Mónica G. López de Luchi

Neoproterozoic to Early Palaeozoic events in the Sierra de San Luis: implications for the Famatinian geodynamics in the Eastern Sierras Pampeanas (Argentina)

The application of the sensitive high-resolution ion microprobe (SHRIMP) U/Pb dating technique to zircon and monazites of different rock types of the Sierra de San Luis provides an important insight into the provenance and timing of deposition of the sedimentary precursors as well as the metamorphic and igneous history of the various basement domains. Additional constraints on the Famatinian metamorphic episode are provided by Pb/Pb stepwise leaching experiments on one staurolite and two garnet separates. The results indicate that the sedimentary precursors of the Conlara Metamorphic Complex have a maximum age of c. 590 Ma, whereas the Pringles Metamorphic Complex metasediments appear to be sourced from the Pampean orogen in the Early Cambrian. Folded xenoliths within the c. 496 Ma El Peñón pluton suggest that the host Conlara Metamorphic Complex underwent a Pampean compression. From a 208Pb/232Th monazite age of 478 Ma for a migmatite from the Nogolí Metamorphic Complex, the structural evolution of this basement complex appears to be entirely post-Pampean. Onset of the Famatinian high-grade metamorphism, between c. 500 Ma and c. 450 Ma, follows a period of crustal extension on the western outboard of Gondwana and might not be related directly to a Mid-Ordovician accretion of the Cuyania Terrane.

Rock and magnetic fabric of the Renca Batholith (Sierra de San Luis, Argentina): constraints on emplacement

Abstract The Renca Batholith belongs to a group of post-collisional plutons that intruded the Early Paleozoic basement of the Sierra de San Luis during Late Devonian to Early Carboniferous. Combined microstructural, petrofabric and anisotropy of magnetic susceptibility (AMS) studies indicate that interactions between magmatism and emplacement related fabrics played the main role in the strain patterns with a very subordinate far field control by regional deformation. This concentrically foliated zoned elliptical granitoid is made up by porphyric granodiorites and equigranular monzogranites that are associated with syenites and monzonites. Two major magmatic associations are present: (i) a Mg–K monzonitic association represented by syenitic, monzonitic and monzodioritic enclaves and synplutonic dykes and a porphyroid monzodioritic to melagranodioritic facies and (ii) a high K calc-alkaline association that constitutes the major part of the concentrically zoned batholith with an external porphyroid granodiorite to monzogranite (Unit 1) and a central equigranular monzogranite (Unit 2). Bulk susceptibility ( K ) shows a bimodal distribution that corresponds, respectively, to Unit 1 (ferromagnetic) and Unit 2 (paramagnetic). Similar distribution is observed in the intensity of natural remanent magnetization (NRM) and AMS degree ( P ′). The steep to vertical inward dipping foliation that is recognized in the porphyroid granitoids is parallel to the X – Y flattening plane of the enclaves, the synplutonic dykes attitude and the contact between the two major units of the batholith. The AMS defines a predominantly foliated fabric, with a distribution of magnetic foliation planes generally subvertical and parallel to the boundaries of the ring-like Unit 1. These internal features are concordant with the axial planes of the meso- and microscale folding of the regional NNE S2/S3 in the ductile aureole that developed in the medium grade metamorphic Ordovician country rocks. Foliation results from a combination of both magmatic and high-temperature solid-state flows; the latter as the result of the emplacement of the central monzogranite, which caused a radial compression. K ≈ 0 as measured using strain indicators suggests flattening. Unit 2 shows a planar magnetic fabric shallowly dipping outwards and dominant subhorizontal lineations. The magnetic fabrics of this body agree with other fabric parameters and are consistent with a magmatic fabric. There is a strong correlation between rock and magnetic fabrics. These fabrics reflect internal magma dynamics whereas regional tectonic exerted a far field control on the expansion of the pluton.

Paleomagnetism and magnetic fabric of Middle Jurassic dykes from Western Patagonia, Argentina

Abstract Jurassic volcanism in Patagonia is widespread. Although associated dyke swarms are conspicuous, they have been almost neglected in previous geologic studies of this region. Radiometric, paleomagnetic and magnetic anisotropy studies are reported from a Middle Jurassic basic to intermediate dyke swarm exposed in the Sierra de Mamil Choique (Western Patagonia) in an area of over 350 km2. Two whole-rock K/Ar determinations indicate that these dykes were intruded at around 170 Ma. An anisotropy of magnetic susceptibility (AMS) study on 15 dykes (74 block samples) shows that they carry different kinds of magnetic fabric (both normal and inverse), apparently governed by compositional differences. A paleomagnetic study of these samples suggests that Ti-poor titanomagnetite is the probable carrier of the characteristic remanence. Mean site characteristic directions pass a reversal test (grade C). A paleomagnetic pole, computed by averaging VGPs from each individual dyke, is situated at 70.2°S, 190.4°E (N=13, α95=9.7°), not substantially apart from other Middle Jurassic poles from South America. Further refinement of the Jurassic South American apparent polar wander path is needed to establish whether or not the Mamil Choique dykes were affected by a small tectonic rotation.

