التفاصيل البيبلوغرافية
| العنوان: |
Recycling in the Puerto Rican mantle wedge, Greater Antilles Island Arc. |
| المؤلفون: |
JOLLY, WAYNE T, LIDIAK, EDWARD G, DICKIN, ALAN P, WU, TSAI-WAY |
| المصدر: |
Island Arc; Mar2002, Vol. 11 Issue 1, p10-24, 15p |
| مصطلحات موضوعية: |
ISLAND arcs, ANALYTICAL geochemistry, SUBDUCTION zones, BASALT |
| مصطلحات جغرافية: |
PUERTO Rico |
| مستخلص: |
Abstract The initial volcanic phase of Cretaceous island arc strata in central Puerto Rico, at the eastern end of the extinct Greater Antilles Arc, comprises a 6-km thick pile of lava and volcanic breccia (Río Majada Group). Preserved within the sequence is a conspicuous shift in absolute abundances of the more incompatible elements, including Th, Nb, and the light rare earth elements (LREE: La, Ce, Pr and Nd). The compositional shift is marked by a decrease in La/Sm from averages of 2.11 in the lowest third of the pile (Formation A) to 1.48 at the top (Formation C), and by a distinctive flattening of LREE segments of chondrite-normalized REE patterns. i 87 Sr/86 Sr and ℇNd average about 0.7035 and 8.2, respectively, in early Formation A basalts. These ranges normally overlap samples from later Formations B and C. Isotope compositions of the latter group are more variable, however, and several samples are considerably more radiogenic than Formation A basalts, such that i 87 Sr/86 Sr averages almost 0.7042 while ℇNd -values decrease to 7.5 in Formation B and C basalts. Theoretical models of non-modal melting processes in both amphibole peridotite and spinel lherzolite sources provide insight into the origin of depleted Th, Nb, and LREE abundances in Puerto Rican basalts. Low Nb concentrations less than normal mid-oceanic ridge basalts in Formation A basalts indicate the wedge was slightly depleted by low-volume decompression fusion due to induced convection in the back-arc region prior to entry of the source into the arc melting zone. However, depleted patterns in Formation C basalts cannot be generated by relatively greater degrees of decompression fusion in the back-arc, because addition of the La-enriched slab-derived component to more depleted source material invariably produces elevated rather than decreased La/Sm. Refluxing of Formation A harzburgitic residua is similarly... [ABSTRACT FROM AUTHOR] |
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