Using the discriminant functions of Roser and Korsch (1988), designed to discriminate between four sedimentary provenance types, the majority of the MA rocks plot within the fields P4 (granitic-gneissic or sedimentary source, similar to PM-derived) and P2 (mature island arc, similar to ARC-derived), supporting the interpretation that they derived from a craton interior or a recycled orogenic terrane. Normalized multi-element plots: Upper continental crustnormalized plots (Floyd et al., 1991) were used to discriminate the tectonic setting of the MA.

The following source-distinguishing anomalies were recognized: (1) Nb/Nb*: 0.340.77, typical of PM or CAAM settings, (2) V, Cr, Ni, Ti and Sc anomalies <1, characteristic for PM environments, and (3) Sr and P anomalies <1, typical of PM (Fig. 3c) Rare earth elements analyses: In a chondrite-normalized diagram (Fig. 3d), the MA rocks show similar REE patterns to PAAS (Post-Archean average Australian sedimentary rocks). Subgreywackes and siltstones are LREE enriched relative to HREE, displaying flat or nearly flat HREE patterns, with small negative Eu anomalies. This suggests that sediments derived from old upper continental crust and/or differentiated arc material compose the MA. According to McLennan et al. (1990) these provenance components are found in several basin types, but rarely in fore-arc settings. Based on other geochemical and petrographical data discussed herein, we considered that the MA is derived from old mature upper continental crust.

Sub-volcanic and volcanic assemblage (SV-VA)

Foliated and folded basic rocks, emplaced as dikes, domes, lava flows and sills constitute the SV-VA. Basic dikes are preferentially emplaced in the micaceous schists (Fig. 2). Between Salamanca and 12 Hermanos Mines, crops out a 200 m-long and 20 m-wide dykes, northeast oriented and foliated according to the MA. Westward of Salamanca Mine, basic dykes display radial arrangements from a major amphibolitic body. Several irregular ellipsoidal basic domes are associated to ultramafic bodies, being the best exposures located between Salamanca and 12 Hermanos Mines.

They follow the regional foliation and are disposed in a sub-vertical position. Several small sills, 30 m thick, were recognized in Quebrada Confusión and Refugio de la Plaza areas, interbedded in the micaceous schists and folded according to the MA structure. Along the western flank of Cuchilla de Guarguaráz, 30 m thick sills are emplaced concordantly within the schistose sequence.

Petrography: SV-VA rocks are petrographically amphibolites. They display a banded texture (S1), formed by nematoblastic and granoblastic layers with ortho- and clino-amphiboles (anthophyllite, hornblende and tremolite), plagioclase (andesine), epidote, sphene, garnet, chlorite, quartz and opaque minerals. Two generations of tremolite were recognized: the older is coeval with hornblende and oriented with S1, while the younger cuts hornblende and foliation S1.

Geochemistry and tectonic setting

Major and trace elements, and REE were analyzed in 15 and 9 samples, respectively. Representative analyses are reported in table 1. Major elements analyses: Metabasites from the SV-VA classify mainly as picrobasalts and basalts in the TAS diagram of LeBas et al. (1986). Rocks are subalkaline and display a tholeiitic trend in the Irvine and Baragar (1971) silica-alkali and AFM plots (Fig. 4a). They show strong negative correlation between TiO2 , Fe2 O3 , MnO, Na2 O and P2 O5 against MgO on Harker-type diagrams. Positive correlation can be observed between MgO and Cr, Ni and Co. These tendencies indicate that all samples belong to a unique magmatic cycle.

Normalized multi-element plots: Plotted in a transition metal diagram normalized to primitive mantle (Sun, 1982), SV-VA rocks display similar patterns to the NMORB (Fig. 4b). Metabasites plot mostly in the EMORB field (Fig. 4cd) on a Th-La diagram (Gill, 1981) and in the Th-Hf/3-Ta ternary diagram of Wood (1980).

Rare Earth Elements analyses: The Guarguaráz Complex amphibolites show low concentration in REE (52135 ppm). Chondrite-normalized REE patterns show 15 to 70 times LREE enrichment, with LaN/YbN: 0.73.3. Small positive Eu anomalies can be recognized in samples M30, N12, N24, N89 and N95, due to enrichment in plagioclase (Fig. 4e). The lowest LaN/YbN ratio belongs to sample GO17, which classify as N-MORB, whereas the other samples are similar to E-MORB.

