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Major-element petrochemistry of some extrusive rocks from the volcanically active Mariana Islands |
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| Authors: | E E Larson R L Reynolds R Merrill S Levi M Ozima Y Aoki H Kinoshita S Zasshu N Kawai T Nakajima K Hirooka |
| Institution: | 1. Cooperative Institute for Research in Environmental Sciences, and the Dept. of Geological Sciences, University of Colorado, 80302, Boulder, Colorado, USA 2. Geophysics Group, University of Washington, Seattle, Washington, USA 3. Geophysical Institute, University of Tokyo, Tokyo, Japan 4. Dept. of Material Science, University of Osaka, Osaka, Japan 5. Geological Laboratory, Fukui University, Fukui, Japan |
| Abstract: | Volcanic rocks from six of the currently or recently active volcances of the Mariana Island are show little variation in major element abundances. SiO2 content averages 51.5 wt.%. The flows are high in Al2O (mean 17.7 wt.%) and Fe oxides (mean 10.1 wt.% calculated as FeO only), and moderate in MgO content (mean 4.7 wt.%), Na2O (mean 2.7 wt.%), and K2O (mean 0.7 wt.%). Only the rocks from Farallon de Pajaros, the northernmost of the Mariana Islands, deviate slightly from the average of the analyses. Three analyses from this island are slightly higher in SiO2 (about 54 wt.%) and Al2O3, and are lower in total Fe oxides and MgO. According to preferred classification, the lavas of the Mariana Islands can be termed mela-andesites, high-alumina basalts, or calc-alkaline (orogenic) basalts. The K2O values (mean 0.7 wt.%) obtained from lavas of the Mariana Islands are significantly higher than the K2O values (about 0.33 wt.%) from volcanics of the Izu chain to the north. Inasmuch as the substantial scatter in location of earthquake foci beneath both arcs prevents accurate delineation of the upper boundary of the Benioff zone, it presently cannot be determined whether this discrepancy in K2O values reflects a difference in depth from the volcanic are to the dipping seismic zone or relates to other phenomena. The older volcanic islands within the Mariana-Bonin island chain apparently defined an island arc system during Eocene to Miocene time. This indicates that the present plane of convergence between the Pacific plate and the Philippine Sea plate has defined the convergence between these plates since Eocene time. |
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