DIVERSITY IN EOARCHEAN METAMORPHISMS FROM ULTRA-HIGH-PRESSURE TO LOW-PRESSURE (<250 TO >1000°C/GPA) EXPLAINED BY PLATE MOVEMENTS IN DEEP TIME

Eoarchean (>3600 Ma) low T/P metamorphism in Greenland’s Itsaq Gneiss Complex (IGC) is reflected by: (i) ~550°C ≥2.6 GPa conditions (≤250°C/GPa) demonstrated by an olivine + antigorite + titano-chondrodite / titano-clinohumite relict assemblage within mantle slivers showing geochemical and crystallographic features of a suprasubduction environment. These slivers were exhumed into the crust by 3712 Ma. (ii) Rare vestiges of 3658 Ma high-pressure (garnet + clinopyroxene) granulite. (iii) Barrovian-style kyanite + staurolite assemblages in metapelites. Cryptic evidence of Eoarchean low T/P on a large scale is given by the tonalites forming >80% of the IGC, that formed by partial melting of eclogitized mafic rocks. On the other hand, high T/P (≥1000°C/GPa) metamorphism is shown by 3669 Ma crustal melts equilibrated with orthopyroxene that formed coeval to the youngest juvenile tonalitic crust in the complex (latter derived by anatexis under low T/P conditions), and a 3670-3570 Ma history of deep crust migmatisation under low pressure, garnet-free conditions. Structural geology of the IGC indicates its low T/P regimes coincide with crustal imbrication of arc-like tholeiites, boninites, andesites, felsic-intermediate volcano-sedimentary rocks and chemical sedimentary rocks, whereas post-3660 Ma high T/P metamorphism was marked by late-orogenic extension/exhumation and deep crustal flow with granitic partial melting and mafic underplating. Thus the diversity of Earth’s earliest-recorded geodynamic settings more resembles ones in observed modern geodynamics, than the lithological and structural relationships expected from theoretical non-uniformitarian scenarios like drip tectonics in a stagnant lid regime. The recognition in the IGC of an ultra-high-pressure ≤250°C/GPa metamorphic regime at >3700 Ma, in a lithological association resembling that of juvenile convergent plate boundaries, indicates a form of plate tectonics operating by the early Archean.