Post-collisional granite - An assessment of the magmatic conditions of Late Neoproterozoic coll

CHAPTER 3. PETROGRAPHY

3.1. Post-collisional granite

14ZA10

Sample 14ZA10 is coarse-grained biotite-granite that is comprised of plagioclase, K-feldspar, biotite, and quartz with accessory amounts of opaques and muscovite (<2 vol. %). This sample is unique amongst the post-collisional granites as it does not have amphibole. Plagioclase (8-10 vol. %) mostly forms subhedral or prismatic crystals (0.4-0.6mm) in the habit, showing parallel extinction as albite twinning. The plagioclase displays dark grey interference color of the first order. Euhedral to subhedral biotite (10-20 vol. %) indicates parallel or extremely low-inclined extinction, flaky, and one set of good cleavage with brown to yellowish interference color. Some interstitial anhedral biotites are enclosed within plagioclase or K-feldspar are also observed. K-feldspar is euhedral to subhedral, prismatic crystals ranging from 0.6 to 1 mm in grain size (45-50 vol. %) that have low pleochroism. They exhibit a poikiolitic texture. Subhedral quartz (25-30 vol. %) is randomly oriented with grey to dark grey or pale brown interference color of the first order. This sample is generally

fresh, but some of the quartz and K-feldspar are cloudy. The texture is characteristic of coarse grain size and the occurrence of antiperthite (in which microcline forms first as a host while plagioclase is internally formed).

Picture 3.1. The thin section photograph of representative crystals characteristic for the sample 14ZA10. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium

feldspar; Apt: anti-perthite; Mus: muscovite; Myr: myrmerkite.

37 14ZA18

Sample 14ZA18 comprises plagioclase (8-10 vol. %), biotite ( 5-10 vol. %), K-feldspar (35-40 vol. %), quartz (30-35 vol. %) and hornblende (2-4 vol. %), and opaques (<1 vol. %). Plagioclase is one of the main constituting minerals in this sample. The euhedral plagioclase (>1.5mm) displays parallel extinction, polysynthetic twinning with low pleochroism from grey to black color and sharp boundary. Biotite is euhedral to subhedral, highly pleochroic, has third-order interference color (reddish-brown to (reddish-brown and yellow-(reddish-brown to dark (reddish-brown), flaky, and perfect cleavage, and parallel extinction. The reddish-brown to brown biotite crystals are larger than the tan-brown to dark tan-brown crystals in grain size, and their typical color is probably due to the high content of Ti regardless of Fe content (Hall, 1941); while the tan-brown to dark brown biotites contain quartz inclusions. K-feldspar is euhedral (>1mm), with the characteristic of Carlsbad twinning and exsolution lamellae showing dark grey blackish color. There are mineral inclusions within K-feldspar of quartz and biotite.

Hornblende is typically subhedral in shape with two sets of good cleavage having an angle of nearly 120^o to 60^o. They are observed as perfect pleochroism (pale green), high birefringence (purple to dark brown). Quartz is yellow-grey, grey and dark grey of the first order of interference color, varying in grain size (coarse to fine) in shape of subhedral and rounded. The quartz has low relief, absence of cleavage planes, good oscillatory/wavy extinction. The main alteration is hornblende converting to biotite whereas some feldspar altered to clay minerals.

Picture 3.2. Some representative crystals for main mineral assemblage of the sample 14ZA18. (+):

crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Hbl:

hornblende

39 14ZA19B

Sample 14ZA19B comprises euhedral plagioclase (8-10 vol. %), euhedral to subhedral biotite (2-3 vol. %), euhedral to subhedral K-feldspar (45-50 vol. %), anhedral quartz (25-30 vol. %), and subhedral to euhedral amphibole (4-6 vol. %), with minor opaque (<1 vol. %). Plagioclase is variable in size from 0.5 to 2.25 mm, having low birefringent (grey to dark-grey of the first order of interference color). It occasionally encloses hornblende. Biotite has mostly third-order brown interference color, good cleavage, and appears to be weakly foliated. It sometimes occurs as minute inclusions trapped in interstitial grain position. K-feldspar commonly occurs as perthite (subsolidus exsolution) with the first order black-grey interference colors, a poikilitic relation with biotite, quartz, and probably hornblende. Quartz exhibits a fine to medium-grained size in euhedral crystal habit, low relief, and wavy extinction. It also occurs as megacrystal mymerkite (intergrowth of quartz in plagioclase).

