Macroscopic description

The fine-grained granite outcrop has the geographical coordinates N10˚00, E19˚23 with an altitude of 831 m south of Lake Iro (Massadjanga village). The structure is micro-grained to fine-grained but has two facies. Sample 16IRO 02. The dominant minerals are: quartz, biotite, orthoclase and feldspar. Structurally the directions observed on the fractures are N53˚ 85NW and N60˚ 85NW ( Figure 4(a) ).

Microscopic description Fine-grained granite has a microgranular texture. Mineralogically, the rock is composed of quartz, plagioclase, alkali feldspar, and biotite ( Figure 4(b) ) and is also composed of accessory minerals such as zircon, sphene and oxides. Alkali feldspars (35% - 45%) Alkali feldspars are composed of microcline and orthoclase. Microcline is easily identified by the pericline twin on its edges while orthoclase, in elongated sections, is recognized by the Carlsbad twin. Orthoclase also occurs in the form of automorphic to subautomorphic areas of approximately a few millimeters in size containing opaque inclusions. Quartz (20% - 25%) occurs in the form of subautomorphic to xenomorphic crystals of medium size. Some quartz crystals are in interstitial form but others occur as quartz aggregates most often found on the edge of feldspars. Plagioclase (15% - 20%) is automorphic to subautomorphic, colorless and cloudy in appearance due to the phenomenon of alteration. Plagioclase crystals have Carlsbad twins. Their size is medium containing opaque inclusions. Biotite (5% - 10%) occurs in the form of flakes with a fine and regular cleavage. The color of biotite is yellowish-brown. Biotite has a size that varies between 0.15 to 0.30 mm and has inclusions of opaque minerals, apatite and zircon as well as some biotite flakes that alter into chlorite.

Macroscopic description at the outcrop, coarse-grained granites occur in the form of balls, slabs and granitic chaos to the NW of Lac Iro (Karou village) with coordinates N10˚12, E19˚19. It should be remembered that these rocks contain basic enclaves, quartz veins and pegmatites in places ( Figure 5(a) ). They are yellowish-gray in color and sometimes altered with a grainy texture whose observable mineral size is greater than 5 mm. Coarse-grained granites are composed in particular of quartz, alkali feldspar, plagioclase and biotite ( Figure 5(b) ).

Microscopic description Coarse-grained granites have a grainy texture. They are composed mainly of plagioclase, quartz, alkali feldspar, biotite, sphene; opaque, and apatite.

Plagioclase (25% - 30%) Plagioclase is Carlsbad twinned, polysynthetic and of low relief. This mineral shows overall subautomorphic to xenomorphic crystals of size varying between 0.1 mm to 0.5 mm. It occurs in the form of elongated areas showing inclusions of apatite and opaque.

Quartz (20% - 25%) occurs as xenomorphic crystals slightly larger than plagioclase. Some are found as fine quartz grains in the interstices of orthoclase, plagioclase and biotite sections. Alkali feldspar (10% - 20%) identified is orthoclase. Orthoclase occurs as medium-sized, automorphic to subautomorphic crystals. It is Carlsbad twinned and cloudy in appearance. Some crystals are perthitic and show cracks filled with fine quartz grains but apatite and opaques are found as inclusions. Biotite (15% - 10%) occurs as medium-sized, yellowish-brown flakes. Cleavages are fine and regular. Opaque minerals are arranged along the cleavages. Biotite contains zircon as inclusion and alters to chlorite. Apatite occurs as clear elongated sections or as medium-sized stocky sections. It occurs as inclusion in alkali feldspar, plagioclase and biotite crystals.

Opaque minerals appear as brown-colored automorphic crystals abundant in the rock. These crystals are inclusions in plagioclase, feldspar and biotite crystals.

Macroscopic description. Biotite granites are described in the village of Karou located in the NW of the lake, N10˚12/E19˚19 and in Massadjanga located in the South of the lake, N10˚00/E19˚23. They outcrop in slabs, granitic chaos and metric to decametric balls, but generally in the form of large massifs dominating the edges of Lake Iro. These are rocks with a grainy to micrograined texture, sometimes with a porphyritic tendency. They are generally composed of plagioclase, alkali feldspars, quartz, and biotite. At the outcrop, these granites have a gray to brown color with a dark gray weathering patina and are crossed by quartz-feldspathic veins. Aplites are also highlighted in these biotite granites ( Figure 6(a) ).

Microscopic description. The texture of biotite granites is granular to microgranular porphyritic with jointed crystals. They have plagioclase phenocrysts of size that varies between 0.03 mm and 1.5 mm. Their mineralogical composition is mainly quartz, plagioclase, alkali feldspar, biotite and muscovite ( Figure 6(b) ). Accessory minerals are opaque, sphene and apatite as well as secondary minerals such as chlorite.

