Eduardo Jorge Llambías

The Choiyoi Group from central Argentina: a subalkaline transitional to alkaline association in the craton adjacent to the active margin of the Gondwana continent

Abstract Permian and Lower Triassic igneous rocks from La Pampa province, central Argentina, are part of the Choiyoi Group, whose extension in Argentina exceeds 500,000 km2. In La Pampa, the distribution of these outcrops occurs along a NW–SE belt that cuts obliquely across the N–S structures of the Lower Paleozoic rocks. The basement of the Choiyoi Group in western La Pampa consists of Mesoproterozoic to Lower Paleozoic rocks that form part of the exotic Cuyania terrane. In central La Pampa, the basement consists of Lower Paleozoic igneous and metamorphic rocks affected by the Lower Paleozoic Famatinian orogeny. The Choiyoi Group from La Pampa shares features with the Choiyoi Group elsewhere, such as an abundance of mesosilicic to silicic ignimbrites, subvolcanic domes, and granite plutons emplaced at sallow levels. In La Pampa, we recognize two suites: shoshonitic and trachydacitic to rhyolitic. The shoshonite suite is overlain by trachydacites and rhyolites. The plutonic rocks that belong to the cupola of the intrusive bodies are monzogranitic. The most significant difference between the Choiyoi Group from La Pampa and that from the Cordillera Frontal and the San Rafael block is that the San Rafael orogenic phase (Lower Permian) is not obvious in La Pampa. Therefore, we cannot attribute to the Choiyoi Group a postorogenic character, as in the Cordillera Frontal or the San Rafael Block. This difference in the tectonic setting is reflected in the composition of the igneous rocks of La Pampa, in that they generally have a higher alkali content with respect to silica, a weak enrichment in TiO2, and a depletion in CaO. Both suites are transitional from subalkaline to alkaline series. The shoshonitic suite is rich in clinopyroxene and apatite. Whole-rock compositions have high content of P2O5 (0.5–3.9%) and Sr (1320–1890 ppm). Zr is weakly enriched (273–502 ppm), and Nb (29–37 ppm) is depleted. The Th (16–45 ppm) and U (3–14 ppm) content is high. We postulate a crustal origin for the magma with a source with a calc-alkaline signature. The extensional regime that prevailed during the evolution of the Choiyoi Group favors melting processes.

Las Matras Block, Central Argentina (37°S-67°W): the Southernmost Cuyania Terrane and its Relationship with the Famatinian Orogeny

Abstract The Las Matras Block in Central Argentina constitutes the southernmost part of the Cuyania terrane, which was accreted to the southwestern margin of Gondwana during the Early to Mid Ordovician Famatinian orogeny. The Grenville-aged rocks of the Las Matras Block are represented by the tonalitic to trondhjemitic Las Matras pluton. A new U-Pb conventional zircon age of 1244±42 Ma confirms previous Sm-Nd and Rb-Sr isochron ages of this pluton. Mineral composition data are consistent with the tonalitic-trondhjemitic character of the pluton, and constrain its emplacement level to 1.9 to 2.6 kb. This shallow level of emplacement and the undeformed character of the pluton are distinctive features of this southernmost basement. A regional comparison indicates that the igneous-metamorphic evolution of the Grenville-aged basement rocks of the Cuyania terrane occurred over a period of more than 200 million years, with ages older than 1200 Ma up to those close to 1000 Ma. The shallowest crustal level is found in Las Matras, suggesting a southward shallowing of the exposed level of basement. The deformation and metamorphism associated with the collisional Famatinian orogeny affect both the Cuyania terrane and the adjacent western margin of Gondwana, and the Gondwana margin was also the locus of the related arc magmatism, but the compressive effects of the collision decrease in intensity toward the south. The Famatinian metamorphism and magmatism continue even further south into the Patagonia region, but the southern continuity of the Cuyania terrane into this region remains uncertain.

