Paleomagnetism of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite, southern Laramie Range, Wyoming and Colorado

Abstract

We present the results of a combined paleomagnetic and 40Ar/39Ar geochronologic investigation of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite in the southern Laramie Range of Wyoming and Colorado. Anorthosites and monzosyenites of the Laramie anorthosite complex yield a well‐defined characteristic magnetization of northeast declination (D) and moderate negative inclination (I), although antipodal normal and reverse polarity magnetizations are present at three sites. A grand mean direction from 29 of 35 sites in the complex is D = 44.6°, I = −48.7° (k = 77.4, α95 = 3.1°). Alternating field (AF) and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low‐Ti titanomagnetite of single or pseudo‐single domain character that occurs as elongate to rod‐shaped inclusions in plagioclase and potassium feldspar. The Sherman Granite contains a dual polarity magnetization that is less well defined than that of the Laramie anorthosite complex but similar in declination and inclination (D = 53.1°, I= −48.1°, k = 46.5, α95 = 7.6°, n = 8/14 sites); rock magnetic data indicate the primary carrier of remanence in Sherman Granite is magnetite. The 40Ar/39Ar geochronologic data from Sherman Granite hornblende, biotite, and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 Ma. Microcline data indicate that the Laramie anorthosite complex and Sherman Granite have probably not been thermally remagnetized. Paleomagnetic poles from the Laramie anorthosite complex and Sherman Granite are indistinguishable at the 95% confidence level, and individual virtual geomagnetic poles (VGPs) from both units are combined to provide a mean pole at 215.0°E, 6.7°S (K = 46.9, A95 = 3.5°, N = 37 VGPs). The location of this pole is similar to paleomagnetic poles derived from 1480 Ma to 1450 Ma intrusions elsewhere in North America, but it plots significantly north of those from Middle Proterozoic sedimentary strata of the Belt Supergroup and Sibley Group. In addition to the characteristic magnetization, samples from some sites in Sherman Granite contain a remanence of southeast declination and shallow negative inclination (D = 154.9°, I = −16.0°, k = 90.6, α95 = 9.7°, n = 4 sites). This secondary magnetization was probably acquired during late Paleozoic time.