The first evidence for MORB-like lavas from the outer Mariana forearc: geochemistry, petrography and tectonic implications

Abstract

The Mariana forearc, the region between the active Mariana volcanic arc and the trench axis, is considered to be composed of rocks entirely of arc origin. This conclusion was based upon the lack of an accretionary wedge and recovery of arc volcanics from sites throughout the forearc (including the inner trench wall). New chemical data from dredged samples demonstrate that exposures of altered basalts similar to mid-ocean ridge basalt (MORB) and of ocean island basalt (OIB) exist in the Mariana forearc. Several forearc sites including a 2000 m scarp bounding a large graben at 19°30′N, two small scarps in the inner trench wall, and a seamount were dredged. MORB-like and OIB lavas in addition to island arc tholeiite (IAT) and boninite were recovered. All samples exhibit low-temperature seafloor alteration, and some are metamorphosed to lower greenschist facies. IAT samples have low TiO 2 (< 1%) and Zr (< 56 ppm). The MORB samples have TiO 2 of 1–2% and Zr of 60–93 ppm. OIB samples typically have high TiO 2 (> 3%) and Zr (> 180 ppm). Several OIB samples contain modal nepheline and all have high alkali contents. The presence of OIB and MORB-like lavas in the outer forearc may suggest either that entrapment of Philippine Sea or Kula Plate or that obduction of Pacific Plate has occurred in the evolution of the Mariana forearc. However, the intimate association of MORB-like lavas with arc lavas in the fault scarp requires that either IAT magma was injected into entrapped or obducted oceanic crust or that MORB-like magma was injected into an arc crust during forearc rifting or deformation.