Silvana Geuna

Paleomagnetism of Jurassic and Cretaceous rocks in central Patagonia: a key to constrain the timing of rotations during the breakup of southwestern Gondwana?

Abstract A paleomagnetic study in Jurassic and Cretaceous rocks from the Canadon Asfalto basin, central Patagonia, indicates the occurrence of about 25–30° clockwise rotation in Upper Jurassic–lowermost Cretaceous rocks, whereas the overlying mid-Cretaceous rocks do not show evidence of rotation. This constrains the tectonic rotation to be related to a major regional unconformity in Patagonia, which in turn seems to be close in time with the early opening of the South Atlantic Ocean. The sense and probably the timing of this rotation are similar to those of other paleomagnetically detected rotations in different areas of southwestern Gondwana, suggesting a possible relationship between these and major tectonic processes related with fragmentation of the supercontinent. On the other hand, the mid-Cretaceous rocks in the region yield a paleopole located at Lat. 87° South, Long. 159° East, A95=3.8°. This pole position is consistent with coeval high-quality paleopoles of other plates when transferred to South American coordinates, implying it is an accurate determination of the Aptian (circa 116 Ma) geomagnetic field in South America.

Paleomagnetism of the Carboniferous-Permian Patquía Formation, Paganzo basin, Argentina: implications for the apparent polar wander path for South America and Gondwana during the Late Palaeozoic

The magnetic properties of the Carboniferous-Permian red beds of the Patquia Formation at Punta del Viento, Sierra de Umango and some previously reported localities, all in the Paganzo Basin (Argentina), have been studied. Whereas all sites are characterized by hematite as the main magnetic carrier and a reversed-polarity magnetic remanence, we found a pattern of variation in magnetic properties along the integrated column for Patquia Formation. The Lower Member (Late Carboniferous) showed higher intensity of natural and saturation isothermal remanent magnetisation (NRM and SIRM, respectively) than the Permian Upper Member. The fall in NRM intensity from the Lower to Upper Member of the Patquia Formation may be related to a change in quantity and/ or grain-size of the hematite pigment, which may reflect the change in environmental and/or depositional setting. As for directional values of NRM, paleomagnetic poles reported for both sections are clearly different. The lower section provided a pole position coincident with Late Carboniferous poles for Gondwana, while the upper section poles are departed from the Early Permian position. We cannot decide whether the Upper Member pole is due to a primary magnetisation at 290 Ma or to a remagnetisation at ~260-270 Ma; even so, the obtained paleomagnetic pole is robust and indicates a rapid apparent polar wander in a ~30o counter clockwise rotation of the region, after deposition of the Late Carboniferous lower section, and in coincidence with the San Rafael Orogenic Phase.

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.

The Late Palaeozoic of Western Gondwana: New insights from South American records

Late Palaeozoic basins of southern South America offer an excellent opportunity to study sequences deposited in contrasting tectonic regimes, shifting climatic conditions and different sea level positions. From the tectonic point of view, these Late Palaeozoic basins developed in three scenarios (Limarino and Spalletti, 2006): arc-related or Proto-Pacific basins, retroarc or peripheral basins, and large intracratonic or intraplate basins (Fig. 1). Arc-related or Proto-Pacific basins formed along the westernmost and highly mobile area of Gondwana, in which volcanism played a fundamental role. In turn, retroarc or peripheral basins which are characterized by a thick fossiliferous Late Mississippian up to Late Permian record, registered less magmatic activity and minor deformation. Finally, large intracratonic or intraplate basins developed under more stable tectonic regimes and their sedimentary record was mainly controlled by significant climatic and eustatic oscillations (e.g. Parana Basin). The effects of dramatic climatic changes and/or eustatic oscillations on Late Palaeozoic basins have been widely studied. Alternating glacial and interglacial periods as well as processes of progressive aridification towards the Middle and Late Permian times have been recognized (Fig. 2). Moreover, a complete record of sea level changes was defined in the intracratonic basins from the Pennsylvanian to the Latest Permian (Fig. 2).Despite the importance of the aforementioned issues, the stratigraphic and sedimentological information obtained from South American basins has been frequently overlooked in regional or global-scale studies. Although this may be attributed to the lack of knowledge about the Carboniferous and Permian Systems in southern South America, there is abundant literature on the sedimentary and palaeontological record of these basins. This is clearly documented in specific papers and in comprehensive compilations, such as those of Bigarella et al. (1966), Archangelsky (1987, 1996), and Veroslavsky et al. (2006), among others.