Alkali feldspars (35% - 45%) are essentially composed of microcline and orthoclase, the orthoclase of which is in automorphic to xenomorphic sections of varying size with Carlsbad twins. Quartz (25% - 30%) is in the form of xenomorphic crystals. The crystals are of medium size. It sometimes occurs in aggregates of contiguous crystals arranged in the interstices left by the alkali feldspar and plagioclase.

Biotite (10% - 15%) occurs in subautomorphic flakes, striated by very fine and regular cleavage with an average size and yellowish-brown color. The biotite sections are oriented in the rock. It is in inclusions in the large crystals of alkali feldspars. Biotite is destabilized and altered into chlorite. Plagioclase (5% - 10%) occurs in the form of large, medium-sized, automorphic to subautomorphic crystals. It presents the twinning of polysynthetic albite. Some crystals have quartz grains in contact with alkali feldspar. Plagioclase contains opaque inclusions and alters into epidote. Sphene is in small, corroded diamond-shaped sections. It is attached to biotite flakes. Apatite occurs in the form of small rods scattered abundantly in the rock. Opaque minerals appear in the form of automorphic to xenomorphic dark brown.

Geochemical study of the main granitoids with the aim of comparing to groups of known rocks. Representative rock samples were analyzed (major elements, traces and rare earths) at the Petrographic Research Center and Geochemical (CRPG) of Nancy/France. In total, 12 samples of granitoids including 06 to Massadjanga (Iro 04, Iro 08, Iro 09, Iro 15, Iro 17 and Iro 18) and 06 in Karou (A1, A2, A3, A4, A5 and A6) with respectively a Loss on Fire (PF) of 0.6% to 0.5% were analyzed ( Table 1 ).

The Na₂O + K₂O vs SiO₂ diagram of [30] adapted to plutonic rocks ( Figure 7 ) reveals at first glance that all granitoids are acidic with one exception (one sample falls in the Intermediate domain) and that they are located in the field of sub-alkaline rocks: these are subalkaline acid rocks.

In the K₂O vs SiO₂ diagram ( Figure 8 ). All samples have behavior of high K and plot to the cal-alkaline field characteristic of the subduction zone.

In the A/CNK (mol.) vs SiO₂ diagram, the Karou and Massadjanga granites with an A/CNK ratio > 1 are hyperaluminous ( Figure 9 ). These granitoids are of type S, i.e. they are derived from magma originating from the fusion of igneous rocks [33] .

Massadjanga granites

Chemical analysis shows that the Massadjanga granites show a high content of SiO₂ (75.35%), Al₂O₃ (15.09%), Fe₂O₃ (9.44%), K₂O (6.39%), Na₂O (3.83%), CaO (3.52%), MgO (1.62%) and TiO₂ (1.49%) but they have a low content of MnO (0.16%) and P₂O₅ (0.64%).

Karou granites

The Karou granites have a high content of SiO₂ (77.04%), Al₂O₃ (13.23%), Fe₂O₃ (5.66%), K₂O (5.35%), Na₂O (3.30%), CaO (2.18%) but a low MnO content (0.08%), MgO (0.65%), TiO₂ (0.76%) and P₂O₅ (0.32%).

The high contents of SiO₂ and alkaline (Na₂O + K₂O = 6.55% - 9.55%) show a significant fractionation of plagioclase and alkali feldspar. Fe₂O₃, MgO and TiO₂ fractionate in biotite, TiO₂ and CaO fractionate in apatite P₂O₅ and Fe₂O₃ fractionate in opaque minerals as well.

Hacker diagrams Al₂O₃, CaO, MnO, Na₂O, K₂O vs SiO₂ identify two groups of rocks: the group of Massadjanga microgranites and Karou granites. The P₂O₅, TiO₂ and MgO diagrams show a single group of rocks composed of microgranites from Massadjanga and granites from Karou. The only Fe₂O₃ diagram shows also a group made up of Massadjanga granites and Karou granites with a tendency negative development. The set of diagrams defines a negative trend as shown in Figure 10 .

The description of trace elements takes into account Zr, Y, Rb, Cr, Sr and Zn ( Table 2 ).

The Masssadjanga granites are rich in Zr (271 ppm), Cr (180 ppm), Zn (137 ppm), Y (110 ppm), Rb (85 ppm) and Sr (81 ppm). The Karou granites are characterized by their richness in Zr (270 ppm), Cr (180 ppm), Zn (136 ppm), Y (128 ppm), Rb (88 ppm) and Sr (79 ppm).

The Hacker diagrams (Cr, Zr, Sr, Rb and Y) vs SiO₂ do not show two groups of rocks the Karou series and the Massadjanga series. Zr, and Zn, have a profile of almost identical evolution with two negative sloping parts. The Y, Cr and Sr have a only part with negative slope. The Rb presents a part with a positive trend. The behavior of Sr shows its substitution by Ca in plagioclase and alkali feldspar. It is incorporated in apatite and sphenes. That of Rb is incompatible with the fractionation of plagioclase and biotite.

