TIMING OF CRYSTALLIZATION AND EMPLACEMENT OF THE TWO-PYROXENE BASALT IN SOUTHWESTERN PUERTO RICO FROM CROSS-CUTTING RELATIONSHIPS AND AR40/AR39 DATING

The Two-Pyroxene Basalt (TPB), also known as the Río Loco Formation, is a porphyritic basaltic-andesitic rock exposed near Mesozoic serpentinite in southwestern Puerto Rico. Studies have suggested a wide range of ages (early Campanian and Late Cretaceous-Paleogene) of crystallization and emplacement of this rock. In addition, none of the previous studies used radiometric dating to determine the age of the TPB, relying instead upon field relationships and geochemical comparisons with other units. In our study, we conducted a field and isotopic study of the TPB to constrain its age of crystallization and mode of emplacement. Field observations indicate that the TPB intrudes both the Late Cretaceous rocks and a thrust fault between the Monte del Estado serpentinite and the Yauco and Sabana Grande formations. Two samples of the TPB were collected at this thrust fault for Ar⁴⁰/Ar³⁹ analysis. Results indicate crystallization ages for plagioclase in the matrix ranging from 44-53 Ma in one sample and 48 Ma for the plagioclase in the other sample. This indicates that the TPB crystallized within the middle- and early-Eocene, which is much younger than previously thought. In addition, the cooling and emplacement of the TPB constrains the age of serpentinite thrusting from late Maastrichtian to middle-Eocene, prior to the collision of the Caribbean Plate with the North American Plate in the late-Eocene. This result supports the hypothesis that an earlier collision at the Caribbean-North American Plate Boundary occurred in the early Paleogene. Moreover, it suggests that geochemical evolution models that used the TPB must be revised in light of the younger ages indicated in this study.