COOLING HISTORY OF THE CORONEL JOÃO SÁ PLUTON, BAHIA, BRAZIL

Rb-Sr mineral isochrons from plutonic rocks commonly exhibit poor alignment of data points (large MSWD). Initially, all of the minerals in a deeply buried pluton are above their respective closure temperatures for retention of radiogenic 87Sr, and diffusion continually homogenizes the 87Sr/86Sr amongst minerals in grain-to-grain contact. As cooling proceeds, diffusion eventually ceases in the mineral species with the highest closure temperature, while continuing rapidly throughout the “residual whole rock”—the other minerals with lower closure temperatures. Closures are not simultaneous, but rather, the constituent minerals become closed systems at different times and initial 87Sr/86Sr, as described by Giletti (1991).

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.