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. Copyright 1992 by the American Geophysical Union.