A view from the roof: magmatic stoping in the shallow crust, Chita pluton, Argentina

Nearly 2 km of vertical relief and exposure of continuous pluton roof-wall corners around the Chita pluton, northwest Argentina, provides constraints on the 3-D host rock displacement field attending magma emplacement in the shallow crust. The pluton is rectangular in crosssectional view and consists of weakly to non-deformed granite to granodiorite. Structures in surrounding metasedimentary host rocks are truncated at a knife-sharp contact and are only weakly deflected from their regional orientations. Final emplacement occurred by stoping together with minor doming of the earth’s surface. Structural restoration of a cross-section indicates that host-rocks above the pluton may have been domed upwards during emplacement by as much as a few hundred meters, accounting for up to 20% of the exposed pluton volume. The remaining 80% of the space for pluton emplacement was made by downward removal of host-rock from the present level of exposure. Although mechanisms such as floor-subsidence are permissive, late stoping has destroyed evidence for the early emplacement history. However, geometrical constraints provided by exposure of continuous wall-to-roof contacts indicate that early emplacement mechanisms at this level in the crust and immediately below were also dominated by the downward displacement of host rocks through a conduit that now is represented by the solidified pluton. q 2002 Elsevier Science Ltd. All rights reserved.

Early Paleozoic Structural and Metamorphic Evolution of Western Sierra de San Luis (Argentina), in Relation to Cuyania Accretion

Abstract Structural, metamorphic and isotopic data obtained from the Nogoli Metamorphic Complex of western Sierra de San Luis indicate that the Early Paleozoic Famatinian Orogeny overprinted an already structured and metamorphosed older basement. The older geological features are relict NW trending fabric associated with high-grade (amphibolite facies) regional metamorphism preserved within thin strips of schists and paragneisses and in the core of mafic to ultramafic lenses. Arc magmatism, medium P (Barrovian type)/high T (amphibolite to granulite facies) regional metamorphism and penetrative NNE to NE trending foliation are related to the building of the Famatinian orogenic belt. The P-T conditions of the Famatinian prograde metamorphism reached a pressure peak of ca. 8 kb, with a thermal peak from -750°C up to -820°C. U-Pb conventional and chemical dating and Ar-Ar plateau ages constrain the peak of the main orogenic phase related to the Famatinian belt to 470–457 Ma (Early to Mid-Ordovician). Greenschist facies retrograde metamorphism closely associated with shear zones and secondary Ar-Ar plateau and Sm-Nd ages suggest that a late to post-orogenic phase of the Famatinian belt was active at least since -445 Ma. This phase continued during the Silurian to Late Devonian times through multiple reactivation of early shear zones. The Famatinian Orogeny reset a previous thermal history and therefore, the timing of the relict fabric could not be constrained conclusively with radiometric dates. Despite this difficulty, a range of 520 to 490 Ma suggests some inheritance from Pampean events registered by the older NW-SE fabric. The Early to Mid-Ordovician regional metamorphism and ductile deformation of the western Sierra de San Luis is interpreted as the orogenic effects of the collision of the allochthonous Cuyania terrane with the autochthonous proto-Pacific margin of Gondwana during the Famatinian Orogeny.

The Rodeo de la Bordalesa Tonalite Dykes as a Lower Devonian Magmatic Event: Geochemical and Isotopic Age Constraints

One of the ‘pre-Carboniferous units’ from the San Rafael Block is the sedimentary Rio Seco de los Castanos Formation, which is distributed in isolated outcrops within the Block. At the Rodeo de la Bordalesa area two small intrusives in the mentioned unit were mapped, composed of tonalitic rocks, lamprophyre (‘spessartite-kersantite’) and aplite dykes. We present in this paper, geochemical and isotopic data from the gray tonalitic rocks with abundant mafic enclaves and late magmatic aplite veins. The country rocks are a folded sequence of feldspathic sandstones, wackes, and shales. The Rodeo de la Bordalesa tonalite dykes are characterized by high to medium potassium concentration, with metaluminous composition and I-type calc-alkaline signature. The 401 ± 4 Ma U–Pb zircon age corresponds to the emplacement time and it is confirmed by the K–Ar biotite age. The Rb–Sr whole rocks and biotite age of 374 ± 4 Ma could be related to deformation during the ‘Chanic’ tectonic phase. Nd model ages (TDM) show an interval between 1 and 1.6 Ga, indicating Mesoproterozoic age derivation, whereas the negative e Nd is typical from crustal sources. The crystallization age for the Rodeo de la Bordalesa tonalite corresponds to a Lower Devonian time and suggests that part of the Late Famatinian magmatic event is present in the San Rafael Block . The dykes are contemporaneous with the large peraluminous batholith in Pampeanas Ranges, with the transpressional shear belts during ‘Achalian’ event and could be correlated with the Devonian magmatism present in the southern part of the Frontal Cordillera. The geochemical and geochronological data allow us to differentiate the Rodeo de la Bordalesa tonalite from the mafic rocks exposed at the El Nihuil area.