NEOPROTEROZOIC MAGNETIZATION AND PALEOZOIC OVERPRINT IN THE SOUTHEASTERN NEW ENGLAND AVALON ZONE

Results presented here are part of an ongoing geochronologic-paleomagnetic investigation into the drift history of the Southeastern New England Avalon Zone based on igneous and sedimentary rocks in and around the Boston Basin, eastern Massachusetts. The majority of rocks in this area are ca. 625-590 Ma plutonic and volcanic units formed during Avalonian arc magmatism in late Neoproterozoic time. Although Peri-Gondwanan paleogeographic position is well accepted for this arc, its exact location continues to be debated. Unraveling multiple magnetization directions in 608.3 +/- 1.4 Ma Dedham Granite from Hingham, MA (new upper concordia intercept date from ID-TIMS U/Pb analyses of four single zircons; MSWD=0.36) sheds some light on this problem.

The Hingham sample contains multiple magnetizations, the relative ages of which can now be interpreted by comparison with directions from other dated rocks around Boston. A generally higher temperature component, characterized by due south declinations and moderately low inclinations has also been identified in younger rocks including ca. 596 Ma rhyolite from the Mattapan Volcanic Complex north of the Boston Basin and a 491 Ma syenite associated with the Nahant Gabbro on Boston's north shore. The high temperature component thus appears to be a secondary magnetization in the 608 Ma granite.

The lower temperature component, characterized by WNW-ESE declinations and moderate inclinations, agrees with directions obtained from mafic and intermediate Mattapan units with a minimum age of 595 Ma in center of the Boston Basin. This sequence also shows both normal and reverse polarity suggestive of a primary Neoproterozoic magnetization. Similar directions are found in ca. 598 Ma and ca. 596 Ma siliceous Mattapan volcanic rocks located, respectively, north and south of the Basin. Paleopoles derived from this regionally recognizable direction resemble the published paleopole from ca. 606 Ma Harbour Main rhyolite in the Newfoundland Avalon Zone and suggest a moderate to low southerly paleolatitude during arc activity.