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

Amphibole (Am), biotite (Bt) and K-feldspar (Ksp) from early and late Devonian plutons in east-central Maine were dated using the ⁴⁰Ar/³⁹Ar method. Cooling ages of Am (~ 392 and 365 Ma) and Bt (390 and 360 Ma) indicate fast cooling rates of ~7°/m.y. In contrast, step-heating experiments on ten Ksp separates produce climbing age spectra (~200-350 Ma) that would typically be interpreted to reflect slow regional cooling. Significant differences in ages from Ksp’s both regionally and between samples from the same pluton suggest that factors other than slow cooling may cause the climbing age spectra and prompt a more detailed characterization of the Ksp’s.

For the entire suite of samples, no clear correlation was found between age spectrum shape and the Ksp microstructure detected with cathodoluminescence (CL) and optical petrography (OP), and with the mineralogy determined by powder X-ray diffraction (XRD) analysis. XRD of the Ksp separates revealed that each sample contains a range of structural order and that polymorph content varies between samples. The Ksp’s contain perthitic textures with a variety of sizes and periodicity, turbid domains with micro-pores, and tartan and Carlsbad twinning with variable degrees of development. CL images reveal complex hidden textures and structures, including healed fractures and the occurrence of variable sizes and shapes of alternating domains of non-luminescent and luminescent Ksp.

A potential correlation was found between two samples collected ~1 km apart in one pluton that have maximum age steps of 285 and 340 Ma, respectively. XRD reveals similarities in the modes of Ksp polymorphs. CL images reveal a hidden complex of fractures healed by dark Ksp and variable degrees of luminescence in both samples. OP shows similarities in the textures of Ksp in both samples, but extensive sericitization of plagioclase and chloritization of Bt in the younger sample. These observations are consistent with the Ar isotopic system being affected by fluid-assisted recrystallization and possible growth below closure of Ksp, which would account for the younger age. The minimum ages in these spectra may represent the timing of this fluid flow event. These results add to a growing body of evidence that Ksp must be characterized in detail to obtain geologically meaningful ages.