T77 A Life in Mineralogy and Petrology: A Session in Honor of Robert J. Tracy

Monday, 23 September 2019
8:00 AM - 12:00 PM
Room 126ABC, North Building

THE PRESSURE-TEMPERATURE-TIME HISTORY OF CALEDONIAN KYA-KFS-MUS ASSEMBLAGES FROM LOFOTEN, NORWAY, BASED ON MODELING OF ELEMENTAL ZONING IN GARNET AND 40AR/39AR AGE GRADIENTS IN MUSCOVITE

Paper number: T77-14
Program: Technical Sessions
Time: 11:20 AM – 11:35 AM

Authors

Abstract

Prograde major and trace element zoning and mineral inclusions preserved in garnet enable reconstructions of the path a rock experienced to peak metamorphic conditions. However, in cases where zoning (chemical and isotopic) has been extensively modified by intracrystalline diffusion during protracted high-temperature conditions, following peak metamorphism, an interpretation of the overall history is uncertain as many combinations of inferred crustal depth, temperature and time could produce similar diffusion profiles. Pelitic gneisses and schists from the district of Røst (the farthest southeast islands of the Lofoten archipelago, Norway) contain the assemblage Kyanite+K-feldspar+Muscovite, that formed thorough deep partial subduction of the Baltic Margin during the Silurian (Scandian) phase of Caledonian orogeny. Millimeter-radius garnets in these rocks typically show little to no zoning with respect to Fe, Mg, Mn and Ca. In contrast, the garnets have conspicuous core-rim patterns of zoning for P, Y, and Cr, consistent with preserved prograde zoning of these trace elements. Phase equilibria indicate minimum conditions of ca. 720°C and 8.5 kbar for the assemblage, though the present mineral chemical compositions and classic thermobarometry yield temperatures and pressures far lower. Muscovite from these rocks has intracrystalline zoning of 40Ar/39Ar ages that are consistent with a path that remained at ca. 500-480°C from ~420-320 Ma, followed by more rapid cooling through final 40Ar retention in muscovite by ~270 Ma. We suggest that prograde concentration gradients for major elements in garnet in these rocks were homogenized by lattice diffusion at conditions above or near 500°C, whereas millimeter-radius muscovite crystals were partially retentive of radiogenic 40Ar from ~ 500-400°C. Combining the diffusion data of these systems with phase equilibria constraints on peak P-T conditions leads to a reconstructed pressure-temperature-time path that is more fully constrained. Overall, the metamorphic history of sequences in Røst are consistent with ~35 km subduction, with an ensuing exhumation history characterized by episodes of rapid cooling separated by spans of very slowly changing temperature that lasted for tens to hundreds of millions of years.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-339890

© Copyright 2019 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.