DELAMINATION OF HADEAN MAFIC PROTOCRUST, THE KEY TO ARCHEAN MAGMATISM AND TECTONICS

Archean rock assemblages are utterly unlike modern plate-interaction complexes and lack evidence for seafloor spreading and subduction, which abounds in Phanerozoic orogens. Popular plate-tectonic rationales incorporate ignorance of modern systems, dismissal of most information, and blind faith in uniformitarianism, trace-element numerology, and 1950s-60s mantle-crust conjectures.

Cosmologic, Archean, and modern-Earth multidisciplinary data fit an early Earth strikingly unlike popular models. Heat of accretion, including the giant impactor, and core separation repeatedly melted the growing Earth, and it fractionated before 4.45 Ga into severely depleted mantle, with uncrossable 650-km discontinuity, and thick global mafic and ultramafic protocrust that contained most of the material that has ever since been part of continental or oceanic crust. The protocrust, with chondritic Sm/Nd, lost its low-melting components mostly as ”juvenile” TTG, starting by 4.40 Ga and continuing, in increments varying in space and time, until after 2.5 Ga. Depleted protocrust incrementally delaminated and sank into the lighter harzburgitic upper mantle, allowing rise of mantle material, much hotter than now, and further partial melting. By 3.6 Ga, possibly 3.8, the global(?) TTG crust was covered by seawater and cool enough for surface accumulation of mafic and ultramafic volcanic rocks, from protocrust and mantle, that produced density inversions. Early volcanics sank and mixed into the TTG, whereas later ones retained coherence and sank slowly as synforms while “juvenile” and secondary granites rose as domes. Simultaneously, the composite upper crust, floating on slowly flowing TTG lower crust, variably deformed laterally. Neither oceanic nor rigid-continental plates existed in Archean time. Greenstones are ensialic sheets. Fertile material in modern upper mantle represents progressive downward enrichment by delamination and, later, subduction.