TITANITE CRYSTAL CHEMISTRY AND U-P ISOTOPIC DATA: A PETROGENETIC INDICATOR FOR PRECAMBRIAN GRANITOID PLUTONS OF THE EASTERN BRAZILIAN SHIELD

Résumé

This paper presents crystal chemistry and U-Pb isotopic data for titanite (or sphene, CaTiSiO5) fromPrecambrian granitoid plutons of distinct tectonic settings within the Eastern Brazilian shield. Despite thefact that most titanite ages are related to partial lead-loss, resetting or crystallization during metamorphicevents, chemistry data reflect the magmatic history of the plutons, specially the Ti4+/Al+Fe3+ ratio. It isdemonstrated that for plutons of different crystallization ages (Archean, Paleoproterozoic and Neoproterozoic)the measured Ti4+/Al+Fe3+ ratios of titanite grains are characteristic of the source region from which themagma was extracted, and the ratios remain unchanged during subsequent metamorphic events. HigherTi4+/Al+Fe3+ values (> 9) are found in predominantly mantle-derived granitoid plutons whilst crust-derivedgranitoid rocks have lower ratios. Therefore, titanite crystal chemistry may be used to discriminate magmasources of granitoid rocks regardless their age and metamorphic history.