Paleomagnetism of Proterozoic mafic dikes from the Tobacco Root Mountains, southwest Montana

Abstract

Paleomagnetic data from Proterozoic mafic dikes in southwestern Montana provides evidence for two distinct episodes of subparallel dike emplacement at ca. 1450 and 780 Ma. Published geochemical data from dikes in the southern Tobacco Root Mountains has identified three distinct compositional groups, termed groups A, B, and C. Geochronological data from the group A dikes yielded a Sm-Nd age of 1448 ?? 49 Ma. Emplacement of these dikes is thought to reflect mafic magmatism associated with extension accompanying development of the adjacent Mesoproterozoic Belt Basin. Paleomagnetic results from these dikes and a group C dike yield antipodal magnetizations with a group-mean direction of D = 225.0??, I = 61.8?? (k = 27.9, ??95 = 7.7??, N = 14 independent means/24 sites). The average paleomagnetic pole (8.7??N, 216.1??E, A95 = 10.3??) is considered to be primary on the basis of positive baked contact tests and similarity to poles of ca. 1.45-1.4 Ga from intrusions elsewhere in North America, but is discordant with respect to poles from age equivalent sedimentary rocks of the Meosoproterozoic Belt Supergroup. 40Ar/39Ar dates from geochemical group B dikes are consistent with published U-Pb dates that demonstrate dike emplacement at 780 Ma as part of the regional Gunbarrel magmatic event. Hornblende concentrates from the group B dikes yield 40Ar/39Ar apparent ages of 778-772 Ma, whereas biotite from a baked contact zone yielded a plateau date of 788 Ma. Paleomagnetic results from the group B dikes yield a mean direction of D = 301.5??, I = -17.1?? (k = 65.7, ??95 = 4.0??, N = 12 independent means/23 sites) with a paleomagnetic pole at 14.6??N, 127.0??E (A95 = 3.2??). The combination of geochronologic data, results of a baked contact test, and spatial agreement of the paleomagnetic poles with poles of similar age elsewhere in North America indicates that this is also a primary magnetization associated with dike emplacement. Paleomagnetic data from some of the Tobacco Root Mountains dikes provide evidence that they were partially to completely remagnetized during latest Cretaceous to early Tertiary time, perhaps due to thermal affects associated with emplacement of the Late Cretaceous Tobacco Root Batholith. The overall agreement of paleomagnetic poles from the Proterozoic dikes with those of age equivalent rocks elsewhere in North America and agreement of the secondary magnetization with expected directions for the latest Cretaceous/early Tertiary indicate that the rocks of the Tobacco Root Mountains have not experienced significant tilting or vertical axis rotation since the Mesoproterozoic. The new paleomagnetic poles from this study thus provide key data for refining Meso- and Neoproterozoic parts of the North American APW path. ?? 2008 Elsevier B.V. All rights reserved.