Rare-earth and LIL element fractionation in high-grade charnockitic gneisses, south Norway
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Rare-earth element
Anorthosite
Breccia fragments expected from a well-studied boulder of Stillwater anorthosite have been modeled to test the ability to identify parental rock types from examination of breccia and soil fragments. Depending on their size, the boulder fragments give distributions that suggest mixtures of rock types, including monominerallic anorthosite with subordinant amounts of more gabbroic anorthosite, anorthosite, and gabbro for small fragments. The distribution of FeO in samples of lunar ferroan anorthosite (FAN) indicates that FAN has a heterogeneous distribution of mafic minerals like the boulder.
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The relationships of the Adirondack anorthosite and associated rocks have been reinvestigated. The order of intrusion is interpreted as anorthosite, gabbro, quartz‐syenite, diabase, granite, diabase (Keeweenawan). The evidence is clear that gabbro and quartz‐syenite form dikes in anorthosite, and it is believed that they also show evidences of chilling against anorthosite. The anorthosite‐magma has given rise to profound pyrometasomatic replacements of limestone country rock of a type peculiar to it. The metamorphosed rocks are locally injected by quartz‐plagioclase veinings related to the anorthosite. All the phenomena are consistent in indicating formation of the anorthosite through differentiation from a gabbroic anorthosite‐magma with a distinct content of volatiles. The hypothesis of a rest‐magma of the composition of the quartzsyenite on the one hand, squeezed out from an anorthositic fraction on the other, does not satisfy the requirements of the new data.
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Magma chamber
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The Chicoutimi Gneiss Complex was previously considered to host the huge 1160–1140 Ma Lac St. Jean anorthosite suite (20,000 km2), the 1082 ± 3 Ma Chicoutimi Mangerite, and the 1067 ± 3 Ma La Baie Granite. New geological mapping and geochronological data from the Chicoutimi Gneiss Complex now demonstrate clear correlations among some gneissic units and the partially deformed Chicoutimi Man-gerite and La Baie Granite. The remaining gneissic units in the Chicoutimi Gneiss Complex have been grouped into six new lithodems. The three oldest were generated during two distinct events. The Saguenay Gneiss Complex, which is one of...
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The New Russia gneiss complex in the northeastern Adirondack Highlands of New York includes meta-anorthosite gneiss and anatectic gneiss derived from metagabbro, mangerite, and charnockite. Metamorphic conditions during anatexis (850-950°C, highest near the anorthosite, with pressure ~750 MPa) are inferred from minerals and textures produced by dehydration melting of pargasitic hornblende and from ternary feldspar in anatectic segregations. The complex abuts and is crosscut by the eastern margin of the ~1130 Ma Marcy anorthosite massif. The crosscutting contact, the presence of meta-anorthosite gneiss within the complex and undeformed meta-anorthosite in the massif, and the occurrence of deformed and undeformed anatectic segregations within the New Russia gneisses indicate an approximate synchroneity of penetrative deformation, very high temperature metamorphism, and emplacement of anorthosite with both intrusion and anatexis outlasting deformation. The position of the New Russia gneisses, the metamorphic gradient within them, and the contemporaneity of anatexis with intrusion of anorthosite imply that the complex is the metamorphic aureole of the Marcy anorthosite.
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Several controversial points in the geology of the Adirondack anorthosite are briefly discussed, and some new observations are added. Balk's postulated evolutionary series, leading from gabbroic anorthosite to large, spherical gabbros, needs revision in view of Buddington's proof that anorthosite is cut by discordant gabbro dikes. The possible derivation of gabbroic anorthosite from Grenville skarn is discussed. A certain amount of pyroxene, and perhaps even of andesine, has been precipitated by volatiles. Accumulations of mafic minerals, therefore, need careful examination to establish their origin. A rather prolonged stage during which diminishing flowage and initial fracturing overlapped in place and time is thought to explain, to some extent, the observed occurrence of mineral-veneered short fractures with, or without, marginal flexures in the surrounding anorthosite. The origin of ill-defined siliceous veins and of the rare anorthosite dikes may be related to this condition of the anorthosite. The evidence for a chilled border of the anorthosite is judged to be inconclusive. Some features bearing on the shape of the anorthosite massif are reviewed, and interpretations of syenite dikes in anorthosite are briefly discussed.
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During the geological mapping in the Fiskenæsset region the origin of the different gneiss types has been much debated, and it has been suggested that some of the gneisses may represent metasomatically altered anorthositic and amphibolitic rocks. F. Kalsbeek suggested that to test this hypothesis a study should be made of possible mineralogical and chemical changes at boundaries between anorthositegneiss and amphibolite-gneiss. The first results of such an investigation are reported here.
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Layered intrusion
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