Global Geodynamics and Hf isotope arrays: implications for supercontinental cycles and mantle convection patterns

Abstract:

Combined U-Pb-Hf isotope studies of zircons provide new insights into the evolution of large-scale, long-term orogenic systems, applicable to understanding supercontinental cycles throughout Earth history. Three types of Hf arrays exist on Phanerozoic Earth: (1) long term isotopic contraction to positive epilson Hf values, reflecting circum-Pacific type orogenesis; (2) fanning isotopic array associated with ongoing collisional orogenesis involving repeated Wilson-cycles; (3) crustal reworking arrays typical of backarc opening and closing during slow convergence of continents, such as Phanerozoic Europe.

Each Phanerozoic array has its Precambrian precursors, at least to 3.0 Ga. Global isotopic contraction similar to the Phanerozoic circum-Pacific system is also recorded between 2.0-1.6 Ga, and less obviously between 2.8-2.5 Ga, suggesting global scale symmetrical subduction systems dominated by accretionary orogenesis since the Late Archean. Fanning arrays between 2.2-1.8 Ga and 1.2-0.6 Ga reflect dominantly collisional processes associated with amalgamation of Nuna and Gondwana, whereas the Mesoproterozoic (1.6-1.2 Ga) produces a dominantly reworking array like that of Phanerozoic Europe.

Circum-Pacific subduction reflects hemispheric-scale, whole mantle convection and probably began approximately 2.8 Ga ago. Global scale accretionary orogens associated with this arrangement include the widespread Late Archean greenstone belts and the circum-Nuna Late Paleoproterozoic orogenic belts. The Mesoproterozoic era possibly represents progressive, slow collapse of a Panthalassic ocean surrounding Nuna, ultimately forming the Grenvillean orogenic system. This climactic event rearranged global mantle convection patterns, observed in the Neoproterozoic “Pan-African” Hf arrays, triggering a peculiar toroidal-type mantle convective cell that transitioned Rodinia to Gondwana.