Pablo H. Alasino

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.

Temporal histories of Cordilleran continental arcs: Testing models for magmatic episodicity

Abstract Magmatic activity in continental arcs is known to vary in a non-steady-state manner, with the mechanisms driving magmatic activity being a matter of ongoing discussion. Of particular importance is the question of what extent episodic magmatism in continental arcs is governed by external factors (e.g., plate motions) and internal factors (e.g., feedback processes in the upper plate). To test existing models for magmatic episodicity, which are mostly based on temporally and spatially limited records, this study uses large data sets of geochronological, geochemical, and plate kinematic data to document the Paleozoic to Mesozoic development of the North and South American Cordilleras in eight transects from British Columbia to Patagonia. The temporal distribution of U/Pb bedrock and detrital zircon ages, used as a proxy for timing of magmatic accretion, shows that some minima and maxima of zircon abundance are nearly synchronous for thousands of kilometers along the arc. Some age patterns are characterized by a periodicity of 50–80 Ma, suggesting a cyclic controlling mechanism. Other magmatic lulls or flare-ups find no equivalents in adjacent sectors, indicating that either discrete events or variable lag times may also be important in governing magmatic activity in continental arcs. Magma composition in Mexico, the Peninsular Ranges, and the Sierra Nevada varies episodically and proportionally with the temporal record of arc activity. During flare-up events, there is an increase in Sm/Yb, indicating deeper melting, and a decrease in eNdb suggesting a higher degree of crustal assimilation. Geochemical scatter also increases during the initiation of flare-up events. Plate kinematic data provide a means of evaluating mantle heat input. The correlation between plate convergence rate and magmatic accretion varies for each sector, suggesting that different flare-ups or lulls likely reflect variable combinations of processes.

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.

AN ISOSTATIC MASS BALANCE MODEL OF CONTINENTAL ARCS AND ITS APPLICATION TO PALEOZOIC-MESOZOIC ARGENTINEAN CORDILLERAN OROGENIC SYSTEMS

The Argentinean Cordilleran arc has been a hot spot to study arc evolution because of its well-preserved arc history. The proto-Andean arc system is on the Pacific side of the South American continent, and has at least 600 Myr of history (Franz et al., 2006). Previous studies have focus on a limited set of parameters. A newly proposed isostatic mass balance model, originally used to study the Sierra Nevada arc, is applied to the Argentinean Cordilleran arc. The one-dimensional model takes into account volumetric fraction of magma (β), tectonic shortening strain ( 3), erosion response time (τE), and mass-of-root to mass-of-melt ratio (γ). The model then reconstructs elevation and crustal thickness history of the arc.