Textural evidence for dilatant (shear thickening) rheology of magma at high crystal concentrations

Investigation of the rheology of magmas at high crystal concentrations by e xperimental means has proved problematic. An alternative approach is to stu dy textures of igneous rocks that not only preserve evidence of the kinemat ics of magma flow, such as flow direction, but can also preserve evidence o f rheology. Flow textures in multiply intruded trachyte dykes on Fraser Isl and, eastern Australia record evidence of dilatant flow during solidificati on. This conclusion is reached by interpretation of microscopic ductile she ar zones that disrupt the groundmass of aligned feldspar laths. Detailed th ree-dimensional investigation demonstrates that the dihedral angle between conjugate micro-shear zones is approximately 65 degrees. This conjugate ang le is equivalent to that observed in dilatant granular materials such as sa nd. Dilatant behaviour is synonymous with shear thickening rheology indicat ing that the magma flow is time-dependent and resists high flow rates. Some of the dykes contain auto-brecciation fragments that may represent localit ies where the ductile flow rate threshold was exceeded. Newtonian or pseudo plastic (shear thinning) rheology of crystal-poor magmas must progressively give way to shear thickening rheology during cooling and increasing crysta l concentration. (C) 2000 Elsevier Science B.V. All rights reserved.