A garnet-hornblende geothermometer; calibration, testing, and application to the Pelona Schist, Southern California

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Authors:Graham, Colin M.; Powell, Roger
Author Affiliations:Primary:
Edinburgh Univ., Dep. Geol., Edinburgh, United Kingdom
Other:
The Univ. at Leeds, Dep. Earth Sci., United Kingdom
Volume Title:Journal of Metamorphic Geology
Source:Journal of Metamorphic Geology, 2(1), p.13-31. Publisher: Blackwell, Oxford, United Kingdom. ISSN: 0263-4929
Publication Date:1984
Note:In English. 76 refs.; illus. incl. 4 tables, sect., geol. sketch maps
Summary:A garnet-hornblende Fe-Mg exchange geothermometer has been calibrated against the garnet-clinopyroxene geothermometer of Ellis & Green (1979). Data for the Fe-Mg exchange reaction between garnet and hornblende have been fitted to the equation. In KD = ΔS°/R-ΔH°/RT-PΔV°/RT-W/RT (XCa,g) where KD is the Fe-Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: T(°K) = 2880 + 3280XCa,g/In KD + 2.426 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 a02. 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. 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 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 8-9 kbar 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. Modified journal abstract. [R.A.H.]
Subjects:Alkaline earth metals; Amphibole group; Amphibolite facies; Chain silicates; Clinoamphibole; Clinopyroxene; Eclogite; Facies; Faults; Garnet group; Geochemistry; Geologic thermometry; Granulite facies; Hornblende; Iron; Magnesium; Metals; Metamorphic rocks; Metamorphism; Nesosilicates; Orthosilicates; P-T conditions; Pelona Schist; Petrology; Phase equilibria; Precambrian; Pyroxene group; Schists; Silicates; Thrust faults; California; Southern California; United States; Sierra Pelona; Vincent Thrust
Coordinates:N343700 N344500 W1181000 W1181700
Abstract Numbers:84M/4520
Record ID:1984055659
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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034 0 |a a  |d W1181700  |e W1181000  |f N344500  |g N343700 
040 |a ViAlAGI  |c ViAlAGI 
072 7 |a 05A  |2 georeft 
100 1 |a Graham, Colin M.  |e analytic author  |u Edinburgh Univ., Dep. Geol., Edinburgh 
245 1 2 |a A garnet-hornblende geothermometer; calibration, testing, and application to the Pelona Schist, Southern California 
300 |a p. 13-31 
500 |a In English. 76 refs. 
500 |a Abstract number: 84M/4520 
500 |a Abstractor: R.A.H. 
500 |a Affiliation: Edinburgh Univ., Dep. Geol.; Edinburgh; GBR; United Kingdom 
500 |a Affiliation: The Univ. at Leeds, Dep. Earth Sci.; ; GBR; United Kingdom 
500 |a Key title: Journal of Metamorphic Geology 
500 |a Source note: Journal of Metamorphic Geology, 2(1), p.13-31. Publisher: Blackwell, Oxford, United Kingdom. ISSN: 0263-4929 
500 |a Publication type: journal article 
504 |b 76 refs. 
510 3 |a GeoRef, Copyright 2019 American Geosciences Institute. 
520 |a A garnet-hornblende Fe-Mg exchange geothermometer has been calibrated against the garnet-clinopyroxene geothermometer of Ellis & Green (1979). Data for the Fe-Mg exchange reaction between garnet and hornblende have been fitted to the equation. In K<D` = ΔS°/R-ΔH°/RT-PΔV°/RT-W/RT (X<Ca,g`) where K<D` is the Fe-Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: T(°K) = 2880 + 3280X<Ca,g`/In K<D` + 2.426 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<02`. 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. 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 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 8-9 kbar 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.—Modified journal abstract. 
650 7 |a Alkaline earth metals  |2 georeft 
650 7 |a Amphibole group  |2 georeft 
650 7 |a Amphibolite facies  |2 georeft 
650 7 |a Chain silicates  |2 georeft 
650 7 |a Clinoamphibole  |2 georeft 
650 7 |a Clinopyroxene  |2 georeft 
650 7 |a Eclogite  |2 georeft 
650 7 |a Facies  |2 georeft 
650 7 |a Faults  |2 georeft 
650 7 |a Garnet group  |2 georeft 
650 7 |a Geochemistry  |2 georeft 
650 7 |a Geologic thermometry  |2 georeft 
650 7 |a Granulite facies  |2 georeft 
650 7 |a Hornblende  |2 georeft 
650 7 |a Iron  |2 georeft 
650 7 |a Magnesium  |2 georeft 
650 7 |a Metals  |2 georeft 
650 7 |a Metamorphic rocks  |2 georeft 
650 7 |a Metamorphism  |2 georeft 
650 7 |a Nesosilicates  |2 georeft 
650 7 |a Orthosilicates  |2 georeft 
650 7 |a P-T conditions  |2 georeft 
650 7 |a Pelona Schist  |2 georeft 
650 7 |a Petrology  |2 georeft 
650 7 |a Phase equilibria  |2 georeft 
650 7 |a Precambrian  |2 georeft 
650 7 |a Pyroxene group  |2 georeft 
650 7 |a Schists  |2 georeft 
650 7 |a Silicates  |2 georeft 
650 7 |a Thrust faults  |2 georeft 
651 7 |a California  |2 georeft 
651 7 |a Southern California  |2 georeft 
651 7 |a United States  |2 georeft 
653 |a Sierra Pelona 
653 |a Vincent Thrust 
700 1 |a Powell, Roger,  |e analytic author  |u The Univ. at Leeds, Dep. Earth Sci. 
773 0 |t Journal of Metamorphic Geology  |d Oxford : Blackwell, Mar. 1984  |x 0263-4929  |n Journal of Metamorphic Geology, 2(1), p.13-31. Publisher: Blackwell, Oxford, United Kingdom. ISSN: 0263-4929 Publication type: journal article  |g Vol. 2, no. 1  |h illus. incl. 4 tables, sect., geol. sketch maps