Oxygen isotope regional pattern in granitoids from the Cachoeirinha Belt, northeast Brazil

Four groups of granitoids are present within the Cachoeirinha belt and in the adjacent migmatitic basement, between 370 and 400 W long. and 70 and 8015' S lat., States of Pernambuco and Paraiba: a) K2O - enriched, very porphyritic; b) a calc-alkalic slightly porphyritic group; c) group with trondjemitic affinities; and d) peralkalic group. Petrology and oxygen isotope geochemistry for over 100 samples from these groups were studied. Almost all plutons for which 5 or more samples were analyzed, exhibit a total range of gamma 18O less than 2% o. A broad range of mean oxygen isotope composition is observed, varying from 6.93 to 12.79% o. There is a systematic regional trend in which the calc-alkalic granitoids (conceicao-type) found within the Cachoeirinha space are the most 18O - enriched rocks (10.6 to 12.9% o) while the lowest mean gamma 18O values (4.5 to 9.7% o) are found in the K2O - enriched granitoids (Itaporanga-type). Intermediate gamma 18O values were recorded in the bodies with trondhjemitic affinities (8.9 to 9.8% o) which intruded metasediments of the Salgueiro Group and in the peralkalic granitoids of Catingueira (8.1 to 9.8% o) which intruded Cachoeirinha metamorphics. Among the potassic granitoids, mean gamma 18O increases from Bodoco to Itaporanga (from west to east). As a whole, the W.R. gamma 18O of these plutons correlate with the type of grade of metamorphism of the host rocks and, therefore, with the tectonic framework, increasing from those which intruded the gneiss-migmatites to those which intruded the low-grade metamorphics of the Cachoeirinha Group. The possible origin of each rock group is discussed in light of the oxygen isotope geochemistry. (Author)