COMPOSITION OF THE CRUST IN THE GRENVILLE AND APPALACHIAN PROVINCES OF NORTH-AMERICA INFERRED FROM V-P V-S-RATIOS/

We use the ratios between P and S wave velocities (V-P/V-S), derived f rom seismic refraction data, to infer the composition of the crust in the Grenville and the Appalachian Provinces of North America. The crus t exhibits V-P/N-S increasing with depth from 1.64 to 1.84; there is a cleat distinction between the Grenville Province (average V-P/V-S = 1 .81) and the Appalachian Province (average V-P/V-S = 1.73) which persi sts at all depths. The boundary between these provinces is east dippin g extending for 100 km east of the Champlain thrust. In the Appalachia n Province the increase in V-P/V-S ratios with depth from 1.67 to 1.74 +/- 0.02 may reflect a normal decrease of silica content in the conti nental crust. In the Grenville Province beneath the Central Granulite Terrane, an anomalous V-P/V-S ratio of 1.82 +/- 0.02 is observed exten ding to a depth of 10 km; this correlates with the abundance of Ca-pla gioclase in the Marcy Anorthosite. At greater depth (15-20 km), where seismic lamination and high electrical conductivity is observed, V-P/V -S is 1.84 +/- 0.02 and correlates with the Tahawus Complex, a layered mafic intrusion. Within the 25-km-thick lower crust of the Grenville Province the V-P/V-S is 1.84 +/- 0.02 and P-velocity is 7.0 +/- 0.1 km /s, which are typical for plagioclase-bearing rocks (gabbro-norite). T he high V-P/V-S ratio in the Grenville Province has not been reported in crust of any other age. Since the Grenville Province contains 75% o f the world's known anorthosites, high V-P/V-S ratio is related to hig h plagioclase. We suggest that the composition of the Grenville lower crust was significantly modified by the emplacement of the anorthosite s in the mid-Proterozoic.