2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 24-7
Presentation Time: 9:35 AM

KINETIC AND FLUID CONTROL OF MINERAL GROWTH AND ISOTOPE DISTRIBUTION DURING VERY LOW- TO LOW-GRADE METAMORPHISM


WILLNER, Arne, Institut fuer Mineralogie und Kristallchemie, Universitaet Stuttgart, Azenbergstr. 18, Stuttgart, 70174, Germany, BARR, Sandra M., Department of Earth and Environmental Science, Acadia University, Wolfville, NS B4P2R6, Canada, GLODNY, Johannes, Deutsches GeoForschungsZentrum, Potsdam, 14473, Germany, MASSONNE, Hans-Joachim, Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, Stuttgart, D-70174, Germany, SUDO, Masafumi, Universität Potsdam, Institut für Erd- und Umweltwissenschaften, Potsdam-Golm, 14476, Germany, VAN STAAL, Cees R., Geological Survey of Canada, 625 Robson Street, Vancouver, BC V6B 5J3, Canada, WHITE, Chris E., Nova Scotia Department of Natural Resources, P.O. Box 698, Halifax, NS B3J2T9, Canada and ZAGOREVSKI, Alexandre, Geological Survey of Canada, 601 Booth Street, Ottawa, K1A 0E8, Canada

Successful geothermobarometry and dating of metamorphic rocks which had experienced temperatures below 320°C (subgreenschist facies) strongly depends on the understanding of the kinetic processes and hydrous fluid access at these conditions. According to our experience, small unzoned grains of solid-solution phases such as chlorite and mica are formed continuously below ~320°C. These grains are characterized by notable compositional heterogeneity acquired during changing PT conditions. Above 320°C grain growth results from recrystallization or specific mineral reactions and larger grains are characterized by zoning. This important difference is interpreted by nucleation rate exceeding growth rate below ~320°C presumably caused by local supersaturation of dissolved cations due to sluggishness of fluid transport, whereas growth rate dominates at higher temperature. By PT pseudosection modelling it can be shown that between 220°C and 320°C (depending on pressure) most hydrous fluids are released in common rock types such as metapelite and metabasite. This has not only consequences for deformation, but particularly for attainment of local equilibria at very low grade and systematic grain growth at low grade conditions during variable local water access. With examples from Nova Scotia and Newfoundland it will be shown that PT pseudosections have advantages to treat these equilibria quantitatively compared to calculations of multivariant equilibria. Rb-Sr mineral isochrons obtained at very low-grade conditions further indicate attainment of equilibrium conditions, whereas the prograde continuous nucleation of phases with variable composition must have occurred within the time range defined by the errors of the isochron method. In some cases strong spot age heterogeneity at thin-section scale was detected applying 40Ar/39Ar in situ laser ablation. This is interpreted as due to heterogeneous fluid access in rocks with multiple deformational overprint.

Session No. 24

T217. Mechanisms, Rates, and Timescales of Texture Formation in Metamorphic Rocks I
Sunday, 19 October 2014: 8:00 AM-12:00 PM
122 (Vancouver Convention Centre-West)
Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.81
© Copyright 2014 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.
Back to: T217. Mechanisms, Rates, and Timescales of Texture Formation in Metamorphic Rocks I
<< Previous Abstract | Next Abstract >>