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

Abstract

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.