SIMS U-Pb age of zircons in Archean granulites from the Limpopo Belt, southern Africa

Tsunogae, T. and Yurimoto, H. (1994)@U.S. Geological Survey Circular 1107, p.329.


New U-Pb age of single zircons in Archean granulites was examined by the secondary ion mass spectrometry (SIMS). A Cameca IMS-3f ion microprobe at the University of Tsukuba was employed for the present zircon analysis. Mass resolution of secondary ions for the SIMS analysis was set up to 6500 (M/M, 10% valley) which is sufficient to resolve all major interferences from U, Pb, and Th isotopes. Although the Pb intensity analyzed by the 3f (about 1 to 2 cps/ppm) is lower than that by SHRIMP, it is sufficient to apply to the U-Pb method of zircon with high radiogenic Pb concentration. The relationship between 206Pb+/238U+ and UO+/U+ ratios was used for the 3f analysis to obtain unknown 206Pb*/238U ratio in terms of standard zircon (SL13 provided by the ANU) as mentioned by Compston et al. (1984). 207Pb*/206Pb* ratio was estimated as measured, because Pb isotopic fractionation on the SIMS analysis was negligibly small.

The analytical procedure was applied to zircons in granulites from the Limpopo Belt in southern Africa. The belt can be lithologically subdivided into the Central Zone (CZ; T=750 -850°C, P>12 kbar) and the Northern and Southern Marginal Zones (NMZ and SMZ; T=700-750°C, P=7.0-7.5 kbar). Preliminary analyses of zircons in the tonalitic Sand River Gneiss (CZ) display ages of 3200-3350 Ma, which show good agreement with previous Sm-Nd age (3200-3300 Ma; Harris et al., 1987) and U-Pb age (<3290 Ma; Retief et al., 1990).

The CZ shows SIMS U-Pb ages of 3100-3350 Ma, while the NMZ shows younger ages of 2650-2800 Ma. Rim of the 2780 Ma zircon from the NMZ suggests a later thermal event at about 1800 Ma. There are two possible explanations for the differences in U-Pb ages between the CZ and NMZ; (1) timing of peak metamorphism of the CZ was older than that of the NMZ, which is consistent with the differences in P-T path. (2) older relict zircon from the CZ did not suffer significant reset of U-Th-Pb system during peak metamorphism, because of rapid uplifting and cooling history of the CZ.

Compston,W., Williams,I.S., and Meyer,C., 1984, JGR, 89, (suppl.) B525-B534.

Harris,N.B.W., Hawkesworth,C.J., Van Calsteren,P., and McDermott,F., 1987, EPSL, 83, 85-93.

Retief,E.A., Compston,W., Armstrong,A., and Williams,I.A., 1990, In J.M.Barton,Jr., Eds., Extended Abstracts, The Limpopo Belt. RAU, Johannesburg, 95-99.