Home – Home – Jornals – Russian Geology and Geophysics 2009 number 12

The "unexpected" (the word is from H.G.F. Winkler, 1974) discovery of CO₂-rich inclusions in granulites has initiated a debate, which, after more than 35 years, is still an important issue in etamorphic 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 CO₂, 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), and 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, and 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. High-temperature 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.