ALBERT

Description: This study documents the geochemical heterogeneity introduced into mantle lithosphere at the onset of subduction, where an essentially homogeneous mantle asthenosphere is converted into a compositionally heterogeneous mantle lithosphere as a result of increasingly focused melt ascent at shallower levels. We report field observations and geochemical results from the Red Hills ultramafic massif, part of the Dun Mountain ophiolite belt of New Zealand. In the east, the Red Hills massif contains geochemically homogeneous harzburgites (Two Tarns Harzburgite). On its western and southern edge, the harzburgites are overprinted by a petrologically zoned sequence of foliated plagioclase lherzolite and plagioclase harzburgite on the periphery (Plagioclase Zone), and banded harzburgite and dunite (Plateau Complex) in the interior. The various lithological units of the Red Hills massif are interpreted to have formed as a result of a polygenetic, multi-stage melting and refertilization history. The Two Tarns Harzburgite is interpreted to record c. 10–15% melting in the garnet stability field followed by an additional 10% melting in the spinel stability field. Early garnet field melting is defined as Stage 1, and probably occurred in a mid-ocean ridge setting in a relatively reduced environment of around log f O 2 (FMQ) –1·5 (where FMQ is the fayalite–magnetite–quartz buffer) or lower. Stage 2 is defined as spinel field melting and melt interaction. Melt interaction trends for Stage 2 indicate that Stage 2 melts were relatively oxidized, at around log f O 2 (FMQ) +1, and boninitic in composition, suggesting a forearc subduction initiation setting for Stage 2. In the western massif, later Stage 3 melts petrologically overprinted rocks once part of the Two Tarns Harzburgite. The plagioclase-rich contact (Plagioclase Zone) between the Two Tarns Harzburgite and the Plateau Complex is interpreted to represent a refertilization front between the Two Tarns Harzburgite and an upper mantle melt transport network (Plateau Complex; Stage 3), where melts became focused into kilometer-scale conduits at shallow depths in the plagioclase stability field. Highly anorthitic plagioclase suggests that Stage 3 melts were also associated with a forearc, subduction-zone setting. Within the Stage 3 melt network, pervasive major element and cryptocrystalline metasomatism altered peridotite originally similar to the Two Tarns Harzburgite. Our results indicate that pulses of pervasively migrating melts (Stage 2) rose and were focused into narrower conduits (Stage 3) towards the surface in a forearc suprasubduction-zone setting. Because shallow melt migration is localized into narrow zones, a significant percentage of the mantle lithosphere in forearcs may be relict from earlier pervasive mid-ocean ridge melting or deeper forearc melting, as seen in the Red Hills.