- UW Department of Geology and Geophysics
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 K2O wt. % (generally > 5%), molar Na/K generally < 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 (ϵ = −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 (∼0.7), 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 (ϵNd = ∼−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# (< 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 porphyritic rocks record the direct interaction of mantle-derived magma and crustal materials to produce lithosphere with intermediate characteristics; 4) Na-rich rocks record melting of metabasalt emplaced prior to or during widespread magmagenesis; and 5) the Sherman Granite records direct partial melting of juvenile lithosphere and subsidiary fractional crystallization locally accompanied by crustal assimilation.