Thermobarometry of the Furua granulites, Tanzania: a comparative study
published: Apr 15, 2004
ArtNo. ESP154018001004, Price: 29.00 €
Mineral thermometers and barometers have been applied to the various assemblages of the Furua granulite complex, SW Tanzania, using individual temperature and pressure calibrations, most of which being collected in the TPF program of Fonarev et al. (1991), and the Thermocalc (Powell & Holland 2001) and TWQ (Berman 1991) multi-reaction equilibria programs. The primary mineral assemblages comprise garnet, ortho- and clinopyroxene, amphibole, biotite, plagioclase and quartz. With the availability of many calibrations between these minerals, the possibilities of attaining consensus peak-metamorphic temperatures and pressures have been investigated. Thermocalc temperatures and pressures correspond best with an assumed a-H2O of 0.10. The TWQ and Thermocalc programs only give comparable temperature and pressure estimates when anhydrous equilibria are taken into consideration. Several individual thermometers and barometers prove to be just as useful as results from the multireaction Thermocalc and TWQ programs. CO2 activities were estimated with the method advocated by Newton (1995) for assemblages carrying scapolite, plagioclase and garnet. Two-pyroxene-garnet granulites have high mean CO2 activities of 0.8, clinopyroxene-garnet granulites and amphibolites have varying CO2 activities and calcsilicate granulites generally low ones. With additional analytical data, the original peak P-T box for the central zone of two-pyroxene-garnet granulites defined by Coolen (1980) may be constrained to 800-850 C and 10-11kbar, with average values of 825 C and 10.5 kbar. Evidence for a temperature gradient in the central zone is absent, whereas a pressure gradient from (south)west to (north) east, from 11kbar to 9kbar, is obvious. Many modern geothermobarometric tools, including TWQ and Thermocalc, do not guarantee, even with a comprehensive and reliable data set, an unambiguous estimation of peak metamorphic temperature and pressure conditions. Moreover, the latest calibrations are not necessarily always improvements, at least not for these Furua granulites; particularly, the calibration of hornblende-bearing equilibria needs further investigation.