Portrait of a pluton: Magmatic perspectives from the mid-crustal Bergell Intrusion, Central Alps

Abstract

The study of Earth is an inherently historical scientific enterprise. Geochronology, i.e., the science of determining the ages of rocks and minerals, the dates of geologic events, and the durations or tempos of naturally-occurring phenomena, is therefore essential to the de- velopment of holistic models of Earth system processes. Uncertainties persist as to the characteristic rates and mechanisms of magma transport, differentiation and cooling in the crust, with direct implications for the spatiotemporal scales of magma chambers, degree of crustal compositional stratification, and processes of volcanism. Debate is also ongo- ing as how to best apply geochronological methods to better constrain igneous phenomena. Chapter 1 provides an introduction to geochronology; the status of geochronology both as a tool and a discipline in and of itself; and the application of geochronological methods to problems in igneous petrology. Chapter 2 introduces a multi-method geochronological– geochemical workflow for precise characterization of the “gold standard” geologic timekeeper, the mineral zircon. This workflow is applied to the Bergell Intrusion, a tilted mid-crustal magmatic system exposed in the Swiss-Italian Alps, with temporal trends in zircon com- position indicating protracted magmatism in the deep crust. Chapter 3 bridges Bergell zircon age–temperature–compositional spectra with thermodynamic and diffusive models of zircon crystallization, highlighting to an unprecedented degree zircon’s capacity as a tracer of magma evolution. Chapter 4 presents the results of original field mapping, structural analysis and titanite thermochronology across a range of magma emplacement depths, es- tablishing a comprehensive framework for modeling Bergell assembly. Chapter 5 synthesizes the preceding findings in the form of a numerical model that describes the emplacement and thermal evolution of the Bergell, with field, structural and geochronological data incorpo- rated as model constraints. Cumulatively, this contribution details the history of a dynamic, long-lived, incrementally-assembled intrusive suite, and provides a template for innovative application and interpretation of geochronological data in future studies.