Argon retentivity of hornblendes: A field experiment in a slowly cooled metamorphic terrane

Hornblende from samples of amphibolite and granitic gneiss, collected within a single outcrop in the central Adirondacks, yield significantly different ⁴⁰Ar^{* 39}Ar_k dates of 948 ± 5 and 907 ± 5 Ma. Assuming that this terrane cooled slowly following high-grade metamorphism and that the samples have experienced the same thermal history, the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples. The Fe/(Fe + Mg + Mn) of the hornblende samples are 0.8 and 0.6, the higher ratio corresponding to the younger ⁴⁰Ar^{* 39}Ar_k date. Transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution, but contain zones of fibrous phyllosilicates ~0.1 to 2 μm wide parallelling (100) and (110). These alteration zones probably formed during post-metamorphic cooling as a result of the migration of fluids through the hornblendes, and are obvious pathways for argon escape from hornblende. As these features are more abundant in the hornblende sample with the younger ⁴⁰Ar ³⁹Ar date and higher Fe/(Fe + Mg + Mn), they may influence the argon blocking temperature by effectively partitioning the hornblende grains into diffusion domains of varying size. Biotite from the granitic gneiss yields an ⁴⁰Ar^{* 39}Ar_k date of 853 ± 2 Ma, with a mildly discordant stepheating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite. Plagioclase from the amphibolite yields a ⁴⁰Ar^{* 39}Ar_k integrated date of 734 ± 3 Ma. All the ⁴⁰Ar ³⁹Ar data are consistent with postmetamorphic cooling rates of 1° to 5°C/Ma.