PETROLOGY AND U-PB GEOCHRONOLOGY OF BURIED AVALONIAN PLUTONIC ROCKS ON SOUTHEASTERN CAPE-COD

Plutonic rocks have been intersected by two separate drill holes on so utheastern Cape Cod. Hole CC2 is located at Chatham Harbor about 7 km south of the Nauset anomaly. an east-northeast-trending magnetic linea ment that was considered to separate the distinct plutonic zones of Av alon terrane. This drill hole intersected weakly foliated, fairly homo geneous biotite granite. Zircons from this granite give a U-Pb age of 584+9/-8 Ma. Hole CC1 is located near North Eastham, about 12 km north of the Nauset anomaly. The drill core intersected foliated, sheared, biotite granodiorite and biotite-hornblende-clinopyroxene-quartz gabbr o, metamorphosed to greenschist facies. The deformed and altered state of these rocks, as well as their geochemistry, suggest that their ori gin and possibly their ages are distinct from the granite in hole CC2. No datable zircons were obtained from rocks in CC1. The age of 584 Ma for the CC2 granite sample is within the range of published ages for plutonic rocks of the Avalon terrane and confirms the suggestion of Hu tchinson et al. (1988) that the southern plutonic zone is a part of th e Avalon terrane. The data also indicate that the Nauset anomaly is no t the Avalon-Meguma terrane boundary in this area.