Volume 38: January - December 1997
Issue 5: May 1997
and AA .Nemchin RT .Pidgeon School of Applied Geology, Curtin University of Technology, CPO Box U1987, Perth , WA 6001,Australia
ABSTRACT Primary and secondary internal structures observed in zircons from granites from the Late Archaen Darling Range Batholith record stages in the evolutionary history of the granites and provide a basis for a SHRIMP U-Pb study on the timing of granite evolution. Many granite zircons contain cores. A few low-U cores retain concordant to slightly discordant U-Pb ages of 2.8 Ga, some sow discordant intermediate ages but most have nearly concordant ages within the range 2690-2650 Ma. these ages are interpreted as dating the source rocks of the Darling Range granites, or as representing different degrees of isotopic resetting owing to recrystallization of protolith zircon during prograde metamorphism and subsequent melting. The zircon cores are enclosed by inner rims of oscillatory zoned zircon, which are interpreted as zircon growth during the main crystallization phase of the granite magma. SHRIMP ages of zoned zircon, of between 2648 and 2626 Ma, suggest an extended period of granite crystallization. The oscillatory zoned inner rims are surrounded by weakly zoned to unzoned outer rims which transgress primary zoning of the inner rim, suggesting corrosion followed by new zircon deposition. However, the preservation of weak zoning in the outer rim and the euhedral nature of external zircon faces, which are identical to those developed in the inner rim, suggests that the outer rims are in fact recrystallized outer parts of inner rims. This conclusion is supported by the younger ages (2628-2616 Ma) determined for outer rims. These results indicate that formation of outer rims and accompanying loss of radiogenic Pb occurred during or soon after granite crystallization and before zircons had time to accumulate significant radiation damage, suggesting that the recrystallization process is independent of the degree of metamictization. The history of formation of the Darling Range granites contained with the zircon crystals suggests initial magma formation between 2690 and 2650 Ma, crystallization and emplacement of the granite magma at 2648-2626 Ma, and slow cooling, indicated by marginal recrystallization and continued Pb loss from the zircons, until 2628-2616 Ma.
: Archaean granites; zircon U-Pb ages; zircon structures Keywords
Pages:
625 -649
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