Shoshonitic volcanism in the Northern Mariana Arc: 1. Mineralogic and major and trace element characteristics

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Abstract
Petrographic and geochemical characteristics of samples dredged from 23 submarine volcanic edifices in the northern Mariana and southern Volcano arcs define two distinct rock series. The central and northern Mariana Arc (to 23°N) and the northern Volcano Arc (north of Iwo Jima) are characterized by plagioclase‐clinopyroxene‐orthopyroxene‐titantomagnetite bearing subalkaline rocks, including both low‐K and medium‐K series. The northern Mariana Arc and southern Volcano Arc, from 23°N to Iwo Jima, are erupting rocks of a shoshonitic series with phenocrysts of plagioclase, clinopyroxene, olivine, and biotite. These rocks are less saturated than those of the subalkaline provinces and are substantially enriched in Ba (400–900 ppm), Sr (600–1000 ppm), K2O (1–4.5%), and K2O/Na2O (0.4–1.2) relative to the subalkaline lavas. Ba/Y and Ba/Zr increase by a factor of 3 to 4 in the shoshonitic rocks, but K/Rb, Ba/Sr, and K/Ba are relatively constant throughout the arc. There is no relationship between degree of enrichment and volcano volume or degree of fractionation: seamounts and islands within each province have the same range of compositions. Much of the intra‐edifice variation in lava composition can be modeled by 10–70% crystallization of plagioclase and clinopyroxene, with smaller amounts of olivine, orthopyroxene, and titanomagnetite, or by accumulation of 10–40% phenocrysts, dominantly plagioclase. The differences between parental lavas in the alkalic and subalkalic provinces require melting of two distinct mantle sources. The occurrence of this enriched mantle in the northern Marianas may be a consequence of the propagation of the Mariana Trough spreading center into the Volcano Arc.