Tectonic setting and U‐Pb geochronology of the Early Tertiary Ladybird Leucogranite Suite, Thor‐Odin ‐ Pinnacles Area, Southern Omineca Belt, British Columbia

Abstract

The Thor‐Odin ‐ Pinnacles area is a structural culmination in the Shuswap complex of the southern Omineca Belt of the Canadian Cordillera. It comprises amphibolite‐facies rocks that were deformed during Mesozoic‐Paleocene compression and were exhumed in the footwalls of Eocene normal faults during crustal extension. The Ladybird leucogranite suite coincides with the extended terrane in the southern Omineca Belt. It is generally restricted to a midcrustal level which lies in the hanging walls of deep‐seated thrust faults and the footwalls of extensional faults. Field relationships of the leucogranites and U‐Pb geochronology place timing constraints on compressional and extensional shear zones. The last thrust motion on the Monashee décollement occurred in the latest Paleocene, and the shear zone had stopped by 58 Ma. Crustal‐scale normal faults were active in the early Eocene, indicating that crustal extension closely followed the compressional regime. Geological and geochronological data are consistent with an anatectic crustal origin for the Ladybird granite. The granites apparently postdate the thermal peak of metamorphism (Carr, 1990) and were generated during the final stages of thrusting, perhaps due to decompression melting as the midcrustal rocks were carried up a thrust ramp and unroofed and/or due to the introduction of hydrous fluids into the system. In situ magma and hot intrusions probably played an important role in the nucleation of extensional shear zones. The extensional regime then facilitated the intrusion of vast late‐synkinematic to posttectonic plutons. U‐Pb systematics reveal that zircons in high‐temperature shear zones may have suffered high‐temperature Pb loss, perhaps due to deformation‐ or fluid‐enhanced diffusion, and that monazite systematics from samples from high‐grade terranes are complex. Magmatic monazite populations contain crystals of different ages that do not coincide with zircon ages and apparently represent neither a crystallization age nor a cooling age.

The thermal–mechanical evolution of crustal orogenic belts at convergent plate boundaries: A reappraisal of the orogenic cycle

“…The duration of the HT conditions and partial melting of the orogenic crust is recorded by the age spread of granites issued from the mid to lower crust and emplaced at higher structural levels. In the Cenozoic Canadian Cordillera, leucogranites cover an age range of more than 10 Myr (Carr, 1992;Parrish, 1995;Vanderhaeghe et al, 1999b;Johnston et al, 2000;Gordon et al, 2008;Kruckenberg et al, 2008). In the Himalaya-Tibet, the age of the oldest leucogranites emplaced in the Higher-Himalayan Crystallines is Oligocene (Deniel et al, 1987;Edwards and Harrison, 1997;Harrison et al, 1999;Chung et al, 2005) that might provide a minimum age for the onset of partial melting beneath Tibet (Vanderhaeghe and Teyssier, 2001b).…”

Burial and temperature evolution in thrust belt systems: Sedimentary and thrust sheet loading in the SE Canadian Cordillera

“…This forward deformation sequence is also followed in our kinematic modeling, and more details are given by Hardebol et al [2007]. Contractional deformation started post-80 Ma farther to west, constrained by the dating of decollement level to polydeformed sillimanite-orthoclase paragneisses [Parrish, 1995;Carr, 1992], and overall shortening ceased at $58 Ma [Fermor and Moffat, 1992;Sears, 2001;van der Pluijm et al, 2006]. Cooling curves suggest rapid uplift and erosion of the Purcell Anticlinorium (just west of the study area) between 65 and 55 Ma [Archibald et al, 1984] and FT T-t modeling for Lewis thrust hanging wall cooling 110-60°C between 75 and 35 Ma.…”

Kinematic and Petroleum Modeling of the Alberta Foothills and Adjacent Forelant - West of Calgary

“…Late Paleocene-Middle Eocene and younger crustal extension and the uplift of mid-crustal metamorphic core complexes in the Omenica belt (Carr, 1992;Lorencak et al, 2001;Price, 1999, 2002;Vanderheghe et al, 2003) was accompanied by westward increasing erosional exhumation of the Foreland thrust and fold belt and the adjacent margin of the undeformed Interior platform. The thickness of strata eroded increases westward from about 2-3 km at the eastern edge of the fold and thrust belt (Magara, 1976;Hacquebard, 1977;Nurkowski, 1984;England and Bustin, 1986a;Issler et al, 1990;Majorowicz et al, 1990), to >8 km in the southern portions of the Front range (Osadetz et al, 2003).…”

“…Younger, Late PaleoceneMiddle Eocene ductile extension and normal faulting developed in the hinterland, west of the transect (Carr, 1992;Lorencak et al, 2001;Price, 1999, 2002;Vanderheghe et al, 2003). This postcompressional crustal deformation event had no direct impact on the architecture of the Subtrap transect, although coeval asthenospheric rise in the hanging-wall of the east-dipping subduction of the Pacific oceanic lithosphere is likely to account for the postorogenic uplift and erosion observed farther to the east in the Alberta foothills, and even in the adjacent foreland autochthon (Burgess et al, 1997).…”

“…Farther west, in the metamorphic core complexes of the Omineca crystalline belt (Fig. 1), isostatic uplift of mid-crustal rocks is commonly >25 km, and the basal detachment of the Cordilleran foreland thrust and fold is exposed locally at >2 km above sea level (Parrish et al, 1988;Carr, 1992). t e r r a n e P u rc e ll a n ti c li n o ri u m 0 300 km 150 Figure 1 Synthetic map outlining the main structural subdivision of the North American Cordillera in southwestern Canada, and the location of the studied transect.…”