Progress toward paleomagnetic identification of ca 1.4 Ga regional metamorphic pulse in central New Mexico
Suzanne T. Meuret and John W. Geissman
We interpret initial paleomagnetic results to indicate that some regionally metamorphosed Mesoproterozoic rocks of central New Mexico retain magnetizations as old as 1.4 Ga with high fidelity. Recognition of ca. 1.4 Ga ChRMs relies on comparison of our data with the relatively well defined 1.45-1.40 Ga apparent polar wander path for North America. Paleomagnetic poles for this time period differ markedly from those provided by younger rocks. Preservation of ca. 1.4 Ga ChRMs is plausible given that the same rocks yield thermochronologic data with high temperature cooling ages of around 1.45 Ga. Of over 290 total samples collected from the Sandia, Manzano, and Santa Fe ranges, about 69% carry ChRMs, 49% of these are well grouped and interpreted to be of 1.4 Ga affinity. Most samples carry younger secondary magnetizations as well. The mean pole from all accepted VGPs is Lon. =210.2°E, Lat. =19.9°S in situ and Lon.=218.7°E, Lat. =22.5°S str. COLT., k=10.2, 7.2, and a95=11.1°, 13.4°, N=19 sites from six localities. This ancient magnetization is carried by magnetite of relatively high coercivity and laboratory unblocking temperatures (400° to 580°C) or by hematite with unblocking temperatures > 600°C. ChRM directions show no correlation with directions of maximum anisotropy. Our interpretation of the preliminary data involves remanence acquisition in response to a ca. 1.45 –1.40 Ga regional thermotectonic event of temperatures greater than 500-550°C, followed by rapid cooling to below ~300°C.
- Meuret, Suzanne T.; Geissman, John W., 1995, Progress toward paleomagnetic identification of ca 1.4 Ga regional metamorphic pulse in central New Mexico, in: Geology of the Santa Fe Region, Bauer, Paul W.; Kues, Barry S.; Dunbar, Nelia W.; Karlstrom, K. E.; Harrison, Bruce, New Mexico Geological Society, Guidebook, 46th Field Conference, pp. 201-208.