# Erosion mode and history of Eastern Adamaoua landscapes (Cameroon): Superimposed lateritic weathering of granites and basalts

Mathieu Nouazi Momo, Anicet Beauvais, Gatien Romuald Kenfack Nguemo, Ethel Nkongho Ashukem, Edima Roland Yana, Salomon César Nguemhe-Fils, Rachel Ngo Nyouma, Alex Chimeu Akaba

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Mathieu Nouazi Momo, Anicet Beauvais, Gatien Romuald Kenfack Nguemo, Ethel Nkongho Ashukem, Edima Roland Yana, et al. Erosion mode and history of Eastern Adamaoua landscapes (Cameroon): Superimposed lateritic weathering of granites and basalts. CATENA, 2023, 226, pp.107071. 10.1016/j.catena.2023.107071. hal-04050321

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Erosion mode and history of Eastern Adamaoua landscapes (Cameroon):

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Superimposed lateritic weathering of granites and basalts

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Mathieu Nouazi Momo¹, Anicet Beauvais², Gatien Romuald Kenfack Nguemo³, Ethel

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Nkongho Ashukem¹, Edima Roland Yana¹, Salomon César Nguemhe-Fils¹, Rachel Ngo

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Nyouma¹, Alex Chimeu Akaba⁴

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¹Institute of Geological and Mining Research (IRGM), P.O. Box 4110 Yaoundé, Cameroon

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²Aix-Marseille Univ, CNRS, IRD, INRAE, CEREGE, BP 80, 13545, Aix-en-Provence,
Cedex 4, France

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³ University of Dschang, Faculty of Science - Department of Earth Sciences. PO Box 67
Dschang - Cameroon

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⁴ University of Yaoundé 1, Faculty of Science, Department of Earth Sciences. PO Box 337
Yaoundé, Cameroon

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https://doi.org/10.1016/j.catena.2023.107071

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*Corresponding author Email: nouazimat@yahoo.fr
25 Abstract

26 Landscapes of eastern Adamaoua highlands have been shaped by successive
27 weathering and erosion processes of Pan-African crystalline rocks and Neogene volcanics
28 cover. Basalts have outpoured through major inherited structural discontinuities mostly in
29 shallow incisions of lateritic pediplains previously shaped on the granitic basement. That has
30 resulted in formation of a singular composite lateritic weathering profile on basalt
31 superimposed to a truncated profile composed of mottled clays and saprolite on granite. The
32 composite profile studied here is little evolved, mostly kaolinised on granite while basalt is
33 weakly to moderately lateritised owing to differences in silica and iron oxide contents of the
34 two contrasted parent rocks. During lateritic weathering, Cu, Ni, and Co are enriched in
35 profile on granite but depleted on basalt, while Zr/Ti is relatively constant. Behavior and
36 fractionation of REE are comparable except higher Eu* and Ce* anomaly on granite profile
37 than on weathered basalt, owing to parent minerals differences, mostly feldspars in granite
38 versus plagioclase and Fe-Mg minerals in basalt. The contrast in Eu* is also linked to
39 differences in the Index Of Laterization (IOL) in weathered horizons of the two parent rocks.

40 Beyond quite classical litho-dependent geochemical differences in the composite profile,
41 persistence of per-humid climate and good drainage over Neogene have sustained rock-
42 weathering and local surface erosion processes upon volcanic outpours and their contact
43 surface with pediplains formed on Pan-African granitoids of Adamaoua highland.

