THE EXTENT OF THE SAN LORENZO BATHOLITH: AN ACCESSORY MINERAL PERSPECTIVE

Puerto Rico experienced plutonic activity between 85 Ma and 38 Ma (Pérez, 2008). The island can be divided in three main igneous provinces: the southwest (SIP), central (CIP), and northeast (NIP) igneous provinces (Schellekens, 1998) each divided by major faults. This research focuses on the aerial extent (>500 km²) of the late Campanian-Maastrichtian San Lorenzo Batholith (SLB), located in the CIP. A comparative study of the accessory minerals titanite, apatite and zircon from the Caguas and Vieques plutons was made to test the hypothesis that these rocks share a common petrogenesis with the SLB. The techniques used in this study include SEM characterization, analysis of accessory minerals in transmitted and reflected light microscopy, UV imaging, petrographic analysis, as well as synthesis of data from previous studies (U-Pb, δ¹⁸O, WR). Petrographic analyses show similar mineral assemblages of qtz+ plag+ hbl +bt+ Kfs; both samples are granodiorite. SEM and transmitted light microscopy characterization of accessory minerals show similar morphologies and inclusion characteristics. The U/Pb age of the Caguas stock (67.1±1.2 Ma) is identical to the Vieques stock (66.8±1.6 Ma); this age is similar to granodiorite and diorite from the interior of the SLB that yield ages of 75.1 and 74.1 Ma (Cavosie, unpublished). The δ¹⁸O values of zircons are 6.32±0.04 ‰ (Caguas), 5.77±0.04 ‰ (Vieques), 5.51±0.10‰ and 5.63±0.01‰ (SLB granodiorite and diorite) (Pérez, 2008). In general, δ¹⁸O (Zrc) values in the range of 5.3±0.6‰ (VSMOW) represent primitive mantle magma, while elevated values indicate a crustal influence (Valley et al., 2005). The elevated δ¹⁸O (Zrc) values for the Caguas granodiorite suggests that crustal material was incorporated into melts on the west side of the batholith, while the Vieques pluton and the interior granodiorite preserve primitive mantle oxygen values. One interpretation of the data from this study is that the Caguas and Vieques stocks are coeval, end-stage magmas that represent terminal melts along the margins of the SLB. Based on the four available U-Pb ages, the SLB accumulated over an 8 Myr span from 75 to 67 Ma.