Dating cleavage formation in slates and phyllites with the ⁴⁰Ar/³⁹Ar laser microprobe: an example from the western New England Appalachians, USA

Abstract

Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high‐spatial‐resolution 40Ar/39Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S2, S3, and S4) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S3 and S4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40Ar/39Ar geochronology. Mean 40Ar/39Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S3 and 345.5 ± 1.7 Myr for S4. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S3 and S4, these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region.

Interpreting and reporting 40Ar/39Ar geochronologic data

“…The limiting factor for the optimum size of the laser pit is the necessity to detect the five Ar isotopes and measure their concentrations with a precision deemed sufficient for the intended purpose. UVLAMP technology has been used to constrain 40 Ar exchange by diffusion and alteration in natural samples (Pickles et al, 1997;Kelley and Wartho, 2000;Smith et al, 2005), the role of deformation and recrystallization in 40 Ar loss (Cosca et al, 2011;Mulch and Cosca, 2004;Mulch et al, 2002), as well as the ages of multiple fabric-forming events in polymetamorphic tectonites (Chan et al, 2000;Janak et al, 2001;Mulch et al, 2005), pseudotachylytes in fault zones (Condon et al, 2006;Cosca et al, 2005;Fornash et al, 2016;Muller et al, 2002), authigenic K-feldspar growth in sedimentary rocks (Sherlock et al, 2005), and multiple impact-melting events in lunar breccias (Mercer et al, 2015(Mercer et al, , 2019.…”

Thermochronology in Orogenic Systems

“…Many deformed rocks display multiple generations of tectonite fabrics, and techniques such as laser microprobe 40 Ar/ 39 Ar dating of polished thin sections permit the dating of specific minerals in structural context and, thus, determination of the absolute ages of different fabric-forming events. Although care must be taken to ensure that the dates for older grains are not compromised by deformation-induced loss of radiogenic 40 Ar (Dunlap and Kronenberg, 2001;Kramar et al, 2001;Mulch et al, 2002;Reddy and Potts, 1999), some successful studies have been carried out (e.g., Chan et al, 2000;Di Vincenzo et al, 2001;Hames and Cheney, 1997;Mulch and Cosca, 2004;Mulch et al, 2005b). This approach ultimately may prove to be particularly valuable for the study of the duration of and interval between fabric-forming events during orogenesis.…”

Geological-structural mapping and geocronology of shear zones: A methodological proposal

“…In brittle deformation conditions, methods such as K-Ar and Ar-Ar in neoformed phyllosilicates, such as chlorite, white mica, sericite, muscovite and illite have been implemented (cf. Crone, 1996;Chan et al, 2000;Jefferies et al, 2006;McWilliams et al, 2007;Vinasco, 2001;Löbens et al, 2011;Oyhantçabal et al, 2012;Wang et al, 2016), in fault gouge (cf. Vrolijk and van der Plujim, 1999;van der Pluijm et al, 2001;Zwingmann and Mancktelow, 2004;Haines and van der Plujim, 2008;Duvall et al, 2011;Fitz-Díaz and van der Plujim, 2013;Viola et al, 2016;Scheiber et al, 2019) and in the pseudotachylite matrix (cf.…”