Timescales of magmatic processes in accretionary orogens: Implications for crustal growth

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The continental lithosphere has a thin cap of siliceous crust (~10-40 km) underlain by a denser Fe-Mg rich (ultramafic) continental lithospheric mantle. The upper crust is broadly granitoid in composition. The granitoid magmas originate in the deep crust and provide a window for studying deep crustal processes. Most researchers agree that large, granitoid intrusions form in continental arcs above subduction zones (the angled descent of oceanic lithosphere into the underlying mantle). However, many, fundamental questions persist as to whether magma generation and emplacement spans the duration of subduction or whether magma emplacement is concentrated in discrete episodes that reflect collisional orogeny and post-collisional crustal relaxation. Are magma genesis and emplacement continuous or episodic processes? If magma genesis is continuous, is magma emplacement in the upper crust episodic and facilitated by discrete tectonomagmatic events? To address these questions, I will focus on the Caledonian mountain belt in northwestern Europe. In Ireland and Scotland, the field relationships are remarkably preserved and well documented. Norway's exceptional vertical exposures allow a three-dimensional investigation of granitoid intrusions. Deducing the processes responsible for granitoid magma generation and emplacement requires combining field observations with lithogeochemistry, precise age constraints, and sensitive isotopic ratios that record the magma source(s). As individual minerals grow in magmas, they become zoned and their geochemical and isotopic compositions from core to rim record the evolving relative contributions from the mantle and recycled crust. Only recently have analytical techniques become sufficiently precise to provide fundamental insights into magma generation and emplacement processes. My HQP and I will use petrochronology (the determination of rock or mineral growth with time) to investigate the genesis of granitoid magmas, the evolution of magma sources during the subduction, collision, and post-collisional phases of an accretionary mountain belt, and the timescales of the magmatic processes. We will analyze the trace element and isotopic compositions of robust minerals, which systematically record magma evolution as they grow. These coupled petrological and temporal frameworks will be incorporated into models that link grain-scale processes to complex tectonic and magmatic histories. My research experiences form the foundation of the proposed research program that includes many HQP opportunities. The objectives are to contribute to the understanding of nature and timescales of magmatic processes in mountain belts. The proportion of crustal recycling to new crustal growth is perquisite for models of crustal evolution and for investigating potential connections between episodes of crustal growth, climate, the evolution of life, and metal endowments.