INVESTIGATING THE TRACE-ELEMENT RECORD OF MAGMA MIXING IN MINGLED MAFIC SHEETS, HALFMOON PLUTON, STEWART ISLAND, NEW ZEALAND

The Halfmoon Pluton located on Stewart Island, New Zealand is a classic example of a shallow-level magma chamber, and provides an opportunity to investigate the role of magma mixing in the geochemical evolution of arc magmas. The Halfmoon Pluton occurs within the composite Bungaree Intrusives and outcrops along the northern coast of Stewart Island. It was emplaced at ca. 140 Ma along the southeastern margin of Gondwana and is characterized by a sequence of mingled mafic sheets and enclaves that are preserved with intermediate to felsic host rocks. Although field evidence for mingling is ubiquitous, we investigate whether magma mixing also played an important role in the geochemical evolution of the Halfmoon pluton.

We focus on four areas of the pluton which from top to bottom include: The Neck, Ringaringa, Evening Cove and Ackers Point. Twelve samples were selected to include all rock types including mingled sheets, mafic enclaves and host leucodiorite. Data from thirty-six single-crystal laser ablation transects of magmatic amphiboles reveal complex results that cannot be explained by simple, closed-system fractional crystallization processes. Instead, our results strongly suggest complex mixing interactions between the leucodiorite host and episodically injected mafic melt in the Halfmoon Pluton.