ISOTOPIC PECULIARITIES OF AN ARCHEAN PEGMATITE (UNION MINE, MICA, SOUTH-AFRICA) - GEOCHEMICAL AND GEOCHRONOLOGICAL IMPLICATIONS

An isotopic investigation (Rb-Sr, U-Pb, Pb-Pb, and Sm-Nd) of the Archa ean Union Mine pegmatite, Northern Province, South Africa, is reported . The intrusion of this pegmatite, devoid of zircon and monazite, has been dated at 2912 +/- 2.6 Ma by Pb-Pb step leaching of magmatic garne t. This age is supported by an identical two garnet-apatite Sm-Nd isoc hron of 2917 +/- 27 Ma. A Rb-Sr K-feldspar-garnet-albite isochron of 2 023 +/- 11 Ma corresponds to a later (hydro)thermal overprint. Isotope resetting during this overprint was incomplete and revealed that vari ous parent-daughter isotope pairs within minerals of the Union Mine pe gmatite behaved in exceedingly different manners under the same physic o-chemical conditions. Sr isotope resetting was incomplete and resulte d in open system behaviour of the pegmatite. K-feldspar almost quantit atively lost its (radiogenic) Sr, which had accumulated between the ti me of intrusion and the overprint. Sr loss from muscovite was variable and incomplete as indicated by a wide range in ages from 2088 +/- 74 Ma to 2744 +/- 13 Ma. Only insignificant portions of this Sr were inco rporated into adjacent albite las revealed by a micro-drilled Rb-Sr pr ofile through a muscovite-albite grain boundary). The Sr-87/Sr-86 init ial ratio is 0.8301 +/- 11 for the Proterozoic isochron, which is far less radiogenic than the calculated value of 2.24 for a closed system. It is therefore concluded that Sr was lost from the system and that t he Rb/Sr ratio of the whole-rock was changed. Apatite, on the other ha nd, seems to have behaved as a closed system and retains a low initial Sr ratio of 0.7125. U-Pb analyses of apatite reveal that this mineral has behaved as an almost closed system for Pb diffusion as well, but was open for U (significant loss) during the overprint. This has resul ted in high reverse discordance of apatite U-Pb analysis and in a geol ogically meaningless secondary Pb-Pb isochron. The data demonstrate th at both preferential loss of daughter (Sr) over parent (Rb), and paren t (U) over daughter (Pb) isotopes can occur in some minerals of one an d the same rock. This process altered the parent/daughter ratios to su ch an extent that, in the case of apatite, a secondary Pb-Pb isochron with an erroneous age of 4119 +/- 66 Ma was established. It appears li kely that preferential loss of daughter or parent isotopes is the reas on for 'spurious' secondary whole-rock isochrons reported and reviewed by Moorbath and Taylor (1986) and Moorbath et al. (1986). Such losses , far exceeding predictions based on volume diffusion experiments, may occur by means of fast-transport diffusion, which in part may depend on the micro-textures of the host minerals. Geochemical studies of ear ly Archaean rocks require substantial back-correction for radiogenic d ecay, and the example of the Union Mine pegmatite shows that changes o f parent/daughter isotope ratios may occur long after rock formation. However. such changes may go unnoticed in high precision geochronology (i.e. garnet dating in this study), but will lead to erroneous correc tion for radiogenic decay and hence to erroneous conclusions regarding the origin of the studied samples. (C) 1998 Elsevier Science B.V.