**4. Petrography**

This section contains the detailed petrographical characteristics of different rock types which are described as follows:

#### **4.1 Extrusive phase**

#### *4.1.1 Basalt*

Basalt reveals textures that are ophitic (**Figure 4A**) and sub-ophitic (**Figure 4B**). Plagioclase labradorite (feldspar) and augite (clinopyroxene) are the dominant in basalt flows (**Figure 4A**). As accessories minerals, fine grained quartz, hematite and magnetite are found in the groundmass. The lath-shaped crystal (plagioclase) takes place as a phenocryst and also occurs in ground mass (**Figure 4B**). In basalt, large vesicles (4–6 mm) are often packed with secondary minerals such as quartz and calcite. Labradorite is in shape from euhedral to subhedral and finely grained. It shows lamellar twinning and is changed to kaolin. Often it gives a cloudy appearance due to alteration. Augite has a medium grained and pale brown and grey color.

#### *4.1.2 Trachyte*

Trachyte illustrates the porphyritic texture (**Figure 4C**). Trachytic texture is often the directive flow and parallelism of elongated crystals (**Figure 4D**). Trachyte's petrographic properties are somewhat similar to rhyolite, with relatively less quartz and more ferromagnesian minerals viz. riebeckite, magnetite, arfvedsonite and hematite. It consists of quartz, orthoclase and riebeckite phenocrysts as important minerals in the groundmass of quartzofeldspathic. Orthoclase occurs in quartzofeldspathic groundmass as euhedral crystals. This illustrates the twinning and kaolin alteration of Carlsbad. The orthoclase phenocrysts are fractured and often filled with crystals of riebeckite. Quartz, with embayed margin and fractured shapes, happens as euhedral to subhedral crystals. The quartz veins cut the ground-mass quartzofeldspathic. The riebeckite is fine grained, blue in color, needle form and embedded in the ground mass.

#### *4.1.3 Rhyolite*

When viewed under a microscope, Rhyolite displays flow bands, porphyritic, aphyritic, spherulitic (radiating growth of feldspar and quartz from a common center) (**Figure 4F**) and perlitic textures. The rhyolite consists of orthoclase, quartz, and arfvedsonite phenocrysts as important minerals in the ground mass quartzofeldspathic. In some samples, high temperature alkali feldspar, i.e. euhedral

#### **Figure 4.**

*(A). Labradorite and augite are present in basalt and shows ophitic texture; XPL. (B). Labradorite and augite are showing sub-ophitic texture in basalt; XPL. (C). Quartz, orthoclase which is showing Carlsbad twinning in trachyte; XPL. (D). Trachyte shows cryptocystalline nature of flow direction; PPL. (E). Quartz, orthoclase occur in a matrix of fine grained quartz and feldspar of rhyolite, which shows a little flow structures; PPL. (F). Quartz, orthoclase, aniegmatite shows equigranular texture by granite; XPL.*

sanidine crystals, are also found. Microcrystalline aggregates of quartz, alkali feldspar, blue amphibole (riebeckite), pyroxene (light green aegirine), blood red aenigmatite, magnetite, and hematite are de-vitrified to the ground mass. The fine quartz-feldspar ground mass reflects the lava flow directions.

Phenocrysts of quartz occur in different ways, i.e. drop like, fractured and embayed. The orthoclase and ground mass are cut by the quartz veins and parallel to the lava flow path (**Figure 4E**). Alkali feldspar phenocrysts illustrate twinning in Carlsbad. Altered and fractured orthoclase is also found at a few stages. Arfvedsonite's fine crystals are correlated with alkali feldspar. Bluish green arfvedsonite occurs with pleochroic (X = dark bluish green, Z = yellowish green) as fine to medium grained prismatic crystals.

The spheroidal rhyolite displays layers of mafic (dark/light brown) and felsic (light grey), reflecting the texture of the flow. Hematite, magnetite and arfvedsonite are formed of the mafic layer and they are fine grained. The felsic layer is composed of quartz, perthite, and orthoclase and medium-grained. Medium

*Petrology and Geochemistry of Nakora Ring Complex with Emphasis on Tectonics… DOI: http://dx.doi.org/10.5772/intechopen.98609*

grained, lath/prismatic shaped, colorless and showing Carlsbad twinning are the orthoclase crystals. Orthoclase is scattered, sericitised spontaneously and combined with quartz intergrowth. Fine quartz veins are cut by the sericitised feldspars and display high order interference colour.
