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ABSTRACT: Late Cenozoic Forearc Volcanism in the Marianas Island Arc Revealed from Drilling at ODP Site 781

Michael S. Marlow, Julian A. Pearce, Ledabeth G. Pickthorn, Michael Hobart

Geophysical surveys of the Marianas forearc revealed a graben about 13 km northwest of Conical Seamount in an area equidistant from the Marianas Trench and the active Marianas Island Arc. The graben and its bounding horst blocks are part of a fault zone that strikes northwest-southeast beneath Conical Seamount. One horst block was drilled on Leg 125 of the Ocean Drilling Program (ODP Site 781) in an attempt to sample the basement rocks within the forearc and beneath the adjacent Conical Seamount. The drilling target was a high-amplitude seismic reflector identified within the horst block and 60 to 70 m beneath the seafloor.

Samples from three lithologic units were recovered at Site 781: an upper sediment unit, a middle basalt unit, and a lower sediment unit. The upper unit, between 0 and 72 m below the seafloor, consists of upper Pliocene to Holocene diatomaceous and radiolarian-bearing silty clay that grades down into vitric silty clay and vitric clayey silt. This unit overlies the vesicular, porphyritic basalt unit corresponding to the high-amplitude reflector on the reflection profiles. Because of poor core recovery (28-55%), the thickness of the basalt unit is estimated to be between 13 and 25 m. Beneath the basalt, the drill encountered middle to upper (and possibly some lower) Pliocene vitric silty clay and vitric clayey silt. The basalt's massive character and absence of internal flow structures s ggest that the unit is either a single lava flow or a near-surface silt. The top of the unit contains fresh glass and the base includes altered glass. The basalt samples contain abundant plagioclase and subordinate augite and olivine phenocrysts in a groundmass composed mainly of plagioclase, augite, olivine, and glass. The basalt is an island-arc tholeiite enriched in large-ion-lithophile elements relative to high-field-strength elements.

The basalt unit is an enigma because it could be either a sill or a lava flow. The sediment layers above and below the basalt unit indicate that the basalt is younger than 3 m.y. (samples are now being isotopically dated). Its geochemistry is consistent with a magma source from the mantle wedge above the subducting Pacific plate. The basalt may have been emplaced either along an inclined feeder dike originating from the mantle wedge beneath the island arc about 100 km west or vertically from depth beneath the middle of the forearc. Vertical emplacement has significant constraints for thermal models of, and magma genesis in, active forearc regions.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990