Paper No. 18-12
Presentation Time: 11:40 AM
THE LATEST NEOPROTEROZOIC HAFNIUM ISOTOPE EXCURSION: SNOWBALL EROSION OR PAN-AFRICAN OROGENS?
The late Neoproterozoic trend towards isotopically enriched (i.e., evolved) Lu-Hf in zircon (εHft) has been interpreted as global phenomena related to putative outsized erosion during Cryogenian Snowball Earth glaciations or to secular changes in plate tectonics, rather than spatially biased sampling of regional orogenesis. In particular, resampling methods have attempted to augment gaps of low data density which in turn risks obscuring tectonic signals and artificially weighting outliers. We test hypotheses calling on global phenomena to explain the late Neoproterozoic isotopic excursion by evaluating density patterns of 155,061 igneous and detrital εHft values using interpretations of paleogeography. With the new compilation we demonstrate that the εHft nadir in values occurs at 550 Ma, 170-85 Myrs after the Snowball Earth glaciations, whereas the Cryognian is characterized by relatively positive, juvenile values. We tie the recent εHft database to a published continuous full-plate model spanning the past one billion years with GPlates software. Restoring the GPlates model to the latest Neoproterozoic (550 Ma) shows that the εHft excursion is restricted to terranes of modern Africa, Antarctica, Australia, and South America, all continents involved in the assembly of the Gondwana supercontinent. In contrast, εHfT of terranes not involved in the Gondwana supercontinent (Baltica, North America, various fragments of modern central Asia) do not display a negative εHft isotopic excursion. We propose that this negative excursion is the product of the Tonian to Cryogenian rifting of Rodinia, which formed new passive margins on old radiogenic crust, followed by the Ediacaran subduction of these margins and crustal thickening in the Pan-African and Transantarctic orogens. These continental collisions involved old crust produced zircon in melts with highly evolved εHft that were subsequently exhumed and distributed throughout Gondwana.