RIS ID
101768
Abstract
A synthesis of the geological record of Earth's ten remaining oldest surviving gneiss complexes, each containing >3.6 Ga rocks, reveals a common history. We propose that the simplest scenario compatible with all observations is that of formation of an ancient continental mass, here named Itsaqia, by 3.66 Ga from amalgamation of earlier quartzofeldspathic crust, followed by initiation of continental break-up at 3.53 Ga by rifting. Evidence for this is reconstructed from the remaining oldest rock record (only ca. 10,000 km2 globally). Dominating the surviving fragments of the proposed Itsaqia continent are 3.9 to 3.66 Ga tonalites that represent juvenile crustal additions with whole-rock initial εNd >+1 and zircon initial εHf ≈ 0. Their trace element chemistry shows that they were derived by ca. 30 percent partial melting of garnetiferous, mostly eclogitized basic rocks, leaving behind a subcrustal garnet-rich restite. The tonalites contain inclusions of mafic rocks with chemical signatures diagnostic of mantle wedge fluxing, such as enrichment in the light rare earths and depletion of Nb and Ti. We interpret that this juvenile crust formed repeatedly in arc-like constructs at convergent plate boundaries. The Acasta Gneiss of Canada is the only undisputed surviving rock record of the proposed Itsaqia continent where crust formation extends back to the Hadean. Before ca. 3.66 Ga, individual gneiss complexes show distinct chronologies of crust formation, yet despite their present-day isolation, they underwent identical 3.66 to 3.6 Ga high temperature orogenic events (Isukasian orogeny) - which we contend indicates that from 3.66 Ga these complexes had amalgamated into a single continental mass. Rare surviving 3.66 Ga high-pressure granulite rocks that underwent rapid decompression indicate tectonic crustal thickening then collapse during amalgamation. This was followed by almost 50 million years of high heat flow and lower pressure metamorphism, most probably in an extensional setting. Starting from ca. 3.53 Ga, we propose that komatiite and basalt eruption and dike emplacement marked the start of Itsaqia's dismemberment by rifting. We further speculate that the deep mantle upwelling responsible for this plume-related magmatism was triggered by either the cascade of pre-3.66 Ga sub-Itsaqia high density garnet-rich restitic subduction graveyards into the lower mantle or the thermal insulation effect of Itsaqia. This resembles the mechanisms of supercontinent breakup throughout Earth's history. Hence we propose that Wilson Cycles of continent amalgamation and breakup were already initiated by the Eoarchean, near the start of the rock record. Australia's East Pilbara region was over the top of the plume, where the thermal impact destroyed Itsaqia by melting to give rise to felsic igneous rocks coeval with komatiites. Greenland's Itsaq Gneiss Complex was peripheral to the plume, and hence was heavily diked at ca. 3.5 Ga, but was not melted.
Grant Number
ARC/DP120100273
Publication Details
Nutman, A. P., Bennett, V. C. & Friend, C. R. L. (2015). Proposal for a continent 'Itsaqia' amalgamated at 3.66 Ga and rifted apart from 3.53 Ga: initiation of a Wilson Cycle near the start of the rock record. American Journal of Science: an international earth science journal, 315 (6), 509-536.