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

The Grenville Orogeny produced a large volume of granitoid rocks containing elevated zirconium (Zr) concentrations as high as ten times the upper crustal average. Several potential mechanisms have been proposed for high-Zr granitoid magmatism: (1) zircon inheritance from the melt source; (2) enrichment of the lithospheric mantle prior to high-Zr granitoid melt production; (3) increased delivery of sediment to Grenville subduction zones; or (4) prior melting of mid to lower crust (Samson et al., 2018). Mechanism 4 (“crustal reworking”) could produce a residue enriched in refractory zircon. Subsequent high temperature melting of this residue during the Grenville could produce melts with elevated Zr content. To constrain crystallization temperatures, titanium (Ti) contents were measured in zircon from seven plutons in the Hudson Highlands (NJ), Shenandoah massif (VA), and French Broad massif (NC). Ti content for all analyzed samples range from 2-63 ppm (n = 260; mean = 16 ppm); calculated temperatures range from 650-1005 °C. Granitoids from VA and NC have some of the highest Ti-in-zircon temperatures yet recorded in felsic plutonic rocks (900-1005 °C).

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 ε_Hf(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.