GREENSCHIST FACIES METAMORPHISM AND MULTI-PHASE DEFORMATION OF AN EARLY PALEOZOIC BASALTIC-ANDESITE; A SLIVER OF ANCIENT CRUST IN A SEA OF DEVONIAN INTRUSIONS, EASTERN COASTAL MAINE

The Flander's Bay greenstone (Gilman and Lash, 1988) is an isolated ~11km² metavolcanic body of unknown age within the central coastal Maine magmatic province. This body is intruded by the bimodal ~420 Ma Gouldsboro pluton in the south and the ~366 Ma Tunk Lake pluton in the north. Along Flander's Bay, the greenstone is unconformably overlain by the Siluro-Devonian Bar Harbor formation. Field relations show that the greenstone pre-dates the Bar Harbor formation as well as all surrounding intrusive bodies. These observations suggest that the latest possible age of emplacement is Ordovician. Plots of total alkalis vs. quartz imply a basaltic-andesite protolith.

The greenstone body records varying degrees of metamorphism and deformation. The actinolite+chlorite+feldspar+hornblende±quartz±epidote assemblage restricts the grade from greenschist facies in the south to epidote-amphibolite facies in the north. A pervasive lineation is best displayed by elongate qtz+ep pods that plunge consistently gently southeast. Thin (<1-2cm thick) but abundant quartz veins define pervasive cm-scale asymmetric, tight folds that typically verge southwest and rarely north. A top-to-the-northwest shear sense is indicated by sigmoidal quartz veins. The primary metamorphic foliation is commonly sub-parallel to veining. The overall fabric is warped into a series of broad open folds in the southern portion of the greenstone. No metamorphic fabric is present in the eastern-most portion of the body. In the northwest, near the intrusion of the granitic Tunk Lake pluton, there is a pervasive, consistent south dipping foliation along with isolated localities that display gneissic banding. Bands defined by alternating chl+act+hbl vs. qtz are characteristic of more strongly deformed zones subparallel with the Tunk Lake contact. Grain shape preferred orientation of act+chl+feldspar define the foliation where banding is not present. Evidence of multiple phases of deformation present in the south are overprinted in the north by contact metamorphism and deformation related to the intrusion of the Tunk Lake pluton.

Future textural analysis and detailed geochemical and isotopic studies will focus on improving correlation with other regional metavolcanic units such as the Castine, North Haven, or Ammonoosuc volcanic successions.