DETRITAL ZIRCON IN THE CLOUGH QUARTZITE CENTRAL CONNECTICUT:EFFECTS OF STRATIGRAPHIC POSITION AND GRAIN SIZE

Our study of detrital zircons as a function of grain size in 3 samples of Silurian Clough quartzite from the base (A), middle (B), and top (C) of the 35 m thick unit near Rockville, CT shows that a single sample of metasediment, and especially of only coarse grains can give erroneous results. We analyzed ~100 zircon grains in the 3 samples from using laser-ablation inductively coupled plasma mass spectrometry and a high-resolution ion microprobe. The quartzite non-conformably overlies amphibolite of the Middletown Formation. The base of the unit contains pebbles stretched to an aspect ratio of 20:1, and X–ray powder diffraction data show that the basal sample (A) contains ~5 wt.% each of paragonite (interpreted to have been deposited as plagioclase) and muscovite. Sample B contains 9% muscovite and 5 % microcline, and C contains 5% muscovite, all in addition to dominant quartz. We interpret this mineralogical trend from samples A to C to reflect deposition of the original sediment as a subarkose that evolved to slightly feldspathic quartz arenite.

Zircons from each of these three samples were divided into larger (>~125 µm) and smaller (<~75 µm) grain size fractions. The results of isotopic analysis show that the coarse-grained fractions are dominated by Mesoproterozoic (Mp) grains (~80%), but the ages of zircons in sample A peak at ~1300 Ma, whereas in B and C they peak at ~1050 Ma - more typical of “Grenvillian” provenance. The proportion of Neoproterozoic (Np) and younger grains drops from 14, 7 to 0% in samples A, B, and C, respectively. However, in the finer-grained fractions Mp grains exceed 50% of the total grains only in sample C; the proportion of Np grains falls from 30, 20 and 7 % of total, and Paleoproterozoic concentrations fall from 28, 16 to 11 % up section. Paleozoic grains are present only in samples B and C. We conclude that significant differences in age populations can exist among samples from different stratigraphic levels, and that at least in this quartzite, the smaller grains display a significantly wider range of ages than do the coarser grains. The absence of any Paleozoic grains in sample A further shows that stratigraphic position can also impact age distribution. The results suggest that caution is necessary in the interpretation of provenance from single samples especially if dominated by larger zircon grains.