Descripción: The Arroyo La Chilca-Zanjón del Buitre alteration zone, San Rafael Massif, Mendoza. The Arroyo La Chilca-Zanjón del Buitre alteration zone is located in the San Rafael Massif (35°24'8" S; 68°19'25,8" W), Mendoza. Previous works defined preliminary this deposit as a porphyry copper system genetically linked to the gondwanan magmatism. This alteration zone is hosted by a sequence composed of ignimbritic breccias and ignimbrites of trachyandesitic composition with interbedded tuffaceous sandstones and andesitic lavas flows intruded by trachyandesitic-dacitic and andesitic hypabyssal bodies. The lithology and geochemistry of the volcanic sequence confirm the link with the Choiyoi Magmatic Cycle lower section (Lower Permian). The alteration zone consists of a potassic halo with a parageneses of K-feldspar-quartzmagnetite overprinted by phyllic alteration with an assemblage of quartz-muscovite-pyrite-(rutile) and quartz -pyrite veins. A late carbonatization process overprints both the potassic and phyllic alteration. Outside the alteration zone pyrite-chalcopyrite- galena-sphalerite veins with quartz gangue crop out. Metals geochemistry shows moderate Ag and Au anomalies and little Cu, Mo, Pb and Zn anomalies. Statistical analyses allow defining one Cu-Mo mineralizing stage and another Mo mineralizing stage only recognized in Zanjón del Buitre. Moreover, two Zn mineralizing stages, one of them probably genetically related to the Cu stage, were also recognized.

Descripción: Magmatism related to the growth and migration of the Andean arc was developed from the 36°40'S to the 38°20'S latitude, in the northwestern sector of the Neuquén Province, by the end of the Mesozoic. Hypabyssal and extrusive facies, which outcrop in the Pichaihue area, were described based on petrologic studies. The first ones are composed of andesites that are emplaced as lacoliths, dikes or sills intruding the Mesozoic sedimentary sequences. In addition, basalts outcrops were recognized. The extrusive facies consist of breccia or massive lavas and little domes. The geochemistry reveals an arc-like calc-alkaline signature with a normal crust and trace-element patterns which suggest a similar source. Based on regional knowledge, three groups are proposed: Cerro Mocho dikes with Ti anomalies and higher Sm/Yb; extrusive facies of the Upper Cretaceous- Paleocene (Rivera Andesite); and the hypabyssal facies of thr Eocene (Colipilli Dikes). Finally, the occurrences of these rocks in this segment of the Andes indicate an important shift to the East of the Upper Cretaceous-Eocene volcanic arc that reveals its easternmost position.

Descripción: Cerro Falkner is composed of granitic rocks underlying volcanic rocks of the Ventana Formation of Paleogene age. Those intrusives are graniodioritic in composition, metaluminous to peraluminous, subalkaline, with characteristics of volcanic arc granites that are concordant with other granitoids of the Cordillera Patagónica Septentrional's northern segment. The age obtained by the K-Ar method on tschemakitic amphiboles (125 ± 20 Ma) points out a minimun age from the lower Cretaceous for the Cerro Falkner's granodiorite. Therefore the use of the term Los Machis Formation (Upper Cretaceous) would be incorrect for these rocks. This age, allows us to relate the granitic rocks on the study area to the lower Cretaceous, coeval with the volcanism of the Divisadero Group.

Descripción: In the northern side of the extra andean Patagonia, in portions of Rio Negro and Chubut provinces, extensive basaltic lava flows plateaux type are exposed. On the southern side of the plateaux, in the Ranquil Huau stream valley, close to Telsen locality, a large succesion of volcanic and vocaniclastic rocks is exposed. The basaltic lava flows of Somuncura Formation and the Basalto La Mesada (Quin̄elaf Superunit) and the volcanisclastic deposits of the Sarmiento Group are exposed in the area. The different groups of basalts are identified by petrographyc and geochemical distinctive features. In the upper level of Somún Curá Formation a basic pyroclastic flow is interbedded. The basic rocks exposed in the medium level of the valley have distinctive characteristics and they are named as Basanitas Ranquil Huao. The analized units have geochemical parameters consistent with the evolution of melt showing that were produced by differents degree of partial melting.

