Steady state creep of fine grain granite at partial melting

Abstract

Steady state creep under constant stress has been measured in a fine grain granite (aplite) from Schauinsland, Germany for the temperature range of partial melting 860°–1060°C, under a hydrostatic pressure of 4.2 kbar, and at low shear stresses of 5–50 bar. The apparatus used is described briefly. Rheological measurements are complemented by microscopic investigations. With a melt fraction of up to about 20%, creep can be described by a power law with a stress exponent of 3-4 and an activation energy of 80 kcal/mole, typical for creep in solids. Above 1010°C or 20% melt, the activation energy increases rapidly to a value of 200 kcal/mole simultaneously with a rapid increase of the melt fraction and a decrease of feldspar content. From the grain structure and from etching tests it is concluded that quartz contributes little to the plastic deformation which is governed mainly by the stress and temperature induced recrystallization of feldspar. The large temperature dependence of the creep rate above 1010° C may be caused by the decreasing area of grain contacts and consequent rise in local stress. These results support those of Arzi (1974) and Roscoe (1952).

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