Low-P partial melting in the Nordre Isortoq area: Petrology of spinel-bearing, partially melted pelites

Leo Kriegsman1 and Geoff Nichols2

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

2GEMOC, School of Earth Sciences, Macquarie University, NSW, 2109, Australia.

Introduction

The Nordre Isortoq supracrustals form a conspicuous metasedimentary realm, with orthogneiss units, in the coastal part of the orthogneiss-dominated CNO ("qgn" on 1: 500 000 Geological Map of Greenland: Escher 1971). A 1994 near-ice traverse (Marker and Kriegsman, Nichols and Van Gool) showed the presence of similar metasediments at the same tectonic level 100 km further east along strike. A one-day helicopter survey (Marker and Kriegsman) in 1994 corroborated the lateral persistence of this metasedimentary belt N of Ilivigdlup Tasia ("ms" on 1: 500 000 Geological Map of Greenland: Escher 1971). It has therefore been suggested that one metasedimentary belt crops out along strike in most of the central CNO in the western Nagssugtoqidian (Kriegsman, 1995a,b; Marker, 1995).

The geometry of the Nordre Isortoq supracrustals can be summarized as doubly plunging upright folds bordered by a steep zone in the N. Stretching lineations (mineral lineations) defined by Q-rods and well-aligned sillimanite crystals plunge ~WSW/10-20 on moderately SSE-dipping foliation planes and ~ENE/50-60 on vertical to steeply NNW-dipping foliation planes (Kriegsman, 1995a,b). Fold axes are subhorizontal. These observations can be explained by refolding of a subhorizontal foliation with a horizontal ~NE-SW oriented stretching lineation. The consistent angle of ~50o between fold axes and stretching lineations in the steep zone suggests little or no reorientation of the fold axes. In view of the high strains in this zone (b = 3-5 from boudinaged metabasites), this also fits refolding of a previously subhorizontal high-strain zone (op. cit.). Passchier & Den Brok (1995) proposed a consistent top-to-the-E displacement for this area and the rest of the CNO.

The Nordre Isortoq metasediments have been metamorphosed to Gt-Sil gneisses and show abundant evidence for partial melting. Most localities show the assemblage Gt-Sil-melt, but Crd is also present in a number of outcrops. Here we describe an outcrop containing the rare assemblage garnet-spinel-corundum-plagioclase-melt and discuss its P-T evolution and the significance for the Nordre Isortoq supracrustal belt.

Outcrop and sample description

Metapelitic rocks were collected in July 1994 from a locality in the western part of the Nordre Isortoq supracrustals, ~2 km from the contact with the Holsteinsborg Charnockite. This locality shows evidence for pervasive partial melting. The melt has segregated from the parent rock, but does not seem to have migrated far. Instead, it has accumulated in pods and layers surrounding the restitic material.

Specimens 415217 and 415462 are partially melted metapelites containing garnet-spinel-plagioclase assemblages with variable amounts of corundum, sillimanite, ilmenite, biotite and quartz. In hand specimen the rocks display quartzofeldspathic layers juxtaposed with attenuated garnet-rich horizons. Layering is further defined by elongate xenoblastic spinel and ilmenite. Garnet occurs in two textural forms: as coarse, sometimes elongate aggregates up to 15 mm in diameter, and as discrete idioblastic grains within leucocratic layers. These idioblastic garnets apparently postdate an isoclinal folding event that deforms both the leucocratic layers and the layer-parallel garnet aggregates.

Mineral Textures

The sampled rocks are heterogeneous with adjacent specimens preserving different mineral assemblages. Specimen 415462 shows the equilibrium assemblage garnet + spinel + corundum + plagioclase. Early garnet occurs in deformed, elongate aggregates. Texturally late garnet overgrows a strong planar fabric defined by elongate spinel , biotite and ilmenite grains. Deformation of corundum is less conspicuous. Biotite is notably absent from the matrix, with the exception of late biotite randomly overgrowing all fabrics. We infer that spinel, corundum and early garnet aggregates were the product of biotite dehydration melting. The restitic assemblage and the segregated melt phase were deformed and subsequently overgrown by late garnet.

Thin section textures in specimen 415217 display idioblastic garnet aggregates and porphyroblasts within a granuloblastic matrix of perthitic plagioclase, quartz and minor K-feldspar. Coarse grained spinel and ilmenite predominantly occur as attenuated aggregates or inclusions in garnet. They are rare in the leucocratic matrix and occur only in regions adjacent to garnet. Biotite is not common and is usually restricted to coarse grained inclusions, or inclusion aggregates, within garnet. Sillimanite only occurs as fine acicular inclusions in garnet, and displays a weak preferred orientation, subparallel to the layering of the rock. The presence of preferentially oriented biotite, spinel, sillimanite and ilmenite in garnet interiors indicates that this was the earliest recorded assemblage.

