PYROCLASTIC ERUPTIONS OF THE NORTHFIELD INTRUSIVE COMPLEX DURING FILLING OF THE CONNECTICUT VALLEY GASPÉ TROUGH

A major structural and stratigraphic boundary divides central Vermont approximately in half along a roughly north-south line parallel to the primary fabric of the Appalachians. East of the boundary is the Connecticut Valley Gaspé Trough (CVGT), filled with marine sediments primarily of Devonian age. This study focuses on the westernmost margin of the CVGT, now marked in central Vermont by the Dog River Fault Zone. Trapped within this fault zone are slivers of the Shaw Mountain Fm., dominated by quartz pebble metaconglomerate with associated crinoid- and conodont-bearing white “marble” derived from calcareous arenite. This suite of rocks represents nearshore Silurian deposits most likely deposited near the mouths of rivers entering the newly developing CVGT.

Where Shaw Mountain rocks are absent, or immediately east of them where present, the Northfield Fm. can be traced continuously from Massachusetts to the Quebec border. This unit is dominated in central Vermont by dark gray slaty phyllites that exhibit graded bedding with a varying carbonate component in the coarser silty portions. These rocks were most likely deposited in relatively deep, quiet water at considerable distance from shore. Bedding is now near vertical and isoclinal folding repeats the section on the scale of tens of meters.

Six horizons of olistostromal conglomerate mapped near the western portion of the Northfield Fm. contain abundant clasts of volcanoclastic rock as well as rip up clasts black mud. The volcanic clasts are felsic with >70wt% SiO₂, commonly with laminated fabrics that retain soft-sediment deformation features. They vary from rounded and highly elongated to fragmented with angular terminations. Two of the largest clasts contain euhedral 1 mm quartz and albite crystals along with fiamme. Their mineralogy and texture of the crystals and matrix are strikingly similar to that of the peraluminous trondhjemitic dike swarm of the Northfield Intrusive Complex (NIC) located 3 km to the west in the Moretown Fm. ISCPMS-LA ages for those dikes and an associated small plug are 412.7 ± 4 Ma and 409.7 ± 8 Ma, respectively. It is theorized that the NIC was explosively eruptive late in the filling of the CVGT and that unstable ash deposits were caught up in submarine sliding to form olistostromes of the Northfield Fm.