14. DETRITAL CHROMIAN SPINELS FROM SITE 960 IN THE CÔTE D’IVOIRE-GHANA TRANSFORM MARGIN1

Ken-ichiro Hisada,2 Shoji Arai,3 and Takako Yamaguchi2

ABSTRACT

Ocean Drilling Program Leg 159 drilled the Côte d’Ivoire-Ghana Marginal Ridge in the Atlantic Ocean. Among four drilled sites (959–962), Site 960 yielded sediments ranging in age from the Holocene to the Cretaceous. Fifteen detrital chromian spinel grains were found during petrographic examination of 30 thin sections of sandstone recovered from probably the middle Cretaceous (late Albian) Unit V at Site 960, which comprises Subunits VA and VB. High and relatively constant Cr/(Cr+Al) atomic ratios of 0.7 to 0.8 and variable TiO2 content characterize these spinels. The spinels can be classified into low- and high-Ti groups. Low-Ti chromian spinels are more abundant in Subunit VB. The high-Ti group is very similar in chemistry to chromian spinels from intraplate and flood basalts (e.g., the Hawaiian tholeiites). It is inferred that the magma for the source rocks of these high-Ti chromian spinels was of an intraplate- or plume-type. The magma for the source rocks of the low-Ti group may have originated from the relatively Ti-depleted lithospheric mantle below the rifting continental crust. The igneous activity that produced the chromian spinels found in Leg 159 sediments may have been related to the continental breakup, having formed the Atlantic Ocean during the Barremian to Aptian stages.

1Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998, Proc. ODP, Sci. Results, 159: College Station, TX (Ocean Drilling Program).
2Institute of Geoscience, University of Tsukuba, Tsukuba, Ibaraki 305, Japan. hisadak@arsia.geo.tsukuba.ac.jp
3Department of Earth Sciences, Kanazawa University, Kanazawa 920-11, Japan.