Journal of Petrology Advance Access published online on January 30, 2008
Journal of Petrology, doi:10.1093/petrology/egm087
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Petrogenesis of Volcanic Rocks from Saipan and Rota, Mariana Islands, and Implications for the Evolution of Nascent Island Arcs
1Department of Geoscience, University of Iowa, Iowa City, IA 52242, USA
2Department of Geological Sciences, San Diego State University, San Diego, CA 92182, USA
3New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Technology, 801 Leroy Place, Socorro, NM 87801, USA
4Department of Earth Sciences, Florida International University, Miami, FL 33199, USA
Received October 1, 2006; Revised typescript accepted December 19, 2007
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Abstract |
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An 40Ar/39Ar age of 45·1 Ma determined for lavas from northern Saipan confirms that these high-silica rhyolites erupted during the ‘proto-arc’ stage of volcanism in the Izu–Bonin–Mariana system, which is characterized elsewhere by eruption of boninitic lavas. Incompatible trace element concentrations and Sr, Hf, Nd, and Pb isotope ratios for these rhyolites are transitional between those of c. 48 Ma boninitic lavas and post-38 Ma ‘first-arc’ andesites and dacites from Saipan and Rota that have typical subduction-related compositions. These transitional compositions are modeled by crystal fractionation of parental tholeiitic basalt combined with assimilation of young boninitic crust. A second stage of Rayleigh fractionation in the upper crust is required by SiO2 concentrations that exceed 77 wt % and near-zero compatible element concentrations. First-arc magma compositions are consistent with fractionation of basalt and assimilation of crust similar in composition to the first-arc magmas themselves. The mantle sources of the proto-arc and first-arc lavas from Saipan and Rota are similar to those of Philippine back-arc basin basalts based on Nd and Hf isotopic compositions. The Pb isotope compositions of these lavas are between those of Pacific sea-floor basalts and Jurassic and younger cherty and clay-rich sediments. This contrasts with the boninitic proto-arc volcanic rocks from Guam and Deep Sea Drilling Project Sites 458 and 459 that have Pb isotope compositions similar to Pacific basin basalts and volcaniclastic sediments. The preferred explanation for the difference in the nature of proto-arc volcanism between Saipan and other fore-arc locations is that the crust ceased extending 3–4 Myr earlier beneath Saipan. This was caused by a change from mantle upwelling, fore-arc extension, and shallow melting to an environment dominated by more normal mantle wedge convection, stable crust, and deeper melting.
KEY WORDS: rhyolite; andesite; Mariana arc; isotope ratios; trace elements
*Corresponding author. Telephone: 1-319-335-1802. Fax: 1-319-335-1821. E-mail: mark-reagan{at}uiowa.edu