Petrography
3.2.1 Optical Microscopy
The MSF hosts numerous facies of syenite and nepheline syenite varying in colour from red to pink to white. The unit was subdivided into three concentric members by Bradshaw (1988) termed the MSF – Marginal arfvedsonite syenite, MSF – Altered syenite and the MSF – Nepheline syenite. Throughout the present study classification of units into these three members has been avoided due to the unusually high degree of heterogeneity found with each.
Syenite samples located at low-levels in the MSF are more homogeneous and display a much lower degree of alteration that units at higher levels in the formation. Generally the rock is a coarsely crystalline syenite, though occurrences of microsyenite of similar mineralogy are also common. Typically this rock comprises alkali-feldspar primocrysts making up 60-70% of the mode. Feldspar are coarsely exsolved patch and braid perthite up to 1 cm long and ~1 mm wide (Fig. 3.1). Large crystals often show Carlsbad twins, perpendicular to which perthitic exsolution lamellae form, giving the feldspar a ‘herringbone’ texture. Although alteration is less intense in these units, feldspars often show turbidity, which in many samples is localised to one of the exsolved phases. Nepheline is present up to 15 modal %. In hand specimen this often has a pale pink coloration but in thin section nepheline often appears less turbid and altered than feldspars. The mafic mineralogy is dominated by characteristic pale olive green to brown pleochroic arfvedsonite amphiboles and bright green to yellow pleochroic aegirine pyroxene intercumulus to the felsic phases. Several occurrences of concentric zoning have also been observed under PPL, characterised by radiating pleochroism. Biotite is common, forming secondary to amphibole or growing as ragged red-brown crystals associated with magnetite. Fluorite is common in many syenite facies of the Motzfeldt centre and is present largely as an intercumulus phase or in microveins, often having a strong purple colouration. Accessory phases identified during the present study include elongate euhedral apatite, interstitial and vein filling calcite, zircon and euhedral pyrochlore crystals, which often show strong alteration haloes in the host feldspars. Other secondary accessory phases reported by Bradshaw (1988) include rinkite and låvenite, though these were not identified during the present study.
widespread. Within ~200 m of the inferred roof of the formation there are a number of textural variants of syenite. These generally have similar mineralogy, although vary in mineral mode, textural character, type and degree of alteration. This section details the general petrology of these units and highlights some of the more unusual mineralogical and textural occurrences of the formation.
Figure 3.1. Textural relations of alkali feldspars from the Motzfeldt Sø formation. (a) Coarsely exsolved patch perthite and simple twinning in alkali-feldspar from low levels in the MSF (GJM06-13). (b) Altered exsolved alkali-feldspar with relatively fresh albite rims (GJM06-17). (c) Fine microperthite from altered alkali-feldspars (GJM06-32). (d) Coarsely exsolved and altered patch perthite with Carlsbad twinning from high-levels in the MSF (GJM06-36).
At high-levels in the intrusion, the most common facies of syenite is typically a coarse- grained and highly altered syenite composed predominantly of tabular brick shaped euhedral alkali-feldspars (mode: 70-80%) 5-15 mm in length with aspect ratios of ~2:1. Feldspar in this unit is coarsely perthitic displaying similar patch and braid textures to feldspar from lower in
reflecting the colour in hand specimen. Although very turbid (interpreted as highly altered), feldspar often shows rims which are relatively transparent in thin section (Fig. 3.1b). The brick red colour which characterises the feldspars of the MSF can be attributed to presence of microgranular iron oxides within the feldspar. In hand specimen many of the samples show specular hematite (Fe2O3) coating grain boundaries of feldspars giving samples a blue-black
colour, this likely occurs as sub-microscopic inclusions within the altered areas of feldspar. Altered feldspar showing red/pink colour has been discussed by Putnis et al. (2007). The characteristic red-oxide colouration in all samples studied by these authors is generated through sub-microscopic growth of hematite rosettes and needles in the pore spaces of feldspar crystals, widely interpreted as forming during subsolidus re-equilibration in the presence of a fluid phase. The possible sources of Fe to form hematite are numerous, and include breakdown of primary Fe-bearing mafic phases, re-equilibration in the presence of Fe- rich externally sourced fluids and/or re-equilibration of a feldspar containing Fe in its structure (e.g. Fe3+ substituted for Al3+ in the tetrahedral site) with a solution, into which Fe is
released and precipitated as hematite (Putnis et al., 2007). Finch & Klein (1999) have shown that Gardar feldspars contain tetrahedral Fe3+ and discuss the possibility that they also contain
nanoparticles of magnetite.
Nepheline is rare and only present in small amounts (<10%) in most samples but is typically heavily replaced by secondary zeolites and micas (an intergrowth referred to as ‘gieseckite’). In many samples nepheline is entirely replaced and is only inferred from the relict crystal habit. The mafic mineralogy of high-level syenite facies of the MSF is dominated by dark green-brown pleochroic arfvedsonite amphibole. Despite the pleochroic scheme observed (which would make compositional zoning particularly obvious), few samples show optical zoning. Amphibole textures in the MSF are highly variable and dependent on the textural character of the particular facies of occurrence. In many samples arfvedsonite occurs intercumulus as anhedral to subhedral crystal clusters. A common feature in some samples (e.g. GJM06-34) is a feathered or embayed texture where amphibole is found in contact with relatively fresh albite rims of alkali-feldspars (Fig. 3.2). In the high levels, pyroxene is less common although striking radial rosettes of aegirine needles are found (e.g. GJM06-34). Sample GJM05-41 also has rare granular clusters of fine elongate aegirine crystals giving samples a spotty appearance with clusters up to 10 mm in diameter. In samples where clusters
Figure 3.2. Feathered intergrowth between altered and exsolved alkali-feldspars and primary green-blue arfvedsonite amphibole (GJM06-34).
