Testing the 'MASH' Hypothesis: An Investigation of Magma Chamber Geometry and Interconnectivity 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 interconnectivity of lower-crustal melts in the Early Cretaceous Median Batholith. The Malaspina Pluton represents the exhumed lower crust (45-50 km depth) of a continental arc, which was emplaced at ca. 118-115 Ma during a high-volume magmatic event. Related igneous assemblages include plagioclase + clinopyroxene + orthopyroxene + amphibole ± biotite. In conjunction with whole-rock data, 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 by dividing the pluton into four different zones based on the dip of magmatic foliations, we aim to investigate the extent and length scale of melt homogenization. Amphibole trace-element data reveal two distinct groups within the Malaspina Pluton: one relatively enriched and one relatively depleted when compared with one another. The enriched amphibole group, for example, shows Zr values ranging from ~25-110 ppm, Nb values ~5-32 ppm, and Th values ~0-2.4 ppm. It shows enrichment in terms of light rare-earth element values (10-500x chondritic values) and middle and heavy rare-earth element values 10-70x chondritic values. The depleted group, in contrast, shows Zr values ranging from ~0.03-35 ppm, Nb values ~0-0.25 ppm, and Th is generally below the level of detection. Depleted group light rare-earth element values range from 0.2-3x chondritic values, and middle and heavy rare-earth element values are ~2-40x chondritic values. Calculated amphibole-equilibrium melts based on mineral-melt partition coefficients also show the same trace-element enriched and depleted groups. Calculated amphibole crystallization temperatures have an overall range of ~965-845 °C, and the samples show wide variability in terms of the temperatures at which they initially began to crystallize. The enriched and depleted groups, however, are not significantly different from one another in terms of their calculated crystallization temperatures. Based on the presence of multiple distinct melt groups, we conclude that rather than homogenizing in one large magma chamber, the melts were instead emplaced as individual discrete subhorizontal sheets which had limited chemical communication with one another. Based on enrichment of fluid-immobile elements such as Zr, Th, and Nb, we interpret these differences to be the result of sediment melt contributions that enriched some of the melts in terms of trace-element abundances.