THE ORIGIN AND SIGNIFICANCE OF GARNET PHENOCRYSTS AND GARNET-BEARING XENOLITHS IN MIOCENE CALC-ALKALINE VOLCANICS FROM NORTHLAND, NEW-ZEALAND

Xenoliths, considered to be of igneous origin and consisting of hornblende +/- garnet +/- plagioclase +/- clinopyroxene, occur in association with high-pressure phenocrysts in early Miocene high-silica andesites and dacites, Northland, New Zealand. Microstructures of these xenoliths range from coarse, even-grained sub-ophitic types to others with coarse glomerocrysts set in a finer-grained mesostasis. The xenoliths are commonly flow-banded and are argued to represent direct crystallization products and crystal aggregations from the calc-alkaline host or related magmas at depth. Many garnets within these high-pressure aggregates and also discrete garnet phenocrysts are rimmed by medium-coarse-grained, interlocking hornblende +/- plagioclase, representing partial adjustment to an assemblage stable at shallower levels. The garnets are typically pyrope-almandine with 17-28 mol.% grossular and show normal, reverse and oscillatory zoning; the associated amphibole is pargasite trending to hornblende in phenocryst rims and reaction rims. Metamorphic xenoliths with plagioclase-hornblende-quartz assemblages are also found in the rocks and are characterized by fine-grained granoblastic mosaic microstructures with well-developed foliation defined by preferred orientation of elongate grains and a mineral layering. These metamorphic xenoliths are interpreted as fragments of lower-crustal country rocks accidentally incorporated into rising andesitic magma. Application of established experimental high-pressure phase diagrams for andesites indicates crystallization of these assemblages at depths corresponding to 10-20-kb pressure, and appropriate geothermometers indicate the following temperatures for equilibration of assemblages at a nominal pressure of 12 kb: garnet-augite approximately 980-degrees-C; garnet-augite-hornblende approximately 920-1020-degrees-C. Geobarometry on a single garnet-orthopyroxene-bearing xenolith indicates a pressure of 10-12 kb for a likely temperature range of 950-1000-degrees-C. Thus the xenoliths point to the generation of host andesite-dacite magmas at subcrustal depths of 35-45 km, from fractional crystallization of more mafic mantle-derived magmas, and demonstrate that relatively silicic calc-alkaline magmas may evolve in the mantle. The rarity of evidence for such a process may be linked with the obduction-related tectonic events operative in Northland just before the magmatic episode, and to the unusually high water content in the magma.