The composition and emplacement of basaltic magmas produced during the development of continental-margin basins: the Gulf of California, Mexico

Abstract

The Gulf of California is a region of active ensialic basin development and transform faulting Legs 64 and 65 of the Deep Sea Drilling Project have demonstrated the diversity of basement stratigraphy: drilling at Sites 474, 482, 483 and 485 at the mouth of the Gulf recovered massive basaltic flows and occasional pillow lavas and sills, whereas drilling in the central part of the Gulf, in the Guaymas Basin, recovered a sequence of sediments and massive doleritic sills. Very high sedimentation rates (over 1000 m/Ma) are characteristic of the Guaymas Basin and prevent extrusion of the basaltic magma onto the seafloor, thus resulting in at least partial intercalation of oceanic layers 1 and 2. Such a process of ocean crust formation could be locally important during the early stages of ocean and marginal basin development, when sedimentation rates may be very high Over 200 samples of basalts and dolerites from the Gulf of California have been analysed by X-ray fluorescence and neutron activation techniques. Basalts from the mouth of the Gulf are tholeiitic and contain very low abundances of the more-hygromagmatophile (more-HYG) elements (K, Rb, Ba, Th, Ta, Nb and light REE) relative to the HYG elements (Y, Zr, Ti and heavy REE), thus resembling depleted N-type MORB from elsewhere along the E Pacific Rise (EPR). Dolerites from the Guaymas Basin also have a tholeiitic chemistry and some EPR-basalt characteristics (e.g. La N ,/Sm N , < l), but they have higher La/Yb, Sr/Zr, Zr/Ti and Th/Hf ratios than the basalts from the Gulf mouth region. It is suggested that these differences are due in part to variations in mantle source composition, the mantle underlying the central and northern parts of the Gulf containing a minor sub-continental (residual calc-alkaline) component.