New evidence on the nature of the Frontal Cordillera ophiolitic belt — Argentina

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  New evidence on the nature of the Frontal Cordillera ophiolitic belt — Argentina
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  Pergamon Joumal of Sourh zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP nwican Earth Scimces. Vol. IO. No. 2, pp. 147-155. IYY7 8 IYY7 Published hv Elrvier Scimcc Lad PII: so89~9811(97)ooo124 ll rights cd. Printed in Great Britain 08YWRI I/Y7 sl7.Ml+ O.(M) 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 arieties of basaltic dikes can be dif- ferentiated. One group, whose mineralogy comprise essentially tremolite-actinolite, lagioclase, pyroxene and limited amounts of olivine as the main phases, display metamorphic extures. The other group show porphyritic extures 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 srcin for both groups of basaltic rocks. Their geochemical and petrographical haracateristics re 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 n 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 n el rea el 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 exturas porfirfticas, e 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 unta con otras caracterfsticas petrogr6ficas geoqufmicas sugieren un modo corntin de srcen para ambos grupos de row bas kas. Los dates petrognlficos y geoquimicos de ambos grupos de row son coherentes on un ambiente extensional a escala continental durante el Eopaleozoico. Tal tipo de ambiente podrfa ncluir un estadio emprano 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 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF l., 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 srcin 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 srcinal 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 srcinated 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. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH wpetiinimd MelemorphiC lAbb88ib lenses Comprex 111 Fault Fig. 1: Simplified geological map of the Frontal Cordiliera between 33” 15’ S and 33’ 27’ S showing ocation of the Rio de Las Tunas area.  New Evidence on the Nature of the Frontal Cordillera Ophiolitic Belt 149 zyxwvut 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 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA l., (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 srcinal 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’ 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 arbles 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 srci- 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 srcinal 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 zyxwvutsr 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 zyxwvuts 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.
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