Friday, 20 December 2013

Rarities: xenoliths on Idanre Batholith, SW Nigeria

Idanre_olokuta rocks
Wikkipedia defines xenolith as “a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption.” To be considered a true xenolith, the included rock must be identifiably different from the rock in which it is enveloped.
In other words, let’s take for instance, you find yourself exploring an outcrop of granite intrusion, and you suddenly stumble on a distinct and well defined chunk of metamorphic rock, that metamorphic rock can be called a xenolith; and it also means that the metamorphic rock pre-exists (is older than) the igneous intrusion itself. To explain this better, now, let’s go back in time….like rewinding to the farthest beginning….
Let’s assume that loose sediments, volcano-sedimentary rocks etc. were poured into a basin area, after which a metamorphic episode occurs, resulting in the folding and modification of the previously deposited rocks, thereby forming some new type of rocks which we call metamorphic rocks. We can name these newly formed rocks “Rock-A”.
Ok. Sometimes later, activities gets serious in the earth’s mantle and magma wells up from the asthenosphere and begins to intrude joints and faults in the subsurface, and as the magma rises through the previously metamorphosed volume (Rock-A), it chops off chunks and fragments of the rock and carries it along as it rises towards the earth’s surface. Often times, these ‘chunks’ gets partially modified within the magma as a result of the influence of the heat and circulating hydrothermal fluid in the magma. This often initiates chemical reactions between the magma and the ‘chunks’. This process of modification experienced by the ‘chunk’ is called digestion. So, in a case where by the ‘chunk’ is totally modified, such that its mineral contents have been pulled out into the body of the molten magma and the chunk seizes to exist, it is referred to has complete digestion; but in a case whereby only a part of the ‘chunk’ is modified, while the other parts of the chunk still bear distinct and identifiable properties of the original “Rock-A”, we say the ‘chunk’ was partially digested. The uprising magma may eventually cool down and solidify before reaching the earth surface, forming granites, which we can then refer to as ‘Rock-B’. These granite rocks, which are often referred to as intrusions, are later exposed to the earth surface as a result of erosion. The embedded ‘chunk’, when partially or not digested, found on an exposed outcrop of granitic intrusion, is referred to it as a Xenolith.
Now let’s go to Idanre Hills, Southwest Nigeria, and see what I found….
Idanre town is an ancient sleepy town located on a massive batholith which covers an area of about 450km2 in Ondo State, SW Nigeria. It has been dissected by numerous fractures into a broad cluster of steep-sided and dome-shaped inslbergs which is popularly known as Idanre hills. I had been to this awesome location countless times and I keep discovering new things on each of the trips. Anyways, let’s focus on our topic for the day. I forgot to mention that the highest point in SW Nigeria, called Orosun Peak (1,052m), is located on Idanre hills., below is a map of Nigeria (with an inset of Africa) showing the location of Idanre, a satellite imagery of Idanre Batholith and a generalised geologic map of the area.
Nigeria map_olokuta rocks
- Map of Nigeria showing the location of Idanre (Inset: Map of Africa)

Idanre sat image_olokuta rocks
- Satellite image of Idanre area (7° 6'37"N, 5° 6' 52"E). Source: Google Earth 2003

idanre geo map_olokuta rocks  - Sketch of simplified geology of Idanre area.

Now, let’s quickly analyze the geologic map above:
The oldest rock unit here is the ‘Migmatite Gneiss & Quartzite’ (this unit also includes Schists)…migmatite gneisses and quartzites are metamorphic rocks. Generally, gneiss is formed from the metamorphosed granite, while quatrzite forms from the metamorphism of sandstone; Schist form from the metamorphism of shale and metabasalts; migmatite is a mixture of different types of metamorphic and igneous rocks. In order words, our country rock is the Migmatite Gneiss & Quartzite unit. The country rock was later intruded by a large volume of up-rising magma which solidifies in the subsurface to form the granitic Batholith which we call ‘Idanre Batholith’. The charnockite body in the map is just a type of intrusive igneous rock that later intruded the granite batholith itself.
By the way, a batholith is simply “a large emplacement of igneous intrusive (also called plutonic) rock that forms from cooled magma deep in the Earth's crust”.

So, on one of my trips to Idanre Hills, at a height of about 450m up the hills, I found a big xenolith of schist rock on the vertical wall of a narrow valley (fracture) on the granite. Thanks to the fracture that exposed the xenolith. The whole stretch of the xenolith was sandwiched within the granite. The features of the schist was quite distinct and unmistakable.
xenolith1_olokuta rocks - Red dotted line showing the boundaries of the schist xenolith, sandwiched within the granite… Thanks to the fracture that exposed the xenolith.
xenolith2_olokuta rocks  - A closer view of the xenolith.

Idanre sat2_olokuta rocks
- Satellite image showing the point where the xenolith of schist was found.

A closer look at the xenolith reveals parallel trending quartz veins, set in the dark background of biotite, hornblende etc. matrix. The platy mica minerals (biotite) are easily identifiable with the naked eyes. The foliation of the schist (which controls the alignment of the quartz veins) lies parallel to the trend of the sandwiched body. Also, the relatively higher susceptibility of the xenolithic body to weathering that its enclosing granitic rock shows its distinct mineralogy. The higher susceptibility to weathering is apparently because of its abundant constituent mica minerals, which naturally breaks down easily when attacked by water.
An observation of the extents of the xenolith around this area revealed that it is wedge-shaped. This ‘chunk’ of old rock was chopped off the country rock during the upward rise of this batholith, but luckily, it wasn’t completely digested before the magma finally cooled and solidified. Therefore, the ‘rock-A’ in this case is the schist, while ‘rock-B’ is the granite. Above is a satellite image showing the point where the xenolith was found.

Then, down the hills, at the floor of a deep valley (deep fracture) on the batholith, see what we found again:
xenolith3_olokuta rocks - A body of migmatite gneiss found within the granite batholith.
idanre sat3_olokuta rocks - Satellite image showing the point where the xenolith of migmatite gneiss was found.
The body of migmatite gneiss lying along the road at the bottom of the valley, surrounded by granitic rocks, was such an interesting find; and it bears similar composition and deformation characteristics as the migmatites found just outside the batholith.
idanre sat4_olokuta rocks  - Satellite image showing relative positions of the two xenoliths.
Generally, xenoliths are important features because they provide information about the composition of the otherwise inaccessible mantle. This applies to xenoliths that were chopped off the walls of the upper mantle during the rise of the magma. However, for those xenoliths as the ones featured in this simple study, they show the relationship between the intruding magma and the country rock which was intruded as well as the condition of the environment at the time of intrusion.
- Anifowose, A.Y.B. and Kolawole, F. (2012). Emplacement Tectonics of the Idanre Batholith, South-western Nigeria. Comunicações Geológicas, 99, 2, pp 13-18.
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