The Isua supracrustal belt lies approximately 150 km northeast of Nuuk,
Greenland (Fig. 3). It forms arcuate tract (Fig. 5). The Isua belt is,
at present, exposed as a fault-bounded tectonic slice, and is in tectonic
contact with the tonalitic Amitsoq orthogneiss on both sides (Fig.12) (McGregor,
1973; Bridgwater et al., 1974), but was originally formed as an enclave
in the Amitsoq gneiss. The Isua belt comprises five Early Archean major
lithologic units: (1) a mafic-felsic turbidite sequence, including minor
interbedded conglomerate (Fig. 6), (2) chert (Fig. 7a) and banded iron-formation
(Fig. 7b), (3) pillow lava (Fig. 8), massive lava, pillow breccia, and
hyaloclastite, (4) dolerite-gabbro intrusives related to (3), and (5) ultramafic
rocks (Fig. 9), plus mid-Archean and early Proterozoic intrusions (Fig.
10). The northeastern part of the Isua supracrustal belt is subdivided
into three units, Northern, Middle, and Southern, by structural breaks
defined by low-angle thrusts. The lower half of the Northern Unit is a
highly disrupted melange (Type II, Cowan, 1985), with blocks of greenstone
and chert. More than 37 angular to subrounded blocks of greenstone are
randomly embedded in a highly sheared muddy matrix, together with several
blocks of chert with or without BIF. The upper half is composed of more
than four subunits bounded by layer-parallel thrusts. Each subunit is composed
of pillow lava or sheet flow greenstone at the bottom, overlain by layered
chert/BIF. BIF layers occur at the bottom of the sedimentary sequence,
whereas white chert becomes more abundant through the transition of white/blue
chert in the upper part. The Middle Unit is mainly composed of dominant
NS-trending chert layers, and contains mafic turbidite sequence associated
with felsic sandstone, mudstone, calcareous sandy shale (Fig. 11), and
minor conglomerate near the southern margin.
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