- UW Department of Geology and Geophysics
Proterozoic plutonism in Colorado and Wyoming was initiated ∼1.8 Ga with scattered tholeiitic mafic complexes coeval with widespread synorogenic bimodal volcanism. Limited Nd and Sr isotopic data for the metavolcanic rocks show derivation from depleted mantle. Major Paleoproterozoic granitic plutonism followed at 1.67–1.77 Ga. Most of the earliest plutons are distinctly calc-alkaline; they range largely from quartz diorite to granodiorite to trondhjemite in composition, and have trace-element signatures similar to plutons within magmatic arcs related to subduction zones. Later Paleoproterozoic plutons at 1.71 Ga include increased volumes of felsic rock types and, independent of silica, are shifted to more peraluminous and iron-rich compositions. The earliest appearance of anorthosite occurs with the 1.76-Ga Horse Creek anorthosite complex of Wyoming, and the earliest occurrence of A-type granite includes the late-kinematic, 1.66-Ga Garell Peak batholith of southern Colorado. Elemental and isotopic compositions of the younger Early Paleoproterozoic granitic plutons are consistent with a systematically increasing crustal component as a function of age in waning orogenic stages of crust formation in the region.
After a 200 m.y. hiatus, renewed granitic plutonism occurred at 1.36–1.44 Ga. Plutonism was associated with emplacement of over a dozen Mesoproterozoic A-type granite batholiths and many smaller intrusions as part of a global “anorogenic” mid-Proterozoic event that commonly includes associated intrusions of anorthosite and charnockite. Across the former Laurentia supercontinent, three geographic and petrologic subprovinces merge in Colorado and Wyoming. An ilmenite-series granitic province, which includes the Sherman Granite and associated Laramie anorthosite complex of Wyoming, extends northeastward through Wisconsin to Labrador and the classic rapakivi granite-anorthosite intrusions of the Baltic region. A magnetite-series granite subprovince ranges across the southern mid-continent to California and includes the San Isabel and Eolus batholiths of southern Colorado. The third subprovince is peraluminous, comprised of two-mica granite, and geographically extends from central Colorado to Arizona. Granites of this suite are the most common in Colorado and include the Silver Plume batholith. Granites defining the three Mesoproterozoic provinces have distinctly different elemental and oxygen isotopic compositions, which presumably reflect fundamental shifts in composition of the lower continental Laurentian crust. The mid-Proterozoic intrusions of Colorado and Wyoming coincided in time with emplacement of regional, north-trending mafic dike swarms, implying widespread extension during this period.
After another magmatic hiatus, one of ∼300 m.y., intrusion of the A-type, 1.08-Ga Pikes Peak batholith formed the last Proterozoic magmatic episode of the Colorado-Wyoming Front Range.