HF ISOTOPIC COMPOSITIONS OF ZIRCONS FROM MESOPROTEROZOIC LAURENTIAN TERRANES

Studies of paired Lu-Hf and U-Pb analyses indicate distinct Hf isotopic signatures may exist within zircons of similar U-Pb ages, indicating that Lu-Hf data may be used to identify distinctions among terranes with similar U-Pb ages.

In this study, single zircon grains from several Grenvillian (sensu lato; 900 to 1250 Ma) orogens of North America were analyzed via LA-ICP-MS for U-Pb and Lu-Hf compositions to determine whether differences in Hf isotopic compositions exist among the units. Units examined included Grenville-aged intrusive rocks of the southeastern Appalachian Blue Ridge, Mars Hill and Pine Mountain terranes, and plutonic rocks from the Adirondacks and West Texas.

The most juvenile Hf signatures among zircons with Grenvillian U-Pb ages were from West Texas (εHf init. = +4 to +8, depleted mantle Hf model ages (T_DMHf) =1.17 to 1.25 Ga). Slightly less depleted Hf signatures were obtained from zircons from granitic gneisses of the Pine Mountain terrane (εHf initial = +2 to +8, T_DMHf = 1.27 to 1.43 Ga) and the Corbin Gneiss of the Blue Ridge in Georgia (εHf init. +3 to +7, T_DMHf = 1.26 to 1.38 Ga). The Laurel Creek pluton (North Carolina Blue Ridge) had slightly lower initial εHf values (-2 to +3 ) and slightly older T_DMHf (1.31 to 1.50 Ga); a nearby granitic dike (U-Pb age = 1040 +- 19 Ma) had initial εHf = -9 to 0, and T_DMHf =1.46 to 1.77 Ga. Grenville-aged zircons from a slightly older (1165 +- 18 Ma) dike displayed wider ranges of initial εHf (-17 to +8) and T_DMHf (1.27 to 1.97 Ga), reflecting incorporation of pre-Grenvillian material into the Grenville-aged zircons. This dike also contained some Paleoproterozoic zircons. More enriched Hf signatures were also observed in zircons from amphibolite and granitic gneiss of the Mars Hill terrane (εHf init. = -9 to -1; T_DMHf = 1.46 to 1.72 Ga). Hf systematics of zircons from charnockites of the Adirondacks were similar to those from Blue Ridge zircons, with εHf init. = -3 to +9 and T_DMHf =1.28 to 1.45 Ga.

These Lu-Hf data show similarities between the sources of Grenvillian intrusions in the Blue Ridge, Pine Mountain terrane, and Adirondacks. The granite in West Texas, however, appears to have been derived from somewhat more juvenile materials, while Mars Hill rocks and the xenocryst-bearing Blue Ridge dike demonstrate that older crust contributed to Grenvillian orogenesis in portions of the southern Appalachians.