Initial results and implications of [sup 40]Ar/[sup 39]Ar mica age dating in the southern Black Hills, South Dakota

Conference · Thu Apr 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5125837

Holm, D K; Dahl, P S; Gardner, E T ^[1]; Lux, D R ^[2]

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1. Kent State Univ., OH (United States). Dept. of Geology
2. Univ. of Maine, Orono, ME (United States). Dept. of Geology

Intrusion of the 1,700 Ma Harney Peak Granite, southern Black Hills, South Dakota resulted in the formation of a large thermal aureole reaching second-sillimanite grade metamorphism and local migmatization. [sup 40]Ar/[sup 39]Ar ages for micas were obtained from staurolite-zone country rock samples collected over a wide region surrounding the pluton. Excess argon appears to be a problem in only one sample which exhibited a total gas age much older than the pluton. A sample ([number sign]87) located [approximately]15 km SW of the pluton yielded a biotite near-plateau age (and concordant total gas age) of 1,669[+-]16 Ms. Muscovite from the sample yielded a concordant plateau and total gas age of 1,648[+-]12 Ma. The concordant mica pair ages (within error) suggests cooling from above [approximately]350 C to below [approximately]300 C about 30--40 Ma after intrusion of the main pluton. Five separates from three localities (W and NW of the pluton) yield concordant plateau and near-plateau mica ages which cluster around 1,620--1,630 Ma ages as having been reset after initial cooling to below [approximately]300 C at about 1,660 Ma. Since no igneous activity of this age has been reported in the Black Hills, the authors favor a resetting event associated with a low-grade metamorphic event long recognized regionally east and west of the Black Hills and perhaps visibly represented within the Black Hills by a late (post-Harney Peak Granite) widespread but non-penetrative (F4) foliation.

• OSTI ID: 5125837
• Report Number(s): CONF-9305259--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:5
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
ARGON 39
ARGON 40
ARGON ISOTOPES
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
DEVELOPED COUNTRIES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOLOGIC FORMATIONS
GEOLOGY
GEOTHERMOMETRY
GRANITES
IGNEOUS ROCKS
ISOTOPE DATING
ISOTOPES
LIGHT NUCLEI
MICA
MINERALS
NORTH AMERICA
NUCLEI
PETROGENESIS
PETROLOGY
PLUTONIC ROCKS
RADIOISOTOPES
ROCKS
SILICATE MINERALS
SOUTH DAKOTA
STABLE ISOTOPES
USA
YEARS LIVING RADIOISOTOPES