Abstract

Detailed geologic mapping in the northern Mummy Range, north-central Colorado, reveals seven distinguishable igneous granitoid rock units intruded into a sequence of high-grade metamorphic host rocks. U-Pb dating of zircons from twelve samples indicates that these granitoids were emplaced during two separate periods of magmatism about 300 m.y. apart, one in the Paleoproterozoic and another in the Mesoproterozoic. Zircon dating confirms that all of this magmatic activity post-dated the peak of Paleoproterozoic metamorphism in this area.

Based on field, geochemical and mineralogical analyses, four distinct Paleoproterozoic granitoid rock units are interpreted to represent three separate, but geochronologically indistinguishable, magmatic events at ∼1.7 Ga. The earliest igneous event is recorded by a body of leucocratic tonalite which yields a zircon age of 1702 ± 6 Ma. The second event is recorded by small bodies of granodiorite which are mineralogically and geochemically very similar to the approximately contemporaneous Boulder Creek Granodiorite exposed to the south. The third Paleoproterozoic igneous event produced foliated granitoid rocks that yield a composite zircon age of 1695 ± 20 Ma and which comprise two distinct lithologies: a melanocratic roof facies and a strongly foliated monzogranite. This foliated Paleoproterozoic monzogranite, which is geochemically and mineralogically indistinguishable from the approximately contemporaneous Rawah batholith exposed to the northwest, is the most widespread rock unit exposed in the northern Mummy Range, accounting for over 65% of the area of the Comanche Peak quadrangle north of the Skin Gulch shear zone.

The earliest Mesoproterozoic igneous event in the northern Mummy Range is represented by small bodies of coarse-grained quartz diorite. The second Mesoproterozoic event is represented by the strongly porphyritic granodiorite of the Hagues Peak pluton. The third and final Mesoproterozoic igneous event in the area produced widespread bodies of equigranular “Silver-Plume-type” peraluminous monzogranite that yields a zircon age of 1393 ± 25 Ma.

Most of our zircon ages for the Mummy Range granitoids are consistent with the most recent synthesis of models for the tectonic evolution of the Proterozoic Colorado province. Our dates for certain individual samples of the Paleoproterozoic granodiorite and foliated Paleoproterozoic monzogranite suggest that Paleoproterozoic syntectonic plutonism may have continued slightly longer in this area than previously recognized. The mineralogy and geochemistry of most of the Mummy Range granitoids are consistent with current tectonic syntheses of the region. The composition of the foliated Paleoproterozoic monzogranite in the Mummy Range suggests a source that included pre-existing continental crustal material, consistent with recent suggestions of <1.8-Ga crustal basement beneath parts of the Colorado province.

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