Geology and Petrographic Studies of Rocks in Anka Sheet 52, Northwestern, Nigeria

Over the years, models of evolution of the schist belts of Nigeria was proposed and classified under ensialic and ensimatic processes of formation. The study area (Anka Sheet 52) lies between latitudes 12 o 00′N and 12°30′N and longitudes 5°30′ E and 6°00′E and covers an area of about 3,086.76 km² (Anka Sheet 52). Field mapping and laboratory studies was considered for this study by taking every detail in the field necessary such as geologic boundaries, structures, physical appearances of rocks and orientation noted in field notebook. Structural characteristics on outcrop was also recorded with the help of photographs and sketches along with text description. Representative rock samples mostly granitoids were prepared into thin for microstructural observations to reconstruct the deformation features and mineralogical assemblages under polarized light microscope. The study area comprises of gneiss of sedimentary protolith and is the oldest rock unit forming a massive sheared and elongated ridge trending north-south for about 1800m with a porphyroblastic texture of gradual lineation of quartz and feldspar in places. Metasediments comprises of quartzite, quartz schist, pellitic schist, phyllite, hornfels and metaconglomerate accounting for more than 35 percent of the study area. Pan-African granites intruded both the gneiss and metasediments and comprises of coarse grained granites, porphyritic granites, diorite and rhyolite to accounts for about 20 percent of the study area. Field and petrological evidences indicates that the Pan-African reworking may have led to the recrystallization of the protolith rocks to form gneiss (paragneiss) resulting from partial melting signifying that metamorphism possibly reach lower amphibolite facies in part of the study area. These rocks were overlain by sedimentary rocks of Cretaceous age.


Introduction
The study area (Anka Sheet 52) in part of Anka schist belt includes a post-orogenic element of Pan-African age; the unmetamorphosed volcanic and sedimentary rocks of the Maradun and Kiserni areas which rest unconformably on the schists and granites and are dated provisionally at 516 + 20 Ma (McCurry, 1976). The Anka schist belt lies west of Maru schist belt (Figure 1), the two being separated by the Pan-African Maiinchi granodiorite and by a probably older gabbro-granite-Anka pegmatite complex (APC) (McCurry, 1971). The rocks contrast with those of the Maru schist belt and include metaconglomerates, sandstones, slates, phyllites and acid volcanic rocks (Holt,Figure 2: Generalized geological map of Nigeria within the framework of the geology of West Africa (Wright, 1985).

Field Mapping
Field mapping is an important approach for this study by taking every detail in the field deem necessary. Different places for the geological study were selected and location determined from maps produced from desk studies. Observations such as geologic boundaries, structures, physical appearances rocks and orientation with details were noted in field notebook. The different rock types at various locations and their properties were noted and marked in the map while samples obtained were properly numbered. All the available information such as the changes in rock type or structural characteristics on outcrop was also recorded with the help of photographs and sketches along with text description. Instruments used in the course of field mapping are; Hammer, Compass clinometer. Others includes; field vehicle, motorbike, measuring tape, field sampling bags, marker, bar magnet, dilute hydrochloric acid, digital camera and GPS.

Sample collection
Rock samples were collected from the exposed rocks. Seventy-six (76) rock samples were collected from the field regionally and labelled (A1-A76) see Table 1.

Petrographic Studies
Representative rock samples were prepared into thin section at Department of Geology, Ahmadu Bello University, Zaria. Standard Olympus BX51 transmitted light microscopy (magnification range between 4x to 50x) was used to study individual minerals in the rocks under plane and cross polarised light for microstructural observations to reconstruct the deformation features and mineralogical assemblages. The slides were also examined with a polarized light microscope at the regional office of the Nigeria Geological Survey Agency (NGSA) Abuja. An external digital camera, connected to the microscope eye piece and a laptop with the use of (AmScope software) aided clear and better view compare to eye strain through the eye piece of the microscope.

Results
Field sample points and coordinates for the study area are shown in (Table 1) and the physical study of the rocks during field mapping comprises the colour, texture, mineralogy, rock type and field relationships. Four major rock types make up the geology of the study area (Anka Sheet 52) (Figure 3), they are; i) Gneiss (grey and porphyroblastic in places) ii) Metasediments, comprising of quartzite, quartz schist, pellitic schist, phyllite, hornfels and metaconglomerate. iii) Pan-African Granites, comprising of coarse grained granites, porphyritic granites, granodiorite and rhyolite iv) Cretaceous Sedimentary Rocks, comprising of basal conglomerate and sandstone.

