Syn- and post-kinematic intrusions

6.2.1. Petrography (thinsections no. 1533, 1549, 2175)

Samples described in the following paragraph are representatives of syn- and postmetamorphic plutonic bodies that intruded into the basement lithologies (see chapter 5.2).

Macroscopically, the light-grey alkali-feldspar syenite and quartz-alkali-feldspar syenite (samples no. 2175 and 1549) are coarse grained, seriate to inequigranular with granitic-type texture. Euhedral K-feldspar phenocrysts may reach up to c. 20.0 mm in size. Major and minor mineral components are K-feldspar, plagioclase, quartz, biotite, and green hornblende. Accessory phases include muscovite, black and dark red opaque phases, apatite, zircon, rutile and titanite.

Microscopically, grain size of euhedral to subhedral K-feldspar ranges between c. 0.1 and 20.0 mm. Some crystals display perthitic unmixing, and microcline phenocrysts show twinning after Carlsbad twin law. Fractured crystals may be altered to sericite and calcite along fissures. K-feldspar is often invaded by myrmekite, and contains solid inclusions of quartz, biotite, hornblende, apatite, plagioclase and/or opaque phases.

Euhedral to anhedral plagioclase varies between c. 0.08 to 4.0 mm in size. Fine polysynthetic twinning after albite and pericline twin law is common and myrmekitic intergrowth with quartz occurs. Growth lines and pervasive sericitisation restricted to crystal cores indicate magmatic zoning. Additional sericitisation has developed along intracrystalline fractures.

Anhedral quartz varies from 0.1 to 6.0 mm in size and fills interstices between subhedral feldspars. It displays fissures, undulous extinction, subgrains and is vermicularly intergrown with, or included in, K-feldspar or biotite.

The common grain size of subhedral biotite crystals is c. 0.4 to 2.4 mm and pleochroitic colouring changes from yellowish to dark brown. Mineral phases overgrown by single biotite crystals are apatite, quartz, opaque phase and zircon, the latter being surrounded by black pleochroitic halos. Crystal margins show opacitisation and some grains are intergrown with either green hornblende or quartz.

Table 6.4: Modal compositions (in vol%) of different younger plutonic rocks that intruded into the metamorphic basement lithologies described in chapter 5.2. x ≤ 2 vol%.

sample no. 1533 1549 2175

Sample no. 1533 contains orthopyroxene and is therefore classified within the charnockite suite of rocks. Its texture is not unequivocal magmatic or metamorphic.

This is characteristic of meta-igneous charnockitic rocks, as they typically form under high-pressures, and igneous textures are inevitably modified in some way.

The medium grained greenish sample macroscopically displays bimodal grain size distribution, and preferred orientation of large deformed feldspar and pyroxene crystals. K-feldspar, plagioclase and quartz were identified as main mineral compounds, whereas orthopyroxene and clinopyroxene are minor phases. The group of accessories comprises muscovite, opaque phase, apatite, rutile, titanite, zircon and chlorite.

Elongated subhedral K-feldspar grains show preferred orientation, and may be up to 10.0 mm in length. However, the most crystals range in size from c. 0.05 to 0.8 mm.

They are often fractured and exhibit irregular grain boundaries, subgrains, and undulous extinction. Anhedral to subhedral plagioclases are in the range of 0.1 to 0.5 mm and thus more evenly grained. They display the same deformation features as the K-feldspars. Additionally, polysynthetic twinning and straight grain boundaries that sometimes form dihedral angles of 120° can be detected.

Anhedral ortho- and clinopyroxene crystals are aligned within the foliation, and vary in size from c. 0.05 to 0.5 mm (Fig. 6.6f). Orthopyroxene shows pleochroitc colours of light green and light red. The grains are fractured and mantled by green hornblende

6. The basement lithologies of the central Petermannketten

and biotite. The latter also occurs as fracture-filling. Clinopyroxene reveals light green colours and very weak pleochroism. Fracturing and mantling by hornblende or biotite are less strongly developed.

Quartz nearly exclusively occurs as myrmekitic intergrowth with plagioclase in K-feldspar.

6.2.2. Fluid inclusion studies (thicksections no. 1533 & 2175)

The only type of fluid inclusions detected in the mangerite sample no. 1533 contains pure CO2 and is hosted by plagioclase. The inclusions uniformly reveal negative crystal shapes and sizes ranging between 6.0 and 15.0 µm. They are arranged on intracrystalline clusters and trails and the volume fraction of the vapour bubble present at room temperature is 40 to 50 vol%. Melting of the solid carbonic phase consistently appears at -56.6 °C and homogenisation into the liquid phase occurs between 16.9 and 26.6 °C (Fig. 6.12). Calculated densities lie in the range of 0.68 to 0.80 gcm^-3.

The majority of fluid inclusions observed in quartzes of the syenite intrusion (sample no. 2175) are arranged on intracrystalline planar arrays and trails, and are characterised by nearly perfect negative crystal shape with sizes that range between 3.0 and 60.0 µm.

A subordinate number of inclusions reveal similar arrangement but irregular and elongated forms. During microthermometry measurements, the only phase transitions observed were melting of a pure carbonic phase at c. -56.6 °C, and homogenisation of this fluid at temperatures between 16.3 and 28.4 °C (Fig. 6.12). In inclusions with negative crystal shape, homogenisation occurs into the liquid phase. Irregular inclusions generally display critical behaviour or homogenisation into the vapour phase. Investigation with Raman spectrometry has proved though, that the fluid also contains small amounts of H2O, and minor amounts of N2. Solid phases enclosed were identified as being accidentally trapped crystals of calcite, rutile and opaque phases.

Densities calculated for this inclusions range between 0.21 to 0.81 gcm^-3. Bulk fluid properties of inclusions that had been proved to contain H2O were calculated taking into account a maximum value of 15 vol% "hidden" H2O (cf. Roedder, 1984). The resulting densities are in a similar range of 0.40 and 0.84 gcm^-3.

Feldspar crystals rarely contain single, dark fluid inclusions with highly irregular to negative crystal shapes. Melting temperatures of -56.6 °C indicate the presence of a pure carbonic phase, which was confirmed by Raman spectrometry. Homogenisation of these inclusions was at 27.9 and 28.5 °C into the liquid phase, which corresponds to calculated densities of 0.64 and 0.66 gcm^-3 (68.42 and 66.92 cm³mol^-1).

Fig. 6.12: Homogenisation and melting temperatures of the carbonic phase of quartz and plagioclase hosted inclusions of the syenite and plagioclase-hosted inclusions of the mangerite samples.

6.3. Summary and Discussion - metamorphic charnockitisation and