Influence of the Mode and Distribution of Garnet and Biotite on Grt-Bt Thermometry: Evidence from a Single-Sample Case Study

J. V. Owen and J. D. Greenough
Department of Geology, Saint Mary's University, Halifax, N.S., Canada B3H 3C3
Department of Geological Sciences, Okanagan College, 3333 College Way, Kelowna, B.C., Canada V1V 1V7

Abstract: Detailed microprobe and modal data for a sample of layered, garnetiferous, quartzose paragneiss reveal significant differences in garnet-biotite Mg-Fe distribution coefficients (Kd) — and hence paleotemperatures — determined for leucocratic (modal Grt+Bt<25 vol.%) and mesocratic (Grt+Bt> 25 vol.%) layers. In leucocratic layers, lnKd determined from both the core and rim compositions of minerals shows a range of values that varies sympathetically with the absolute amount of garnet and biotite, and, as demonstrated in other studies, inversely with the distance separating both minerals. Due to the small size (<2 mm) of garnets, which facilitated diffusional re-equilibration during cooling from peak metamorphic temperature, lnKd does not correlate to modal Bt/Grt ratios. The largest garnets, which occur in mesocratic layers, nonetheless tend to preserve the most pronounced (retrograde) zoning patterns (i.e. rimward increase in Fe/Mg), consequently mineral core composition lnKd values correlate with grain diameter except where garnets contain abundant biotite inclusions. The highest Grt-Bt temperatures (∼700°C are recorded by: (1) the composition of relatively widely-separated (>0.3 mm) grains in highly leucocratic layers; and (2) the core compositions of relatively large (>1 mm), inclusion-free grains in mesocratic layers. More closely spaced garnets and biotites in leucocratic layers, and small grains in mesocratic layers, give a range of temperatures intermediate between Tmax and diffusional blocking temperatures (∼ 560°C) recorded by the rim compositions of contiguous grains.

Keywords: garnet • biotite • thermometry • palaeotemperatures • paragneiss

Mineralogical Magazine; September 1995 v. 59; no. 396; p. 497-504; DOI: 10.1180/minmag.1995.059.396.11
© 1995, The Mineralogical Society
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