PALEOMAGNETISM OF THE MIDDLE PROTEROZOIC LARAMIE ANORTHOSITE COMPLEX AND SHERMAN GRANITE, SOUTHERN LARAMIE RANGE, WYOMING AND COLORADO

We present the results of a combined paleomagnetic and Ar-40/Ar-39 geo chronologic investigation of the Middle Proterozoic Laramie anorthosit e complex and Sherman Granite in the southern Laramie Range of Wyoming and Colorado. Anorthosites and monzosyenites of the Laramie anorthosi te complex yield a well-defined characteristic magnetization of northe ast declination (D) and moderate negative inclination (I), although an tipodal normal and reverse polarity magnetizations are present at thre e sites. A grand mean direction from 29 of 35 sites in the complex is D = 44.6 degrees, I = -48.7 degrees (k = 77.4, alpha(95) = 3.1 degrees ). Alternating field (AF) and thermal demagnetization behavior and roc k magnetic experiments indicate that magnetization is carried by low-T i titanomagnetite of single or pseudo-single domain character that occ urs as elongate to rod-shaped inclusions in plagioclase and potassium feldspar. The Sherman Granite contains a dual polarity magnetization t hat is less well defined than that of the Laramie anorthosite complex but similar in declination and inclination (D = 53.1 degrees, I = -48. 1 degrees, k = 46.5, alpha(95) = 7.6 degrees, n = 8/14 sites); rock ma gnetic data indicate the primary carrier of remanence in Sherman Grani te is magnetite. The Ar-40/Ar-39 geochronologic data from Sherman Gran ite hornblende, biotite, and microcline indicate that subsolidus cooli ng 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 c omplex and Sherman Granite have probably not been thermally remagnetiz ed. Paleomagnetic poles from the Laramie anorthosite complex and Sherm an Granite are indistinguishable at the 95% confidence level, and indi vidual virtual geomagnetic poles (VGPs) from both units are combined t o provide a mean pole at 215.0 degrees E, 6.7 degrees S (K = 46.9, A(9 5) = 3.5 degrees, N = 37 VGPs). The location of this pole is similar t o paleomagnetic poles derived from 1480 Ma to 1450 Ma intrusions elsew here in North America, but it plots significantly north of those from Middle Proterozoic sedimentary strata of the Belt Supergroup and Sible y Group. In addition to the characteristic magnetization, samples from some sites in Sherman Granite contain a remanence of southeast declin ation and shallow negative inclination (D = 154.9 degrees, I = -16.0 d egrees, k = 90.6, alpha(95) = 9.7 degrees, n = 4 sites). This secondar y magnetization was probably acquired during late Paleozoic time.