TRACE ELEMENT EVIDENCE FOR LATE ORDOVICIAN TO EARLY SILURIAN CRUSTAL THICKENING, SOUTHWEST CONNECTICUT

Geochemical affinities of two orthogneiss in southwestern Connecticut suggest a transition from volcanic arc granitic (VAG) to a within plate granitic (WPG) setting from late Ordovician to early Silurian (Ludlow). The Beardsley orthogneiss is a foliated, medium-grained, hornblende-biotite-plagioclase-quartz-titanite orthogneiss. It falls in the calc-alkalic field of Frost et at. (2001) and is metaluminous, and probably I-type. Concordant U-Pb ages of zircon yield 446 ± 2 Ma (Sevigny and Hanson, 1993). In contrast, the Pumpkin Ground (PG) orthogneiss is a variably foliated, med-grained, plagioclase-biotite-K-feldspar-quartz orthogneiss characterized by very large (2-7cm) plagioclase phenocrysts. It lies in the calc-alkalic field of Frost et al. (2001) and is peraluminous, and S-type. Concordant U-Pb ages of zircon yield 428 ± 2 Ma (Sevigny and Hanson, 1993). The PG orthogneisses intrudes the Beardsley orthogneiss in Fairfield and New Haven Counties in southwestern Connecticut. The tectonic affinities of these two gneisses, only 20 Ma different in age, is determined by their trace element concentrations. Trace element data from 5 Beardsley orthogneiss samples plot in the VAG field of the Rb vs. (Y + Nb) discrimination diagram of Pearce et al. (1984), while 14 samples of the PG orthogneiss plot in transition from VAG to WPG. If subsequent high grade metamorphism of these rocks occurred in an open system it is possible that Rb was gained or lost during metasomatism. To test for this possibility, analyses were also plotted in a discrimination diagram employing only the refractory elements Y and Nb (Pearce et al., 1984). The analyses plot in the same fields, confirming the volcanic arc granite and transitional crust interpretations of the Rb vs. (Y + Nb) diagram, and suggesting that the metamorphism occurred in a system moderately closed to Rb. These results suggest that the Beardsley orthogneiss developed in a Late Ordovician volcanic arc setting, which from plate tectonic reconstructions was likely on the eastern margin of Laurentia. About 20 m.y. later, the S-type PG orthogniess developed over a transitional crust from VAG to WPG. This is consistent with merging of the Gander and Laurentian lower crusts, and may reflect tectonic activity during the Salinic orogeny.