C. D. Deering

Cannibalization of an amphibole-rich andesitic progenitor induced by caldera-collapse during the Matahina eruption: Evidence from amphibole compositions

Abstract The diverse range of calcic-amphibole compositions in eruptive products from the ca. 330 ka Matahina eruption (ca. 160 km3 rhyolitic magma) of the Okataina Volcanic Complex, Taupo Volcanic Zone, including crystal-rich basalt-dacite pumice from post-collapse deposits, reveals several pre- and syn-eruption magmatic processes. (1) Amphibole phenocrysts in the basaltic-andesite and andesite crystallized at the highest pressures and temperatures (P to 0.6 ± 0.06 GPa and T to 950 °C), equivalent to mid-crustal depths (13-22 km). Inter- and intra-crystalline compositions range from Timagnesiohastingsite → Ti-tschermakite → tschermakite → magnesiohornblende and some display gradual decreases in T from core to rim, both consistent with magma differentiation by cooling at depth. (2) The largest amphibole crystals from the basaltic-andesite to andesite display several core to rim increases in T (to 70 °C), indicating that new, hotter magma periodically fluxed the crystal mush. (3) The dominant population of rhyolite amphibole is small and bladed (magnesiohornblende) and crystallized at low P-T conditions (P = 0.3 GPa, T = 765 °C), equivalent to the eruptive P-T conditions. Dacitic and low-silica rhyolitic amphibole (tschermakite-magnesiohornblende) form two distinct populations, which nucleated at two different T (high: 820 °C and low: 750 °C). These compositional variations, governed primarily by differences in T conditions during crystal growth, record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive temperature. Therefore, the diversity in amphibole compositions can be reconciled as an exchange of crystals + liquid between the basaltic-andesite to dacite from the mid-crust and rhyolite from the upper crust, which quenched against one another, modifying the dacite to low-silica rhyolite compositions as the eruption progressed.

A model for the origin of rhyolites from South Mountain, Pennsylvania: Implications for rhyolites associated with large igneous provinces

High-silica rhyolites, ubiquitous features of continental volcanism, continue to evoke controversy as to their petrogenesis and evolution. We utilized the geochemical characteristics of late Vendian high-silica rhyolites erupted in the Catoctin Volcanic Province at South Mountain in Pennsylvania to probe the origin of the parental magmas and assess heterogeneities in the subsequent fractionation paths. We identified high- and low-Ti signatures within the South Mountain rhyolites, a common feature in many large igneous provinces, and these signatures are suggestive of a genetic link between basalts and rhyolites erupted in the Catoctin Volcanic Province. Two evolutionary trends are superimposed on the Ti-based subdivisions that reflect variable control of plagioclase and amphibole in the fractionating assemblage of the South Mountain rhyolites. Such distinctive evolutionary trends are evident in rhyolites from other tectonic settings (e.g., arcs), where they have been interpreted in terms of cold-wet and hot-dry conditions within the differentiating magmas. We interpret the amphibole-dominated fractionation path of the South Mountain rhyolites as following a cold-wet fractionation path compared to the hot-dry plagioclase-dominated trends. This study, which examines the geochemical implications of cryptic amphibole fractionation, has implications for assessing the role of amphibole and volatile content in the development of rhyolites in other large igneous provinces.