Modeling the thermal evolution of fault-controlled magma emplacement models: implications for the solidification of granitoid plutons

A two-dimensional finite difference model is used to simulate the conductive thermal regime attending construction and maintenance of a continental magma chamber by intrusion of granite dikes into granodiorite host rocks displaced at various spreading rates. Final intrusion shapes include tabular, square, and vertical rectangular bodies emplaced in the shallow crust (5–15 km) and tabular bodies emplaced in the middle crust (15–20 km) fed by dikes with widths of between 20 and 100 m. The formation of a steady-state chamber is defined as the point at which the ambient temperatures surpass the intrusion solidus forestalling the solidification of subsequently intruded material. For spreading rates <10 mm year−1, construction of a steady-state magma chamber in the shallow crust took 260 ka (rectangular), 360 ka (square), and 1 Ma (tabular), whereas in the mid crust a steady state was reached in less than 30 ka (tabular). At faster spreading rates (25 and 50 mm year−1) ambient temperatures pass the solidus isotherm forming a steady-state reservoir within 55 ka, depending on intrusion depth and size. For 10–25 mm year−1 spreading rates, sheeted dikes make up from 10 to 100% of the intrusion. ermal modeling supports the following conclusions: (a) episodic magma emplacement into a fault-controlled setting is a thermally viable means of constructing a steady-state chamber at moderate to fast spreading rates only if the duration of faulting and intrusion are long enough to elevate ambient temperatures above the intrusion solidus, (b) isotherms will migrate outward during successive intrusion before converging back on the center of the intrusion after chamber construction, (c) the margins of most intrusions formed by this scenario should contain sheeted dikes, (d) the solidus isotherm, and thus the solidification front that it tracks, will become progressively curviplanar during the construction of the magma chamber and will not represent the initial shape of the intrusions (i.e. sheets), (e) the steady-state chamber will be smaller than the total intrusion dimensions, and (f) magmatic fabrics will form diachronously and not always parallel to sheet margins as they track the migrating solidification front. Because it is unlikely that most large intrusions formed instantaneously, the effects of continued addition of heat on the migration of solidification fronts may have significant implications for magmatic processes in many emplacement scenarios.