Retrograde Assemblages in the Muscovite-Biotite Gneiss of Oluyole Southwestern Nigeria, an Indication of Shear-Zone Environment

Petrographic and whole-rock geochemical study of biotite-muscovite gneiss was determined in order to interpret the metamorphic evolution of the Basement Complex of Southwestern, Nigeria. The gneiss shows a millimetric banding, and in some cases the quartzo-feldspathic bands running up to 10 cm. The gneiss has mineral assemblage biotite + plagioclase + quartz + garnet + K-feldspar + muscovite + chlorite + ilmenite ±titanite. Chlorite occurs along cleavage planes of biotite, and in some cases forms reaction rims around porphyroblasts of garnet. K-feldspar crystals are surrounded by muscovite. Titanite crystals are sub-idioblastic to xenoblastic in form, and have inclusions of ilmenite. Titanite, where present, occurs in close association with biotite and opaque minerals (ilmenite). Also, titanite forms a reaction rim around apatite. Mylonitic texture, fine-grained matrix of mica and quartz ribbons were observed. In addition, there is stretching of the quartz crystals. The SiO2 content is greater than 60 wt %, while CaO ranges from 3.05-6.91 wt %. The M1 foliation comprise of mineral biotite some of which are included in the opaque mineral, M2 represents the metamorphism which gave rise to porphyroblasts of ilmenite, while the M3 gave rise to foliations that forms a wraparound structure on the porphyroblasts of ilmenite. The last metamorphism gave rise to retrograde minerals; chlorite, titanite, and muscovite. The study suggests that this area of the Basement Complex has been subjected to multiple deformations, as well as multiple episodes of metamorphism. The structures observed are similar to those associated with shear zone environment.


Introduction
The Basement Complex of Nigeria lies within the Neoproterozoic to Early Paleozoic, Pan-African (ca 0.6 Ga.)province east of the West African Craton.This can be classified into three major subdivisions i.e the ancient migmatite-gneiss-quartzite complex, the schist belts and the Pan-African (ca 600 Ma.) granitic series [1].Zircon age-dating from southeastern Nigeria, has made possible the documentation of multiple high-grade Precambrian metamorphic episodes of Pan-African and Eburnean ages.[2].Archaean to Palaeoproterozoic age has been ascribed to migmatite-gneiss complex [3].The migmatised gneiss of Ibadan has been reported comprise of the undifferentiated biotite-gneiss and hornblende-biotite gneiss, intercalated with amphibolite [4].The migmatite-gneiss is a group of rock reported to have formed at Ca 2.5 Ga [5], while its migmatization occurred through the Pan African orogeny.The Basement Complex of Nigeria is reported to have undergone polyphase form of deformation during the Precambrian [6][7][8][9][10][11].More than one episode of deformation has been reported for some gneisses in southwestern Nigeria [12][13][14].Retrograde processes have been reported to be associated with fault and shear-zone environment [15], while fluids have been linked to reducing mechanisms such as the disintegration of strong minerals into weaker minerals [16][17][18].The gneiss under IOP Publishing doi:10.1088/1755-1315/1342/1/012037 2 study is dissected by quartzo-feldspathic veins ranging from few centimeters to several centimeters and has been subjected to high intense folding and shearing (Figure 1).The quartzo-feldspathic veins show prominent pinkish feldspars.Even though migmatised, gneiss is the most prominent and widespread rock of the Nigerian basement [19].In this paper, we pay close attention to the textural feature of chlorite and biotite around garnet.This paper documents retrograde as well as prograde microstructures that are present in the migmatised muscovite-biotite gneiss of Ibadan southwestern Nigeria.This study will help to deduce the possible chemical reactions that have occurred and also help to write the metamorphic evolution of rocks of the Basement Complex of Ibadan area, southwestern Nigeria.

Study Location
The area of study, Olonde/Aba-nla of Oluyole lies 35.9 km south of the city of Ibadan, Southwestern Nigeria (Figure 2a), which is underlain by crystalline basement rocks of variably migmatised, undifferentiated gneiss (comprising biotite, and biotite-hornblende) with intercalated amphibolite.(Figure 2b).

