DOI: 10.25549/usctheses-c40-411564
title: Depth-dependent physical, chemical and temporal evolution of crustal differentiation in magmatic arcs

description: The compositional similarity between the average trace element composition of magmas erupted at island arcs and the bulk continental crust implies that new continental crust is predominately formed in subduction zone settings and associated magmatic arcs. However, partial melts generated in the mantel of a subduction zone are dominantly basaltic and further differentiation is required to achieve the andesitic to dacitic bulk composition of the continental crust. What the exact differentiation processes are, where continental crust differentiation takes place, how material deforms and is accommodated in the arc crustal column and what the pace of continental crust formation is, are controversial in the scientific community. ❧ This study presents new insights to the ongoing debate of continental crust formation by studying arc crustal sections, which are solidified, tilted snapshots of arc crustal columns, now brought to the surface through tectonic processes so that different depths can be investigated. By integrating structural, chemical and geochronologic studies, depth-dependent changes in deformation, magma reservoir formation, differentiation at different crustal levels and the interplay of magmatism and tectonism in the arc crustal column can be observed. In this study, three locations spanning a depth range from volcanics to 35 were closely studied: the shallow crustal Guadalupe igneous complex in the Jurassic Sierra Nevada arc, California, the mid-Cretaceous Cascades arc (~ 5 to 35 km depth), Washington and the Ordovician Famatinian arc (volcanics to ~ 30 km depth), Argentina. ❧ In order to constrain the timescale of differentiation and magma reservoir construction a new workflow is developed to interpret high-precision U-Pb zircon TIMS ages in terms of the emplacement durations of mafic and felsic magmas in the Guadalupe igneous complex. This methods allows to estimate the emplacement time scale of the bimodal complex to ~ 300 ka. Results also indicate that although a magma chamber can form at these shallow crustal levels, assimilation below the emplacement level is required to explain the isotopic composition and in-situ generation of felsic liquids from basaltic parental melts is restricted. ❧ Investigated intrusive complexes in the mid-Cretaceous arc crustal column do not show a depth-dependent dominance of certain ifferentiation mechanism but indicate that at all crustal levels, magma chambers large enough for local assimilation and fractionation to take place are able to form. Magma addition rates (MARs) calculations show a generally low baseline value of ~ 0.0012 km3/yr, which is only interrupted by the mushroom unit in the Mt. Stuart intrusive complex showing exceptional high MARs. This unit forms at the temporal peak of the flare-up and sustained a large, melt-rich magma reservoir genrating the observed compositional variability in-situ. Comparably large and long-lived magma chambers are uncommon in the mid-Cretaceous Cascades upper and deep crust. Thus, the majority of compositional variability observed at each crustal level is largely a consequence of lower crustal assimilation, fractional crystallization and mixing processes. ❧ Structural studies in the Famatinian arc in Argentina have shown that magmatic fabrics and host rock structures imply regional orthogonal east-west shortening and crustal thickening during arc magmatism with stronger intensities in the mid to deep-mid crust (~ 2 to 5 kbar) of the main arc section. The different amounts of crustal shortening between upper (~ 10 %) and mid-deep crust (~ 60 %) need to be accommodated for. Possible explanations are the following: (a) presence of a crustal-wide detachment zone; (b) a vertical deformation gradient, partly accommodated by changing deformation mechanisms with depth such as the development of large shear zones at depth; or (c) the greater shortening at depth is accommodated by the greater volumes of magmatism. Mass balance calculations indicate that magmatism and tectonism equally coontribute to crustal thickening (~ 240 %) and elevation gain during arc activity.