César Casquet

The Pampean Orogeny of the southern proto-Andes: Cambrian continental collision in the Sierras de Cordoba

Abstract A detailed study of the pre-Silurian geology of the Sierras de Córdoba, Eastern Sierras Pampeanas, is used to define the sequence of magmatic and metamorphic events during the Pampean orogeny. This primarily involved Early to Mid-Cambrian subduction and terrane collision at the western margin of Gondwana during the amalgamation of the super-continent. Evidence for this is based principally on new information concerning (a) regional mapping and field relations, (b) analysis of the structures, deformational history and meta-morphic evolution and (c) geochronology and geochemistry of the igneous and metamorphic rocks. The main events recognized are (1) Late Proterozoic break-up of Rodinia (Nd model ages of 1500 ± 200 Ma, inherited zircons 800–1400 Ma), (2) development of an Early Cam-brian passive margin sequence (Puncoviscana Formation and equivalents), (3) emplacement of metaluminous calc-alkaline granitoids (G1a, dated at 530 ± 3 Ma) as a result of NE-directed subduction, (4) crustal thickening, ophiolite obduction, compression and high-grade metamorphism (M2: 8.6±0.8 kbar, 810 ± 50°C, c.525 Ma) related to collision, and culmina-ting in (5) isothermal uplift and widespread low-P anatexis (M3, 4.0±0.5 kbar, 715 ± 15°C, c.520 Ma). The last event is responsible for the linked generation of highly peraluminous granites (G1b) and cordieritites. Subsequent emplacement into the accreted terrane of Ordovician trondhjemite-tonalites (500-470 Ma) and dextral wrench shear are interpreted as inner cordilleran counterparts of the Famatinian arc, which developed to the west along the newly-formed proto-Andean margin.

Early evolution of the Proto-Andean margin of South America

From a detailed study of a 500 km transect in the Sierras Pampeanas, central-west Argentina, two pre-Silurian tectono-magmatic episodes are recognized and defined, each culminating in micro-continental collisions against the proto-Andean margin of Gondwana. The Pampean orogeny started in Early Cambrian time with short-lived subduction, indicated by ca. 535 Ma calc-alkaline granitoids. Following Pampean terrane collision, burial to granulite facies conditions (ca. 9 kbar) generated widespread migmatites and ca. 520 Ma highly peraluminous granites in the Eastern Sierras Pampeanas. After brief quiescence, a second major episode, the Famatinian orogeny, started with subduction ca. 490 Ma, forming a wide continental arc and ensialic backarc basin. This heralded the approach of Laurentia to Gondwana, during which the Precordillera terrane separated from the southern Appalachian region, finally colliding with Gondwana in Silurian–Devonian time.

Evolution of the eastern volcanic ridge of the Canary Islands based on new KAr data

The results of 64 new KAr age determinations, together with 32 previously published ages, show that after a period of erosion of the basal complex, Miocene volcanic activity started around 20 Ma in Fuerteventura and 15 Ma in Lanzarote, forming a tabular succession of basaltic lavas and pyroclastics with a few salic dykes and plugs. This series includes five separate volcanic edifices, each one with its own eruptive history. In Fuerteventura, several Miocene eruptive cycles have been identified: in the central edifice one around 20–17 Ma, followed by two others centred around 15 and 13 Ma; in the southern edifice the maximum of activity took place around 16–14 Ma, whereas in the northern one the main activity occurred between 14 and 12 Ma. In Lanzarote a first cycle of activity took place in the southern edifice between 15.5 and 14.1 Ma, followed by another between 13.6 and 12.3 Ma. In the northern edifice three pulses occurred: 10.2–8.3, 6.6–5.3 and 3.9–3.8 Ma. An important temporal gap, greater in Fuerteventura than in Lanzarote, separates Series I from the Plio-Quaternary Series II, III and IV, formed by multi-vent basaltic emissions. In Fuerteventura the following eruptive cycles have been identified: 5, 2.9–2.4, 1.8–1.7, 0.8–0.4 and <0.1 Ma. In Lanzarote, the activity was fairly continuous from 2.7 Ma to historic times, with a maximum in the Lower Pleistocene. Eruptive rates in the Series I edifices were on the average 0.1–0.01 km3/ka, comparable but slightly smaller than in similar edifices in Tenerife and La Gomera, but much lower than in Gran Canaria. Average post-Miocene eruptive rates were about 0.013–0.027 km3/ka in Lanzarote and 0.003–0.007 km3/ka in Fuerteventura. All these volcanic edifices show a similar general sequence (fissural eruptions, erosion, multi-vent volcanism), repeated at different periods in different parts of the eastern islands of the Canaries. The model of growth of the Series I edifices is comparable to those in Tenerife and La Gomera: long periods of activity, sometimes greater than 6 m.y., with pulses separated by gaps. However, salic and intermediate differentiates, frequent in Tenerife and La Gomera, are very scarce in these islands. The Fuerteventura-Lanzarote ridge shows a decrease in volcanic activity with time, and also a certain SSW-NNE polarity in the temporal development of volcanism.

