IMPLICATIONS OF C. 550 MA CRYSTALLIZATION OF THE SABOT AMPHIBOLITE PROTOLITH IN THE GOOCHLAND TERRANE OF THE CENTRAL VIRGINIA PIEDMONT

The Goochland terrane is an enigmatic crustal block in the Piedmont of central Virginia. For decades nearly all rocks in the Goochland terrane were inferred to have Mesoproterozoic initial crystallization or depositional ages, but recent dating indicates that the protoliths for at least part of the most widely-exposed rock unit (Maidens gneiss) crystallized in the Devonian. Exposures of ~1023-1046 Ma rocks are confined to several antiformal domes on the eastern side of the terrane, and the Sabot amphibolite partially encircles two of these domes.

There are multiple fine-grained felsic layers at a variety of scales within the Sabot amphibolite at its type locality. These rocks appear to be derived from a felsic igneous protolith, and a whole-rock analysis of one sample shows it to be rhyolitic with A-type characteristics. The layers are not obviously cross-cutting, and our preferred interpretation is that they represent felsic volcanic rocks coeval with basaltic rocks that now make up the Sabot amphibolite.

Zircons in one felsic rock have inclusion-rich cores and inclusion-poor rims, and cores and rims in most grains show distinct cathodoluminescence zoning. The (U-Th)/Pb systematics from individual grains match this textural zoning. LA-ICPMS spot analyses of most cores yielded low U/Th ratios and the weighted mean age of twelve concordant core analyses was 552 +/- 11 Ma (2-sigma). In contrast, rim analyses yielded much higher U/Th ratios and a range of ages between 450 and 410 Ma. We interpret the older age from the cores to indicate the time of initial crystallization of the protolith and the younger ages to reflect metamorphism.

If our interpretations are correct, the Sabot amphibolite thus represents basaltic magmatism in the latest Neoproterozoic. Further, the bimodal nature of rocks at the type locality, coupled with the A-type characteristics of the felsic rock, suggest magmatism in a rift setting. The timing is consistent with Iapetan rifting. Combined, the new data support the following interpretations: (1) Late Mesoproterozoic rocks are exposed only in the cores of several domes on the east side of the Goochland terrane; and (2) Compared to the Blue Ridge, there is a later record of rifting associated with opening of the Iapetus ocean in the Goochland terrane, perhaps indicating an outboard position for this terrane.