Multi-stage assembly of the Utuado Pluton, Puerto Rico: evidence from enclaves and amphibole geochemistry
Journal Article
Overview
abstract
; 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 that records 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 pressure–temperature (; P; –; T; ) fields: Zone 1 records the deepest, highest temperature conditions (110–415 MPa;; c; . 800–940°C); Zone 2 captures intermediate hybridized conditions (120–220 MPa;; c; . 800–850°C); and Zone 3 reflects the shallowest, coolest crystallization (55–130 MPa;; c; . 730–820°C) indicating episodic recharge rather than monotonic cooling. Calculated amphibole-equilibrium melts form a continuous array (; c; . 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 (; c; . 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.;
