Whole Rock Geochemistry and Geodynamic Evolution of Paleoproterozoic Gneisses, Ako’ozam-Njabilobe Area, Southwestern Cameroon

: The Ako’ozam–Njabilobe area, which is part of the Nyong group, is characterized mainly by highly deformed gneisses that host amphibolites, and metadolerites as enclaves. The gneisses within the Ako’ozam-Njabilobe area were investigated to constrain their geotectonic environment as well as understand their geodynamic evolution within the Nyong group. Three litho-types of gneisses outcrop within the Ako’ozam-Njabilobe area, consisting of horblende-biotite, quartzofeldspathic, and garnet gneisses. They are strongly peraluminous, with low Yb N , Rb/Sr, K 2 O/Na 2 O, and enriched in large ion lithophile elements (LILE). The gneisses within the Ako’ozam-Njabilobe area have a trachy-andesite parentage. The negative anomalies in high field strength elements (HFSE) and enrichments in LILE and light rare earth elements (LREE) suggest a subduction setting for this magmatism. The geochemical signatures including, low K 2 O/Na 2 O ratio, silica content (67.18 to 73.65 wt%), Yb and Y contents less than 1 and 10 ppm respectively, and low heavy REE values of the studied gneisses are akin to chemical signatures of archean crustal rocks. The Nb/Th < 1 and Th/Yb >1 of the studied gneisses indicate that the magma that sourced their protolith is mantle-derived with some degree of crustal contamination. The similarity in whole rock data of the three lithotypes of gneisses within the Ako’ozam-Njabilobe area suggests a possible mechanical mixing during emplacement. The chemistry of the gneisses indicates a possible Archean inheritance during the emplacement of their protoliths, as such could provide insights into the petrogenetic processes controlling crustal growth and aid in understanding the Archean-Proterozoic transition within the Nyong group.


Introduction
The Nyong group, located in southwestern Cameroon, represents a Paleoproterozoic suture zone between the Congo shield and the São Francisco craton.It is comprised of metasedimentary and metaigneous rocks consisting of various gneisses, amphibolites, pyroxenites, eclogites, charnockites, banded iron formations, and granitoids which constitute fragments of greenstone belts, magmatic grey gneisses and intrusive rocks [1,2].Gneisses constitute the dominant rock unit and make up the basement of the Nyong group.Greenstone belt rocks (amphibolite, pyroxenites, BIF) within the Nyong group have been at the center of recent works, as these rocks have been associated with iron mineralization [3][4][5][6][7] as well as gold mineralization [8][9][10][11][12].Equally metasedimentary and metavolcanic rocks have been very useful in geochronologic studies confirming an early-middle Paleoproterozoic age of the Nyong group [2,7,[13][14][15].Despite their contribution to the geochronology of the Nyong group, the chemistry of metavolcanic rocks (gneisses) has been understudied compared to other lithologies (charnockites, metasedimentary and plutonic rocks) within the Nyong group.The geochemical composition of gneisses in the Paleoproterozoic Nyong group is poorly constrained and available data is limited.The origin of the gneisses and their geodynamic evolution is equally not well documented.This paper highlights the results of the petrography and geochemistry of gneisses within the Ako'ozam -Njabilobe area, to determine their geochemical characteristics with emphasis on the geotectonic environment.This will contribute to understanding how the complex history of these rocks has impacted their chemistry and equally add to the geochemical database initiated by previous workers.

