Laser ablation ⁴⁰ Ar/ ³⁹ Ar dating of metamorphic fabrics in the Caledonides of north Ireland

Abstract

In situ 40 Ar/ 39 Ar dating of two texturally distinct biotite generations within a phyllite from the Caledonian orogen of Ireland was performed using a high spatial resolution laser microprobe. In this sample, an older generation of biotite (Bt 1 ) is aligned subparallel to a steeply dipping crenulation foliation, which itself is overprinted by a regionally developed shallow-dipping foliation. The second biotite generation (Bt 2 ) occurs as randomly oriented porphyroblasts that overprint and therefore postdate both foliations. A dataset of 89 ages from a single thick section was acquired for six Bt 1 and eight Bt 2 crystals. The two biotite generations yielded two statistically distinct age distributions with Bt 1 maxima at c. 433 Ma and Bt 2 maxima at c. 417 Ma. A small number of analyses (11% Bt 1 and 26% Bt 2 ) from within crystals yielded 'main population' ages that conflict with the dominant age populations and relative chronology established by field relationships. Discordant analyses from the earlier Bt 1 crystals are younger (than the Bt 2 population) and probably reflect intracrystalline partial resetting during a later thermal perturbation. The discordant Bt 2 analyses are predominantly older (than the Bt 2 population) and are interpreted as indicating localized excess 40 Ar. Such dis cordant analyses indicate intracrystalline variation reflecting both excess 40 Ar (Bt 2 population) and 40 Ar loss (Bt 1 population), and were therefore discarded when determining geologically meaningful ages.

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.…”

In Situ and Step‐Heating 40Ar/39Ar Dating of White Mica in Low‐Temperature Shear Zones (Tenda Massif, Alpine Corsica, France)

“…The significance of 40 Ar/ 39 Ar ages in deformed rocks has been the focus of many studies since 1990. A central motivation of such studies is the potential of the 40 Ar/ 39 Ar dating to provide (i) temporal control on deformation structures and crustal shear zones (e.g., Brunet et al, 2000; Di Vincenzo et al, 2016; Isik et al, 2004; Kligfield et al, 1986; Rolland et al, 2013; Turrillot et al, 2011), (ii) determine and correlate the sequence of events in polydeformed rocks (e.g., Augier et al, 2005; Beltrando et al, 2009; Castonguay et al, 2007; Condon et al, 2006; Hames & Cheney, 1997), and (iii) infer the kinematics, evolution, and partition of shear in tectonic systems undergoing progressive deformation (e.g., Bellanger et al, 2015; Cardello et al, 2019; Dunlap et al, 1991; Kellett et al, 2016; Rolland et al, 2009; Sanchez et al, 2011; Schneider et al, 2013; West & Hubbard, 1997). Practically, the question of dating deformation in such contexts is whether or not the 40 Ar/ 39 Ar targets (typically micas) can actually record deformation as a result of structural‐stoichiometric recombination (recrystallization and neoblastesis) and whether these phenomena are kinetically more efficient than temperature for Ar exchange and resetting (e.g., Di Vincenzo et al, 2001, 2004, 2006; Scaillet et al, 1992).…”

The chemical character of the Late Caledonian Donegal Granites, Ireland, with comments on their genesis

“…Indeed, very recent U-Pb work together with assessment of the Rb-Sr data suggests emplacement of the Donegal plutons occurred c. 428-400 m.y. ago (Condon et al 2006), closer to the Late granites of mainland Scotland.…”