44 Keywords: Lateritic weathering; Erosion; Granite; Basalt; Neogene; Cameroon.
45 **1. Introduction**

46 Lateritic landscape dynamics and formation of lateritic profiles on tectonically-stable
47 cratonic continental platforms across tropical belts have been primarily driven by climatic and
48 lithologic factors that constrained the amplitude of weathering, erosion and geomorphic
49 processes over Paleogene (Thomas, 1994; Tardy and Roquin, 1998; Colin et al., 2005;
50 Beauvais et al., 2008; Beauvais and Chardon, 2013; Beauvais et al., 2016; Chardon et al.,
51 2018; Vasconcelos and Carmo, 2018; Jean et al., 2020). West African lateritic paleo-
52 landscapes, whose relics are currently preserved for million years in West Africa (Beauvais
53 and Chardon, 2013) have formed under Paleocene – Eocene humid climates but afterward
54 evolved under post-Eocene drying climatic conditions across the West African platform (e.g.,
55 Chardon et al., 2018). Once swells like the Jos plateau plateau in Nigeria and the Adamaoua
56 highland in Cameroon have established under Paleocene - Eocene epigenic uplift driven by
57 under-plating mantle activity (Burke, 2001), lateritic rock weathering process have operated
58 on the uplifted surfaces. Afterwards, lateritic paleo-landscapes evolved under long-lasting
59 humid climatic conditions (post-Eocene to late Neogene) that effectively prevailed upon these
60 uplifted platforms. Long-term preservation of lateritic paleosurfaces and regolith formed on
61 granitic basements depends on weathering and erosion processes intensity, which have been
differentially mostly amplified by major trajectories of climatic regimes and the possible
62 effects of eventual tectonics. For example, these factors combined together may have induced
63 deformation of previously formed paleo-landscapes and their drainage networks across and
64 around intra-cratonic swells (Burke and Gunnell, 2008; Grimaud et al., 2014; Chardon et al.,
65 2016).
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67 The ancient lateritic landscapes (mainly pediplains) formed on the Pan-African
68 basement in Adamaoua mainly evolved under persistent humid climatic regimes during the
69 middle and late Cenozoic. Neogene basaltic flows outpoured over the pediplains through
major structural discontinuities, and their lateritic weathering has completed the edification of composite profiles. Nguetnkam et al., (2020) described paleosols and saprolite on granite and superimposed basalts, respectively, and interpreted the current lateritic profile edifice as the result of local relief inversion of a previous valley on granite filled by late Neogene basalts.

Eventual tectonic effect of basalt outpourings (among other volcanics) on landscape incision remains however a matter of debate that is out of the scope of this paper. Here, we document the history and erosion mode of Eastern Adamaoua landscape by describing in details the formation of such a composite lateritic profile on the basis of major petro-geochemical evolution paths across that profile, complemented by regional scale field observations on geomorphological surfaces and regolith types on granite and basalt in and around the studied profile. We considered the major influence of long-lasting per-humid climatic conditions on the geochemical differentiation and elements fractionation across the lateritic profiles formed on the different lithologies and upon the landscape erosion pattern of eastern Adamaoua highland. The known ages (~9 to ~11 Ma; see Marzoli et al., 1999) of basaltic flows over lateritic pediplains previously formed on granite basement impose a relative chronology of weathering and landscape erosion. Our study opens perspectives to larger scale analysis of volcanically-driven regolith geomorphological markers that seal the minimum age of underneath lateritic pediplains on Adamaoua highlands.

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## 2. Major geological and geomorphological features of Eastern Adamaoua

The granitoid basement of the Adamaoua plateau belongs to the Pan-African orogenic belt of Central Africa ($^{207}$Pb/$^{206}$Pb zircon ages of ~ 500-600 Ma, Ganwa et al., 2011). Pulses of volcanic activities with evidences of mantle sources (lherzolites, Girod et al., 1984) resulted in establishment of strato-volcanos (~ 400 m, Marzolli et al., 2000) upon previously-formed lateritic sequences on granitoids, in eastern Adamaoua highland (Figs. 1a-b). Basalts

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outpoured through structural discontinuities (opened faults and fractures) inherited from post-orogenic distensile tectonics that affected the Pan-African basement of Adamaoua (Deruelle et al., 1991). Detailed chronological and petrological studies revealed successive stages of volcanic activities (Marzoli et al., 1999, 2000; Delor et al., 2021). Voluminous stratoïd alkaline basalts and basanites at c. 11 Ma, are then covered, from 11 Ma to 9 Ma, by silicic magmas extrusions mainly nepheline-rich trachytes (Marzoli et al., 1999) with spots of rhyolites and phonolites (Lasserre, 1961) at Tchabal Nghana (Fig. 1a). A last stage is represented by minor basaltic flows and pyroclastic deposits dating from Quaternary (Kagou Ndongmo et al., 2010). The wide age range from Miocene to Quaternary is suggestive of multiple magmatic hot-spots operating beneath the Cameroon Volcanic line (Marzolli et al., 1999; 2000).

