HAVE CONODONTS SURVIVED KYANITE-GRADE METAMORPHISM IN THE MARBLES OF TRUMBULL, CONNECTICUT?

No.

The age of deposition of a stratigraphic assemblage of discontinuous quartzite, marble, and amphibolite overlain by the pelitic Straits schist is critical to unraveling the stratigraphic record of west-central Connecticut because it could lie between Ordovician and older Laurentian shelf/slope sediments and overlying accreted Acadian forearc sediments. In an attempt to establish the age of deposition we have processed three samples (base, middle, top) of marble from the Old Mine Park, Long Hill, Trumbull, Connecticut in an attempt to recover conodonts. Finding them was already a long shot, because conodont yields in the Silurian are often low, and conodonts have not been reported from kyanite-grade rocks. Calcareous metasediments here are locally intercalated among layers of amphibolite. The upper-most calcareous sediment is overlain by a 30 cm thick quartzite followed by ~2 m calcareous meta-arkose (plagioclase >>K-feldspar), and this by an amphibolite. It is overlain by the gray-weathering muscovite > biotite Straits schist. We tentatively correlate this unit with the Russell Mountain Fm. of Hatch et al. (1970, USGS Bull 1324B) because the lithologic progression is similar. However, its Silurian age is only inferred by long-range stratigraphic correlations.

We sampled the only ~6 m thick cleaner marble sandwiched between two layers of amphibolite. The concentrations of minerals (wt. %) in the marble determined by X–ray powder diffraction analysis varies with stratigraphic position: calcite climbs from 50 to >90 %, quartz is present (40%) in only the lower sample, plagioclase falls from 5.5 to ~2%, amphibole is present (2-5 wt. %) and the biotite and chlorite content is constant at ~1% wt. all from base to the top of the section. Twenty kilograms of each of the three samples were processed for conodonts using the double-buffered formic acid technique of Jeppsson and Anehus (1995; J Paleontology, 69(4):790-794). Residues then underwent cryogenic density separation following the method of Morrow and Webster (1989), using the heavy liquid lithium metatungstate (LMT). No conodonts were recovered from the three samples, as of yet. However, it is possible that highly metamorphosed conodonts are hiding amongst the high concentration of insoluble silicates with a similar density to biogenic apatite.