No abstract

Los Esquistos del Silgará consisten en metapelitas, metasemipelitas, rocas cuarzofeldespáticas, rocas carbonatadas puras y rocas calcosilicatadas. El contenido y patrón de REE y las relaciones Zr/Sc, Th/Sc y los altos contenidos de Hf y Zr, apuntan a una fuente de composición félsica de la corteza superior con alto grado de reciclaje. Los Esquistos del Silgará fueron metamorfizados en condiciones de pico indicado por la paragénesis Bt + Grt + St + Qz + Ky + Pl a condiciones de T = 670 – 770° C y P = 7.8 – 11.3 Kbar. La litología y las condiciones P-T del metamorfismo barroviense de los Esquistos del Silgará de la franja de Silos son muy similares a las del área de Mutiscua.

<p>The most recent activity of Piton des Neiges (La Réunion) is characterized by explosive behavior and relatively evolved magma compositions. These eruptions occurred roughly over the past 200 thousand years producing thick pyroclastic deposits, lava domes and block and ash flow deposits. We here present a detailed petrologic and geochronologic characterization of these deposits providing insights into the timing of explosive eruptions and pre-eruption magma storage conditions.</p><p>The early phase of explosive activity is characterized by up to 15 m-thick pyroclastic deposits found on the southeastern and western flanks of Piton des Neiges. These deposits have been regarded as individual discrete eruptions that occurred between 220 and 110 ka. Our detailed petrographical and geochemical study on juvenile fragments and the main mineral phases indicate that all deposits share similar geochemical fingerprints. High-precision single crystal <sup>40</sup>Ar/<sup>39</sup>Ar dates on 70 alkali feldspars from 6 samples reveal significant dispersion but the youngest population of dated crystals from each sample yield overlapping weighted mean dates around 200 ka, supporting their geochemical correlation. The wide spread in <sup>40</sup>Ar/<sup>39</sup>Ar dates of up to 88 kyrs prior to eruption, uncommon for alkali feldspar in volcanic rocks, argues for the presence of excess and/or inherited argon in those crystals. Together, our findings suggest that the early pyroclastic deposits are the product of a Plinian-type eruption that covered a large area of the island around 200 ka. The eruption was fed by a long-lived magma reservoir that produced differentiated magmas in response to lower recharge fluxes after the main active center migrated to the currently active Piton de la Fournaise. A wide range of mineral compositions and the strong disequilibrium between crystals and the trachytic groundmass is an indication of the pronounced heterogeneity of the magmatic reservoir following a deep recharge event that triggered the eruption.</p><p>The younger eruptive episode of Piton des Neiges occurred between 70 and 30 ka with dome-forming lavas of trachytic to rhyolitic composition that collapsed into pyroclastic density currents resulting in block and ash flow deposits found closer to the current summit. This eruptive style, infrequent in this geotectonic setting, has not yet been well recognized for Piton des Neiges. Pristine zircon crystals, found in a sample from a block and ash deposit, were dated with a total of 192 LA-ICP-MS spot analyses using the U-Th disequilibrium method, and constitute the first zircon geochronology study for this volcano. The results yield a well-defined isochron with a date of 44.80 ± 1.32 ka (2 S.E., MSWD = 1.2). Single crystal <sup>40</sup>Ar/<sup>39</sup>Ar dates on alkali feldspars show a similar dispersion as for the older eruptive phase, but the youngest dates overlap with the zircon U-Th date, providing robust estimates of the eruption age.</p><p>This detailed characterization of the youngest eruptive episode of Piton des Neiges documents its explosive potential during the past 200 thousand years and has significant implications regarding the current view of Piton des Neiges as an extinct volcano.</p>

<p>In porphyry copper deposits, where ore grades are low, the volume of the ore body determines its economic potential. However, the factors that control their size are still an open question and understanding them is crucial for finding new and large porphyry copper deposits (PCDs); especially in a society with an ever-increasing Cu demand. One of the most important, but highly debated size-modulating factors, time, could make the difference for a magmatic system to form small or large PCDs. The limited access to the plutonic roots of PCDs hinders our ability to study the plutonic and the porphyritic systems as a whole. However, Cenozoic tilting of the Yerington district (Nevada, USA) make it the perfect place for a key study as the Yerington batholith and associated volcanic sequences, porphyritic dikes and Cu mineralized centers are exposed. The complex upper-crustal batholith is comprised of three consecutive plutons that with time increase in silica content, granulometry and depth of emplacement while decreasing in volume: the McLeod quartz monzodiorite, the Bear quartz monzonite and the Luhr Hill granite with associated porphyritic dikes that formed Cu mineralization. Field observations show sharp intrusive contacts between the three plutons, until now have been interpreted as periods of magmatic quiescence that separate the emplacement of the three intrusions, overall accounting for 1 Ma of magmatic activity (Dilles & Wright, 1988; Schöpa et al., 2017). However, our new high-precision zircon U-Pb ID-TIMS dates spread over 2 Ma and show a continuum in zircon crystallization from the McLeod quartz monzodiorite and coeval volcanics to the Luhr Hill granite and porphyritic dikes with no hiatuses. In-situ trace element LA-ICPMS analyses on zircon further indicate a continuous geochemical evolution from intermediate compositions and higher Ti-in-zircon temperatures in the oldest zircons towards colder and evolved ones in the youngest ones, following normal fractional crystallization trends with the onset of titanite crystallization during evolution. These data argue for a sustained crystallization of the three main plutonic bodies. Our new lifetime of the magmatic system in view of zircon crystallization ages changes the previously defined thermal models for the Yerington district and affects how we assess its mineralizing potential. This questions the thermal budget of these upper crustal magma chambers, which should remain partially molten for about a million years to produce the observed zircon age spectra in each pluton. Such considerations open the discussion of zircon crystallization in the deep crust, reconciling these new high-precision data and the well-stablished field crosscutting relationships, and impacting the current understanding and application of zircon petrochronology in porphyry copper systems. <span> </span></p><p> </p><p>John H. Dilles, James E. Wright; The chronology of early Mesozoic arc magmatism in the Yerington district of western Nevada and its regional implications. GSA Bulletin 1988; 100 (5): 644–652.<span> </span></p><p>Anne Schöpa, Catherine Annen, John H. Dilles, R. Stephen J. Sparks, Jon D. Blundy; Magma Emplacement Rates and Porphyry Copper Deposits: Thermal Modeling of the Yerington Batholith, Nevada. Economic Geology 2017; 112 (7): 1653–1672.<span> </span></p>