Abstracts
Abstract
Plutonic rocks have been intersected by two separate drill holes on southeastern Cape Cod. Hole CC2 is located at Chatham Harbor about 7 km south of the Nauset anomaly, an east-northeast-trending magnetic lineament that was considered to separate the distinct plutonic zones of Avalon terrane. This drill hole intersected weakly foliated, fairly homogeneous biotite granite. Zircons from this granite give a U-Pb age of S84+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 gabbro, metamorphosed to greenschist facies. The deformed and altered state of these rocks, as well as their geochemistry, suggest that their origin 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 Hutchinson et al. (1988) that the southern plutonic zone is a part of the Avalon terrane. The data also indicate that the Nauset anomaly is not the Avalon-Meguma terrane boundary in this area.
RESUME
Des roches plutoniques ont 6t6 recouples par deux forages distincts dans le sud-est de Cape Cod. Le forage CC2 est situ* a Chatam Harbor a environ 7 km au sud de l'anomalie de Nauset, un lineament magn*tique d'orientation est-nord-est qui *tait consider comme s*parant les zones plutoniques distinctes du terrain d'Avalon. Ce forage a recoup* un granite a biotite faiblement foli* et tres homogene. Des zircons de ce granite donnent un age U-Pb de 584 + 9/-8 Ma. Le forage CC1 est situ* pres de North Eastham, a environ 12 km au nord de l'anomalie de Nauset. Le sondage a recoup* de la granodiorite a biotite et du gabbro a biotite-homblende-clinopyroxene-quartz foli*s et cisaill*s, m*tamorphis*s au facies des schistes verts. La nature d*form*e et alt*r*e de ces roches, ainsi que leur geochimie, suggere que leur origine et possiblement leur age soient distinct du granite du forage CC2. Aucun zircon datable ne fut obtenu des roches dans le CC1.
L'age de 584 Ma pour l'*chantillon de granite CC2 tombe dans l'intervalle des ages publics pour les roches plutoniques du terrain d'Avalon et confirme la suggestion de Hutchinson et al. (1988) que la zone plutonique m*ridionale est une partie du terrain d'Avalon. Les donn*es indiquent aussi que l'anomalie de Nauset n'est pas la frontiere des terrains d'Avalon et de M*guma dans cette region.
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