Rarities: xenoliths on Idanre Batholith, SW Nigeria

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.

Ok..so, 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.

	- Map of Nigeria showing the location of Idanre (Inset: Map of Africa) - Satellite image of Idanre area (7° 6'37"N, 5° 6' 52"E). Source: Google Earth 2003

  - 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.

- Red dotted line showing the boundaries of the schist xenolith, sandwiched within the granite… Thanks to the fracture that exposed the xenolith.

  - A closer view of the xenolith.

- 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:

- A body of migmatite gneiss found within the granite batholith.

- 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.

  - 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.

Sources:
- Anifowose, A.Y.B. and Kolawole, F. (2012). Emplacement Tectonics of the Idanre Batholith, South-western Nigeria. Comunicações Geológicas, 99, 2, pp 13-18.
Download link- http://www.lneg.pt/download/6009/2_ART_CG11-036-1309_FINAL_A.pdf

- www.wikipedia.org

A Voxler beyond A Surfer

 

 

Here comes another great three-dimensional visualization software from the stables of Golden Software Inc. Overtime, Surfer had taken the prominent place in the domain of ‘basic/simple’ three-dimensional visualization of geophysical or geospatial data; but Voxler is here to put spice up our plots with its its amazing speedy creation of stunning models that visualize the relationships across your data set. This robust, yet user-friendly program gives you the power to display your data in a variety of formats and colours, capture video animation of your moving model, and select from several image and data export options.

One cool feature I like about Voxler is that it suspends layers of imported datasets on one another, while Surfer only merges the layers on one another. The suspension capability allows for independent as well as relative observation of the plots.

Voxler can plot datasets in variety of ways which include 3D-surface maps (height field), contour maps, point maps (scatter-plots) etc. It can also work with datasets of a wide variety of formats, although there is limitation to the type of graphic output obtainable with different data formats; for example you can not plot a surface map (height-field) with a .dat dataset but you can plot surface maps and many more types of graphic output with a .grd dataset.

Try out the wonders of Voxler and you’ll get hooked as I’ve been :)

- Contour map suspended above surface map, allowing for both independent and relative observation of datasets.

- 3D observation of the relationship between datasets (surface maps only).

 

- Observation of the relationship between datasets (Surface map and scatter points).

The fear of asking questions.

 
One of the many challenges preventing the bulk of Nigerian geoscience students from understanding geoscientific concepts is their inability to ask questions during classes.

Students are unable to ask questions in class for various reasons. These reasons  include the fear of coming across as arrogant to the lecturer and risk being tagged an "I too know (I.T.K)", as well as the fear of being perceived as dull by other students in the class who are not asking questions. In some cases, the students are just simply too confused about the topic being taught to even know which question to ask.

Being unable to ask questions during class is a challenge I personally battled with for the major part of my undergraduate education. For me, it was the fear of coming off as a proud or arrogant student to the lecturer. This fear started from before I gained admission into the university. I had heard several stories of some lecturers in Nigerian universities who would get extremely embarrassed and offended when their students ask them questions they could not answer, and may eventually regard the inquisitive student as "an enemy". Such stories have probably contributed substantially to the fear of asking questions in class by many students and consequently led to failure of many students in Nigerian universities.

However, in order to weather the challenge, I spent extra time in the library and browsing the  internet in order to learn more and find answers to the many questions I had on the topics I was taught in class. Finally, in my final year, assured that I had built my grade point to a level that no lecturer could pull it down below "comfort zone", I became bolder and began to ask my lecturers question in class.

Surprisingly, it produced amazing results. I began to learn more and understand better, as my questions turned the lecture classes into interesting discussions. I could remember one of the classes in which the lecturer gave some tips beyond the scope of the lecture at the time, which broadened my understanding of the tectonic evolution of the African plate. These tips were never stated in class notes, yet they remained entrenched in my mind up to this day.

As opined by Blosser (1990), if one of the objectives of scientific knowledge impartation is the development of higher level thinking processes in students, how else can this goal be achieved without intensive communication between the teachers and students involved? And how better can this communication grow between a teacher and a student without at least one of the two parties asking relevant questions?

Arthurs (2011) highlighted the propensity of students to develop alternate conceptions, misconceptions and even their own personalised but incorrect cognitive models of geoscience concepts. This will definitely be typical of a teacher-dominated geoscience class.

Therefore, if Nigerian geoscience teachers expect their students to develop into independent scientific thinkers and future problem solvers, they must begin to provide opportunities during their lectures that allow for greater student involvement and less teacher domination of the learning process. With this in place, it'll be easier for them to identify the various misconceptions and difficulties their students are having with understanding the topics and concepts being taught in class.

References:
1. Arthurs, L. (2011). What college-level students think: Student alternate conceptions and their cognitive models of geoscience concepts. Geol. Soc. of America Special Papers, v.474. Pp.135-152.

2. Blosser, E. P. (1990). Using questions in science classrooms. Research Matters - to the science teacher. NARST. No.9001.

A threatening earthquake in SW Nigeria?

The increase in reports of tremors in the south-west Nigerian sub-region within the past two-three decades has become disturbing; and several research works (Elueze, 2003, Odeyemi, 2006, Anifowose et al., 2006, Kolawole & Anifowose, 2011) had referred to the Ifewara-Zungeru mega-structure and its connecting Southern-Atlantic transform faults as the major sources.

Below is a historical record of earth tremors in Nigeria (Akpan & Yakubu, 2010):

 

However, these questions may arise:

Considering the spatial coverage of the effects of the tremors, is it possible that these tremors were produced by more than a single fault (Ifewara Fault)? or are there possibly more active faults in the region?

 

I will like to know your thoughts on this issue. Kindly drop your opinions in the comment box.

   

References:

1. Akpan, O. U. and Yakubu, T. A. (2010). A review of earthquake occurrences and observations in Nigeria. Earthq Sci. (2010) 23. Pp. 289-294.

2. Anifowose, A. Y. B., Odeyemi, I. B. and Borode, A. M. (2006). The tectonic significance of the Ifewara-Zungeru megastructure in Nigeria. In: Teme S C and Ezeigbo C U. Proceedings of the 1st International Workshop on Geodesy and Geodynamics. Centre for Geodesy and Geodynamics, Toro, Nigeria. Pp. 17–28.

3. Elueze, A. A. (2003). Evaluation of the 7 March 2000 earth tremor in Ibadan area, southwestern Nigeria. Jour Min Geol 39(2). Pp. 79–83.

4. Kolawole, F. and Anifowose, A.Y.B. (2011). Remote sensing analysis of a dextral discontinuity along Ifewara-Zungeru area, Nigeria, West Africa. Ind. Jour. of Sci. & Tech. Vol. 4 No. 1. Pp. 46-51.

5. Odeyemi, I. B. (2006). The Ifewara fault in southwestern Nigeria: Its relationship with fracture zones along the Nigerian coast. Centre for Geodesy and Geodynamics, Toro, Bauchi State. Pp. 1–13.