ORIGIN AND PROVENANCE SIGNATURE OF MESOPROTEROZOIC A-TYPE GRANITES IN LAURENTIA BASED ON O-ISOTOPE COMPOSITIONS

The ca. 1.4 Ga A-type granites are a distinctive element of Laurentia that provide valuable paleogeographic controls in supercontinent reconstructions due to their unique identity as a source of sedimentary detritus. These granites are important in the chemical evolution of Laurentian continental crust; they have alkaline to peralkaline compositions, are mostly metaluminous, show variable f(O₂), and are variably involved in Mazatzal deformation. With few exceptions, they intrude only Proterozoic crust. This distinctive belt of rocks is truncated along the Neoproterozoic rift margin at the western edge of the Mojave Province and appears to continue into East Antarctica as a critical piercing point. Their geographic distribution, unique age, and distinctive compositions means that zircons from these granites are an important isotopic and provenance tracer. Zircon Hf-isotope compositions indicate that the Laurentian granites have both juvenile and evolved isotopic characters derived from the melting of heterogeneous 2.0-1.6 Ga lower crust. Importantly, the granite compositions vary by the Proterozoic host terrains in which they were emplaced, reflecting discrete sources and crustal interactions. New zircon O-isotope data from ~30 samples of Laurentian granites ranging from 1.34 to 1.48 Ga were obtained to further constrain the crust-mantle contributions and establish a tracer reference. Zircons from individual samples have distinctive δ¹⁸O compositions ranging from about 4.5‰ to 8‰, and most samples have crustal O-isotope compositions ≥6‰. There is a general secular trend from an early mantle-like group (4.5-5.5‰) at 1.48-1.46 Ga to higher values of 6-8‰ between 1.44-1.42 Ga and then to somewhat lower values of 6-7‰ between 1.40-1.36 Ga. Ages and O-isotope compositions correspond generally to the host basement provinces, with those in the Mojave and southern Yavapai provinces showing the strongest crustal signatures. O-Hf relations reflect mostly juvenile crustal melt compositions (initial εHf > 0), in which Yavapai granites have radiogenic Hf but crustal-like O, whereas the Mojave has lower O values but more evolved Hf. There is a gross similarity to Antarctic glacial erratics, confirming earlier correlations, but the latter are more mantle-like in both O and Hf isotopic characteristics.