Middle Proterozoic cooling ages in the Cimarron Mountains, northern New Mexico: U-Pb and Ar-40/Ar-39 constraints

Conference · Wed Jan 01 00:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5935869

Grambling, J A ^[1]; Bowring, S A ^[2]; Dallmeyer, R D ^[3]

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1. Univ. of New Mexico, Albuquerque, NM (United States). Dept. of Earth and Planetary Sciences
2. Massachusetts Inst. of Tech., Cambridge, MA (United States). Dept. of Earth and Space Sciences
3. Univ. of Georgia, Athens, GA (United States). Dept. of Geology

Early Proterozoic rocks in the Cimarron Mountains define three tectonic units, each with a unique metamorphic grade and cooling history. Gneissic rocks of the Eagle Nest tectonic unit (ENTU) preserve high-grade regional metamorphism. Quantitative thermobarometry based upon Grt-Bt-Pl-Sil yields P-T conditions of 6--7 kb and 700--725 C. These rocks followed an apparent P-T path of cooling and decompression, which is interpreted to reflect extensional deformation. A low-angle ductile shear zone separates these rocks from the structurally overlying Tolby Meadow tectonic unit (TMTU). Metamorphic grade in the TMTU, constrained by Cld + Sil and Chl + Ms, was 3.5--4.5 kb and 500--550 C. A steeply-dipping right-slip fault isolates the Cimarron River tectonic unit (CRTU) from the ENTU and TMTU. Rocks from the CRTU record a low-grade metamorphism at P-T conditions of 4--6 kb and 300--400 C. The CRTU preserves Ar-40/Ar-39 Hb and Ms cooling ages of 1,678 and 1,350 Ma. The older age, obtained from plutonic Hb, appears to date igneous formation of the CRTU. The younger age reflects the final cooling of the CRTU. The cooling history of the ENTU and TMTU is remarkably different. Zircon and monazite (ENTU) have yielded nearly concordant U-Pb ages of 1,430 Ma, whereas Ar-40/Ar-39 hornblende ages are 1,395--1,400 Ma. These appear to reflect cooling during extensional juxtaposition of the ENTU and TMTU. Muscovite Ar-40/Ar-39 ages from the ENTU and TMTU are notably younger, 1,250--1,260 Ma. These younger ages may reflect a dramatic slowing of cooling rates or a younger, overprinting metamorphism. Thus, metamorphic grade and cooling ages differ across two ductile shear zones in the Cimarron Mountains. An outstanding question is whether the young cooling ages are representative of a regional event, or of a local event associated with unrecognized plutonism.

• OSTI ID: 5935869
• Report Number(s): CONF-921058-; CODEN: GAAPBC
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
MOUNTAINS
METAMORPHISM
NEW MEXICO
AGE ESTIMATION
ARGON 39
ARGON 40
COOLING
GEOBAROMETRY
GEOTHERMAL GRADIENTS
ISOTOPE RATIO
LEAD ISOTOPES
PALEOTEMPERATURE
PRECAMBRIAN ERA
RESERVOIR PRESSURE
TECTONICS
URANIUM ISOTOPES
ACTINIDE ISOTOPES
ARGON ISOTOPES
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
DEVELOPED COUNTRIES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOLOGIC AGES
ISOTOPES
LIGHT NUCLEI
NORTH AMERICA
NUCLEI
RADIOISOTOPES
STABLE ISOTOPES
TEMPERATURE GRADIENTS
USA
YEARS LIVING RADIOISOTOPES
580000* - Geosciences