Publication Date:
2009-06-26
Description:
Eoarchaean crust in West Greenland (the Itsaq Gneiss Complex, 3870-3600 Ma) is 〉80% by volume orthogneisses derived from plutonic tonalite-trondhjemite-granodiorite (TTG) suites, 〈10% amphibolites derived from basalts and gabbros, 〈10% crustally derived granite, 〈1% metasedimentary rocks and 〈〈1% tectonic slices of upper mantle peridotite. Amphibolites at 〉3850, c. 3810 and c. 3710 Ma have some compositional similarities to modern island arc basalts (IAB), suggesting their origin by hydrous fluxing of a suprasubduction-zone upper mantle wedge. Most of the Eoarchaean tonalites match in composition high-silica, low-magnesian adakites, whose petrogenesis is dominated by partial melting of garnetiferous mafic rocks at high pressure. However, associated with the tonalites are volumetrically minor more magnesian quartz diorites, whose genesis probably involved melting of depleted mantle to which some slab-derived component had been added. This assemblage is evocative of suites of magmas produced at Phanerozoic convergent plate boundaries in the case where subducted crust is young and hot. Thus, Eoarchaean subduction' first gave rise to short-lived episodes of mantle wedge melting by hydrous fluxing, yielding IAB-like basalts{+/-}boninites. In the hotter Eoarchaean Earth, flux-dominated destructive plate boundary magma generation quickly switched to slab melting of ( subducted') oceanic crust. This latter process produced the voluminous tonalites that were intruded into the slightly older sequences consisting of tectonically imbricated assemblages of IAB-like pillow lavas+sedimentary rocks, gabbros and upper mantle peridotite slivers. Zircon dating shows that Eoarchaean TTG production in the Itsaq Gneiss Complex was episodic (3870, 3850-3840, 3820-3810, 3795, 3760-3740, 3710-3695 and 3660 Ma). In each case, emplacement of small volumes of magma was probably followed by 10-40 Ma quiescence, which allowed the associated thermal pulse to dissipate. This explains why Greenland Eoarchaean crustal growth did not have granulite-facies metamorphism directly associated with it. Instead, 3660-3600 Ma granulite-facies metamorphism(s) in the Itsaq Gneiss Complex were consequential to collisional orogeny and underplating, upon termination of crustal growth. Similar Eoarchaean crustal history is recorded in the Anshan area of China, where a few well-preserved rocks as old as 3800 Ma have been found including high-MgO quartz diorites. For 3800 Ma rocks, this is a rare, if not unique, situation outside of the Itsaq Gneiss Complex. The presence of volumetrically minor 3800 Ma mantle-derived high-MgO quartz diorites in both the Itsaq Gneiss Complex and the Anshan area indicates either that Eoarchaean subduction' zones were overlain by a narrow mantle wedge or that the shallow subduction trapped slivers of upper mantle between the conserved and consumed plates.
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