AVALON OFF NORTH AFRICA AT 595 MA AND OTHER IMPLICATIONS OF U-PB GEOCHRONOLOGY AND PALEOMAGNETISM IN NEOPROTEROZOIC ROCKS AROUND BOSTON, MA

The paleogeographic origin of Avalon terranes is central to tectonic understanding of terminal Neoproterozoic time, especially in relation to global "snowball" glaciations hypothesized for that interval. Integrated geochronologic and paleomagnetic study of arc-related magmatic rocks and overlying glaciomarine deposits in the Boston portion of the Southeastern New England Avalon Zone was undertaken to address these questions.

ID-TIMS U/Pb zircon dates show that members of the Lynn-Mattapan volcanic complex with ages clustered around 595 Ma rest regionally on ca. 610 Ma Dedham Granite, defining a major unconformity and implying significant pre-volcanic topographic relief. The mean direction calculated from the A component of magnetization at eight Lynn-Mattapan sites is D= 322.5°, I=59.6° (a95=5.6°), and the paleopole based on this mean direction is 209°E, 62°N. These results pass both reversal and fold tests and are regionally consistent among lithologies including basalt, andesite, rhyodacite and rhyolite representing both volcanic and subvolcanic phases. The calculated paleolatitude of approximately 40° for Southeastern New England at around 595 Ma is similar to those reported from Avalonian terranes in maritime Canada. The most likely peri-Gondwanan position for West Avalonia consistent with these paleomagnetic data lies off the northern African margin oriented to lie in the southern hemisphere to achieve reasonable drift velocities after 595 Ma. Using poles from the East Sahara craton (Egypt) and the Rio de la Plata craton (Brazil) has permitted reconstruction of West Gondwana without recourse to Laurentian proxies.

A broadly comparable A direction with D=111.6°, I=-51.9° (a95=8.8°, n=9 samples) has been obtained from one site in the Squantum Member of the Roxbury Conglomerate (maximum age 595 Ma) overlying the volcanic sequence. Previously published U/Pb dates for igneous clasts and detrital zircons in the Squantum beds match ages of plutonic and volcanic components of the Avalonian basement around Boston. The localized detrital provenance, the rugged topography inferred at the time of volcanism and the paleomagnetically determined mid-latitude location of these deposits seem more consistent with Neoproterozoic alpine glaciation than the extremes of Snowball Earth.