DUNBAR, N.W., KELLEY, S., and MIGGINS, D. New Mexico Bureau of Geology and Mineral Resources and Earth and Environmental Science Department, New Mexico Tech, Socorro, NM, 87801.

Argon 40/39 dating and fission track analysis of K-metasomatized rocks have provided insight into the timing and thermal regime of this geochemical alteration in the Socorro area. During K-metasomatism, which is thought to be caused by alkaline-saline brines in a closed-basin playa system, the rocks of the basal Popotosa formation and underlying ignimbrites underwent alteration that resulted in selective replacement of non-K-bearing phases, such as plagioclase, by a combination of adularia, clay, and quartz. 40Ar/39Ar dating was carried out on material selectively hand-picked from altered plagioclase crystals in basaltic-andesite units as well as plagioclase-bearing silicic ignimbrites, and allows resolution of the alteration age from the eruption age of the original unit. Isochron ages for hand-picked rhyolitic material and a mafic clast from the basal Popotosa conglomerate in the Box Canyon area are 8.7±0.1 and 7.4±0.1 Ma respectively. A rhyolitic clast from an underlying lithic-rich tuff at a nearby location yields a plateau age of 10.4±0.1 Ma. An ignimbrite sample collected from the metasomatized Water Canyon area yields a plateau alteration age of 14.1±0.1 Ma. 40Ar/39Ar ages determined on sanidine crystals from the above-mentioned samples are slightly younger than true eruption age, suggesting that the sanidine has been chemically affected by alteration.

Fission track analysis of apatite and zircon in ignimbrites from the areas mentioned above have also been performed. One clast of crystal-rich tuff collected from an intensely metasomatized outcrop of Popotosa Formation in the Box Canyon area contained apatite and yielded an apatite fission track (AFT) age of 8.7±5.1 Ma. Zircon FT ages determined for other two other clasts yielded ages of 23.3 ± 1.4 (Hells Mesa Tuff) and 23.9 ± 2.2 Ma (upper Lemitar Tuff). These ages are slightly younger than true eruption ages and suggest that the clasts may have been exposed to temperatures on the order of 170 to 200°C. Two clasts of crystal-rich tuff were collected from the Popotosa Formation in Water Canyon area. A clast tentatively identified as Upper Lemitar Tuff has an AFT age of 15.0 ± 3.3 Ma and a clast of Hells Mesa Tuff has an AFT age of 18.0 ± 3.6 Ma. The AFT results can be used to infer that temperatures were above 120°C in this area, and cooling occurred rapidly in the middle Miocene.

The combination of ages determined by 40Ar/39Ar and FT analysis suggests that both techniques are dating the same thermal/alteration event, and we would infer that this event is the K-metasomatism. The dates suggest that alteration began at least 15 Ma, and continued until approximately 7 Ma. This chronological information is consistent with the inferred timing of playa deposition from independent geological evidence. We believe that metasomatism in the Box Canyon area may have been caused by the deepest and most central part of the playa system, consistent with the high temperatures shown by FT analysis, high degree of K enrichment, and youngest date of metasomatism. The Water Canyon area may represent a more distal part of the playa where alteration occurred earlier, and at lower temperature. This pattern of alteration would be consistent with that of paleolake T'oo'dichi (Turner and Fishman, 1991).