MOBILIZATION OF INTERMEDIATE TO FELSIC MELTS IN A LOWER CRUSTAL MUSH ZONE FIORDLAND, NEW ZEALAND

The generation of continental crust in the deep crust of arcs is a longstanding problem with no consensus about the mechanisms of melt diversification and production of intermediate to felsic magmas. Here, we study the diversification of arc melts in a lower crustal magmatic mush (crystal + magma) zone located in the base of the Cretaceous Median Batholith, New Zealand. We focus on the Misty pluton which was emplaced at ca. 45-35 km depth and offers a unique opportunity to evaluate magma diversification processes in the lower crust of a cordilleran arc.

The Misty pluton is part of a tilted section that exposes three magma zones. From base to top these are: 1) an ultramafic section of hornblendite and hornblende peridotite; 2) a hybridization zone of host hornblende and pyroxene diorite and variably disaggregated hornblendite injections and 3) an upper zone of dominantly hornblende diorite and abundant felsic dikes. In the hybridization zone, host diorites were repeatedly injected by meter- to cm-scale sheets of hornblendite, which are spatially associated with felsic leucosomes that coalesced at hornblendite-diorite contacts. Centimeter-scale leucocratic veins emanate from these zones and cut across the diorite and hornblendite. In the upper zone, leucocratic melt channels range from small dikes and veins to large dikes. These features suggest that injected ultramafic magmas reheated and rejuvenated the dioritic crystal-rich mush leading to melt segregation and hybridization. Field observations suggest that melt extraction was also driven by compaction, non-coaxial viscous flow, and diking.

Petrographic evidence for mush processes is highlighted by relationships between magmatic hornblende, apatite, epidote, and plagioclase within the host diorites and hornblendite injections. Disequilibrium textures include sieve textures in hornblende and plagioclase, and late-stage apatite and epidote growth along grain boundaries and in fractured grains. In some cases, late-stage apatite-rich veins occur at contacts between hornblendite and diorites. Collectively, we interpret these textures to reflect evidence for mush rejuvenation in the base of the Median Batholith during an Early Cretaceous Separation Point Suite flare-up event.