The Mavuradonha Layered Complex: Neoproterozoic ... - ArchiMeD

3 Analytical methods 34
wavelength dispersive mode with a focused beam of about 2 µm. Matrix corrections were
made with the EPMA internal φρz correction program.
Structural formulae and endmembers of minerals were calculated following the
procedure described in Deer et al. (1986) for plagioclase, in Morimoto et al. (1988) for
pyroxene, in Spear (1993) for garnet, and in Leake et al. (1997) for amphibole. Structural
formulae of scapolites are calculated following the recommendations of Shaw (1960) and
Teertstra & Sherriff (1997), whereas endmembers are defined as suggested by
Shaw (1960) and Ellis (1978). Fe 3+ corrections in pyroxene, amphibole and garnet
formulae followed the procedure of Droop (1987). Mineral abbreviations follow the
suggestions of Kretz (1983).
Thermobarometry
Temperature calculations were performed using the garnet-pyroxene
geothermometer of Ellis & Green (1979) and Krogh (1988) and the garnet-hornblende
geothermometer of Graham & Powell (1984). Pressures were calculated applying the
geobarometers of Newton & Perkins (1982), Powell & Holland (1985; 1988), Holland &
Powell (1985) and Eckert et al. (1991) and using the mineral assemblage garnetclinopyroxene-plagioclase-quartz.
The geobarometer of Kohn and Spear (1989) was used
for pressure calculations of the assemblage garnet-hornblende-plagioclase-quartz.
Total iron was assumed to be ferrous iron for PT-estimates following the
suggestions of Schuhmacher (1991) and Canil & O’Neill (1996). This is because of the
still unsolved problem of ferric iron estimation in many ferromagnesian minerals as
mentioned by several authors (e.g., Bucher & Frey, 1994; Krogh Ravna, 2000;
Schumacher, 1991). Comparative studies (Canil & O’Neill, 1996; McCammon et al., 1998)
between Mössbauer spectrometry and the method of Droop (1987) have shown that ferric
iron calculations for pyroxenes which are based on stoichiometry lead to imprecise ferric
iron estimations caused by higher SiO2 levels and lower total iron contents compared with
spinel or garnet. Ferric iron estimates based on the method described by Droop (1987) may
lead to erroneous results in thermobarometry, due to the extreme sensitivity to microprobe
errors. Most of the geothermobarometers used in this study are calibrated using
experimental materials and/or natural assemblages with poorly constrained amounts of
ferric iron.
To obtain information about the PT evolution, quantitative PT-conditions have been
evaluated applying the TWEEQ/TWQ (Thermobarometry With Estimation of
EQUilibration state) computer application of Berman (1991) versions 1.02b (for garnetamphibole-plagioclase
calculations) and 2.02 (garnet-clinopyroxene-plagioclase). This
enables the combination of the PT estimates with mineral reactions deduced from