TEMPORAL EVOLUTION OF THE MARIANA ARC DURING RIFTING OF THE MARIANA TROUGH TRACED THROUGH THE VOLCANICLASTIC RECORD

The sedimentary sequences that accumulate around volcanic arcs may be used to reconstruct the history of volcanism provided the degree of al ong-margin sediment transport is modest, and that reworking of old sed imentary or volcanic sequences does not contribute substantially to th e sediment record. In the Mariana arc, the rare earth and trace elemen t compositions of ash layers sampled. by Deep Sea Drilling Project (DS DP)) site: 451 on the West Mariana Ridge, and sites 458 and 459 on the Mariana Forearc, were used to reconstruct the evolution of the arc vo lcanic front during rifting of the Mariana Trough. Ion microprobe anal ysis of individual glass shards horn the sediments shows that the glas ses have slightly light rare earth element (LREE)-enriched composition s, and trace element compositions typical of are tholeiites. The B/Be ratio is a measure of the involvement of subducted sediment in petroge nesis, and is unaffected Toy fractional crystallization, This ratio is variable over the period of rifting, increasing up-section at site 45 1 and reaching a maximum in sediments dated at 3-4 Ma, similar to 3-4 million years after rifting began. This may reflect increased sediment subduction during early rifting and roll-back of the Pacific lithosph ere. Parallel trends are not seen in the enrichment of incompatible hi gh field strength (HFSE), large ion lithophile (LILE) or rare earth el ements (REE), suggesting that flux from the subducting slab alone does not control the degree of melting. Re-establishment of are volcanism on the trench, side of the basin at ca 3 Ma resulted in volcanism with relative enrichment in incompatible REE, HFSE and LILE, although thes e became more depleted with time, possibly clue to melt extraction fro m the mantle source as it passed under the developing back-ar-e spread ing axis, prior to melting under the volcanic front.