EARTH'S FIRST TWO BILLION YEARS---THE ERA OF INTERNALLY MOBILE CRUST

Magmatic and tectonic processes in the hot, young Earth differed profoundly from those now operating. Ancient rocks differ in individual and collective composition, occurrence, association, and structure from modern ones. Preserved Archean crust records extreme internal mobility, not rigid-plate behavior. Despite widespread forcing of inappropriate plate models on Archean geology, no geologic indicators of subduction (ophiolite, melange, high-P/low-T rocks), arc magmatism, continental sundering and convergence, and ensimatic oceans have been documented. (Speculations from trace-element ratios are not documentation.) The only known basement to mafic and ultramafic lavas and other supracrustals is tonalite-trondhjemite-granodiorite (TTG) gneiss, zircons recycled from which go back to 4.4 Ga. Archean mantle is extremely depleted (Mg ol>>opx), and komatiite shows it was still ~300 degrees C hotter than modern asthenosphere ca. 3.5 Ga. A thick global melabasaltic protocrust likely formed by 4.45 Ga, and from it TTG suites were extracted by partial melting over the next 2 b.y. Incremental delamination of restitic protocrust enabled rise of hot, light mantle and more melting.

Earth may have had a dense greenhouse atmosphere, not a hydrosphere, before 3.6 Ga, the maximum proved age of supracrustal rocks, older felsic crust having been too hot to permit rise of dense melts. Mafic lavas erupted atop felsic crust after 3.6 Ga produced a density inversion that was partly righted by sinking of supracrustals and rising of subjacent TTG. Where felsic mobility was greatest, dismembered early dense rocks sank deep into it; but where least, they sank as synclinal keels between diapiric batholiths that often rose slowly, with variable partial melting and with additions of new TTG from mafic protocrust, for hundreds of millions of years. Variably severe prolonged lateral deformation (with little change in area) simultaneously affected the felsic crust. Hot lower crust flowed and mixed pervasively, and was partly coupled to quasi-floating upper crust.

Paleoproterozoic orogens may have been derived mostly from ensialic basins, not plate tectonics. Early-depleted upper mantle was progressively re-enriched over geologic time by delaminated and subducted crustal materials, opposite to conventional assumptions.