Paleomagnetism of Ordovician alkalic intrusives and host rocks from the Pedernal Hills, New Mexico: positive contact test in remagnetized rocks?

Abstract

A set of thin dikes from central New Mexico, dated at 469 +/- 7 Ma (Rb-Sr; Loring and Armstrong, 1980), have yielded a virtual geomagnetic pole which lies on the Late Paleozoic segment of the North American apparent polar wander path. The remanence of the dikes appears to be a product of Late Paleozoic hydrothermal alteration. Paradoxically, however, the magnetization of the host rocks is most simply explained in terms of a positive contact test. Samples collected between 0.2 and 0.5 dike-widths from the contact contain a component of remanence parallel to the magnetization in the dikes, with unblocking temperatures which decrease with distance from the dikes. Host rocks from a distance of more than 1 dike-width show no evidence of the characteristic dike magnetization.There are two possible resolutions of this paradox: 1. (1) the magnetization of the host rocks is secondary, despite the apparent positive contact test, and is a product of hydrothermal fluid migration through the dikes or along the contact zones; or2. (2) the magnetization of the dikes is primary, but not representative of the Ordovician paleofield for North America.Possible reasons for inaccurate representation include: 1. (a) incomplete averaging of secular variation;2. (b) tectonic rotation with respect to the stable craton; or3. (c) erroneous age determination for the rocks.We argue that explanation (1) is the most likely.