Abstract:
Volcanic rocks of the Kyushu-Palau Ridge (KPR) from Deep Sea Drilling Project (DSDP) site 448 and from Belau comprise a low-to-medium-K arc tholeiitic series. Belau rocks include (probable) Mid-Eocene low-Ca type-3 boninite and pre-Early Oligocene-Early Miocene low-K arc tholeiitic basalt, basaltic andesite, andesite and dacite. Palau Trench samples include sparsely phyric high-Mg, -Cr and -Ni rocks which resemble the Belau boninite and Izu-Bonin-Mariana (IBM) system boninites. The high-Mg Palau Trench samples also resemble other primitive arc lavas (e.g. arc picrites). Their chemistry suggests an origin involving steep thermal gradients in multiply depleted mantle. Subduction of hot, young lithosphere under a young hot upper plate is postulated to explain this occurrence. The KPR is inferred to be the source of Eocene boninite and arc tholeiitic terranes presently in forearc regions of the IBM system. A model is presented here showing how many IBM boninites may have originated in a small area near Belau. These have migrated eastward by episodic back-arc opening accompanying eastward migration of arcs and trenches. Oldest known KPR rocks (ca 47.5 Ma at DSDP site 296), and presumed KPR-derived exotic terranes of Guam (ca 43.8 Ma), presage the postulated Eocene (ca 42-43 Ma) change in Pacific plate motion invoked as the cause of subduction initiation at the KPR. The KPR has been rotated more than 40 degrees clockwise since the Eocene, thus the age mismatch may indicate a different tectonic style, for example transtension or transpression, in earliest KPR history.