A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

TLDR

The garnet–hornblende Fe–Mg exchange geothermometer was calibrated against the garnet–clinopyroxene thermometer of Ellis & Green and applied to the high‑pressure greenschist‑to‑amphibolite facies Pelona Schist of Southern California. Using a robust regression of the Fe–Mg distribution coefficient, the authors derived a thermometer valid below ~850 °C for Mn‑poor garnet and common hornblende in low‑oxygen metamorphic rocks. The thermometer produced 520–610 °C for Dalradian amphibolites and 480–650 °C for the Pelona Schist, agreeing with clinopyroxene estimates, suggesting minimal systematic errors, and revealing that eclogitic amphibolites rarely equilibrate while inverted gradients of 170–250 °C/km were inferred beneath the Vincent Thrust.

Abstract

Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In K D =Δ (X Ca,g ) where K D is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn‐poor garnet and common hornblende of widely varying chemistry metamorphosed at low a O2 . Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo‐feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.