40AR 39AR DATING OF AMPHIBOLES FROM ZABARGAD ISLAND (RED-SEA) IS PRECLUDED BY INTERACTION WITH FLUIDS

Stepwise heating data on four amphiboles separated from various Zabargad rocks (a dolerite dike, a peridotite, a gneiss and a pyroxenite) invariably show saddle-shaped age spectra. The age information in the spectrum is generally masked by 1-2 nl/g excess 40Ar. The spectrum least affected by excess Ar constrains the emplacement age of the dolerite dike (Z-2137) to be ≤ 20 Ma. The greatest concentration of excess Ar is found in the peridotite, implying that the ultramafic rocks were metasomatized by a very 40Ar-rich fluid. A complex history of subsolidus reactions with hydrothermal fluids is suggested by the irregularities in the 37Ar 39Ar spectra. © 1990.