An overview of the petrology and geochemistry of the Sherman batholith, southeastern Wyoming: Identifying multiple sources of Mesoproterozoic magmatism

Abstract

The 1.43-Ga Sherman batholith of southeastern Wyoming and northeastern Colorado is a texturally and geochemically heterogeneous intrusion that comprises rocks derived from at least four different sources. The coarse-grained, metaluminous, biotite-hornblende Sherman Granite is volumetrically the most significant unit in the batholith. It has geochemical characteristics at the extreme end of A-type suites with high Fe# > 88, high q0 wt. % (generally > 5%), molar Na/K generally c 1, and high abundances of incompatible elements. The source for the Sherman Granite is constrained to be a Fe-rich, low oxygen fugacity source mafic material by initial isotopic ratios (E,~ = -0.8-1.1 and initial 87Sr/86Sr = 0.7024-0.7126), low oxygen fugacity (-0.1 to -0.5 log units below the fayalite-magnetite-quartz buffer reaction) and water activity ( NO. 7), comparisons to experimental melt compositions, and mineralogical and geochemical similarities to monzonitic intrusions from the Laramie anorthosite complex (LAC). The medium-grained, peraluminous Lincoln granite, which is volumetrically subordinate to the Sherman Granite, has less extreme A-type geochemical characteristics. Initial isotopic ratios (ENd ---1 and initial 87Sr/86Sr = 0.7189 and 0.7238), comparisons to experimental melt compositions, and geochemical similarities to intrusions from the peraluminous Silver Plume suite favor a parental magma for the Lincoln granite derived from intermediate to felsic crustal rocks from the Colorado province, which forms the basement to the Sherman batholith. A suite of mineralogically heterogeneous biotite-hornblende porphyritic quartz monzonites and granites has geochemical characteristics that also fit the A-type classification but are less extreme than that for the Sherman Granite, with distinctly lower Fe# ( c 88) and molar Na/K (generally > 1). Field and geochemical observations are consistent with many of the porphyritic granites having formed by mixing between granitic and monzodioritic magmas and highlight the importance of magma mixing in the formation of the batholith. Mafic rocks are present in the batholith in minor quantities and include gabbro, ferrodiorite, and monzonite in addition to monzodiorite. All of the mafic rocks are geochemically and mineralogically similar to rocks from the adjacent LAC, and the gabbro presumably is derived from a mantle source similar to that for gabbroic rocks from the LAC. Geochemically distinct sodic granodiorite, which occurs in minor quantities in the batholith, represents a distinct unit likely derived from partially melted metabasalt.The rocks of the Sherman batholith record the evolution of Mesoproterozoic lithosphere in five ways: 1) gabbro records asthenospheric input of heat and mass(?) into the juvenile, Paleoproterozoic lithosphere; 2) peraluminous granite records partial melting of the Paleoproterozoic lithosphere in response to the influx of asthenospheric melts; 3) hybrid porRocky Mountain Geology, u. 35, no. I , p . 113-137, 17 figs., July, 2000 113 B. R. EDWARDS AND C. D. FROS...

Critical Zone Structure Under a Granite Ridge Inferred From Drilling and Three‐Dimensional Seismic Refraction Data

“…Variations in bulk geochemistry can have a profound effect on rock weathering (Bazilevskaya et al, ; Brantley et al, ; Brantley & White, ; Hahm et al, ; Hilley et al, ; Lebedeva et al, ), so it is worth considering whether some of the observed heterogeneity in CZ structure can be explained by bedrock geochemical variability across our study area. Although bedrock geochemistry data are limited within the confines of our study area, the surrounding Sherman Batholith has been well studied, so bulk geochemical data are available for a wide range of sites spanning the region (Bell, ; Edwards & Frost, ; Frost et al, ; Galipeau & Ragland, ; Geist et al, ; Houston & Marlatt, ; Zielinski et al, ).…”

“…The Sherman Batholith was uplifted to its present location during the Laramide Orogeny. Metamorphism has been minor since batholith emplacement (Peterman &Hedge, 1968), and although the batholith harbors geochemically diverse bedrock (Edwards &Frost, 2000), our site is developed on relatively uniform Sherman granite (Figure 1a).…”

Estimating the water holding capacity of the critical zone using near‐surface geophysics

“…The Sherman granite contains large phenocrysts of potassium feldspar that weathers to a friable coarse‐grained material, commonly referred to as grus (Eggler, Larson, & Bradley, ; Evanoff, ; Frost, Frost, Chamberlain, & Edwards, ). The Sherman granite is composed of 30–40% microcline, 15–30% quartz, 20% plagioclase, 10–15% perthite, and 5–10% biotite (Edwards & Frost, ; Frost et al, ; Geist, Frost, Kolker, & Frost, ). The study site receives ~620 mm of precipitation annually, 90% of which occurs in the form of snow (Natural Resources Conservation Service, ).…”

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

“…The Sherman batholith hosts three distinct granites: the Lincoln, the Sherman, and porphyritic granite (Geist et al, 1989; Frost et al, 1999; Edwards and Frost, 2000). The Sherman granite is coarse grained and has the highest concentrations of Fe and K. The Lincoln granite is finer grained and has more quartz and plagioclase.…”