Multiple thermotectonic events in a continuous
metamorphic sequence,Mica Creek area,
southeastern Canadian Cordillera
J.L.Crowley1*, E.D.Ghent1,
S.D.Carr2,
P.S.Simony1, M.A.Hamilton3
1Department of Geology and Geophysics,University of Calgary,Calgary,AB
T2N 1N4,
Canada, 2Department of Earth Sciences and Ottawa-Carleton
Geoscience Centre,Carleton
University,Ottawa,ON K1S 5B6,Canada, 3Geological Survey
of Canada,601 Booth Street,
Ottawa,ON K1A 0E8,Canada, *now at Department of Earth Sciences, Memorial
University of Newfoundland, St.Johns,NF A1B 3X5, Canada <jcrowley@sparky2.esd.mun.ca>
(Received January 10, 2000; Published May 24, 2000)
Abstract
U-(Th)-Pb isotope dilution and SHRIMP dating indicates that multiple
periods of metamorphism and deformation occurred between ~160 and 60
Ma in the Mica Creek area, southeastern Canadian Cordillera. The metamorphic
sequence (Barrovian assemblages in the garnet zone through the sillimanite
- K-feldspar zone), fold generations, and Neoproterozoic stratigraphy
are apparently continuous across the area, yet it broadly lies between
regions that were tectonized at different times, Middle Jurassic to
the east and middle Cretaceous - Paleocene to the west. Our dating along
a north (lowest metamorphic grade) to south (highest grade) transect
shows that three, km-scale tectonic age domains exist. In garnet schist
of domain 1, peak metamorphism occurred at ~163 Ma and a minor thermal
overprint occurred at 70-60 Ma. In migmatitic kyanite schist of domain
2, monazite inclusions in garnet grew at 110 Ma, matrix monazite and
monazite inclusions in kyanite grew at ~85-73 Ma, and rims of some grains
grew at ~60 Ma. Kyanite growth during peak metamorphism must have occurred
after 73 Ma, possibly during intrusion of 61 Ma leucosome. In migmatitic
kyanite schist and sillimanite schist of domain 3, peak metamorphism
occurred at 99-93 Ma, and lesser thermal events occurred at ~280, 160,
110, and 70 Ma. We thus conclude that the thermal peak of metamorphism
took place at different times in each domain, with peak events coinciding
with secondary events in other domains. Age constraints on S1+2
are provided by dating variably deformed granitoid rocks.S1+2
developed before 72 Ma in domain 1, at least partly at 61-58 Ma and
122-63 Ma in domains 2 and 3, respectively. Our findings show that the
Mica Creek area is the only known region in the southeastern Canadian
Cordillera that was affected by the five recognized major periods of
tectonism (175-160, 140-120, 110, 100-90, and 75-50 Ma). The key to
understanding the geologic history is determining why several metamorphic
and deformation events spanning 100 Myr occurred within a single tectonometamorphic
complex with apparent geologic continuity. If continuity is only apparent,
the domains may have been tectonically assembled along cryptic, unrecognized
shear zones. However, if continuity does indeed exist, the events must
have been locally superimposed on the complex.
Keywords: Canadian Cordillera, metamorphism, monazite, U-Th-Pb
geochronology
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Copyright © 2000 by the Mineralogical Society
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