Amphibole (hornblende) has deep-green, and red-brown pleochroism, high relief, and two sets of cleavage. It is commonly euhedral to subhedral crystal, associated with biotite, ranges from fine to coarse-grained size and contains mineral inclusions of biotite. Locally, some amphibole crystals embay the plagioclase. Fractured crystals such as quartz or feldspar are more abundant than other samples in the same group.

Some feldspar altered to clay minerals.

Picture 3.3. The thin section photograph of main mineral assemblage typical for the sample 14ZA19B (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Pt:

perthite; Myr: myrmerkite; Hbl: hornblende.

41 14ZA20B

Sample 14ZA20B is coarse-grained and comprises subhedral to euhedral plagioclase (8-10 vol. %), subhedral to euhedral biotite (5-7 vol. %), subhedral to euhedral K-feldspar (30-35 vol. %), anhedral quartz (20-25 vol. %), subhedral to euhedral amphibole (15-20 vol. %) and accessory amounts of opaque minerals (1-3 vol. %). Plagioclase crystals have albite and Carlsbad twinning and display parallel extinction. It is commonly intergrowth with K-feldspar to create the mesoperthitic texture. Greenish and reddish-brown biotite exist both primary magmatic crystal and as inclusions. It is generally associated with amphibole, showing perfect cleavage, parallel to very low extinction. K-feldspar is the dominant mafic mineral, medium to coarse-grained size with grey to dark grey interference color of the first order, and has inclusions of biotite and quartz. The feldspars have microcline twinning, but there are crystals with perthite exsolution. Quartz is variable in grain size, showing grey-yellow and dark-grey in interference color, and low relief. It is often interstitial to other minerals or occurs as an inclusion. Fractured crystal of quartz also observed in the thin section. The amphiboles are strongly pleochroic and medium to coarse-grained. The minerals show the characteristic high relief and two sets of cleavage with an angle of approximately 120^o to 60^o are visible under the microscope. Some feldspar crystals altered to clay minerals.

Picture 3.4. The subhedral crystals of the main minerals observed in the sample of 14ZA20B. (+):

crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Hbl:

hornblende.

43 14ZA21B

Sample 14ZA21B comprises plagioclase (10-15 vol. %), biotite (2-5 vol. %), K-feldspar (25-30 vol. %), quartz (35-40 vol. %), amphibole (5-7 vol. %) and accessory amounts of zircon and opaques (1-3 vol. %). The plagioclase crystals are subhedral to euhedral and display albite twinning. Intergrowth between plagioclase and K-feldspar is often observed. The amount of biotite in this sample is lower in comparison to other post-collisional granites, more anhedral to subhedral, and mainly is interstitial to other minerals. However, it can still be identified based on its pleochroism (light to dark brown), parallel extinction, massive. K-feldspar is subhedral to euhedral, medium-grained size (0.75-1mm). The characteristic perthitic texture is frequently visible.

Fine- to medium-grained quartz is observed in the thin section with anhedral shapes, and low relief. Some coarse crystals are also present in a cluster or showing a poikilitic relation to other crystals. Inclusion of quartz is occasionally found. Most of the amphibole crystals are subhedral, fine- to medium-grained size, high relief, two sets of good cleavage, deep-green to deep red-brown pleochroism, and show a poikilitic texture. Accordingly, such amphiboles are likely hornblende. In some cases, the deep-green hornblendes are breaking down to oxide minerals that have a red-brown color, due to the change in the oxidation state of iron from Fe²⁺ to Fe³⁺. Murphy et al. (2000) considered that is opacitization can proceed via hornblende oxidation during the growth of an extrusion dome, but, the possibility that alteration was caused by the weathering cannot be ruled out.

Picture 3.5. Microscope thin section photograph of the sample 14ZA21B, showing the euhedral to subhedral crystals of K-feldspar, plagioclase, quartz, and hornblende. (+): crossed polarized light;

(-) plane polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Hbl:

hornblende.

K-feldspar is medium- to coarse-grained, subhedral to euhedral crystal, performing low pleochroism from pale grey to deep grey of the first order of interference color.

