Os isotopic systematics in mantle xenoliths; age constraints on the Canadian Cordillera lithosphere

Abstract

The 187 Os/ 188 Os ratios of lherzolites from eight xenolith suites from the Canadian Cordillera show a correlation with Al 2O 3 and heavy rare earth elements (HREE). The best interpretation of these correlations appears to be ancient melt depletion followed by a long period of radiogenic ingrowth. The 187 Os/ 188 Os–Lu correlation is used to calculate an Os model age of 1.12±0.26 Ga for the lithospheric mantle throughout the Canadian Cordillera. This single melting age suggests that the mantle lithosphere now underlying the entire Canadian Cordillera may have formed by melting events closely spaced in time. This is consistent with seismic evidence of the extension of crustal basement under much of the Canadian Cordillera that is independent of the upper-crustal terranes overlying it. Indeed, this Proterozoic Os model age for the mantle contrasts with the younger formation ages (Nd model ages and U–Pb ages of zircons) of most crustal terranes of the region which are around 0.5 Ga. Early Proterozoic basement is exposed only in southeastern British Columbia and has the same age (1.9 to 2.3 Ga) as the ancestral North American crust, but is older than the Os model age of the mantle lithosphere underlying the Canadian Cordillera. The Canadian Cordilleran mantle is thus probably not a simple extension of the North American cratonic lithosphere beneath the adjacent mobile orogenic belt of the Canadian Cordillera. The difference in age between the formation of the Canadian Cordillera upper-crust and the formation of the underlying mantle suggests that this mantle lithosphere does not represent the mantle roots of the crustal terranes overlying it. Instead, these crustal terranes were thrust onto the mantle lithosphere during Canadian Cordillera orogeny. This contrasts strongly with Archean cratonic zones and Early Proterozoic belts where oldest crustal rocks and mantle may have the same formation age.