Low-relief lateritic landscapes are marked by a NW sloping regional composite surface on granitoids and volcanics (Fig. 1b). On syn-tectonic granites, lateritic pediments form kilometers wide rolling pediplain between c. 1200 m a.s.l. to c. 1420 m a.s.l (Fig. 1b), which are interrupted by c. 40 m height hills of poorly weathered recent basalts/basanites (Fig. 2a). Lateritic duricrusts formed on pediments at c. 1200 m altitude show a composite assemblage of nodular and massive ferricretes (Fig. 2b). At higher altitudes, hillsides of truncated weathering profiles on pediments are covered by saprolites or sandy mottled clays.

Scarped erosion slopes mark major lithological unconformity between the upper pediplain on syn-tectonic granites and the lower pediment surface on migmatites and gneiss (Fig. 1b). This is linked to a regional extension of a faulting system N70°E that established on the Adamaoua highland as a result of Albian-Cenomanian extensive tectonic (Allix et al., 1984) with major lithological control (see Fig. 1). Below the scarped slope at c. 1100 m a.s.l. (Fig. 1b), piedmont surfaces are covered by ferricrete (Figs. 2c-d).
In contrast to landscape on granitoids, landsurfaces on mid-Miocene basalts are marked by a succession of smoothed hills covered by consolidated ferricretes/bauxites, that are dismantled mostly on the edges of eroded plateaus (Fig. 2e), which the incision led to the formation of a lower and younger pediment surface (Fig. 2f).

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## 3. Methodological approach

Field geomorphological observations at regional scale (Fig. 1a) allowed to characterize lateritic landsurfaces deriving from lateritic weathering of granitoids and basalts, and landscape dissection (Fig. 1b). This detailed geomorphic characterization was undertaken along a NW-SE prospect in area exposing a patchwork of multiple weathered volcanic covers and surfaces of lateritic pediplains formed on granite. Field stations (Fig. 1a) were chosen to also analyze the broader ranges of altitudes in the studied area (Fig. 1b) including a major structural discontinuity of the Adamaoua that may constrain the lateritic surfaces distribution.

At a given field station, elevations and petrographic descriptions of preserved regolith/duricrusts on granite or basalts were recorded and linked to specific landscape morphology.

Based on a detailed geological map (Lasserre, 1961), field survey allowed to determine outcrops of composite weathering profiles, that are arranged preferably around geological contacts between crystalline basement and overlying lavas flows, e.g. Mangoli (see Nguetnkam et al., 2020, and this study, Fig. 1). A road trench exposing 35° NW-dipping structures permitted to describe the weathering profile MEI01 and its different horizons formed on granite and basalt (Fig. 3a). The composite weathering profile exemplified regional scale regolith formed on low-reliefs lateritic pediplains and erosional slopes on granitoids, which have been then covered by mid- to late Miocene volcanics (basalts and/or andesites)

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143 before renewed weathering and laterization under propitious climatic conditions over the late
144 Neogene.

145 Eleven samples including the two parent rocks (granite and basalt) were collected in
146 the different horizons of the weathering profile section (Fig. 3a). Weathered samples were
147 described according to reference nomenclature of Nahon (1991) and Tardy (1997), while their
148 dominant hue was estimated using codes from the Munsell color chart (Mc).

149 Samples were prepared and analyzed by the ALS Laboratory Group SL, Spain.

150 Samples were crushed to a sieved size of 2 mm that underwent further pulverization such that
151 more than 85% of grounded sample be sieved to 75 µm. Prior to 4 acid dissolution and ICP-
152 MS analysis, the lithium borate fusion was applied to samples, that is suited for quantitative
153 analyze of whole rock elements. The quality control here is based on a number of blanks,
154 reference materials (standards) and duplicates that are analyzed along with the samples. The
155 ICPMS (Inductively coupled plasma-Mass spectrometry) method was used for analyzing a
156 combination of Major (wt.%), Trace and Rare Earth elements in ppm (ME-MS81TM + whole
157 rock package ME-ICP06; www.alsglobal.com). The detection limits of each element are
158 given in the table 1 caption, table 2 and 3.

