GEOLOGY OF THE TALLY POND GROUP: A MIDDLE CAMBRIAN ISLAND-ARC SEQUENCE IN THE EXPLOITS SUBZONE, NEWFOUNDLAND APPALACHIANS

The Cambro-Ordovician Victoria Lake Supergroup is located within the Exploits Subzone of the Newfoundland Appalachians and is a composite and structurally complex collection of volcanic, volcaniclastic, and epiclastic rocks of varying ages, geochemical groupings and tectonic environments. It consists of mafic and felsic volcanic rocks, chert, greywacke and shale, which formed in a variety of island-arc, rifted arc, back-arc and mature-arc settings.

The proposed Tally Pond Group is a sequence of volcanic, volcaniclastic and sedimentary rocks which crop out as a 60 km long northeast trending linear belt in central Newfoundland. The group contains roughly equal proportions of island-arc felsic pyroclastic rocks and intercalated mafic volcanic rocks that are interbedded with epiclastic volcanic and sedimentary turbidites.

The mafic rocks within the Tally Pond Group comprise a sequence of tholeiitic, mainly pillowed, commonly vesicular and amygdaloiodal mafic lavas with lesser amounts of pillow breccia and mafic agglomerate. The felsic rocks consist of an assemblage of ash flow tuffs, commonly quartz and/or feldspar porphyritic, massive and flow-banded rhyolite and rhyolite breccia, and felsic quartz and feldspar porphyritic intrusions. The mafic and felsic rocks are intruded by fine- to medium-grained dominantly gabbro intrusions with lesser amounts of diorite. The intrusions are weakly altered and undeformed.

Clastic sedimentary rocks of the Tally Pond Group include thin to massive bedded siltstone, medium-grained greywacke, pebble conglomerate and chert. Many of these rocks are rich in volcanic quartz, feldspar, and quartz porphyry clasts, indicating that the clastic sedimentary rocks were derived from the adjacent volcanic units. Numerous beds of fine-grained, thinly bedded graphitic black shale are present and are highly strained and in fault contact with both the felsic and mafic volcanic rocks.

The Tally Pond Group is the oldest well-dated Iapetan island-arc sequence in the Appalachian Orogen, as zircons derived from a quartz porphyritic rhyolite intrusion in the area northeast of Tally Pond have yielded a U/Pb age of 513 ±2 Ma. This age is a minimum age and current geochronological studies on volcanic flows and ash tuff beds will help constrain the timing of arc volcanism in the Tally Pond Group.