A SISTEMÁTICA Rb/Sr EM ROCHAS GRANITÓIDES: CONSIDERAÇÕES INTERPRETATIVAS, LIMITAÇÕES E EXEMPLOS BRASILEIROS

Resumo

A literature revíew about the Rb/Sr systematícs in undertaken, as applied to granitoid rocks, poiting out potentials and interpretatíon limitations. Ages derived from Rb/Sr isochrons pinpoint either crystallization or post-crystallízatíon homogenization events. Isochroníc Sr/⁸⁶Sr inítial ratios are a clue to the Sr evolution stage at the site of magma generation, and can evaluate the relative crustal-mantle contributions. Cogenetic granítoids, generated from isotopically homogeneous sources, show identical initial ratíos: ísochroníc ages should pinpoint the true emplacement age, since magma cooling is generally thought to be a fast geologíc process. Neverthless, supposed cogenetic granitoid often present real variations in initial ratios, a behavior which prcbably can be explained by several petrogenetíc processes (e.g., mixing of magmas, crystal separation from magma mushes during protracted crystallization, assimilation of country rocks). Some of the cited mechanisms can be simulated using several Rb/Sr variation díagrams. Provided data dispersion is not linked to systematic errors of various kinds, it is clear thar "anomalous" Rb/Sr data reflect the influence of the cited genetic mechanísms, and may thus become valuable tool for petrogenetic interpretations. Rb/Sr data of selected Brazilian granitoid massifs show significant spread; a brief discussion of these and of the corresponding geological settíng indicate that many discrepancies arise from analytical and/or interpretation errors, although ít is clear that several ones are actually due to the influence of complícatíons in the petrogenetic process, such as pointed out above.