EMPLACEMENT-RELATED DEFORMATION OF GRANITOID MAGMAS, SOUTHERN ADAMELLO MASSIF, ITALY

The Lago della Vacca Suite in the Tertiary Adamello Massif, Italian Al ps, is a small (approximately 10 km2), crudely zoned granitoid pluton emplaced at a pressure of approximately 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 e arly, passive stoping by the mafic to intermediate composition margina l units, to later forceful emplacement of the more silcic core. Margin al units of the pluton have highly discordant, stoped outer contacts a gainst folded Triassic carbonates and precursor mafic/ultramafic rocks . In contrast, the core units have steeply inward-dipping contacts aga inst these marginal units. Magmatic foliations defined by the planar a lignment of plagioclase, amphibole, and biotite, and by mafic enclaves , are well developed in all units. They describe steeply inward-dippin g concentric trajectories that approximately parallel unit contacts an d in general increase in intensity toward the pluton margin. Lineation s are rarely present. The most intensely foliated rocks occur in the m arginal units, as anastomosing, sinuous bands that wrap around lenses of less intensely foliated granitoid. Three-dimensional strain analysi s using mafic enclaves from throughout the pluton indicates that these bands represent zones of very high strain. Strain ellipsoids are weak ly prolate within the core of the pluton, becoming increasingly oblate as strain increases toward the pluton margin. The short axis (Z) of t he strain ellipsoid is uniformly subnormal to the foliation, suggestiv e of radial flattening. Textures of the marginal rocks, which record h igh strain, coupled with amphibole-plagioclase thermometry, indicate t hat deformation occurred in the magmatic or ''submagmatic'' state (app roximately 650-780-degrees-C) and was accompanied by significant migra tion of interstitial melt. The documented transition from passive to f orceful emplacement, and final geometry of the suite, resulted from te mporal 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 mag mas contracted by 15%-20%, creating an unpressured volume, which allow ed subsequent magma pulses to be emplaced into the pluton core. These pulses distended and flattened their partially molten carapace and tho se parts of the aureole that had been thermally softened by intrusion of earlier magmas. Emplacement of the Lago della Vacca Suite was there fore the product of several internal or magma-related processes, an in teraction 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 temperatu re and, most importantly, in the distribution of interstitial melt dur ing emplacement.