Regional Geology
The Precambrian basement in southern Cameroon is comprised of the Archean congo shield (Ntem group), Paleoproterozoic Nyong group, and the Neoproterozoic Yaounde group (Figure 1a).The Paleoproterozoic Nyong group located in southwestern Cameroon represents parts of the archean congo shield reworked during the Eburnean/Trans-Amazonian Orogeny by the collision of the Congo and Sao Francisco shields [2,13,14].It is comprised of meta-sedimentary and metaigneous rocks consisting of gneisses, banded iron formations, amphibolites, pyroxenites, and intrusive rocks such as charnockites, granitoids, metadiorite, TTG suites, syenite, and metadolerites [1,2].Greenstone eclogite facies metamorphic rocks equally outcrop within the high-grade Nyong gneisses.These include garnet-clinopyroxenites, garnet-clinopyroxene bearing amphibolites, and serpentinized peridotites [16][17][18], with SHRIMP U-Pb analyses constraining the metamorphic age of these eclogite rocks at 2.09 Ga [16].New U-Pb zircon ages infer the deposition age of metasedimentary rocks of the Nyong group to be early-middle Paleoproterozoic [13,14], with the Pan-African high-grade recrystallization overprinting the granulite-amphibolite metamorphic assemblage of the Nyong group [2,14,15,19].Structurally, the Nyong group underwent a polycyclic metamorphic evolution typified by a regional S 1 /S 2 foliation with low to moderate dip, associated with a variably oriented stretching lineation and local large open folds more or less associated with broadly N-S sinistral strike-slip shear zones [1,2,19,20].

Local Geology
The Ako'ozam-Njabilobe area is characterized by gneisses hosting amphibolites, and metadolerites as enclaves (Figure 1b).The tectonic evolution of the area is polycyclic, with four deformation phases observed (D 1 , D 2 , D 3 , and D 4 ) on the field.D 1 corresponds to an early compression in ductile conditions and is observed only as relict structures.D 2 also corresponds to a compression associated with S 2 foliation.D 3 corresponds to a strike-slip deformation associated with C 3 shear zones.D 4 corresponds to a late brittle tectonic phase associated with fractures.Rocks within the area are heterogeneously affected by deformation.To the west of the area around Adjap and Ako'ozam, gneisses display a penetrative foliation (S 1/2 ) trending mostly NE-SW with low moderate dips (Figure 1c).In contrast in the eastern part around Njabilobe (Figure 1d), two foliation trends are observed.A dominant NE-SW trend and a less conspicuous NW-SE trend, both with low to moderate dips (5°-65°).The early formed S 1/2 foliation is overprinted by shallowly to moderate dipping shear zones associated with C 3 mylonitic foliation.At the outcrop scale around Njabilobe, the penetrative foliation (S 1/2 ) is crosscut by shallowly to moderately dipping (3°-55°) shear zones.Shear criteria such as recumbent folds overprint the early S 1/2 banding and are more conspicuous around Akok, Akanga, and Njabilobe.Fractures and joints observed in both sectors of the field are related to late brittle tectonics (D 4 ), with N-S, NW-SE, and NE-SW orientations.

Sampling and Analytical Methods
Systematic mapping and sampling were completed in the study area and thirty-seven (37) fresh and representative samples of gneisses were collected from the Ako'ozam-Njabilobe area.Thin sections were prepared at the Department of Geology, Obafemi Awolowo University, Ile-Ife (Nigeria), and the petrographic observations were carried out at the Institute for Geological and Mining Research, Yaounde, Cameroon.Whole Rock geochemical analysis was conducted at Bureau Veritas Minerals (BVM) Pty Ltd, Canada using Inductively Coupled Plasma-Mass Spectrometry method, with a lithium metaborate fusion digestion technique.Before analysis, the rock powders were pulverized to 85% passing 200 mesh.A 60-element analytical suite was selected comprising of major, trace, and rare earth elements including Total Carbon and Sulphur, with Loss on Ignition and Sum of Oxides calculated.Inductively Coupled Plasma Emission Spectrometry (ICP-ES) was used for major elements while Mass spectrometry (ICP-MS) was employed for trace elements.During analyses, STD SO-19 standards were used and the accuracy of major oxide and trace element analyses were 0.002-0.04%and 0.01-0.5ppmrespectively.The discriminant function (DF) was defined as DF= 10. 44

Petrography
Field and petrographic investigation of gneisses in the Ako'ozam-Njabilobe area indicate that they are comprised of three litho-types which include hornblende-biotite gneiss, quartzo-feldspathic gneiss, and garnet gneiss (Figure 2).

