Precambrian thermal régimes

Abstract

Regional studies of Archaean and Proterozoic complexes provide the basis for one approach to the problem of reconstructing the thermal regimes of Precambrian eras. Such studies have a bearing both on the PT conditions under which metamorphism and deformation of continental crustal rocks took place and on the conditions of magma-generation at depth. Although several lines of evidence, including the wide distribution of low-pressure metamorphic facies series, are consistent with the inference that Archaean geothermal gradients tended to be steep, the characters of many complexes formed before 2700 Ma suggest PT ranges not far from those indicated by rocks which suffered deformation and metamorphism in Phanerozoic zones of high heat flow. Tectonic patterns developed on a global scale in early Proterozoic times suggest that lateral variations of heat flow in continental crust did not conform to the present pattern, zones of high heat flow being developed within as well as at the borders of continental rafts. There are, nevertheless, indications that the lithosphere had locally attained thicknesses comparable with those found today in many cratonic regions soon after the end of the Archaean era.