HOW DOES VARIABLE WELDING AND DEVITRIFICATION AFFECT THE MAGNETIC FABRIC OF ASH FLOW TUFFS?: AMS AND AARM INVESTIGATION OF THE OLIGOCENE CARPENTER RIDGE TUFF, SAN JUAN MOUNTAINS, COLORADO

Anisotropy of magnetic susceptibility (AMS) data can be a powerful means of quantifying the transport direction, and thus inferred source region, for regionally extensive ash-flow tuffs (ignimbrites). Anisotropy of anhysteretic remanent magnetization (AARM) data, although difficult to obtain, have elucidated the controls by different assemblages of magnetic particles on magnetic fabrics. Effects of variable degrees of welding and subsequent devitrification on the magnetic fabric of ignimbrites are not well understood. We have collected magnetic fabric data from a continuously exposed sequence of outflow facies of the ca. 27.55 Ma Carpenter Ridge Tuff (CRT), one of the major large volume ash-flow tuffs of the Southern Rocky Mountain Volcanic Field, southwest Colorado, about 3 km east of Buffalo Pass on Colorado Highway 114. The CRT, a compound cooling unit, is of reverse polarity, with considerable internal variability in paleomagnetic record through the section, and was sourced from the Bachelor Caldera, southwest of the sample locality. Some 21 total independently oriented samples, as very large oriented blocks, were collected in traverse fashion through most of the lowermost cooling unit of the tuff, and an additional ten block samples were collected from the moderately welded and completely devitrified third cooling unit. Typically, about 20 discrete specimens were obtained from each block. AMS data are internally consistent at the oriented sample level and, overall, throughout the section sampled, and imply a west to east transport orientation, consistent with several other sampling sites in the CRT in this area. AARM data at the block sample level are usually more dispersed. All samples have positive T values, implying strong foliations. The degree of anisotropy (P’) varies from sample to sample, but is about 1.10. AARM P’ values are always higher than AMS P’ values. Between samples, there is no consistent relation between AMS T and AARM T values. In comparing AARM and AMS fabric data, the well-welded vitrophyre near the base of the lower cooling unit is typified by inverse fabric relations, and the completely devitrified parts of the CRT are characterized by normal fabric relations. Magnetic fabric results from ignimbrites can reflect variations in the degree of welding and devitrification of these deposits.