CRUSTAL ORIGIN OF ~1.4 GA LAURENTIAN A-TYPE GRANITES FROM HF ISOTOPE COMPOSITIONS

Granitic rocks are commonly used as a means to study chemical evolution of continental crust. In particular, their isotopic compositions reflect the relative contributions of mantle and crustal sources in their genesis. In Laurentia, a distinctive belt of Mesoproterozoic “anorogenic” A-type granites of ~1.4 Ga age was emplaced within composite, heterogeneous Proterozoic crust. Zircons are an ideal mineral to constrain granite petrogenesis because they contain both age (U-Pb geochronology) and tracer (Lu-Hf isotopic) information. We measured the Hf-isotope composition of zircons from 31 dated A-type granites intruding Proterozoic basement from the southwest U.S. to the upper mid-continent. Hf isotopic measurements were done by MC-ICPMS both on solutions prepared by separation chemistry and, for comparison, on individual zircons using laser ablation. Group averages from the different provinces yield present-day epsilon Hf values between -33.9 and -23.9. Calculated initial espilon Hf values discriminate the granites by age of the crust which they intrude, as follows (with basement formation ages in parentheses): Mojave (2.3-2.0 Ga), -1.0 ± 0.8; Penokean (1.9-1.8 Ga), -1.2 ± n/d; western Yavapai (1.8-1.7 Ga), +2.1 ± 1.1; central Yavapai (1.8-1.7 Ga), +4.2 ± 0.9; Granite-Rhyolite (1.5-1.3 Ga), +0.2 ± 0.7; and southern Granite-Rhyolite (1.5-1.3 Ga), +5.8 ± 1.0. The uncertainty reported here represents the 2-sigma standard deviation of Hf isotopic compositions within each province's population. Thus, isotopic compositions for all 1.4 Ga granites are broadly similar, yet within individual crustal provinces they have distinct, low-variance Hf-isotope values independent of intrusion age. Granites with high initial espilon Hf values in the southern Granite-Rhyolite and central Yavapai provinces reflect more juvenile sources, whereas Mojave and Penokean granites show contributions from more evolved crustal sources. The Hf-isotope compositions of the 1.4 Ga granites therefore appear controlled predominantly by melting of heterogeneous 2.0-1.6 Ga lower crust, consistent with other geochemical indicators. In addition to constraining granite petrogenesis, the distinct age and Hf isotope composition of these zircons can be used to constrain crustal sources for detrital zircon suites.