Hornblende-Biotite Gneiss
Hornblende-biotite gneisses are banded coarse-grained porphyroblastic rocks.The banding of mafic and felsic minerals is visible in the hand specimen (Figure 2a), and foliation is distinct on outcrops.In the thin section, the hornblende-biotite gneisses have a heterogranular granoblastic texture (Figure 2b).Two generations of quartz crystals can be distinguished, that is, randomly oriented euhedral-anhedral sutured grains with undulatory extinction and recrystallized subhedral-anhedral (micro-grains) filling micro-fractures within grains and interstitial spaces between the grains (e.g plagioclase).Biotite and hornblende occur as subhedral to euhedral elongated crystals and are preferentially oriented in the gneisses.Alternating domains Ako'ozam-Njabilobe Area, Southwestern Cameroon of plagioclase characterized by bird-eye extinction and sericite-quartz-rich domains, define another foliation in these rocks.Quartz within this domain is dominantly recrystallized.Plagioclase is also albitized with few micro-grains occurring as inclusions in quartz, with oxides generally occurring as small blebs and usually forming rims around hornblende, biotite, and muscovite crystals.Some plagioclase crystals show evidence of deformation such as myrmekitic intergrowth textures.

Quartzo-Feldspathic Gneiss
Quartzo-feldspathic gneisses are moderately mesocratic to leucocratic (Figure 2c).They are coarse-grained porphyroblastic rocks that are weakly foliated.The leucocratic gneisses have a heterogranular granoblastic texture.Quartz crystals range from 0.1 to >5 mm and contain inclusions of plagioclase in certain portions.Quartz crystals show a range of forms from subhedral to anhedral.
Microcline and plagioclase feldspar are abundant in these rocks and are generally bounded by quartz crystals (Figure 2d).Muscovite and feldspars crystals make up over 50 % of the groundmass with some phenocryst of quartz embedded in the groundmass.In some parts, plagioclase is often altered to sericite.Platy biotite interlocks with quartz.Oxide phases in these rocks have anhedral and euhedral forms.

Garnet Gneiss
Two types of garnet gneisses were identified in the field.They include coarse and fine-grained garnet gneisses.The former was more prevalent on the field.Garnet gneisses are massive and weakly foliated (Figure 2e), exhibiting a granoblastic heterogranular texture.Their mineral composition includes garnet, quartz, plagioclase, biotite, hornblende, microcline, pyroxene, and oxides, (Figure 2f).Hornblende contains inclusions of quartz.Porphyroblasts of garnet often contain inclusions of quartz, hornblende, and oxides.

Whole-Rock Geochemistry
Thirty-seven representative samples of hornblende-biotite, quartzo-feldspathic, and garnet gneisses were selected for whole-rock geochemistry.The summarized data is presented in table 1.The entire dataset is available in the supplementary data.

Major Element Geochemistry
Gneisses in the Ako'ozam-Njabilobe area are acidic in composition with SiO 2 content ranging from 67.18 to 73.65 wt%.Al 2 O 3 contents are high ranging between 13.37 to 16.08 wt% except for sample EL05 which is 9.11 wt%.The nonvariability of alumina suggests a calc-alkaline affinity (Figure 3).The Fe 2 O 3 +MgO+MnO+TiO 2 sum varies from 2.19 wt% to 5.24 wt%.NaO 2 is slightly higher than K 2 O in all samples except for Nj03, El04, and Nj16.The total alkali content ranges from 5.15 to 9.29 wt%.The gneisses are calcalkalic and have a higher molar A/CNK ratio indicating their peraluminous nature.Major elements have a negative correlation with SiO 2 in hornblende-biotite gneisses, similar to those of quartzo-feldspathic gneiss, while major elements in garnet gneiss have a negative correlation with SiO 2 except K 2 O (Figure 4).

Trace and REE Element Geochemistry
Large ion lithophile elements (LILE) contents in the gneisses are high except for Cs which is low.Ba ranges between 770 to 977 ppm except for sample El05 which is 541 ppm.Rb shows high and relatively constant values ranging from 120. 5  Harker's diagram indicates that trace elements have a negative correlation with silica except for Cs which is positive for hornblende-biotite and garnet gneisses and Rb which is positive for hornblende-biotite gneisses (Figure 5).Plagioclase fractionation possibly accounts for the decrease in Sr and Ba and the slight increase in Rb with increasing silica.The studied gneisses display enrichment in light rare earth elements (LREE~89.97-144.74) and depletion in heavy rare earth elements (HREE~3.72 -6.85).The chondrite-normalized REE patterns of the studied gneisses show LREE enrichment (La N /Yb N = 19.9-45.5) relative to HREE (Gd N /Yb N = 2.09 -5.44) with a negative Eu anomaly (Eu/Eu* = 0.73 -0.84), indicating plagioclase removal by fractional crystallization (Figure 6).[24] normalized REE plot of gneisses in the Ako'ozam-Njabilobe area.

