GRENVILLE GRANITOIDS IN THE SOUTHERN APPALACHIANS – FURTHER EVIDENCE OF THE “HOT” GRANITE HYPOTHESIS

Granitoids produced during the Grenville orogeny provide an excellent opportunity to test the notion of “hot” versus “cold” granitic magmas and the role of crustal recycling in their petrogenesis. Miller et al. (2003) clearly demonstrated the possibility of hot and cold granites based upon zircon saturation thermometry, where melts with minimal zircon inheritance provide zircon-saturation temperatures reflecting minimum melt temperatures and inheritance rich melts provide maximum saturation temperatures. Previous workers have shown a significant portion of crystalline rocks produced during various phases of the Grenville orogeny contain elevated Zr concentrations, sometimes approaching ten-times the upper crustal average. One aspect of this study was to determine if zircon inheritance was a significant contributor to the high whole-rock Zr concentrations. Six Grenville plutons were sampled from the French Broad massif in North Carolina. Their whole-rock Zr concentrations range from 140-575 ppm. New zircon U/Pb dates were determined for these samples and all but one show relatively few (< 12%) xenocryst populations. Corresponding zircon-saturation temperatures range from 720-865 °C. Additionally, Ti concentrations measured in zircon range from 2-35 ppm, with the majority having values higher than zircon from typical granitoids. Temperatures calculated from the high end of Ti contents reach values approaching 1000 °C, lending support to the conclusion of Miller et al. (2003) for the existence of high temperature granites.

Crustal recycling could produce magmas with elevated Zr and other high field strength element concentrations. Two possibilities are envisaged, either remarkably high rates of sediment delivery and resultant subduction or high-temperature, low water content melting of mid-level continental crust. Preliminary zircon ε_Hf(t) values for all analyzed samples range from -2.9 to +8.9; the majority of samples have epsilon values that are mildly negative. These results suggest most of the Grenvillian magmas were influenced by a significant degree of preexisting crustal material. The Hf data, high Ti-in-zircon values, and detailed petrographic study lead to the suggestion that these granitoids were produced in mid-crustal levels under unusually high temperature conditions.