Abstract:
K-Ar and (super 40) Ar/ (super 39) Ar dates are presented for locations in the Izu-Bonin-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 arc of the Kyushu-Palau Ridge. For a number of these locations, the (super 40) Ar/ (super 39) Ar plateau and (super 36) Ar/ (super 40) Ar versus (super 39) Ar/ (super 40) Ar isochrons give older ages than the K-Ar results. The most important results are: (i) at site 786, initial construction of the proto-IBM (now forearc) basement occurred at least by ca 47-45 Ma, consistent with the age of the immediately overlying sediments (middle Eocene nannofossil Zone CP13c); the younger pulse of construction dated at ca 35 Ma by K-Ar could not be confirmed by (super 40) Ar/ (super 39) Ar analysis; (ii) (super 40) Ar/ (super 39) Ar ages for the initial construction of the Mariana portion of the IBM system are as old as those of the Izu-Bonin portion, for example at site 458, initial construction commenced at least by ca 49 Ma and at ca 47 Ma at Saipan (Sankakayuma Formation); and (iii) a combination of K-Ar and (super 40) Ar/ (super 39) Ar ages indicate continued boninite magmatism in the Izu-Bonin forearc (and remnant arc 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 forearc for approximately 15 million years after arc inception concurrently with low-K tholeiitic series eruptions from a subaerial arc system, established at > or =40 Ma, on the Kyushu-Palau Ridge. For the Mariana portion of the IBM system, reconstruction of the proto-arc places this activity adjacent to the concurrent but orthogonally 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-arc 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 arc development is incomplete.