NEW DETRITAL ZIRCON DATA FROM WESTERN CONNECTICUT PROVIDE INSIGHT INTO TERRANE AFFINITIES AND MAP PATTERNS OF REGIONAL LITHOLOGIES

New LA-ICP-MS U-Pb detrital zircon (DZ) data from quartzose metasedimentary rocks in western Connecticut (CT) provide insight into sediment provenance and terrane affinities and helps to resolve a geologic mapping problem. DZ spectra for samples east of Cameron’s Line, mapped as Ordovician Ratlum Mtn. Schist on the CT state map, contain prominent 1.1-1.0 Ga peaks and fewer Mesoproterozoic grains from 1.5 to 1.1 Ga, consistent with a provenance from Laurentian sources without input from other terranes. Similar spectra from a nearby Rowe Schist sample agree with published data for Rowe in Massachusetts (MA) and Vermont (VT) and indicate that all these rocks have an exclusively Laurentian provenance. Published correlations of the Ratlum Mtn. Schist with peri-Gondwanan units such as Moretown Fm. in MA and VT need reconsideration.

Two additional samples previously mapped as Ratlum Mtn. Schist contain prominent 1.1-1.0 Ga DZ peaks but also 600-500 Ma peaks, indicating a mixed Laurentian/peri-Gondwanan provenance, and some 470-440 Ma DZ that suggest a source from Ordovician arcs. The mixed provenance indicates that these samples are neither Laurentian Ratlum Mtn. nor peri-Gondwanan Moretown. Their DZ data are similar to DZ spectra from nearby Early Devonian Straits Schist and published data from the correlative Goshen Fm. in MA. Significantly, one of these samples with mixed provenance is found near coarse-grained, lustrous, mica-quartz schist that is indistinguishable from nearby Straits Schist. Consequently, we now interpret these rocks as basal Straits Schist, and their reassignment to a Silurian-Devonian (S-D) belt resolves the problem of a gap in continuity of this belt of rocks on the state map.

South of and structurally below the proposed S-D belt, a DZ sample from schist mantling the Waterbury dome contains a strong 1.0 Ga Grenville peak and lesser peaks at ~550 Ma and in the 1.9-1.5 Ga range, but no grains younger than 550 Ma. These DZ again indicate a mixed Laurentian and peri-Gondwanan provenance and resemble spectra from rocks to the south mapped as Ordovician Collinsville Fm. or more distally mapped Cobble Mtn. Fm.