TESTING THE "MASH" HYPOTHESIS: AN INVESTIGATION OF MAGMA CHAMBER GEOMETRY ANDINTERCONNECTIVITY IN THE LOWER CRUST OF A CONTINENTAL ARC, FIORDLAND, NEW ZEALAND

We present single-mineral major and trace element data from 14 magmatic samples collected from the Malaspina Pluton (Fiordland, New Zealand) with the goal of testing the geometry and interconnectivitiy of lower-crustal melts in the Cretaceous Median Batholith. The Malaspina Pluton represents the exhumed lower crust (40-50 km depth) of a continental arc, which was emplaced at ca. 118-115 Ma. Igneous assemblages include plagioclase + clinopyroxene + orthopyroxene + hornblende +/- biotite. We analyzed 364 spots on 51 in situ igneous amphibole grains via electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry. Our samples cover 550 km² of the lower crust, and we divide the pluton into four different magmatic zones based on the dip of magmatic foliations.

Amphibole trace-element data from the four zones reveal two distinct geochemical groups within the Malaspina Pluton: one relatively enriched and one relatively depleted. The enriched amphibole group, have Zr values ranging from ~25-110 ppm, Nb values ~5-32 ppm, and Th values ~0-2.4 ppm. Enriched amphiboles show strongly enriched chondrite-normalized LREE/HREE patterns. The depleted group, in contrast, shows lower Zr values ranging from ~0.03-35 ppm, Nb values ~0-0.25 ppm, and Th is generally below the level of detection. In chondrite-normalized plots, depleted group amphiboles have strongly depleted LREE/HREE patterns similar to N-MORB. Average crystallization temperatures are 920 +/- 20 (1sd) for the enriched group and 900 +/- 26 (1sd) for the depleted group. We propose that melts in equilibrium with enriched amphiboles were sourced predominantly from mantle-wedge melts with a small sediment contribution. In contrast, melts in equilibrium with depleted amphiboles had little to no sediment influence. We concluded that Malaspina melts were emplaced as discrete sheets which had limited chemical communication, and there was no long-range (km-scale) homogenization.