Reply to Comment by Y.-F. Zheng on "Pb-isotopic evidence for U-Th-Pb behaviour in a prograde amphibolite to granulite facies transition from the Lewisian complex of north-west Scotland: Implications for Pb-Pb dating"

Details Reply to Comment by Y.-F. Zheng on "Pb-isotopic evidence for U-Th-Pb behaviour in a prograde amphibolite to granulite facies transition from the Lewisian complex of north-west Scotland: Implications for Pb-Pb dating" Reply to Comment by Y.-F. Zheng on "Pb-isotopic evidence for U-Th-Pb behaviour in a prograde amphibolite to granulite facies transition from the Lewisian complex of north-west Scotland: Implications for Pb-Pb dating"

Jun 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54.1839w&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 54, Issue 6, pp.1839-1842

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Scientific paper

In a recent paper ( 1989a), Pb-isotopic arrays obtained from a prograde amphibolite to granulite facies transition in the Lewisian Complex were interpreted in terms of different degrees of Pb-isotopic homogenisation and different styles of U / Pb fractionation during the 2660 Ma Badcallian metamorphism. (1990) comments that post-2660 Ma U-Pb disturbance might have contributed significantly to the observed Pb-isotopic arrays, thus requiring a re-evaluation of the proposed model for U-Th-Pb behaviour at 2660 Ma. This comment raises an important point with regard to the general interpretation of Pb- (and other) isotopic data from ancient poly-phase metamorphic terranes, namely the extent to which particular events may be evaluated as significant in multi-stage isotopic evolution or dismissed as insignificant, thus reducing the number of stages and permitting more definitive modelling of earlier events. In this reply, it is argued that, for the samples studied by (1989a), post-Badcallian events are not represented or are unable to generate the observed Pb-isotopic arrays. The modelling of (1989) for these samples is considered to be inappropriate and misleading. The Pb-isotopic data of et al. (1969), for which measured U / Pb ratios are available, appear to indicate an U-Pb disturbance at ~900 Ma when reduced using a three-dimensional isochron plane model (ZHENG, 1990). Similar results are obtained using the approach of Gale and Mussett (1973) for reducing three-stage data to two-stage. However, ages obtained from both these methods are highly dependent upon the assumption of initial ratio homogenisation during Badcallian metamorphism. Geochemical considerations discussed by Whitehouse (1989a) and reiterated here question the validity of this assumption and any modelling based upon it.

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