Discussion

Abstract

Dr. M. Brown observed that both of the examples of Archaean diapirism quoted by Professor Stephansson required a layer of granitic composition in the lower part of the Earth’s crust which the author suggested might be derived from the upper mantle by some kind of phase transformation. The speaker proposed an alternative origin for the granitic layer by anatexis of lower crustal material and segregation of a granitic melt from a refractory residue. Would Professor Stephansson comment on this alternative mechanism? The speaker also asked for the author’s opinion on the state of the granitic material during its diapiric ascent. Was it liquid, crystal mush or solid? Was this also its initial state or could the nature of the granitic material change during ascent? Professor Stephansson replied that the origin of a granitic layer by anatexis of lower crustal material is suggested as a possible mechanism to form the low density material of the synorogenic or late-kinematic granite intrusions of the Svecofennian orogeny of central Sweden. These granites all intrude along the margin of the batholiths. The size of granite intrusions is proportional to the thickness of the source layer and the viscosity contrast between the source layer and the overburden. A thick layer and a moderate viscosity contrast gives rise to large intrusions. The lack of low viscosity properties of the metasediments and metavolcanics surrounding the granite intrusions in the Archaean and their size of the order of 20–40 km in diameter indicate intrusion in a solid state. To