The emplacement of silicic magma bodies in the upper crust may be controlled by density (such that there is no buoyancy to drive further ascent) or temperature (such that surrounding rocks are too cold to deform significantly over geological timescales). Evidence for the latter control is provided by negative gravity anomalies over many granitic plutons. Conditions of diapir ascent and emplacement in this case are studied with a numerical model for deformation and heat transport allowing for ductile, elastic and brittle behavior. A large-strain formulation is used to solve for temperature, stress, strain, and strain rate fields as a function of time for a range of diapir sizes, density contrasts, and background geotherms. The method allows for large viscosity contrasts of more than 6 orders of magnitude and determines the dominant deformation mechanism depending on the local values of temperature, strain, and strain rate. Emplacement depth and final deformation characteristics depend on diapir size and buoyancy. Small diapirs (less than about 5 km in diameter) cannot reach shallow crustal levels and do not involve brittle deformation. In the ductile regime the diapir flattens significantly upon emplacement due to stiff roof rocks and to the free surface above. Late stage deformation proceeds by horizontal spreading, with little upward displacement of roof rocks and is likely to be interpreted as ballooning. Large diapirs (more than about 5 km in diameter) rapidly rise to shallow depths (1--5 km) and induce brittle faulting in the overlying rocks. In this regime, buoyancy forces may lead to faulting in roof rocks. In this case, late stage ascent proceeds by vertical intrusion of a plug of smaller horizontal dimensions than the main body. Buoyant diapirs keep on rising after solidification, long after the relatively short-lived high-temperature magmatic stage. This may account for some phases of late caldera resurgence in extinct volcanic systems.