¹ Geology and Geochemistry Department, Stockholm University, S-10691 Stockholm, Sweden carlo.dietl@geo.su.se
² Hans Ramberg Tectonic Lab, Dept. of Earth Sciences, Uppsala University, Villav. 16,
75236 Uppsala, Sweden
Hemin.Koyi@geo.uu.se

Abstract

Concentrically expanded plutons (CEPs) are a common igneous feature. They are characterized by compositional zoning, mainly concentric magmatic fabric inside the pluton and ductile fabric in the contact aureole which are concordant with the pluton / host rock contact. Two intrusion mechanisms have been proposed for CEPs: dyking + ballooning and diapirism. Here, we present results of a centrifuge model to study the kinematics and dynamics of CEPs. The model consisted of three layers from bottom to the top; a 5 mm thick buoyant lower layer of Rhodorsil Gomme simulating a partially molten magma, a 50 mm thick non-Newtonian Rhodorsil Gomme + Plastilina overburden simulating a natural silicic overburden and a 10 mm thick layer of PDMS simulating a less dense overburden. The model was centrifuged twice with two differently stained buoyant layers attached. After the first stage of the experiment two mushroom-shaped diapirs had intruded and deformed the overburden to spread below the less-dense PDMS layer. The second-stage intrusion occurred along the stem of the preexisting diapirs using them as a mechanically weak pathway. This intrusion was not diapiric, but the buoyant material rose passively similar to a dike. Once reaching the level of neutral buoyancy, the intrusive material spread laterally resulting in extensive spreading and expansion of the overhang of the preexisting diapirs. Model results show that CEPs can be the result of combined initial diapirism and subsequent dyking. Multiple diapirs can form only when the overburden units deform ductiley during the different stages of diapirism.