159 The geochemical Index Of Laterization, IOL, was also calculated for better
160 understanding and comparing lateritic weathering processes intensity in each section of the
161 composite profile. The IOL considers major elements, Al, Si and Fe, involved in the lateritic
162 processes such as:

$$
163 \qquad \text{IOL} = 100 \times \left[ \frac{(\text{Al}_2\text{O}_3 + \text{Fe}_2\text{O}_3)}{(\text{SiO}_2 + \text{Al}_2\text{O}_3 + \text{Fe}_2\text{O}_3)} \right] \qquad (1)
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$$
166 The IOL values were then reported on a scale alongside a ternary diagram (SiO₂-Al₂O₃-Fe₂O₃), which allow to distinguishing different laterization grades (Schellmann, 1994; see also Babechuk et al., 2014).

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170 **4. Results**

171 **4.1. Petrological differentiations across the composite profile**

172 The profile is composed of weathering horizons formed on granite and overlying basalt defining two sections separated by an inconformity marking a rippled contact (Fig. 3a), which dips ca. 35° roughly parallel to surface slope gradient toward the local valley incision to the NW. From the bottom to the top of the profile on granite, a ca. 4 m thick nodular horizon M1g is overlaid by a 3.3 m thick mottled clay M2g on granite, while a 2.4 m thick mottled clay M1b topped by 2.3 m thick pebbly horizon compose the profile on basalt (Fig. 3).

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Granite (Fig. 4a) is composed of quartz, feldspars, amphiboles and biotites. Blocks of granite also outcrop in river beds and are widespread in adjacent lowlands where lateritic regolith has been stripped. At the bottom of the profile, the nodular horizon (M1g) is made up of a brown-reddish clay-ferruginous matrix (Mc: 2.5 YR 4/8) embedding few nodules with some preserved centimeter-sized whitish saprolitic relics (Fig. 4b). In higher part of this horizon (Fig. 3b), numerous pluri-centimeter sized dark-red nodules (Mc: 10R 3/6) are embedded in a reddish (Mc: 2.5 YR 3/5) friable internodular matrix (Fig. 4c); the horizon is limited upward by a centimeter-sized dark brown crusting (Fig. 3a-b). The overlying yellowish-red (Mc: 5YR 4/5) mottled clays (M2g) are formed in-situ. The upper part is made up of a yellowish-red to yellow clay-ferruginous matrix (Mc: 7.5 YR 7/6) studded with quartz grains (Fig. 4d). The lower part of M2g shows the original structure of the fresh granite rock crossed by a network of fractures marked by iron crystallizations coating (Figs. 3a-b and 4e).
Basalt coarse structure is preserved in the upper section of the weathering profile with a columnar-jointed vertical network in the mottled clays horizon (Figs. 3d). Basalt is dark grey massive and microcrystalline (Fig. 5a), with some phenocrysts of olivines and pyroxenes observed under a magnifying glass. The base of the profile on basalt is made up of a yellowish (Mc: 7.5 YR 7/6) mottled clay horizon (M1b), which shows a sharp transition with the underlying horizons on granite (Fig. 3c). This horizon is massive and made up of undifferentiated yellowish clay studded with preserved structures of micrometric inherited primary minerals, and crossed by centimeter sized voids filled with a dark brown (Mc: 7.5 YR 5/5) loose fine earth (Fig. 5b). The basaltic structure is progressively indistinct in the pebbly horizon M2b composed of 70% pebbles and 30 % fine earth (Fig. 5c).

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## 4.2. Major and trace elements across the composite profile

Graphical distributions of major elements and IOL values are showed in figures 6 (see also analyses in Tab. 1). Data analysis highlights decreasing SiO₂ content and increasing contents of Fe₂O₃ and Al₂O₃ relative to the respective parent rocks (Fig. 6). The maximum depletion of SiO₂ and element enrichment of Fe₂O₃ occur at the top of the nodular horizon (M1g) on granite (Figs. 6 and 7a).

Laterization index IOL progressively increases from parent rocks to kaolinized weathering horizons on granite, and to weakly and moderately lateritised horizons on basalt (Fig. 6; see also Tab. 1).

