Paper Number: 1205
Metamorphic processes in the Eoarchaean: evidence from the 3.7-3.8 Ga Isua Greenstone Belt
Rollinson, H.R.1
1Geoscience, University of Derby, DE22 1GB, UK; email: h.rollinson@derby.ac.uk
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The rocks of the Isua Greenstone belt in west Greenland represent the world’s largest and best preserved suite of Eoarchaean (3.7-3.8 Ga) meta-volcanic and meta-sedimentary rocks. As such they represent an important region in which to obtain evidence of geodynamic processes in the early Earth. This region however has also become a ‘battleground’ of competing ideas, not least the tectonic setting in which these rocks might have formed. For example Furnes et al. [1] have argued that, in part, this suite represents a former ophiolite, whereas others have argued from the presence of boninitic lavas for an arc affinity for these metavolcanic rocks [2].
Unravelling the deformation and metamorphic history of this area is not trivial. Geological mapping and detailed U-Pb zircon chronology shows that the rocks of the Isua Greenstone belt comprise two structural packages of different ages. A 3.8 Ga suite of metavolcanic and metasedimentary rocks intruded by 3.8 Ga tonalitic gneisses making up the outer, southerly arc of the greenstone belt was juxtaposed against a younger, northerly 3.7 Ga suite associated with 3.7 Ga tonalitic gneisses. The two packages were assembled at about 3.66 Ga [3] and both are metamorphosed to amphibolite facies. Later 3.65-3.6 Ga deformation of the younger suite has given rise to a number of separate, shear-zone bounded units in the younger, northern terrane [3].
Detailed mapping of garnet textures and compositions in pelitic lithologies reveals a complex sequence of garnet growth chronologies with up to three episodes of garnet growth and in detail variations between the different domains of the greenstone belt [4,5]. The older, southern terrane is least well characterised with respect to metamorphic grade. There are three episodes of garnet growth and garnet-biotite temperatures are up to 590oC. Metamorphic pressures are not well constrained. In the northern terrain three episodes of garnet growth can be recognised and peak garnet-biotite temperatures are up to 610oC. However in the northwestern subdomain only two phases of garnet growth are recorded and peak garnet biotite temperatures are up to 650oC. Kyanite but not sillimanite is recorded in this terrane suggesting a minimum pressure of ca 7 kb. In the northeastern subdomain only a single phase of garnet growth is recorded and calculated P-T conditions are 470oC, 4 kb [6] although these rocks are part of a prograde sequence with temperatures rising to 550 oC. A further constraint on metamorphic conditions is the absence of partial melting in felsic lithologies. Garnet geochronology indicates that garnet growth was at 3.74-3.7 Ga [4] although it is possible that the latest garnet growth episode was at 3.63 Ga [7] synchronous with titanite in the tonalitic gneisses.
The Isua metasediments record thermal gradients from ca. 3.7 Ga which plot in the ‘higher T/P gradient than normal’ category of Brown [8], indicative either of higher mantle heat flow in the early Archaean, or of their structural location between two magmatic/tonalitic massifs. There is no evidence for high pressure metamorphism analogous to that found associated with modern subduction zones.
References:
[1] Furnes H et al. (2007) Science 315: 1704-1707
[2] Friend CRL and Nutman AP (2010) Amer J Sci 310: 826-861
[3] Nutman AP and Friend CRL (2009) Precamb Res 172: 189-211
[4] Rollinson HR (2002) In: Geol Soc London Sp Publ 199: 329-350
[5] Rollinson HR (2003) Precamb Res 126: 181-196
[6] Appel PWU et al (2001) Precamb Res 112: 27-49
[7] Crowley JL (2003) Precamb Res 126: 235-257
[8] Brown M (2014) Geoscience Frontiers 5: 553-569