INFLUENCE OF THE MODE AND DISTRIBUTION OF GARNET AND BIOTITE ON GRT-BT THERMOMETRY - EVIDENCE FROM A SINGLE-SAMPLE CASE-STUDY

Citation
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
Citations number
20
Categorie Soggetti
Mineralogy
Journal title
ISSN journal
0026461X
Volume
59
Issue
396
Year of publication
1995
Pages
497 - 504
Database
ISI
SICI code
0026-461X(1995)59:396<497:IOTMAD>2.0.ZU;2-E
Abstract
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.