SUBDUCTION ZONE GEOCHEMICAL CHARACTERISTICS IN OCEAN RIDGE BASALTS FROM THE SOUTHERN CHILE RIDGE - IMPLICATIONS OF MODERN RIDGE SUBDUCTION SYSTEMS FOR THE ARCHEAN

The southern portion of the Chile Ridge is one of few sites where acti ve subduction of a spreading center and its consequences for ridge axi s magmatism can be investigated. New major element, trace element, and isotopic data for lavas recovered from the ridge axis between 43 degr ees S and 46 degrees 20'S of the southern Chile Ridge have revealed a suite of mid-ocean ridge basalts which possess typical major element v ariations, but diverse and sometimes unusual trace element characteris tics, For several Chile Ridge lavas, key trace element ratios, such as Rb/Cs, Ce/Pb, Nb/U, La/Ta, Hf/Th and Nb/La, extend well outside the f ields for normal MORE or ocean island basalts and have values more com monly associated with are volcanics and continental crust. This hybrid mixture between MORE-like major elements and are-like trace element s ignatures has only previously been seen in back-are basins, and is con sidered to primarily reflect contamination of a depleted MORE source m antle with slab-derived components. Along the southern Chile Ridge, co ntamination with slab components is occurring in advance of the subduc tion zone, possibly as a result of slab break-up or shearing in conjun ction with subduction of young, buoyant lithosphere, and subsequent en trainment of these slab components into the sub-ridge mantle. Interest ingly, many Archean greenstone basalts share the unusual hybrid MORE-a re geochemical characteristics found along the southern Chile Ridge. O n the basis of theoretical modeling, it has been suggested that the ma ntle was hotter, plate motions were more rapid and ridge-trench intera ctions were more frequent during the Archean. Although use of geochemi cal signatures to discriminate tectonic setting must be approached wit h caution, the observed geochemical affinity of modem lavas from the s outhern Chile Ridge and some Archean greenstone lavas lends support to the idea that ridge subduction may have been an important mechanism i n the formation of Archean greenstone basalts.