Los óxidos de Fe-Ti de las rocas calco-alcalinas del sureste de España

Abstract

The Fe-Ti oxides appearing in the calc-alkaline rocks from SE Spain are titanomagnetite and ilmenite, although there are also sorne crystals of maghemite resulting from the oxidation of the rnagnetite. Titanomagnetite is to be found in all petrologic types, but the second is absent from the basaltic andesites and even from the less siliceous andesites. The titanomagnetites present a low proportion of molecule ulvospinel (10.6-28.9%), because of the low content in TiO₂ of these lavas . The percentage of R.O. in the ilmenites analysed is relatively small (19.7-26.5%). However, both oxides are progressively richer in the above-mentioned molecules as they pass from andesites to dacites . From the chemical composition of both coexisting oxide phases it has been estimated, using the curves of Buddington and Lidsley (1964), that the andesites crystallized at temperatures between 880°C and 855°C and under oxygen fugacities of around 10^-11.2 and 10^-11.4 atm., while the temperatures in the case of the dacites were between 880°C and 8l0°C, with oxygen fugacities of 10^-11.4 and 10^-12.3 atm. The fO₂-T curve of these rocks is therefore situated above that of nickel-nickel oxide (NNO) buffer. These crystallization temperatures are somewhat lower than those determined by using the pair orthopyroxene-c1inopyroxene (Wood and Banno, 1973 and Wells, 1977). However, since it is widely considered that the titanomagnetite-ilmenite geothermometer is the most accurate, these temperatures are considered more correct than those determined from the composition of pyroxenes .