Geology topics
Thomas W. Stern
Publications and source records attributed to Thomas W. Stern.
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 PB 207 /Pb 206 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 SiO 2 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.
Boulder Creek batholith, Colorado part III: Fingerprinting discordant zircon ages in a complex intrusion
The apparent ages (32 lead/alpha and 6 Pb 206 /U 238 ) of zircon as plotted on an isochron map of the Boulder Creek batholith define the following pattern: (1) very high ages (1600 to 1900 m.y.) within the outermost border zone on the southwest, south, and southeast; (2) transitional high ages (1300 to 1600 m.y.) within an inner border zone on the east and widening to the south and west to include about one-third of the batholith; (3) transitional low ages (1000 to 1300 m.y.) throughout much of the interior of the northern half; and (4) very low ages (1000 m.y. or less) limited to a small area within the northeast corner. The area of minimum age is shown to be part of the reduced-age aureole surrounding the 77 m.y. hornblende granodiorite stock at Jamestown that intrudes the Silver Plume Granite of the Longs Peak-St. Vrain batholith in the region immediately to the north of the minimum-age area of the Boulder Creek batholith. A southeastward elongation of the area of minimum age is attributed to channelway control of the solutions responsible for the recrystallization of the zircon by those northwest-trending breccia reefs that are cut by, or strike toward, the Laramide intrusion. Statistical studies of five zircon separates used for isotopic work showed that the frequency of grains having partial, or complete, rims of colorless zircon on purple to semiopaque zircon cores increased inversely with measured Pb 206 /U 238 age along a smooth curve that, when extrapolated, connected the point representing age of emplacement (0 percent rims) and the point representing the approximate age of re-crystallization (100 percent rims). Consequently, in the Boulder Creek zircon rim frequency gives a useful estimate of the amount of lead lost relative to uranium and thorium from a given sample during its recrystallization. The microstudy indicated: (1) the surface separating core and rim is a major discontinuity; (2) the greater part of the rims appear to be true overgrowths; and (3) the highest frequency of rims is found in the most metamict zircon. However, in any one sample a significant fraction of the most metamict zircon has been sheltered from reaction, presumably by inclusion within relatively impervious minerals, and remains free of rims. These observations coupled with the map evidence of selective channelway control point to warm solutions rather than dry heat as the agent of recrystallization and lead loss.