Clinopyroxene REE geochemistry of the Red Hills peridotite, New Zealand: Interpretation of magmatic processes in the upper mantle and in the Moho transition zone

The Red Hills peridotite in the Dun Mountain ophiolite of South Island, New Zealand, is assumed to have been produced in a paleo-mid-ocean ridge tectonic setting. The peridotite is composed mostly of harzburgite and dunite, which represent residual mantle and the Moho transition zone (MTZ), respectively. Dunite channels within harzburgite blocks of various scales represent the MTZ component. Plagioclase- and clinopyroxene-bearing dunites occur sporadically within common dunites. These dunites represent products of melt-wall-rock interaction. Chondrite-normalized rare earth element (REE) patterns of MTZ clinopyroxenes show a wide compositional range. Clinopyroxenes in plagioclase dunites are extremely depleted in light REE (LREE) ([Lu/La](N) > 100), and are comparable with clinopyroxenes in abyssal peridotites from normal mid-ocean ridges. Interstitial clinopyroxenes in the common dunite have flatter patterns ([Lu/La](N) similar to 2) comparable with those for dunite in the Oman ophiolite. Clinopyroxenes in the lower part of the residual mantle harzburgites are even more strongly depleted in LREE ([Lu/La](N) = 100-1000) than are mid-ocean ridge peridotites, and rival the most depleted abyssal clinopyroxenes reported from the Bouvet hotspot. In contrast, those in the uppermost residual mantle harzburgite and harzburgite blocks in the MTZ are less LREE depleted ([Lu/La](N) = 10-100), and are similar to those in plagioclase dunite. Clinopyroxenes in the clinopyroxene dunite in the MTZ are similar to those reported from mid-ocean ridge basalt (MORB) cumulates, and clinopyroxenes in the gabbroic rocks have compositions similar to those reported from MORB. Strong LREE and middle REE (MREE) depletion in clinopyroxenes in the harzburgite suggests that the harzburgites are residues of two-stage fractional melting, which operated initially in the garnet field, and subsequently continued in the spinel lherzolite field. The early stage melting produced the depleted harzburgite. The later stage melting was responsible for the gabbroic rocks and dunite. Strongly LREE-MREE-depleted clinopyroxene in the lower harzburgite and HREE-enriched clinopyroxene in the upper harzburgite and plagioclase dunite were formed by later reactive melt migration occurring in the harzburgite.