Alkaline magmas in shallow arc plutonic roots: a field and experimental investigation of hydrous cumulate melting in the southern Adamello batholith.

Despite the first-order importance of crystallisation-differentiation for arc magma evolution, several other processes contribute to their compositional diversity. Among them is the remelting of partly crystallised magmas, also known as cumulate melting or 'petrological cannibalism'. The impact of this process on the plutonic record is poorly constrained. We investigate a nepheline-normative dyke suite close to the Blumone gabbros, a large amphibole-gabbro unit of the Tertiary Southern Alpine Adamello igneous complex. The compositions of the studied dykes are characterised by low SiO2 (43-46 wt. %), MgO (5.0-7.2 wt. %), Ni (18-40 µg/g), and high Al2O3 (20.2-22.0 wt. %) contents. Phenocrystic plagioclase in these dykes exhibits major, trace, and Sr isotope compositions similar to Blumone cumulate plagioclase, suggesting a genetic link between the nepheline-normative dykes and the amphibole-gabbro cumulates. We tested this hypothesis by performing saturation experiments on a nepheline-normative dyke composition in an externally heated pressure vessel at 200 MPa between 975 and 1100 °C at fO2 conditions close to the Ni-NiO buffer. Plagioclase and spinel are near-liquidus phases at and above 1050 °C, contrasting with the typical near-liquidus olivine ± spinel assemblage in hydrous calc-alkaline basalts. The alkaline nature of the dykes results from the abundance of amphibole in the protolith, consistent with melting of amphibole-gabbro cumulates. We modelled the heat budget from the repeated injection of basaltic andesite into a partly crystallised amphibole-gabbro cumulate. The results of this model show that no more than 7% of the cumulate pile reaches temperatures high enough to produce nepheline-normative melts. We propose that such nepheline-normative dykes are a hallmark of hydrous cumulate melting in subvolcanic plumbing systems. Therefore, ne-normative dykes in arc batholiths may indicate periods with high magma fluxes. Supplementary Information: The online version contains supplementary material available at 10.1007/s00410-023-02047-3.

Fluids as primary carriers of sulphur and copper in magmatic assimilation.

Magmas readily react with their wall-rocks forming metamorphic contact aureoles. Sulphur and possibly metal mobilization within these contact aureoles is essential in the formation of economic magmatic sulphide deposits. We performed heating and partial melting experiments on a black shale sample from the Paleoproterozoic Virginia Formation, which is the main source of sulphur for the world-class Cu-Ni sulphide deposits of the 1.1 Ga Duluth Complex, Minnesota. These experiments show that an autochthonous devolatilization fluid effectively mobilizes carbon, sulphur, and copper in the black shale within subsolidus conditions (≤ 700 °C). Further mobilization occurs when the black shale melts and droplets of Cu-rich sulphide melt and pyrrhotite form at ∼1000 °C. The sulphide droplets attach to bubbles of devolatilization fluid, which promotes buoyancy-driven transportation in silicate melt. Our study shows that devolatilization fluids can supply large proportions of sulphur and copper in mafic-ultramafic layered intrusion-hosted Cu-Ni sulphide deposits.