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

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 . Los óxidos de Fe-Ti que aparecen en las rocas calco-alcalinas del sureste de Espa a son titanomagnetita e ilmenita, si bien también hay algunos cristales de maghemita, producto de la oxidación de la magnetita. La primera está presente en todos los tipos petrológicos existentes , pero la segunda está ausente en las andesitas basálticas e incluso en las andesitas menos silíceas. Las titanomagnetitas presentan una baja proporción de moléculas de ulvoespinela (10.6-28.9%), reflejo del bajo contenido en TiO2 de estas lavas. Asimismo, el porcentaje de R2O3 de las ilmenitas analizadas es relativamente reducido (19.7-26.5%). No obstante, tanto unas como otras muestran un progresivo enriquecimiento en dichas moléculas al pasar de las andesitas a las dacitas . A partir de la composición química de ambas fases mineralógicas, utilizando las curvas de Buddington y Lindsley (1964), se ha estimado que las andesitas han cristalizado a temperaturas comprendidas entre 880°C y 855°C y bajo fugacidades de oxígeno del orden de 10-