Most of the K-feldspar exhibits cross-hatched twinning similar to microcline.

Anhedral quartz is predominantly equigranular, low relief, wavy extinction, and contains inclusion minerals of zircon and opaque. Fractured quartz crystals are observed too. Biotite is subhedral to euhedral, flaky and exhibits strong pleochroism from yellow-brown to pale brown, one set of cleavage, and parallel extinction.

Amphibole is commonly subhedral crystal in the habit and distinguished from others by birefringence color and pleochroism (deep green), high relief, two sets of cleavage.

Some crystals have interrupted cleavage. Zircon crystals have prismatic and tabular shape and pleochroic haloes when they are within biotite. This sample is more altered compared to the other samples. Biotite and feldspars are altered to chlorite, clay minerals; meanwhile, some amphibole and biotite crystals are intergrowth. The sample is characteristic of a hypidiomorphic granular texture.

Picture 3.6. The mineral assemblage of the sample 14ZA01. (+): crossed polarized light. Qz: quartz;

Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Pt: Perthite; Ms: muscovite; Hbl: hornblende;

Chl: chlorite; Opq:opaque; Zrn: zircon.

47 14ZA23

Sample 14ZA23 is porphyritic with ~45-50 vol. % phenocrysts set in a fine-grain groundmass of feldspar, quartz, and biotite. The phenocryst population consists of predominantly subhedral K-feldspar (0.8 to 1 mm long), anhedral to subhedral plagioclase having albite twins, subhedral biotite with green to brown pleochroism, and rounded quartz (<0.65 mm in length) with first order interference color. Subhedral crystals of quartz are also present. The groundmass is made up of microcrystalline quartz, plagioclase, biotite, and K-feldspar that are interstitial together growth. The phenocrysts are mainly fractured and clouded. The sample is characteristic of porphyritic texture.

Picture 3.7. Microscope thin section photograph of the sample 14ZA23, showing phenocryst of K-feldspar, quartz, and biotite. (+): crossed polarized light. Qz: quartz; Bt: biotite; Kfs: potassium

feldspar.

48 3.2. Collisional granite

14ZA12D

This sample comprises K-feldspar (40-45 vol. %), plagioclase (10-15 vol. %), quartz (20-25 vol. %), biotite (8-10 vol. %), amphibole (1-2 vol. %) and opaques (1-3 vol. %). The K-feldspar crystals are predominantly subhedral, ranging from fine to medium in grain size. They are typically associated with quartz. The subhedral to euhedral plagioclase crystals have albite twinning. The euhedral to subhedral biotite crystals are less abundant compared to other samples in this group. They have strong pleochroism of dark brown to tan color, parallel extinction, and have mineral inclusions (apatite, zircon). The quartz crystals are anhedral to subhedral, ranging from medium to coarse-grained size (0.75-1cm) with the interference color of dark grey to yellow-grey. They commonly enclose the crystals of plagioclase or are found in the form of mymerkite. Occasionally, cloudy and/or fractured crystals are observed in the thin section. The main alteration is K-feldspar converting to clay minerals.

Picture 3.8. The various crystal form of feldspar, quartz, biotite, and myrmerkite. (+): crossed polarized light, (-): plane polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium

feldspar; Myr: myrmerkite.

50 14ZA16A

Sample 14ZA16A consists of euhedral to subhedral plagioclase (15-20 vol. %), subhedral K-feldspar (35-40 vol. %), euhedral biotite (5-7 vol. %), anhedral to subhedral quartz (25-30 vol. %) with accessory minerals of opaques, hornblende, and zircon (1-3 vol. %). The plagioclase crystals typically have albite twinning with interference color ranging from grey to pale white. The K-feldspar crystals are medium- to coarse-grained size (0.5-1mm or larger) ranging from black to deep grey interference color, contain inclusions of biotite and quartz. They form as subsolidus exsolution (Picture 3.9a, cross-hatched twinning perthite) and exsolution lamellae (Picture 3.9c). The fractured crystals are often observed. The biotite crystals are coarse-grained size up to 2mm and characteristic of one set cleavage, high pleochroism of deep brown to dark green. Locally, it can be found that some crystals form bands. The quartz crystals are fine- to coarse-grained size and easily identified under the microscope by the low relief and no cleavage planes. They also occupy interstitial mineral positions or are observed as inclusions within other minerals. The hornblende crystals occur as a minor mafic mineral phase and are a typical character of two set cleavages and pleochroism of deep brown. The zircon crystals are found in the form of prism or rectangle and developed surrounding pleochroic haloes.

