Gabbroic rocks recovered from Holes 923A and 921E in the MARK area include troctolite, clinopyroxene-olivine gabbro, olivine-clinopyroxene gabbro, gabbro, gabbronorite, oxide gabbro, and quartz diorite. Clinopyroxene and plagioclase occur in all rock types, whereas olivine is supplanted by orthopyroxene in the evolved rock types and in the intercumulus stage. Forsterite (Fo) content of olivine, magnesium number (Mg#) of pyroxenes, and anorthite (An) content of plagioclase systematically decrease with decreasing modal olivine content, and plagioclase is most sodic in the quartz diorite. Sr- and Nd-isotopic compositions are characteristic of N-type MORB.

Date of initial receipt: 6 September 1995
Date of acceptance: 1 February 1996

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