Charnockitic Gneisses Research Articles

Precambrian basement at Molodezhnaya Station, East Antarctica

The Precambrian basement exposed around Molodezhnaya Station (lat 67°40′S, long 45°50′E) in western Enderby Land, Antarctica, consists largely of well-layered, in part migmatitic, gneisses and of plutonic gneisses. The most abundant rock types in the well-layered gneisses are pyroxene gneiss, hornblende gneiss, garnet-biotite gneiss, garnet-pyroxene gneiss, and quartzo-feldspathic gneiss. The plutonic gneisses crop out in three bodies, each of a different rock type: charnockitic, granodioritic, and enderbitic. Field relations show that the charnockitic gneiss locally cuts the compositional layering in the well-layered gneisses and that it was emplaced prior to granulite-facies metamorphism and folding. Total-rock Rb-Sr ages calculated on seven samples of quartzo-feldspathic gneiss range from 460 ± 250 to 2,120 ±155 m.y. (assumed initial Sr 87 /Sr 86 ratio of 0.715). Eight samples of charnockitic gneiss give an isochron age of 987 ± 60 m.y. and an initial ratio of 0.7109 ± 0.0015. Seven samples of granite give an isochron age of 512 ± 155 m.y. U-Th-Pb data on monazite, sphene, and allanite from pegmatites indicate an age of crystallization of 500 to 550 m.y. for the pegmatites. Rb-Sr ages of 460 and 465 m.y. were obtained on biotites from gneiss. The geochronologic data are consistent with the following sequence of events: (1) Formation of the well-layered gneisses, possibly 2,000 m.y ago. (2) Plutonism, granulite-facies metamorphism, and folding, 1,000 m.y. ago. (3) Dikes of granite and pegmatite, amphibolite-facies metamorphism, and faulting, 500 to 550 m.y. ago. (4) Secondary mineralization, possibly 400 to 500 m.y. ago. The event 1,000 m.y ago appears to have been important in this part of Antarctica, and some Antarctic charnockites may have been emplaced at that time.

The chemistry and origins of Proterozoic low-potash, high-iron, charnockitic gneisses from Tromøy, south Norway

New major and trace element data for 79 acid-intermediate charnockitic gneisses (the Tromøy gneisses) and 16 associated metabasites from the island of Tromøy show that this part of the 1200–900-m.y. Sveconorwegian zone is occupied by rocks of unusual composition. Overall values for K and Rb are the lowest yet reported for any granulites, and K/Rb ratios are very high. Cs and Th are also low and, abnormally for granulites, so are Ba, Sr and Zr. Ba/Sr ratios are similar to those in other suites, but K/Ba and K/Sr are higher. These features may partially be reflecting unusual pre-metamorphic lithologies, but it is considered more likely that they are largely the product of metamorphically induced depletion processes involving metasomatism. There is some indication that the Na 2O/CaO and normative Ab/An ratios may also have been modified during metamorphism. Data for the presumed relatively immobile elements Cr, Co, Ni and V support an igneous origin for the Tromøy gneisses, but the presence of a paragneiss component cannot be ruled out. A characteristic of the gneisses is their high iron content, and spatial and temporal considerations point towards a genetic link with the iron-rich, intrusive rapakivi suites of Finland, Sweden and south Greenland. If the Tromøy gneisses do represent material of this type, it would seem to follow that potash fractionation has been extreme.

ON THE GEOLOGY OF THE MIRNYY STATION AREA

The U. S. S. R. -IGY Antarctic station, Mirnyy [66°31'S. 93°E.], is situated on four outcrops of Precambrian charnockitic granite, crystalline schist, and gneiss. The four mounds contain numerous angular xenoliths, generally dark schist and gneiss, denoting a palingenetic origin for the bedrock. Quartz veins and sulfide mineralization indicate later hydrothermal activity. The mounds are cut by two major fissure systems perpendicular to each other and several secondary fissure systems. In general, the offshore islands (the Haswell Islets) and the mounds are similar to the ancient Antarctic shield and are, therefore, considered to be a part of it. --Editor.