A structural and metamorphic transect across the West Greenland Nagssugtoqidian orogen, near the inland ice

Leo Kriegsman1, Jeroen van Gool2, Geoff Nichols3, Mogens Marker2

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

2Danish Lithosphere Centre, Øster Voldgade 10, 1350 Copenhagen K, Denmark

3GEMOC, School of Earth Sciences, Macquarie University, New South Wales 2109, Australia

Introduction

The Nagssugtoqidian Orogen in West Greenland is a Paleoproterozoic (~2.1-1.7 Ma), E-W trending orgenic belt separating two Archaean blocks. While the Nagssugtoqidian Orogen has been regarded as ensialic in the past, the recognition of Palaeoproterozoic, juvenile intrusive rocks (Kalsbeek et al., 1987; Kalsbeek & Nutman, 1996) suggests significant addition of new material to the crust, which is more typical for subduction-collision orogens. A traverse across the central part of this orogen, near the inland ice, was carried out in 1994 under the aegis of the Danish Lithosphere Centre and extended in 1995 by JvG and MM. This contribution describes lithologies and structures along the eastern transect and attempts to correlate them with coastal data. It also summarizes preliminary P-T data and their implications.

Field Observations

SNO (Fig. 1.1)

The southern part of the traverse, located within the SNO between Søndre Strømfjord and the inland extension of the Ikertôq "thrust zone" (Grocott, 1978) is dominated by granitic to dioritic gneisses of Archean age (Kalsbeek & Nutman, 1996). Intrusive relationships show that several generations of gneisses are present, of which the oldest, more dioritic gneisses have a well-defined gneissic/compositional layering, while the younger (still Archean) phases are more granitic and more homogeneous in composition. Supracrustal rocks are not common in this part of the orogen. These gneisses are intruded by abundant mafic dykes, bound to the N by a moderately N-dipping lithological (SNO-CNO) boundary interpreted as the eastern continuation of the Ikertôq "thrust zone". Folded and metamorphosed, coarse-grained mafic dykes are restricted to the south (footwall) of the unexposed contact and are correlated with the early Proterozoic (~2050 Ma: Kalsbeek & Nutman, 1996) Kangâmiut dykes which intrude the southern Archean craton. Most rocks in the SNO equilibrated in the amphibolite facies, but some of the older gneisses contain relict granulite facies assemblages (Grt-Opx-Plag).

The gneissic layering in the SNO dips consistently to the NNW at a dip angle varying from ~40-80°, most commonly 50-60°. The dip angle is 45-50° at the SNO-CNO boundary. Mineral and stretching lineations plunge to the NW at an angle of 10-30°. Folds on the scale of hundreds of metres to kilometers have been recognized mainly in the southern part of the SNO, where they are open, upright, with shallowly to moderately west-plunging fold axes. Some mylonite zones in the northern SNO show deformed pegmatites with asymmetric structures (s-clasts and shear bands) indicating sinistral strike-slip in horizontal sections. The moderate, W-ward plunge of the stretching lineation implies a smaller dip-slip component with W-block up. Retrogression to amphibolite facies conditions suggests that these mylonites postdate the main foliation-forming event. Since upright folds in adjacent blocks have fold axes parallel to the mylonitic stretching lineation, they may have formed simultaneously, which would suggest a transpressional tectonic setting at this stage.

A km-scale tectonic lens in the footwall of the SNO-CNO boundary forms a relatively low-strain domain in the higher-strain gneisses. It comprises upto 50 m thick metabasic layers alternating with dioritic gneisses and minor garnet-biotite gneisses of possible metasedimentary origin. It also contains a 10 x 3 m lens of sapphirine-spinel-bearing amphibolite described in more detail by Kriegsman (1995a). These rocks display S>L tectonite fabrics with a moderately NNW dipping gneissic layering and a subhorizontal stretching lineation that is 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, indicate high strains.

CNO (Fig. 1.1)

The tectonostratigraphy N of the inferred structural contact, in the southern part of the CNO, shows a complex of supracrustal rocks, of up to 150 metres in cross-sectional width, consisting of psammitic to pelitic sediments, the latter partially melted, and banded mafic rocks, presumably volcaniclastics, interleaved with orthopyroxene-bearing orthogneisses that form sheets of up to several kilometres thick. This sequence is repeated several times and is interpreted as a thrust stack with a top-to-the-S displacement.

N of the thrust stack, but still within the southern part of the CNO, lies a large mass of weakly deformed hypersthene bearing gneiss containing some amphibolite lenses and only a few rocks of undoubted metasedimentary origin. Reconnaissance age determinations showed that this intrusive body has an Archean age (~2750-2800 Ma: Kalsbeek & Nutman, 1996). Alternating metasediments and charnockitic gneisses crop out further N.