Trace elements behavior relative to parent rocks are given in the figure 7 (see also Tab. 2) and their relations to each other and major elements in figures 8. Ba, Rb, Sr and Zr are the most abundant trace elements in the granite with up to 1035 ppm for Ba (Tab. 2); trace elements in weathering horizons are further enriched relative to the granite except Sr, Rb, Ba, Y and Zn, which are depleted (Fig. 7a). Nb, Zr, Ga, Cu, Ni, Co, Cr, V, Sc, Th, Hf, U and Ta
216 contents increase from the fresh granite to the top of the lower section of the profile (Fig. 7a;
217 Tab. 2) and vary together in the weathering horizons (e.g., r²Cr-Th=0.81; Fig. 8a-b). Trace
218 elements are also IOL dependent with a significant enrichment with increasing laterization
219 (e.g., r²IOL-Th=0.98; Fig. 8c-d). Their distribution in the section is linked to major oxides like
220 TiO₂, Al₂O₃, and Fe₂O₃ to some extent (e.g., r²Ni-Al₂O₃=0.90; Fig. 8e-h).

221 In the upper litho-section of the profile on basalt, Nb, Y, Sr, Ba, Zn, Ni, Co and Sc are
222 the most abundant trace elements in the parent basalt (Fig. 7b; Tab. 2). Two groups of trace
223 elements are further distinguished based on their content relative to the basalt. As in the lower
224 unit, the first group comprises high field strength elements (HFSE) Zr, Ga, Cr, V, Pb, Th, Hf,
225 U and Ta, which are known to be stable during lateritic weathering (Mordberg, 1996), and are
226 thus enriched relatively to their amount in the basalt (Fig. 7b; Fig. 8a-b). The second group
227 comprises depleted elements relative to basalt as Sr, Rb, Ba, Zn, Cu, Ni, Co and As. Enriched
228 trace elements in weathering horizons (ΣHFSE) of this litho-section show increasing contents
229 from the mottled clay M1b to the surface pebbly horizon M2b (Tab. 2). In contrast, Nb, Ba,
230 Zn, Ni, Cu, Co, As, Sc, U and Ta contents decrease toward the surface. At the top of mottled
231 clay (M1b), Nb, Zr, Y, Ni, Ga, Cr, V, Sc, Pb, Th, Hf, U and Ta are relatively enriched (Fig.
232 7b and Tab. 2). In this profile section, trace elements behave differently, except some HFSE
233 (e.g., r²Hf-Ga= 0.90; Fig. 8a-b). They instead vary with IOL and major elements like Al₂O₃ and
234 TiO₂ (e.g., r²Ni-TiO₂=-0.87; Fig. 8d-e) owing to kaolinite and anatase control, while negative
235 relation to Fe₂O₃ content (e.g., r²Ni-Fe₂O₃=-0.63; Fig. 8g-h) is suggestive of their release from
236 Fe-rich primary minerals (olivine, pyroxene) upon still ongoing basalt weathering. The
237 geochemical discrimination diagram Zr/Ti vs Nb/Y (Fig. 8i; based on Winchester and Floyd,
238 1977) further strengthens the parental lithological differences (Figs. 8b-d-f-g). Basalt is richer
239 in Ti, Zr, Nb and Y than granite, and the behaviour of these elements is quite contrasted in the
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two weathered litho-sections (Tabs. 1 and 2) that portrays maintained geochemical signatures

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during sequential profile maturation.

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4.3. REE behavior across the composite profile

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REE contents vary from 0.13 ppm to 516 ppm in the composite profile (Tab. 3) with

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maximum abundances for LREE. In the lower section, LREE and HREE are mostly depleted

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in the nodular horizon M1g while enriched in the yellowish-red mottled clays M2g relative to

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the granite, except La, Ce and Tb that are depleted in the overall lower unit of the profile (Fig.

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9a; Tab. 3). In the upper section, LREE namely La, Pr, Ce and Nd are enriched while Sm, Eu,

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Gd and LREE show contrasted behavior and are either depleted or enriched relative to the

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basalt (Fig. 9b; Tab. 3). REE vertical distribution displays a slight increase along the profile

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section with apparent enrichments at the top of the nodular duricrust M1g and the top of the

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yellowish mottled clay (Tab. 3).

