THE MYTH OF PRE-NEOPROTEROZOIC PLATE TECTONICS

Popular interpretations of pre-Neoproterozoic tectonics and magmatism are based on obsolete speculations that (1), plate tectonics operated throughout Earth history; (2), plate settings and structures can be defined merely with ratios of ratios of a few trace elements; and (3), Earth accreted to full size before mantle and crust fractionation began, lower mantle is still “primitive”, and upper mantle has been progressively depleted by net removal of crust. (1) is disproved by lack of pre-Neoproterozoic analogues for any of the distinctive lithologic, associational, structural, and crustal products of Phanerozoic subduction and seafloor spreading. (2) is falsified by phase petrology, utter misfits, and more. (3) is invalidated by the upper-mantle sample, disproof of deep-mantle plumes, and more. A new paradigm is needed.

A contrary scenario accords with abundant multidisciplinary evidence that is widely overlooked. Thick mafic global protocrust separated from highly depleted mantle, mostly at low pressure, concurrently with hot, fast accretion. The protocrust contained most material multiply recycled subsequently into both continental and oceanic crust. Initially depleted upper mantle has been enriched from above by delaminated and, later, subducted crust in excess of extracted new crust. Partial melting of protocrust produced Archean TTG, recycling of which yielded more-potassic granites. Together, these dominate preserved Archean crust, which was kept too hot and mobile for “plates” by high radioactivity. Paleoproterozoic and Mesoproterozoic orogens developed mostly from ensialic basins, with mafic melts from protocrust that was either still above the mantle or had sunk into it, tonalitic melts from protocrust, and voluminous potassic granites mobilized from thick basin fills and older granitoids. No waterlaid materials are yet proved older than ~3.6 Ga (TTG was forming by 4.4), so there may have been an early superdense CO₂-H₂O greenhouse atmosphere, not a hydrosphere.

Following Neoproterozoic transition, modern-style plate tectonics began in the early Paleozoic. It was enabled by enrichment of upper mantle from the top, it is driven and organized by top-down cooling, and its circulation is limited to the upper mantle. There is yet no proof of pre-Neoproterozoic lithospheric oceans.