The ultramafic rocks drilled at Site 920 are intruded by dikes, dikelets, and impregnation lenses of texturally, mineralogically, and chemically diverse gabbroic rocks. These rocks represent about 4% in volume of the core recovered at Site 920. They range from impregnation lenses with magnesium-rich clinopyroxene and calcic plagioclase, to zircon-bearing dikelets with iron-rich clinopyroxene and sodium-rich plagioclase. In terms of mineralogy and chemical diversity, they are similar to gabbroic rocks that form large outcrops a few tens of kilometers north of Site 920 (Sites 921-924) and to rocks drilled near the Southwest Indian Ridge at Site 735. The chemical diversity of gabbroic rocks recovered at Site 920 is reflected in their order of crystallization in the peridotites, the most primitive rocks crystallizing first, and the most fractionated ones, last. Chemical modifications in the ultramafic rocks near the gabbroic intrusions range from the formation of decimeter-thick dunite screens to cryptic variations in olivine Mg# and nickel content, in clinopyroxene sodium content, and in spinel Mg# and titanium content. We propose a model in which the gabbroic rocks recovered at Site 920 crystallized from melts that had undergone variable degrees of differentiation in a root of lithospheric mantle beneath the Mid-Atlantic Ridge.

Date of initial receipt: 7 August 1995
Date of acceptance: 25 January 1996

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