EAST MARIANA BASIN THOLEIITES - CRETACEOUS INTRAPLATE BASALTS OF RIFTBASALTS RELATED TO THE ONTONG JAVA PLUME

Studies of seafloor magnetic anomaly patterns suggest the presence of Jurassic oceanic crust in a large area in the western Pacific that inc ludes the East Mariana, Nauru and Pigafetta Basins. Sampling of the ig neous crust in this area by the Deep Sea Drilling Program (DSDP) and t he Ocean Drilling Program (ODP) allows direct evaluation of the age an d petrogenesis of this crust. ODP Leg 129 drilled a 51 m sequence of b asalt pillows and massive flows in the central East Mariana Basin. Ar- 40/Ar-39 ages determined in this study for two Leg 129 basalts average 114.6 +/- 3.2 Ma. This age is in agreement with the Albian-late Aptia n paleontologic age of the overlying sediments, but is distinctively y ounger than the Jurassic age predicted by magnetic anomaly patterns in the basin. Compositionally, the East Mariana Basin basalts are unifor mly low-K tholeiites that are depleted in highly incompatible elements compared to moderately incompatible ones, which is typical of mid-oce an ridge basalts (MORB) erupted near hotspots. The Sr, Nd and Pb isoto pic compositions of the tholeiites (Sr-87/Sr-86(init) = 0.70360-0.7037 4; Nd-143/Nd-144(init) = 0.512769-0.512790; Pb-206/Pb-204(meas) = 18.3 55-18.386) also overlap with some Indian Ocean Ridge MORB, although th ey are distinct from the isotopic compositions of Jurassic basalts dri lled in the Pigafetta Basin, the oldest Pacific MORB. The isotopic com positions of the East Mariana Basin tholeiites are also similar to tho se of intraplate basalts, and in particular, to the isotopic signature of basalts from the nearby Ontong Java and Manihiki Plateaus. The Eas t Mariana Basin tholeiites also share many petrologic and isotopic cha racteristics with the oceanic basement drilled in the Nauru Basin at D SDP Site 462. In addition, the new 110.8 +/- 1.0 Ma Ar-40/Ar-39 age fo r two flows from the bottom of Site 462 in the Nauru Basin is indistin guishable from the age of the East Mariana Basin flows. Thus, while ma gnetic anomaly patterns predict that the igneous basement in the Nauru and East Mariana Basins is Jurassic in age, the geochemical and chron ological results discussed here suggest that the basement formed durin g a Cretaceous rifting event within the Jurassic crust. This magmatic and tectonic event was created by the widespread volcanism responsible for the genesis of the large oceanic plateaus of the western Pacific.