Materials and Methods
The thin sections of four gneiss samples (Figure 1c-1f) obtained from two different locations were prepared and studied at the Department of Geology, Obafemi Awolowo University laboratory in detail.Also, detailed petrography was performed at the laboratory of the Department of Geosciences, University of Lagos and photomicrographs of minerals and microstructures that are of interest were taken using a Sony digital camera.The whole rock chemical analysis was conducted at the laboratory of the Nigerian Geological Survey Agency (NGSA), Kaduna.Samples were pulverized using Retsch PM 400 planetary ball mill.The ground samples were sieved using 150 micro mesh sieves.The pulverized rock samples were then packaged for XRF (X-ray fluorescence) analysis using Energy Dispersive X-ray fluorescence spectrometer of model "Epsilon 4" by PanAlytical, Netherlands.A 20g of the prepared powdered sample was weighed each into a sample cup, and these cups were meticulously positioned in their designated measurement locations on the machine's sample changer.A current of 14kv was used for the major oxides, and selected filters were "kapton".Time duration measurement of 100 sec using air as the medium was employed for each sample.

Petrography
The following mineral assemblages were observed; biotite, chlorite, muscovite, K-feldspar, quartz, titanite, ilmenite, apatite + epidote + garnet.Chlorite occur along the cleavage planes of biotite with both exhibiting a preferred alignment (Figures 3 & 3b).Also, chlorite form reaction rim around prophyroblasts of garnet (Figures 3c-3f).Biotite forms reaction rims around porphyroblasts garnet (Figure 3a), some of these garnet crystals are idioblastic and elongate parallel to the foliation defined by mica (muscovite and biotite).Greenish brown biotite forms a rap around structure on porphyroblasts of opaque minerals (Figures 4a & 4b) defining an external fabric (Se), while the porphyroblasts of opaque mineral (ilmenite) have an internal fabric (Si) defined also by biotite (Figures 4a & 4b).4c & 4d).The idioblastic to subidioblastic garnet have no inclusions of other minerals.Mica (biotite and muscovite) surround porphyroclasts of microcline.Mylonitic texture was observed, with fine-grained matrix of mica and stretching of the quartz crystals (Figure 4f).Quartz crystals were observed to exhibit undulose form of extinction.No equilibrium texture was observed between quartz crystals but rather serrated edges of quartz crystals (Figure 4f).Idioblastic crystals of apatite were observed (Figure 5).These crystals have reaction rims of titanite around them (Figures 5a-5d).Platy crystals of apatite were observed within matrix of micas (Figures 6a & 6b).The white mica (muscovite) is closely associated with K-feldspar (Figures 6c-6e) which in some cases exhibits crosshatched twinning.K-feldspar crystals were surrounded by xeonoblastic quartz grains (Figure 6e).There is reaction rim of titanite around ilmenite (Figures5e-5f, 6f & 7a-7d).There are inclusions of opaque (ilmenite) and apatite in xenoblastic crystals of titanite (Figure 7a & 7b).Epidote crystals were also observed in matrix of plagioclase feldspars (Figures 7e-7f).
The rock samples (C and D) comprise amphibolite facies mineral assemblage of plagioclase, K-feldspar, quartz, biotite, muscovite, epidote, ilmenite, titanite, and garnet.The first quarry where 'C' samples were obtained has the mineral assemblage chlorite, muscovite, plagioclase, garnet, quartz and ilmenite, while the other quarry where 'E' samples were collected has the mineral assemblage, biotite, muscovite, plagioclase, quartz, titanite, epidote and ilmenite.The chlorite occurring within the cleavages of biotite and as well as around garnet can be described as late and could have resulted from retrograde processes [15,21].Retrograde assemblages have been widely reported in the gneisses of southwestern Nigeria [15,22,23].In migmatites, retrograde chlorite has been reported to replace biotite along with some titanite, and retrograde muscovite occurs as lenses within matrix biotite [24].Retrograde chlorite is a common feature in the migmatized gneiss of southwestern Nigeria [15].The older foliation defined by biotite, may have developed at garnet-grade conditions.The dominant foliation wraps around garnet porphyroblasts and in some cases K-feldspar, and most likely crystallized during biotite-grade metamorphic conditions as presented in equation ( 1); The retrogressive chlorite and muscovite could have developed through the above retrograde hydration reactions.Biotite cleavages are a very good pathway for fluid which facilitate retrograde process as above.Titanite and K-feldspar are typically complimentary to the biotite breakdown reaction, as are chlorite and muscovite, whose compositions are directly related to biotite composition.