Element Mobility
The elevated concentrations of LILE relative to HFSE in the studied gneisses are a general feature recognized in arc magmas [25].The LILE enrichment in arc magmas is regarded as a result of the deposition of these elements by HFSE-poor fluids ascending from the subducted crust [26].The elements Rb, Sr, Sm, Nd, U, and Pb are generally redistributed during the movement of fluids associated with regional metamorphism.On the diagrams of Zr versus Nb, Nd, Sm, Ti, Y, and HREE, the samples display systematic strong correlations with Zr (Figure 7), indicating the low mobility of these elements.Contrary to HFSE and HREE, Rb, K, Na, and Sr, have scattered trends with Zr abundances (Figure 7) and display significant enrichments and depletions on the primitive mantle-normalized diagrams (Figure 10).This is an indication that they are relatively more mobile [27,28].The Ngovayang orthogneisses show impoverishment in LILE [29], contrary to the studied gneisses.Ndong, F. B., et al. attributed this LILE impoverishment to be a consequence of dehydration reactions that occur during the passing from the amphibolite to the granulite facies [29].The observed difference in orthogneisses in the Ngovayang area and those presented in this study could be an indication that the gneisses which make up the basement of the Nyong group fall within the transition between amphibolite and granulite facies metamorphism [30].The loss on ignition of these acidic formations is slightly higher than 1% except for a few samples where it is lower than 1%.This indicates that the alteration phenomena of some minerals as noted in the petrography such as the sericitization of quartz and albitization of feldspars are reduced [28].

Protolith
The protolith of gneisses can be determined using a discriminant function (DF) as long as the MgO<6% and SiO2<90% [21], with positive DF values suggesting an igneous origin while negative DF values point to a sedimentary source.DF is positive for all samples (Table 1), indicating an igneous protolith of the studied gneisses.To determine the original igneous rock from which the studied gneisses were derived, less mobile elements such as Nb, Zr, Hf, and Y, were used because these elements are hardly affected by post-magmatic processes such as metamorphism and thus will represent the initial parent material.On the Nb/Y -Zr/Ti diagram (Figure 8), all samples plot within the trachy-andesite field except two samples that fall within the trachyte field.

Tectonic Setting
To constrain the tectonic setting under which the igneous protolith of the studied gneisses was emplaced, a trace element discrimination diagram was employed.The data plot (Figure 9) indicates a major emplacement within a volcanic arc regime with few samples plotting in the syn-collision regime.This is akin to data from garnet and amphibole-rich gneiss in Eseka [32], pyroxene gneisses in Kelle Bidjoka [5].More recent data confirms the arc setting for the Nyong group crustal rocks associated with the Eburnean/Trans-Amazonian orogeny in Cameroon [33].

