Garnet–monazite rare earth element relationships in sub-solidus metapelites: a case study from Bhutan

Clare J. Warren, Lucy V. Greenwood, Tom W. Argles, Nick M. W. Roberts, Randall R. Parrish, Nigel B. W. Harris

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    Abstract

    A key aim of modern metamorphic geochronology is to constrain precise and accurate rates and timescales of tectonic processes. One promising approach in amphibolite and granulite-facies rocks links the geochronological information recorded in zoned accessory phases such as monazite to the pressure–temperature information recorded in zoned major rock-forming minerals such as garnet. Both phases incorporate rare earth elements (REE) as they crystallize and their equilibrium partitioning behaviour potentially provides a useful way of linking time to temperature. We report REE data from sub-solidus amphibolite-facies metapelites from Bhutan, where overlapping ages, inclusion relationships and Gd/Lu ratios suggest that garnet and monazite co-crystallized. The garnet–monazite REE relationships in these samples show a steeper pattern across the heavy (H)REE than previously reported. The difference between our dataset and the previously reported data may be due to a temperature-dependence on the partition coefficients, disequilibrium in either dataset, differences in monazite chemistry or the presence or absence of a third phase that competed for the available REE during growth. We urge caution against using empirically-derived partition coefficients from natural samples as evidence for, or against, equilibrium of REE-bearing phases until monazite–garnet partitioning behaviour is better constrained.
    Original languageEnglish
    JournalGeological Society Special Publications
    Volume478
    Early online date7 Apr 2018
    DOIs
    Publication statusEarly online - 7 Apr 2018

    Keywords

    • RCUK
    • NERC
    • NE/H016279/1
    • IP-1177-0510
    • NIGFSC

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