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The C1 chondrite normalized REE spectra (Mc Donough and Sun, 1995) for granite

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and weathered samples are similar and evidence important HREE fractionation relative to

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LREE along the lower section of the profile (Fig. 9c). The normalization pattern further

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highlights Ce* and Eu* anomalies (Fig. 9c; Tab. 3). Indeed, the granite and the yellowish-red

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mottled clay M2g show Ce/Ce* values of ~1 (Tab. 3) indicating no depletion/enrichment

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relative to La and Pr (Fig. 9c). Lowest Ce/Ce* values with minimum of 0.41 in the nodular

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horizon M1g (Tab. 3) further imply negative Ce anomaly in this horizon (Fig. 9c).

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Normalization patterns also highlight highest Ce/Ce* for the basalts (0.95) while decreasing

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values in the yellowish mottled clay M1b and the pebbly surface horizon M2b to 0.65 (Tab. 3)

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are also significant of reducing conditions upon basalt weathering (Fig. 9d).

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The C1 chondrite normalized Eu/Eu* further display negative anomalies across the

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whole profile (Fig. 9c-d). Granite (Eu/Eu*=0.53) and basalt (Eu/Eu*=0.91) anomalies (Tab.
3) first indicate a depleted Eu granite relative to Eu- and plagioclase rich basalt. Eu/Eu*
variations across the profile further show lower values in the granite section (~0.6) with well-
marked anomalies (Fig. 9c), while the basalt litho-section has higher values and lower
anomalies in (~0.8; Fig. 9d). Eu/Eu* increases with IOL in the granite section of the profile
(Fig. 10a) with no clear trend but higher IOL and Eu/Eu* in the basalt section (see also Tab. 1
and Fig. 6). The overall ΣREE dependence on IOL is also better expressed in the lower part
of the litho-section on granite (r²ΣREE-IOL= 0.96) than in the higher litho-section on basalt (Fig.
10b).

Highest La/Y values occur in the nodular horizon M1g, while the lowest values are
recorded upper in the yellowish-red mottled clay M2g in the lower section; then La/Y
decreases upward in the upper section of the profile (Tab. 3). Ratio La/Y evolves like ΣREE
in the higher and the lower section of the profile (r²ΣREE-La/Y = 0.97 vs.0.87), while (La/Yb)N
varies less with ΣREE (r²=0.66; Fig. 10c). The (Gd/Yb)N ratio show lowest values (2-4) in the
nodular horizon, the yellowish red mottled clay and the surface pebbly horizon (Tab. 3). Its
highest value 10.33 along with highest (La/Yb)N in yellowish mottled clays M1b of the upper
section on basalt (Fig. 10d) highlight LREE enrichment relative to HREE in early stage of
lateritic weathering. Eu/Eu* vs (Gd/Yb)N differentiates end terms fractionation between the
lower litho-section on granite and the upper litho-section on basalt (Fig. 10e), while (La/Yb)N
and Ce/Ce* differently evolve (Fig. 10f).

In the lower litho-section on granite, ΣREE are highly related to Fe₂O₃ and P₂O₅
amounts (r²=0.97, and r²=0.95, respectively) indicating iron oxides and phosphates as
potential hosts minerals (Braun et al., 1993). Inversely, their opposite behavior from SiO₂ and
Al₂O₃ (r²=-0.94, and r²= - 0.74, respectively) is indicative of their release during dissolution of
primary Si-Al minerals without secondary trapping by kaolinite. However, in the upper
section on basalt, ΣREE variations with SiO₂, Al₂O₃, TiO₂, MnO₂ and P₂O₅ (053 < r² < 0.99)
290 indicate kaolinite, iron oxides, ilmenite and phosphates as potential carriers. But, their
291 behavior different from FeO₂ (r²=-0.88) is instead indicative of ongoing REE release from
292 residual ferruginous primary minerals (olivines and pyroxenes) without trapping by secondary
293 iron oxy-hydroxides given the low IOL calculated in the upper section of the profile (see Tab.
294 1; and Fig. 6).
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296 **5. Discussion**

