U-PB ZIRCON AGES FROM THE INNER PIEDMONT TERRANE: IMPLICATIONS FOR A LAURENTIAN ORIGIN

Zircons from lithologic units within the Inner Piedmont terrane of Georgia (IP) were analyzed via ion microprobe. Samples examined included the Elberton Granite, Stone Mountain Granite, Palmetto Granite, and Athens Gneiss. The granites contained zircons with complex U-Pb systematics, yielding a variety of magmatic and pre-magmatic U-Pb dates reflecting both their time of emplacement (Alleghanian) and the ages of the sources from which the magmas were derived or interacted. Pre-magmatic Precambrian zircons from the granites had concordant or near-concordant 207Pb/206Pb ages that ranged from 1039 ± 50 to 1600 ± 25 Ma (1-sigma). The Athens Gneiss also contained abundant Precambrian zircons, with ages spanning a similar range. A Gaussian probability distribution for 207Pb/206Pb ages from < 10% discordant Athens Gneiss zircons yields maxima at 1070, 1141, 1235, 1435, and 1494 Ma. An additional zircon, excluded from the probability plot because of a discordance of 11%, had a 207Pb/206Pb age of 1552 ± 24 Ma (1-sigma).

All of the Precambrian ages derived from zircons within these rocks correspond to Laurentian tectonomagmatic events, such as the Grenville orogenic event (1.0-1.2 Ga), the mid-continent granite/rhyolite event (1.3-1.5 Ga), and the Central Plains, Yavapai, and Mazatzal orogenies (1.6-1.8 Ga). Carrigan et al. (2003) reported similar age ranges for zircons from basement units in the eastern and western Blue Ridge. The Forbush gneiss, a possible basement unit along the border of the northeastern IP, was reported by Carrigan et al. (2003) to contain Grenville-age zircons only. The question of whether the IP is exotic with respect to Laurentia, or whether it is a rifted, reattached fragment has been a matter of debate. The variety of pre-Grenvillian ages found in the Athens Gneiss, Stone Mountain Granite, and Palmetto Granite, and their similarity to those reported from the Blue Ridge basement strengthens the proposed connection between these tectonic elements, suggesting that the IP is not exotic with respect to the Blue Ridge province. Furthermore, the absence of zircons with characteristic Gondwanan ages (e.g., 500-600 Ma, 2.2 Ga, or 3.0 Ga) suggests the IP was not proximal to any Gondwanan or peri-Gondwanan terranes prior to its accretion to North America.