Metasomatic Rocks

Metasomatic Rocks

 

rocks formed as the result of metasomatism. Depending on the mode of occurrence, formation temperature, and stages of the hydrothermal process at which they formed, metasomatic rocks are divided into different groups.

Among the high-temperature products of the early alkaline stage are magnesian and calcareous skarns, which usually form at the contacts of granites and syenites with carbonate rocks. Magnetite, borate, borosilicate, and phlogopite ores are associated with magnesian and calcareous skarns. The formation of greisens (with tin, tungsten, and beryllium mineralization) and secondary quartzites (with copper and molybdenum mineralization) occurs during the acid stage. The products of metasomatism developing near ore veins—beresite, listvenite, and chlorite-carbonate rocks—are from the late alkaline stage and the stage that is transitional to it. In volcanic regions propylites are widely distributed. Fenites form at contacts with intrusive alkaline rock, and albitite and albite-spodumene rocks with rare-earth mineralization develop in association with pegmatites. In eugeosynclinal ophiolitic belts, chlorite-albite, glaucophane, aegirine, and jadeite rocks and spilites form as a result of sodic metasomatism. Serpentinites, steatites, and anthophyllitic and quartz-magnesite rocks develop by replacement of dunites and peridotites.

REFERENCE

Osnovnye problemy v uchenii o magmatogennykh rudnykh mestorozhdeniiakh, 2nd ed. Moscow, 1955.
References in periodicals archive ?
The Bamble and Kongsberg-Modum lithotectonic domains of south Norway represent classic high-grade metamorphic terrains [12-18], which contain a series of different metasomatic rocks. An early scapolitisation event, where Cl-rich scapolite coexisted with enstatite, phlogopite, amphibole, and rutile is constrained at 600 to 700[degrees]C at mid-crustal levels [19-21].
Petrography and mineral chemistry of the metasomatic rocks have been described in detail by Engvik et al.: albitisation of granitoid [35], scapolite metagabbro and vein-related albitisation of mafic rock [20, 29], albite-carbonate-rich deposits and albitic felsites [23], and scapolite metagabbro, clinopyroxene-bearing albitite, and rutile-albitite [50].
For the major elements, systematic geochemical changes are seen for the elements Na, Ca, Fe, and Mg in the metasomatic rocks compared to the protoliths.
In addition, fluid compositions can also have been varied, even on a local scale, and scales of equilibrium might also have been local, regardless of the widespread regional occurrence of the metasomatic rocks.
Separation of hydrous and C[O.sub.2]-dominated fluid phases and brines could possibly explain the complex pattern in the spatial distribution of metasomatic rocks containing scapolite metagabbros, different varieties of albitites, and carbonate deposits in the Bamble lithotectonic domain.
and Pollard, P.J.: 1989, Albite-rich, silica-depleted metasomatic rocks at Emuford, Northeast Quennsland: mineralogical, geochemical and fluid inclusion constraints on hydrothermal evolution and tin mineralization, Econ.
Residual bauxites developed in the weathering, interlaminated, carbonate rocks, schists, and metasomatic rocks of Rifean age.