AR-40 AR-39 AND K-AR GEOCHRONOLOGICAL AGE CONSTRAINTS FOR THE INCEPTION AND EARLY EVOLUTION OF THE IZU-BONIN-MARIANA ARC SYSTEM/

K-Ar and Ar-40/Ar-39 dates are presented for locations in the Izu-Boni n-Mariana (IBM) forearc (Ocean Drilling Program (ODP) sites 786 & 782, Chichijima, Deep Sea Drilling Program (DSDP) sites 458 & 459, Saipan) , and Palau on the remnant are of the Kyushu-Palau Ridge. For a number of these locations, the Ar-40/Ar-39 plateau and Al-36/Ar-40 versus Ar -39/Ar-40 isochrons give older ages than the K-Ar results. The most im portant results are: (i) at site 786, initial construction of the prot o-IBM (now forearc) basement occurred at least by ca 47-45 Ma, consist ent with the age of the immediately overlying sediments (middle Eocene nannofossil Zone CP13c); the younger pulse of construction dated at C a 35 Ma by K-Ar could not be confirmed by Ar-40/Ar-39 analysis; (ii) A r-40/Ar-39 ages for the initial construction of the Mariana portion of the IBM system are as old as those of the Izu-Bonin portion, for exam ple at site 458, initial construction commenced at least by ca 49 Ma a nd at cn. 47 Ma at Saipan (Sankakayuma Formation); and (iii) a combina tion of K-Ar and Ar-40/Ar-39 ages indicate continued boninite magmatis m in the Izu-Bonin forearc (and remnant are at Palau) until ca 35 Ma. Subduction inception including boninite series rocks along most of the exposed length of the IBM: system, clearly preceded by some 5 million years the Middle Eocene (ca 43.5 Ma) change in Pacific plate motion. Boninitic series magmatism persisted at locations now exposed in the f orearc for similar to 15 million years after are inception concurrentl y with low-K tholeiitic series eruptions from a subaerial are system, established at greater than or equal to 40 Ma, on the Kyushu-Palau Rid ge. For the Mariana portion of the IBM system, reconstruction of the p roto-are places this activity adjacent to the concurrent but orthogona lly spreading Central Basin Ridge of the West Philippine Basin. It is possible that a combination of subduction of a young North New Guinea Plate beneath newly created back-are basin crust may account for some of the features of the Mariana system. It is clear, however, that the understanding of the processes of subduction initiation and early IBM are development is incomplete.