Descripción: A major and trace element based characterization of the metasedimentary protoliths of three metamorphic units of Sierra de San Luis (Pringles Metamorphic Complex, San Luis Formation and Conlara Metamorphic Complex) is presented. Geochemistry indicates a dominance of shales in the protoliths of San Luis Formation, whereas greywackes and shales made up the Conlara Metamorphic complex and mainly greywackes, the Pringles Metamorphic Complex. Both major element data and trace element ratios (i.e. Th/Sc, Th/U,) indicate a source with an average upper crustal composition for the protoliths of the Pringles Metamorphic Complex, the San Luis Formation and the shales of the Conlara Metamorphic complex. A component with less evolved signature may be inferred for the metagreywackes of the Conlara metamorphic Complex. Mixed sourced detritus are indicated for the three units with clastic material resulting mainly from both andesitic and acidic/recycled detritus. The overall data consistently suggest a continental island arc and/or active margin setting as the more probable geodynamic scenario for the deposition of the sedimentary precursors of the studied units. In this context, a back-arc setting can account for the mixed nature of the inferred source areas with uplifted old basement and arc-related detritus as the end members of the mixtures. The inferred back-arc basin would have evolved through the Cambrian receiving the sediments derived from the Pampean Orogen to the east combined with probably some old crust exposures and to the west the source might have been controlled by the active continental margin. © 2003 Asociación Geológica Argentina.

Descripción: Geochemical features and neomorphism of Chachao Formation calcareous rocks (Valanginian)-Malargüe Anticline, Mendoza. The Lower Cretaceous Chachao Formation in the Malargüe anticline area consists of wackestone, packstone, grainstone and minor mudstone rich in benthic fauna (mainly bivalves) which were deposited in a shallow carbonate ramp as previous sedimentological studies have proposed. Standard petrographic, XRD, SEM, cathodoluminiscense and trace element determinations composition of micrite and microsparite mosaic crystals were undetaken in order to characterize their original mineralogy. The SEM shows that the mud fraction is composed of a mosaic texture of subhedral to anhedral micrite and microsparite crystals, both showing similar features such as smooth (unpitted) surfaces and lack of aragonite relicts. Crystal morphology also rules out the possible presence of aragonite. These features together with the microfabric, low Sr content (average: 592 ppm) of both micrite and microsparite point to an original high-magnesium calcite composition of the muds. Geochemical data from the micrite and microsparite of the mud-supported rocks of the Chachao Formation show two contrasting trends : decreasing-upward Sr and Na concentrations and increasing-upward Mg and Mn concentrations. The loss of Sr and Na are related to mineralogical stabilization of metastable carbonate particles involving the inversion of high-magnesium calcite to low-magnesium calcite, whereas local dolomitization in the upper part of the studied sequence has formed dolomite crystals showing different stages of dissolution due to meteoric and marine water influences. © 1999 Asociación Geológica Argentina.

Descripción: In this paper new basaltic outcrops from Ramírez de Velasco ranger (Santiago del Estero province) are described. They are basaltic dikes that vary from few meters to two kilometers long across the igneous basement. All these rocks show dark colors with aphyric (S9 sample) to porphyritic (S17 and S20 samples) textures. When these basalts present phenocrysts, they are composed of lagioclase within a groundmass of subophitic texture. The low alteration of the groundmass allows to recognise microliths of labradorite, augite and opaque minerals of about 0.2 and 0.5 mm. These sizes suggest a hypabisal emplacement of the biggest dikes. Sericite, chlorite, epidote, sphene and opaque minerals compose the alteration assemblage on these rocks. Quartz and carbonates mainly fill small cavities. The geochemical compositions of all analyzed samples support the petrographic classification. They have negative slopes of their trace elements and evidence affiliation with calc-alkaline basalts from magmatic arcs. Their similar textures, mineralogy and chemical compositions suggest that all these basalts where crystallized from the same source and due to the same magmatic process. On the basis of all these data and the features of the basalts that crops out in the surrounding of the study area, this volcanism is attributed to the post-orogenic stage of the Pampean orogeny and therefore it is correlated with the Balbuena Formation (Sierra Norte of Córdoba).

