Petrogenesis of Lavas along the Solomon Island Arc, SW Pacific: Coupling of Compositional Variations and Subduction Zone Geometry

The Solomon island arc, SW Pacific, is of particular interest for understanding subduction zone volcanism, as magmatism in the active part of the arc is dominated by mafic melts, thus permitting direct insights into mantle processes. Along the Solomon island arc, the Indian-Australian plate is subducting at present beneath the Pacific plate. However, until at least c. 12 Myr ago, the Pacific plate was subducting beneath the Indian-Australian plate until the Cretaceous Ontong Java Plateau collided with the northern Solomon island arc. To evaluate the effects of the changes in tectonic regime on lava compositions, we present a comprehensive Sr-Nd-Hf-Pb isotope, major element and trace element dataset, covering lavas erupted along the entire island arc (c. 1000 km). Basalts and andesites represent the most abundant rock types. Picrites and ankar-amites occur in the New Georgia Group of the Solomon Islands, where they erupted above the subducting Woodlark spreading center, and also in the Santa Cruz archipelago, north of Vanuatu, where the Rennell Fracture Zone is subducting. Recent work has also identified the presence of adakites (Sr/Y up to c. 200), and high-Mg andesites (MgO>5 wt%, Sr/Y c. 11-46). Most of the high-Mg andesites are genetically linked to the adakites, but some of the high-Mg andesites show affinities to boninitic compositions. Large ion lithophile element abundances in most Solomon island arc magmas indicate a strong source overprint by subduction components. Sr-87/Sr-86 and epsilon Nd values along the arc range from 0.7029 to 0.7052 and from +5.8 to +8.3., respectively. The Sr-Nd values partially overlap the compositions of oceanic basalts from the Indian-Australian plate. Measured epsilon Hf values range from +10.5 to +14.6. If corrected for contributions from subduction components, combined epsilon Hf epsilon Nd systematics also indicate that most of the studied Solomon arc lavas were generated within the Indian-type mantle domain. However, a few samples display epsilon Hf-epsilon Nd compositions resembling those of the Pacific-type mantle domain. These samples either originate from older Pacific basement (basalts) or represent melts derived from subducted Pacific crust (adakites). Lead isotope compositions, controlled by subduction components, can be used to identify the presence of two distinct types of subduction components that originate (1) from the Pacific plate including Ontong Java Plateau material (>6 Myr old) and (2) from the more recently subducted Indian-Australian plate. Combined Hf-Nd-Pb isotope data also reveal that lower parts of the Ontong Java Plateau entered the mantle wedge, as previously postulated by geophysical models.