ORIGIN OF NAURU BASIN IGNEOUS COMPLEX - SR, ND AND PB ISOTOPE AND REE CONSTRAINTS

The tholeiitic basalts and microdolerites that comprise the Cretaceous igneous complex in the Nauru Basin in the western equatorial Pacific have moderate ranges in initial Sr-87/Sr-86 (0.70347-0.70356), initial Nd-147/N-144(0.51278-0.51287), and measured Pb-206/Pb-204 (18.52-19.15), Pb-207/Pb-204 (15.48-15.66) and Pb-208/Pb-204 (38.28-38.81). These isotopic ratios overlap with those of both oceanic island basalts (OIB) and South Atlantic and Indian mid-ocean ridge basalts (MORB). However, the petrography, mineralogy, and bulk rock chemistry of the igneous complex are more similar to MORB than to OIB. Also, the rare earth element contents of Nauru Basin igneous rocks are uniformly depleted in light elements (La/Sm(ch) < 1) indicative of a mantle source compositionally similar to that of MORB. These results suggest that the igneous complex is the top of the original ocean crust in the Nauru Basin, and that the notion that the crust must be 15 to 35 m.y. older based on simple extrapolation and identification of the M-sequence magnetic lineations [1,2] may be invalid because of a more complicated tectonic setting. The igneous complex most probably was extruded from an ocean ridge system located near the anomalously hot, volcanically active, and isotopically distinct region in the south central Pacific which has been in existence since approximately 120 Ma.