Laura Ratcliffe

The Central Gneiss Belt (CGB) in the western Grenville Province of the Grenville orogen (~1190 - 980 Ma) in southern Ontario exposes a Himalayan scale mountain belt at deep levels. The recently identified North Tea Lake mylonite zone (NTLMZ) is a late extensional, greenschist facies mylonite zone lying conformably within an amphibolites facies nappe-bounding thrust sense North Tea Lake shear zone (NTLSZ) which separates the Kiosk and Bonfeld domains within the western CGB.

Detailed mapping of a transect across the NTLMZ shows lithological and structural variations within the shear zone. The NTLMZ has normal-sense shear determined from microstructural kinematic indicators and extension direction determined from orientation of rotated fold hinges to the down dip direction.

A temperature estimate at the time of mylonitization is based on 1) identification of dynamic recrystallisation processes in quartz and feldspar (400-500^o C), 2) a mineral assemblage suggestive of greenschist facies conditions (300-500o C) and 3) Electron Backscatter Diffraction (EBSD) analysis indicating the dominant slip system active in quartz (basal , 300-400^o C).

An ⁴⁰Ar/³⁹Ar geochronology investigation of bulk rock samples and mineral separates produced a reliable cooling age of hornblende to be 1025 ± 19 Ma. A model assuming a hornblende closure temperature of 500^o C and cooling rate of 2^o C Myr-1 suggests a NTLMZ age of ~995 - 975 Ma. This age range suggests the NTLMZ was a late Grenville process.

In the extensional NTLMZ the footwall is brought closer to the surface indicating exhumation of the study area, while PT changes from the earlier amphibolite facies NTLSZ to the later green schist facies NTLMZ indicates cooling of the study area. Therefore the NTLMZ suggests cooling and exhumation during late Grenville processes.

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Pages: 95
Supervisor: Nick Culshaw