Gabbroic rocks drilled at Sites 923 and 921 are cumulate rocks that crystallized from at least two distinct parental magmas. One parental magma was geochemically similar to basalts erupted near the Kane Fracture Zone. The second parental magma was substantially more depleted in incompatible trace elements. Primitive gabbros at Site 923 and 921 have abundant poikilitic clinopyroxene more magnesian (Mg# 88-84) than should crystallize from most mid-ocean ridge basalts (MORBs) at low pressure. This observation suggests that the gabbros drilled during Leg 153 could have formed at elevated pressure (2-5 kbar) or that they could have formed at low pressure from melts with compositions unlike erupted MORBs. Mass-balance modeling of incompatible trace-element abundances in gabbroic whole rocks suggests that these rocks are adcumulate to orthocumulate, with contents of trapped liquid that vary from near zero to about 30%. Electron probe and ion probe data show enrichments of incompatible trace elements in portions of clinopyroxene grains that are attributed to postcumulus crystal growth from or interaction with evolving intercumulus melts. Trace-element-enriched zones in clinopyroxene commonly show little or no change in major-element compositions. The zirconium-bearing phase baddeleyite has been observed in 11 samples of the 55 gabbros examined in this study. Its occurrence in these rocks is anomalous, because it is expected that silica-saturated magmas will ultimately crystallize zircon (ZrSiO4) at extreme degrees of fractionation, and not baddeleyite. The origin of baddeleyite in these rocks is enigmatic. Hornblende, apatite, and ilmenite are also common accessory phases in these rocks, occurring in samples with refractory mineral compositions. The presence of these phases suggests extensive crystallization of trapped intercumulus liquid, and yet extreme major-element zoning of plagioclase and mafic phases is not observed in rocks with late-stage phases.
Date of initial receipt: 31 July 1995
Date of acceptance: 13 May 1996
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