Examples of twinning and parallel growth in zircons from some Precambrian granites and gneisses
SUMMARY. Growth twins, parallel growths, and necked crystals are described from zircon popula-tions from granitic gneisses from the Precambrian of SW. Greenland. From observations of the distribution of internal growth zones it is concluded that whereas twinning takes place early in the growth histo...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.625.2037 http://www.minersoc.org/pages/Archive-MM/Volume_39/39-305-587.pdf |
| Summary: | SUMMARY. Growth twins, parallel growths, and necked crystals are described from zircon popula-tions from granitic gneisses from the Precambrian of SW. Greenland. From observations of the distribution of internal growth zones it is concluded that whereas twinning takes place early in the growth history of elbow twins it can occur at any time during crystallization of the zircon. Parallel growth is attributed to a process of synneusis or attachment ofzircons that have initially undergone separate growth histories. This implies that viscosity conditions of the rocks were low enough to permit he movement and collision of growing zircon crystals. The common occurrence of zircons with central constrictions, ometimes superimposed on transverse fractures, in zircon suites from the granitic gneisses i explained in terms of late-stage chemical corrosion accompanied by brittle fracturing. SECONDARY growth features have been observed in zircons from rocks suspected of having an anatectic or metasomatic history (Hoppe, 1959; Harris, 1959; Dalziel, 1963; Poldervaart and Eckelman, 1955; Ozhogin, 1968; Berzina and Sotnikov, 1972). Poldervaart and Eckelman (1955) regarded features uch as outgrowths, aggregate crystals, and overgrowths as characteristic of autochthonous granites. Similar mor- |
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