (1991).

154 D. A. GREGORI and E. A. BJERG

this paper as well as their REE spider diagrams indicate that the rocks show an evolutionary trend from slightly enriched MORB for the metabasites (E-MORB signature) to 100 times chondrite enrichment in the case of the basic dikes (OIB signature).

The presence of rocks with both E-MORB and OIB sig- natures within the Metales belt would suggest that the basic magmatism was a complex process which involved inter- ruptions in time and slight changes in the composition of the parental magma. Rocks of similar characteristics can be found in diverse locations around the North Atlantic plume. It is believed to have been caused by decompres- sional melting of a hot mantle plume head (White & Mor- ton, 1995).

According to Kay et al. (1984) the Precordilleran basalts were erupted in transitional and plume ocean ridges such as the Reykjanes ridge in southern Iceland.

Haller & Ramos (1984) explained the ofiolitas famatini- anas as generated by oblique collision of several seg- mented ocean ridges against the Gondwana margin.

The age difference between the basaltic rocks interbed- ded in the Caradocian sedimentary rocks from Precordill- era and the metamorphic rocks related to the Metales belt can be explained as the result of inconclusive radiometric ages in the second case. Haller & Ramos (1984) used simi- lar arguments to explain the observed age differences along the belt.

In conclusion, the geochemical evolution trend of the Metales belt metabasites can be broadly correlated with the Precordilleran basalts (the ofiolitas famatinianas). Both groups display chemical similarities with oceanic intra- plate magmas. The Metales belt metabasites show charac- teristics of basalts erupted in transitional and plume ocean ridges such as the Reykjanes ridge in southern Iceland.

The basaltic dykes are interpreted as parts of seamounts or ocean islands situated on oceanic crust. The geochemi- cal and petrographic data of both groups of rocks are con- sistent with a continental-scale extension environments during Eopalaeozoic times that could include an oceanic rift in an early stage of development, a transitional ridge segment or oceanic intraplate magmas.

Acknowledgements -We would like to thank M. Cabane and R. Bartroli

for helping during XRF determinations at Barcelona. D.A.G. is particu-

larly grateful to A. Lopez Soler and I. Jarvis for their guidance during his

EC fellowship at Jaune Almera Institute, Barcelona, Spain and Kingston

University, UK. Our thanks are to B. Delakowitz at the Technical Univer-

sity of Munich, Germany for the REE. E.A. Bjerg would like to thank E.

F. Stumpfl for his guidance during his CONICET fellowship at Mining

University, Leoben, Austria. Special thanks: to the Directorate General

XII of the Commission of the European Communities for the “Marie

Curie” bursary to D.A.G and to V. A. Ramos and A. Mogessie for their

critical reviews and valuable comments which greatly improved a previ-

ous version of this manuscript. This work was supported by SGCyT of the

Universidad National del Sur, Argentina grant CU-0027/95.

REFERENCES

Astini, R.A., Ramos, V.A.. Benedetto, J.L, Vaccari. N.E. and Cafias, EL.

1996. La Precordillera: Un terreno exotic0 a Gondwana. Xl/l Con-

peso Geol6gico Argentina y III Congreso de Exploruci6n de Hidro-

curburos, 5: 293-324.

Bjerg. E. A., 1985. Aspectos geneticos del proceso de mineralization y su

relation con 10s cuerpos serpentinicos del area de las minas Sala-

manta, La Luisa y La Banera, departamento Tupungato. provincia de

Mendoza. Unpublished thesis, Universidad National del Sur, Bahia

Blanca, 82 p.

Bjerg, E.A., Brodtkorb, M.K. de y Stump& E.F., 1994a.Variaclones com-

posicionales de cromitas ricas en Zn. area mina Salamanca, provincia

de Mendoza. II Jum. min. Pet,: Met. Rows Ultmb., Actas (PubI. N”

3): 483-490, La Plata.

Bjerg, E.A., 1984. Identilicaci6n de las fases monoclinica y hexagonal de

la pirrotina de mina Salamanca. Su posible usa coma geotermbmetro.

IX Congreso GeoMgico Argentina, 3: 269-274.

Bjerg. E.A., 1994b. Platinum Group Elements and Gold in serpentinized

ultramalic rocks from the Cordillera Frontal, Argentina. Asoc. Geol.

Arg., Rev., 49 (l-2): 48-54.

Bjerg, E.A.; Brodtkorb, M. and Stumpfl. E.E. 1993. Compositional zon-

ing in Znchromites from the Cordillera Frontal Range, Argentina.

Minerulogical Magazine, 57: I3 l-l 39.

Bjerg. E.A.; Gregori, D.A.; Losada Calder6n. A. y Labudfa, C.H.. 1990.

Las metamorhtas del Faldeo Oriental de la Cuchilla de Guarguaraz.

Cordillera Frontal, prov. de Mendoza. Asociuci6n Geol6gicu Argen-

tina Revistu, 45 (3-4): 234-245.

Bjerg. E.A.; Stumpfl, E.F. and Prochaska, W.. 1991. Rh-Pt-Pd-Au en

rotas ultram&icas de la Cordillera Frontal, Mendoza, Argentina. I

Reunion de Minerulogiu y Metulog6ne.k~. I: 371-378.

