Abstract

The Precambrian rocks of the Front Range have undergone a long and complex history involving multiple periods of metamorphism, deformation, and intrusive activity. Before 1750 m.y. ago sedimentation and volcanism resulted in the accumulation of many thousands of feet of sandstone and shale, and lesser amounts of calcareous and volcanic rocks on a basement terrane that has not yet been recognized.Regional and dynamic metamorphism at 1700 to 1750 m.y. ago converted the sedimentary-volcanic pile to a metamorphic terrane of dominantly medium- to high-grade gneisses and schists. Plutonic rocks typified by Boulder Creek Granite were emplaced during this event. After this orogeny, there is a time span of 250 to 350 m.y. for which there is no obvious geologic record. The interval 1390 to 1450 m.y. was a period of major igneous activity during which the extensive Silver Plume Granite, Sherman Granite, granitic and mafic dikes, and pegmatites were emplaced. Regional heating attendant with this plutonism was effective in resetting most K-Ar and Rb-Sr mineral ages of the older rocks. Tectonic activity along the great shear zone that trends northeast through Idaho Springs probably began during this orogeny and may have continued intermittently until about 1200 m.y. ago.The Pikes Peak batholith and smaller related granites were emplaced at 1040 m.y. ago. This event was not accompanied by regional metamorphism. There is no recognized sedimentary record between the earlier orogeny and this igneous activity. The Pikes Peak igneous activity is the youngest recognized Precambrian event, but it appears to be limited to the Pikes Peak batholith and satellite plutons.

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