Geodynamic Implications
Primitive mantle [24] normalized trace elements of the studied Nyong gneisses (Figure 10) show enrichment of LILE and LREE elements, negative anomalies of Nb, Ta, Zr, and Pr, a slightly positive Sr anomaly, which is typical of subductionrelated settings [34][35][36].Nédélec, A., et al. reported positive Na 2 O/K 2 O ratio in Eseka gneisses and that such ratios, coupled with the range in silica content (67.18 to 73.65 wt%) and trace element values are similar to those of Archean TTG [37][38][39].According to Ganwa, A. A., et al. the characteristic chemical signatures of the Archean crust include extremely low content in Y and heavy REE [40].As such, TTGs will usually display Yb and Y contents less than 1 and 10 ppm respectively.Typical calc-alkaline rocks have Yb and Y contents which are two to three times higher than TTGs.The authors attribute these low values to the partial fusion of a source containing garnet as a residual phase.Gneisses in the Ako'ozam-Njabilobe area have Yb and Y values less than 1 and 10 respectively, with Sr/Y ratios far greater than 30 and low HREE.The chemistry suggests an archean inheritance during the emplacement of their protoliths, such as those of Paleoproterozoic orthogneisses in the Meiganga area [40] and gneisses from Edea and Eseka [32].The negative sub-vertical trends of Co, Sr, V, Nb, and Ba with increasing silica (Figure 6) is an indication that fractional crystallization is the major mechanism of differentiation.Similar trends were observed in Paleoproterozoic orthogneisses (Caico complex), NE Brazil [44].Contrary to the Caico gneisses, which have higher Yb N , Rb/Sr, K 2 O/Na 2 O, and lower A/CNK, (La/Yb) N and Zr/Sc (Table 2), gneisses of the Ako'ozam-Njabilobe area have lower Yb N , Rb/Sr, K 2 O/Na 2 O and higher A/CNK, (La/Yb) N and Zr/Sc, akin to late Archean calc-alkaline granites [43].The Nb/Th < 1 and Th/Yb >1 of the studied gneisses indicate that the magma that sourced their protolith is mantle-derived with some degree of crustal contamination [45].

Conclusions
The Ako'ozam-Njabilobe area is located in the Eburnean belt, which covers Central Africa and extends to northeastern Brazil.The geochemical composition of gneisses within this area was studied to ascertain their tectonic setting and geodynamic evolution within the Nyong group.Three lithotypes of gneisses were identified within the Ako'ozam-Njabilobe area.These include hornblende-biotite, quartzofeldspathic and garnet gneisses.The mineral assemblages of the studied gneisses suggest an upper amphibolite facies metamorphism.They are sourced from trachy-andesites.The studied rocks are peraluminous, high-Na, LILE, and LREEenriched sourced from deep-seated calc-alkaline magmas emplaced in a subduction-like tectonic setting.The magma that sourced the protolith of the gneisses is mantle-derived with some degree of crustal contamination.The similarity in the major, trace, and REE chemistry of these meta-volcanic rocks suggests a possible mechanical mixing during emplacement.A further isotopic and geochronologic study of the rocks is recommended to provide more clarity on the similarity in whole rock chemistry and their petrogenesis.

Figure 1 .
Figure 1.Simplified geologic map of Southwestern Cameroon (a) modified after Soh Tamehe, L. [22], the geological sketch map of the Ako'ozam-Njabilobe area (b) with sample locations.Orientation diagrams displaying the S1/2 foliation in the west (c) and east (d) sectors of the study area.

Figure 5 .
Figure 5. Harker's variation diagram of some trace elements in gneisses of the Ako'ozam-Njabilobe area.

Figure 7 .
Figure 7. Zr vs. selected element variation diagrams to highlight the limited effects of alteration or mobility.
to 162.8 ppm, except samples El04 and Nj16 which are extremely high reaching 410.7 ppm and 421.6 ppm respectively.Sr values are high (725.8-817.1 ppm) except in sample El05 which is 426.8 ppm.Cs values range from 3.4 to 9.3 ppm except for samples El04 and Nj16 which are 13.1 and 13.2 ppm respectively.High field strength elements (HFSE) such as Hf and Nb contents are low in all samples, ranging from 2.9 to 4.7 ppm and 4.6 to 8 ppm respectively.Ta is depleted with extremely low values (<1 ppm).Zr has elevated contents varying from 104.4 to 145.7 ppm except in sample Nj16 which is slightly lower (99.5 ppm).Base metals have averagely low values, with Zn ranging from 26 to 72 ppm, Pb from 4 to 12.6 ppm, and Cu ranging from 9.6 to 16.8ppm except in samples Ny01 and Ad02, with Cu values of 21.7ppm and 37.3ppm respectively.All samples are depleted in Sn with uniform values ranging from 1 to 3 ppm.Au values in some samples are higher than the crustal abundance.It ranges between 0.6 to 3.3 ppb except for samples El04, Nj15, and Nj03 with 6.7, 5.7, and 5.2 ppb respectively.

Table 2 .
Selected average element ratios of the Ako'ozam-Njabilobe orthogneisses compared with Archean granitoids and Paleoproterozoic orthogneisses of the Caico complex.