Prograde Metamorphic Stage Research Articles

Contribution à l'étude du métamorphisme des gisements cupro-cobaltifères stratiformes du Sud-Shaba, Zaire. Le district minier de Lwishia

The late precambrian copper-bearing strata of southern Shaba (‘Série des mines’) have undergone metamorphism during the latest proterozoic to early paleozoic times. Structural, textural and crystallochemical analysis of this metamorphic event shows a very peculair evolution. During a first phase, prior to folding, a static recrystallization is accompanied by several chemical chanes such as Na, then KMg metasomatism. This hydrothermal character is due to ‘hydrostatic’ stress conditions and fluid circulation linked with surrounding evaporites. During this prograde stage of metamorphism, kyanite + carbonate + sulphide paragenesis suggest low water partial pressure and high pH-low Eh conditions. Geothermometers and fluid inclusions indicate moderate to medium temperatures (300–400°C). Sparse synergetic CuCo sulphide mineralization is overconcentrated to its present disposition. A second hydrothermal event following the main folding phase is probably linked with halokinetic or halotectonic structures. It is essentially retrograde with oxidizing and low-temperature (200°C) conditions, resulting in the total leaching of evaporites and the forming of clay minerals, such as kandites or smectites.

Regional retrograde metamorphism of a high grade terrain: the Willyama Complex, Broken Hill, Australia

Retrograde metamorphism has been a major influence in the development of the presently observed lithologies of the Willyama Complex, Broken Hill. Two broad types of retrogression are distinguished: pseudomorphous and kinematic retrogression. The former type of retrogression involves replacement of prograde phases without complete loss of the high grade fabric; hence the prograde assemblage can frequently be inferred. Kinematic retrogression involves the development of a new schistose fabric and, like pseudomorphous retrogression, is commonly related to F 3 deformation. Retrogression was initiated during the waning stage of prograde metamorphism and was accompanied by an influx of aqueous fluid at similar pressure, but lower temperature conditions than prograde metamorphism. The source of the water is believed to be the crystallization of cooling partial melts. The regional nature of this ‘Willyama-style’ of retrogression is attributed to the metasediment-rich nature of the sequence, the high geothermal gradient and the considerable amount of in situ partial melting.

Stability and paragenesis of Fe-Ti oxide minerals and sphene in the basic schists of the Sanbagawa metamorphic belt in central Shikoku, Japan.

The Sanbagawa basic schists in the Shiragayama area, central Shikoku contain hematite, ilmenite, rutile, sphene and some magnetite. Mineral assemblages of Fe-Ti oxide and silicate minerals in hematite-bearing ones are significantly different from those in hematite-free variety. Excluding albite, quartz, epidote, chlorite and phengite, the mineral assemblages of hematite-bearing ones are hematite+riebeckitic actinolite+sphene or hematite+crossite+sphene in the lower grade zone and hematite+ilmenite+rutile+hornblende±magnetite in the higher grade zone. In the other variety, the mineral assemblages are actinolite+sphene±rutile in the lower grade zone, and ilmenite+hornblende in the higher grade zone. Combining the mode of occurrence and chemistry of Fe-Ti oxides and sphene as well as the Mn-Fe2+ partitioning among magnetite, hematite and ilmenite, stability and paragenesis were determined. Magnetite, ilmenite and rutile in the hematite-bearing basic schists occur in the garnet and biotite zones, whereas, in the hematite-free ones, rutile is restricted to the garnet zone at the prograde stage, and ilmenite occurs in the biotite zone. Sphene is widespread in all the zones, but its occurrence is restricted to the lower grade zone at the prograde stage of metamorphism; its occurrence in the higher grade zone postdates major mineralization. The stable oxide mineral assemblages are magnetite+hematite, hematite+rutile, magnetite+hematite+ilmenite, magnetite+hematite+rutile, hematite+ilmenite+rutile and magnetite+hematite+ilmenite+rutile. Ilmenite and hematite contain significant amounts of MnO; the maximum MnO content of ilmenite and hematite are 24.1 and 2.1 wt. per cent, respectively. Therefore, the paragenesis of Fe-Ti oxide minerals can be determined only in the FeO-Fe203-Ti02-MnO system. The stability of Fe-Ti oxide minerals and sphene is controlled by the bulk-rock chemistry as well as pressure, temperature and oxygen fugacity.