Discussion
An outcrop of porphyroblastic gneiss along Abare-Dereta road with gradual lineation of quartz and feldspar in a preferred orientation is shown in (Figure 4a) and also mapped on the outcrop is a sheared gneiss showing a xenolith of gneiss within granitic textures as an evidence of variation in temperature-pressure (Figure 4b Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol.10, No.1, 2020 thereby enriching this mineral at the growing edge ( Figure 5). Primary (NNW-SSE) and (b) secondary (NNE-SSW) joints were studied on the Abare-Derita gneiss ( Figure 6). Figure 7 is an outcrop photograph of a quartzite ridge trending in 058 o and petrographic studies on the quartzite revealed quartz as major constituent mineral with minor biotite and muscovite. The quartz schist (Figure 9a) has medium to large, flat, sheet-like grains in a preferred orientation (between 040 o -360 o in places), dips to the west and to the east in places (Figure 9b). Foliation on the quartz schist were studied in the field and data plotted as shown in rose plot (Figure 10a) and joints orienting (NE-SW) 048 o (Figure 10b). The pelitic schist trends in 010 o and dipping at 20 o to the west (Figure 11). It has multiple and parallel joints in an average of 088 o . At Masafa village it trends 170 o and dips at 60 o to the west with multiple and parallel joints in an average of 072 o . Phyllite occurred as low-lying outcrop and mostly exposed along a river channel, it trends at 180 o with a dip of 60 o to the west (Figure 12a) and at 150 o with a dip of 020 o in places. In another location the phyllite is intruded by sugary textured rhyolite (Figure 12b). Hornfels (Figure 13a) were mapped arround Rafin Gero village with a granitic textured rock as veins. Petrographic studies of the hornfels show that biotite grades into hornblende ( Figure 14) with variation visible at the rims between the biotite and hornblende with iron concentration along the cleavage. The meta-conglomerate was mapped extensively along river Zamfara around Bunkasau village. Figure 15a shows a slippers shaped clasts of medium grained granite which inherited quartz vein before metamorphism (Figure 15b). Recovery of gold along the river channel by Artisans was encountered during mapping. Petrographic studies on granitic rock extract from the metaconglomerate ( Figure 16) revealed a medium to fine grained textures crystals of predominant quartz and biotite with minor hornblende in addition to mafic minerals. The granites within the study area comprises of coarse grained granites, porphyritic granites, diorite and rhyolite. These rocks accounts for about 20 percent of the total area mapped. The coarse grained granite forms the basic constituents of the granite suites in the study area and are more widespread (Figure 17a). The rock is weathered and shattered at Rafin Gero with NO visible field contact with host rock and also occurred as inselberg. Two (2) generation of granitic veins were noted as primary (V1) and secondary (V2) on the coarse grained granite mapped at location A14 (Figure 17b). Petrographic studies revealed quartz and zoned plagioclase with stains of biotite in coarse grained granite sample studied at location A8 ( Figure 18). A grey coloured porphyritic granite within the study area occurs mostly as flat-lying and as small enclaves around Ruwan Jema (Figure 19a) and at Sabon Geri Sesawa. Diorite is less widespread in the study area and are only mapped as low to flat-lying outcrop along Anka-Bukkuyum road (Figure 19b) and are too small to be represented on the regional map. In the porphyritic granite sample, petrographic studies revealed hornblende, plagioclase, biotite and quartz as the major mineral constituent and the plagioclase occurred as phenocryst with lamellar twining (Figure 20) while in the hornblende, plagioclase, biotite and quartz are the major mineral constituent (Figure 21).

Conclusion
The Precambrian to Pan-African basement rocks which occur in northwestern Nigeria and cretaceous sedimentary rocks of Sokoto basin are duplicated in the study area. These rocks are gneiss (greyish and porphyroblastic in places) representing the migmatite-gneiss group of sedimentary protolith, quartzite, quartz schist, pellitic schist, phyllite, hornfels and meta-conglomerate representing the metasediments; coarse grained granites, porphyritic granites, diorite and rhyolite represents the Pan-African granites while basal conglomerate and sandstone represents the sedimentary group. The gneiss is the oldest rock unit forming a massive sheared and elongated ridge trending north-south for about 1800m with a porphyroblastic texture of gradual lineation of quartz and feldspar in places. Two cross-cutting joints of primary (NNW-SSE) 160 o and secondary (NNE-SSW) 008 o developed on the gneiss such that the later which obliterate the earlier suggests evidence of post tectonic deformation after the initial tectonism that form the primary joints inherited in the rock before displacement and post metamorphic tectonic deformation on the rock. Other structures observed on the grey gneiss include mylonitic textures resulting from a rather ductile fault shear zone signifying modification of the regional textures characterized by plastic flow predominantly due to dynamic recrystallization. The well-defined foliation bands of micas and felsic minerals studied as a common feature on the grey gneiss probably indicate that metamorphism in this area possibly reach a higher green schist facies. Rocks in the study area is composed chiefly of very poorly exposed, commonly homogeneous quiet-water argillites are in line with this studies as coarse clastic units and the magmatic rocks both yielded information concerning palaeo-environments and geotectonics evolution of the belt respectively. The Pan-African granites intruded both the gneiss and metasediments and the coarse grained granite forms the major constituents of the granite suites and are more widespread. It is light-coloured with grains of quartz and feldspar with minor amounts of mica which may have resulted from slow crystallization of magma below Earth's surface. Basal conglomerate and sandstone form the cretaceous sedimentary rocks unconformably overlying the basement complex rocks in close contact. In quintessence, the study area comprises of low grade metasedimentary rocks of Proterozoic time and intruded by a sedimentary sourced granitic plutons active during Pan African time. Field and petrological studies indicates that the Pan-African reworking may have led to the recrystallization of the protolith rocks to form gneiss (paragneiss) resulting from partial melting signifying that metamorphism possibly reach lower