Age of Precambrian events in the Northeastern Front Range, Colorado

Abstract

Precambrian metasedimentary rocks of the northeastern Front Range have undergone a long and complex geologic history involving multiple periods of metamorphism, deformation, and intrusive activity. The first event following sedimentation involved deformation which produced large east‐west isoclinal folds and metamorphism in the greenschist facies and possibly higher. It has been impossible to date this first event by radiometric techniques. The second major event was a period of dominantly medium‐ to high‐grade regional metamorphism accompanied by folding and syntectonic intrusions, such as the Boulder Creek granite. Rb‐Sr dating of high‐grade gneisses, pegmatites, and Boulder Creek granite places this event at 1700–1800 m.y. ago. Precambrian metasedimentary rocks younger than 1700 m.y. have not been recognized in the northeastern Front Range. A major period of plutonism is dated at 1390–1450 m.y. ago during which time the Sherman and Silver Plume granites, pegmatites, and basalt and andesite dikes were emplaced. The regional heating of the country rock at this time was effective in lowering most of the Rb‐Sr and K‐Ar mineral ages determined on prebatholithic rocks and probably caused the retrogressive metamorphism which locally affected the gneisses and schists. Post‐batholith faulting and cataclasis, possibly at about 1300 m.y., may have been effective in further reducing some mineral ages.

The Landscape Continuum: A Model for High-Elevation Ecosystems

“…The rocks underlying this area are of similar origin and are extremely old (Peterman et al 1968, Madole 1982. Variations in the rock's chemical content and weathering rates, especially in the cold alpine environment, are unlikely to produce the differences seen in figure 10.…”

Strontium 87/strontium 86 as a tracer of mineral weathering reactions and calcium sources in an Alpine/Subalpine Watershed, Loch Vale, Colorado

“…The [Peterman et al, 1968], which underlies 80% of the watershed and crops out mostly above tree line, and Precambrian Silver Plume granite (1450 Ma) [Peterman et al, 1968], which intrudes the gneiss and underlies the lower 20% of the basin. The mineralogy of the two units is similar; primary minerals consist of quartz (28-41%), plagioclase (25-30%), biotite (6-16%), microcline (9-34%), and sillimanite (0-6%) [Cole, 1977].…”

Early Proterozoic oceanic crust in the northern Colorado Front Range: Implications for crustal growth and initiation of basement faults

“…[11] M 3 metamorphism was long regarded as a purely retrogressive event during the waning stages of M 2 [Peterman et al, 1968;Nesse, 1984]. However, recent thermobarometric and 40 Ar/ 39 Ar studies [Hodgins, 1997;Selverstone et al, 1997; correlate M 3 metamorphism with a regional heating event throughout the Front Range circa 1.4 Ga that locally resulted in prograde growth of new garnet, andalusite, and staurolite.…”

“…Subsequent deformation accompanied peak Early Proterozoic metamorphism and locally resulted in the formation of two additional fold generations and cleavages. M 2 is regarded as the major regional metamorphism that produced garnet, staurolite, andalusite, and sillimanite in the northern Front Range and has been broadly constrained between 1.70-1.75 Ga [Hedge et al, 1967;Peterman et al, 1968;Braddock and Cole, 1979;Nesse, 1984]. The timing of M 2 metamorphism is roughly coeval with the intrusion of Routt Suite plutonic rocks.…”