Involvement of the Argentine Precordillera terrane in the Famatinian mobile belt: U-Pb SHRIMP and metamorphic evidence from the Sierra de Pie de Palo

New data suggest that the eastern margin of the Argentine Precordillera terrane comprises Grenvillian basement and a sedimentary cover derived from it that were together affected by Middle Ordovician deformation and metamorphism during accretion to the Gondwana margin. The basement first underwent low pressure/temperature ( P/T ) type metamorphism, reaching high-grade migmatitic conditions in places (686 ± 40 MPa, 790 ± 17 °C), comparable to the Grenvillian M2 metamorphism of the supposed Laurentian counterpart of the terrane. The second metamorphism, recognized in the cover sequence, is of Famatinian age and took place under higher P/T conditions, following a clockwise P-T path (baric peak: 1300 ± 100 Mpa, 600 ± 50 °C). Low-U zircon overgrew detrital Grenvillian cores as pressure fell from its peak, and yields U-Pb SHRIMP ages of ca. 460 Ma. This is interpreted as the age of ductile thrusting coincident with early uplift; initial accretion to Gondwana must have occurred before this. The absence of late Neoproterozoic detrital zircons is consistent with a Laurentian origin of the Argentine Precordillera terrane.

The age and origin of the barite-fluorite (PbZn) veins of the Sierra del Guadarrama (Spanish Central System, Spain): a radiogenic (Nd, Sr) and stable isotope study☆

Fluorite and barite-rich veins ( + sulphides) of the Sierra del Guadarrama (Spanish Central System) are usually hosted by Hercynian granitoids. Zoned fluorites from one of these veins have a Sm-Nd age of 145 ± 18 Ma. This age is similar to the K-Ar ages obtained in other hydrothermal rocks of the Spanish Central System (156-152 Ma) and confirms the existence of an important hydrothermal event during the Late Jurassic, probably related to extensional fracturing accompanying the opening of the North Atlantic Ocean. The 87Sr/86Sr ratio found in fluorite and barite (0.7154-0.7207) is in most cases lower than that found in the host granitoids (0.7194 and 0.7209), suggesting more than one source of strontium. This, in turn, supports previous models that involve mixing of two fluids during mineralization, an ascending hot ( > 300 ° C) and low-salinity fluid and a shallow, cooler ( < 100 ° C) and more saline brine. The ascending fluid probably reacted to a variable extent with the host granitoids, and the most likely source for the shallow fluid derived from a marine/evaporitic environment, with a low 87Sr/86Sr ratio (0.7069 in the Late Jurassic) . The calculated ENd composition of the fluorites (-8.8 to -7.6) as well as the 147Sm/144Nd variation found in this mineral (0.153-0.405) do not conflict with the mixing model although they can be largely explained by interaction of the ascending fluid with the host granitoids. Sulphur isotopic compositions of sulphides range between - 3.4 and - 0.8 ‰ and those of barites from + 15.5 to + 15.8‰. The near δ34S-value of the sulphides suggests that their sulphur was derived from the hydrothermal leaching of the granitoids by the ascending fluid. The δ34S-value of barite is consistent with Upper Jurassic marine sulphate as the source of sulphate in the shallow fluid. The absence of equilibrium between reduced and oxidized sulphur species can be easily interpreted as a consequence of the short residence time of the mixed fluid in the open fractures at temperatures between 250° and lOO°C. Although no pre-Cretaceous sediments are found on this part of the Iberian Hercynian basement, the data presented here support the existence of a shallow platform joining central Spain with the Iberian or the Subbetic realm in Late Jurassic times.

