Prograde, peak and retrograde metamorphic fluids and associated metasomatism in upper amphibolite to granulite facies transition zones

Abstract

Granulites constitute a major part of the (lower) continental crust, occurring on a regional scale in many metamorphic belts. Their origin is generally discussed in terms of vapour-absent melting and fluid-assisted dehydration. This last model is notably supported by the occurrence of two immiscible free fluids at peak- and retrograde conditions, viz. CO2 and highly saline brines. Evidence includes fluid remnants preserved in mineral inclusions, but also large scale metasomatic effects. The current paper discusses the presence and action of these fluids in granulites, with special attention to amphibolite to granulite facies transition zones (e.g. the Bamble sector, south Norway). Metasomatic effects induced by fluid percolation at different scales and stages include: (1) Control of state variables (H2O activity or O2 fugacity), regional oxidation and so-called ‘granulite facies’ islands. (2) Small scale metasomatism at mineral intergrain boundaries (e.g. K-feldspar microveins and/or myrmekites). (3) Large scale metasomatism at the amphibolite to granulite facies transition zone, evidenced by: (a) Incipient charnockites, (b) Metasomatic redistribution of elements traditionally considered as immobile (e.g. Zr, Th, REE), (c) Peak metamorphic to retrograde bulk chemical processes (scapolitization, albitization), (d) Long distance action of granulite fluids. The importance and widespread occurrence of these effects call for large fluid quantities stored in the lower crust at peak metamorphic conditions, later expelled towards shallower crustal levels during retrogradation. Fluid origin, only briefly discussed in this paper, is complicated, not unique. Some fluids are crustal, either far remnants of sedimentary waters (brines) or linked to metamorphic/melt reactions. But, especially for high-temperature granulites, the greatest amount, notably for CO2, is issued from the upper mantle, which contain also the same fluid remnants as those found in the lower crust.