TY - JOUR

T1 - Alkaline magmas in shallow arc plutonic roots: a field and experimental investigation of hydrous cumulate melting in the southern Adamello batholith

AU - Pimenta Silva, Manuel

AU - Marxer, Felix

AU - Keller, Tobias

AU - Giuliani, Andrea

AU - Ulmer, Peter

AU - Müntener, Othmar

N1 - Funding Information: Numerical calculations were run at the supercomputer cluster Euler at ETH Zürich. Lydia Zehnder, Julien Allaz, and Marcel Guillong are thanked for their analytical support. MPS thanks Luiz Grafulha Morales of ScopeM for supporting and assisting with this work. Julia Neukampf is acknowledged for scientific discussions and encouragement during this project. Peter Tollan is thanked for providing initial assistance to the LA-MC-ICP-MS analyses and reference materials. Florian Viher and Carl Spandler are acknowledged for assistance during sampling. Sébastien Pilet and Benjamin Klein provided constructive feedback on this work. Thoughtful and constructive reviews from Bruno Scaillet and an anonymous reviewer as well as the editorial handling by Dante Canil are gratefully acknowledged. This project was funded by the Swiss National Science Foundation project SNSF 200020_184867. FM was supported by the ETH research grant ETH-14 16-1 and the DFG project HO1337/47 (part of the Forschungsgruppe FOR 2881 “Diffusion chronometry of magmatic systems”).

PY - 2023/9/3

Y1 - 2023/9/3

N2 - 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 SiO 2 (43–46 wt. %), MgO (5.0–7.2 wt. %), Ni (18–40 μg/g), and high Al 2O 3 (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 fO 2 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.

AB - 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 SiO 2 (43–46 wt. %), MgO (5.0–7.2 wt. %), Ni (18–40 μg/g), and high Al 2O 3 (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 fO 2 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.

KW - Amphibole

KW - Arc batholiths

KW - Arc magmatism

KW - Chemical diversity

KW - Cumulate melting

KW - Experimental petrology

KW - Magma plumbing systems

KW - Nepheline-normative rocks

KW - Petrological cannibalism

KW - Thermal modelling

UR - http://www.scopus.com/inward/record.url?scp=85169695110&partnerID=8YFLogxK

U2 - 10.1007/s00410-023-02047-3

DO - 10.1007/s00410-023-02047-3

M3 - Article

VL - 178

JO - Contributions to Mineralogy and Petrology

JF - Contributions to Mineralogy and Petrology

SN - 0010-7999

IS - 9

M1 - 64

ER -