U-Pb isotopic constraints on diachronous metamorphism in the northern Monashee Complex, southern Canadian Cordillera

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doi: 10.1046/j.1525-1314.1999.00210.x
Authors:Crowley, J. L.; Parrish, R. R.
Author Affiliations:Primary:
Carleton University, Department of Earth Sciences, Ottawa, ON, Canada
Other:
NERC Isotope Geosciences Centre, United Kingdom
Volume Title:Journal of Metamorphic Geology
Source:Journal of Metamorphic Geology, 17(5), p.483-502. Publisher: Blackwell, Oxford, United Kingdom. ISSN: 0263-4929
Publication Date:1999
Note:In English. 59 refs.; illus., incl. sects., 2 tables, geol. sketch maps
Summary:U-Pb isotopic data indicate that the age of metamorphism varies according to structural position in a 6 km thick section. Metamorphism resulted in an unusual sequence in which rocks with the lowest-grade assemblage (kyanite-sillimanite-staurolite-muscovite) are underlain and overlain by higher-grade rocks. Xenotime and monazite U-Pb dates vary progressively from 64 m.y. in the structurally highest rocks to 49 m.y. in the deepest rocks. Discordant U-Pb ages from Proterozoic and Cretaceous monazite and titanite are used to interpret the thermal significance of the early Tertiary dates. The discordant analyses define linear arrays with lower intercepts that broadly overlap with the early Tertiary. The amount of discordance varies with structural level, being least in the deeper rocks and greatest in the higher rocks. EPMA data show that monazite discordance in the deeper rocks resulted from Tertiary mineral overgrowth and recrystallization rather than Pb diffusion. The downward younging 64-49 m.y. U-Pb dates are interpreted as reflecting xenotime and monazite growth ages rather than cooling ages of substantially reset ages based on the lack of Pb diffusion in monazite and on previously obtained 40Ar/39Ar data which suggest that rapid cooling occurred immediately after the U-Pb dates. Metamorphism was diachronous according to structural level with higher rocks attaining peak T and cooling rapidly while deeper rocks were still heating towards a thermal peak attained a few m.y. later. [P.J.T.]
Sections:Age determination
Subjects:Absolute age; Cenozoic; Cooling; Cretaceous; Dates; Grade; Mesozoic; Metamorphism; Mineral assemblages; Monashee Complex; Monazite; Nesosilicates; Orthosilicates; Overprinting; Paleogene; Phosphates; Silicates; Tectonics; Tertiary; Thermal history; Titanite; Titanite group; U/Pb; British Columbia; Canada; Canadian Cordillera; North America; North American Cordillera; Western Canada
Coordinates:N503000 N520000 W1173000 W1190000
Abstract Numbers:00M/2325
Record ID:1999063486
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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