Checking the vital signs of arc magmatism from the fossil reservoirs of a NW Argentinian crustal section and elsewhere
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2022
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Doctoral Thesis
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Abstract
Checking the last vital signs of magmatism concealed in fossil reservoirs of ancient arcs is key to retracing fundamental processes related to the growth of continents, the volcanic-plutonic connection, the cycle of important greenhouse gases, and the distribution of precious metals. For decades, geological research employed indirect methods of observations to explore these processes, such as analyzing volcanic emissions, lava flows, upper crustal plutonic reservoirs, experiments, remote sensing, and numerical modeling. However, it becomes more evident that direct observations could greatly help resolve some of the biggest conundrums in this field. Over the past two decades, the discovery of crustal sections bearing crustal-scale fossilized magmatic remnants of arcs provided a useful tool to directly observe the processes controlling the growth and chemical stratification of juvenile continental crust and the recycling of dense crustal roots (e.g., delamination). These advances were essential to estimate the pace of crustal growth over time, which records crucial moments in the planet's history, such as the emergence of continents or the great oxygenation event.
In light of these developments, this thesis aims to further deepen our understanding of crustal-scale arc magmatism by exploring three main research avenues. To this end, this work focuses primarily on the crustal section bearing the remnants of the Ordovician Famatinian continental arc in NW Argentina presented in the first chapter. This work also focused on two other systems, e.g., the crustal section of the Kohistan island arc in Pakistan and the Adamello Batholith in northern Italy.
The second chapter explores whether granitic fossil reservoirs characterize failed eruptions or silicic cumulates (e.g., crystal mush) used to feed volcanic systems. This distinction is critical as one considers granites to be representative of liquid compositions and the other does not. The study used statistical analyses on a large geochemical database of bulk-rocks and minerals coupled with petrological observations and thermodynamical modeling on samples from the Adamello Batholith. The results show the majority of granitic rocks from the databases to be, in fact, silicic cumulates that underwent different amounts of melt-loss. Adamello Batholith samples provide a case study supporting these statistical results and show the most distinctive geochemical characteristics of silicic cumulates.
The third chapter treats the question of crustal growth over time by shedding light on the balance of crustal generation, reworking, and recycling at convergent plate margins. The study assembled a geochemical database for the Famatinian arc with bulk-rock analyses for major and trace elements, Ca-isotopes, zircon Oxygen (O) – Hafnium (Hf), Uranium – Lead isotopes, and trace element compositions. The results show that the remnants of the Famatinian arc, representing different levels of the paleo-crust but displaced on a N–S paleo-arc axis, originated from similar source magmas, and settled at the same peak period characteristic of the area, i.e., 470 ± 2 Ma ago. However, O – Hf isotopes in zircon reveal that the mafic lower crustal remnants have “crustal” signatures and the upper crustal remnants have more “mantle” signatures. The study argues that these variations originate from reworking crustal materials at the source rather than within the host crust (e.g., “source” vs. “path” contamination, respectively) because the pre-existing host rocks are homogenous in the whole arc. In light of these results, the study challenges the current paradigms that attribute the isotopic diversity of magmatic rocks mostly to “path” contamination and proposes “source” contamination as the most viable process. This conclusion implies significant continental growth since the Archean as “source” contamination decreases the flux of recycled materials in arcs.
The fourth chapter explores the distribution of precious metals and the role of volatiles in arcs by deciphering the nature, distribution, and composition of sulfides in lower crustal arc cumulates. This study used petrological and petrographic methods to track the evolution of sulfides from ultramafic to tonalitic rocks in the Kohistan island arc and the Famatinian continental arc. This work also provides the trace element compositions for each phase recognized. The main results of this study can be divided into three parts. Firstly, dense cumulates susceptible to recycling back to the mantle have scant proportions of sulfides. Secondly, there are different sulfide assemblages in similar types of rocks between the two arcs. Thirdly, the sulfides assemblages in both arcs have textures attesting of the interaction with a magmatic volatile phase. Interestingly, it is possible to see that the global trends in bulk-rock data react to the different sulfide assemblages and to the effect of the magmatic volatile phase. These results diverge with the model in which delamination plays the major role in the chalcophile deficit observed in arcs but instead supports the role of volatiles in carrying these elements to the upper crustal and hydrothermal system, hydrosphere, and atmosphere.
Overall, these three studies provide additional insights about crustal-scale arc magmatism that can serve future research on the topic. For example, the fact that granitic materials are not liquids supports that, in arc environments or continental rifts, the upper crustal reservoirs are dynamic engines that contribute further to the chemical stratification of the continental crust. This model would further improve our understanding of the origin of granites, “the building blocks” of our continents. Furthermore, the sustainability of upper crustal silicic reservoirs is dependent on a constant heat-flux from an underlying magmatic refinery. The latter may require that the entire magmatic column is active simultaneously from the lower to the upper crust. The ages and geochemical compositions collected for the Famatinian arc in the second chapter and the different field observations strongly support this concept. Therefore, these observations would support the model of transcrustal polybaric magmatic columns. The exsolution of volatiles in these magmatic columns, commanded by mafic recharges, mixing or simply silicate differentiation would cause the transfer of chalcophiles and greenhouse gases to the upper crust to form porphyry copper deposits or to the atmosphere regulating the climate and the chalcophile deficit over time.
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Examiner : Bachmann, Olivier
Examiner : Otamendi, Juan
Examiner : Müntener, Othmar
Examiner: Ulmer, Peter
Examiner : Scaillet, Bruno
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ETH Zurich
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Subject
MAGMATIC ROCKS + IGNEOUS ROCKS (PETROGRAPHY); Continental subduction; ISOTOPE GEOCHEMISTRY; SULFUR CYCLE (GEOCHEMISTRY)
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03958 - Bachmann, Olivier / Bachmann, Olivier