A GONDWANAN AFFINITY FOR THE GNEISS DOME BELT, SOUTHWESTERN NEW ENGLAND APPALACHIANS

Geochronologic and geochemical data from the Hartland Belt (HB) and the Gneiss Dome Belt (GDB) of western Connecticut indicate that these high-grade rocks from southwestern New England are a collage of tectono-stratigraphic units formed in continental margin, backarc, and arc settings. The terrane affinities of key units remain problematic. In Caradoc to Llandovery plutonic rocks that intrude the HB, discordant zircons and Pb isotopic data from some of the pre-Silurian ones show them to have Grenvillian inheritance (Sevigny & Hanson, 1993, 1995). The timing of final assembly and accretion of the HB to Laurentia was early Silurian based on concordant TIMS U-Pb xenotime ages of 435±3 and 438±2 Ma from blastomylonitic granite (Sevigny & Hanson, 1995) that forms part of the ductile shear zone of Cameron’s Line.

The whole-rock geochemistry of another set (n=16) of metabasites from the pre-Llanvirn Collinsville Formation (CF) of the GDB is consistent with the hypothesis that they formed in a backarc basin (Chocyk-Jaminski and Dietsch, 2002). The majority of metabasites in this set can be classified as basaltic andesite with subalkaline signatures. They have a range of Ti/V ratios with 10 samples having values between 20 and 50 characteristic of ocean floor basalt. The chemistry of previously analyzed arc-like CF metabasites and their association with evolved meta-tonalites suggest that this backarc had an ensialic character. Chocyk-Jaminski (1998) and later Dietsch (2002) proposed that it was built on a rifted fragment of Laurentian crust.

The idea that rocks of the GDB have, in fact, affinity to the Gondwanan side of Iapetus is supported by re-determined electron microprobe U-Pb monazite ages from migmatitic paragneiss from the Waterbury dome (WD). Rounded, xenoblastic monazites yield consistent sets of preliminary mean ages between 565 and 568 Ma [for example, 566 and a [(std error x 2) of 4] Ma, n=9]. In WD migmatitic schist, a monazite inclusion in garnet has yielded 437 [12] Ma, n=7, consistent with a concordant TIMS U-Pb monazite age of 432±2 Ma from the same rock, and indicating the older ages can be interpreted as relic detrital grains. In addition, conventional ages of abraded zircon fractions from WD bt-ms-grt granitic gneiss whose REE pattern shows it to be derived from shale, yield ²⁰⁷Pb/²⁰⁶Pb ages no older than 618 Ma.