Petrography of Allanite-bearing Tonalite from Iwo Region, Osun State, Nigeria

The rock is porphyritic in texture, with phenocrysts of K-feldspar and plagioclase in matrix of biotite, amphibole and quartz. The main minerals are microcline, biotite, plagioclase, whereas green non-pleochloric pyroxene, amphibole (green hornblende) and quartz occur as minor minerals. Chlorite and muscovite occur as secondary minerals. Accessory minerals are allanite, zircon, titanite (sphene), apatite and opaque minerals. Allanite crystals are zoned, reddish brown in colour and strongly pleochroic with euhedral and subhedral crystals (Figures 5–7). Some crystals of allanite are altered whereas others are not; those that are altered have inclusions of opaque minerals mostly concentrated at the core (Figure 5c). Biotite is brownish in colour and subhedral in form, intergrown with allanite (Figure 5a).

Figure 5

Figure 6

Figure 7

Biotite consists of other minerals such as apatite (Figure 6a). Crystals of biotite show altered surfaces and display a form of preferred alignment. There is a close association between biotite and chlorite which is in turn surrounded by titanite (Figures 8a and 8b), all of which are in the proximity of microcline (Figure 8b). The plagioclase feldspar shows sign of (i) compositional zoning and (ii) deformation (Figures 8c and 8d). Plagioclase exhibits a combination of albite and Carlsbad twinning. Crystals of biotite occur as inclusions in plagioclase. A few crystals of titanite are euhedral, but majority are subhedral/anhedral. The subhedral grains occur as aggregates and are clustered around biotite, chlorite and opaque minerals. The anhedral form of titanite forms reaction rim around opaque minerals (Figures 9–11). Titanite is concentrated in and around biotite (Figures 8a, 9 and 10). There is a close association between titanite and biotite, amphiboles, plagioclase, chlorite and opaque minerals. In some samples, the titanite crystals that form reaction rim round opaque minerals are in contact with biotite, amphibole and plagioclase (Figures 10 and 11).

Figure 8

The early formed minerals observed in the tonalite are allanite, amphiboles, plagioclase and quartz. The presence of allanite within biotite and hornblende can be attributed to early crystallisation. Amphibole (hornblende) is not present in all of the samples studied. Primary allanite has inclusions of ilmenite and is included in biotite as observed in the rock samples [10]. Primary allanite has been found in andesitic [11] and tonalitic [12] igneous rocks. Allanite is a high-Th-REE mineral of the epidote group and has been used in recent times in Th–U–Pb geochronology [9, 13,14,15,16,17,18].

The accessory mineral allanite is highly prone to alteration as a result of its metamict ability and there are inclusions of opaque minerals within the altered region of the crystals (Figure 5c). The biotite crystals surrounding allanite have radiation haloes that are darker at the contact edges closer to allanite (Figures 5a and 6a). This complex radioactive rim is an evidence of post-crystallisation process [19]. Allanite is one of the most common sources of primary uranium-bearing minerals [19]. In addition to the radioactive rim, there are numerous fractures on the altered allanite (Figure 6a) which can be attributed to the radioactive process. The degree of metamictisation of allanite depends on the degree of alpha-recoil damage [20]. This metamictisation coupled with hydrothermal alteration results in a variety of alteration products that tend to immobilise the REE and Th⁴⁺ in more stable secondary phases [21]. These formed phases has been found to be dependent on the physiochemical condition of the fluid which is present. Allanite and titanite have the potential of controlling REE in granitic magmas [5]. Titanite observed are of both primary and secondary types: the primary types are subhedral to eudedral in shape with some occurring as inclusions in biotite and amphiboles. The titanite-opaque mineral phase observed is an indication of the breaking down of titanite to opaque mineral, a common phenomenon in granitoids indicating a late fluid activity [22]. Results from experiments suggest that titanite forms as an early phase during crystallisation of magma [23, 24]. Secondary titanite observed is anhedral and occurs as reaction rim round opaque minerals (Figures 9–11), and this reaction can be supported by the equation given below: Plagioclase 1 (Ca-rich)+Ilmenite=>Titanite+Plagioclase 2 (Na-rich)\matrix{{{\rm{Plagioclase}}\,1\,({\rm{Ca}{\rm{ - }}{rich}}) + {\rm{Ilmenite}} = > } \hfill \cr {{\rm{Titanite}} + {\rm Plagioclase}\,2\,({\rm Na}{\rm{ - }}{\rm rich})} \hfill \cr }

Figure 9

Figure 10

Figure 11

The titanite grains observed show no visible form of zoning. The secondary titanite has been found to be associated with sericitisation of plagioclase and chloritization of biotite as observed in the tonalite [25]. These set of reactions represent retrogressive events probably prompted by moderate heating and fluid activity [22, 26] and can be represented by an equation such as: Plagioclase 1 (Ca-rich)+Biotite (Ti-rich)=>Titanite+Chlorite+Plagioclase 2 (Na-rich)\matrix{{{\rm{Plagioclase}}\,1\,({\rm{Ca}} {\rm{ - }} {\rm{rich}}) + {\rm{Biotite}}\,{\rm{(Ti}} {\rm{ - }} {\rm{rich)}} = > } \hfill \cr {{\rm{Titanite + Chlorite}} + {\rm Plagioclase}\,2\,({\rm{ Na }} {\rm{ - }} {\rm{ rich }})} \hfill \cr }

This type of alteration reaction liberates titanium that is locked within biotite [19]. In this regard, titanite can be said to be replacing early biotite and coeval with the chlorite formation (Figure 8a). The opaque mineral is likely to be ilmenite, because ilmenite is an opaque mineral with titanium in its structure. The accessory mineral assemblages such as allanite and titanite have been found to be sensitive to parameters such as temperature, oxygen fugacity, melt composition, as well as the dissolution history of evolving magma [27, 28]. The apatite which occur as inclusions in biotite can be said to have formed very early during the magmatic process because of the euhedral nature of crystals and occurrence as inclusions in biotite (Figure 6a) and, thus, can be regarded as primary igneous mineral. Plagioclase crystals observed have some of the following features: Carlsbad/albite twinning sericitisation, compositional zoning and deformed twin lamellae (Figures 8c and 8d). The sericitisation can be supported by the reaction involving plagioclase, biotite, opaque (ilmenite), muscovite, titanite and quartz [22]. Zoned plagioclase could have resulted from rapidly cooled magma leading to disequilibrium growth occurring during large undercooling. Compositional zoning in crystals is a tool in determining the magmatic history of plutonic rocks [29, 30]. The foliation observed on the outcrop and the deformation of the twin lamellae in the plagioclase is an indication that the rock has been subjected to tectonic activity. Twin lamellae of plagioclase with bent surfaces are common, an indication of syn-magmatic or post-magmatic crystallisation feature.