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Neoproterozoic kimberlite emplacement in the Front Range, Colorado

Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399, U.S.A.

A paucity of isotopic age determinations for the more than 100 kimberlitic diatremes, dikes, and sills within the Colorado-Wyoming kimberlite province has fostered the prevailing notion that emplacement of these bodies occurred during a discrete interval of ultramafic magmatism spanning Early to Late Devonian time. However, new geochronologic data for two kimberlite bodies (Chicken Park and Green Mountain kimberlites) support a Neoproterozoic emplacement age for these bodies.

⁴⁰Ar/³⁹Ar analysis of fine-grained matrix phlogopite from the Chicken Park dike indicates emplacement between 620 and 640 Ma. ⁴⁰Ar/³⁹Ar dating of the Green Mountain diatreme, utilizing both phlogopite megacrysts and mineral separates from xenoliths, is somewhat equivocal because of excess Ar, but suggests emplacement between 500 and 800 Ma. A Sm-Nd isochron, based on megacrystic phases from the Green Mountain diatreme, yields an age of 572 ±49 Ma.

Neoproterozoic emplacement indicates that the Colorado-Wyoming kimberlite province apparently has undergone multiple intervals of kimberlitic magmatism, similar to other kimberlite provinces worldwide. The episodes of kimberlite emplacement coincide with periods of relative orogenic and magmatic quiescence in Colorado and Wyoming.

Key Words: Kimberlites • geochronology • Front Range • Colorado

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