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 cond uctive thermal regime attending construction and maintenance of a cont inental magma chamber by intrusion of granite dikes into granodiorite host rocks displaced at various spreading rates. Final intrusion shape s 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. Th e formation of a steady-state chamber is defined as the point at which the ambient temperatures surpass the intrusion solidus forestalling t he solidification of subsequently intruded material. For spreading rat es < 10 mm year(-1), construction of a steady-state magma chamber in t he 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-sta te 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 10 0% of the intrusion. The thermal modeling supports the following concl usions: (a) episodic magma emplacement into a fault-controlled setting is a thermally viable means of constructing a steady-stale chamber at moderate to fast spreading rates only if the duration of faulting and intrusion are long enough to elevate ambient temperatures above the i ntrusion 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 th is scenario should contain sheeted dikes, (d) the solidus isotherm, an d thus the solidification front that it tracks. will become progressiv ely curviplanar during the construction of the magma chamber and will not represent the initial shape of the intrusions (i.e. sheets), (e) t he steady-state chamber will be smaller than the total intrusion dimen sions, 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 instan taneously, the effects of continued addition of heat on the migration of solidification fronts may have significant implications for magmati c processes in many emplacement scenarios. (C) 1998 Elsevier Science L td. All rights reserved.