HAFNIUM ISOTOPIC EVIDENCE FOR PALEOPROTEROZOIC CRUST IN THE MARS HILL TERRANE AND COMPARISON WITH OTHER TERRANES

The Mars Hill terrane has been identified as a suspect terrane (e.g., Bartholomew and Lewis, 1988; Raymond, 1987; Brewer and Woodward, 1988) and contains Paleoproterozoic material based on Nd whole-rock and U-Pb zircon data (e.g. Carrigan et al., 2003; Ownby et al., 2004).

New hafnium isotope data on rocks from the southern Mars Hill terrane confirm the antiquity of Mars Hill crust with respect to known Blue Ridge units, and offer a basis of comparison with Hf data from other terranes. 

Zircon grains were extracted from three samples. These were analyzed for U-Pb and Lu-Hf systematics via LA-MC-ICP-MS.  Two samples, a granitic gneiss and an amphibolite,  yielded Grenvillian U-Pb ages (1.0 to 1.2 Ga).  Zircons from a pegmatitic granite yielded  Neoproterozoic ages consistent with the age of the Bakersville dikes.  Although the zircon data do not strongly indicate pre-Grenville-aged sources, depleted mantle Hf model ages are Meso-to Paleoproterozoic. T_DM-Hf values are 1.47 to 1.72 Ga for granitic gneiss zircons and 1.48 to 1.57 Ga for amphibolite zircons.  T_DM-Hf values for the Neoproterozoic granite are younger: 1.14 to 1.34 Ga.

The T_DM-Hf values for the Grenville-aged Mars Hill rocks are older than those from the western Blue Ridge Corbin Gneiss (T_DM-Hf = 1.17 to 1.30 Ga) and Grenville-aged gneisses from the Pine Mountain terrane (T_DM-Hf =  0.94 Ga to 1.43 Ga), and are consistent with involvement of older sources in the petrogenesis of the protoliths of the Mars Hill rocks. However, xenoliths within a Grenville-aged gneiss from the Pine Mountain terrane also bear an older Hf signature (T_DM-Hf = 1.40 Ga to 1.87 Ga), suggesting that Mars Hill-like sources may be present within the Pine Mountain terrane, although they were not melted in sufficient quantities to contribute significantly to the Grenville-aged magmatism.  Hf values from the Grenville-aged Mars Hill units are comparable to those reported from the Granite-Rhyolite provinces (Goodge and Vervoort, 2006).