Descripción: The finding of peperite breccias on Marambio Island allows to confirm the existence of at least two different pulses in the instrusion of basaltic dykes within the James Ross Island Volcanic Group. Two important bodies of peperites were recognized at Filo Negro and Geoantar hill. They were formed by the intrusion of basaltic dykes in the unconsolidated Paleogene sediments of Sobral Formation. The breccias are composed of vesicular glassy fragments, and volcanic and sedimentary lithic constituents in a sand matrix. The intrusions were probably emplaced at very shallow depth and are possibly related to a younger volcanic pulse, dated at 1.3 Ma. © 1997 Asociaciôn Geolôgica Argentina.

Descripción: In this paper new geologic evidences allow us to complete the stratigraphy of Ramez de Velasco ranges. Granitoids of the igneous basement are included into the Ojo de Agua-Ambargasta complex as the Sumampa Granite which is composed of three facies: facies Para Yacu, Sumampa Viejo and Los Telares. Thin aplitic dykes were included into the same basement as the last magmatic event of the same geological setting. The geochemical signature of these rocks shows meta to peraluminous compositions, which evidence magmatic arc granitoides. Basic dykes, few meters wide and more than one kilometer long, also intrude these granitoids. They are composed of basalts and andesites with calcoalkaline signatures that were correlated with Balbuena formation due to their petrological features. Finally, few and small outcrops of sandstones appears as the only expression of the Neopaleozoic sedimentation. On the basis of all the data obtained we suggest that the Ramíez de Velasco ranges preserve evidences of several geologic processes that took place since the Paleozoic.

Descripción: The study of mineral assemblages and metamorphic facies in rocks exposed on the eastern margin of Brazo Huemul, Nahuel Huapi Lake, allowed the reconsideration of the temporal succession of units whose outcrops are isolated and difficult to locate due to the exuberant forest that covers the southestern part of Neuquén province. The rocks exposed in the study area are mainly of tonalitic composition and have been correlated with the Subcordilleran Patagonian Batholith because they are crosscut by two different types of dikes. One group of dikes shows an albite -chlorite - epidote - cummingtonite/grunerite -hornblende mineral assemblage as a consequence of a thermal metamorphic event transitional between albite - epidote hornfels facies and hornblende hornfels conditions. This process has been attributed to the emplacement of the Cordilleran Patagonian Batholith of Cretaceous age. Because of this, the hornfels dikes were correlated with Montes de Oca Formation. The other group of dikes shows a chlorite + pumpellyite + kumontite + chlorite/esmectite + iron oxides assemblage indicating metamorphism in zeolite facies of high temperature or, sometimes shows propilitic alteration and these dikes have been correlated with the Ventana Formation. The differences between these two types of dikes allow the discussion of the relative age of the tonalitic rocks that host them. In this paper it is proposed that this tonalitic body is of Low to Middle Jurassic age. © 2006 Asociación Geológica Argentina.

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: 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: Continuity and evolution in time of the horizontal stress direction in the Neuquen Basin area, derived from the analysis of recent borehole data and orientation of volcanic dykes measured in outcrop in the Cara Cura and Reyes Ranges is presented. The bitumen dykes along the Neuquén Basin were formed during Paleocene - Eocene in a context where the maximum stress was horizontal and had a NE. Within the analyzed volcanic dykes two major groups have been recognized, group i (NE orientation) and group ii (NNE orientation) and a secondary group iii (NW orientation). The age of these dykes is still not well established, but they were related to a Late Oligocene - Miocene magmatic event. Both for the volcanic and bitumen dykes a type i group (ENE-NE orientation) has been recognized, evidencing the maximum stress direction at the time they were formed. Beyond the uncertainties in the volcanic and bitumen dykes age, it is interpreted that during great part of Tertiary, the orientation of the horizontal maximum stress was NE. However, the breakout data shows that the actual horizontal maximum stress is a little different with a ENE mean direction. This change in the direction of stress is consistent with the change in the subduction vector produced between the Eocene and today.

Descripción: In the cliffs of the Ladrillero cape, situated on the Atlantic coast of the Isla Grande de Tierra del Fuego, the uppermost records of the Austral or Magallanes foreland basin infill crop out. This locality contains Lower Miocene deposits informally named "Cabo Ladrillero beds" and "Cabo San Pablo beds" being the "upper Cabo Ladrillero beds" analyzed in this paper. This locality is situated around ten kilometers to the north of the emerging orogenic front (Punta Gruesa locality) where four facies association have been defined from gravitationally-driven supra batial to prograding and shallowing upward deltaic wedges. Deformed and massive beds by liquefaction processes and resedimentation are connected with abundant clastic dykes and synsedimentary faults. The statistic study suggests they are associated tectonic activity related to a transtensive episode rather than tectonic quiescence sometimes suggested in previous works. We conclude that microfossils documented in the area and used as a tool to support a deep marine environment are situated in underlying stratigraphic intervals that form part of the Desdémona Formation and the "lower Cabo Ladrillero beds" cropping out to the south of the study area. Sedimentological evidence are not conclusive respect to the batimetry but absence of classical turbidites, hyperpycnal flows and abundant deformed and resedimented deposits are more compatible with deltaic deposits, with high detritus supply generating unstable slopes rather than deep marine depositional settings, suggesting a more complex depositional evolution than the foraminiferal-based framework dominant in the literature.