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Active felsic magmatism has been rarely probedin situby drilling but one recent exception is quenched rhyolite sampled during the 2009 Iceland Deep Drilling Project (IDDP). We report finding of rare zircons of up to ∼100 µm in size in rhyolite glasses from the IDDP-1 well products and the host 1724 AD Viti granophyres. The applied SHRIMP U-Th dating for both the IDDP and the Viti granophyre zircons gives zero-age (±2 kyr), and therefore suggests that the IDDP-1 zircons have crystallized from an active magma intrusion rather than due to the 20–80 ka post-caldera magmatic episodes recorded by nearby domes and ridges. Ti-in-zircon geothermometer for Viti granophyre reveals zircon crystallization temperatures ∼800°C–900°C, whereas IDDP-1 rhyolite zircon cores show Ti content higher than 100 ppm, corresponding to temperatures up to ∼1,100°C according to the Ti-in-zircon thermometer. According to our thermochemical model at such elevated temperatures as 1,100°C, rhyolitic magma cannot be saturated with zircon and zircon crystallization is not possible. We explain this controversy by either kinetic effects or non-ideal Ti incorporation into growing zircons at low pressures that start to grow from nucleus at temperatures ∼930°C. High temperatures recorded by IDDP-1 zircon together with an occurrence of baddeleyite require that the rhyolite magma formed by partial melting of the host granophyre due to basaltic magma intrusion. Zr concentration profiles in glass around zircons are flat, suggesting residence in rhyolitic melt for >4 years. In our thermochemical modeling, three scenarios are considered. The host felsite rocks are intruded by: 1) a basaltic sill, 2) rhyolite magma 3) rhyolite sill connected to a deeper magmatic system. Based on the solution of the heat conduction equation accounting for the release of latent heat and effective thermal conductivity, these data confirm that the rhyolite magma could be produced by felsic crust melting as a result of injection of a basaltic or rhyolite sill during the Krafla Fires eruption (1975 AD). 

Kiruna-type iron oxide−apatite (IOA) deposits constitute an important source of iron and phosphorus, and potentially of rare earth elements (REE). However, the origin of IOA deposits is still a matter of debate with models that range from a purely magmatic origin by liquid immiscibility to replacement of host rocks by hydrothermal fluids from different sources. In order to better constrain the origin of Andean IOA deposits, we focused on the Cretaceous Cerro Negro Norte deposit located in the Chilean Iron Belt, northern Chile. The Cerro Negro Norte magnetite ore is hosted in andesitic rocks and is spatially and genetically associated with a diorite intrusion. Our results show that the deposit is characterized by three main mineralization/ alteration episodes: an early Fe–oxide event with magnetite and actinolite followed by four stages that comprise the main hydrothermal event (hydrothermal magnetite + actinolite; calcic–sodic alteration + sulfides; quartz–tourmaline and propylitic alteration) and a minor supergene event. Based on textural and chemical characteristics, four different types of magnetite are recognized at Cerro Negro Norte: type I, represented by high-temperature (~ 500 °C) magnetite cores with amphibole, pyroxene, and minor Ti–Fe oxide inclusions; type II, an inclusion-free magnetite, usually surrounding type I magnetite cores; type III corresponds to an inclusion-free magnetite with chemical zoning formed under moderate temperatures; and type IV magnetite contains abundant inclusions and is related to low-temperature (~ 250 °C) hydrothermal veinlets. Electron probe and laser ablation ICP-MS analyses of the four magnetite types show that the incorporation of Al, Mn, Ti, and V into the magnetite structure is controlled by temperature. Vanadium and Ga concentrations are relatively constant within each magnetite type, but are statistically different among magnetite types, suggesting that both elements could be used to discriminate between magmatic and hydrothermal magnetite. However, our results show that the use of elemental discrimination diagrams should be coupled with detailed textural studies in order to identify superimposed metasomatic events and evaluate the impact of inclusions on the interpretation of microanalytical data. The presence of a distinct textural and chemical variation between magnetite types in Cerro Negro Norte is explained by a transition from high- to low-temperature magmatic-hydrothermal conditions. The microanalytical data of magnetite presented here, coupled with new δ34S data for pyrite (− 0.5 to + 4.3‰) andU–Pb ages of the diorite (129.6 ± 1.0 Ma), are indicative of a genetic connection between the diorite intrusion and the magnetite mineralization, supporting a magmatic-hydrothermal flotation model to explain the origin of Kiruna-type deposits in the Coastal Cordillera of northern Chile.