PROTEROZOIC ACCRETIONARY OROGENS OF SOUTHERN LAURENTIA REVISITED (Invited Presentation)

The core of Laurentia formed 1.85-1.78 G by assembly of Archean cratons within Nuna. Southward addition of Proterozoic accreted terranes took place along a long-lived convergent margin from 1.78-1.0 Ga culminating in formation of Rodinia. Earthscope images show weak correlation of mantle velocity structure to crustal provinces suggesting that the cratonic keel, with Archean and Proterozoic elements, was cratonized in the Proterozoic. Now better resolved by Hf isotope studies: the Mojave- Yavapai province boundary is a midcrustal imbricate suture overlapped by 1.75 turbidites; the 1.78-1.70 Yavapai province is dominantly juvenile (1.78 in WY; 1.75 in AZ) but has portions (in CO and AZ) underlain by older 1.84 Ga crust. 1.65-1.60 Ga Mazatzal province is locally underlain by Yavapai crust and is interpreted as a continental magmatic arc, not a colliding terrane. 1.46-1.35 Ga A-type magmatism (Granite Rhyolite provinces) perforated Yavapai/Mazatzal crust as well as accreted juvenile 1.5 Ga crust south of the Missouri line. Most Grenville crust reworks 1.5 Ga crust, and some is juvenile. Deformation was semi-continuous along the long-lived margin and inboard terranes were reactivated by distant plate convergence. The southward-progressing locus of deformation reflects docking events: 1.75-1.78 Ga along the Cheyenne Belt (Medicine Bow orogeny), 1.75-1.70 Ga suturing of Mojave with Yavapai (Yavapai orogeny), 1.68-1.60 Ga continental arc deformation (Mazatzal orogeny), 1.45-1.35 Ga docking of 1.5 Ga crust and its inboard response (Picuris orogeny), 1.3 Ga continental margin arc, 1.2-1.0 continent-continent collision (Grenville orogeny). The Laurentian 1.6-1.5 Ga tectonic and magmatic gap is interpreted as a time of erosion of Yavapai-Mazatzal mountains after tectonism shifted southwards. 1.5 Ga basins received outboard (1.6-1.5 Ga) detritus, probably from Australia. 1.45-1.35 Ga intraplate granites formed by basaltic underplating (not subduction) during formation of an orogenic plateau. Laurentian connections to Australia/ Antarctica in both Nuna and Rodinia include birth of both cratons within Nuna, a long-lived Cordilleran-style contractional “southern” margin facing an open ocean, and jostling along Laurentia’s “western” margin with Australia/Antarctica until Rodinia breakup.