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 consistingof hornblendegarnetplagioclase clinopyroxene, occur in associationwith high-pressure phenocrysts in early Miocene high-silicaandesites and dacites, Northland, New Zealand. Microstructuresof these xenoliths range from coarse, even-grained sub-ophitictypes to others with coarse glomerocrysts set in a finer-grainedmesostasis. The xenoliths are commonly flow-banded and are arguedto represent direct crystallization products and crystal aggregationsfrom the calc-alkaline host or related magmas at depth. Manygarnets within these high-pressure aggregates and also discretegarnet phenocrysts are rimmed by medium—coarse-grained,interlocking hornblendeplagioclase, representing partial adjustmentto an assemblage stable at shallower levels. The garnets aretypically pyrope—almandine with 17–28 mol.% grossularand show normal, reverse and oscillatory zoning; the associatedamphibole is pargasite trending to hornblende in phenocrystrims and reaction rims. Metamorphic xenoliths with plagioclase-hornblende-quartzassemblages are also found in the rocks and are characterizedby fine-grained granoblastic mosaic microstructures with well-developedfoliation defined by preferred orientation of elongate grainsand a mineral layering. These metamorphic xenoliths are interpretedas fragments of lower-crustal country rocks accidentally incorporatedinto rising andesitic magma. Application of established experimental high-pressure phasediagrams for andesites indicates crystallization of these assemblagesat depths corresponding to 10–20-kb pressure, and appropriategeothermometers indicates the following temperatures for equilibrationof assemblages at a nominal pressure of 12 kb: garnet-augite980C; garnet-augite-hornblende 920–1020 C. Geobarometryon a single garnet—orthopyroxene-bearing xenolith indicatesa pressure of 10–12 kb for a likely temperature rangeof 950–1000C. Thus the xenoliths point to the generationof host andesite-dacite magmas at suberustal depths of 35–45km, from fractional crystallization of more mafic mantlederivedmagmas, and demonstrate that relatively silicic calc-alkalinemagmas may evolve in the mantle. The rarity of evidence forsuch a process may be linked with the obduction-related tectonicevents operative in Northland just before the magmatic episode,and to the unusually high water content in the magma.