High-Mg andesites and related lavas from southwest Puerto Rico (Greater Antilles Island Arc): Petrogenetic links with emplacement of the Late Cretaceous Caribbean mantle plume

Abstract

Two-pyroxene-bearing high-Mg andesite, hornblende basalt and andesite, and high-Fe augite basalt were erupted simultaneously in southwest Puerto Rico between 85 and 65 Ma. An analogy with geologic settings in Cenozoic arcs indicates that hornblende-bearing lavas and high-Mg andesites, restricted to the southwestern-most corner of Puerto Rico, represent the forearc assemblage, whereas high-Fe basalts, concentrated in an adjacent volcanic belt toward the northeast, represent the arc-axis suite. This arrangement implies northeast-dipping subduction of refractory Jurassic chert from the Caribbean Basin, and is, therefore, consistent with relatively low Sr-isotope ratios in all three lava suites compared with correlative strata in Eastern Puerto Rico. Moreover, Pb- and Nd-isotope ratios and trace element melting models for both high-Mg andesites and hornblende-bearing lavas are consistent with the presence of a slab melt component generated by high-pressure fusion of incompatible element-enriched plateau basalts. The most likely source for such a basaltic component is the Caribbean basalt plateau, which is represented in southwest Puerto Rico by the Upper Cajul Formation. The models indicate that up to 5% slab melt was added to the source of hornblende-bearing lavas, but higher proportions, as much as 10%, are required to generate high-Mg andesites. The elevated buoyancy of the more enriched and siliceous high-Mg andesite source apparently destabilized the mantle wedge and induced combined mantle-mass assimilation and fractional crystallization of orthopyroxene (AFC ≈ 1), which ultimately produced elevated MgO and low Al 2O 3 concentrations characteristic of the high-Mg andesites. The tectonic setting in southwest Puerto Rico was unlike Cenozoic analogues, because the pre-arc basement was already old (Early Jurassic, 185–155 Ma) at the time of initial island arc volcanism (∼ 85 Ma). However, geothermal gradients in the region were increased again immediately preceding arc volcanism by emplacement of the Caribbean mantle plume (92–88 Ma), during which the original N-MORB-type upper mantle in the region was replaced by incompatible element-enriched material. The elevated heat flow produced by plume emplacement, supplemented by ascent of plume basalts from depth and associated gabbroic underplating, is inferred to have promoted slab melting. The presence of a low Zr/Sm component in both plateau basalts and arc lavas in southwest Puerto Rico is consistent with the incorporation of a small biogenic supra-subduction zone component of Atlantic origin, introduced into the back-arc region of an older (from 115 Ma) southwest-dipping subduction zone in eastern Puerto Rico.