THE CHAIN LAKES MASSIF, MAINE AND QUÉBEC: A PRODUCT OF EXTENSION?

The origin and history of the Chain Lakes massif, which lies within the axial zone of the northern Appalachian orogen, is uncertain, and many models attempt to explain its role in Appalachian orogenesis. Based on our recent work, we suggest that the most likely scenario includes the protolith of the Chain Lakes massif as a sedimentary deposit laid down on extending Grenville(?) crust. Possibly as a consequence of continued extension, the massif partially melted at low pressure (approximately 4 kb) during the Ordovician. Following metamorphism, intrusion of arc-related plutons, and cooling, the massif remained relatively rigid during regionally extensive middle Paleozoic contractional events.

The southern half of the Chain Lakes massif contains relatively abundant exotic clasts and a widespread subhorizontal to shallowly-dipping compositional banding defined predominantly by sillimanite-biotite lenses. Many of these lenses contain isoclinal microfolds defined by sillimanite (fibrolite). The quartzofeldspathic matrix is granofelsic; no penetrative foliation is present. We interpret well-defined leucosomes containing plagioclase, K-feldspar, and quartz as fossil melt. Pseudomorphs of andalusite, garnet, and possibly cordierite are sporadically present, and there is no clear systematic variation in metamorphic grade across the massif. In places along and near the southern border of the massif, subvertical shear zones up to a few meters wide overprint the compositional banding. We attribute these shear zones to Devonian deformation. The northern half of the massif is dominantly massive, with xenoliths set in a granofelsic matrix. We interpret these rocks as anatectic products, possibly related to the partial melting evident in the southern half of the massif.

The above observations are most consistent with petrogenesis in an extensional setting. If it formed during extension, the massif records little evidence related to the collisional history of the Laurentian margin. Thus, although it may have been a microcontinental fragment that accreted to Laurentia in the Ordovician or later, the Chain Lakes massif could equally well represent an extended portion of the Laurentian margin.