LATE PALEOZOIC MAGMATISM IN SOUTHEASTERN MA: INSIGHTS INTO ALLEGHANIAN REJUVENATION OF AVALONIAN BASEMENT

Until now there are no documented examples of Late Paleozoic magmatism east of the Narragansett Basin, perhaps because the Basin delineates the eastern terminus of the source region for such magmas. We present evidence for the occurrence of similar plutonic granitoids in the New Bedford MA area, 50 kilometers east of the Narragansett Basin. On the state geological map, New Bedford consists of Proterozoic alaskitic (Zagr) and biotite gneiss (Zgn), which are well displayed at Fort Phoenix, where they are discordantly to concordantly cut by undeformed granite. Fabric in the gneisses is defined by segregations and discrete grains of oriented micas and feldspar laths, and defines an E-W trending foliation. Biotite gneiss contains rootless folds of undeformed granite that lie within the foliation and occur as concordant, boudinaged seams that grade, without truncation, into discordant granite. These field relationships are reminiscent of those seen in Late Paleozoic granitoids from west of the Narragansett Basin, which have been interpreted as having crystallized at mid-crustal levels during deformation. Textural analysis confirms this interpretation, as gneisses show little evidence of crystal plastic deformation, suggestive of melt segregation and redistribution of a deforming crystal mush. Monazites separated from alaskitic and biotite gneiss show highly concordant U-Pb ages of 278±2 Ma and 267±2 Ma respectively, and are considered to date their magmatic crystallization. These age relationships are also akin to those ascribed to granitic gneisses to the west, an interpretation supported by trace element chemistry.

These results have important implications for our understanding of Alleghanian tectonism. Late Paleozoic magmatism in southern New England is more widespread than previously realized, and extends at least from south-central Conn. to Buzzards Bay. New Bedford granitoids occur as E-W trending sills, in contrast to the slightly older (?) NNE-trend of folding, shearing, and faulting that epitomize Alleghanian contractional tectonics in the region. Finally, the fact that granitic and gneissic rocks, long assumed to be Proterozoic basement, are apparently of Late Paleozoic age argues for a careful reassessment of other alleged Proterozoic gneisses in southern New England.