Abstract

Twelve small serpentinite diatremes are now known near the Colorado-Wyoming border in the Precambrian core of the northern Front Range and southern Laramie Range. Five of them contain blocks of fossiliferous and nonfossiliferous sedimentary rocks of Early, Middle, and Late Ordovician and Early, Middle, and Late Silurian age. The unweathered matrix of the diatremes is serpentinite and talc, with pyrope, chrome diopside, ilmenite, and other minerals in small quantities.

The diatremes occur in an area previously thought to have been above sea level during Early Paleozoic time, but the fossil-bearing xenoliths they contain show that the area was certainly covered by the sea during much of Ordovician and Silurian time. They provide the first positive identification of marine Silurian rocks within the state of Colorado.

The diatremes were formed subsequent to Silurian deposition. Regional relationships indicate that they were formed before the erosional interval which removed Silurian sediments from this region prior to Devonian sedimentation. They were certainly formed before regional stripping of the pre-Pennsylvanian section that preceded deposition of Pennsylvanian red beds.

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