PHYSICAL AND CHEMICAL EVIDENCE FOR MAGMA DIVERSIFICATION IN A WET, LOWER-CRUSTAL MUSH ZONE, FIORDLAND, NEW ZEALAND

We use field observations and mineral geochemistry to investigate physical and chemical processes involved in mush remobilization in the lower crust of an Early Cretaceous continental magmatic arc (Fiordland, New Zealand). In the Misty Pluton, field mapping reveals three vertically stratified zones that record diorite crystallization, mafic melt injection, and melt remobilization at 40 to 25 km depth. From bottom to top, these zones include: 1) a basal ultramafic complex injected into diorite, 2) a middle zone of mostly hornblende and pyroxene diorites, and 3) an upper zone of hornblende diorites and felsic dikes. In the middle zone, mush textures are commonly observed where hornblende and pyroxene diorites are intruded by syn-magmatic ultramafic hornblendite sheets and dikes. Leucocratic segregations are concentrated near hornblendite sheets and are sometimes associated with peritectic garnets. Crystal exchange between hornblendite and diorite is common, suggesting physical mixing and hybridization in the presence of melt.

To explore chemical processes involved in melt remobilization, we studied 4 suites of coeval hornblendite, host diorite, and mush. Major-element data from amphiboles show that hornblendites crystallized at the highest temperatures (990-910°C), whereas host diorite and mush amphiboles record comparatively lower temperatures (930-850°C). All three groups were in equilibrium with wet (8-12% H₂O), andesite to dacite melts that range from 55 to 68 wt. % SiO₂. Mush amphiboles often show higher temperatures and lower SiO₂ contents compared to their host diorites indicating that they experienced chemical hybridization with hornblendites during reheating. Magmatic apatites and epidotes in mush samples also plot at intermediate compositions between diorites and hornblendites consistent with hybridization. In ΣREE vs. La/Sm space, apatites and epidotes in mush samples display relatively lower ΣREE and La/Sm compared to host samples, features which we interpret to reflect progressive dissolution-reprecipitation facilitated by reheating due to hornblendite injections into host diorites. We conclude that geochemical data show evidence for both hybridization and dissolution-reprecipitation, and these processes played important roles in promoting the generation of intermediate to felsic melts in a wet, lower-crustal mush zone.