The Isua supracrustal belt (ISB) rocks are dated at about 3.8 Ga and constitute
the oldest accretionary complex in the world. Petrochemical and geothermobarometric
studies of over 1,500 rock samples in ISB enabled us to estimate the extent
of regional metamorphism, petrotectonic environment and subduction- zone
geothermal gradient in the Archean. The following lines of evidence indicate
the first discovery of progressive, prograde metamorphism from greenschist
(Zone A) through Ab-Ep-amphibolite (Zone B) to amphibolite facies (Zones
C and D) in the northeast part of the Isua supracrustal belt: (1) systematic
change of mineral paragenesis in metabasites and metapelites; (2) progressive
change of composition of major metamorphic minerals, including plagioclase,
amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole
and garnet; and (4) absence of any vestige of high-grade metamorphism even
in the lowest metamorphic zone (Fig. 20). Geology and geochronological
constraints of ISB indicate that the regional metamorphism was related
to the subduction of Archean lithosphere. Metamorphic pressures and temperatures
of the metamorphism are estimated to be 5 to 7 kbar from Grt-Hbl-Pl-Qz
geobarometry and 380‹ to 550Ž from the Grt-Bt geothermometry in Zones B
to D (Fig. 21). These P-T estimates indicate that ISB was affected by progressive
metamorphism of an intermediate P/T ratio metamorphic facies series, and
that it records a much higher geothermal gradient of a subduction zone
in the Archean than is known from the Phanerozoic. The high geothermal
gradient may have resulted from the subduction of young lithosphere and
a high potential temperature of mantle.
The Archean high geothermal gradient led to melting of thick oceanic crust
in a thin oceanic plate, creating many huge granitic (tonalite, trondhjemite,
and granodiorite) batholiths. The slab melting changed the oceanic crust
(density=3.07) into a denser Grt-bearing residue (density=3.55), implying
that TTG melt extraction provided a potential driving force for Archean
plate tectonics. In addition to the preservation of the oldest accretionary
complex, this suggests that Precambrian-type plate tectonics, whose driving
force is slab-pull due to densification of the residue of oceanic crust
as a consequence of slab melting, was already operating in the Early Archean.
The transition from Precambrian-type to Phanerozoic-type plate tectonics
may be caused by thinning of oceanic crust and thickening of oceanic lithosphere
in the late Archean, due to decrease of mantle temperature (Fig. 21).
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