MULTI-STAGE MAGMATIC EVOLUTION OF THE UTUADO PLUTON, PUERTO RICO: EVIDENCE FROM AMPHIBOLE PETROGRAPHY AND THERMOBAROMETRY

Investigating the magmatic processes and conditions that drive pluton formation is crucial for understanding island arc evolution. This study evaluates the formation conditions of the Late Cretaceous Utuado pluton in the fossil island arc of Puerto Rico. Through major element analyses of amphiboles in enclaves and host granodiorite, we assess the P-T conditions and magma composition of the Utuado pluton.

Petrographic analyses categorize the enclaves into Type 1 (porphyritic intergranular micro-gabbro) and Type 2 (poikilitic micro-granitoid). Both enclave types and granodiorite are dominated by Mg-hornblendes with rare tshermakitic pargasite and Mg-hastingsite. Amphiboles in the enclaves show patchy zoning, primarily with diffuse but occasionally sharp boundaries. Three compositional zones were identified based on greyscale contrast in BSE imagery and X-ray maps: bright (SiO₂ 43 wt.%; FeO 16 wt.%), intermediate (SiO₂ 48 wt.%; FeO 14 wt.%), and dark (SiO₂ 50 wt.%; FeO 12 wt.%). Amphibole thermobarometry provides P-T estimates of 53-417 MPa ± 12% (D= 2-16km) and 732-962 ± 22 °C for the enclaves, and 50-284 MPa ± 12% (D= 2-11km) and 706-898 ± 22 °C for the granodiorite. These ranges are recorded in single crystals, with bright zones correlating to deeper, hotter conditions and dark zones indicating shallower, cooler conditions. Calculated melt compositions in equilibrium with amphibole suggest that bright zones crystallized from melts with 58-69 wt.% SiO₂, intermediate zones with 70-76 wt.%, and dark zones with 77-84 wt.%.

The diverse enclave types within the granodiorite at Utuado suggest that various magmas contributed to its formation. Evidence of this is recorded in the patchy zoning, which suggests re-equilibration via diffusion with chemically distinct magmas. Melt compositions, correlated with P-T estimates, point to the contribution of three melts: 1) mid-crust intermediate melt (<16 km), possibly the parent magma to Type 1 enclave; 2) upper-crust felsic melt (<11 km), possibly the parent magma to Type 2 enclave; and 3) shallow-crust highly felsic melt (~2 km) that likely mixed with other felsic melts to form the granodiorite and various felsic units in the Utuado pluton. These findings reveal a complex magmatic history involving mixing, hybridization, and multi-stage cooling of intermediate and felsic magmas across mid to upper crustal depths in Puerto Rico.