HIGH-Mg ARC-ANKARAMITIC DIKES, GREENHILLS COMPLEX, SOUTHLAND, NEW ZEALAND

Minor dikes 8–70 cm thick of ankaramitic composition were emplaced late in the crystallization of the Greenhills Complex, a layered dunite – wehrlite – hornblende gabbronorite intrusion of Permian age in the volcanic arc-derived Brook Street Terrane of southern New Zealand. Mg contents range from 10 to 28 wt % MgO. The interior of a hornblende ankaramite dike at Steep Head contains phenocrysts of olivine Fo93–82, in some cases skeletal, and scattered phenocrysts of diopside, set in a groundmass of magnesiohornblende. The chilled margins of this dike consist almost wholly of amphibole (91%) and anorthite, An99–94 (9%). In other dikes, crystallization of amphibole followed that of olivine, diopside, enstatite, and overlapped with plagioclase. Olivine in these dikes varies in composition from Fo90 to Fo72. Diopside contains oriented blebs of magnesiohornblende and edenite. We believe that the components of these amphibole inclusions were in part introduced and in part derived from the host pyroxene by a process of reactive exsolution. Spinel varies from chromite containing 53 wt % Cr2O3 through compositions at first increasingly Al-rich, and then through chromian magnetite to virtually Cr-free magnetite. Amphiboles range from magnesiohornblende to edenite, pargasite, and tschermakite, with minor late-stage high-Si magnesiohornblende. The aphyric margin of the Steep Head dike contains 17.4 wt % MgO on a volatile-free basis. Some of the melts may have exceeded this MgO content. Dike centers are more Mg-rich than margins, reaching 28.3 % in one case, in part an effect of concentration of phenocrysts by flow differentiation, and in part a result of changing composition of the melt. The presence of abundant magmatic amphibole, especially in the most