The Aguablanca Cu–Ni ore deposit (Extremadura, Spain), a case of synorogenic orthomagmatic mineralization: age and isotope composition of magmas (Sr, Nd) and ore (S)

Abstract The Aguablanca Cu–Ni orthomagmatic ore deposit is hosted by mafic and ultramafic rocks of the Aguablanca stock, which is part of the larger, high-K calc-alkaline Santa Olalla plutonic complex. This intrusive complex, ca. 338 Ma in age, is located in the Ossa-Morena Zone (OMZ) of the Iberian Variscan Belt. Mineralization consists mainly of pyrrhotite, pentlandite and chalcopyrite resulting from the crystallization of an immiscible sulphide-rich liquid. Isotope work on the host igneous rocks (Sr, Nd) and the ore (S) suggests that contamination with an upper-crustal component took place at some depth before final emplacement of the plutons (eNd338=−6 to −7.5; Sr(338)=0.7082 to 0.7100; δ34S(sulphides) near +7.4‰). Assimilation–fractional crystallization (AFC) processes are invoked to explain early cumulates and immiscible sulphide-magma formation. Intrusion took place at the beginning of the type-A oblique subduction of the South Portuguese Zone under the Ossa-Morena Zone and was probably driven by transpressive structures (strike-slip faults). The mineralization is thus synorogenic. Aguablanca is probably the first case referred to in the literature of a magmatic Cu–Ni ore deposit hosted by calc-alkaline igneous rocks.

K-bentonites in the Argentine Precordillera contemporaneous with rhyolite volcanism in the Famatinian Arc

New U–Pb radiometric dates for K-bentonite horizons within the Lower Cambrian to Middle Ordovician platform carbonates from the Precordillera terrane of NW Argentina provide further constraints on models for the allochthonous or parautochthonous accretion of this terrane. Two K-bentonite layers from the Talacasto section yield indistinguishable sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dates of 469.5 ± 3.2 Ma and 470.1 ± 3.3 Ma respectively. These are within uncertainty of the U–Pb SHRIMP zircon date of 468.3 ± 3.4 Ma for a porphyritic rhyolite from the Famatinian magmatic arc, Sierra de las Planchadas, near Rio Chaschuil. Geochemical and isotope data also demonstrate the similarity of the K-bentonite and Chaschuil rhyolite parent magmas. Thus, it is highly probable that the Famatinian arc volcanoes provided the ash for the K-bentonite horizons, suggesting proximity to the Precordillera terrane during the deposition of the Lower Cambrian to Middle Ordovician platform carbonates. This implication supports a mid-Ordovician collision model, but could also be compatible with a parautochthonous model for docking of the Precordillera terrane, by movement along the Pacific margin of Gondwana, rather than across the Iapetus Ocean.

Grenvillian massif-type anorthosites in the Sierras Pampeanas

We report the discovery of massif-type anorthosites in the Andean basement of the Western Sierras Pampeanas of Argentina. U–Pb zircon dating (by sensitive high-resolution ion microprobe) of a cogenetic gabbronorite dyke yields ages of 1070 ± 41 Ma for igneous emplacement and 431 ± 40 Ma for metamorphism. These anorthosites are petrologically and geochemically comparable with those of the Grenville province of Laurentia. Palaeogeographical reconstructions of Rodinia at 1.0–1.1 Ga suggest that the Sierras Pampeanas anorthosites were part of a large anorthosite province in the late Mesoproterozoic.

