Operation of subduction factory and production of andesite

Abstract

The subduction factory processes raw materials such as oceanic sediments and basaltic crust, selectively extracts particular subduction components and manufactures magmas, their solidified materials and continental crust as products. The waste materials from the factory, such as chemically modified oceanic materials and delaminated mafic arc lower crust are transported down to the deep mantle modified their compositions and ultimately recycled as mantle plumes. Andesite composes the bulk continental crust and therefore is the major product in the subduction factory. Two types of andesites, calk-alkalic and tholeiitic series, are commonly recognized in a single arc volcano. We propose a new mechanism for production of these two magma series on the basis of data obtained by Sr isotopic micro-analyses of plagioclase in volcanic rocks from Zao Volcano, NE Japan. Tholeiitic magmas having constant and enriched isotopic signatures are produced by anatexis of the preexisting mafic lower crust, whereas calc-alkalic magmas, having compositions similar to the bulk continental crust, are products of mixing a mantle-derived, hence isotopically depleted, basaltic magma and crust-derived felsic tholeiites.