The structural and petrologic evolution of the Red Hills ultramafic massif: Coupled melt migration and deformation during subduction initiation.

摘要

The Red Hills ultramafic massif, located on the South Island of New Zealand, contains a three-stage history of overprinting deformation which occurred during the onset of Permian subduction along the Australian-New Zealand portion of the Gondwanan margin. Geologic mapping of the massif delineates four compositional units which are, from east to west: Massive harzburgites (two tarns harzburgite); Plagioclase lherzolites and plagioclase harzburgites (plagioclase zone); Banded dunite, harzburgite, and lherzolite (Plateau complex); Banded dunite and harzburgite (Ellis Stream complex). Geothermobarometry, major and trace element geochemistry, geochronology, and structural analysis are combined with the geologic mapping to place the history of the rocks in an incipient subduction environment. The kinematic history of the rocks is ultimately tied to transtensional plate boundary conditions imposed by the foundering of oceanic crust and initiation of subduction along an active oceanic transform margin. The style of strain accommodating transtensional deformation was largely a function of the distribution of melt during exhumation and cooling of the massif. At 1200° and at pressures >6 kbar (Stage 1), melts were homogeneously distributed, and strain was homogeneous and highly constrictional. At 1000°C and 5 kbar pressure (Stage 2), melts were focused into kilometer scale conduits, and deformation produced lineated and foliated rocks that overprinted the earlier constrictional fabrics. When no melts were present at temperatures less than 600°C (Stage 3), deformation became highly localized into serpentinized faults. In addition to its influence on the kinematic history of the massif, the two stages of melt migration altered the composition of the massif. Early melting associated with the first stage of deformation produced depleted harzburgites with a small range in whole-rock compositions. During Stage 2, melts were channelized and initially undersaturated in orthopyroxene, producing dunite bands. During the waning phases of Stage 2, melts became saturated in clinopyroxene +/- plagioclase, resulting in local refertilization of the massif. Refertilization was particularly prevalent along the margins and upper terminations of Stage 2 melt conduits, forming a refertilization front enriched in plagioclase and clinopyroxene, but presumably due to lower melt fluxes, lacking dunite bands. A key result is that melt migration, not any pre-existing compositional heterogeneity, plays a fundamental role in the kinematic history of the massif. Rather, the compositional heterogeneity within the massif is the end result of multiple episodes of melt migration and refertilization during the earliest stages of subduction. Thus the Red Hills ultramafic massif provides a pristine record of how the mantle flows and changes composition as a result of melt migration during the earliest stages of subduction.