Descripción: Los Chilenos granite: a new Jurassic granite unit in Cerro Colorado, Sierras Australes de Buenos Aires. Tectonic implication. The Sierras Australes of Buenos Aires are located in the south-west corner of the Province about 100 km west of Bahia Blanca. On the south-west flank of these hills, younger Palaeozoic sedimentary formations overlie poorly exposed granitic and volcanic basement rocks. Granitic Cerro Colorado outcrops, located 16 km strike to the south of the town of San Martin de Tours, and near Lago Los Chilenos, have been included in this broadly interpreted "basement" in many previous geological studies. This contribution is mainly based on detailed structural mapping of the cerro Colorado area, and systematic sampling of granitic and sedimentary rocks for petrographical, microtectonic and Rb/Sr isotopic age analysis. In contrast to previous studies, two Phanerozoic granitic units have been recognised and mapped in the cerro Colorado area of Sierras Australes. The older one, the Cerro Colorado Granite (CC), is composed of foliated and sheared anisotropic granitic rocks with a Rb/Sr isochron age of 381±9 Ma (ir= 0.7035). The younger one, Los Chilenos Granite (LCh), consists of isotropic granophiric microgranite with a Rb/Sr isochron age of 140 ± 14 Ma (ir= 0.7126). This granite has intrusive contacts with both, the folded quartzite of the Mascota Formation and Cerro Colorado Granite (CC).The older tectomagmatic event (CC) is correlated with the compressive Chañica Orogeny, which was associated in the Sierras Australes region, with the eruption of La Mascota - La Hermita rhyolites (360±21 Ma) and the unconformity between Lolén Formation (Middle Devonian) and Sauce Grande Formation (Carboniferous).The younger intrusive event of Los Chilenos Granite (LCh) is considered to be associated with an extensional tectonic regime during the Late Jurassic initial rifting of the Atlantic Ocean. © 1999 Asociación Geológica Argentina.

Descripción: A 25 m thick, rhythmic, limestone-marl alternation of Los Catutos Member (Vaca Muerta Formation, Neuquén Basin), was studied bed by hed in several quarries near Zapala. Beds up to 1 m thick are of massive or laminated, lithographic, fossiliferous limestone, locally mottled or weakly bioturbated (?Chondrites), interbedded with thinner marl beds. Limestones and marls can be classified as pelbiomicrites and biopelmicrites, mainly composed of pellets, radiolaria, forams, ostracods, equinoids, spicles of sponges and gastropods, with minor amounts of peloids, feldspar, chert, muscovite, glauconite and phosphates. Values for δ18O and δ13C are similar to those of lithographic limestones from the northern hemisphere. δ13O values are most probably result of diagenetic modification of the primary signal but the δ13C agrees well with the published curves for the Tithonian sea water and with other records from Tethyan limestones. A preliminary analysis of negative excursions of δ13C point to productivity crisis in the sedimentary system with a periodicity of 400 ka, which can be a result of changes in the orbital eccentricity of the earth. Smectite and interlayers reflect pedogenic processes developed on volcanogenic source-rocks, whereas more abundant kaolinite in marls might reflect more humid conditions. With TOC up to 1.95% the rocks offer prospects as a regular to good source rock for hydrocarbons, although thermally immature. A low-energy, dysaerobic, open-marine environment is inferred for Los Catutos Member, with high sedimentation rates as a result of high productivity at the sea-surface and incoming plumes of suspended sediment from the inner shelf. Sediment accumulation in wide and gently dipping calcareous bulges in an outer ramp system is envisaged.