EGU General Assembly 2019
The General Assembly is the biggest international conference on Earth sciences in Europe, organized by the European Geosciences Union. This congress takes place in Vienna Austria. This year it was held from April 7 to 12 and gathered 16273 people from 113 different countries. In the context of the scientific research and outreaching of our activities, three GeoExpedition geologists Valentina, Italo and Francisco, participated in the international congress European Geophysical Union (EGU 2019).
We presented three works in poster mode:
- Developing geosciences near the high mountain range next to Santiago, Chile: From Giant Volcanoes to Giant Ore Deposits
- Host rock architecture controlling the emplacement of shallow magma reservoirs: Inferences from Miocene plutonic bodies in central Chile
- High magma flow rates during the emplacement of shallow felsic laccoliths
The first work was presented on April 7th in the session Inter- and transdiciplinary research, education and practice in mountain regions: field experiences, challenges and opportunities. The following is a summary:
Developing geosciences near the high mountain range next to Santiago, Chile: From Giant Volcanoes to Giant Ore Deposits
We present high mountain activities involving geosciences research, in close proximity to Santiago, Chile, including: (1) our field experience on high altitude volcanoes (gas and water sampling) and oriented sampling (drill) during rock climbing in cliffs in Cajón del Maipo; (2) our geoturistic guide of Cajón del Maipo and (3) educational videos.
Santiago is the capital and the most densely populated city of Chile, with over 6 million people. It sits in a basin beside the Cajón del Maipo (valley of the Maipo river) and Mapocho Valley, in the Cordillera de los Andes. The Andes range next to Santiago is a natural laboratory for geosciences, characterized by a fluvial and glacial geomorphology, active fault systems, landslides, glaciers, giant ore deposits and active volcanoes that elevate
over 6 km above the sea level, feeding hydrothermal systems, including crater lakes and hot springs. More than 2.5 million tourists visit Chile every year, positioning it as the fourth most visited country in Latin America. Chile has been considered a privileged tourist destination because of the spectacular Andean nature and the security and stability of the country. Even though visitors usually stay a few days in Santiago, they generally go on to San Pedro de Atacama, Torres del Paine, or Easter Island, but rarely to the Cajón del Maipo. We identify an opportunity to educate people and increase the touristic activity in the city by the development of geological activities and information about geotouristic material. Our presentations make use of videos, maps and educational talks in local schools. Our goal is to educate the Chilean population, as well as visitors that stay for a few days in Santiago, using our research material by including science projects close to Santiago. Our goal is to use our research material to raise awareness of the geological richness of the area, both for local school children and visitors to Santiago.
The other two papers were presented on April 12th in the session Magma generation and differentiation: Field, geochemical, experimental and numerical investigation of magmatic and volcanic systems. Below is a summary:
Host rock architecture controlling the emplacement of shallow magma reservoirs: Inferences from Miocene plutonic bodies in Central Chile
Miocene magmatism in Central Chile is characterized by intense coeval tectonic activity changes, which controlled, for example, the composition and location of the volcanism: shifting from an extensional setting to a compressive one, volcanic sequences became more silicic and plutonic activity migrated slightly eastward. Under this scenario, the San Gabriel (SGP, 12.4-13.5 Ma U-Pb in zircon and ~60 km3 exposed volume) and La Gloria plutons (LGP, 9.9-11.1 Ma U-Pb in zircon and ~120 km3 expose volume) were emplaced at the eastern border of an intra-arc basin (Abanico basin), spatially associated to several N-S syncline and anticline folds. The geometry of both intrusives is dominantly elongated in a N-S direction, which are located at the same level of a stratigraphic discordance. These field observations suggest that the emplacement of SGP and LGP was strongly controlled by the architecture of the host rocks. Petrographic, compositional and internal structural features of both plutons allow inferring the magmatic evolution of the such reservoirs. SGP presents broad compositional and textural variations, suggesting low emplacement rates with limited internal homogenization by convective stirring. On the contrary, the LGP is a more homogeneous, indicating higher injection rates and enhanced convective stirring. Both plutons were likely constructed by magma input of similar compositions (intermediate 60-65 wt.% SiO2), but geochemical and textural data indicate that melt extraction from crystallizing mushes was more efficient in SGP compared to LGP, giving place to the more pronounced compositional variability.
The detailed examinations of both plutons suggest that: (1) blinded inverse faults and axial plane faults that progressively folded the country rocks controlled the magma ascent, whereas the stratigraphic discordances acted as rheological barrier that promoted lateral magma flow. Injection rate did not seem to play a major role in the emplacement location. Ubiquitous presence of host rock blocks into the plutons indicate that magmatic stoping was key both cases. (2) The internal differentiation and compositional diversification of both reservoirs was controlled mainly by differing injection rates, favoring the heterogeneity in a lower injection rate scenario, but not necessarily favoring a greater volume of the bulk extracted silicic melts.
High magma flow rates during the emplacement of shallow felsic laccoliths
Magma flow through the crust is typically directed vertically, although lateral emplacement had been documented during shallow magma reservoir construction in both volcanic and plutonic environments. We present a new numerical model to constrain magma flow conditions of shallow magma reservoirs that emplaced laterally by horizontal magma flow. The main goal is quantifying cooling rates to be compared with U-Pb zircon crystallization ages and temperatures. We apply these simulations on two case studies: La Gloria Pluton and San Gabriel Pluton, central Chile. Both are northwestward elongated plutons of 20 km in length and 4-6 km in width. They present ages between 10 to 13 Ma, and intrude volcanic sequences. Within-pluton age progression and subhorizontal mineral and magnetic lineations in these case studies suggest a southeastward Horizontal propagation of the magma during the reservoir emplacement.
Assuming continuous lateral magma propagation, thermo-mechanical numerical simulations indicate that the minimum injection rate needed to preserve a liquid-dominated reservoir (i.e. <50 vol% crystals) for hundred thousand years is a few times larger than that obtained by dividing the estimated pluton volume by the age range. This indicates that most of the magma that flowed through the magma reservoirs was evacuated away from the site of emplacement, implying that injection rates inferred from the plutonic record are underestimated with respect to the actual magma fluxes. An advective transfer of magma through plutonic bodies, particularly in shallow systems with a horizontal flow component such as La Gloria pluton, can partly explain why the averages of pluton-filling rates are lower than the injection rates obtained in volcanic systems or by numerical simulations of incremental magma emplacement. Magma advection rates necessary to maintain reservoirs’ temperature above solidus are typically inverse to their preserved volume, because the faster cooling of relatively small reservoirs. The higher advection of smaller reservoirs could be favored by their ability for reaching a critical overpressure to evacuate magma, with respect to more voluminous reservoirs.