Contact Metamorphism around Shallow Level Granitoid Plutons: the role of Magmatic Fluids

Bjørn Jamtveit Department of Geology, University of Oslo, Oslo, Norway

Contact metamorphism around shallow level granitoid plutons is to a major extent affected by infiltration of externally derived fluids. Granitoid magmas may release about 3 wt% water upon crystallization even at shallow crustal depth (< 5 km). Continuum models of the hydrodynamics of contact metamorphism indicate that flow of magmatic fluids will mainly be directed upwards, away from the intrusive contact. Maximum average fluid fluxes of about 10-10 m3/m2s and integrated fluxes of 100-1000 m3/m2 are expected in the contact aureoles above such intrusives. Permeability increases during cooling of the aureole may arise from thermal fracturing processes. This may lead to a systematic evolution in flow systems during contact metamorphism from lithostatic fluid pressure and pervasive expulsion of internally generated (magmatic and metamorphic) fluids, to hydrostatic conditions with channelized flow dominated by surface-derived fluids (e.g. Hanson, 1995). However, in reality the spatial distribution of fluid fluxes will be critically depended on permeability heterogeneities in the country rocks and continuum models may not always be adequate. The appropriate modelling approach to flow in such environments can only be determined by a careful assessment of the statistical properties of the permeability distribution for the field area of interest. Field and stable isotope data from selected contact aureoles in the Permian Oslo rift demonstrate how lithological and structural controls on country rock permeability variations have affected fluid release from various intermediate to silicic plutons.

Unmixing of the magmatic fluid into a viscous and dense saline brine and a less viscous H2O-dominated vapour in shallow level systems has large consequences for mass transfer and transport. The saline brine tend to pond near apical regions of the intrusive bodies due to the low mobility (viscosity) of hydrosaline liquids. Flow of such fluids into the country rocks will be strongly focused through highly permeable fracture zones generated during thermal contraction, by hydrofracturing or by tectonic extension. Periodic release of such fluids may account for oscillatory zoned skarn minerals formed in fracture zones around granitoid intrusives and periodic inflation and deflation of calderas in modern hydrothermal systems.


Hanson, R.B., Geol. Soc. Am. Bull. 197, 595-611 (1995).