Interpretation of 40Ar/39Ar age results
The Silurian age (429.3±3.5 Ma) obtained from the La Calera pseudotachylyte sample (98109) is coincident, within error, with the published age of this fault zone (428±12 Ma; Northrup et al., 1998), and calculated ages from all of the individual analyses in this sample lie within the error range of the Northrup et al. (1998) results. This concordance suggests that the excimer laser technique for dating pseudotachylyte is capable of producing results in line with other established techniques. In addition, the lower error constraints on the cumulative age produced by excimer probe analyses highlight the potential advantages of this technique for fine-grained materials.
Figure 4. Frequency distribution plot
Frequency distribution plot of 40Ar/39Ar age results from all pseudotachylyte and ultramylonite samples. Peaks indicate 2σ error range for individual spot analyses of each sample.
The closure temperature for Ar-retention in pseudotachylyte is not nearly as well-defined as for K-bearing minerals such as amphibole and muscovite (McDougall and Harrison, 1999; c.f. Carroll, 1991). Recent experimental analyses by Hazelton et al. (2003) suggest that the closure temperature for high-silica, low-CaFeMg pseudotachylyte glass is no higher than 175˚C. An estimation of the ambient temperature of pseudotachylyte host rocks can be obtained with the geothermometer of O’Hara (2001). Temperatures of 250-253˚C (Table AP3) were calculated for the Los Tuneles samples, which is greater than the Hazelton et al. (2003) closure temperature for pseudotachylyte. The predominantly quartzofeldspathic composition of the Los Tuneles host-gneisses likely falls within the high-silica category of Hazelton et al. (2003), and therefore the Los Tuneles results may represent regional cooling ages that are younger than the actual period of fault movement.
Existing 40Ar/39Ar thermochronology indicates that regional ambient temperatures along the Tres Arboles fault zone dropped below the amphibole closure temperature (~500˚C; McDougall and Harrison, 1999) no later than 478 Ma, through the standard closure temperature for biotite (~300-350˚C) by 440 Ma, and below 250˚C by about 380 Ma (Northrup et al, 1998; Krol and Simpson, 1999). These constraints indicate that host rocks were cooler at the time of faulting than the standard closure temperature for medium-grained biotite. However, recalibration of the closure temperature for 10-20µm biotite grains, using the equation from Dodson (1973), yielded temperatures of 237-252˚C (Table AP4), similar to host-rock temperatures. Therefore, it is possible that the 40Ar/39Ar biotite ages represent regional cooling below about 250˚C, instead of directly dating movement along the fault zone.
Existing 40Ar/39Ar thermochronology (Northrup et al, 1998; Krol and Simpson, 1999) suggests that regional cooling of the western margin of the Sierra de Córdoba progressed at an approximate rate of 4 - 5˚C/m.y. during the Late Cambrian to Middle Ordovician. If these cooling rates were consistent throughout the Middle Paleozoic, the 40Ar/39Ar ages presented here would underestimate the period of fault zone movement by a maximum of 20 million years. Thus, deformation in the Los Tuneles and Tres Arboles fault zones may have occurred as early as 368 Ma and 363 Ma, respectively. This is consistent with existing K-Ar ages for amphiboles from the Los Tuneles fault zone that yielded ages of 373 ± 11 Ma and 365 ± 10 Ma (Rapela et al., 1998).
40Ar/39Ar ages from the Los Tuneles pseudotachylyte samples (348-345 Ma) are 3-8 million years older than those obtained from the Tres Arboles ultramylonites (343-341 Ma), although error constraints allow for overlap between almost all of the samples. The small age discrepancy may represent slight differences in the time of movement between the fault zones, or it may be due to differences in the cooling temperature profiles of pseudotachylyte versus fine-grained biotite. Either way, the coincidence, within error, of 40Ar/39Ar ages from the northern Los Tuneles fault zone and the southern Tres Arboles fault zone is consistent with the existing model, which depicts one extensive deformation zone along most of the western margin of the Sierras de Córdoba (Whitmeyer and Simpson, 2003).
Intrusive rocks of the Achala batholith suite truncate deformation fabrics of the Tres Arboles fault zone (Figure 2c), which implies that movement along the zone must have ceased prior to 368 ± 2 Ma (Dorais et al., 1997). The U/Pb zircon crystallization age for the Achala batholith was obtained from samples in the central "Achala series" (Demange et al., 1993) section of the poly-phase granitoid. However, field relations suggest that undated southwestern regions of the batholith (the "Cumbrecita" and "Champaquí series") that cross-cut the Tres Arboles fault zone (Figure 2c) are the youngest of the Achala-related intrusions, and may postdate the Late Devonian - Early Carboniferous estimates of movement along the fault zone reported here.
The constraints on the regional ambient temperature at the time of pseudotachylyte and ultramylonite generation suggest that the early Carboniferous age results may reflect regional cooling and not directly date the time of deformation. Thus, these results are interpreted as minimum ages for movement along the Los Tuneles and Tres Arboles fault zones. Regional cooling rate estimates allow for reactivation as early as 368 Ma, which would temporally equate late movement along the fault zones with intrusion of the Achala batholith. An important consideration is that 40Ar/39Ar analyses in both the Los Tuneles and Tres Arboles fault zones date reactivation fabrics and provide no temporal constraints on earlier phases of deformation in either zone.