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Geochemistry of Oceanic Island-Arc and Active Continental Margin Volcanic
Suites: Some Statistical Evaluations and Implications Using the Database
GEOROC
Article · November 2002
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Baerbel Sarbas
Max Planck Institute for Chemistry
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Geochemistry of Oceanic Island-Arc and Active Continental Margin Volcanic Suites –
Some Statistical Evaluations and Implications Using the Database GEOROC
B. Sarbas
The major- and trace-element and the isotopic composition of volcanic rocks from
oceanic-island arcs and active continental margins are compared using the comprehensive
dataset available in the geochemical database GEOROC (http://georoc.mpchmainz.gwdg.de). The Aleutian, Kurile, IBM, Indonesian, Vanuatu, Tonga, Kermadec,
Lesser Antilles, and Scotia Arcs and the Andes, Central America, Mexico, and the
Cascades are used as representatives for oceanic-island arcs and active continental
margins, respectively.
Average major-element compositions are tabulated for basalts, basaltic andesites,
andesites, dacites, and rhyolites from the individual oceanic and continental arcs. Majorelement variation diagrams show similar patterns for volcanic rocks from oceanic island
arcs and active continental margins. In comparison with the island-arc volcanic suites,
low-K series magmas are poorly represented, while high-K and shoshonitic magmas are
more common, in the active continental margin settings.
With the GEOROC database, the generally described overall depletion of high-field
strength elements such as Nb, Ta, Ce, Zr, Hf, Sm, Yb, and Sc relative to large-ionlithophile elements such as Rb, Sr, Ba, Pb, U, and Th can be verified for arcs from both
tectonic settings. It also allows more detailed insights into the geochemical signature of
individual arcs.
The Sr, Nd, and Pb isotopic composition of most oceanic island-arc volcanic rocks shows
an overlap with the composition of OIB and MORB, with the exception of samples from
the Indonesian and Lesser Antilles arcs, consistent with the involvement of subducted
upper-crustal material in these arcs. Among the active continental margin settings, the
Cascades, the Mexican, and the Central American arcs as well as the Southern and
Northern Volcanic Zones of the Andean Arc overlap with the composition of OIB,
whereas volcanic rocks from the central Andean Arc show high $^{87}$Sr/$^{86}$Sr
and low $^{143}$Nd/$^{144}$Nd ratios.
There appears to be no distinctive grouping of arc compositions in any “island-arc”
versus “continental” type, rather a continuous range from Tonga-Kermadec and Scotia at
one extreme with Indonesia and the Andes at the other.
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