Oxygen isotope geochemistry of plutonic rocks from Puerto Rico

Puerto Rico is a complex island arc terrane with plutonic intrusive activity from 85 million years (Ma) to 38 Ma (Cavosie et al., 2008). Schellekens (1991) divided the island into three igneous provinces named the southwest, central and northeast igneous provinces based on stratigraphy, lithology, petrology and geochemistry. Plutonic intrusions are heterogeneously distributed among the three igneous provinces; exposed areas vary from ∼1 to 500 km 2 (stock to batholith scale). The origin of these rocks has been studied by several authors but is still speculative since the parental magmas for the plutons are not known. Petrographic analyses, whole rock (WR) major, minor and trace elements, and oxygen isotope ratios (delta18O) of zircon (Zrc), quartz (Qtz) and titanite (Tnt) were used to constrain the origin of the granitoids. The data were combined with U/Pb zircon dating to place time constraints on island arc evolution. (delta18O) in igneous rocks record magmatic compositions if primary, and are used in this study to evaluate alteration or incorporation of sediments into Puerto Rico magmas, as proposed by Schellekens (1993). These processes will increase delta 18O to values higher or lower than the mantle (delta18O=5.3+/-0.6‰; 2sd). Analyses for delta18O were performed with laser fluorination by Isotope Ratio Mass Spectrometry (IRMS) at the University of Wisconsin, Madison. Thin sections were used to evaluate whether or not delta 18O values are primary (magmatic) or represent alteration. Plutonic rocks in Puerto Rico show a remarkable limited range in WR major and minor elements, suggesting a common origin. The distribution of plutonic intrusions among the three igneous provinces, delta18O(Zrc) values, and age of crystallization shows a continuous and complex magmatic history during the evolution of the Cretaceous island arc. Plutons with the same age of crystallization and in close proximity to each other record isotopically bimodal ratios over the entire magmatic evolution of the arc beginning in the Santonian [Coamo Arriba stock (85.6+/-1.3 Ma, delta18 O(Zrc)= 5.34‰) and Morovis stock (85.3+/-1.8 Ma, delta 18O(Zrc)= 6.69‰)] continuing into the Campanian/Maastrichtian [San Lorenzo granodiorite (75.1+/-2.1 Ma, delta18O(Zrc)= 5.63‰), San Lorenzo diorite (74.1+/-1.4 Ma, delta18O(Zrc)= 5.51‰), Utuado pluton (70.8+/-1.2 Ma, delta18O(Zrc)= 5.71‰), Caguas pluton (66.8+/-1.2 Ma, delta18O(Zrc)= 6.32‰) and Vieques pluton (67.1+/-1.6 Ma, delta18O(Zrc)= 5.77‰)] and during the Eocene [Cuyon stock (47.6 +/-0.8 Ma, delta 18O(Zrc)= 5.33‰) and Barranquitas stock (47.7+/-1.6 Ma, delta 18O(Zrc)= 6.75‰)]. Bimodal oxygen isotope ratios during the island arc evolution (85-38 Ma) indicates the incorporation of crustal material into the magmas.;The results of this study suggest that any regional tectonic changes that resulted in changes in magmatic processes in the island arc occurred prior to ∼85 Ma, the age of emplacement of the oldest plutons in Puerto Rico. Smith et al. (1998) suggest that the emplacement of the batholiths, stocks and volcanism in Puerto Rico was related to subduction and magma generation in the northeastern Caribbean due to tectonic changes. This conclusion contrasts with the findings of this work, where the generation of bimodal plutons from ∼85 to 48 Ma is consistent along the evolution of the arc. The generation of batholiths in Puerto Rico is therefore not interpreted to represent markers of changes in regional tectonism. Any changes in magmatism as a consequence of regional tectonism are here constrained to have occurred prior to 85 Ma.