The Hadean is the most mysterious period because no rocks and
geologic bodies are preserved except for only the zircons in
Western Australia, Canada, China and Greenland [1]. But, it is the
most important period because the early evolution possibly clinched
the earth’s history. We try to find the earliest supracrustal rocks in
the world to investigate the Hadean tectonics. As far, the oldest
supracrustal rocks are found in Akilia association in West
Greenland, Nuvvuagittuq in Quebec, and Nain Complex in Labrador
[2,3].
We made geological survey in the Nain Complex, and
reinvestigated the occurrence of the supracrustal rocks and their
relationship with the ambient orthogneisses. Previous works focused
on distribution of the supracrustal belts within the orthogneisses
[e.g. 4], but the detailed field occurrence of the supracrustal rocks
within the belts is still ambiguous. Therefore, we focus on their
internal structures.
The supracrustal belts are repeatedly intruded by granitic
intrusions with some ages and their original structures are obscured,
but their lithostratigraphies are relatively well preserved in Nulliak,
Big and Shuldham islands and St Jones Harbor. The supracrustal
belts in Nulliak and Big islands comprise ultramafic rocks, mafic
rocks and mafic sediments intercalated with feldspathic sediments
and banded iron formations in ascending order. In the St Jones
Harbor, it is composed of ultramafic rocks, mafic rocks, banded iron
formation, and clastic sediments, intercalated with chert in the
middle and with bedded carbonate rocks in the upper part,
respectively, in ascending order. In the Shuldham Island, it consists
of ultramafic rocks, layered gabbro with precursors of plagioclase
and pyroxene accumulation layers, mafic rocks and terrigenous
sediments in ascending order. The lithostratigraphies are very
similar to oceanic plate stratigraphy. The fact that some supracrustal
belts are intruded by Uivak I orthogneisses and presence of >3.86
Ga zircons in the supracrustal rocks [e.g. 3] suggest that the
supracrustal belts have early Archean ages. In addition, despite of the
still ambiguous relationship between Nanok Gneiss and supracrustal
rocks, presence of Nanok Gneiss (3.85 to 3.91 Ga) in this area [5]
implies that the supracrustal belts date back to the earliest Archean.
[1] Froude et al. (1983) Nature 304, 616-618; Nelson et al.
(2000) EPSL 181, 89-102; Mojzsis & Harrison (2002) EPSL 202,
563-576; Iizuka et al. (2006) Geology 34, 245-248; Wang et al.
(2007) CSB 52, 3002-3010. [2] Bowring & Williams (1999) CMP
134, 3-16; Nutman et al. (1996) Precamb. Res. 78, 1-39; O'Neil et
al. (2008) Science 321, 1828-1831. [3] Schiøtte et al. (1989) Can
Jour Earth Sci. 26, 2636-2644. [4] Bridgwater et al. (1974) Geol
Surv Canada, Paper 75-1 Part A, 282-296. [5] Collerson (1983)
in Abstracts for Early Crustal Genesis Field Workshop, LPI,
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