TEMPERATURE AND PRESSURE VARIATIONS IN SUITES OF ARCHEAN FELSIC PLUTONIC ROCKS, BERENS RIVER AREA, NORTHWEST SUPERIOR PROVINCE, ONTARIO, CANADA

Abstract

The Berens River area is representative of a broad belt of Archean felsic plutonic rocks in the northwestern Superior Province of Ontario. The plutonic belt (Berens River subprovince) occupies the southern margin of the North Caribou terrane, a regional-scale enclave of pre-2.8-Ga plutonic and greenstone sequences. Plate-tectonic models maintain that the North Caribou terrane is an early continental mass against which other terranes (subprovinces) were rapidly accreted from 2.75 to 2.70 Ga. During accretion, a magmatic arc characterized by voluminous felsic intrusions developed on the southern margin of the old continent. In the Berens River area, five major groups of felsic plutonic rocks have hornblende-bearing assemblages suitable for application of amphibole + plagioclase thermometers and Al-in-hornblende barometers. Cross-cutting relations show that the biotite tonalite suite is oldest; it is cut consecutively by hornblende tonalite of the hornblende suite, hornblende granodiorite of the hornblende suite, the biotite granite suite and the sanukitoid suite. Average temperatures for the major plutonic suites range from 716 to 773°C. Temperature maps show irregular isotherms, with some low-T zones associated with faults in the hornblende tonalite, but not in hornblende granodiorite. The average temperature-corrected Al-in-hornblende pressures for the plutonic suites range from 4.4 to 2.3 kbar and decrease with age of the suites. The distribution of P measurements indicates that the Berens River area was uplifted significantly during magmatism, with uplift concentrated at the southern margin of the North Caribou continent.