GEOCHRONOLOGY OF THE QUEEN MAUD BLOCK: IMPLICATIONS FOR THE EVOLUTION OF THE NORTHWESTERN CANADIAN SHIELD

The Queen Maud block (QMB) is a large but little studied portion of the Canadian Shield that occupies a key tectonic position between the Slave and Churchill cratons. The QMB offers an opportunity to investigate Mesoarchean rocks, which are relatively rare in the global rock record, as well as the tectonic history of western Laurentia in the time period between 2.5 and 2.0 Ga. Unlike other parts of western Laurentia, 2.5-2.0 Ga events in the QMB are not strongly overprinted by later Taltson-Thelon (2.01-1.91 Ga) or Trans-Hudson (1.86-1.75 Ga) orogenic events.

To obtain a better understanding of the QMB, we conducted a 200 km long transect of spectacular wave-washed outcrops along the southern coast of the Queen Maud Gulf. U-Pb LA-ICP-MS dating of zircon grains in rocks collected during field work, combined with those examined in an earlier study¹ lead to the following conclusions. The western part of the QMB is dominated by Meso- and Neoarchean (3.2-3.1 and 2.7-2.6 Ga) granitoid gneisses interspersed with supracrustal rocks of unknown depositional age. The eastern QMB consists of a 2.50-2.45 Ga suite of bimodal granitoid and mafic rocks as well as supracrustal rocks of the so-called Sherman Basin that were deposited after ca. 2.45 Ga.

U-Pb dating of monazite grains from 11 samples reveals multiple age populations associated with several metamorphic events. A subordinate age population is clustered around 2.47 Ga, concurrent with intrusion of the Queen Maud granitoids. The dominant age population is clustered at 2.39-2.35 Ga, and is likely associated with the Arrowsmith Orogeny (AO) documented elsewhere in the western Churchill Craton. A relatively uniform spreading of ages between 2.35 and 2.10 Ga is present as well and suggests ongoing or perhaps episodic monazite growth after the main phase of the AO.

A revised model for the 2.5-2.0 Ga evolution of western Laurentia is proposed. Intrusion of the 2.50-2.45 Ga bimodal igneous suite is thought to be the result of crustal extension¹. Circa 2.39-2.35 Ga metamorphism and monazite growth associated with the AO may be the product of Slave-Churchill collision. There is also evidence for post-AO mafic magmatism of unknown tectonic origin. The effects of the Thelon orogenic event (2.01-1.91 Ga) are only manifested in the westernmost QMB.

¹Schultz et al., 2007, Geol, 35: 707-710.