P-T-D Evolution of a Transpressional Orogen: a Structural and Petrological Traverse through the Nagssugtoqidian Orogen, Western Greenland

KRIEGSMAN, LEO M.1, NICHOLS, G.T.2, VAN GOOL, J.3, MARKER, M.3

1Dept. of Applied Geology, University of New South Wales, Sydney NSW 2052, Australia

2School of Earth Sciences, Macquarie University, New South Wales 2109, Australia

3Danish Lithosphere Centre, Oster Voldgade 10, 1350 Copenhagen K, Denmark

The Nagssugtoqidian Orogen in West Greenland is a ~1.9 Ma old, E-W trending orgenic belt separating two Archean blocks. While the Nagssugtoqidian Orogen has been regarded as ensialic in the past, recent recognition of Palaeoproterozoic, juvenile intrusive rocks suggests major addition of new material to the crust, which is more typical for subduction-collision orogens. A traverse across the central part of this orogen was carried out in 1994 under the aegis of the Danish Lithosphere Centre.

The southern part of the traverse is dominated by granitic gneisses with abundant mafic dykes, bound to the N by a major structural contact. Folded and metamorphosed, coarse-grained mafic dykes are restricted to the footwall of this contact and are correlated with the early Proterozoic Kangâmiut dykes cross-cutting the southern Archean craton. A km-scale tectonic lens in the footwall is comprised of ~50% mafic material and contains one lens of sapphirine-spinel-bearing amphibolite. The stretching lineation is subhorizontal and parallel to the ENE-WSW long axis of both the amphibolite lens and the large-scale tectonic lens. Isoclinal, intrafolial folds occur locally and, together with the presence of boudins of garnet-rich layers, testify to high strains.

The tectonostratigraphy N of the structural contact shows interleaved partially melted metapelites and orthogneisses. This sequence is repeated several times and is interpreted as a thrust stack with a southward displacement. Evidence for strike-slip movements along the same contact indicates a transpressional setting. Foliation dips in this part of the traverse vary from ~NNW/30 in the S through NNW/70-80 in steep belts to subvertical in the N. Stretching lineations are particularly well-developed in steep belts and have an average plunge of ~30o towards WSW. Folds are mainly preserved outside the steep belts and have subhorizontal fold axes. Asymmetric fabrics in the steep belts generally indicate sinistral strike-slip in horizontal sections. The combination of sinistral strike-slip, WSW-plunging stretching lineations and upright folds is also suggestive of a transpressional tectonic setting.

The northernmost part of the traverse shows Archaean orthogneisses interleaved with a complex of Proterozoic supracrustal and intrusive rocks, which are in tectonic contact. Ultramafic lenses in the Proterozoic complex, containing olivine and Cr-spinel, may represent peridotites and suggest the existence of a major tectonic boundary.

P-T estimates on the early assemblage garnet + clinopyroxene + quartz + plagioclase + orthopyroxene (inclusions in quartz) in metamorphosed Kangâmiut dykes from the southern part of the traverse indicate pressures of 13-14 kbar at temperatures of ~700-800 oC. Garnet breakdown to orthopyroxene + plagioclase occurred at ~7-9 kbar and ~650-700 oC, which is similar to P-T estimates from metapelites N of the thrust contact.

These results suggest that the Proterozoic Kangâmiut dykes in this part of the orogen may have witnessed a high-P metamorphic event which does not seem to have been recorded in nearby metapelites and subsequently equilibrated at intermediate pressures. Strong, near-isothermal decompression recorded in these Kangâmiut dykes may be interpreted in terms of extensional collapse of the orogen after the high-P stage. It is uncertain at present at which level the extensional strain was accommodated, but possible candidates include the structural boundary described above and the high-strain zone comprising the large-scale tectonic lens and adjacent gneisses. We propose that ENE-WSW stretching in this part of the orogen may be simultaneous with thrusting and transpression in the rest of the Nagssugtoqidian Orogen.

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