Paleogeographic and Kinematic Constraints in the Tectonic Evolution of the Pre-Andean Basement Blocks

Formation of the south Central Andes basement cannot be unlinked from the final stages of Western Gondwana amalgamation. This process is still matter of much debate and continuous research. In the last two decades, a picture has emerged that suggests that the accretion and/or displacement of discrete crustal blocks in the Ediacaran and Early Paleozoic was a major process that finally configured the basement upon which three hundred million years later the Andean orogen developed. A main crustal fragment of the basement of central Argentina is the Pampia terrane. Recent, but still scarce, paleomagnetic results on Cambrian rocks from this terrane show anomalous pole positions whose interpretation is still ambiguous. Whether Pampia is a cortical fragment linked to the Amazonian craton that collided against the Rio de la Plata craton coetaneously with the closure of the hypothetical Clymene Ocean, whether it was attached previously and subsequently collided with a western crustal sliver (Western Pampia) associated with the Arequipa-Antofalla terrane, or whether it was a fragment detached from southern Kalahari that was displaced along the margin of the Rio de la Plata craton in the Cambrian, cannot be unambiguously solved due to the scarcity of paleomagnetic data. It is generally accepted that the very long magmatic belt along the western boundary of Pampia in the Ordovician (the Famatinian arc) developed on stretched continental crust. Systematic large clockwise rotation of Ordovician magmatic and sedimentary rocks along this belt has been confirmed in recent years, but the originally para-autochthonous rotated terrane model has been replaced by one in which a systematic pattern of small crustal block rotations accompanied deformation due to collision of the allochthnous Cuyania terrane. The very large counterclockwise rotations in the Western Puna (Antofalla terrane) of Chile and Argentina are still interpreted by some authors as evidence of closure of a V-shaped back-arc basin between Antofalla (and Arequipa?) and the Western Gondwana margin in the Late Ordovician. However, the interpretation of such rotations as due to tectonic escape of the Antofalla block during Cuyania collision may be more compatible with geochemical signatures that suggest crustal links between the Sierras Pampeanas and the Puna basements. Paleomagnetic support for the Laurentian origin of the Cuyania terrane has endured the significant improvement of the apparent polar wander path for Gondwana in the Ediacaran–Cambrian. Alternative models suggesting a para-autochthonous origin of Cuyania are difficult to reconcile with different lines of evidence (biogeographic, isotopic) and are not supported by available paleomagnetic data for this terrane. Whether Chilenia is a truly allochtonous terrane or a fragment of Cuyania that rifted apart to collide back in the Devonian is still controversial. Unfortunately, paleomagnetic data are not available to test these models yet.

Magnetic susceptibility mapping of the Cambrian El Hongo pluton, Eastern Sierras Pampeanas, Argentina

A map of bulk magnetic susceptibility was obtained on El Hongo trondhjemite, a small Cambrian pluton intruding the metamorphic basement in Eastern Sierras Pampeanas, Argentina, based on systematic magnetic susceptibility measurement at 450 sites using a SM30 susceptibility meter. Samples were collected on 58 sites and their bulk magnetic susceptibility was measured in laboratory with a Bartington MS2 system. Point-to-point comparison showed differences, that were attributed to the effect of roughness of the surveyed surfaces, and to the development of a weathered cap. However, the difference was systematic and in accordance with expected values predicted by manufacturer tables, whereby, once corrected with the appropriate factor, the obtained values with SM30 susceptibility meter were regarded as representative of fresh rocks. The resulting map was interpreted in terms of variation in abundance of magnetite, which is present in the rocks as a magmatic mineral, altered to hematite (martitized) in varying degrees. The map revealed that El Hongo trondhjemite is a weakly magnetic pluton, with a typical bulk susceptibility of about 500 × 10−6 SI, which would correspond to an abundance of magnetite below 0.2 vol%, but with conspicuous variations. Lows in the outer sector and in the vicinity of metamorphic xenoliths were interpreted as due to destruction of magnetic minerals linked to reactions between magma and host rock. A distinct concentric pattern in the western area could indicate the presence of a separate intrusion. Finally, alternate highs and lows in susceptibility follow the undulations in regional schistosity, which in turn would have controlled the emplacement of the pluton. Thus we provide a good example of the utility of magnetic susceptibility mapping in granitoid terrains, as an expeditious way for preliminary mapping that could guide further and more detailed research.

Early Cretaceous Volcanism in Central Argentina

The main exposed site of Early Cretaceous volcanism in central Argentina is located in Sierra Chica of Cordoba Province (SCC), within the Central Rift System. Also to the south, in Levalle basin, a thick Early Cretaceous volcanic pile lies buried in the subsurface. Other localities where volcanism is exposed are Sierra de las Quijadas and Cerrillada de las Cabras of San Luis Province. In SCC, as in the other mentioned localities, a volcanic–sedimentary complex was developed under rifting tectonics. Lava flows are frequently associated with scoria fall, pyroclastic and phreatomagmatic breccias within a strombolian-type volcanism. A new 40Ar/39Ar dating performed on sanidine phenocrysts of a trachyte from Almafuerte locality indicated an age of 129 ± 1 Ma. Diverse groups of rocks, mainly of potassic character, were distinguished: (1) alkali basalt—trachyte suite, (2) transitional basalt—latibasalt suite, (3) basanite—phonolite suite and (4) ankaratrites. Magma evolution must have taken place at crustal level(s) from distinct parental melts, mainly through fractional crystallization in an open-system magma chamber. Mantle source composition supports residual garnet and phlogopite, it does not exhibit features related to slab-derived metasomatism despite its location over Pampean mobile belt, and bears a lithospheric nature. SCC volcanism is of high Ti, display similarities with potassic Brazilian localities around Parana basin as Alto Paranaiba and Goias, pointing out analogies in their mantle sources.