A several km wide zone consisting of supracrustal rocks - highly anatectic, sillimanite-bearing metapelites alternating with psammites, marble and banded amphibolites, forms the eastern extension of the Nordre Isortoq supracrustal belt (see Marker et al, this volume). Extension lineations in the S>L fabric are variable in orientation but generally show a shallow plunge to the east or west. Kinematic indicators show non-coaxial flow with a statistical predominance of sinistral strike-slip.

The northernmost part of the traverse in the Inner Nordre Strømfjord-Arfersiorfik area shows Archaean orthogneisses interleaved with a complex of Proterozoic supracrustal and intrusive rocks, which show thrust contacts. Ultramafic lenses at the tectonic contacts suggest the existence of a major tectonic boundary (Kriegsman, 1995b). They contain olivine and Cr-spinel and have been interpreted as peridotites (Manatschal and van Gool, this volume). This area is described in more detail by Manatschal et al. (this volume) and van Gool et al. (this volume).

Metamorphic conditions

So far we have calculated P-T data for 10 localities in the SNO along the traverse, using Berman's (1988) dataset. It is a main target of near-future probe analyses to obtain P-T data for the CNO. Metabasites in the southern SNO consist mainly of garnet + clinopyroxene + quartz + plagioclase with variable amounts of green amphibole. Orthopyroxene is locally present as small inclusions in quartz, which may reflect relics enclosed during an up-pressure P-T path crossing the divariant CFMAS reaction Opx + Plg = Gt + Cpx + Q. Preliminary results indicate pressures of 13-14 kbar at temperatures of ~700-800 oC when compositions of orthopyroxene inclusions are combined with core compositions of garnet and clinopyroxene (Fig. 1). Subsequent garnet breakdown to orthopyroxene + plagioclase occurred at ~7-9 kbar and ~680-800 oC, which is similar to P-T estimates for the area N of the thrust contact. We favour the high-T estimates of the low-P stage in view of the fact that sillimanite is the dominant aluminosilicate in metasedimentary gneisses to the N. We attribute the lower-T estimates to retrograde Fe-Mg exchange.

Table 1. Correlation between lithological and structural phenomena in western and eastern sections.

phenomena in western section (from N to S)	corresponding phenomena in eastern section
Nordre Strømfjord steep belt	Nordre Strømfjord steep belt
mixed Archean and Proterozoic gneisses, refolded into open to close upright folds(northern CNO "flat belt")	alternating stack of Archean and Proterozoic gneisses, refolded into open to close upright folds (northern CNO "flat belt")
Steep zone separating supracrustals (S) and orthogneisses (N); L ~070/50, shear-sense unknown	Steep zone separating supracrustals (S) and orthogneisses (N); L~255/40, sinistral shear
Alternating supracrustals and charnockitic gneisses(Nordre Isortoq zone)	Alternating supracrustals and charnockitic gneisses; (Nordre Isortoq zone); syntectonic porphyritic granites
Weakly deformed charnockites, banded orthogneisses of Proterozoic age ("Holsteinsborg charnockite"); few outcrops with metasediments	Weakly deformed charnockites, banded orthogneisses of Archean age; one outcrop with metasediments
N-dipping steep belt in supracrustals, possibly a thrust with top-to-the-S dispacement, reworked during sinistral strike-slip (Ikertoq)	N-dipping zone of alternating supracrustals and granitic gneisses interpreted as thrust stack with top-to-the-S displacement
Archean gneisses cross-cut by Kangâmiut dykes (SNO)	Archean gneisses cross-cut by Kangâmiut dykes and transected by steep, retrograde sinistral shear zones (SNO)
mafic material S of Itivdleq (Davidson, 1978; Jack, 1978); 30 km S of CNO-SNO boundary	Kilometre-scale lens with abundant mafic material, locally with sapphirine; 10 km S of CNO-SNO boundary

Discussion

Correlation with coastal data

Table 1 shows some similarities and differences between the western and eastern sections. Features that are more prominent in the western section are the abundant Proterozoic metasediments (Nordre Isortoq) and the Holsteinsborg charnockite. Phenomena that are more prominent in the eastern section are the tectonic boundary between inferred Archean basement gneisses and overlying Proterozoic ortho- and paragneisses (eastern Nordre Strømfjord) and the occurrence of syntectonic porphyritic granites. The kilometre-scale tectonic lens with abundant, locally sapphirine-bearing, mafic material in the eastern section of the SNO may have its counterpart in a large-scale funnel-shaped structure S of Itivdleq (Jack, 1978; Davidson, 1978), although the latter is situated 20 km further away from the CNO-SNO boundary. Similar sapphirine-bearing mafic rocks have also been described from the Sukkertoppen area (Herd et al., 1969) and are generally of Archean age.

