Mantle to lower-crust fluid/melt transfer through granulite metamorphism

Abstract

The “unexpected” (the word is from H.G.F. Winkler, 1974) discovery of CO 2-rich inclusions in granulites has initiated a debate which, after more than 35 years, is still an important issue in metamorphic petrology. Experimental and stable isotope data have led to the conception of a “fluid-absent” model, opposed to the “fluid-assisted” hypothesis, derived from fluid inclusion evidence. Besides CO 2, other fluids have been found to be of importance in these rocks, notably concentrated aqueous solutions (brines), also able to coexist with granulite mineral assemblages at high P and T. Brines also occur in inclusions or, more impressively, have left their trace in large scale metasomatic effects, typical of a number of high-grade areas: e.g., intergranular K-feldspar veining and quartz exsolution (myrmekites), carbonate metasomatism along km-scale shear zones (Norway, India), “incipient charnockites” (India, Sri Lanka, Scandinavia), highly oxidized Archean granulites. All together, this impressive amount of evidence suggests that the amount of fluids in the lower crust, under peak metamorphic conditions, was very large indeed, far too important to be only locally derived. Then, except for remnants contained in inclusions, these fluids have left the rock system during postmetamorphic uplift. Fluid remnants identical to those occurring in deep crustal granulites are also found in mantle minerals, including diamonds. Major mantle fluid source is related to the final stages of melting processes: late magmatic emanations from alkalic basaltic melts, carbonate-metasomatizing aqueous fluids issued from igneous carbonatites. Even if a local derivation of some fluids by crustal melting cannot be excluded, most lower-crustal granulite fluids have the same origin. They are transferred from the mantle into the crust by synmetamorphic intrusives, also responsible for the high thermal gradient typical of granulites, notably HT- or UHT-types. These are mostly found in Precambrian times, generated during a small number of time intervals: e.g., around 500, 1000, 1800, 2500 Ma. HT-granulites forming events occur at world-scale in supercontinents or supercratons, either at the end of amalgamation, or shortly before breaking-off. They provide a mechanism for a vertical accretion of the continental slab, which complement the more classical way of lateral accretion above subduction zones at convergent boundaries.