Magma genesis of Miocene adakites in the southern Patagonia by Zircon geochemistry

HIRATA D; ORIHASHI Y; MOTOKI A; HALLER MJ; RAMOS VA; YOSHIDA H; ANMA R

Pucón

Congreso; International Association of Volcanology and Chemistry of the Earth's Interior, General Assembly; 2004

Zircon occurs extensively in most igneous rocks, and remains stable against weathering, alteration and plutonic/metamorphic processes due to chemical resistance and refractory. Some of igneous zircons are possibly crystallized in earlier stage of magmatic process, and therefore zircon geochemistry combined with U-Pb dating can provide advanced petrogenetic information. Recent studies have demonstrated geochemical signature for igneous zircons could be used to discriminate most rock type. However, the geochemical characteristics of zircon in adakite are still unknown. We have determined major and trace element compositions and U-Pb ages, using EPMA and LA-ICP-MS, of about 50 zircon crystals each extracted from the Mid-Miocene adakite bodies in Cerro Pampa (CP; 12 Ma) and Puesto Nuevo (PN; 16 Ma, unpublished data), Patagonian province. Our result has shown that all CP zircons were recycled and originated from pre-Tertiary country rocks (123–1,766 Ma). In contrast, most PN zircons were crystallized at the time of generation of the adakitic magma in Miocene times [14.1±3.6 Ma (SD)]. The REE patterns for PN zircons showed positive Ce and slightly negative Eu anomalies and their slopes from La to Lu were gentler than those of arc-granitic rocks. Estimated coexisting felsic melt from average of PN zircon compositions indicated 3–5 times higher concentrations in REE and slightly larger Gd/YbN than whole rock composition of PN adakite. This suggests that PN zircons were disequilibrated to the whole rock composition, and that they might be crystallized in melt pockets in which over 80% of original adakitic magma interacted with mantle peridotite above the subducted slab.