Temporal Geochemical Variations in Glass and Minerals from Early Oligocene to Miocene Volcanic Sediments, DSDP Site 296, Kyushu Palau Ridge: Is There a Geochemical Signal for Arc Rifting?

Abstract:

Volcaniclastic sediments and sedimentary rocks from DSDP Site 296, located within a basin at the crest of the northern Kyushu Palau ridge (KPR), record the latter part of the first stage of Izu Bonin Mariana (IBM) arc evolution, up to the cessation of volcanism caused by arc rifting and opening of the Shikoku basin. The lower section consists of early to late Oligocene coarse volcaniclastic sedimentary rocks, and is overlain by late Oligocene to Pleistocene nannofossil chalks and oozes with volcanic sand and ash-rich layers. We have studied the chemical composition of pyroxene, feldspar and glass grains separated from the coarse volcaniclastic rocks at depths from 435 to 1082 meters below sea floor, and of glass shards in layers in the overlying sediments of late Oligocene to early Miocene age. Overall, pyroxene and feldspar compositions show little systematic variation with depth in the core, although for pyroxene, highest En and highest Al2O3 contents are found in the interval from 600-900 meters bsf. An contents in feldspars show a bimodal distribution throughout the core, with most values > 90 or in the range 60-70, with more abundant intermediate compositions in the 600-900 meter interval. Compositions of glass shards vary widely, from basalt to rhyolite, and from low K, light rare earth (LREE)-depleted to high K, strongly LREE-enriched character, without systematic variation with depth in the core. However, all cores sampled from early Oligocene to early Miocene contain relatively low K basalt and basaltic andesite glass. Like the pyroxenes, a wider range of compositions is found in glass from the 600 to 900 mbsf interval. The Site 296 sequence overlaps in age with the uppermost sedimentary section of recently drilled IODP Site 1438, located 230 km to the southwest in the Amami Sankaku basin, thus the two sites may contain volcanic debris shed from contemporaneous sections of the KPR.