Empirical garnet–muscovite geothermometry in low‐grade metapelites, Selwyn Range (Canadian Rockies)

Abstract

Partitioning of Fe and Mg between garnet and phengitic muscovite was calibrated as a geothermometer by Green & Hellman (1982) using experimental data at 2S30 kbar. #en the thermometer is applied to pelites regionally metamorphosed at pressures of between 3 and 7 kbar it yields temperatures much higher than those from the garnet-biotite thermometer. A new empirical calibration is proposed for use with such rocks, with particular application where garnet occurs at lower grades than biotite. The new calibration is:In K+4.13 T (K) = where K is given by:In K = In Kd andXii are mole fractions in the garnets.The calibration was derived from comparison with the garnet-biotite thermometer of Ferry & Spear (1978), assuming no pressure-dependence for the partitioning between garnet and muscovite, no ferric iron partitioning, ideal mixing in muscovite, and the garnet mixing model of Ganguly & Saxena (1984) modified for a non-linear Ca effect. This latter garnet mixing model was selected because it gave the geologically most reasonable results. It has not proved possible to distinguish a pressure effect from a ferric-iron effect.Despite the simplifying assumptions used to derive the calibration, it yields temperatures generally within 15°C of those given by the garnet-biotite thermometer, and has been used to supply thermometric data in a low-grade region of the Canadian Rockies.

Petrologic modeling of chloritoid–glaucophane schists from the NW Iberian Massif

“…The presence of garnet and phengite in the absence of biotite is typical of pelites in high-P belts (Kurata and Banno, 1974;Raheim, 1977;Hynes and Forest, 1988;Arenas et al, 1995 ). In the Cean 5chists, biotite is absent in the 51 micro-inclusions fabric, but is a common mineral in some of the samples.…”

Evolution métamorphique des métapélites du Massif hercynien des Rehamna (Maroc): implications tectonothermales

“…Skhour. Th ermometres: grt-chl (Ghent et aI., 1987); grt-bio (Ferr y et Spear, 1978;Hodges et Spear, 1982;Hoinkes, 1986); grt-ms (Hynes et Forest, 1988). 8arometre : GRIPS (Sohlen et Liotta, 1986), applique, d'une part, aux assemblages syn-D, (1) et syn-D2c (2) de la zone du grenatet, d'autre part, aux assemblages syn-D, (3) et syn-D2c (4) de la zone de la staurotide.…”

“…Les resultats significatifs ont ete obtenus par les reactions d' echange Fe-Mg entre biotite et grenat (Ferry et Spear, 1978;Hodges et Spear, 1982;Hoinkes, 1986) ' chlorite et grenat (Ghent et al , 1987) et muscovite et grenat (Hynes et Forest, 1988) . Le calibrage utilise dans ce dernier thermometre correspond it I'equation 9 de Hynes et Forest (1988), en raison de sa dependance pratiquement nulle de la pression .…”

“…De ce fait, les resultats privilegies sont ceux obtenus par les calibrages grenat-biotite (Hoinkes, 1986), grenat-chlorite (Ghent et al , 1987) et grenat-muscovite (Hynes et Forest 1988) , qui prennent en compte I ' activite de Mn dans le grenat. Quant it la pression, elle a ete evaluee par le barometre 9 re nat-ruti le-i Ime nite-plag i ocl ase-q u a rtz ('GRIPS ' ; Bohlen et Liotta, 1986).…”

“…Dans la zone de la staurotide, la temperature a ete estimee par les thermometres grenat-biotite (Ferry et Spear, 1978;Hodges et Spear, 1982) et grenat-muscovite (Hynes et Forest, 1988;Figs 9 et 10). Le calibrage utilise dans ce dernier thermometre correspond toujours it I ' equation 9.…”