HIGH-TEMPERATURE MEMORY IN CALCIC AMPHIBOLES AND CONSTRAINTS ON COMPOSITIONAL CONTROL OF THEIR AR-40 AR-39 AGES/

Well-characterized calcic amphiboles from an incompletely mixed magmat ic rock association of mainly intermediate dioritic composition were s tudied by incremental heating experiments of Ar-40/Ar-39. The rocks in question were thermally overprinted by late Hercynian granites, Micro probe and scanning electron microscopy analyses show that the amphibol e grains consist of two different calcic hornblende varieties: (1) a S i-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 hig her CaO/K2O ratios (similar to 30-50) compared to the Al-Ti-Fe-K-Na-ri ch hornblende (similar to 12-15), Three hornblende separates that prev iously had yielded conventional K Ar ages of 320, 317, and 302 Ma have been dated by the Ar-40/Ar-39 step-heating technique, The low-and med ium-temperature parts of the Ar-40/Ar-39 age spectrum show discordant Ar systematics with single-step ages generally younger than 325 Ma, Th e gas released at the highest-temperature steps preserves an exception al record of old and remarkably similar integrated ages: 342, 346, and 334 hla, The high-temperature ages correspond to phases with higher a pparent CaO/K2O ratios (similar to 30-45) and are interpreted to resul t from Ar release from Mg-Si-hornblende domains that are assumed to ha ve been very resistent to Ar loss during the late Hercynian thermal ev ents, The results of this study suggest that Ar retentivity differs am ong calcic amphiboles and Mg-Si-rich hornblendes are more Ar retentive than their Al-Ti-Fe-K-Na-rich counterparts.