THE PRESENT STATUS OF ZIRCON

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SUMMARY

Saxena (1966a) proposed that zircon could be authigenic and low-grade metamorphic, as well as magmatic in origin. He questioned the mechanical and chemical stability of zircon, the use of crystal morphology in correlation, and the use of roundness as a criterion of detrital origin. In so doing, he challenged the role of zircon in problems of stratigraphy, pedogenesis, and sedimentary, igneous, and metamorphic petrology.

SAXENA'S(1966a, b) evidence for authigenic and low-grade metamorphic zircon is unconvincing, since conventional explanations adequately encompass his obser- vations, and do not conflict with established knowledge of zircon behaviour.

During weathering, authigenesis, and metamorphism, zircon is shown to conform with the concepts that:

(1) its removal and transport in solution is minimal, (2) zircon formation during authigenesis and low-grade metamorphism is insignificant. Rare occurrences of hydrozircon confirm this paucity of authigenic zirconium minerals. At high metamorphic grades zircon-transformation is favoured by the “wet” metasomatic processes of the amphibolite facies and impaired by the “dryness” of the granulite facies. The occurrence of zircon transformation in the granulite facies may also be prevented by incorporation of zirconium in the lattice of newly-formed pyroxene. Release of this zirconium during diaphthoresis of pyroxene-bearing granulites could give rise to new zircon.

The formation of zircon during authigenesis, and its formation and transfor- mation during low-grade metamorphism, would diversify initially homogeneous assemblages; but Saxena has failed to prove that either process does in fact occur. It is contended that, since the positive correlation of zircon assemblages from co-eval sedimentary and metamorphic rocks of the Kingsbridge area is opposed to Saxena's views regarding the origin and behaviour of zircon, the roles of authigenesis in the formation of zircon, and of low-grade metamorphism in its formation and transformation, are insignificant or non-existent.

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