10-6 Multi-Level Construction and Magma Hybridization in the Utuado Pluton, Puerto Rico Fossil Island Arc

Session: How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution

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Understanding how island arc crust evolves requires insights into deep magmatic processes. Petrographic, bulk geochemical, and mineral chemistry data from the Late Cretaceous Utuado Pluton in the Puerto Rico fossil island arc reveal a complex interplay of magma mixing, enclave formation, and progressive differentiation across mid- to upper-crustal levels in this region. 

Previous work identified two enclave types: Type 1, porphyritic micro-gabbro with intergranular textures and abundant plagioclase; and Type 2, poikilitic micro-granitoid with oikocrystic K-feldspar enclosing fine-grained mafic chadacrysts. Amphibole thermobarometry constrained crystallization depths to ~2–16 km. Amphibole zoning yielded melt compositions that group into intermediate dioritic (58–69 wt.% SiO₂), granodioritic (70–76 wt.% SiO₂), and highly evolved granitic melts (77–84 wt.% SiO₂), consistent with progressive differentiation and multi-level pluton construction. 

New whole-rock trace element data reveal differences in Sr/Y and La/Nd ratios between enclave and host granodiorite. Enclaves show lower Sr/Y ratios (15–35) and slightly lower La/Nd ratios (0.9–1.1) compared to host granodiorite (Sr/Y 45–60; La/Nd 1.1–1.3) which is consistent with greater plagioclase fractionation in the enclaves and evolving source characteristics. Amphibole trace element data supports this interpretation: Type 1 enclave amphiboles have lower Zr (10–40 ppm) and Ti (2200–4800 ppm) concentrations than both Type 2 and host amphiboles (Zr: 40–90 ppm; Ti: 8200–12,500 ppm). These trends are consistent with crystallization from more primitive melts in the enclaves and more evolved magmas in the host. A transitional group (Zr: 20–50 ppm; Ti: 4500–7500 ppm) likely reflects hybridization between mafic and felsic magmas during pluton assembly. 

These data suggest the Utuado Pluton formed via the interaction of at least three compositionally distinct magmas emplaced at different crustal levels: a primitive mafic magma (recorded in Type 1 enclaves), a more evolved felsic magma (host granodiorite and Type 2 enclaves), and a hybrid magma identified through transitional amphibole compositions and melt chemistries. Magma recharge, hybridization, and fractional crystallization likely played key roles in the Utuado Pluton assembly. Variations in source characteristics may also reflect differences in crustal thickness during magma generation and ascent. These findings support incremental construction of island arc crust via episodic magmatic input and emphasize the important role of hybridization. Ongoing work will incorporate geochronology to further constrain plutonic construction timescales. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7144

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