PB-PB DATING OF APATITE BY A STEPWISE DISSOLUTION TECHNIQUE

Apatites from different geologic settings have been dated by a new seq uential dissolution method. The procedure is based on stepwise exposur e of apatite to weak-intermediate acids. A pegmatite sample from the K erala Khondalite Belt in southern India yielded an age of 509 +/- 25 M a. Apatites from an enderbite sample from the Northern Marginal Zone o f the Limpopo Belt in southern Africa yielded an age of 1970 +/- 26 Ma which is in agreement with the high-grade metamorphism at around 2.0 Ga, reported from the Triangle Shear Zone and amphibolite-facies metam orphism in the Transition Zone. Apatites from an amphibolite-facies gn eiss from the Transition Zone south of the NMZ yielded an age of 1878 +/- 18 Ma. This age is somewhat younger than a sphene age on the same rock of 1971 +/- 17 Ma and is believed to reflect the time of retrogra de metamorphism in the presence of a fluid phase. The sequential disso lution method is a new tool that allows dating of apatites which have relatively unradiogenic Pb ratios because the spread can be significan tly increased and no correction for common lead is needed. This study further demonstrates that apatites from magmatic and high-grade metamo rphic rocks yield ages very close to those of monazite, zircon or garn et, unless the rock subsequently interacted with a fluid phase. (C) 19 97 Elsevier Science B.V.