Abstract

Late Cretaceous batholith, quartz monzonite ore, Precambrian isotope age, partial-melting source material model; Isotope ratios of Pb in feldspar range widely from pluton to pluton: 16.9-18.1 for 206/204, 15.4-15.7 for 207/204, and 37.7-38.5 for 208/204.Whole-rock initial Sr-87/Sr-86 is 0.706-0.709. Variation in Pb isotope ratios within Butte Quartz Monzonite, plus differences in ratios between megacryst and groundmass feldspars in Donald pluton suggest incomplete isotopic mixing; similarity of Butte ore Pb to feldspar Pb of Butte Quartz Monzonite, therefore, has genetic significance. Isotope data suggest that batholith rocks result from partial melting of lower crust or upper mantle; assimilation of upper crustal material is necessary if source material resembled oceanic tholeiites.

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