Petrology and U-Pb geochronology of buried Avalonian plutonic rocks on southeastern Cape Cod
DOI :
https://doi.org/10.4138/1993Résumé
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 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 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. RÉSUMÉ Des roches plutoniques ont été recouplées par deux forages distincts dans le sud-est de Cape Cod. Le forage CC2 est situé à Chatam Harbor à environ 7 km au sud de l'anomalie de Nauset, un lineament magnétique d'orientation est-nord-est qui était considéré comme séparant les zones plutoniques distinctes du terrain d'Avalon. Ce forage a recoupé un granite à biotite faiblement folié et tres homogène. Des zircons de ce granite donnent un âge U-Pb de 584 + 9/-8 Ma. Le forage CC1 est situé près de North Eastham, à environ 12 km au nord de l'anomalie de Nauset. Le sondage a recoupé de la granodiorite à biotite et du gabbro a biotite-homblende-clinopyroxène-quartz foliés et cisaillés, métamorphisés au faciès des schistes verts. La nature déformée et altérée de ces roches, ainsi que leur géochimie, suggère que leur origine et possiblement leur âge soient distinct du granite du forage CC2. Aucun zircon datable ne fut obtenu des roches dans le CC1. L'âge de 584 Ma pour l'échantillon de granite CC2 tombe dans l'intervalle des âges publics pour les roches plutoniques du terrain d'Avalon et confirme la suggestion de Hutchinson et al. (1988) que la zone plutonique méridionale est une partie du terrain d'Avalon. Les données indiquent aussi que l'anomalie de Nauset n'est pas la frontière des terrains d'Avalon et de Méguma dans cette région. [Traduit par la rédaction]Téléchargements
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