Paleomagnetism of upper Precambrian to lower Paleozoic intrusive rocks from Colorado

Abstract

The paleomagnetism of the Iron Mountain–McClure Mountain alkalic intrusive complex in the northern Wet Mountains of Colorado has been investigated. Published radiometric ages for the complex, and for similar intrusive rocks nearby, range between 485 and 704 m.y. Eighty-five samples were collected from 35 sites, and the samples were demagnetized using thermal and alternating-field techniques. In many samples thermal cleaning has yielded better isolation of the primary magnetization, but the two methods yield very similar directions. The resulting directions have a trimodal distribution. One of the three groups of direction could possibly be interpreted as a later (Ordovician or late Paleozoic?) magnetization (pole position at lat 48°N, long 107°E) and agrees well with the previous findings of Larson and Mutschler, who studied these and similar intrusive complexes. However, the other two groups of directions observed in this study are interpreted with confidence as representing late Precambrian to Cambrian magnetizations. Their pole positions are 30° apart (lat 5°N, long 174°E and lat 15°N, long 142°E) and may well reflect a trend of apparent polar wandering during this time. When averaged, these two groups have a mean direction of 104° (declination) and −2° (inclination) for 21 sites (48 samples), k = 9, and α 95 = 11°. This direction yields a pole position at lat 12°N, long 156°E ( dp = 5.5°, dm = 11°), which is consistent with published Cambrian poles, but considerably different from the pole obtained by Larson and Mutschler.