U-Pb sphene dating of metamorphism: the importance of sphene growth in the contact aureole of the Red Mountain pluton,Laramie Mountains,Wyoming |
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| Authors: | L A Verts Kevin R Chamberlain C D Frost |
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| Institution: | (1) Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, USA, US |
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| Abstract: | The relative importance of thermal diffusion versus new growth or recrystallization 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. Samples 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 contact. The 207Pb/206Pb ages of sphene from amphibolite samples range between 1.43 Ga, the intrusive age of the Red Mountain pluton, to 1.78 Ga,
the age of regional metamorphism. The 207Pb/206Pb ages of sphene in rocks metamorphosed above 700° C are within error of the intrusive age of the pluton, and appear to have
resulted from diffusional resetting of preexisting sphene and the metamorphic growth of additional sphene at 1.43 Ga. At greater
distance from the contact the 207Pb/206Pb ages range from 1.45 to 1.72 Ga. This 300 million year spread in ages is interpreted to result from two periods of sphene
growth, one produced during regional metamorphism at 1.78 Ga and another generation of newly grown or recrystallized sphene
that formed during contact metamorphism at 1.43 Ga. These two age populations may be identified 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° C, sphene growth was the dominant process controlling the response of the U-Pb isotope system
to contact metamorphism. Sphene grew well outside the zone of obvious contact metamorphism. The U-Pb sphene ages were reset
by diffusion only at high temperatures, supporting the experimentally determined closure temperature estimates for the U-Pb
system in sphene of around 650° C (Cherniak 1993). This study demonstrates that U-Pb ages of sphene can be used to date metamorphism
not only in areas with a simple 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.
Received: 22 August 1995 / Accepted: 17 April 1996 |
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