Boulder Creek batholith, Colorado part II: Isotopic age of emplacement and morphology of zircon

Zircon separated from six rocks whose compositions spanned the range of differentiation in the Boulder Creek batholith yielded a “discordia” age of emplacement of 1725 m.y., close to the average PB²⁰⁷/Pb²⁰⁶ age 1720 m.y.) and indicating that the constituent rocks are cogenetic within approximately ± 20 m.y. Statistical studies show that from 20 to 80 percent of the zircon in any one sample (1) is no-neuhedral, (2) has lower (length/width) ratios than the associated euhedral zircon, and (3) in direct contrast to the euhedral, increases markedly interior of the batholith toward contacts with the older metasediments and internal zones of contamination; it is inferred to have been “inherited” via assimilation. Unlike the noneuhedral fraction the euhedral zircon shows a linear decrease in length/width ratio with an increase in SiO₂ content of the containing rocks; it is inferred to be magmatic in origin. Regardless of the relative abundance of inherited versus magmatic zircon, all samples closely fit a single discordia chord, indicating that both zircon fractions formed at about the same time. This conclusion is compatible with field relationships that indicate the emplacement of the syntectonic Boulder Creek rocks took place during a period of metamorphism notable for the widespread development of new minerals in the country rocks.

(1) Zircon from a Silver Plume Granite dike intruding the Boulder Creek batholith, (2) zircon from Silver Plume correlatives immediately to the north (Tilton and co-workers), and (3) uraninite from a probable Silver Plume correlative in the Central City district, together yield a sharply defined discordia age of emplacement of 1415 m.y.

The separate “discordia” chords for the Boulder Creek and Silver Plume zirconconverge close to their lower intercepts with “concordia” indicative of a one-step lead loss during the Laramide disturbance. The Silver Plume “thermal event” left no age imprint on the Boulder Creek zircon, presumably because insufficient time had elapsed to produce significant metamictization.