ZR SATURATION THERMOMETRY FOR PUERTO RICO GRANITOIDS

The volcanic island arc of Puerto Rico (PR) had intrusive igneous activity from 85 Ma to 38 Ma. It has been divided into the Northwest Igneous Province (NIP), the Central Igneous Province (CIP), and the Southeast Igneous Province (SIP) by their stratigraphy, lithology, geochemistry, petrology and tectonism. The general geology of PR consists of volcanic, volcaniclastic, sedimentary and plutonic rocks. This study compares the Zr saturation temperatures for coarse-grained, zircon-bearing plutonic rocks from PR obtained using the zircon solubility model of Watson and Harrison (1983), and also evaluates the applicability of the solubility model for the granitoid suite of PR.

The solubility model is: T = 12,900/[lnD_Zr^zircon/melt +3.80 +0.85(M-1)]

where D is the concentration ratio of Zr in zircon (497,646 ppm) to whole rock (WR) Zr. M is the molar cation ratio M=(Na+K+2Ca)/(Al*Si), experimentally calibrated for values of M = 0.9-1.7.

XRF geochemical data from 15 plutonic rock samples from the three igneous provinces were used (Pérez, 2008). The range of calculated Zr saturation temperatures for the suite is 593 °C to 760 °C over a range of SiO₂ from 48.84 to 64.73 wt%. The low end of this temperature range is unrealistic for granitoid solidus temperatures. Plotting M vs Zr saturation temperatures shows that M values are inversely proportional to temperature; lower values of M yield higher temperatures. Only two samples in the PR granitoid suite (Ciales and Cuyon stocks) yield M values within the calibration (M = 1.53 to 1.70, T= 742-760°C), and two additional samples (Utuado and Tea dikes) are near the calibration (M= 1.72-1.75, T = 719 to 722°C); thus the temperature range of 719-760°C may approximate true crystallization temperatures for these samples. The remaining samples from the plutonic zircon-bearing suite are outside the calibration (M= 1.81 to 3.07) and yield temperatures representing the entire range from 593 to 760°C that may not be meaningful. Petrographic analysis and BSE imaging of zircon morphologies is consistent with zircon-melt equilibrium for many of the plutonic samples, however the model assumption of zircon-WR melt equilibrium is not met for the more mafic bodies where zircon crystallization may have occurred in isolated fractionated melts.