We welcome the opportunity to discuss further our ideas regarding translational tectonics along the Iapetan margin of Laurentia. We look forward to seeing published papers on the Alleghanian history of the Goochland terrane based on recent abstracts of our colleagues from William and Mary, but strongly caution against extrapolating results from strain studies of local outcrops to orogen-scale reconstructions without independent supporting data. They contest our restoration of the Goochland terrane ~600 km northeast to an area proximal to the Manhattan and Reading Prongs, with which it has clear petrologic affinities. This palinspastic location is ~250 km outboard from the Blue Ridge (Bartholomew and Tollo, 2004, Fig. 2) and is consistent with 40%–70% orogen-normal shortening (Bailey et al., 2004). Their discussion contends that “the Goochland terrane is likely to restore to a position outboard of southern New England (Bailey, 2004)” (~1200 km from the Blue Ridge), but Bailey (2004) actually states that such retrodeformation “places the Goochland terrane at least 500 km northeast of the Virginia Blue Ridge prior to the Alleghanian orogen.” Their repositioning of the Goochland terrane is relative to the fixed present-day location of the Blue Ridge but we recognize that the Iapetan Blue Ridge was located ~100 km southeast (Bartholomew and Lewis, 1992), thus accounting for part of our ~600 km separation between these terranes. The amount of transpression and/or west-directed shortening from the Manhattan Prong northward is unknown, hence the Goochland terrane may have been farther north or east along the Laurentian margin and thus separated by more than 600 km from the Blue Ridge. However, other types of constraining data are needed before a restored location proximal to New England becomes a viable option.
They contend that our “best evidence for this correlation seems to be the similarity in age and composition of A-type granitoids in the Goochland terrane to several felsite dikes in the Reading Prong…” [but that] “age similarities do not require that they be in close proximity at this time.” Tollo et al. (2004) and Cawood et al. (2001) show that Neoproterozoic A-type granitoids become progressively younger northward through the Appalachian orogen and that ca. 600 Ma extension-related granites were grouped geographically during a failed rifting event. Moreover, despite metamorphism, the Yonkers and Pound Ridge Gneisses (Manhattan Prong) are petrologically similar to A-type granitoids of comparable age within the Goochland terrane, suggesting derivation from similar sources and implying geographic proximity. Thus, we placed the Goochland terrane within the southernmost area of ca. 600 Ma rifting in proximity to rocks with which its A-type granitoids have close petrologic and age affinities.
Their questioning of our correlation of the Sabot Amphibolite (Goochland terrane) with Grenville-age amphibolites (Manhattan Prong) contends that “the absolute crystallization age of the protolith of the Sabot Amphibolite is unknown at present…[and] ...geochronologic evidence… for Devonian plutonism and metamorphism in the Maidens Gneiss (which is interlayered with amphibolite in many places) should raise serious doubts about a Mesoproterozoic age for the Sabot Amphibolite.” Amphibolite constitutes a significant proportion of Laurentian basement in the New Jersey Highlands–Manhattan Prong region (Volkert, 2004) but not in the Blue Ridge (Bartholomew and Lewis, 1992). The Sabot Amphibolite, which directly overlies the Grenville-age State Farm Gneiss and underlies the Maidens Gneiss, has not been demonstrated to be correlative with amphibolite interlayered within the Maidens Gneiss. Nor are protolith ages of either the Sabot Amphibolite or Maidens Gneiss known. These protoliths could be of Grenvillian age or of late Neoproterozoic age and, like the volcanics of eastern Madagascar, represent rift-related products. Alternatively, they could have formed during some other Devonian or older Paleozoic igneous event. In all three alternatives, the Sabot Amphibolite is not precluded from experiencing Devonian metamorphism.
Our palinspastic placement of the Goochland terrane proximal to the Manhattan-Reading Prong region was based on five criteria: (1) petrologic, isotopic, and geochronological correlation of the Montpelier and Roseland alkalic anorthosites and associated rocks of the Goochland terrane and the Blue Ridge coupled with a lack of correlation of late Neoproterozoic plutonism between these two terranes; (2) correlation of 630–600 Ma, rift-related, A-type plutonism in the Goochland terrane, Reading Prong, and the Iapetan margin in the northern Appalachians; (3) possible correlation of the Sabot Amphibolite with amphibolites of the Manhattan Prong which also experienced late Neoproterozoic A-type plutonism; (4) palinspastic location of the Blue Ridge along the Iapetan margin and interpretation that the Goochland terrane is a lateral extension of that Grenvillian belt; and (5) Alleghanian transpression of the Goochland terrane as well as its rifting and possible lateral translation during breakup of Rodinia. Our data constrain the original position of the Goochland terrane to the vicinity of New York. Much more data on Grenvillian and late Neoproterozoic rocks of the northern Appalachians need to be shown to correlate with the Goochland terrane before a proposed location significantly north of the Manhattan Prong would be convincing to us.