Normalization of trace elements

The trace elements of the Massadjanga granites are standardized to the Primitive Mantle considered magma source in the logarithmic diagram.

Rare earths

Massadjanga granites

In the rock/primitive mantle diagram, the alkaline granites present a spectrum of rare earths marked by a positive anomaly in Yb, and a negative anomaly in Eu and an appearance decreasing from La to Sm and from Gd to Lu.

The rare earth spectra of the Massadjanga granites show a sloping appearance decreasing from light rare earths to heavy rare earths via intermediaries. They present negative anomalies in Europium (Eu) and one positive anomaly in Ytterbium (Yb) ( Figure 11 ).

The Massadjanga granites present a profile reflecting a rate of fractionation weak. The sum of the rare earth contents of these rocks is (REE = 156 ppm). Quite different by their rare earth content, these rocks present a small negative anomaly in Eu which is confirmed by the ratio [Eu/Eu* = EuN/((SmN) × (GdN)) 1/2], Eu/Eu* = 0.484 ppm. This brings us to consider that the evolution between these rocks would be controlled by the fractionation of zircon, apatite, sphene and plagioclase. The ratio (Sm/Nd)N = 0.881 ppm is less than unity while that of (La/Lu)N = 2.16 is greater than unity.

Rock normalization to MORB

The Massadjanga granites display spectra marked by positive anomalies in K₂O, Rb, Ba, Th, La, Ce, Sm, Hf, y and Yb, and negative anomalies in Sr, Nb, P₂O₅ and TiO₂ ( Figure 12 ).

The positive anomaly in K₂O, Rb and Ba reflects the enrichment of biotite granites in these elements. The negative anomaly in P₂O₅ shows the fractionation of apatite and that of TiO₂ reveals the fractionation of ferrotitanium oxides (TiO₂).

The positive anomaly in Rb suggests substitution potassium in alkali feldspar. The slight positive anomaly in Zr highlights the possible substitution of Zr, Ti in accessory minerals such as zircon.

The positive Yb anomaly reflects the enrichment of the rocks in zircon.

Negative anomalies in (Sr, Nb, P₂O₅ and TiO₂) and positive in (K₂O, Rb and Ba) show that granitoids have for the source the continental crust. The negative anomaly in Ta and TiO₂ shows that the granites of Massadjanga were set up in a subduction environment.

Karou granites

In the rock/primitive mantle diagram, the alkaline granites present a spectrum of rare earths marked by a positive anomaly in Yb, and a negative anomaly in Eu and an appearance decreasing from La to Sm and Gd to Tm.

( Figure 13 ) Rare Earth spectra of Karou granite from the Lake Iro massif normalized by relation to the Primitive Mantle of [34] .

The rare earth spectra of the Karou granites show a sloping appearance decreasing from light rare earths to heavy rare earths via intermediaries. They present negative anomalies in Europium (Eu) and one positive anomaly in Ytterbium Yb.

The Karou granites have a profile reflecting a low fractionation rate. The sums of rare earth contents of these rocks are respectively (REE = 156 and 239 ppm).

These rocks present a negative anomaly in Eu which is confirmed by the ratio [Eu/Eu* = EuN/((SmN) × (GdN)) 1/2], Eu/Eu* = 0.484 ppm and a positive Yb anomaly. This brings us to consider that the evolution between these rocks would be controlled by the fractionation of zircon, apatite, sphene and plagioclase.

Rare Earth profile shows weak fractionation in the Karou granites: (La/Yb)N = 2.13. In this profile, we note that the Karou granites have a ratio (Gd/Yb)N = 1.123, this value greater than unity suggests that these rocks are highly fractionated. The ratio (Sm/Nd)N = 0.881 is less than unity while that of (La/Lu)N = 2.16 is greater than unity.

Normalization of phased arrays

Rock normalization to MORB

The Karou granites display spectra showing positive Rb anomalies, and Th, and negative anomalies in Ba, Cs, P₂O₅, TiO₂ and Sc ( Figure 14 ).

Normalization of phased arrays

The positive anomaly in Rb and Th reflects the enrichment of the Karou granites in these elements. The negative anomaly in Cs shows the fractionation of apatite and that of TiO₂ reveals the fractionation of ferrotitanium oxides.

The positive anomaly in Rb suggests the substitution of potassium in the alkali feldspar. The slight positive anomaly in Th highlights the possible substitution of Th and Ba in accessory minerals such as zircon. Negative anomalies in Ba, Cs, P₂O₅, TiO₂ and Sc and positive in Rb and Th show that granitoids have a source in the continental crust. The negative anomaly in Cs and TiO₂ shows that the Karou granites were emplaced in a subduction environment. According to [36] , the TiO₂ contents >0.20 characteristic is indicative of his non-mantelic origin.