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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