Picture 3.9. (a) perthite with cross-hatched twinning, (b) the lamellar twinning of plagioclase is visible in the most of crystals, (c) a cluster of biotite performing the high pleochroism associated with

exsolution lamellae of K-feldspar, (d) the anhedral crystals of quartz and myrmerkite enclose a discontinuous band of biotite. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite;

Kfs: potassium feldspar; Myr: myrmerkite; Hbl: hornblende.

52 14ZA02

Sample 14ZA02 comprises subhedral to euhedral K-feldspar (45-50 vol. %), subhedral to euhedral plagioclase (16-18 vol. %), and quartz (25-30 vol. %).

Accessory minerals are mica (biotite, muscovite), and opaques (1-2 vol. %). The medium- to coarse-grained size of K-feldspar crystals commonly occur as exsolution lamellae (Picture 3.10b, c, d), or perthite (Picture 3.10a) with interference color from grey to dark grey of the first order. Plagioclase commonly displays albite twinning and occurs as lamellae in microcline (antiperthitic texture). The quartz crystals are coarse-grained (up to 1.5 mm or larger), have deep grey to grey, and yellowish first-order interference color, low relief, no cleavage plane, and have wavy extinction.

Occasionally, wormy crystals of quartz develop within plagioclase to give a specific myrmekitic texture. Mica includes biotite and muscovite showing high birefringence from purple to greenish interference color of the third order. The anhedral mica crystals occur interstitial to quartz and microcline, whereas, inclusions of mineral appear on the surface of the feldspar crystals. Muscovite is an alteration phased that is often found associated with quartz and K-feldspar.

Picture 3.10. Main mineral assemblage of the sample 14ZA02. (+): crossed polarized light. Qz:

quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Pt: Perthite; Mus: muscovite; Myr:

myrmerkite.

54 14ZA06

Sample 14ZA06 consists of a typical assemblage of granitic rocks such as plagioclase (10-15 vol. %), K-feldspar (35-40 vol. %), biotite (15-17 vol. %) and quartz (20-25 vol. %) that are accompanied by muscovite, opaque and inclusion of minerals (1-3 vol. %). Plagioclase is subhedral to euhedral or tabular shape, occurring in grain sizes from fine to medium phenocrysts. Most of the crystals have albite- or Carlsbad-twinning, with some that have a zoning-Carlsbad twinning. Biotite is the primary mafic minerals and is subhedral, with greenish to brown pleochroism. Many tiny inclusions of biotite are found within other minerals or in the interstitial spaces.

K-feldspar is subhedral to euhedral with Carlsbad and cross-hatched twinning texture and range from black to white of interference color. Quartz is usually anhedral in crystal habit and is typically 1-1.6mm in size. Some crystals are intergrowth with plagioclase to produce mymerkite. Generally, most of the crystals are cloudy and contain inclusions of biotite, plagioclase or quartz.

Picture 3.11. Representative crystals for the chiefly mineral assemblage of the sample 14ZA06. (+):

crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Apt: anti-perthite; Mus: muscovite; Myr: myrmerkite; Hbl: hornblende.

56 14ZA25C

Sample 14ZA25C comprises of mostly medium- to coarse-grained of plagioclase (15-20 vol. %), K-feldspar (25-30 vol. %), biotite (15-17 vol. %), quartz (25-30 vol.

%), and minor zircon, apatite and opaque (1-3 vol. %). The subhedral to euhedral crystals of plagioclase have albite twinning, moderate relief, parallel or very low inclined (3-4^o) extinction and are often associated with quartz and biotite. The subhedral to euhedral biotite crystals have strong pleochroism (red-brown to blue-green) and contain inclusions of prismatic or rounded zircon. It appears that the biotite forms a banding, but some crystals are interstitial to feldspars. K-feldspar occurs as subhedral to euhedral in the habit but is volumetrically less abundant than other samples in this group. Similar to other samples, quartz is the most abundant crystalline phase of Guéra granites, occurring in both subhedral to euhedral and inclusion of crystal shape. They usually range from deep grey to pale-white in interference color, and other distinguishable properties of quartz such as low relief, wavy extinction are also observed. Many tiny crystals of inclusion within quartz and biotite are found under the microscope.