Modelo gravimétrico en el borde noreste del macizo norpatagónico

We present a gravimetric study on the northeast edge of the North Patagonian massif in the province of Rio Negro, to ascertain the depth distribution of the different outcrop units in the study area, as well as the geometry of the Navarrete Plutonic Complex. The negative anomalies in the Bouguer residual map reproduce the lateral density contrast between what would be the bedrock of Cambrian age and the lithologic units of the Mesozoic age. The solutions of the analytical signal along with variations in the curve of the horizontal gradient of gravity, obtained along a profile across the outcrops of the Navarrete Plutonic Complex, show contacts between the intrusive body and the bedrock with different densities. Over this profile we obtained a 2D (dimensional) gravimetric forward model adjusted with the density data of the surface rocks. Above the outcrop of the Yaminue Complex there exists a gravimetric high which does not correspond to the density in this area. This is a new evidence that proves that it is a series of intrusions and thin subhorizontal tectonic slices and that its thickness is probably thin. On the other hand theNavarrete Pluton shows the greatest development in size and depth to the east of Treneta stream.

Untangling the Neoproterozoic-Early Paleozoic Tectonic Evolution of the Eastern Sierras Pampeanas Hidden in the Isotopical Record

The Sierras Pampeanas of Central Argentina are an example of a continuous and fast overlap of episodes of high- to medium- and low-grade metamorphism, deformation, anatexis, magmatism and mineralization along the belts that bounded the margins of the South American cratons during the Neoproterozoic and early Paleozoic. A compilation and critical revision of the massive isotopic and geochemical data for the basement rocks of the Eastern Sierras Pampeanas is presented. The Eastern Sierras Pampeanas are defined by three main events: the Ediacaran to early Cambrian (580–530 Ma) Pampean, the late Cambrian–Ordovician (500–460 Ma) Famatinian and the Devonian-Carboniferous (400–350 Ma) Achalian orogenies. The mean average crustal residence age (Sm–Nd TDM) varies between 1.8 and 1.7 Ga with eNd(540) (−6 to −8). Pampean and Famatinian granitoids exhibit a similar TDM interval except for the Ordovician TTG suites of the Sierras de Cordoba (TDM 1.3–1.0 Ga). Achalian magmatism exhibits more radiogenic eNd(540) values (0.5 to −4) and TDM ages younger than 1.3 Ga. Two types of Pampean-related mafic rocks are recognized: one with a depleted mantle signature and another younger group with an enriched mantle signature, which is associated with the peak of metamorphism. Ordovician mafic-ultramafic rocks result from mixing/assimilation of depleted mantle and crustal magmas. Detrital zircon data for the metaclastic sequences indicates mainly Grenvillian and Brasiliano sources. The difference between the measured crystallization age for detrital zircon grains and the depositional age of the succession indicates that most of the Pampean basins are collisional, i.e. foreland basins except for Sierra Norte metaclastic host rocks that correspond to a convergent setting signature. The results for the post-Pampean Famatinian basins indicate mostly collisional convergent settings for the Ambato and La Cebila (type locality) metamorphic complexes and the Olta (northern sector) and Achavil formations. The Negro Peinado Formation is the only post-Pampean basin that corresponds to a collisional setting. Data from Green Quarry, Nogoli Metamorphic Complex, and Olta Formation in the central part of the Sierra de Chepes, Suri Formation and La Cebila Metamorphic Complex at Quebrada La Rioja yielded convergent margin settings. These exhibit the highest correlation among the post-Pampean basins showing a relatively large number of Middle Cambrian age detrital zircons apart from having Ordovician magmatic zircons in the detrital record. Sources were apparently more restricted than in the rest of the Famatinian post-Pampean basins. The two samples of post-Pampean basins that exhibit Rio de la Plata age peaks (2.2–2.0 Ga; Paleoproterozoic) belong to the collisional convergent group. Therefore the exhumed Pampean rocks probably formed a drainage divide that blocked westward transport of the Rio de la Plata-derived sediments. Intense erosion owing to an unstable tectonic scenario would have led to the progressive appearance of the Rio de la Plata signature.