Factors Affecting the Thickness of Thermal Aureoles

Catherine Annen

doi:10.3389/feart.2017.00082

Abstract

Intrusions of magma induce thermal aureoles in the country rock. Analytical solutions predict that the thickness of an aureole is proportional to the thickness of the intrusion. However, in the field, thermal aureoles are often significantly thinner or wider than predicted by simple thermal models. Numerical models show that thermal aureoles are wider if the heat transfer in the magma is faster than in the country rock due to contrasts in thermal diffusivities or to the effect of magma convection. Large thermal aureoles can also be caused by repeated injection close to the contact. Aureoles are thin when heat transfer in the country rock is faster than heat transfer within the magma or in case of incrementally, slowly emplaced magma. Absorption of latent heat due to metamorphic reactions or water volatilization also affects thermal aureoles but to a lesser extent. The way these parameters affect the thickness of a thermal aureole depends on the isotherm under consideration, hence on which metamorphic phase is used to draw the limit of the aureole.

Highlights

Contact metamorphic aureoles are due to the elevation in temperature in country rocks induced by the proximity of an igneous intrusion
The temperature evolution in the country rock after the instantaneous emplacement of a sheet-like magma intrusion depends on the thickness of the intrusion, the country rock initial temperature and composition, the magma emplacement temperature and latent heat of crystallization, and the thermal diffusivity
Thermal diffusivity controls the time that is needed for a given point to reach a maximum in temperature (Tmax) and the time it takes for temperatures to decay (Figure 4)

Summary

Introduction

Contact metamorphic aureoles are due to the elevation in temperature in country rocks induced by the proximity of an igneous intrusion. Simple analytical models predict that the thickness of a thermal aureole, i.e., at what distance from the intrusion contact a metamorphic mineral appears that is characteristic of a certain temperature, is proportional to the thickness of the igneous intrusion. In reality, contact aureoles are often thinner, and sometimes wider than predicted. A compilation of contact aureole thicknesses (Reverdatto et al, 1970; Barton et al, 1991) shows that for a given depth and intrusion composition, i.e., for a given initial country rock and magma temperature, aureole thicknesses relative to intrusion thicknesses vary significantly (Figure 1). A compilation of contact aureole thicknesses (Reverdatto et al, 1970; Barton et al, 1991) shows that for a given depth and intrusion composition, i.e., for a given initial country rock and magma temperature, aureole thicknesses relative to intrusion thicknesses vary significantly (Figure 1). Galushkin (1997) reports reduced thermal effects around a number of relatively small intrusions (from 1.3 to 20 m thick) and explains low temperatures by emplacement of the intrusion in a shell of cooler magmatic rocks but without explaining how this shell formed without transiting through high magmatic temperatures

Methods

Results

Conclusion