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

Jolly, Wayne T., Johannes H. Schellekens, and Alan P. Dickin. 2007. 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. Lithos 98 (1-4) (OCT): 1-26.

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 elementenriched 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 Al2O3 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.

Sr isotopes as a tracer of weathering processes and dust inputs in a tropical granitoid watershed, Luquillo Mountains, Puerto Rico

Pett-Ridge J. C., Derry L. A. and Kurtz A. C. (2009) Sr isotopes as
a tracer of weathering processes and dust inputs in a tropical
granitoid watershed, Luquillo Mountains, Puerto Rico. Geochim.
Cosmochim. Acta 73, 25–43.

Sr isotope data from soils, water, and atmospheric inputs in a small tropical granitoid watershed in the Luquillo Mountains of Puerto Rico constrain soil mineral development, weathering fluxes, and atmospheric deposition. This study provides new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on major elements alone. 87Sr/86Sr data reveal that Saharan mineral aerosol dust contributes significantly to atmospheric inputs. Watershed-scale Sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is 2100 ± 700 mg cm2 ka1. Nd isotope analyses of soil and saprolite samples provide independent evidence for the presence of Saharan dust in the regolith. Watershed-scale Sr isotope mass balance calculations are used to calculate the overall short-term chemical denudation velocity for the watershed, which agrees well with previous denudation rate estimates based on major element chemistry and cosmogenic nuclides. The dissolved streamwater Sr flux is dominated by weathering of plagioclase and hornblende and partial weathering of biotite in the saprock zone. A steep gradient in regolith porewater 87Sr/86Sr ratio with depth, from 0.70635 to as high as 0.71395, reflects the transition from primary mineral-derived Sr to a combination of residual biotite-derived Sr and atmospherically-derived Sr near the surface, and allows multiple origins of kaolinite to be identified
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