COOLING HISTORY OF THE CORONEL JOÃO SÁ PLUTON, BAHIA, BRAZIL
The unfoliated Coronel João Sá granodiorite pluton, Bahia, Brazil, is emplaced into host schist of the Sergipano Foldbelt. Its concordant U-Pb age of 625 ± 2 Ma on zircon dates the earliest (highest T) event during cooling, followed by a 621 Ma U-Pb age of titanite. We have modeled the following information: (i) the zircon and titanite U-Pb ages, (ii) Rb-Sr isotopic data for whole rock and for separated titanite, plagioclase, K-feldspar, apatite, and biotite, (iii) modal abundances, which provide relative sizes of the mineral Rb-Sr reservoirs, and (iv) the Dodson (1973) mathematical relationship of closure temperature to the cooling rate, diffusion coefficient, effective grain size, etc. The objective was to determine a beginning temperature and rate of cooling that provides optimum agreement between 87Sr/86Sr as measured today, and as calculated from the model, in the whole rock and each mineral phase.
Excellent agreement is achieved if the cooling rate=36°/myr, and initial temperature=840°C. The cooling rate is broadly consistent with the paleodepth of the pluton inferred from independent data. At 840°C, the granodiorite would have consisted of crystals + melt. However, the model does not require that the rock be totally solid in characterizing open or closed chemical systems.