Wide-angle seismic imaging of a Mesoproterozoic anorthosite complex: The Nain Plutonic Suite in Labrador, Canada

Abstract

The Mesoproterozoic Nain Plutonic Suite (NPS) of Labrador (Canada), one of the largest anorogenic plutonic terranes, was studied by a refraction/wide-angle seismic experiment. Four ocean bottom seismometers and 18 land stations were deployed along a 330-km profile and recorded air gun shots from the easternmost 160 km with the NPS located in the center of the line at the suture of the Nain and Churchill Provinces. P and S wave velocity models were developed by forward modeling of travel times and amplitudes. Upper and middle crustal P wave velocities outside and beneath the NPS range from 5.9 to 6.5 km/s, lower crustal P wave velocities range from 6.55 to 7.0 kmls. Within the anorthositic rocks, velocities are as high as 6.8 km/s, and reflections define the base of the NPS to be 8 km deep in the SE Churchill Province and 11 km in the Nain Province, a variation that may be the result of lateral density changes within the country rocks or the anorthosites. The total crustal thickness is 39 km west of the NPS but is only 32-34 km beneath the NPS, some 5 km less than Nain Province crust distal from the NPS. The inferred crustal thinning is possibly related to anatexis of the lowermost crust by a thermal plume that generated the plutonism. The Poisson's ratios are 0.275 within the anorthosite plutons, 0.27 in the upper and middle crust, and 0.285 in the lower crust. These values are some 0.03 higher than in the Archean Nain crust distal to the NPS, indicating a higher plagioclase content at all crustal levels as result of the plutonism. We postulate that a crustal root, similar to the root observed farther north in the Torngat Orogen, was completely removed by anatexis and the silicic and basic magmas probably ascended to midcrustal levels along preexisting zones of weakness at the Nain-Churchill boundary.