THE HIDDEN WORLD OF K-FELDSPARS: DO WE REALLY KNOW WHAT WE ARE DATING?

Ten K-feldspar samples from the Devonian Mattamiscontis Mountain, Seboeis Lake, Center Pond, and Passadumkeag River plutons in east-central Maine were dated using the ⁴⁰Ar/³⁹Ar method. Step-heating experiments conducted on these separates produced climbing age spectra from ~190-225 to ~350 Ma that would typically be interpreted to reflect slow regional cooling following Acadian metamorphism. However, the maximum ages of K-feldspar show a range of more than 50 m.y. both regionally and among samples from the same pluton. This range of ages suggests that factors other than slow cooling have affected the age spectra. X-ray powder diffraction (XRD) analyses of the K-feldspar separates reveal that each sample contains K-feldspar of at least two structural states and that polymorph content varies among the samples. Most K-feldspar grains display optically clear domains with Carlsbad twinning. These domains are brightly luminescent in cathodoluminescence (CL) imaging and are cut by darkly or non-luminescent domains and veins of multiple generations of secondary K-feldspar. These younger K-feldspar populations (1) tend to be turbid with abundant fluid inclusions recording microporosity; (2) may display tartan twinning; and (3) are commonly associated with albite intergrowths in different textural settings and with a variety of sizes and periodicity. Coexisting magmatic plagioclase grains and albite inclusions in K-feldspar are luminescent. In contrast, most secondary albite veins and patches contain abundant fluid inclusions, are non-luminescent, and contain cracks with micrometer size euhedral muscovite grains. These albite intergrowth textures and their association with secondary K-feldspar domains suggest the replacement of magmatic feldspars by multiple populations of secondary K-feldspar, albite, and muscovite. These results show that these samples are ill suited for applying the Multi-Diffusion Domain model. On the contrary, the ~350 Ma apparent ages of the high-temperature steps are most easily interpreted as regional cooling ages following cooling of the Devonian pluton hosts. The range of apparent ages in the low-temperature steps probably reflects the times of secondary K-feldspar and muscovite crystallization during known Early Mesozoic magmatic and tectonic events.