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Examples of twinning and parallel growth in zircons from some Precambrian granites and gneisses

Published online by Cambridge University Press:  05 July 2018

J. Jocelyn
Affiliation:
Scottish Universities Research and Reactor Centre, East Kilbride, Scotland
R. T. Pidgeon
Affiliation:
Scottish Universities Research and Reactor Centre, East Kilbride, Scotland

Summary

Growth twins, parallel growths, and necked crystals are described from zircon populations 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 of zircons that have initially undergone separate growth histories. This implies that viscosity conditions of the rocks were low enough to permit the movement and collision of growing zircon crystals. The common occurrence of zircons with central constrictions, sometimes superimposed on transverse fractures, in zircon suites from the granitic gneisses is explained in terms of late-stage chemical corrosion accompanied by brittle fracturing.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1974

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