Juan A. Dahlquist

New SHRIMP U-Pb data from the Famatina Complex: constraining Early-Mid Ordovician Famatinian magmatism in the Sierras Pampeanas, Argentina

New SHRIMP U-Pb zircon ages are reported for igneous and sedimentary rocks of the Famatina Complex, constraining the age of the magmatism and the ensialic basins. Together with whole-rock and isotope geochemistry for the igneous rocks from the complex, these ages indicate that the voluminous parental magmas of metaluminous composition were derived by partial melting of an older lithosphere without significant asthenospheric contribution. This magmatism was initiated in the Early Ordovician (481 Ma). During the Mid-Late Ordovician, the magmatism ceased (463 Ma), resulting in a short-lived (no more than ~20 Ma) and relatively narrow (~100-150 km) magmatic belt, in contrast to the long-lived cordilleran magmatism of the Andes. The exhumation rate of the Famatina Complex was considerably high and the erosional stripping and deposition of Ordovician sediments occurred soon after of the emplacement of the igneous source rocks during the Early to mid-Ordovician. During the upper Mid Ordovician the clastic contribution was mainly derived from plutonic rocks. Magmatism was completely extinguished in the Mid Ordovician and the sedimentary basins closed in the early Late Ordovician.

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.

Sr, C and O isotope composition of marbles from the Sierra de de Ancasti, Eastern Sierras Pampeanas, Argentina: age and constraints for the Neoproterozoic–Lower Paleozoic evolution of the proto-Gondwana margin

The Sierra Brava Complex on the eastern flank of the Sierra de Ancasti consists of marbles, metabasites, calc-silicate rocks, psammo-pelitic schists and gneisses. In the central part of this sierra a thick succession of banded schists (Ancasti Formation) crops out. Regional metamorphism of these rocks is attributed to the Famatinian orogeny (Ordovician), metamorphic grade increasing westwards and southwards and culminating in a migmatite complex on the western side of the Sierra. The meta-carbonate rocks are subdivided into a northeastern group (low-grade calcite marbles), and a southeastern group (high-grade calcite and calcite-dolomite marbles). Twenty-three marble samples were analysed for Sr isotope composition and Rb, Mn, Mg and Ca contents, and six for C and O isotope composition. An Ediacaran depositional age of 570 –590Ma is inferred by reference to the trend of 87Sr/86Sr in Neoproterozoic seawater. Thus the metacarbonates are probably slightly older than the Ancasti Formation (equivalent to the Puncoviscana Formation of northern Argentina), which has a maximum sedimentation age of ca. 570Ma. Ediacaran depositional ages have also been reported for metacarbonates elsewhere in Argentina, Uruguay and Brazil. We propose that the Sierra de Ancasti carbonates on one hand, and those in the Western Sierras Pampeanas (Difunta Correa Sequence) and –tentativelythe Corumba Group of Brazil on the other, represent platforms on opposite margins of the extinct Clymene Ocean, whereas Neoproterozoic carbonate successions such as the Loma Negra Formation (Tandilia, southern Argentina) and the Arroyo del Soldado Group (Uruguay) were deposited on the eastern side (present coordinates) of the Rio de la Plata craton, which at the time occupied a position farther to the north.

Low-pressure emplacement of epidote-bearing metaluminous granitoids in the Sierra de Chepes (Famatinian Orogen, Argentina) and relationships with the magma source

A petrological and geochemical study of Lower Ordovician epidote-bearing granitoids of the Sierra de Chepes (Famatinian Orogen, Sierras Pampeanas, Argentina) indicates low-pressure emplacement of these extensive metaluminous suites. Textural relationships combined with major oxide and REE data support a magmatic origin for the ubiquitous epidote in hornblende-biotite granodiorite and tonalite plutons. Geothermobarometry using mineral assemblages in igneous rocks indicates a temperature interval from 780 to 850°C, and low emplacement pressures in the range of 2.2 to 4.1 kbar, consistent with those estimated from the metasedimentary envelope of the plutons. The widespread occurrence of primary epidote is a distinctive characteristic of the Lower Ordovician granitoids as opposed to the Andean Carboniferous granitoids emplaced at middle pressures ( e.g. , Santo Domingo Complex, 7 kbar), where magmatic epidote-granitoids are scarce and secondary epidote is commonly present. On the other hand, the pistacite value (Ps=[Fe 3+ /(Fe 3+ +Al)] x 100, in %) in magmatic epidote of the Ordovician granitoids (average Ps=28) sharply contrasts with that of magmatic epidote in the Carboniferous Andean granitoids (average Ps=24). This strongly suggests that the occurrence of magmatic epidote-bearing granitoids might be related to different sources, the Famatinian Orogen granitoids being mainly derived by melting of old continental lithosphere with probable partial contribution from subcontinental lithospheric mantle, and the Carboniferous Andean granitoids mainly resulting from mixing of crustal- and mantle- derived magmas. If the relationship of epidote-bearing granitoids to the characteristics of the source is confirmed in future studies, this will constrain the geotectonic environment in which the epidote-bearing magmas occur.

Mafic rocks of the Ordovician Famatinian magmatic arc (NW Argentina): New insights into the mantle contribution

We studied the petrogenesis of mafic igneous rocks in the Famatinian arc in the western Sierra Famatina (NW Argentina), an Early Ordovician middle-crustal section in the proto-Andean margin of Gondwana. Mafic rock types consist of amphibolite, metagabbro, and gabbro, as well as pod- and dike-like bodies of gabbro to diorite composition. Field relations together with geochemical and isotopic data for the mafic rocks of the western Sierra de Famatina (at 29°S) define two contrasting suites, which can be correlated with similar assemblages noted in other parts of the orogen. Amphibolite, metagabbro, and gabbro bodies are mostly the oldest intrusive rocks (older than 480 Ma), with the host tonalite and post-tonalite mafic dikes being slightly younger. The older mafic suite is tholeiitic to calc-alkaline and isotopically evolved, except for most of the amphibolite samples. The younger suite is calc-alkaline, typically displaying subduction-related geochemical signatures, and it is isotopically more juvenile. Whole-rock chemical composition and isotopic analyses are compatible with a progressive mixing of different isotopic reservoirs. Pyroxenite (±garnet) was likely the dominant source of the older gabbroic magmas, whereas peridotite dominated in the source of the younger suite, implying that the mafic magma experienced a progressive shift toward more juvenile compositions though time (over 20 m.y.). Pyroxenite-derived melts could have been generated by lithospheric foundering followed by upwelling of primitive melts by adiabatic decompression of mantle wedge peridotite.