297 **5.1. Differentiated geochemical weathering across the composite lateritic weathering profile**
298 Petro-geochemical differentiations across the composite profile reflect different in-situ
299 weathering paths owing to the different lithologies and age of each profile litho-section that
300 control the distribution of major elements. These differentiations resulted in a Si-rich lower
301 section on granite while Fe-rich horizons developed on basalt. The lower litho-section on
302 granite is mainly characterized by processes of kaolinization to weak laterization (see Fig. 6),
303 which have preserved parent rock structure in the uppermost mottled clays horizon, with
304 obvious ferruginizations in maintained fractures (Figs. 4e). Heating effect of basalt
305 outpouring ~ 9 Myr. ago on clay rich mottled clays may have further exaggerated the pre-
306 existing fractures in a “shrinkage slots” pattern, that favored to Fe-oxyhydroxides
307 crystallizations/coating (Fig. 4e), and/or transfers and nodular segregations of iron in the
308 underlying granitic litho-section during late Neogene weathering (Fig. 11). The lowermost
309 nodular horizon M1g of the granite litho-section therefore results from relative downward
310 enrichment of Fe (Figs. 6 and 7a) that partially deleted the original granite structure (see Figs.
311 4b). At the top of this nodular horizon, formation of centimeter sized ferruginous concretions
312 and nodules (Figs. 4c) in a loose inter-nodular matrix along with increasing Fe₂O₃ content and
313 IOL (Fig. 6; Fig. 7a) emphasize mobilization and further segregation of iron under contrasted
conditions in an area of water table fluctuation (see Beauvais, 1999). In such conditions, 314
formation of nodules results from redox-controlled dissolution/precipitation processes of iron, 315
which preferentially accumulated in kaolinite-rich domains of mottled clays (Tardy and 316
Nahon, 1985; Ambrosi and Nahon, 1986a; Muller and Bocquier, 1986; Tardy, 1997; 317
Beauvais, 1999, 2009; Löhr et al., 2013) to ultimately form ferruginous nodules and/or 318
nodular ferricrete. 319

Trace elements display contrasted behaviors; Cu, Ni, Co are enriched in weathering 320
horizons on granite while depleted in horizons on basalt. Trace elements composition also 321
reveals increasing contents of Nb, Zr, Ga, Ni, Co, Sr, and Rb upward, which vary with major 322
elements in the lower litho-section on granite (Tab. 2). In contrast, these elements decrease 323
from the bottom to the top of the upper litho-section of the profile on basalt (Tab. 2), after 324
their release from relic pyroxene and olivine. Trace elements behavior to major elements 325
(0.56≤r²≤0.71) in the basalt litho-section portrays their enrichment in the weathering 326
profiles by trapping in iron oxides, anatase and possibly P-bearing minerals (Ambrosi and 327
Nahon, 1986b; Mordberg, 1996). 328

Fractionation differences of LREE (La and Ce) vs HREE (Yb) and Ce* anomaly 329
depend on varying redox conditions across the composite profile (Braun et al., 1993; 330
Boulangé and Colin, 1994; see also Brown and Helmke, 2003; Su et al., 2017), e.g., as 331
illustrated in figure 4e. The variation of (La/Yb)N vs. ΣREE (Fig. 10c) corroborates 332
differential REE fractionation with laterization grade (different IOL) and possibly pH 333
variations in the two litho-sections (Zarasvandi et al., 2012). Negative Ce/Ce* anomaly is 334
stronger in the granite litho-section than on basalt (Figs. 9b-c) reflecting differences of redox 335
conditions, (Braun et al., 1993; Beauvais, 1999; Momo Nouazi et al, 2019). Ce depletion 336
relative to La and Pr generally depends on reducing conditions (see Compton et al., 2003; 337
Nyakairu and Koeberl, 2001). The Eu/Eu* negative anomaly is also stronger in the granite 338
339 litho-section than on basalt (Fig. 9c-d) that emphasizes the lithological dependence of less
340 weathered horizons on relatively young basalt compared to granite. Higher Eu/Eu* anomaly
341 rand relatively higher IOL in the basalt litho-section (Fig. 6) depend on residual Fe-Mg
342 minerals in the early-stage weathering but moderate laterization process of basalt (Tab. 1 and
343 Fig. 6).

