Emplacement-related deformation of granitoid magmas, southern Adamello Massif, Italy

Abstract

The Lago della Vacca Suite in the Tertiary Adamello Massif, Italian Alps, is a small (∼10 km 2 ), crudely zoned granitoid pluton emplaced at a pressure of ∼3.5 kbar into folded Mesozoic sediments and precursor intrusive rocks. Field relations and detailed mapping reveal a temporal evolution in the style of emplacement from early, passive stoping by the mafic to intermediate composition marginal units, to later forceful emplacement of the more silicic core. Marginal units of the pluton have highly discordant, stoped outer contacts against folded Triassic carbonates and precursor mafic/ultramafic rocks. In contrast, the core units have steeply inward-dipping contacts against these marginal units. Magmatic foliations defined by the planar alignment of plagioclase, amphibole, and biotite, and by mafic enclaves, are well developed in all units. They describe steeply inward-dipping concentric trajectories that approximately parallel unit contacts and in general increase in intensity toward the pluton margin. Lineations are rarely present. The most intensely foliated rocks occur in the marginal units, as anastomosing, sinuous bands that wrap around lenses of less intensely foliated granitoid. Three-dimensional strain analysis using mafic enclaves from throughout the pluton indicates that these bands represent zones of very high strain. Strain ellipsoids are weakly prolate within the core of the pluton, becoming increasingly oblate as strain increases toward the pluton margin. The short axis (Z) of the strain ellipsoid is uniformly subnormal to the foliation, suggestive of radial flattening. Textures of the marginal rocks, which record high strain, coupled with amphibole-plagioclase thermometry, indicate that deformation occurred in the magmatic or submagmatic state (∼650-780 °C) and was accompanied by significant migration of interstitial melt. The documented transition from passive to forceful emplacement, and final geometry of the suite, resulted from temporal changes in the rheology of both the magma and adjacent country rocks. Pulses of low-density hydrous magma were initially emplaced by stoping denser country rocks. On cooling and solidification, these magmas contracted by 15%-20%, creating an unpressured volume, which allowed subsequent magma pulses to be emplaced into the pluton core. These pulses distended and flattened their partially molten carapace and those parts of the aureole that had been thermally softened by intrusion of earlier magmas. Emplacement of the Lago della Vacca Suite was therefore the product of several internal or magma-related processes, an interaction between stoping, radial expansion, and volume contraction on solidification of the hydrous magmas. The heterogeneous distribution of strain in the marginal units reflects local variations in temperature and, most importantly, in the distribution of interstitial melt during emplacement.