Spinel enclosed by garnet in sample 415217 is usually surrounded by narrow sericite rims; where spinel grains occur within the leucocratic matrix, adjacent to garnet, they are enclosed by coarser rinds composed of radiating crystals of sericite.

Mineral Chemistry

We have performed mineral analyses in polished thin-sections on a Cameca SX50 microprobe (Macquarie University) and JEOL microprobes (Utrecht University and Århus University), at 20 nA and 15 KeV operating conditions, using international standards for calibration.

Spinels are predominantly hercynitic in sample 415217 with average compositions of Hc70Spl26Gah3Chr1. Garnet does not show any zoning. Traverses through crystals as large as 15 mm in diameter demonstrate an essentially homogeneous composition of Alm70Prp22Grs8. Garnet and spinel XMg [=Mg/(Fe+Mg)] values are 0.24 and 0.27, respectively. Plagioclase within the matrix is calcic with a compositional range of An72-78. Sericite at Gt-Sp contacts is a low-(Fe+Mg) muscovite.

Garnet and spinel XMg [=Mg/(Fe+Mg)] values in sample 415462 are slightly higher: 0.27 and 0.30, respectively. Spinel contains ~0.5% magnetite, calculated assuming perfect stoichiometry, no chromite, and ~3.5% Zn-spinel. Plagioclase is less calcic in this sample, with a compositional range of An46-54.

Pressure-temperature constraints

The observed mineral textures are reminiscent of an in-situ partial melting event involving biotite breakdown. This hypothesis accounts for the lack of biotite in the matrix of the rocks, and explains the observations of early deformed, and later idioblastic, garnet forms. A relatively high temperature partial melting event may also account for the compositional homogenization of garnets that presumably formed at different conditions prior to melting.

Several approaches were used to constrain equilibrium pressure and temperature of the assemblages described above. They can be subdivided into constraints derived from petrogenetic grids and P-T calculations using geothermobarometry.

Petrogenetic constraints can be derived from a consideration of reactions involving biotite dehydration melting and FMAS equilibria. Biotite dehydration melting in the system KFMASH implies temperatures in excess of ~800 oC (e.g., Vielzeuf & Boivin, 1984). The intersection of P-T curves for biotite dehydration melting and the FMAS univariant line Gt + Cd + Cor = Sp + Sil lies at about 6 kbar, 800 oC (Fig. 1). Hence, the inferred early stability of Sp + Sil in sample 415217 suggests pressures below 6 kbar at that stage and, possibly, similarly low peak pressures. The low amounts of Zn, Cr and Fe3+ in spinel are unlikely to have a strong influence on the equilibrium P and T of the assemblage.

Biotite dehydration melting in the system KFMASH should give melts containing only Kf, Q and minor amounts of Fe-Mg phases (e.g., Carrington & Harley, 1995). However, the samples discussed above contain abundant plagioclase in the restites and the melt phase is granitic. Hence, the temperature of melting must have been reduced by the additional components Ca and Na. The minimum melting curve for granitic compositions under hydrous conditions lies at ~650 oC, which may serve as an absolute minimum temperature constraint. Since Kf in the granitic melts was derived from biotite breakdown, we infer that fluid-undersaturated conditions are more appropriate. Hence, we estimate the temperature of melting at somewhere between 700 and 750 oC. By implication, peak pressures may even have been lower than those inferred from KFMASH and FMAS considerations.

At P = 9 kbar, the assemblage Gt + Sp + Cor is generally quite magnesian (Gt XMg 0.6-0.7, Sp XMg 0.7-0.8: Kriegsman & Schumacher, submitted). Since XMg numbers in the FMAS system show a consistent decrease with pressure (e.g., Hensen, 1987), the much lower XMg values in the Nordre Isortoq samples suggest pressures much lower than 9 kbar.

The PTAX software after Berman's (1988) dataset gives pressures of 4.0-5.5 kbar and temperatures of 550-700 oC for the assemblage Gt + Sp + Cor + Pl in sample 415462. Fig. 2 shows the extreme values. Importantly, the equilibrium lines for the CFAS and CMAS barometers (dP/dT ~ 10 bar/K) are similar for all mineral pairs and the spread in intersections is entirely due to the variable position of the Gt-Sp Fe-Mg exchange line. In view of the petrogenetic constraints derived above, the maximum P-T values seem to be a close approximation of the peak P-T conditions. We therefore interpret the lower P-T values in terms of retrograde reequilibration causing Fe-Mg exchange without affecting Ca-Al-Si systematics. Such selective behaviour of retrograde reequilibration has been noted by many authors working in other granulite terrains (e.g., Frost & Chacko, 1989; Fitzsimons & Harley, 1994).

