Testing Models for Magma Transfer and Emplacement In 4-Dimensions:The Bergell Intrusion, N. Italy

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Intellectual Merit. The timescales and volumes of magma generation, transport, and emplacement help set the thermal, structural, and geochemical architecture of the lithosphere. Structural and petrologic data, in conjunction with recent advances in high-precision geochronology have led to contrasting models that involve either large volumes of magma transported and emplaced over short timescales or small batches of magma that are emplaced incrementally over many millions of years. Modelling of magma emplacement is hampered by : a) limited exposure of complete plutonic systems, and b) incomplete understanding of timescales timescales for magma emplacement. This proposal outlines a three year project targeting both challenges in the ca. 30 Ma Bergell intrusion, located in the Central Alps in N. Italy. This pluton was chosen for the following reasons: [1] extraordinary exposure and relief in the high Alps, [2] its young age permits very high-precision geochronology (ca. ±10-20 ka on single zircon fragments), [3] it exhibits a range in lithologies from gabbroic to granodioritic with archetypal normal zoning, and [4] an ~12-15 vertical km crustal cross section has been exposed through post-emplacement Alpine uplift and tilting. Based primarily on structural and field data, emplacement of the Bergell pluton is inferred to have been prolonged (ca. 5 Ma) with mid-crustal magma residence followed by diapirism/ballooning into the upper crust. This model is in stark contrast to recent incremental intrusion models for middle and upper crustal plutons and demands corroborating geochronology of samples with well-characterized field relations. The goal of this study is to construct a robust 4D model for pluton construction based on detailed field work and high-precision geochronology of carefully selected samples spanning the feeder zone to the roof. Initial work described in this proposal summarizes field work in the Bergell and presents a novel analytical approach combining zircon geochronology with in situ and bulk mineral trace element analyses on dated zircon fragments. Accessory mineral geochemistry will aid interpretations of high-precision geochronology by identifying xeno- and antecrysts and be used to detail potential magmatic differentiation trends, which place timescales on AFC processes and evaluate the degree to which magmatic intrusions remained closed systems. Integration of whole-rock major/trace element data will be used to decipher the size and frequency of magma pulses. Finally, this information will be used to develop numerical models of magma emplacement and evolution, and its implications for rheological characteristics of middle and upper crust. Broader Impacts. This work will train one PhD student (Kyle Samperton) and at least two senior Under graduates (Christine Chen ?13 in year 1) in a diverse set of tools including field geology, structural geology, and igneous petrology, and to supplement these skills with experience using cutting edge geochemical and isotopic analytical methods. This work involves training in one other laboratory (Prof. John Cottle?s LA-ICPMS lab at Univ. California Santa Barbara) and also in numerical techniques through collaboration with Dr. Catherine Annen (Univ. Bristol). The project will provide support for newly established analytical facilities at Princeton as well as the PI?s participation in the NSF-supported EARTHTIME initiative. The project links with a multi-university Swiss initiative to study the Adamello batholith, located ~100 km SE of the proposed field locale. The Adamello batholith is a classic locality for studies of magmatic processes but, significantly, lacks the vertical resolution through the crust provided by the Bergell intrusion. The PI has served as an external expert for that project and will organize a joint field symposia involving swiss PhD students and collaborators in summer 2013 in order to share the results of both projects.