MID-SILURIAN CAPE ANN PLUTON, NE MASSACHUSETTS: U-PB CA-TIMS GEOCHRONOLOGY AND LINGERING LITHOTECTONIC QUESTIONS

The alkalic character of the Cape Ann pluton has long been established geochemically, but the timing and setting for this intrusion into the SE New England sector of Avalonia en route to collision with North America remain unclear. Clarifying the age and duration of the magmatism is the first step in solving this lithotectonic problem. We previously reported a weighted mean ²⁰⁶Pb/²³⁸U date of ca. 426 Ma for Cape Ann Granite comprising most of the pluton. This date is significantly younger than Late Ordovician or Lower Silurian interpretations shown on existing geologic maps. Additional high-precision dates are now available from other members of the plutonic suite including Beverly Syenite (distinguished from Cape Ann Granite by its low quartz content), Squam Granite (really quartz diorite), and syenites from the zone bordering the pluton in Salem Neck, MA. The collective results record mid-Silurian magmatism over a 2-4 Ma interval.

As yet undated pyroclastic and flow-banded rhyolite at Marblehead, MA and islands to the north show “within-plate” trace element signatures and are included here as volcanic components of the mid-Silurian suite. Gravity and magnetic surveys further imply high-density rock beneath a sheet-like Cape Ann body. The mafic understory inferred to a depth of 5 km in the cross section accompanying the 1983 Bedrock Geologic Map of Massachusetts is most likely to be Salem Gabbro-Diorite (reported ²⁰⁷Pb/²⁰⁶Pb zircon date of 427 ± 1.5 Ma and less precise LA-ICPMS dates between 425-431 Ma). The presumably mantle-derived mafic magma has been invoked as the thermal impetus for crustal melting that produced felsic magma at shallower levels; intra-plate rifting and hot spot activity have both been suggested as driving mechanisms. The observed volume of mid-Silurian magma based on aerial exposure and estimated thicknesses, however, renders either scenario problematic. Alkalic plutonic rocks with estimated volume of ~160,000 km³ are reported in the Norway’s Permian Oslo Graben, dwarfing ~ 325 km³ of felsic Cape Ann intrusives. And even the relatively small McDermott caldera in the early end of the Yellowstone hotspot trace erupted ~ 1700 km³ of peralkaline ash-flow tuff. Erosion has certainly removed some of the elusive Cape Ann structure, but along-strike tectonic dismemberment might also be involved.