NEW TOOLS FOR OLD ARCS: ACCESSORY MINERAL STUDIES OF PUERTO RICAN GRANITOIDS

Islands in the NE Greater Antilles preserve a complex record of island arc construction and evolution from the early Cretaceous to the Oligocene. Puerto Rico (PR) and neighboring islands expose numerous arc granitoids ranging from small stocks (<1 km²) to large batholiths (>500 km²) that are dominated by granodiorite, with lesser occurrences of felsic and mafic plutonic bodies. Available data on PR granitoids consists primarily of field relations and WR geochemical data sets (XRF) including major, minor, and trace elements used for petrogenetic models (Smith et al., 1998). Recent studies have focused on understanding of the origin of PR granitoids through application of accessory mineral geochemical studies on zircon-bearing granitoids from PR ranging from 48 to 64 wt.% SiO₂. Zircon U-Pb studies using LA-ICP-MS have shown that exposed plutonic rocks in Puerto Rico were emplaced from 85 to 38 Ma; no early Cretaceous plutonic record has been identified (Cavosie, unpublished). The largest bodies were emplaced over a narrow interval, including the Utuado pluton at 70.8±1.2 Ma, and the San Lorenzo batholith from 75-68 Ma (Pérez, 2008 UPRM MS; Lebrón et al., this volume). Oxygen isotope studies of zircon, titanite, quartz, and WR by laser fluorination show that the PR plutonic suite is bimodal in δ¹⁸O. Most granitoids record mantle-equilibrated δ¹⁸O values, however several plutons preserve elevated zircon δ¹⁸O values (up to 6.3-7.2‰) indicative of crustal recycling (Pérez, 2008; Pincus et al., this volume). Zr saturation thermometry has been applied to the PR suite, and rocks with appropriate compositions yield temperatures up to 760 °C; most PR granitoids do not fall within the compositional range of the Watson and Harrison (1983) solubility model (Montalvo et al., this volume). In addition to high temperature processes, (U+Th)/He dating studies have been conducted on zircon and apatite to provide constraints on the final stages of exhumation of Puerto Rico (Román, 2013 UPRM MS). Zircons and apatites yield a range of He dates, including widespread Eocene (ca. 50 Ma) ages that record regional events, including the collision of the once separate southwestern and north-central arcs along the southern Puerto Rican fault zone, and the collision with the Bahama bank.