Reference 1618

Abstract

The Boulder Mountain Stock, a shallow Eocene pluton in central Idaho,is composed of two major compositional suites: 1 a mineralogicallyuniform weakly peraluminous high-silica granite suite; and 2 ametaluminous granodiorite suite comprising mostly hornblende andbiotite-rich quartz monzodiorite and quartz monzonite. Field relationssuggest that the granodiorite is the older unit. Detailed traceelement combined with stable and radiogenic isotopic data are reportedhere for this stock and are used to identify respective sources foreach suite and the nature of the underlying craton. Pb isotopic ratiosplace both suites in the field of magmas derived from Precambriancrystalline basement, super 87 Sr/ super 86 Sr, super 143 Nd/super 144 Nd, and delta super 18 O isotopic ratios also suggestcrustal components. Pb-Pb isochron and T super Nd model agedeterminations of 1.6 Ga for the granite and 1.9 Ga for thegranodiorite imply a Proterozoic, rather than Archean, age for thelower crust in central Idaho. Earlier studies have suggested aco-magmatic origin for these intrusive suites, but the combined major,trace element, and isotopic data suggest that the two suites must bederived from different sources. The absence of mantle signaturesimplies that partial melting occurred completely within thecontinental crust, possibly as a direct result of Cenozoic tectonicthickening. The respective suites were derived separately, and theirmagmas are not genetically related by fractional crystallization orsource mixing. The granodiorite was generated by partial melting of agarnet amphibolite or mafic granulite completely within the lowercrust, while the granite is a lower/upper crustal hybrid composed of agarnet-bearing lower crustal granulite severely modified bycontamination of middle to upper crustal Proterozoic units of theBelt-Yellowjacket Terrane. Thus, the Eocene magmas in central Idaho donot appear to be directly related to subduction; however, theirgenesis is probably related to heat transfer from the upper mantle tothe crust and subsequent lithospheric thinning.

57 samples from this reference