Introduction
Micro-crystalline pseudotachylyte glass and fine-grained ultramylonite are frequent products of fault deformation (Sibson, 1977; Magloughlin and Spray, 1992), and precision geochronology of such materials can greatly assist in temporally constraining tectonic events. 40Ar/39Ar thermochronology of K-bearing fault materials has become an accepted method for dating deformation. However, standard 40Ar/39Ar analytical techniques are not well suited for dating pseudotachylyte or ultramylonite due to nuclear recoil effects and the likely incorporation of unwanted materials within analyses. Previous studies have shown that the nuclear recoil effect on fine-grained and/or altered biotite can yield 40Ar/39Ar ages older than the actual crystallization age of the mineral (Mitchell and Taka, 1984; Lo and Onstott, 1989; Ruffet et al., 1991). This is particularly problematic if the analyses are performed using conventional step-heating techniques, which can produce disturbed apparent age spectra (Ruffet et al., 1991; Roberts et al., 2001). Step-heating techniques also increase the potential for incorporating of undesirable materials in 40Ar/39Ar analyses of fine-grained fault rocks. Pseudotachylyte veins almost always include clasts of host rock material within the glassy matrix (Magloughlin and Spray, 1992), and analytical accuracy depends on avoiding as many of these clasts as possible. A similar problem is encountered when analyzing ultramylonite matrix materials, where the intended target mineral (such as biotite) is frequently less than 50µm in size, and intergrown with other minerals that are unsuitable for 40Ar/39Ar analyses.
The interpretation of age results from 40Ar/39Ar analyses of pseudotachylyte can be problematic as well. Several studies (e.g. Reimold et al., 1990; Kelley et al., 1994; Thompson et al., 1998) have interpreted at least some 40Ar/39Ar ages obtained from pseudotachylyte samples as crystallization ages that directly provide the time of fault movement. However, recent work suggests that pseudotachylyte glass may have a closure temperature of 175˚C or less for argon gas retention (Hazelton et al., 2003). Since 40Ar/39Ar ages from mid-crustal fault rocks may record the time of regional cooling below 175˚C rather than the actual period of fault movement, an estimation of the ambient temperature of host rocks during faulting is critical for the correct interpretation of 40Ar/39Ar ages.
This study uses excimer (uv) laser-probe spot fusion 40Ar/39Ar thermochronology to analyze pseudotachylyte and very fine-grained ultramylonite biotite. Pseudotachylyte and ultramylonite were sampled from the La Calera, Los Tuneles and Tres Arboles fault zones located in the Sierras de Córdoba, central Argentina (Figure 1). The La Calera results are compared with existing 40Ar/39Ar ages from the same fault zone (Northrup et al., 1998) to evaluate whether comparable results can be obtained using excimer laser probe fusion techniques. Existing 40Ar/39Ar thermochronology in the region of the Los Tuneles and Tres Arboles fault zones provides a framework within which these 40Ar/39Ar results are evaluated to determine whether they represent geologically reasonable ages for fault movement, or, conversely, represent regional cooling of pseudotachylyte and/or fine-grained biotite below their closure temperatures. Age results from the Los Tuneles pseudotachylytes are compared with biotite ages from the Tres Arboles ultramylonites to evaluate an existing model that depicts the Los Tuneles and Tres Arboles fault zones as a continuous, north-striking deformation zone, which is exposed through a range of crustal depths from 10-12km in the north to 18-22 km in the south (Whitmeyer and Simpson, 2003). Finally, the implications of these age results are evaluated in the context of early to middle Paleozoic tectonism along the western margin of Gondwana.