U-PB SPHENE DATING OF METAMORPHISM - THE IMPORTANCE OF SPHENE GROWTH IN THE CONTACT AUREOLE OF THE RED MOUNTAIN PLUTON, LARAMIE MOUNTAINS, WYOMING

The relative importance of thermal diffusion versus new growth or recr ystallization on U-Pb isotopic data from sphene is assessed through a study of amphibolites and granite gneisses within the contact aureole of the Red Mountain pluton, Laramie anorthosite complex, Wyoming. Samp les were collected along a traverse approximately perpendicular to the margin of the intrusion over a distance of 0.13 to 2.65 km from the c ontact. The Pb-207/Pb-206 ages of sphene from amphibolite samples rang e between 1.43 Ga, the intrusive age of the Red Mountain pluton, to 1. 78 Ga, the age of regional metamorphism. The Pb-207/Pb-206 ages of sph ene in rocks metamorphosed above 700 degrees C are within error of the intrusive age of the pluton, and appear to have resulted from diffusi onal resetting of preexisting sphene and the metamorphic growth of add itional sphene at 1.43 Ga. At greater distance from the contact the Pb -207/Pb-206 ages range from 1.45 to 1.72 Ga. This 300 million year spr ead in ages is interpreted to result from two periods of sphene growth , one produced during regional metamorphism at 1.78 Ga and another gen eration of newly grown or recrystallized sphene that formed during con tact metamorphism at 1.43 Ga. These two age populations may be identif ied on the basis of petrographic textures, the morphologies and color differences of grain separates as well as by the U-Pb systematics. In rocks metamorphosed to temperatures less than 700 degrees C, sphene wa s the dominant process controlling the response of the U-Pb isotope sy stem to contact metamorphism. Sphene grew well outside the zone of obv ious contact metamorphism. The U-Pb sphene ages were reset by diffusio n only at high temperatures, supporting the experimentally determined closure temperature estimates for the U-Pb system in sphene of around 650 degrees C (Cherniak 1993). This study demonstrates that U-Pb ages of sphene can be used to date metamorphism not only in areas with a si mple geologic history, such as igneous intrusion or single metamorphic or deformational events, but also to date multiple events so long as different generations of sphene can be identified and separated.