Large-scale carbon transfer between crust and mantle during supercontinent amalgamation and disruption

Early Cambrian (Pan-African orogeny, ca. 600 Ma) amalgamation of the Gondwana supercontinent through linear hot orogenic belts occurred in Africa and Southern India. These orogenic belts are characterized by ultrahigh temperature (UHT, ca. 1000°C) metamorphic events, which have played a fundamental role in the development and stabilisation of the continents. Granulite and UHT-metamorphism are linked to major episodes of supercontinent amalgamation at least since the late Archean, traces of which having been found in virtually all known supercontinents so far. Depending on the geodynamic setting (continent collision or basin-inversion after lithospheric thinning), the duration of these UHT-episodes is variable, from less than 15 Ma in the recent granulites from Hokkaido (Japan) or Seram (Indonesia) to more than 100 Ma in the long-lived and slowly-cooled Napier complex (Antarctica). But whatever their age or geodynamic settings, minerals of all UHT-granulites worldwide contain a great quantity of primary fluid inclusions, containing dense or superdense (> 1.1 g/cm3) mantle-derived CO2. It shows that, during UHT events, large quantities of mantle-derived CO2 were injected into the continental lower crust. The occurrence of these syn-metamorphic CO2 fluids is so systematic in UHT granulites, that it can be assumed that they played a critical role in the genesis of the extremely high temperatures reached during this metamorphism. In addition to CO2 another fluid is present, namely high-salinity aqueous brines, the source of which can be the sedimentary protolith, the mantle, or both. Both fluids were immiscible at peak metamorphic conditions, but they became miscible and mutually reactive at decreasing pressure and temperature. The amalgamation of the Gondwana supercontinent lasted for more than 400 Ma, starting about 1000 Ma ago in the Trivandrum bloc, India and ending in Eocambrian times (ca. 600 Ma) during the Pan-African orogeny, which affected Africa, Madagascar, Sri-Lanka and Southern India. At this time, the large fluid influx in the lower crust caused instability, leading to breakup and disruption of the supercontinent immediately after its final amalgamation. Elimination of the UHT-granulite fluids occurred rapidly during post-metamorphic uplift, with important consequences for both at local and regional scales. Local scale (10 to 100’s m.) effects are due to the intergranular migration of brines, resulting in the formation of granulite mineral assemblages at the periphery of the main granulite complex (incipient charnockites, granulite “islands”). Less mobile CO2 fluids can only migrate through crustal-scale (10 to 100 km) shear zones, probably caused by major earthquakes. CO2 fluids can either be reduced, resulting in graphite veins, or oxidized, resulting in the quartz-carbonate shear zones found in the vicinity of many granulite terranes

Vein-type graphite deposits in Sri Lanka: the ultimate fate of granulite fluids

The world-best vein graphite deposits in Sri-Lanka occur scattered through the high-grade terrain of the Wanni and Highland Complexes of Sri-Lanka. The Wanni Complex (amphibolite to granulite grade) consists of ~770-1100 Ma metagranitoids, metagabbro, charnockite, enderbitic gneisses, migmatites, clastic metasediments, including garnet-cordierite gneisses, rare to minor calc-silicate rocks as well as late to post-tectonic granites (Kröner et al., 2013). Higher metamorphic grade, reaching in places UHT-conditions (T>1000 °C) characterizes the Highland Complex. Peak metamorphism occurred during the Neoproterozoic Pan-African orogeny (~620-535 Ma), which led to the accretion of terrains in Sri Lanka and played a key role for the amalgamation of the Gondwana supercontinent (Tsunogae and Santosh, 2010). Structurally disposed in extensional fractures post-dating the Pan-African ductile structures (Kehelpannala, 1999), the graphite veins equilibrated at relatively low temperature (500-600 °C). However, the presence of mesoperthites indicate that graphite precipitation may have started at higher temperature. Samples from khondalite host rocks and quartz co-precipitated with graphite from the Bogala and Kahatagana graphite mines in the Wanni Complex were studied. Host-rocks show spectacular decompression reaction aureoles around feldspars and garnet. They contain small CO2 inclusions in garnet cores or quartz in decompression reaction aureoles. Larger, highly transposed brine inclusions are more abundant and are responsible for metasomatic features (feldspar leaching and deposition) observed in the aureoles. Fluid inclusions in vein minerals are dominantly aqueous, rarely mixed H2O+CO2. Fluid inclusions and petrographic data suggest that graphite has been deposited from fluids at decreasing pressure and temperature at relatively reduced redox conditions. Carbon isotope data indicate a dominant mantle source, mixed with small quantities of light C-bearing fluids. It has been proposed that large quantities of mantle-derived CO2 fluid have infiltrated the lower crust during the final stage of Gondwana supercontinent amalgamation (Touret et al., 2016). Formed during strong decompression at the end of a long (up to a few 10 Ma) period of isobaric cooling, the graphite veins in Sri-Lanka (and elsewhere in the former Gondwana) reflects the escape of these granulite fluids to higher crustal levels. In this respect, they are comparable to the quartz-carbonates mega-shear zones found in other granulite terranes (Newton and Manning, 2002). Depending on the redox conditions, former lower crustal fluids (mantle-derived CO2 and/or brines) may either result in mid to upper-crustal quartz-carbonate or graphite veins

Remnants of early Archean hydrothermal methane and brines in pillow-brecia from Isua Greenstone belt, West Greenland

Fluid inclusions containing high-density methane and saline waters (brines), associated with carbonates, have been found in undeformed, annealed quartz-bearing vesicles from pillow-breccia at Isua (West Greenland). Massive quartz veins cementing the pillow fragments contain the same type of carbonate-bearing saline aqueous inclusions as the pillows, but different gaseous inclusions: either trails of low-density methane close to the boundary of the pillow fragment or isolated high-density C

Fluids in metamorphic rocks

Basic principles for the study of fluid inclusions in metamorphic rocks are reviewed and illustrated. A major problem relates to the number of inclusions, possibly formed on a wide range of P-T conditions, having also suffered, in most cases, extensive changes after initial trapping. The interpretation of fluid inclusion data can only be done by comparison with independent P-T estimates derived from coexisting minerals, but this requires a precise knowledge of the chronology of inclusion formation in respect to their mineral host. The three essential steps in any fluid inclusion investigation are described: observation, measurements, and interpretation. Observation, with a conventional petrographic microscope, leads to the identification and relative chronology of a limited number of fluid types (same overall composition, eventually changes in fluid density). For the chronology, the notion of GIS (Group of synchronous inclusions) is introduced. It should serve as a systematic basis for the rest of the study. Microthermometry measurements, completed by nondestructive analyses (mostly micro-Raman), specify the composition and density of the different fluid types. The major problem of density variability can be significantly reduced by simple considerations of the shape of density histograms, allowing elimination of a great number of inclusions having suffered late perturbations. Finally, the interpretation is based on the comparison between few isochores, representative of the whole inclusion population, and P-T mineral data. Essential is a clear perception of the relative chronology between the different isochores. When this is possible, as illustrated by the complicated case of the granulites from Central Kola Peninsula, a good interpretation of the fluid inclusion data can be done. If not, fluid inclusions will not tell much about the metamorphic evolution of the rocks in which they occur. © 2001 Elsevier Science B.V. All rights reserved