The role of peritectic biotite for the chemical and mechanical differentiation of felsic plutonic rocks (Western Adamello, Italy)

Abstract

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 continuous 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 of melt segregation within crystallising tonalitic to granodioritic plutonic rocks, and to compare those with recent silicic eruptions. Here we investigate the processes of interstitial melt segregation in the calc-alkaline Western Adamello pluton (Italy). The Western Adamello tonalite exhibits a coarse-grained, equigranular texture and is mainly composed of hornblende partially replaced by biotite, plagioclase, quartz and alkali feldspar. Within the tonalites, several types of schlieren textures, crystal accumulation zones and dikes are found, comprising: (i) hornblende-biotite-gabbros, spatially-related to (ii) plagioclase- and quartz-rich leucotonalites; and (iii) quartz-, albite- and alkali-feldspar-rich domains forming aplitic to pegmatitic dikes indicative of melt segregation and extraction. Hornblende, biotite and plagioclase phenocrysts have essentially the same compositional range in the tonalites, gabbros and leucotonalites. Together with field observations, this indicates that deformation-driven crystal–melt segregation controls the modal variation within the host tonalite. The calculated melt in equilibrium with the primitive amphiboles has the same trace element composition as the host tonalite to within 5–10 %, indicating that the tonalite did not experience substantial melt loss. Quantitative modal compositions and crystallisation–differentiation calculations suggest that the evolution of the tonalite is controlled by plagioclase and hornblende crystallisation followed by a biotite-forming peritectic reaction. This peritectic reaction can be written as melt1 + amphibole = melt2 + biotite + quartz + plagioclase and decreases the remaining interstitial melt fraction from 40 to 15 % in a small temperature interval (~50 °C), therefore reducing the temperature window for large-scale melt segregation. The biotite-forming reaction initiates in weakly corundum-normative compositions in low to intermediate K calc-alkaline differentiation (e.g., Western Adamello and Peninsular Ranges batholith, California), whereas it seems absent in intermediate to high K, clinopyroxene-normative melts (e.g., Tuolumne intrusive suite, California). This difference is likely controlled by the initial aluminium saturation index and the differentiation path of the parental melt within the middle to lower crust. Textural observations and mass balance models indicate that 75–88 % plagioclase and quartz and 30–70 % interstitial melt was mechanically removed from the Western Adamello tonalite to form hornblende-biotite-gabbros, whereas the leucotonalites result from the accumulation of 40–80 % plagioclase and quartz. Of the emplaced 300–400 km3 of Western Adamello tonalite, only about 0.8–2.4 km3 represent rock types related to physical segregation processes, indicating limited melt extraction. Such crystal–melt segregation processes in tonalitic to granodioritic plutons are observed worldwide and facilitate the extraction of granitic liquids. This mechanism as observed in the Western Adamello tonalite potentially contributes to the accumulation of crystal-poor rhyolites and the segregation of metal-rich brines.