40Ar/39Ar Results
Pseudotachylyte samples were probed with a 42µm beam and rastered over a 300µm-square region that minimized the incorporation of cataclastic fragments (Figure 2c). A total of 13 individual analyses from the La Calera pseudotachylyte sample (98109) produced a weighted mean age of 429.3±3.5 Ma ((Table 1); Figure 3). This result is coincident, within error, with an inverse isochron age of 428±12 Ma reported by Northrup et al. (1998). Analyses of 16-18 spots from the Los Tuneles pseudotachylyte samples (9819, 9820) yielded weighted mean ages of 345.0±5.1 Ma and 348.2±2.2 Ma, respectively (Table 1; Figure 3).
Table 1. Cumulative weighted mean age results for pseudotachylyte and ultramylonite samples, including sampling locations and number of individual spots analyzed for each sample by laser spot fusion. Errors reported at 2 sigma. See Fig. 1 for sampling locations.
| Sample | Material | Location | No. of spots analyzed | Age (Ma) ± 2σ |
|---|---|---|---|---|
| 004 | ultramylonite biotite | Tres Arboles | 15 | 340.6 ± 5.4 |
| 006 | ultramylonite biotite | Tres Arboles | 19 | 342.5 ± 5.3 |
| 9819 | pseudotachylyte | Los Tuneles | 16 | 345.0 ± 5.1 |
| 9820 | pseudotachylyte | Los Tuneles | 18 | 348.2 ± 2.2 |
| 98109 | pseudotachylyte | La Calera | 13 | 429.3 ± 3.5 |
Figure 3. Sample 9819
(a) Photomicrograph of pseudotachylyte sample 9819, with representative 300 µm raster square indicated. Plane light. (b) Enlargement of sampling region of (a), illustrating cataclastic fragments inadvertently incorporated within 300 µm raster square. Plane light. (c) Photomicrograph of ultramylonite sample 004, with representative biotite-rich matrix indicated. Plane light. (d) Backscatter electron image of ultramylonite sample 004, showing relict garnet fragments (white) within a biotite + plagioclase ± quartz matrix. Note that in this image light gray = biotite, medium gray = plagioclase, dark gray = quartz. Scale bars for (c) = 50 µm; (d) = 100 µm.
Tres Arboles ultramylonite samples were probed with a 30µm beam along 3-4 discrete lines that followed regions of biotite-rich matrix (Figure 2d). 15-19 individual analyses of fine-grained biotite from ultramylonite samples 004 and 006 yielded weighted mean ages of 340.6±5.4 Ma and 342.5±5.3 Ma, respectively (Table 1, Figure 3)). Analytical procedures and data from individual analyses are detailed in Appendix A.