Multi-stage Assembly of the Utuado Pluton, Puerto Rico: Evidence from Enclaves and Amphibole Geochemistry

The studied localities within the Utuado Pluton in the Puerto Rico fossil island arc preserve a detailed record of enclave-host interaction and mid-crustal processes consistent with multi-stage assembly. Petrography, whole-rock geochemistry, amphibole major- and trace-elements, and thermobarometry reveal repeated rejuvenation of a vertically connected reservoir. Two enclave types are identified: intergranular Type 1 formed from hotter intermediate injections, and poikilitic Type 2 recording slower cooling and partial equilibration with the host. Amphiboles display well-developed patchy zoning defined by discrete compositional steps. When grouped by intra-crystal zones, amphiboles define systematic P-T fields: Zone 1 records the highest-T, deepest conditions (∼800-940 °C; 110-415 MPa), Zone 2 captures intermediate hybridized conditions (∼800-850 °C; 120-220 MPa), and Zone 3 reflects the shallowest, coolest crystallization (∼730-820 °C; 55–130 MPa) indicating episodic recharge rather than monotonic cooling. Calculated amphibole-equilibrium melts form a continuous array (∼60-80 wt.% SiO 2 ; 0.8-9 wt.% CaO) with zoning domains tracking melt evolution from hotter to more silicic compositions. Coupled whole-rock Eu/Eu*, Sr/Y, and Y variations indicate limited (∼5-10 wt.%) melt extraction from amphibole-plagioclase mushes. These results provide a framework where the Utuado Pluton was built through cyclic replenishment, incomplete mixing, mingling, and localized melt extraction within a vertically integrated arc reservoir.