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