High-temperature metamorphism

13.02-22.02.2008

13-14 + 21-22.02.2008

The course is given by Simon Harley, University of Edinburgh, one of the most highly reputed metamorphic petrologists worldwide.

Course Outline

Granulite facies high-grade terrains form much of the deep continental crust, and are exposed over vast areas in continental shields as well as in young mountain belts. They are, therefore, important components of the evolving Earth system that demand close scrutiny if we are to understand mountain building, the nature of crust-mantle interactions, mass fluxes through the crust, and the evolution of continental crust through time. Within the broad P-T field of granulite metamorphism, ultrahigh-temperature metamorphism (UHT) presents specific challenges to thermomechanical modeling of tectonics that will in time lead to advances in our understanding of the deep crust and its behaviour in accretional, collisional and possibly back-arc settings.

This course will present an overview of granulites and granulite facies metamorphism, considered both in a purely metamorphic sense but also within their broader contexts as examples of high-grade terrains and sources of evidence for tectonic processes via their P-T-time and structural histories.

The course will consist of a series of illustrated lectures and interactive presentations / tutorials, complemented by practical work. The practical work will involve both the examination of high grade metamorphic rocks under the microscope and the construction and calculation of grids and phase diagrams that can provide insight into the interpretation of UHT rocks. The emphasis in the practical exercises will be on understanding principles and applying these in simple ways, rather than on using sophisticated but ‘black box’ software packages that yield results but not insight into the stepwise processes involved in detailed calculations. The application of geochronology to granulites will also be considered, supported by an illustrative practical exercise. Students will need a laptop, pen and paper, and be able to use the standard software package Excel to do the calculations in the practicals. Tailor-made spreadsheets will be provided by Simon Harley.

The course will be given in three parts, corresponding to the three (or so) days over which the lecture material and intervening practicals will be given. The content associated with each part is detailed below.

Day 1: High-Grade Terrains, Granulites, and Metamorphic Principles

Wednesday, February 13., 2008; 09:15h, room 605

1. High Grade Terrains: Significance for the Earth; HGT / granulites in space and time; Constitution – orthogneisses and paragneisses, with examples and overview of the types of information that can be obtained; Field / structural features and chronologies of ‘typical’ HGTs / granulite areas.

2. The PT regime of granulite metamorphism I: The key P-T-assemblage indicators in mafic and felsic rock types; The nature of the amphibolite-granulite transition and importance of low water activities.

Laboratory work:

• Initial petrological study of typical, and not-so-typical, granulites

3. The PT regime of granulite metamorphism II: Overview of the key P-T-assemblage indicators in pelites. Essentials of AFM assemblage analysis. The importance of melt. Definition of UHT. Introduction to P-T paths.

Laboratory work:

• Refresher tutorial / exercises on variance, divariant equilibria and related chemographic principles leading to Schreinemakers’ analysis

Day 2: Pelites and their phase equilibria in HT/UHT Metamorphism

Thursday, Ferbruary 14., 2008; 09:15h, room 605

1. P-T grids and Schreinemakers’ analysis: the key principles and concepts as applied to HT/UHT pelites; worked examples from simple MAS and FMAS systems – doing it from scratch.

Laboratory work:

• Schreinemakers’ analysis from scratch in MAS and FMAS

2. Experimental and calculated grids – from FMAS to KFMASH with melt: Key features, and rationale for UHT. Relationships between univariant and divariant equilibria and resultant contouring of grids.

Laboratory work (to be done prior to Day 3):

• Calculations of MAS to FMAS equilibria about an invariant point

3. P-T pseudosections: More advanced approaches using P-T phase diagrams and P-(T)-u sections; How to develop and use these diagrams; quantification; significance of melt and cordierite. Uncertainties.

4. Migmatites and the importance of Cordierite: Processes in migmatites. Experimental data. Analysis of cordierite as a volatile-bearing phase, and cordierite-melt H2O relations. Calculation of water activities in grids and sections.

Laboratory work (to be done prior to Day 3):

• Activity calculations and melt H2O estimates from cordierite.

Day 3: HT/UHT P-T constraints, Reaction textures and P-T paths

Thursday, February 21., 2008; 09:15h; morning: room 201; afternoon: room 605

1. UHT metamorphism - advances in thermometry and P-T constraints: Al in Opx; Ti in zircon, Zr in rutile and other phases; use of calculated pseudosections involving sapphirine, with discussion of their uncertainties.

Laboratory work:

• Follow up of laboratory work set on Day 2 (P-T and aH2O calculations results and interpretations)

• Estimation of T using Ti-Zr thermometers, with appraisal

2. Reaction textures: Principles of interpretation using contoured grids and pseudosections; examples with P-T paths interpreted; Cordierite in reaction textures and the roles of fluids; Importance of gaining time information.

Laboratory work:

• Petrological analysis of UHT granulites with high-quality reaction textures. Interpretation using pseudosections and P-T-μ

• Activity calculations based on cordierite in reaction textures

Day 4: HT/UHT P-T-time paths and their tectonic implications

Friday, February 22., 2008; 09:15h, room 201

1. Putting the T into P-T-t; some aspects of dating in granulites: Zircon behaviour in HT/UHT metamorphism, and linkage to P-T via reactions and chemistry; Monazite and its behaviour; T-time paths and implications for rates and settings. Examples.

Laboratory work:

• Age calculation using monazite electron probe Th, U and Pb data

• Examination of zircon-garnet trace element data

University of Geneva

2

active participation

please contact Urs Schaltegger

see website 'courses', go to 'practical information, then download the form; reimbursements are made twice a year

via the web

25