Excess argon in amphiboles from fluid interaction and short intrusion interval at the Epizonal Marangudzi Complex, Zimbabwe

The epizonal Marangudzi complex of the Karoo‐associated Nuanetsi igneous province in southern Zimbabwe was emplaced into Precambrian gneisses and consists of a gabbro mass intruded in turn by ring dikes of quartz and nepheline syenites. Amphibole and biotite from various lithologies have been examined using 40Ar/39Ar techniques in order to investigate the temporal distribution of intrusions and the possible occurrence of excess Ar. With one important exception, samples give internally concordant incremental‐release spectra; average 40Ar/39Ar dates range from 177.6 to 181.3 Ma. Three amphiboles give dates which are significantly older than the other samples and contain small yet discernible quantities of excess 40Ar. Excess Ar is restricted to amphiboles displaying substantial Fe/Mg zoning which is interpreted as reflecting late magmatic or early sub‐solidus reaction with fluid phases. Argon was apparently acquired from the fluid during alteration and, ultimately, could have been magmatic or could have come from degassing of the country rocks. The remaining samples (both amphibole and biotite) show only a restricted range (∼1 m.y.) in apparent age and are essentially analytically indistinguishable, implying rapid cooling of the complex. The average for these analyses, 178.1 Ma, reflects the mean age of emplacement of cogenetic intrusions within a short interval ≤1 m.y. in duration. This revised age of the Marangudzi complex fits well within the temporal framework of Karoo province magmatism in the region.