charnockite
 Charnockites are an assemblage of of foliated metamorphosed igneous rocks that share genesis through differentiation of the similar parent magmas. Charnockites typify the deep continental crust of the Earth, and they are commonly found in granulite-facies metamorphic terranes. Charnockites are characterized by the presence of strongly pleochroic, reddish or green hypersthene. Acid members of the charnockite series includes are rich in quartz and microcline; intermediate members correspond mineralogically to norites (cu), quartz-norites and diorites; and, basic members contain pyroxene and olivine. The rocks have a banded structure and many charnockite minerals are schillerized – containing tiny platy or rod-shaped enclosures that lie parallel to particular crystallographic planes or axes. The surfaces of these enclosures reflect light, giving the contained minerals an atypical appearance – quartz appears blue and opalescent, the feldspar has a milky shimmer, and hypersthene gleams bronzy and metalloidal. The banding results predominantly from fluxion in the viscous crystallizing intrusive magma, accompanied by differentiation or segregation of the mass into bands of different chemical and mineralogical composition. Charnockites are of wide distribution in the southern hemisphere, including India (i, 2), Sri Lanka, Madagascar, and Africa. Very similar rocks described as pyroxene granulites, pyroxene gneisses, and anorthosites occur in Norway, Sweden, France, Germany, Scotland, and North America. Charnockites are generally regarded as being of Archean or Precambrian age, and share a history with the Earth's most primitive gneisses. |
| links: images: hand-specimens: charnockite c-u, Norway; charnockite, a crystalline granitic rock (hypersthene monzonite - mangerite), composed largely of two feldspars (perthite and andesine), with 5 volume percent orthopyroxene (hypersthene), with minor amounts of quartz, myrmekite, hornblende, biotite, magnetite and clinopyroxene, and with traces of apatite, sulphides and zircon, Pangnirtung region, seBaffin Island; charnockite, Tanzania; formations: in situ charnockite formation in granitoid gneiss of Wanni Complex, N of Kurunegala, Sri Lanka; charnockite patches make a patterned array, quarry at Kottavattom, South India; dark charnockite forms veinlets and patches obliterating the structure of granodioritic Hbl-Bt gneiss in Kabbaldurga quarry, South India; charnockite; marble on the banks of the Hudson River, Warrensburg, coarse marble with xenoliths of charnockite, Hudson River, Warrensburg, Adirondacks; Sjelset charnockite; charnockite; close-ups: charnockite, 2, 3; in situ charnockite formation in garnet-sillimanite gneiss, Kerala Khondalite Belt, Southern Granulite Terrain, southern India; charnockite vein overprinting foliation in Hbl-Bt gneiss, Ponmudi, South India; xenolith of Proterozoic crustal rock (charnockite) in a dike of alkali-ultramafic rock (greenish black), Turiy Mys (Terskiy Coast, Kola, Russia); blue quartz in charnockite, Old Rag Mountain, Shenandoah National Park; charnockite containing large xenocrysts of gray andesine (2, 3) detached or mixed from anorthosite margin, between Minerva and Newcomb, Adirondacks; charnockite on High Peak, 2, Tobacco Row Mountain, Amherst County; charnockite, India; charnockite (on right), at top; lower contact of an aplitic xenolith in the Bitterfontain charnockite, and slight weathering in the Bitterfontein charnockite provides the means to distinguish the mineralogy and coarse texture; thin-sections: charnockite from Blue Ridge province, Grenville province; charnockite pl, xp, Norway; pl, xp, Charnockite d'Ansignan dans le massif de l'Agly, Pyrénées; Sjelset charnockite - albite-twinned plagioclase (black and white, lower left), orthoclase (gray), quartz (cream, white), myrmekite with circular to oval cross-sections of coarse quartz vermicules (white to gray, lower right); diagrams: PT diagram of high grade metamorphic facies; webpages: Charnockites; Petrological features of granulites, charnockites and migmatites; field trip to eastern and central Adirondacks |
