Other
Scientific paper
Sep 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975gecoa..39.1287w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 39, Issue 9, pp.1287-1302
Other
23
Scientific paper
Pleistocene and Recent lavas from the Sunda arc range from those showing affinities with the island arc tholeiitic series, through a spectrum of calc-alkaline to high-K alkaline rocks. The tholeiitic rocks have relatively low 87 Sr / 86 Sr ratios averaging 0-7043; the calc-alkaline rocks show a wide range (from 0.7038 to 0.7059, averaging 0.7048); the high-K alkaline rocks average 0.7045. A rhyolitic ignimbrite from Sumatra has an 87 Sr / 86 Sr ratio of 0.7139. The relationship between 87 Sr / 86 Sr and major and trace element geochemistry is variable and complex. Lavas from the same volcano sometimes show significant differences in 87 Sr / 86 Sr despite close geochemical relationships. Rocks of the calc-alkaline suite show a regular decrease in 87 Sr / 86 Sr from West Java to Bali and there is some evidence for increasing 87 Sr / 86 Sr with increasing depth to the Benioff zone. Calc-alkaline and tholeiitic rocks from the Sunda arc have significantly higher 87 Sr / 86 Sr ratios than those from other island arcs, except from those arcs where continental crustal involvement has been inferred (e.g. New Zealand). A model of 87 Sr enrichment due to isotopic equilibration of oceanic crust with sea water and disequilibrium melting in the slab and/or mantle is favoured to explain the Sr isotopic composition of the tholeiitic and normal calc-alkaline lavas. Calc-alkaline lavas with high 87 Sr / 86 Sr ratios are best explained by either sialic contamination, or the presence of alkali basalt as a component of the downgoing slab. The Sr isotopic data for the high-K alkaline lavas suggest a mantle origin. The high 87 Sr / 86 Sr ratio in the Lake Toba rhyolite implies a crustal origin.
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