Eocene plutonic rocks in south central Idaho

Abstract

Large areas previously thought to be underlain by Cretaceous granodiorite of the Idaho batholith are now known to be underlain by plutonic rocks of the Eocene Challis magmatic episode (50–44 Ma). Recent mapping conducted in south central Idaho as part of the U.S. Geological Survey's Conterminous United States Mineral Assessment Program (CUSMAP) indicates that roughly 30% of the exposed plutonic rocks are part of this younger event. The Eocene plutonic rocks are subdivided into a pink granite suite, consisting of varieties of biotite granite, and a quartz monzodiorite suite, which includes hornblende‐biotite granite, granodiorite, quartz monzodiorite, diorite, and gabbro. The members of each suite form discrete epizonal stocks, and some exposures of pink granite are of batholithic size. Silica content of the quartz monzodiorite suite ranges from 48 to 70%; the rocks vary considerably in texture and mineralogy. The most mafic rocks of the quartz monzodiorite suite are cumulates. These are layered intrusive phases containing pyroxene, olivine, and plagioclase. Other mafic phases include hornblende‐ and pyroxene‐rich diorites, which contain plagioclase feldspar as an interstitial phase and are “cumulates” in the sense that they probably represent rocks that were enriched in early formed mafic minerals by some fractionation process. Stocks of the pink granite suite are more compositionally restricted and range from 70 to 77% in SiO2 content. The quartz monzodiorite suite appears to be older than the pink granite suite, but age data are limited. Field relations, initial 87Sr86Sr ratios, CaO content, and MgO/(MgO + FeO*) ratios demonstrate that the two suites are not related by fractional crystallization. Rather, the respective magmas probably originated from different sources. Large ranges in MgO, Sc, Cr, and Co content in the quartz monzodiorite suite can be explained by crystal‐liquid fractionation of olivine, pyroxene, and hornblende, but variations in composition of parental magmas are also likely. Large variations of Ba content in the pink granite suite are probably a result of removal or accumulation of variable amounts of K‐feldspar. Mixing between the two Eocene suites has not been extensive, but some varieties of hybrid pink granite were generated that contain mafic inclusions and have less than 72% SiO2. The pink granite has geochemical affinities with A‐type granite, but unlike typical A‐type granite, it is not uniformly enriched in the highly charged cations such as Zr, Nb, Y, and Ce. By analogy to other A‐type granites the pink granite suite probably originated from partial melting of lower crustal rocks under high‐temperature, vapor‐absent conditions.