High-temperature memory in calcic amphiboles and constraints on compositional control of their 40Ar/39Ar ages

Abstract

Well-characterized calcic amphiboles from an incompletely mixed magmatic rock association of mainly intermediate dioritic composition were studied by incremental heating experiments of 40 Ar/ 39 Ar. The rocks in question were thermally overprinted by late Hercynian granites. Microprobe and scanning electron microscopy analyses show that the amphibole grains consist of two different calcic hornblende varieties: (1) a Si- and Mg-rich variety and (2) a variety enriched in Al, Ti, Fe, K, and Na. The Mg-Si–rich hornblende can be further characterized by its higher CaO/K 2 O ratios (∼30–50) compared to the Al-Ti-Fe-K-Na–rich hornblende (∼12–15). Three hornblende separates that previously had yielded conventional K-Ar ages of 320, 317, and 302 Ma have been dated by the 40 Ar/ 39 Ar step-heating technique. The low- and medium-temperature parts of the 40 Ar/ 39 Ar age spectrum show discordant Ar systematics with single-step ages generally younger than 325 Ma. The gas released at the highest-temperature steps preserves an exceptional record of old and remarkably similar integrated ages: 342, 346, and 344 Ma. The high-temperature ages correspond to phases with higher apparent CaO/K 2 O ratios (∼30–45) and are interpreted to result from Ar release from Mg-Si-hornblende domains that are assumed to have been very resistent to Ar loss during the late Hercynian thermal events. The results of this study suggest that Ar retentivity differs among calcic amphiboles and Mg-Si–rich hornblendes are more Ar retentive than their Al-Ti-Fe-K-Na–rich counterparts.