Biotite occurs throughout the altered facies of syenite in three distinct textural relationships. In many samples biotite is found as discrete primary crystals (Fig. 3.3a) or as a secondary phase intimately associated with intercumulus amphibole (Fig. 3.3c&d). Less common are large porphyritic plates of poikiolitic biotite up to 15 mm in diameter enclosing other common rock forming mineral phases, accessory zircon and rare pyrochlore crystals (Fig. 3.3b). In many samples small flakes of biotite are found growing as a rim surrounding clusters and individual crystals of magnetite (Fig. 3.3e&f). Textures like these have been described from other Gardar centres (e.g. Klokken intrusion, Parsons et al., 1991 and Igdlerfigssalik, Finch et al., 1995) and have been termed ‘fringe biotites’. It is likely that the intercumulus and poikiolitic biotite grew directly from the silicate melt. Phase diagram shows that the phlogopite solidus temperature is too high, hence primary biotite phenocrysts must have been annite rich (Parsons et al., 1991). It is not clear whether the fringe biotite grew through reaction of the residual silicate melt and accessory magnetite or through subsolidus reactions (Finch et al., 1995). In samples where biotites grew as a product of subsolidus replacement reactions the term reaction corona may be applied to describe the textural relationship of the biotites. Several rare samples of large (ca. 40 mm) mica books with pseudohexagonal basal sections have also been observed in hand specimen from the MSF. Each of these textural occurrences of biotite is studied in subsequent chapters concerning their halogen chemistry (chapter 5).
Figure 3.3. Textural occurrence of biotite micas in samples from the Motzfeldt Sø formation. (a) Large primary biotite crystal (GJM06-63). (b) Large poikiolitic biotite phenocryst hosting euhedral alkali-feldspar, apatite and magnetite (GJM06-31). (c & d) secondary biotite replacement of primary arfvedsonite (GJM06-63). (e & f) Fringe/reaction corona of secondary biotite (GJM05-23).
Fluorite is a common component of all samples of the MSF and makes up 10% of the rock, in many samples occurring intragranular, as fine veins and as cubic euhedral crystals in mineralised cavities. Associated with some fluorite samples are rims of zeolites on the boundary between fluorite and feldspar. In small miarolitic cavities, euhedral quartz has also been identified in association with fluorite. Both these phases host abundant fluid inclusions which have been the focus of microthermometric investigation (chapter 6). The presence of quartz in nepheline bearing syenites suggests that quartz host in miarolitic cavities is secondary in origin, associated with the late magamtic to subsolidus phase.
Pyrochlore minerals hosting economically important concentrations of Nb and Ta have been identified as an accessory phase in all facies of syenite from the MSF. However, throughout the MSF, distinct facies of fine-grained leucocratic syenite enriched in pyrochlore have been identified. These occur in greatest abundance close to the roof-zone of the formation. The pyrochlore host is composed almost entirely of euhedral, aligned tabular alkali- feldspar laths, giving the rock a strong fabric. Aside from pyrochlore the mafic mineralogy is restricted to rare clusters of highly altered mica, amphibole and magnetite.
Pyrochlore is typically disseminated throughout these samples (Fig. 3.4a&b), although cumulate horizons locally enriched in granular clusters of pyrochlore define particular pyrochlore-rich facies of syenite (Fig. 3.4c-f). Disseminated pyrochlore grains typically range in size from 0.5-2 mm across and are always euhedral. In cumulate-rich horizons, crystals are larger than the disseminated crystals (up to 4 mm) and often have strong octahedral crystal habit. Pyrochlore group minerals are often characterised by their strong honey-yellow colour in transmitted light. However crystals from the MSF show a dark reddish-brown to opaque colour in transmitted light suggesting alteration. Larger crystals from cumulate-rich horizons occasionally show subtle zoning which is enhanced by the dark zones of more intense alteration (e.g. Fig. 3.4e). Pyrochlore crystals in the MSF in general have a thin rim of hematite, similar to that observed mantling many of the primary phases in the formation, and may contain inclusions of alkali-feldspar, magnetite, fluorite and calcite. In cumulate-rich horizons intense fracturing of the host feldspars is common (e.g. Fig.3.4d) running between individual pyrochlore crystals and running parallel the mineral fabric of the rock.
Figure 3.4. Occurrence and textural relationships of pyrochlore mineralisation in the Motzfeldt Sø Formation. (a&b) Disseminated pyrochlore crystals host in a leucocratic feldspar rich groundmass (GJM05-23, GJM06-18). (c) Granular mass of pyrochlore crystals intimately associated with large (1-2mm) high-relief zircon megacrysts (GJM06-65). (d-f) Dense concentration of pyrochlore crystals from locally enriched cumulate layers. Image D shows strong fracturing of host feldspars parallel to the cumulate layer. Image e shows weak zoning in larger crystals. Image f shows concentrated alteration in feldspars intimately associated with pyrochlore mineralisation (A&R 10520, GJM05-21).