Circum-Caribbean Granitoids: Characteristics and Origin

Abstract

Caribbean granitoids occur among a series of widespread magmatic arcs that developed during a period of major oceanic plate convergence and subduction that began in Late Cretaceous time. Evaluation of Caribbean granitoids reveals that two main suites of granitoids are widespread. Low-K granitoids, including gabbro, diorite, quartz diorite, tonalite, and trondhjemite, comprise one of these suites. The second main type is distinctly more potassic and consists primarily of quartz monzodiorite and granodiorite, but also includes monzodiorite, quartz monzonite, and granite. Both groups contain rocks that are transitional to the other group. The granitoids are part of an extensive Caribbean calc-alkaline assemblage that includes low-K, medium-K, and high-K rocks. Island-arc tholeiitic and normal-K calc-alkalic compositions reflect geochemical continua within the orogenic granitoids. The granitoids (including low-K rocks) lie within the calc-alkaline field on FeO^T/MgO and AFM diagrams. Alkali-lime indices generally correlate with potassium content, the low-K varieties being calcic (tholeiitic) and the higher K rocks being calc-alkalic.

Rare-earth-element concentrations range from fairly primitive compositions having flat patterns and abundances of ∼10 to 30 times chondrites to more evolved types that show mild enrichment in the light rare earths and typically are normal calc-alkaline. Mean initial ⁸⁷Sr/⁸⁶Sr ratios are commonly between 0.703 and 0.704. These low ratios are consistent with a mantle source in which assimilation of old sediments is not a factor. Limited isotopic and trace-element data suggest, instead, that the granitoids formed by partial melting in the mantle wedge and that at least some of the melting was accompanied by the addition of an aqueous fluid phase that was derived from the subducted oceanic plate.

Normative An-Ab-Or compositions are consistent with the presence of two groups of Caribbean granitoids. The group that is low in normative Or follows fractional crystallization vectors of parent magmas that typically are depleted in incompatible elements. The second group has higher normative Or and fractionates toward more enriched incompatible-element compositions. Normative compositions also indicate that there are temporal relations between the intrusive and extrusive rocks of Puerto Rico. Granitoids and lavas of Middle Cretaceous (125 to 100 Ma) and Early Tertiary (65 to 35 Ma) age are geochemically primitive types that display only limited degrees of mantle enrichment. In contrast, intrusive and extrusive rocks of Late Cretaceous age (100 to 65 Ma) show a wide range of incompatible-element concentrations, reflecting varying degrees of mantle enrichment.