Las Orogénesis del Paleozoico Inferior en el margen protoandino de América del Sur, Sierras Pampeanas, Argentina.

El margen proto-andino de Gondwana ha sido el escenario de al menos dos orogenesis desde el desmembramiento del supercontinente Rodinia al final del Neoprotrozoico, hasta el reagrupamiento de las masas continentales en Pangea al final del Carbonifero. Ambas orogenesis van precedidas de un periodo de apertura oceanica y sedimentacion en margenes pasivos y culminan en subduccion oceanica con desarrollo de arcos-magmaticos de tipo cordillerano y colision de tipo continente-continente. La primera, orogenesis Pampeana, tiene lugar en el Cambrico, en un intervalo de tiempo relativamente pequeno (535-520 Ma: etapas de subduccion-arco magmatico y colision), y culmina con la acrecion ortogonal de un pequeno terreno continental (terreno Pampeano) de naturaleza semiautoctona. Por el contrario, la orogenesis Famatiniana, tiene lugar en un periodo de tiempo mas dilatado, durante el Ordovicico y Silurico (499-435 Ma). Durante esta orogenesis tuvo lugar la acrecion de un terreno exotico a Gondwana, el terreno Precordillera (460 Ma). Este terreno esta constituido por un basamento grenvilliano (aprox. 1.1Ga) y una cubierta sedimentaria de plataforma carbonatada de edad Cambrico-Ordovicico. La acrecion al margen de Gondwana fue probablemente oblicua, y el margen oriental del terreno Precordillera fue afectado por fuerte deformacion y metamorfismo regional. El basamento de los cinturones andinos del Paleozoico Superior y Mesozoico situados al oeste de la Precordillera, parece estar constituido tambien por rocas metamorficas grenvillianas; con lo cual, gran parte de los Andes centrales entre los 26oS y 34oS se encuentra asentado sobre terrenos aloctonos. En cualquier caso, la paleogeografia de las masas continentales involucradas en la colision de los terrenos exoticos durante el Paleozoico Inferior no se conoce bien todavia.

Palaeostress and geotectonic interpretation of the Alpine Cycle onset in the Sierra del Guadarrama (eastern Iberian Central System), based on evidence from episyenites

Several episodes of hydrothermal activity related to periods of fracturing and/or reactivation of previous structures took place from 300 to - at least - 100 Ma, in the Sierra del Guadarrama, which is part of the crystalline axis of the Iberian Hercynian Fold Belt (Central-Iberian Zone). One of these episodes led to the formation of episyenites, which are de-quartzified and alkalinized granites. Episyenite formation took place on a regional scale and in a short period (approx. at 277 Ma). The episyenites were formed by the action of fluids at temperatures between 350°C and 650°C, at depths of about 6.5 km, and in microfractured dilatancy zones developed under a regional extensional regime. These zones are crosscut by normal faults, developed during the progressive deformation process accompanying the formation of the episyenites. The calculated regional palaeostress tensor has ~r I close to vertical and σ3 between NI0-20E and an average value of the stress ratio (Ф) of 0.19 [Ф = (σ2 - σ3)/(σ1 - σ3)]. Because σ1 is close to vertical the stress tensor is compatible with an extensional deformation field. The analysis also shows that most of the faults that slip under this stress field have an average coefficient of friction of 0.8. This extensional regime was probably accompanied by a regional thermal anomaly, as suggested by the high temperature of the fluids involved, which are amagmatic. This thermo-tectonic episode is interpreted as representative of the generalized extensional regime corresponding to the onset of the Alpine Cycle. The episode was preceded by a wrench-faulting event, equivalent to the Late Variscan event of Arthaud and Matte (1977), for which an age of - at least - 300-290 Ma is indicated by recent radiometric data. In its turn, this event was preceded by the regional extensional gravitative collapse of the Hercynian orogen. A correlation between evidence from the cover (stratigraphy and volcanism) and evidence from the basement (hydrothermal alterations, dyke injection episodes and granitic magmatism) is attempted on the basis of new available radiometric data.