Significance of early Nagssugtoqidian high-P metamorphism

The preliminary results from thermobarometry along the traverse suggest that some metabasic dykes in this part of the orogen may have witnessed a high-P metamorphic event, which does not seem to have been recorded in rocks N of the thrust contact, and subsequently re-equilibrated at intermediate pressures. The results are consistent with high-pressure estimates (²11kbar at 700-750°C) and re-equilabration at 6-8 kbar and 600-700°C in mafic dykes in the SNO along the coast in the Ikertôq-Itilleq area (Mengel et al., 1995). Assuming that these dykes are members of the Kangâmiut dyke suite, constrains the age of this high-pressure event on the coast and inland as being Paleoproterozoic.

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 which levels the extensional strain was accommodated, but it must have affected higher levels of the orogen in order to explain the ~5 kbar decompression. Assuming that the gneissic layering in the CNO and the northern SNO was subhorizontal before upright folding and sinistral strike-slip, gives a pervasive, subhorizontal stretching lineation with an ENE azimuth at all levels. We propose that ENE-WSW horizontal stretching and vertical flattening may have been pervasive in this part of the orogen and occurred under high-grade metamorphic conditions. The subhorizontal fabric was subsequently refolded and deformed under retrograde amphibolite facies conditions in a transpressional setting. The time relations with thrusting are still unresolved.

Conclusions

Lithological and structural data along a N-S traverse through the SNO and CNO near the inland ice show strong similarities with coastal data. However, some features (Proterozoic Nordre Isortoq supracrustals and Holsteinsborg Charnockite) are more prominent in the western section, while other phenomena (stacking of Proterozoic and Archean units in the eastern Nordre Strømfjord and the occurrence of syntectonic porphyritic granites) are more prominent in the east.

Thermobarometry on metabasites correlated with the Paleoproterozoic Kangâmiut dykes reveal an early high-P stage (~12-14 kbar at 700-800 oC) unrecorded in adjacent gneisses. Late-stage equilibration at 7-9 kbar and similar temperatures suggests a phase of near isothermal decompression which may be attributed to extensional collapse of higher levels of the overthickened crust in the Nagssugtoqidian orogen. ENE-WSW horizontal stretching and vertical flattening may have occurred at this stage. Transpression in this part of the orogen was accompanied by retrograde amphibolite facies conditions.

References

Berman, R.G., 1988. Internally consistent thermodynamic data for minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2. J. Petrol., 29: 445-522.

Davidson, L. M. 1978: Granulite facies rocks bordering the Ikertôq shear belt, central West Greenland. Unpublished Ph.D. thesis, Univ. Liverpool.

Grocott, J. 1979: Shape fabrics and superimposed simple shear strain in a Precambrian shear belt, West Greenland. J. Geol. Soc. Lond. 136, 471-488.

Herd, R.K., Windley, B.F. and Ghisler, M, 1969. The mode of occurrence and petrogenesis of the sapphirine-bearing and associated rocks of West Greenland. Grønlands Geol. Unders., 24: 44 pp.

Jack, S. M. B. 1978: The North Atlantic Proterozoic dyke swarm. Unpublished Ph.D. thesis, Univ. Liverpool.

Kalsbeek, F. and Nutman, A.P., 1996. Anatomy of the Early Proterozoic Nagssugtoqidian orogen, West Greenland, explored by reconnnaissance SHRIMP U-Pb dating. Geology, in press.

Kriegsman, L.M., 1995a. Status Report, submitted to DLC on 23 June 1995.

Kriegsman, L.M., 1995b. Report on field activities 1994, Nagssugtoqidian W; submitted to DLC on 8 February 1995.

Marker, M., 1995. Summary of field work in the central Nagssugtoqidian Orogen in 1995. In: DLC Report on 1995-fieldwork in the Nagssugtoqidian Orogen, West Greenland, pp. 42-45.

Mengel, F., van Gool, J. and Marker, M. (1995): Mafic dykes as monitors of orogenic development: an example from the southern margin of the Paleoproterozoic Nagssugtoqidian Orogen, West Greenland. In: Nagssugtoqidian Geology 1995, workshop Proceedings DLC. p. 15-18.

Nichols, G.T., 1995. Notes on metamorphism with special emphasis on the southern part of the Nagssugtoqidian Orogen. In: Proceedings of the April 6-7, 1995 DLC workshop "Nagssugtoqidian Geology 1995", pp. 34-35.

Passchier, C. & Den Brok, B., 1995. Structural evolution of the central Nagssugtoqidian Belt (West Greenland). In: DLC Report on 1995-fieldwork in the Nagssugtoqidian Orogen, West Greenland, pp. 46-47.

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