MAGMATISM DURING GRENVILLE (1.14-1.05 GA) OROGENESIS: DID SOME OF EARTH'S HOTTEST GRANITOIDS FORM BY MELTING OF CHEMICALLY PRIMED CRUST? EVIDENCE FROM TI CONTENTS AND HF ISOTOPES IN ZIRCON
Crustal reworking may produce magmas with elevated Zr content. Previous Nd isotopic studies of central and southern Appalachian Grenville rocks suggest that there was variable amounts of crustal remelting. To determine the relative role of crust and mantle components involved in the petrogenesis of Grenville granitoids, hafnium (Hf) isotopes in zircon were measured. Calculated εHf(t) values for all analyzed samples range from -6.6 to +13.0, with the majority of samples having epsilon values that are mildly positive. These results suggest most of the granitoids were derived by variable mixing of juvenile material with preexisting crustal material. Either significant entrainment of country rock polluted mantle-derived melts while rising through the crust or delamination of lithospheric mantle and subsequent anatexis of the lower crust can explain the Hf isotope composition, high Ti-in-zircon temperatures, and elevated Zr content. The latter interpretation is similar to tectonic models invoked for the formation of the Anorthosite-Mangerite-Charnockite-Granite suite (AMCG) farther north in the Adirondacks.