Rb-Sr and Sm-Nd study of the São Rafael Batholith, Rio Grande do Norte, NE Brazil




Ketcham, Denise H.

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During the Brasiliano (Pan-African) orogeny from 700 to 550 Ma, the geologic province of Borborema in northeast Brazil was intruded by more than 80 granitoid bodies, one of them the primary epidote-bearing São Rafael batholith located in the Seridó foldbelt. This unfoliated granite pluton is thought to be of very late Brasiliano age, perhaps between 600 and 550 Ma, based on field relations. Twenty-four samples were analyzed isotopically, including 4 from a subtly different facies of the pluton (Serra-type), and 2 mafic enclaves. In general, the rocks have high Sr (655 ± 175 ppm) and low Rb (61 ± 9 ppm), with consequent low enrichment in radiogenic Sr. Three mineral-whole-rock isochrons provide cooling ages of 0.49 ± 0.01 Ga. Data of the 18 epidote-bearing facies whole rocks plot as an amorphous "blob" on a Rb-Sr isochron diagram. Inhomogeneous initial Sr ratios preclude the determination of a Rb-Sr age of crystallization. Somewhat more systematic data from the Serra samples suggest an age of about 0.6 Ga. Sr ratios of epidote-bearing facies rocks, projected back to 0.6 Ga, are high (0.713 ± 0.001), pointing to an ancient radiogenic crustal source. Six whole-rock samples provide a Sm-Nd isochron apparent age of ~1.1 Ga, which is inconsistent with field evidence for late Brasiliano emplacement. Anomalous Sm-Nd data indicate that initial Nd ratios were also inhomogeneous, just as Sr initial ratios were. Strongly negative values of εNd [subscript 0.6Ga] (-18 to -21) imply a source relatively enriched in Nd, a characteristic of continental crust. Depleted mantle model ages are consistent with minimum crust-formation ages of 2.7 ± 0.3 Ga. The εNd [subscript 0.6Ga] values, depleted mantle model ages and high initial Sr ratios of the pluton (at 0.6 Ga) all suggest that the São Rafael source rock was ancient crust. This conclusion is also consistent with the existence of Archean basement in northeastern Brazil. Preliminary U-Pb zircon geochronology of a single fraction yielded a highly discordant (44%) data point, with a ²⁰⁷Pb/²⁰⁶Pb age of 2.0 Ga. This result is interpreted as indicating that relict zircon cores are present, containing an inherited component whose age is older than 2.0 Ga. Formed at about 0.6 Ga, apparently by partial melting of Archean crust, the São Rafael magma mixed sufficiently to attain a distinctively uniform physical appearance and chemical-mineralogical composition. The magma, however, consisted of partial melts with different isotopic characters which failed to homogenize completely.