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There is no simple relationship between a PT point derived for a metamorphic complex from mineral assemblage and composition data and the equilibrium geotherm which may be calculated for the crust. Transient or complex geotherms of various slopes may be generated by magmatic intrusion, rapid burial, rapid uplift or tectonic thickening, or almost any combination of these processes. Proterozoic metamorphic complexes yield PT conditions ranging from about 6 kb, 600° in the Wakeham Bay area to 3-5 kb, 750° in the Bear Province and 10 kb, 980° in the Musgrave Ranges. The lowest apparent geotherms lie in linear metamorphic belts, the highest in domal regions in the NW Canadian shield, while intermediate values occur in most granulite terrains. These can be related to equilibrium surface heat flows in the range 80 to 120 mW/m2, with mantle heat flow of about 50 mW/m2 for reasonable crustal compositions. The higher values can be simulated by intruding sloping sill-like bodies of basic magma into the crust at depths around 20 km; the complex heat transfer equations have not yet been solved for such situations, however, and calculations are inexact. Tectonic crustal thickening and erosion can produce similar effects; but in the absence of the former, the latter can most easily be caused by continental underplating by basic magma. Gradients are generally higher than in Phanerozoic orogenic belts, and no blueschist facies rocks have yet been substantiated in pre U. Proterozoic complexes.

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