GROWTH OF CONTINENTAL LITHOSPHERE BY ACCRETION OF ARC TERRANES ALONG THE LAURENTIAN MARGIN THROUGHOUT THE PROTEROZOIC: EVIDENCE FROM HF ISOTOPES

Proterozoic orogenic belts of southern Laurentia have long been interpreted in terms of the accretion of juvenile arc terranes along a long-lived convergent margin. Recently, models of Proterozoic lithospheric assembly have become more complex with the recognition that older material is also incorporated, Proterozoic domains can be underlain by reworked Archean crust and mantle, and discrete boundaries may be less common than mid-crustal imbricate suture zones and transitional isotopic boundaries. Similar characteristics are also found in modern arc settings. U-Pb-Hf isotopic analysis of zircon provides new insight for understanding lithospheric provinces. We present data from >1,500 zircon separated from metasedimentary rocks and the plutonic bodies that intrude them across southwestern Laurentia, and compare them to modern arcs and Phanerozoic orogens. Refinements to long-established lithospheric provinces are as follows: the 1.84-1.71 Ga Mojave Province may be underlain by Archean crust and mantle but the Archean signal comes dominantly from the ca. 1.75 Ga “Vishnu” basin that spans the Mojave and probably was derived from the Gawler craton. The Mojave-Yavapai boundary is an imbricated mid-crustal suture centered on the Crystal shear zone in Grand Canyon. The 1.75-1.71 Ga Yavapai Province is an accreted juvenile arc terrane with associated basins. The Yavapai-Mazatzal province boundary is a transitional and isotopically mixed zone with >1.8 Ga contributions to post-1.7 Ga rocks. The 1.7-1.6 Ga Mazatzal Province is underlain by juvenile arc rocks and sedimentary successions record an increasing older Laurentian component up-section suggesting accretion by 1.6 Ga. The 1.5-1.47 Ga Yankee Joe Formation overlaps older crust and is not derived from Laurentia, but probably from Australia. The 1.46-1.35 Ga A-type plutons are juvenile to the south, and inherit the isotopic character of older provinces they intrude north of the Missouri Line. This compiled Hf isotopic record significantly refines our understanding of Proterozoic lithospheric provinces. When viewed in the context of supercontinent assembly, Hf data show a similar trend to the Phanerozoic circum-Pacific orogenic system, and provide additional support for arc accretion along a long-lived convergent margin during the Proterozoic.