Project

metamorphic petrology; geochronology; high pressure; continental subduction; Western Alps; Rare Earth minerals; Sesia Zone; Dent Blanche nappe; PTt-paths; thermal evolution; polycyclic orogen; in situ geochronology

Lead
Lay summary
In tektonischen Subduktions- bzw. Kollisionszonen entstehen Hochdruck-Gesteine, z.B. Eklogite und Blauschiefer. Die internen Westalpen gelten als klassischer Gebirgsgürtel mit einem Spektrum solcher HP-Gesteine. So finden sich in der Sesia-Zone km-grosse Decken von kontinentaler Kruste, die zunächst in Tiefen von 40-70 km versenkt wurden, um dann rasch wieder zur Erdoberfläche zu gelangen. Obschon von fundamentaler Bedeutung in der Geodynamik, versteht man die komplexen Prozesse an destruktiven Plattenrändern noch immer ungenügend. Speziell ist in den Westalpen die räumlich-zeitliche Entwicklung der HP-Metamorphose kontrovers.Dieses Projekt erforscht die Gesteinsbildung, Deformation und geochemischen Prozesse, welche die Entstehung der HP-Gesteine am Nordrand des adriatischen Kontinents bedingt haben. Strukturell kontrollierte Proben von metamorphen Gesteinen aus mehreren tektonischen Einheiten werden untersucht, von denen die meisten bereits prä-alpin in Gebirgsgürteln gebildet worden sind. Mittels mikrochemischer Analyse von lokalen Mineralgleichgewichten werden die in HP-Gesteinen gespeicherten Druck-Temperatur Bedingungen rekonstruiert und die relative zeitliche Abfolge zur mehrphasigen Deformation geklärt. In geeigneten Proben wird das Alter der HP-Stadien datiert, v.a. mittels Monazit, Allanit und Titanit. Dazu werden Methoden der in situ U-Th-Pb Chronometrie (LA-ICP-MS) verbessert. Diese Mineralalter und P-T-Resultate haben in der ersten Phase des Projekts robuste Daten zur Frühphase der Orogenese in den Westalpen liefern. Die Untersuchung mehrfach metamorpher Gesteine wird das Verständnis der geochemischen Mobilität und Fluid-Gesteins Interaktion im Bereich der tiefen Subduktion vertiefen.High pressure rocks, such as blueschists and eclogites, form in suduction and collision zones. The internal Western Alps (e.g. Sesia zone) are considered a classic area of (largely continental) high pressure metamorphism, encompassing a broad spectrum of HP-rocks. However, the dynamics of processes leading to HP-terranes, notably the timing and conditions of the early collisional history in the Western Alps, remain poorly understood.This project focuses on the processes leading to the formation of high pressure terranes. Structurally controlled samples from several (mono- and polycyclic ones) tectonic units of the Western Alps shall be analyzed. Local mineral equilibria are used for thermobarometry, and assemblages are related to the polydeformation. The best suited samples are used to date the early HP-metamorphism using improved in situ dating techniques based on U-Th-Pb isotopic analysis of allanite, monazite, and titanite by LA-ICP-MS.

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Last update: 21.02.2013

Active participation

Number	Title	Start	Funding scheme

A field-based project aims to reconstruct the tectonometamorphic evolution of the internal Western Alps, a classic high-pressure terrain. Characterized by predominantly continental rocks, most of the polycyclic, the current picture of this important orogenic segment is wrought with discrepant age data and a paucity of data relating to the initial convergence at the Apulian margin. This project concentrates on monometamorphic units uints first, to work out the Alpine history by integrating structural, petrological, and geochronological methods. In the second part, polymetamorphic units are investigated as well, and the processes and scale of crustal recycling as well as the assembly of the orogen are worked out.Much emphasis is laid on extending the possibilities of in situ isotopic age dating of robust mineral chrono¬meters, notably allanite and monazite, using Th-Pb and U-Pb analysis. An improved procedure for LA-ICP-MS analysis is developped and applied in conjunction with ion probe (SHRIMP) dating. Work on isotopic standard materials is proposed, again primarily for allanite. The reaction mechanisms of allanite and its potential for integrated petrochronometry are investigated. Field work in the Dent Blanche nappe, the central and southern Sesia Zone, and in several Austroalpine klippen units includes detailed mapping, structurally controlled sampling, and correlation. Petrological and geochronological analysis of samples aims to quantify the spatial and temporal evolution of tectonic units. Quantitative PTDt-paths are derived, yielding constraints on the rates of tectonometamorphic processes (heating, subduction, mineral reaction, and exhumation). A number of currently hot problems of metamorphic petrology and geochemistry are addressed as part of this comprehensive study. These include the extent of re-equilibration of polymetamorphic rocks (including eclogite facies superimposed on granulite facies), the mobility of trace element, and the REE-budget in various metamorphic environments under high pressure conditions.