Dating Metamorphism with Accessory Minerals - Integrated In-situ Geochronology and Trace Element Budgeting of Metamorphic Reactions

University Of Kansas Center For Research Inc, Lawrence KS

Investigators

Abstract

This proposal seeks funding to investigate two fundamental aspects of petrological research, namely an improved understanding of dating metamorphic processes by accessory minerals and the "granulite enigma". One of the remaining challenges to geochronology is to understand when and by which reactions accessory minerals used for U-Th-Pb dating grow and are modified during metamorphism. The question is: which part of a metamorphic P-T path are we dating with a particular accessory mineral? State-of-the-art imaging and laser-ablation ICP-MS techniques will be used to carry out in-situ geochronology, trace element thermometry and fingerprinting, trace element budgeting, all in a detailed textural context. The "granulite enigma" concerns how granulite facies conditions (high T at moderate P) are attained in the continental crust. Intellectual merit. The proposed research will address three fundamental questions in metamorphic petrology and geochronology: 1) Which part of the metamorphic path can be dated by different accessory phases? 2) Are inclusion textures reliable relative time markers for accessory phases? 3) What is the likely heat source responsible for granulite metamorphism in an alpine style orogen? The target area to be investigated is a young (30 Ma) occurrence of high temperature granulite facies rocks in orogenic belts, the Gruf complex in the Central Alps. Ages can determined with 1-2 % precision by laser ablation ICP-MS dating of U-Th-rich accessory minerals (xenotime, monazite, zircon), enabling correlation of the transient metamorphic event with the regional geological history. Previous studies of the PI have identified accessory mineral textures in the Gruf complex granulites that have the potential to combine mineral textures and inclusion relationships with trace element budgeting of metamorphic reactions and balancing reactions of accessory phase growth, e.g. xenotime found in breakdown-textures of garnet may be produced from HREE and Y stored in high temperature garnet, with or without involvement of monazite and/or apatite. Garnet- or rutile-breakdown may contribute Zr to metamorphic zircon growth. Broader impacts. This study will contribute to the very fundamental scientific questions outlined above. In addition, the project will support one PhD student and provides that student an opportunity for international collaboration with an expert in the field of metamorphic petrological modeling, which enhances the expertise at KU. This will provide the graduate student with important international experience for a future career either in industry or academia. The project also involves undergraduate students in activities that will develop their teaching and leadership skills and provide insight into research, state-of-the-art analytical facilities and international collaborative science. Work with the PI and other academics in the KU laboratories will extend learning beyond the classroom.