volcanism

Bimodal volcanism in northeast Puerto Rico and the Virgin Islands (Greater Antilles Island Arc): Genetic links with Cretaceous subduction of the mid-Atlantic ridge Caribbean spur

Jolly, Wayne T., Edward G. Lidiak, and Alan P. Dickin. 2008. Bimodal volcanism in northeast puerto rico and the virgin islands (greater antilles island arc): Genetic links with cretaceous subduction of the mid-atlantic ridge caribbean spur. Lithos 103 (3-4) (JUL): 393-414.

Abstract: 
Bimodal extrusive volcanic rocks in the northeast Greater Antilles Arc consist of two interlayered suites, including (1) a predominantly basaltic suite, dominated by island arc basalts with small proportions of andesite, and (2) a silicic suite, similar in composition to small volume intrusive veins of oceanic plagiogranite commonly recognized in oceanic crustal sequences. The basaltic suite is geochemically characterized by variable enrichment in the more incompatible elements and negative chondritenormalized HFSE anomalies. Trace element melting and mixing models indicate the magnitude of the subducted sediment component in Antilles arc basalts is highly variable and decreases dramatically from east to west along the arc. In the Virgin Islands, the sediment component ranges betweenb0.5 to ∼1% in Albian rocks, and between ∼1 and 2% in succeeding Cenomanian to Campanian strata. In comparison, sediment proportions in central Puerto Rico range between 0.5 to 1.5% in the Albian to 2 toN4% during the Cenomanian-Campanian interval. The silicic suite, consisting predominantly of rhyolites, is characterized by depleted Al2O3 (averageb16%), low Mg-number (molar Mg/Mg+Feb0.5), TiO2 (b1.0%), and Sr/Y (b10), oceanic or arc-like Sr, Nd, and Pb isotope signatures, and by the presence of plagioclase. All of these features are consistent with an anatexic origin in gabbroic sources, of both oceanic and arc-related origin, within the sub-arc basement. The abundance of silicic lavas varies widely along the length of the arc platform. In the Virgin Islands on the east, rhyolites comprise up to 80% of Lower Albian strata (112 to 105 Ma), and about 20% in post-Albian strata (105 to 100 Ma). Farther west, in Puerto Rico, more limited proportions (b20%) of silicic lavas were erupted. The systematic variation of both sediment flux and abundance of crustally derived silicic lavas are consistent with current tectonic models of Caribbean evolution involving approximately perpendicular subduction of the Caribbean spur of the mid-Atlantic Ridge, which was located approximately midway between North and South America until Campanian times. Within this hypothetical setting the centrally positioned Virgin Islands terrain remained approximately fixed above the subducting ridge as the Antilles arc platform swept northeastward into the slot between the Americas. Accordingly, heat flow in the Virgin Islands was elevated throughout the Cretaceous, giving rise to widespread crustal melting, whereas the subducted sediment flux was limited. Conversely, toward the west in central Puerto Rico, which was consistently more remote from the subducting ridge, heat flow was relatively low and produced limited crustal melting, while the sediment flux was comparatively elevated.

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.

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 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.

Geologic implications of the oxygen isotope profile of the toa-baja drill hole, puerto-rico

SMITH, BM. 1991. Geologic implications of the oxygen isotope profile of the toa-baja drill hole, puerto-rico. Geophysical Research Letters 18 (3) (MAR): 549-52.

Bimodal volcanism in northeast Puerto Rico and the Virgin Islands (Greater Antilles Island Arc): Genetic links with Cretaceous subduction of the mid-Atlantic ridge Caribbean spur

Jolly, Wayne T., Edward G. Lidiak, and Alan P. Dickin. 2008. Bimodal volcanism in northeast puerto rico and the virgin islands (greater antilles island arc): Genetic links with cretaceous subduction of the mid-atlantic ridge caribbean spur. Lithos 103 (3-4) (JUL): 393-414.

Abstract: 
Bimodal extrusive volcanic rocks in the northeast Greater Antilles Arc consist of two interlayered suites, including (1) a predominantly basaltic suite, dominated by island arc basalts with small proportions of andesite, and (2) a silicic suite, similar in composition to small volume intrusive veins of oceanic plagiogranite commonly recognized in oceanic crustal sequences. The basaltic suite is geochemically characterized by variable enrichment in the more incompatible elements and negative chondritenormalized HFSE anomalies. Trace element melting and mixing models indicate the magnitude of the subducted sediment component in Antilles arc basalts is highly variable and decreases dramatically from east to west along the arc. In the Virgin Islands, the sediment component ranges betweenb0.5 to ∼1% in Albian rocks, and between ∼1 and 2% in succeeding Cenomanian to Campanian strata. In comparison, sediment proportions in central Puerto Rico range between 0.5 to 1.5% in the Albian to 2 toN4% during the Cenomanian-Campanian interval. The silicic suite, consisting predominantly of rhyolites, is characterized by depleted Al2O3 (averageb16%), low Mg-number (molar Mg/Mg+Feb0.5), TiO2 (b1.0%), and Sr/Y (b10), oceanic or arc-like Sr, Nd, and Pb isotope signatures, and by the presence of plagioclase. All of these features are consistent with an anatexic origin in gabbroic sources, of both oceanic and arc-related origin, within the sub-arc basement. The abundance of silicic lavas varies widely along the length of the arc platform. In the Virgin Islands on the east, rhyolites comprise up to 80% of Lower Albian strata (112 to 105 Ma), and about 20% in post-Albian strata (105 to 100 Ma). Farther west, in Puerto Rico, more limited proportions (b20%) of silicic lavas were erupted. The systematic variation of both sediment flux and abundance of crustally derived silicic lavas are consistent with current tectonic models of Caribbean evolution involving approximately perpendicular subduction of the Caribbean spur of the mid-Atlantic Ridge, which was located approximately midway between North and South America until Campanian times. Within this hypothetical setting the centrally positioned Virgin Islands terrain remained approximately fixed above the subducting ridge as the Antilles arc platform swept northeastward into the slot between the Americas. Accordingly, heat flow in the Virgin Islands was elevated throughout the Cretaceous, giving rise to widespread crustal melting, whereas the subducted sediment flux was limited. Conversely, toward the west in central Puerto Rico, which wasconsistently more remote from the subducting ridge, heat flow was relatively low and produced limited crustal melting, while the sediment flux was comparatively elevated.
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