Computer Science
Scientific paper
Jun 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984e%26psl..68..443g&link_type=abstract
Earth and Planetary Science Letters, Volume 68, Issue 3, p. 443-458.
Computer Science
56
Scientific paper
Twenty-eight new Pb, 20 Sr, and 9 Nd isotopic compositions are presented for 32 rocks and one galena from Fiji and the South Fiji (back-arc) Basin. The Fijian rocks range in age from 35 to < 0.1 m.y., and are taken from arc tholeiitic, calc-alkaline, trondjhemitic, shoshonitic, and alkali olivine basalt series, and from the hybrid basalts to dacites of Kadavu. They span both the subduction-related and rifting-related episodes of Fijian history.
For at least 30 m.y., andesite-dominated volcanics with arc-like trace element characteristics (e.g. Rb/Zr, Ba/La, Sr/Nd, Pb/U, Ba/Th, and La/Nb ratios higher than in either OIB or MORB) were erupted in the oceanic Vitiaz island arc which included Fiji. During the initial 2/3 of this time, most rocks belonged to an island are tholeiitic series similar to MORB both in some trace element (e.g. K/Rb, La/Yb, Hf/Th, and Y/Zr) and isotope ratios (e.g. relatively high 143Nd/144Nd and low 206Pb/204Pb). Nearly constant 207Pb/204Pb ratios therein are attributed to mixing between a sediment-contaminated MORB source component with low 206Pb/204Pb, and an OIB source component lying within the conventional Sr-Nd-Pb mantle array. In later calc-alkaline and shoshonitic series rocks, these same trace element and isotopic ratios are more like those of OIB. The change was not accompanied by an increase in 207Pb/204Pb or Cs/K, indeed, 207Pb/204Pb is closer to the mantle array in these later series. Consequently, the change indicates a greater contribution from OIB sources, rather than from recycled ocean crust. These interpretations require that both MORB and OIB sources coexist in the uppermost mantle above subducted lithosphere.
Rifting-related volcanism in Fiji, which accompanied opening of the North Fiji and Lau Basins, has been basalt-dominated but was drawn initially (in the shoshonites and related rocks) from much the same mixture of sources that fed the older andesite-dominated calc-alkaline series. Relative to the calc-alkaline rocks, the high K, Rb, Ba, and Sr contents of the shoshonitic group are accompanied by higher 87Sr/86Sr and 206Pb/204Pb but not higher 207Pb/204Pb or lower 143Nd/144Nd, reflecting separation of Sr from Pb and Nd during recycling of sediment-poor ocean crust. Still later rifting-related alkali olivine basaltic volcanism lost both its arc-like trace element and isotopic qualities, being like OIB in most respects.
Present address: Earth Sciences Board, University of California, Santa Cruz, CA 95064, U.S.A.
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