90 kimberlite localities along the eastern margin of the Front Range and
Laramie Range, has fostered the belief that emplacement occurred
during a single magmatic interval spanning the Early to Late Devonian.
New geochronologic evidence, however, suggests that at least two of
these intrusives, the Green Mountain and Chicken Park kimberlites, were
emplaced earlier, during the latest Proterozoic to early Paleozoic.
40Ar/39Ar dating of the Green Mountain diatreme, utilizing both
matrix phologopite and mineral separates from xenoliths, suggests
emplacement between 500 and 800 Ma. In addition, a well-constrained
Nd/Sm isochron, bsaed on whole-rock and megacryst phases yields an
age of 572 +/- 49 Ma.
40Ar/39Ar dating of fine-grained phlogopite from the Chicken Park dike
yields an age spectrum suggestive of emplacement between 620 and
640 Ma. Precambrian emplacement of the Green Mountain and Chicken
Park kimberlites suggests that the Colorado-Wyoming province, similar
to
other kimberlite provinces world-wide, has experienced multiple intervals
of kimberlitic magmatism.
