Origin of Mesoproterozoic A-type granites in Laurentia: Hf isotope evidence

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 A-type or "anorogenic" granites of ∼1.4 Ga age was emplaced within composite, heterogeneous Proterozoic crust. Zircons are an ideal mineral to constrain the granite petrogenetic history because they are repositories of both age (U-Pb geochronology) and tracer (Lu-Hf isotopic) information. We measured the Hf isotope composition of zircons from 31 previously dated A-type granites intruding Proterozoic basement provinces from the southwest U.S. to the upper mid-continent. Isotopic compositions for all granites are broadly similar, with average ¹⁷⁶Hf/¹⁷⁷Hf(i) ratios of 0.281871-0.282153. Averages for granites within different crustal provinces yield present-day ε_Hf values between -31.9 and -21.9. Initial ε_Hf values discriminate the granites by age of the 2.0-1.6 Ga crust which they intrude, but are independent of intrusion age, as follows (basement formation ages in parentheses): southern Granite-Rhyolite (1.5-1.3 Ga), +7.0 ± 0.9; central Yavapai (1.8-1.7 Ga), +5.4 ± 0.9; western Yavapai (1.8-1.7 Ga), +3.3 ± 1.1; Granite-Rhyolite (1.5-1.3 Ga), +1.4 ± 0.6; Mojave (1.8-1.7 Ga), +0.2 ± 0.8; and Penokean (1.9-1.8 Ga), -0.1 ± n/d. The narrow ranges of Hf isotopic signatures within these regional groupings of granites reflect the age and isotopic composition of the basement provinces they intrude. Granites in the southern Granite-Rhyolite and central Yavapai provinces have the highest initial ε_Hf, reflecting their more juvenile sources, whereas Mojave and Penokean granites show contributions from more evolved crustal sources. Simple calculations indicate that all the granites represent dominantly crustal melts; although a mantle contribution cannot be ruled out, if present it must be minor. The Hf isotope compositions of the 1.4 Ga granites appear controlled predominantly by widespread 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 signature of the zircons will prove useful as a provenance tracer in detrital zircon suites from Neoproterozoic and younger siliciclastic deposits worldwide.