EVIDENCE FROM U-PB SHRIMP AGES OF DETRITAL ZIRCONS FOR EVOLVING PROVENANCES FOR PERI-GONDWANAN FOREARC METASEDIMENTS, WESTERN CONNECTICUT

Isotopic analysis of detrital zircons tests the age and provenance of metasediments mapped within and below the Silurian-Devonian Straits Schist (DSt) in the Bristol dome (BD) and greater Southbury (SB) areas Conn. Rodgers (1985) mapped the units as DSt, and Early to Middle Ordovician Cobble Mountain Fm. (CM), Taine Mountain Fm. (TM), Rowe schist (RS) and Collinsville Fm (CV). We analyzed 22, and 50 grains of DSt from 2 samples and about 40 grains in 5 samples of the Ordovician units using the SHRIMP facility of the KBSI, Ochang, Korea. We do not claim that we found all minor populations with this sample size, but the populations we found are significant. The DSt contains grains spanning all of the Ordovician, with the youngest grains ~450 Ma confirming a Late Ordovician or younger deposition age. Other age populations include 600-650 Ma and 1.0-1.6 Ga. The western Collinsville and Rowe fms. contain only Grenville grains from 900-1600 Ma. However, the eastern Cm zircons also contain grains from 570-650 Ma. The TM suite is unique with most grains ~550 Ma and a few at ~2.0 Ga; Grenville grains are almost absent. Significantly, the three Bristol dome samples (DSt, Cm, and TM) contain grains with oscillatory zoned (magmatic) rims with Th/U > 0.1 and ages between 515 and 570 Ma on xenocrystic cores of ~610 Ma and 1.0 and 2.0 Ga. This is evidence that an Ediacaran magmatic arc developed upon and intruded a complex of earlier Proterozoic magmatic arcs. We conclude that the TM sediments were deposited on marginal peri-Gondwanan crust, but by Middle Ordovician CM times, peri-Laurentian sediments were mixing with these east of the BD. However, exclusively Grenvillian grains in SB suggest a sedimentary divide approximately at the Waterbury dome location. Paleozoic grains were not transported to SB until DSt times. An oblique accretion of peri-Gondwanan rocks during the Ordovician might explain this age distribution, with the TM rocks probably deposited only on peri-Gondwanan crust. However, not until DSt times is there evidence of the arrival of a Paleozoic volcanic arc in this detrital zircon record.