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 indicates the following temperatures for equilibration of assemblages at a nominal pressure of 12 kb: garnet-augite ∼980°C; garnet-augite-hornblende ∼920–1020 °C. Geobarometry on a single garnet—orthopyroxene-bearing xenolith indicates a pressure of 10–12 kb for a likely temperature range of 950–1000°C. Thus the xenoliths point to the generation of host andesite-dacite magmas at suberustal depths of 35–45 km, from fractional crystallization of more mafic mantlederived 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.