Borrello, A.V., 1969. Los geosinclinales de la Argentina Direcci6n Gen-

erul de Geologiu y Mine&, Anules 14, Buenos Aires, I88 p.

Brodtkorb, M.K.. 1970. La cobalto-pentlandita del yacimiento Sala-

manta. provincia de Mendoza, Republica Argentina. AsociuciBn Geo-

16gicu Argentina Revista 25 (3): 307-310.

Brodtkorb. M.K., 1971. El yacimiento Salamanca, ejemplo de deposit0

hidrotermal de Cu-Ni, Republica Argentina. ler Congreso Hispanou-

mericuno de Geologia Econbmicu 4: 1001-101 I, Madrid.

Caminos, R.. 1979. Cordillera Frontal: In: Geologiu Regionul Argentinu:

Nrlume I (edited by J.C.M. Turner), pp. 397-453. Academia National

de Ciencias. C6rdoba.

Caminos, R., Cingolani, C.. Her+ F. and Linares, E.. 1982. Geochronol-

ogy of the pre-andean metamorphism and magmatism in the Andean

Cordillera between latitudes 30” and 36” S. Earth Science Review, 18:

333-352.

Caminos, R., 1965. Geologfa de la vettiente oriental del Cordon de1 Plata,

Cordillera Frontal de Mendoza. Asociaci6n Geol6gica Argentina

Revistu, 20 (3). 351-392.

Davis, J.S., Moores. E.M.. Roeske, S.M., Kay, SM., McClelland, WC.

and Snee, L.W. 1995. New data from the western margin of the Pre-

cordillera terrane. Argentina constrain scenarios for the middle Paleo-

zoic tectonics of western South America. International Geologicul

Correlution Ptogrumme IGCP Project 376, Laurentian - Gondwunun

connections before Pangea. Field Conference, Abstracts, 15.

Dessanti, R.N. y Caminos, R., 1967. Edades potacio-argon y position

estratigrsfica de algunas rotas fgneas y metam6rficas de la Precordill-

era, Cordillera Frontal y Sierras de San Rafael, provincia de Mendora.

Asociaci6n GeoMgicu Argentinu Revistu, 22 (2). 135-162.

Gregori, D.A. y Bjerg, E.A., 1992. Los depdsitos de talc0 del distrito min-

ero Salamanca, prov. de Mendoza. Argentina Asociuci6n Geol6gica

Argentina Revistu. 47 (I): 55-59.

Gregori, D.A., 1985: Evaluaci6n de las condiciones geneticas de 10s

yacimientos de origen hidrotermal a traves del estudio quimico, min-

eraldgico y estructural de las cloritas. Unpublished thesis, Universi-

dad National del Sur. Bahia Blanca, I04 p.

New Evidence on the Nature of the Frontal Cordillera Ophiolitic Belt 155

Gmgori, D.A., Femandez-Turiel, J.L., Lopez-Soler, A. and Pefford, N., Ramos. V.A., Jordan, T.E., Allmendinger, R.W., Kay, S.M., Cort6.s. J.M. y 1996 Geochemistry of Upper-Palaeozoic-Lower Triassic granitoids of Pahna, M.A., 1984. Chilenia: un termno al6ctono en Ia evoluci6n Central Frontal Cordillera. Argentina. Journal of South Amcricun paleozoica de 10s Andes Centrales. X Congreso Geol6gico Argentino. Eiu?hSciences, 9(1-2): 141-151. 2: 84-106.

Hailer, MA. y Ramos, VA., 1984. Las Ofiolitas famatinianas (eopale- ozoico) de las provincias de San Juan y Mendoza. IX Congreso Geo- 16gico Argentina 2: 66-83.

Ramos. V.A., Jordan, T.E., Allmendinger. R.W., Mpodozis, SM.. Kay, S.M., Cartes, J.M. y Palma, M.A., 1986. Paleozoic terranes of Central Argentine-Chilean Andes. Tectonics, 5 (6): 855-880.

Hailer, M.A. y Ramos, V.A., 1993. Las Ofiolitas y otras rotas atines. In: Geologiu y Rccursos Naturules de Menairtu (edited by V.A. Ramos), pp. 31-39. Relatorio del XII Congreso Geoldgico Argentino y 11 Con- greso de Exploraci6n de Hidrocatburos.

Ruviitos. M.A. and Gregori, D.A., 19% Nature of the Frontal Cordillera metamorphic rocks in the Rio de las Tunas area, Mendoza province, Argentina. 3rd Intemationul Simposyum on Andeun Geodynumics, 795-798.

HaBer. M.J. 1995. A plume -influenced ridge-segment origin for the Pre- cordillera ophiolite. Internutionul Grobgicul Correlution Pro- grammc IGCP project 376, Luurentiun - Gondwanun connections before Pun geu. Field Conference. Abstructs, 16.

Stappenbeck, R., 1910. La Precordillera de San Juan y Mendoza. Ministe- rio de Agriculturu de la Nucibn. Secci6n Geologia. Minerulogia y Mineriu Anules 4, (3): Buenos Aires. 197 p.

