Insights into the mechanisms and timescales of pluton assembly from deformation patterns of mafic enclaves

Abstract

We propose a novel method to constrain both timescales and assembly styles of intrusive bodies using the strain recorded by mafic enclaves, a common component of granitic rocks. Petrology, thermal modeling, and magma rheology are combined to investigate the evolution of strain recorded by enclaves during the piecemeal assembly of a pluton growing at various rates of magma input. The different compositions (and hence phase relations) of host magma and enclaves limits homogeneous deformation of these two materials to restricted temperature ranges, which we term “windows of mutual deformability.” Outside these windows only the less viscous host granite records any appreciable deformation and enclaves are mainly transported as rigid objects. The temporal and spatial development of the windows of mutual deformability reflects the emplacement rate of magmas into the pluton. Consequently, the radial distribution of strain recorded by enclaves can provide a picture of the thermal and rheological evolution of a magmatic body during its construction. Our approach is applied to deformation patterns of mafic enclaves in the Lago della Vacca Complex (LVC) of the Adamello Massif (Italy), to estimate the timescales of pluton emplacement. Our calculations suggest total emplacement timescales of the order 50 to 150 ky, in excellent agreement with recent high precision radiometric dating of zircons from the LVC.