La. Verts et al., U-PB SPHENE DATING OF METAMORPHISM - THE IMPORTANCE OF SPHENE GROWTH IN THE CONTACT AUREOLE OF THE RED MOUNTAIN PLUTON, LARAMIE MOUNTAINS, WYOMING, Contributions to Mineralogy and Petrology, 125(2-3), 1996, pp. 186-199
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.