Hards, V. L., Kempton, PD. and Thompson, R.N., 1995. The heterogene- ous Iceland plume: new insights from the alkaline basalts of the Snaefell volcanic centre. Journal of the Geologicul Society London, 152: 1003-1009.

Sun, S.S. and McDonough, W.F., 1989. Chemical and isotopic systemat- its of oceanic basahs implications for mantle composition and proc- esses. In; Magmutism in the Ocean Basins (edited by Saunders, A.D. and Norry, M.J.) Geological Society, London, Special Publications 42.313-345.

Kay, S.M., Ramos. V. A. y Kay, R.W., 1984. Elementos mayoritarios y tra- zas de las vulcanftas ordovfcicas de la Precordillera occidental; basaltos de rift oceanic0 temprano (?) pr6ximo al margen continental. IX Congreso Geol6gico Argentina, 2: 48-65.

Kerr, A.C., 1994. Lithospheric thinning during the evolution of continen- tal Large Igneous Provinces: a case study from the North Atlantic Ter- tiary province. Geology, 22:1027-1030.

Villar, L.M. y Donari. E.l., 1989. Geologia y metalogenesis de1 Complejo mrifico ultramatico de Los Gateados, Departamento Tupungato, Men- doza. In: Serie Correlucidn Geol6gicu 3. (edited by Facultad de Cien- cias Naturales, Universidad National de Tucuman). pp 45-50. TucumBn.

Kerr, A.C., 1995. The melting processes and composition of the North Atlantic (Iceland) plume: geochemical evidence from the Early Terti- ary bamhs. Joumul ofthe Geologicul Society. London, 152: 975-978.

Villar. L.M., 1%9. El Complejo ultrabasico de Novillo Muerto, Cordillera Frontal, provincia de Mendoza, Argentina. Asociuci6n GeoMgicu Argentinu Revistu. 24 (3). 23-238.

Le Maitre R.W (editor), 1989. A ClussiJkution of Igneous Rocks und Glossury of Terms. Blackwell, Oxford, 193 p.

Villar. L.M., 1985. Las fajas ultrtxlsicas argentinas, tipos de ultram6ficas. Metalogenia. IVcongreso Geol6gico Chileno, 3 (4): 610-633.

McKenzie, D. and O’Nions, R.K., 1991. Partial melt distributions from inversion of rare earth concentrations. Journul of Petmlogy, 32: 102 I- 1091.

Villar, L.M.. Donari, E.I. y Page, N.J.. 199 I. Presencia de platinoideos en la franja malice-ultrarm6hca de la Cordillera Frontal de Mendoza. Su significado. VI Congreso Geol6gico Chileno, 1: 688-692.

Pearce J.A. and Cann J.R., 1973. Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth und Plunetury Sci- ence Letters, 19,290-300.

White, R.S. and Morton, A.C., 1995. The Iceland plume and its influence on the evolution of the NE Atlantic. Joumul of the Geological Society, London, 152: 933.

Pearce J.A. and Norry, M.J., 1979. Petrogenetic Implications of Ti. Zr, Y, and Nb variations in Volcanic Rocks, Contributions to Minerulogy und Petmlogy 69.33-47.

White, R.S., 1988. A hot-spot model for early Tertiary volcanism in the North Atlantic. In: Eurly Tertiary Wcunism und the Opening of the NE Atluntic. (Edited by Morton, AC. and Parson, L.M.) Geological Society, London, Special Publication, 39, 3-13

Pearce, J.A.., 1975. Basalt geochemistry used to investigate past tectonic environments on Cyprus. Tectonophysics, X41-67.

Winchester J.A. and Floyd, PA.. 1977. Geochemicai discrimination of different magma series and theii differentiation products using immo- bile elements. Chemicul Geology, 20.325-343.

Polanski, J., 1972. Descripci6n Geol6gicu de lu Hoju 24 a-b, Cerro Tupungato, (Pmvinciu & Mendozu). Servicio Geologico National, Secretaria de Estado de Mineria. Buenos Aires. Boletin 165. 117 p.

Ramos. V.A. 1988. Tectonics of the Late Proterozoic-Early Paleozoic: a collisional history of the southern South America. Episodes ll(3) 168- 174.

Winchester, J.A. and Floyd, PA.,1976. Geochemical magma type dis- crimination: Application to altered and metamorphosed basic igneous rocks. Eurth and Plunetury Science Letters, 28: 459-469.

Winkler, H.G.E. 1979. Petrogenesis of metamorphic rocks. Springer Ver- lag, New York, 348 p.

Ramos. V.A. and Kay, S.M., 1991. Triassic rifting and associated basalts in the Cuyo basin, central Argentina. In: Andean Magmatism and its tectonic setting. (Eds. Harmon, R. S. and Rapela, C.W.). Geological Society of Americu, Speciul Puper 265.79-g I.

Wood, D.A., 1980. The application of aTh-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crus- tal contamination of basaltic lavas of the British Tertiary volcanic province. Eurth und Plunetary Science Letters, 50. 1 I-30.