Eoarchean contrasting ultra-high-pressure to low-pressure metamorphisms (< 250 to > 1000 degrees C/GPa) explained by tectonic plate convergence in deep time

Abstract

Greenland’s Itsaq Gneiss Complex (IGC) shows Eoarchean (> 3600 Ma) 250–400 °C/GPa (low T/P – high pressure) and ≥1000 °C/GPa (high T/P) metamorphic regimes, demonstrating a similarity of contrasting metamorphic T/P regimes from the Phanerozoic back to the start of Earth’s rock record. Low T/P metamorphism produced: (i) Deep crustal eclogitised mafic rocks which upon partial melting formed the tonalites dominating the IGC; (ii) ~550 °C ≥ 2.6 GPa conditions (≤250 °C/GPa) demonstrated by an olivine + antigorite + titanochondrodite/titano-clinohumite relict assemblage within mantle slivers showing geochemical and crystallographic features of a suprasubduction environment, that were exhumed into the crust by 3712 Ma; (iii) rare vestiges of 3658 Ma high-pressure (garnet + clinopyroxene) granulite; and (iv) Barrovian-style kyanite + staurolite assemblages. High T/P metamorphism is shown by 3669 Ma crustal melts equilibrated with orthopyroxene. This was 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, garnetfree conditions. Structural geology of the IGC indicates its low T/P regimes coincide with crustal imbrication by compression of arc-like tholeiites, boninite-like lavas, 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 mafic underplating and partial melting generating granites. Thus the diversity of Earth’s earliest-recorded geodynamic settings resembles more those of modern geodynamics, than the lithological and structural relationships expected from theoretical non-uniformitarian scenarios like drip tectonics in a stagnant lid regime. The recognition of an ultra-high-pressure ≤250 °C/GPa metamorphic regime at > 3700 Ma in the IGC removes the last argument against a form of plate tectonics operating throughout the Archean. Hence since the start of the rock record, a mobile lid plate tectonic regime contributed to interior heat loss, facilitating chemical communication and feedbacks between Earth’s surface and its deep interior.