[sup 40]Ar/[sup 39]Ar thermochronology in the northern Bitterroot mylonite zone, Mt

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

OSTI ID:5098418

House, M A; Hodges, K V ^[1]

---

1. Massachusetts Inst. of Tech., Cambridge, MA (United States). Dept. of Earth, Atmospheric, and Planetary Sciences

The extensional Bitterroot mylonite zone defines the eastern and southern border of the Bitterroot metamorphic core complex and is generally interpreted to be the major structure which accommodated unroofing of the metamorphic core. The most commonly cited evidence for the age of mylonitization are [sup 40]Ar/[sup 39]Ar ages for hornblend, muscovite, biotite, and potassium feldspar from the southern Bitterroot mylonite zone that indicate rapid cooling of the core rocks between 45.5 and 43.5 Ma. More recently, an [sup 40]Ar/[sup 39]Ar K-feldspar age of 46.4 [+-] 0.8 Ma for an undeformed rhyolite dike that cuts across the mylonitic fabric places a minimum age constraint on the southern part of the shear zone. The authors have obtained new [sup 40]Ar/[sup 39]Ar data for metapelitic rocks and amphibolites from the northeast border of the Bitterroot metamorphic core complex near an area where mylonitized granitoid rocks yielding 48--52 Ma U-Pb zircon crystallization ages constrain the maximum age of mylonitization. Isochran ages of 47.9 [+-] 0.9 and 49 [+-] 1 Ma for hornblende separated from deformed amphibolite pods in the northeast border zone are within analytical uncertainty of the younger mylonitized granitoid crystallization ages and indicate rapid post-crystallization cooling through temperatures of [approximately]780--800 K. They attribute this cooling to denudation related to shear zone development. Muscovite and biotite isochron ages from metapelitic rocks within the shear zone are significantly younger, between 42 and 44 Ms., and generally agree with mica ages obtained by Garmezy and Sutter for the southern part of the shear zone. However, all mica ages from the Bitterroot shear zone are younger than the minimum age of the shear zone deduced from the age of cross-cutting rhyolite dikes.

• OSTI ID: 5098418
• 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
CHEMICAL COMPOSITION
CHEMISTRY
DEVELOPED COUNTRIES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOCHEMISTRY
GEOLOGIC FAULTS
GEOLOGIC FORMATIONS
GEOLOGIC FRACTURES
GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
GEOLOGY
GEOTHERMOMETRY
ISOTOPE DATING
ISOTOPE RATIO
ISOTOPES
LIGHT NUCLEI
METAMORPHIC ROCKS
MONTANA
NORTH AMERICA
NUCLEI
PETROGENESIS
PETROLOGY
RADIOISOTOPES
ROCKS
STABLE ISOTOPES
USA
YEARS LIVING RADIOISOTOPES