Picture 3.12. The thin section photograph of the sample 14ZA25C. (+): crossed polarized light. Qz:

quartz; Pl: plagioclase; Bt: biotite; Ap: apatite; Opq: opaque.

CHAPTER 4. METHODS

4.1. Principles of electron probe micro-analyzer (EPMA)

The first generation electron microprobe was established and reported by Castaing (1951) in his Ph.D. thesis “Application of electron probes to local chemical and crystallographic analysis.” Electron probe microanalysis (EPMA) is known as a non-destructive analytical technique that uses an electron probe (a focused electron beam) to determine the concentration of almost all of the elements of the periodic table, except for hydrogen and helium in any solid material, crystalline or amorphous. The technical, quantitative EPMA analysis has been applied in geology for chemical analysis at small scales (down to micrometer-scale) with the capabilities including point analysis, line profiles and X-ray mappings, both qualitative and quantitative, as well as the determination of the composition and thickness of thin films and multilayers.

4.1.1. X-ray generation and interaction volume

The basic principle in the generation of characteristic X-rays is based on bombarding and interaction of a focused electron beam with solid materials. The wavelength of the characteristic X-ray from an element is inversely proportional to its atomic number Z. The range of electrons and X-rays generation resulted from the interaction of the electron beam with a sample is defined as interaction or excitation volume. The interaction and excitation volume is constrained by the energy of the electron beam and the average atomic number of the sample, and their shape and size indicate the source from which analytical signals arise.

When the electron beam passes throughout the sample, its electrons hit atoms of the sample’s electrons by either elastic or inelastic collision, be contingent on the situation of energy change. In the case of inelastic collisions, the beam electrons will lose some of their energy. If the lost energy is beyond the necessary ionization energy of an element, it can remove an shell electron from the atom to create an inner-shell vacancy. Then, unstable excited atoms and higher-inner-shell electrons will transfer into the vacancy and release a characteristic X-ray photon or an Auger electron at the same time (Zhao et al., 2015).

4.1.2. Wavelength dispersion and focusing of characteristic X-ray

The EPMA system is equipped with Wavelength Dispersive Spectroscopy (WDS) detectors to detect the characteristic X-rays produced in the interaction volume. The WDS detector is made up of a gas-flow or sealed proportional counter and a few diffracting crystals, by which, diffracting crystals are used to separate wavelengths of characteristic X-rays and direct a specific X-ray wavelength to the gas-flow or sealed proportional counter for measurement. The position of the X-ray source in the sample, the surface of diffracting crystals and detector in spatial define an imaginary circle of constant diameter (Figure 4.1). Because the amount of the possible X-ray photons from the sample reaching the diffracting crystal is relatively low; therefore, the intensity of the X-ray detected by WDS is usually lower than as compared to Energy Dispersive Spectrometer (EDS) with a given beam current (Zhao et al., 2015).

Figure 4.1. A sketch illustrating the imaginary Rowland circle.

60 4.2. Source of data and analytical method

To investigate the mineralogical composition of the Neoproterozoic granites from the Guéra Massif (including 590 Ma-, 570 Ma- and 560 Ma granite), twelve samples containing biotite and/or hornblende-biotite-granite were selected for analysis. A JEOL EPMA JXA-8500F equipped with five WDSs at the EPMA laboratory in the Institute of Earth Sciences, Academia Sinica, Taipei was used. The samples were mounted in into epoxy resin and polished until exposed, then facilitated electron conductance by carbon coating (Q150TE, Quorum Technologies Ltd., UK, Picture 4.1). The target positions of minerals were determined based on the secondary- and back-scattered electron images.

The equipment operated at 16 kV voltage and 6 nA current beam with the electron beam was in diameter of 2mm. The ZAF method using the standard calibration of synthetic chemical-known standard minerals with diverse diffracting crystals used to correct the X-ray intensities, list as follows: rutile for Ti (PETJ), periclase for Mg

在文檔中 An assessment of the magmatic conditions of Late Neoproterozoic collisional and post-collisional granites from the Guéra Massif, South-Central Chad (頁 51-0)