Low pressure crystal accumulation and melt segregation within the Western Adamello tonalite (Italy)

The volcanic–plutonic connection plays a fundamental role for magmatic systems, linking crystallising plutons, volcanic activity, volatile exsolution and ore deposits. Nonetheless, our understanding of the nature of these links is limited by the scarcity of outcrops exhibiting clear relationships between the plutonic roots that feed its volcanic counterpart. One way to better characterise the volcanic–plutonic connection is to quantify the amount and rates of melt segregation within a crystallising plutonic body, and to compare the volumes and rates with recent silicic eruptions. Here we investigate the processes of interstitial melt segregation in the calc-alkaline Western Adamello (WA) pluton. The WA tonalite (WAT) is part of the southern Alps and represents an intrusive body emplaced at 2.5 kbar in ~1.2 Myr (Floess and Baumgartner, 2015; Schaltegger et al., 2019). The WAT exhibits a coarse-grained, equigranular texture and is composed of hornblende partially replaced by biotite, plagioclase, quartz, K-feldspar, apatite, zircon, and secondary epidote. K-feldspar, quartz and albite-rich plagioclase (An_25-40) are late and occur as interstitial phases. Several types of igneous structures, constituting <0.5 vol.% of the WA, are found, comprising: (i) hornblende and biotite accumulations (0.1–30 m) with interstitial K-feldspar, quartz and albite-rich plagioclase (An_25-40) representing 25–45 vol.% of the rock; (ii) plagioclase (An_40-70) accumulations with 40–50 vol.% of the same interstitial assemblage; and (iii) quartz-, albite- and K-feldspar-rich domains (0.1–10 m) containing WAT-derived plagioclase phenocrysts which form either zoned aplitic to pegmatitic dikes or schlieren-shaped bodies probably representing in situ melt segregations. The latter are spatially associated with the accumulation zones. Hornblende, biotite, and plagioclase phenocrysts have essentially the same compositional range in accumulations and segregations. This observation indicates that deformation-driven crystal–crystal and crystal–melt segregation operated within the host tonalite. Quantitative modal compositions and mass balance calculations indicate that the hornblende–biotite accumulations lost 60–90 vol.% of their plagioclase phenocrysts and 20–55 vol.% interstitial melt, whereas the plagioclase accumulations lost up to 15 vol.% melt. Such calculations place the maximum efficiency of crystal–melt segregation to 40–55 % in the WAT, as most of the melt remains trapped within the crystal framework. Based on phase relationships and major element modelling, it is proposed that the peritectic relationship hornblende + melt₁ = biotite + quartz + melt₂ and the efficiency of plagioclase–melt separation are linked to the variable composition of the felsic dikes. Such a reaction is known from experimentally derived phase relationships of tonalite (Marxer and Ulmer, 2019) and probably plays a fundamental role linking pluton solidification and extraction of interstitial liquid.