[25].Granular anhedral titanite (Figures 5, 6f, 7a-7d), which can be considered a product of most reactions involving low-grade disintegration of biotite, being a main depository for Ti contained in biotite [25].Hydration of the prograde assemblages by an influx of H2O has been reported to produce the retrograde assemblages' chlorite-K-feldsparmuscovite-titanite [26].Titanite are common accessory minerals in the rocks of southwestern Nigeria [27][28][29].Ilmenite-titanite relationship, is a common feature in granodiorite and tonalitic rocks from southwestern Nigeria [27,28].Titanite appearing as reaction rims on ilmenite in rocks has been reported to originate from hydration reactions [30].Titanite form retrograde reaction rim on rutile and ilmenite [31].In prograde and retrograde reactions involving amphibolite-facies, titanite crystals can enlarge or recrystallize.[32].The above reactions are hydration reactions which could have resulted from the infiltration of fluids during shearing.Granules of opaque surrounded by chlorite (Figure 4c) could have resulted from the breakdown of biotite which liberates Fe for the formation of ilmenite.The result of the chemical analysis (Table 1), show that there is enough aluminum to form first, feldspar, then biotite and lastly muscovite.During the retrograde process when temperature drops, Ti is released and combines with Si to form titanite.When Ca 2+ is available from plagioclase breakdown, ilmenite is formed.The textural evidences (Figures 5, 6f, 7a-7d) suggest that titanite is of secondary origin of the breakdown of biotite; Plagioclase breaking down to give epidote, such that Ca goes into epidote and Fe goes into the formation of titanite.The presence of secondary titanite most likely indicates crystallization during isolated retrograde reactions aided by mild increase in temperature or influx of fluid [33], and is mostly caused by the degradation of biotite (Ti-bearing annite), as shown by equation (3); Plagioclase (An − rich) + Biotite => Titanite + Epidote + Chlorite + Quartz Evidence of tectonic deformation was observed on the field (Figure 3a) which showed a sheared structure.Three metamorphic events could be interpreted (M1-M3) from the microtextures.Two sets of foliations; Si and Se occur at right angles to each other.Two episodes of deformation were observed D1 and D2, resulting from M1 and M2 respectively.Si fabric encloses biotite, an indication that there was an initial fabric formed by recrystallization M1.D1 can be said to produce the internal fabric Si during M2.The external fabric Se can be said to be produced by D2 during the third episode of metamorphism (M3), thus concluding the tectono-metamorphic evolution.

Conclusion
Garnet can be said to be breaking down to lower grade minerals.The relative replacement of garnet is biotite > chlorite > muscovite > plagioclase > quartz.Mica crystals do not have bent cleavages which is suggestive of post-tectonism.There are two generations of biotite which define foliation (Si & Se).The peak mineral assemblage was garnet + chlorite + biotite + muscovite + plagioclase + quartz representing a garnet zone assemblage.All garnet crystals are retrograded either by muscovite, biotite, plagioclase, and ilmenite or chlorite.The absence of high prograde/peak assemblages in the gneiss might be as a result of prevalent deformation together with the influx of fluid.The reaction rims of titanite on apatite and ilmenite is an indication that titanite is a secondary mineral resulting from retrograde reactions.

Figure 1 :
Figure 1: a) Field photohraphs showing a sheared gneiss, b) Field photograph showing gneiss with pinkish felspar, c) Hand specimen Sample C1 d) Hand specimen Sample E2, e) Hand specimen Sample C2 f) Hand specimen Sample E4

Figure 2 :
Figure 2: a) Nigeria's Geological settings which reveals that the area under study is concealed in the southwestern PreCambrian Basement Complex [20], b) Ibadan's modified geological map from GSN 1: 250,000 series sheet 59[4]