Differential unroofing within the central metasedimentary Belt of the Grenville Orogen: constraints from 40Ar/39Ar thermochronology

Combining sphene data with 40Ar/39Ar and zircon data reveals that the eastern part of the Elzevir domain experienced middle to upper amphibolite facies conditions during the time at which the Frontenac domain had already cooled to <350°C.

“Combining sphene data with 4~ (Cosca et al. 1991(Cosca et al. , 1992) ) and zircon data (Davis and Bartlett 1988;Lumbers et al. 1990;Marcantonio et al. 1990;van Breemen and Davidson 1990) reveals that the eastern part of the Elzevir domain experienced middle to upper amphibolite facies conditions during the time at which the Frontenac domain had already cooled to < 350 ~”

Time-integrated cooling rates, obtained from U-Pb zircon and sphene ages combined with 40Ar/39Ar hornblende ages and peak metamorphic temperatures, are only in the range of 1 to 4 °C/Ma.

“However, time-integrated cooling rates, obtained from U-Pb zircon and sphene ages combined with 4~ hornblende ages (Cosca et al. 1991(Cosca et al. , 1992) ) and peak metamorphic temperatures, are only in the range of 1 to 4 ~”

When 40Ar/39Ar ages for hornblende for the Grenville Orogen are compared with U-Pb sphene ages for the same samples or equivalent areas, the U-Pb sphene ages are significantly older, typically about 50 Ma.

“When the 4~ ages for hornblende (Cosca et al. 1991(Cosca et al. , 1992) ) for the Grenville Orogen are compared with the U-Pb sphene ages for the same samples or for equivalent areas it is always the case that the U-Pb sphene ages are significantly older, typically about 50 Ma.”

Hornblende 40Ar/39Ar ages are also younger in the eastern than in the western Elzevir domain and support the new subdivision within the Elzevir domain proposed by Easton (1992).

“Hornblende 4~ ages (Cosca et al. 1991(Cosca et al. , 1992) ) are also younger in the eastern than in the western Elzevir domain and support the new subdivision within the Elzevir domain proposed by Easton (1992).”

This interpretation is suggested by the similarity of the 40Ar/39Ar hornblende age obtained by Cosca et al. (1991, 1992) for one sample from the Sharbot Lake domain with hornblende ages from the Elzevir domain, and the similarity of the sphene age with sphenes from the Frontenac domain.

“This interpretation is suggested by the similarity of the 4~ hornblende age obtained by Cosca et al. (1991Cosca et al. ( , 1992) ) (Fig. 3) for one sample from the Sharbot Lake domain with hornblende ages from the Elzevir domain, and the similarity of the sphene age with sphenes from the Frontenac domain.”

The sphene data indicate that the Frontenac domain experienced upper amphibolite facies metamorphism at about 1180 to 1160 Ma and immediately thereafter cooled below the closure temperature for sphene and at 1125 to 1104 Ma below the closure temperature for the 40Ar/39Ar system in hornblende.

“The sphene data (Fig. 2, Table 1) indicate that the Frontenac domain, as defined in Fig. 2, experienced upper amphibolite facies metamorphism at about 1180 to 1160 Ma and immediately thereafter cooled below the closure temperature for sphene and at 1125 to 1104 Ma below the closure temperature for the 4~ system in hornblende (Cosca et al. 1991(Cosca et al. , 1992)).”

There is a pronounced difference in the 40Ar/39Ar hornblende ages between the western and eastern Elzevir domain.

“As evident from the data of Cosca et al. (1991Cosca et al. ( , 1992) ) there is a pronounced difference in the 4~ hornblende ages between the western and eastern Elzevir domain.”

The ages for hornblende from unsheared amphibolites in western Elzevir domain are 1021 Ma and 1026 Ma, whereas the ages for hornblende from the southeastern part range from 942 to 919 Ma.

“The ages for hornblende from unsheared amphibolites in western Elzevir domain are 1021 Ma and 1026 Ma, whereas the ages for hornblende from the southeastern part range from 942 to 919 Ma.”

Combining the temperature estimates for peak metamorphism with U-Pb sphene ages from this study and 40Ar/39Ar hornblende ages yields cooling rates of about 16°C/Ma for the western Elzevir domain in the vicinity of the Bancroft shear zone.

“Combining the temperature estimates for peak metamorphism (Anovitz and Essene 1990) with U-Pb sphene ages from this study and 4~ hornblende ages (Cosca et al. 1991(Cosca et al. , 1992) ) yields cooling rates of about 16~ for the western Elzevir domain in the vicinity of the Bancroft shear zone.”

Although the U-Pb sphene ages do not show a systematic change from the southeastern Frontenac domain to the Sharbot Lake domain, the 40Ar/39Ar hornblende ages show an apparent younging from about 1115 Ma in the central Frontenac domain to 1007 Ma in the Sharbot Lake domain.

“Although the U-Pb sphene ages do not show a systematic change from the southeastern Frontenac domain to the Sharbot Lake domain, the 4~ hornblende ages (Cosca et al. 1991(Cosca et al. , 1992) ) show an apparent younging from about 1115 Ma in the central Frontenac domain to 1007 Ma in the Sharbot Lake domain.”

The different cooling history of this block is recorded in 40Ar/39Ar ages obtained on hornblendes.

“The different cooling history of this block is recorded in 4~ ages obtained on hornblendes (Cosca et al. 1991(Cosca et al. , 1992)).”

At about 30 Ma after peak metamorphism they reached the commonly observed rate of 1 to 4°C/Ma as indicated by the 40Ar/39Ar mineral ages.

“At about 30 Ma after peak metamorphism they reached the commonly observed rate of 1 to 4 ~ as indicated by the 4~ mineral ages (Cosca et al. 1991(Cosca et al. , 1992)).”

Movement along fault zones led to the final adjustment of different tectonic blocks.

“Movement along fault zones (e.g., in the eastern Elzevir domain as indicated by the 40Ar/39Ar hornblende ages of Cosca et al. (1991, 1992) led to the final adjustment of different tectonic blocks.”