Contact Metamorphism of Pelitic Rocks Constrains the Depth of Emplacement of the Re di Castello Intrusion (Adamello Batholith, Italy)

Abstract

Re di Castello is the southernmost unit of the composite Adamello batholith (central Alps, Italy). It was emplaced between 43 and 40 Ma, yet its depth of emplacement is not well constrained. We have studied the contact metamorphism of pelitic country rocks in two localities: the upper Caffaro Valley and the Borzago Valley in the southern and the northern sub-units of the Re di Castello, respectively. In the Caffaro Valley, the country rocks comprise the Triassic sedimentary Lozio Shale, a carbon-rich slate–siltstone, which was intruded by quartz-dioritic magma. Near the contact, the hornfelses display the mineralogical assemblage Kfs-Crd-Bt-Ms-Pl-Qz-Gr, with rare fibrolitic sillimanite in a single sample. Andalusite was never observed throughout the aureole at this locality. In the Borzago Valley, contact metamorphism developed on schists of the pre-Permian basement with a Variscan, regional metamorphic overprint. The sequence of contact metamorphic mineral assemblages progresses from And-Bt-bearing parageneses to the Sil-Crd-Bt-Kfs-Ms-Pl-Qz that characterizes peak conditions, at which incipient melting is also observed. K-feldspar forms slightly upgrade the first appearance of sillimanite. For samples at both localities, thermodynamic modelling in the NCKFMASHT system failed at predicting a stability field for the sequences of mineral assemblages developed during contact metamorphism. Also the Ti-in biotite thermometer did not constrain temperatures adequately. The P–T conditions at the thermal peak were thus evaluated by an alternative bathograd-like approach, considering phase relationships in the simplified NKASH-C system. To form sillimanite only as product of the incomplete Ms-Qz breakdown—divariant for the presence of Na in Ms and Kfs and shifted to lower temperature due to the presence of graphite (in the Lozio Shale)—an isobaric path typical of contact metamophism must have crossed above the Ms_ss-Qz-Kfs_ss-Sil-And-fluid invariant point. This constrains an emplacement pressure >3.3 kbar in the Caffaro Valley, and >3.2 kbar in the Borzago Valley. Concerning temperature, the same univariant point also constrains the minimum temperature in the Caffaro Valley as >615°C–620°C, consistent with results of RSCM thermometry on graphite from the Lozio Shale. The evidence of incipient melting in some samples from the Borzago Valley indicates higher temperatures, probably approaching 670°C, near the contact to the intrusion. Assuming an average crust density of 2.7 g cm⁻³, the estimated pressures correspond to a minimum paleo-depth of emplacement of about 12 km. These depths are somewhat greater than normally considered and should be regarded as revised constraints on models of the emplacement dynamics of the Adamello batholith and on paleogeographic reconstructions of this part of the Southalpine domain.