Gt-Sp pairs in sample 415217 were used to derive temperature estimates with the aid of the gahnitic spinel-cordierite-garnet calibration of Nichols et al. (1992), giving only ~600-650 oC. Applying the PTAX software to the assemblage Gt + Sp + Sil + Pl in this sample gave pressures of ~4.0 kbar and temperatures of ~600 oC. The equilibrium lines for the CFAS and CMAS barometers in this assemblage overlap with the corresponding lines for the assemblage Gt + Sp + Cor + Pl in sample 415462. Hence, we similarly interpret the lower P-T values in terms of retrograde reequilibration affecting only the Gt-Sp Fe-Mg exchange line. The growth of sericite at Gt-Sp grain boundaries inside and outside Gt suggests that reequilibration may have been promoted by the influx of a K-rich, hydrous fluid.

The peak P-T estimates are supported by assemblages observed elsewhere in the Nordre Isortoq supracrustals. A nearby mafic enclave in partially melted metasediments adjacent to a mafic dyke contains the assemblage Gt + Opx + Plg + Q, which yielded a P-T estimate of ~7 kbar and ~700 oC (sample 415430). P-T estimates from Gt-Sil-Crd gneisses cropping out on the peninsular separating the Isortuarssuk arm from the Nordre Isortoq fjord are ~6 kbar and 700 oC (sample 415456). Allowing for the possibility of regional variations in P-T conditions throughout the Nordre Isortoq supracrustals and taking uncertainties in thermodynamic calibrations into account, we propose that 6 ± 1 kbar and 700 ± 50 oC are reasonable estimates from the average peak P-T conditions in this part of the Nagsugtoqidian Orogen.

Comparison with other data for the coastal CNO

The above observations and calculations suggest that the Nordre Isortoq supracrustals represent a low-P realm in the coastal section of the CNO in the Western Nagssugtoqidian. In order to assess its importance, we have to look at some other P-T estimates from this section.

An outcrop 10 km further S, in the core of the Holsteinsborg Charnockite, shows Gt + Q breaking down to Opx + Plg. Calculated P-T values for this assemblage are: 6.0-7.5 kbar, 650-700 oC (sample 415442: Kriegsman, 1995b) and 7-8 kbar, 700 oC (sample 414690: Nichols, 1995). Since this reaction assemblage is commonly considered to be a typical decompression texture, the calculated pressures represent minimum values only. Hence, we conclude that 7-8 kbar is the minimum pressure range experienced by these rocks.

Davidson (1978) reports the assemblage Opx + Sil + Q in two samples from the Kangerdluarssuk Tugdleq fjord, about 15 km further S and just 5 km N of Holsteinsborg. This assemblage and the evidence for the stable coexistence of Gt + sapphirine suggest P-T conditions in the order of 9 kbar, > 900 oC. Breakdown of the assemblages Opx + Sil + Q and Gt + Sil + Q to form Crd is clear evidence for decompression. The early stable coexistence of Sp + Q and Spr + Q and their breakdown to Gt + Sil (op. cit.) suggests cooling from very high temperatures at pressures exceeding 8-9 kbar. High temperatures in these samples may reflect heating by the intrusion of the Holsteinsborg Charnockite, causing partial melting of the metasedimentary enclaves and subsequent cooling of the restitic material. This was followed by decompression, similar to the evidence from our samples.

This non-extensive review of data suggests that the coastal section from the Nordre Isortoq supracrustals to Holsteinsborg/Sisimiut may represent an oblique crustal section with maximum pressures increasing from 6 kbar in the north to 9 kbar in the south. By implication, the enveloping surface to the upright folds in this section (e.g., Paschier & den Brok, 1995) must have a shallow to moderate dip to the N. This is consistent with the position of this section in the hanging wall of the northward dipping, southward directed Ikertoq thrust. The southward increase in pressure is also consistent with the presence of kyanite in metasediments near the thrust contact.

Conclusions

1. Gt-Sp-Plg-Cor and Gt-Sp-Plg-Sil assemblages in the Nordre Isortoq supracrustals reflect restitic material left after biotite dehydration melting in the presence of plagioclase, i.e. in the system NCKFMASH.

2. The Nordre Isortoq supracrustals represent a low-P realm in the coastal section of the CNO in the Western Nagssugtoqidian. Reasonable estimates for the average peak P-T conditions in this part of the orogen are 6 ± 1 kbar and 700 ± 50 oC.

3. A comparison with other coastal data suggest the presence of an oblique crustal section with maximum pressures increasing from 6 kbar in the north to 9 kbar in the south, consistent with the position in the hanging wall of the Ikertoq thrust.

Near-future work

These specimens as well as others from adjacent outcrops, are the current focus of on-going studies. We intend to publish a paper on Gt-Sp-Cor-melt assemblages with two aims: (i) to clarify the conditions under which such assemblages form; and (ii) to obtain P-T data on this relatively low-P area in the Nagsugtoqidian. Work on this topic requires some additional probe data and more samples will therefore be analysed shortly.

References

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