Jv. Owen et Jd. Greenough, INFLUENCE OF THE MODE AND DISTRIBUTION OF GARNET AND BIOTITE ON GRT-BT THERMOMETRY - EVIDENCE FROM A SINGLE-SAMPLE CASE-STUDY, Mineralogical Magazine, 59(396), 1995, pp. 497-504
Detailed microprobe and modal data for a sample of layered, garnetifer
ous, quartzose paragneiss reveal significant differences in garnet-bio
tite Mg-Fe distribution coefficients (Kd) - and hence paleotemperature
s - determined for leucocratic (modal Grt+Bt<25 vol.%) and mesocratic
(Grt+Bt>25 vol.%) layers. In leucocratic layers, InKd 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 bi
otite, and, as demonstrated in other studies, inversely with the dista
nce separating both minerals. Due to the small size (<2 mm) of garnets
, which facilitated diffusional re-equilibration during cooling from p
eak metamorphic temperature, InKd does not correlate to modal Bt/Grt r
atios. The largest garnets, which occur in mesocratic layers, nonethel
ess tend to preserve the most pronounced (retrograde) zoning patterns
(i.e. rimward increase in Fe/Mg), consequently mineral core compositio
n InKd values correlate with grain diameter except where garnets conta
in abundant biotite inclusions. The highest Grt-Bt temperatures (simil
ar to 700 degrees C) are recorded by: (1) the composition of relativel
y 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 r
ange of temperatures intermediate between T-max and diffusional blocki
ng temperatures (similar to 560 degrees C) recorded by the rim composi
tions of contiguous grains.