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Journal of Petrology Volume 42 Number 12 Pages 2197-2214 2001
© Oxford University Press 2001
Secular Geochemistry of Central Puerto Rican Island Arc Lavas: Constraints on Mesozoic Tectonism in the Eastern Greater Antilles
1DEPARTMENT OF EARTH SCIENCES, BROCK UNIVERSITY, ST. CATHARINES, ONT., CANADA L2S 3A1
2DEPARTMENT OF GEOLOGY AND PLANETARY SCIENCES, UNIVERSITY OF PITTSBURGH, PITTSBURGH, PA 15260, USA
3DEPARTMENT OF GEOLOGY, McMASTER UNIVERSITY, HAMILTON, ONT., CANADA L8S 4M1
4DEPARTMENT OF GEOLOGY, UNIVERSITY OF WESTERN ONTARIO, LONDON, ONT., CANADA N6A 5B7
Island arc volcanism in the Greater Antilles persisted for >70 m.y. from Middle Cretaceous to Late Eocene time. During the initial 50 m.y., lavas in central Puerto Rico shifted from predominantly island arc tholeiites (volcanic phase I, Aptian to Early Albian, 120–105 Ma), to calc-alkaline basalts (phase II, Late Albian, 105–97 Ma), and finally to high-K, incompatible-element-enriched basalts (phases III and IV, Cenomanian–Maastrichtian, 97–70 Ma). Following an island-wide eruptive hiatus, geochemical trends were reversed in the Eocene with renewed eruption of calc-alkaline basalts (phase V, 60–45 Ma). Progressive increases in large-ion lithophile elements (LILE)/light rare earth elements (LREE), LILE/high field strength elements (HFSE), LREE/HFSE, and HFSE/heavy rare earth elements (HREE) characterize the compositional evolution of the first four volcanic phases. The shift in trace element compositions is mirrored by increasing radiogenic content of the lavas. Pb
8/4 values, representing deviations of 208Pb/204Pb from the Northern Hemisphere Reference Line (NHRL), range from -20 to almost +2·0 in phases I and II, and up to +25 in phase III. Similarly, 
Nd values decrease slightly from +8 to almost +6 between volcanic phases I and III. Finally, initial (i) 87Sr/86Sr values in phase I basalts have a narrow range from 0·7033 to 0·7040, near the upper limit of altered mid-ocean ridge basalt (MORB), whereas values from phases III and IV basalts have a broader range from 0·7034 to 0·7044. N-MORB-normalized incompatible element distribution patterns of Puerto Rican volcanic rocks have uniformly flat HREE segments and Y/Yb is 
1, indicating that garnet and amphibole were insignificant as residual phases and that melting occurred predominantly within relatively dry spinel lherzolite. Yb concentrations, which provide constraints on degree of melting, are consistent with a narrow range from 30 to 35% melting in volcanic phase I, but with a much broader range from 25 to 40% melting during phase III. It seems likely that such high degrees of melting were attained through a combination of flux-related melting and buoyancy-driven pressure-release fusion. Nb abundances, which reflect degree of incompatible element enrichment compared with fertile MORB mantle (FMM), are low in volcanic phase I, consistent with 2% low-degree pressure release melting of source material in the back-arc region before entry into the arc melting zone. Subsequent lavas from phases II and III have N-MORB-like or higher Nb abundances, indicating that (1) back-arc processes peaked in intensity during the first 10-20 m.y. and later declined in significance, and/or (2) the degree of incompatible element enrichment gradually increased as a result of subduction of a thickening accumulation of pelagic sediment. Isotope mixing models indicate that the proportion of authigenic pelagic sediment incorporated into Puerto Rican basalts increased from negligible levels in phase I to as high as 2% in phases III and IV. Although the absolute magnitude of the sediment component increased progressively, a narrow range of Th/La in mafic end-members indicates that the terrigenous contribution remained uniform throughout volcanism, consistent with the insular setting of the eastern Greater Antilles Arc.
KEY WORDS: Cretaceous subduction; fluid flux; mantle melting; Puerto Rico; Greater Antilles; Caribbean
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