Ultramafic bodies (UM)

Ultramafic bodies were fault-emplaced into the MA sequences following parallel belts along the Cuchilla de Guarguaráz, with ophicalcites, rodingites and mylonites associated in the borders. The UM can be divided in three segments: (1) eastern segment, located between Arroyo Ordenes and Refugio de la Plaza; (2) central segment, comprising the Salamanca and 12 Hermanos Mines area, and (3) western segment, along Río de las Tunas (Fig. 2). There are not original magmatic textures in these bodies; they only show ‘mesh’ and interpenetrative textures conformed by serpentine and scarce relictic olivine and diopside. Diopside cores are surrounded by serpentine, talc and calcite. Tremolite, chlorite and opaque minerals constitute minor phases.

Geochemistry

Twelve samples were analyzed by major and trace elements, whereas 7 by REE. Representative analyses are reported in table 1. Major elements analyses: Ultramafic rocks have SiO2 : 36.847.3 wt.%, Al2 O3 : 0.845.39 wt.% and MgO: 26.70 38.21 wt.%. Samples 2120293 and 9120293 have anomalous concentrations of SiO2 , Al2 O3 and Fe2 O3 due to alteration, while TiO2 is exceptionally enriched (1.78 and 1.3 wt.%, respectively) possibly due to abundance of illmenite. All samples are plagioclase, pyroxene and olivine-normative, with Mg#: 6596 (calculated as Mg# = 100*Mg/(Fe+Mg) following Middlemost, (1989)).

According with the CIPW calculation samples of the eastern and central segments have 3.77 to 17.77 wt.% of plagioclase, plotting in the melagabbroids field on a Px-Pl-Ol diagram. Samples from the Río de las Tunas segment have 0.182.9 wt.% of plagioclase, plotting in the plagioclase-bearing ultramafic rocks field. UM classify mainly as lherzolites in the Opx-Cpx-Ol diagram for ultramafic rocks (Fig. 5a). Most samples plot in the picrite basalt field in alkalies-silica diagram of Cox et al. (1979), whereas in the AFM diagram of Irvine and Baragar (1971) they follow a tholeiitic trend. The Al2O3 Mg# diagram indicate that UM rocks are comparable with those from the Hess Deep abyssal peridotites (Gillis et al., 1993).

Rare Earth Elements analyses:

In a chondrite-normalized REE spidergram (Fig. 5b), four groups can be recognized according their REE enrichment. Samples 35260289 and 7130293 (Arroyo Yesera) display nearly flat patterns 5 to 10 times enriched, similar to samples of the Ussuit Komatiite (Kalsbeek and Manatschal, 1999). Samples 9120293, 9260294 and N84F (Río de las Tunas) are 30 to 100 times enriched, and slightly LREE depleted. Eu negative anomalies (Eu/Eu*) vary between 0.32 and 0.64 reflecting some plagioclase fractionation. Samples 15010389 and 2120293 (Arroyo Yesera) have LaN/LuN between 3 and 10. The first one is 200 times enriched, displaying a positive Eu anomaly, typical of plagioclaserich gabbro. Sample 2120293 has LaN/LuN of 10, possible due to retention of garnet in the source; and pronounced Eu/Eu* (0.42), indicative of an important plagioclase fractionation.

Structure and Deformation of the Guarguaráz Complex

The Guarguaráz Complex conforms a decakilometerscale anticline to regional axis 3º/N47º. In the eastern flank of this structure, schistosity S1 dips to the southeast, whereas the northwestern side dips to the north or northwest (Fig. 2). The Guarguaráz Complex overthrusts tertiary sediments to the east in a structure known as Espolón de la Carrera (Polanski, 1958), and is overthrusted by folded Carboniferous rocks to the west. Several SE vergent slices of ten- to thousand-meters thick, including folded metasedimentary and metavolcanic sequences are tectonically imbricated. Thrust sheets transporting mafic sills and domes and ultramafic bodies are exposed predominantly in the eastern flank of Cuchilla de Guarguaráz and along Río de las Tunas (Fig. 2). The MA rocks, deposited in passive margin environments, were progressively deformed and metamorphosed in an accretionary prism at an active margin. During this process, S1 foliation and decameter to hundred-meters folding were developed, while crenulation and S2 foliation were developed later.

Radiometric dating using Rb/Sr and K/Ar isotopes by Dessanti and Caminos (1967), Caminos et al. (1979) and Basei et al. (1998) on biotite and amphibole indicate ages from 425 to 355 Ma for this deformation. Coevally, slices of ultramafic and mafic rocks were tectonically emplaced into the metasedimentary sequences. After Devonian, extensional faulting and subsidence of the western margin of Gondwana conditioned the deposition of Carboniferous marine sediments over the Guarguaráz Complex. Deformation during Permian generated folding of the Carboniferous sediments, and northeast and northwest faulting in the Guarguaráz Complex, were felsic dykes were emplaced. Andean deformation is characterized by reactivation of northeast structures with strike-slip and thrust components.

Correlations