ZIRCON δ18O EVIDENCE FOR CRUSTAL RECYCLING IN ISLAND ARCS: THE CIALES AND MOROVIS STOCKS OF PUERTO RICO

The island of Puerto Rico is an island arc that records igneous growth primarily through extraction of juvenile melts from the mantle, which experienced fractionation and partial melting in the arc crust. Most plutonic rocks are intermediate in composition, and yield primitive mantle-equilibrated oxygen isotope ratios for zircon that fall in the range of δ¹⁸O(Zrc) = 5.3±0.6‰ (Pérez, 2008). However, several stocks were identified previously that yield δ¹⁸O(Zrc) > 6.5‰, values that require crustal contributions to the melt source (Valley et al., 2005). In this study we investigated grain aspects such as morphology, internal structure, and cathodoluminescence (CL) zoning patterns for high δ¹⁸O zircons from the Ciales and Morovis stocks of north-central Puerto Rico. These new observations, together with published data (δ¹⁸O, whole rock compositions and petrology, field relations) allow us to evaluate the evidence for crustal recycling in the Puerto Rican arc.

Ciales and Morovis are small (2 to 20 km²) granodiorite stocks located 1 km apart. Both contain zircons that yield elevated δ¹⁸O values of 6.73±0.04 and 6.69±0.04‰, VSMOW, respectively (Pérez, 2008). Grain imaging with back scattered electrons (BSE) and transmitted light (TL) shows that zircons from the Ciales and Morovis stocks are mostly euhedral and do not show internal complexities. Likewise, CL imaging shows simple oscillatory zoning and sector zoning with no evidence of inherited cores. The BSE, TL, and CL results all indicate a simple crystallization history with no evidence of inheritance; these data are consistent with simple U-Pb spectra for these samples, which yield U-Pb ages of 83.9±1.7 Ma (Ciales) and 85.3±1.8 Ma (Morovis) (Cavosie, unpublished). The similarities in data between the Ciales and Morovis stocks indicate that they are two surface exposures of a single pluton, whose δ¹⁸O(Zrc) values, grain morphologies, and internal structure demonstrate evidence of crustal recycling during the formation of Puerto Rico.