Summary

Description

Cores collected from recent drilling in western Cape Cod, Massachusetts provide insight into the topography and petrology of the underlying bedrock. Cores from 64 drill sites spread over a approximately 140 km2 study area produced samples of granitoids (31), orthogneisses (20), basalts/diabases (4), amphibolites (3), felsic mylonites (2), and dolomitic rock (2). Granitoid composition ranges from granite to tonalite and quartz diorite, but it is dominated by two-feldspar granites. Hydrothermal alteration is common in nearly all cores as evidenced by the secondary growth of chlorite and epidote. Granitoids resemble rocks of the Dedham and Fall River plutons (Wones and Goldsmith, 1991). Gneisses generally contain the mineral assemblage K-feldspar+plagioclase+quartz+/-biotite+/- chlorite+/-muscovite+/-sphene+/-epidote+oxides. Amphibolites contain the assemblage hornblende+plagioclase+quartz+biotite+epidote+/-chlorite+/-sphen+/-K-feldspar+/-sericite+ oxides. Peak metamorphic grade based on mineral assemblages is estimated at lower amphibolite facies. X-ray powder diffraction of unmetamorphosed dolomitic cores shows presence of layered silicates (clays), plagioclase, and possible magnesite. Contours of the bedrock surface show locally irregular topography suggesting erosion by glacial scour. Lithologic distribution mapping suggests a possible continuation of the New Bedford gneissic terrane that outcrops 25 km to the west. Dolomitic rocks may represent a lithified fault gauge material at the eastern edge of the gneissic zone. Basalts/diabases are interpreted to be post-metamorphic dikes associated with Mesozoic rifting.