344 The last stage of profile development after the emplacement of the ~ 9 Myr. old
345 basalts upon weathered granitic regolith was still sustained by per-humid climatic conditions
346 on Adamaoua that also prevailed more widely across the continental intertropical belt during
347 the Mid-Miocene climatic optimum (e.g., Beauvais et al., 2008; Jean et al., 2020).

348

349 **5.2. Landscape erosion signatures from the composite lateritic weathering profile**

350 The unconformity between the weathered basaltic cover and the mottled clay horizon
351 formed in situ on granite (Fig. 3a) marks the erosion surface upon which the basalts outpoured
352 ~ 9 Ma ago. Around Mayo-Bali (Fig. 1), this erosion surface exposes deeper weathering
353 horizons, like mottled clays, after truncation of the top ferricrete (Fig. 11a-b) although
354 remnants may be preserved at lower altitudes (~1200 m) over syn-tectonic granitoids (Fig. 2a-
355 d). The general rolling morphology of the pediplain formed on granite (Fig. 1b) does not
356 show younger stepped relics that suggests slow landscape incision and erosion limited to the
357 duricrusted top of lateritic profiles (see Chardon et al., 2018). Under persistent per-humid
358 climatic conditions, erosion of ferricrete-capped glacis/pediment apparently prevailed over
359 rivers down-cutting (Grandin, 1976). That resulted in a low relief saprolite/mottled clay-
360 mantled erosion surface (Fig. 11b) in between shallow incised valleys, in which Neogene
361 basalts have spilled out covered the downslope eroded portions of the lateritic pediplain
362 previously formed on granite (Fig. 11c).
363 Mid- to late Miocene volcanics outpourings through pre-existing structurally controlled discontinuities across the granitic basement resulted in limited local incision of previous lateritic pediplains formed on granites. Note also that iron-poor Pan-African basement of Adamaoua limited regolith duricrusting and strength allowing lateritic profiles truncation on pediplains. After ~9 Ma, basalts/andesites weathered and their lateritic regolith evolved with marked hillside incisions into a younger surface (Figs. 1b and 2e-f). Late morphogenetic processes related to weathering/duricrusting on basalts also included late Neogene lateritic weathering previously formed profiles on the granitic basement (Fig. 11d).
364
365
366
367
368
369
370
371 Landscape topography has been only locally rejuvenated under persistent per-humid climatic conditions and sustained drainage that have however mostly enhanced weathering and geochemical differentiations in the composite profile (Fig. 11d). In Eastern Adamaoua, rugged erosional terrains with average slope gradient of ~4 to 5% between wide domains of pediplains (Fig. 1b) also portray hillslope erosion that preferentially occurred on inherited litho-structural discontinuities (see also Sawadogo et al., 2020). Landscape physical rejuvenation was also previously observed in western highlands of Cameroon (Momo Nouazi et al., 2019) on Mid-Miocene basalts and trachytes, and possibly occurred during a period (i.e., Pliocene) of mechanical (clastic) erosion (Tchouatcha et al., 2010).
371
372
373
374
375
376
377
378
379
380

381 **6. Conclusion**

Upon Eastern Adamoua highland, Neogene basalts outpourings through inherited structural discontinuities have locally spread in shallow incisions of low-relief lateritic pediplains previously established upon granitic basement. This resulted in the formation of superimposed composite weathering profiles on granite and basalt. Though the studied profile is moderately lateritised, the two sections on granite and basalt are differentiated by their variable index of laterization (IOL), and specific behaviors of some trace and REE elements
382
383
384
385
386
387
388 relative to their respective amounts in each parent rock. Transition elements Cu, Ni and Co
389 are more enriched in the weathering section on granite than on basalt. Owing to contrasted
390 amounts of heavy minerals in the two parent rocks, Zr, Ti, Nb and Y also shows a contrasted
391 pattern, with decreasing (Zr/Ti) and increasing (Nb/Y) in the granite litho-section while
392 (Zr/Ti) remains constant in basalt litho-section. Differences of silicate minerals composition
393 of the two parent rocks also induced a differential weathering with negative Eu* anomaly
394 higher on granite section than on basalt. Eu* also increases with increasing IOL in the granite
395 litho-section, while REE further fractionated as indicated by the amplitude of co-variations of
396 (La/Yb)N and Ce*. After Neogene basalt outpourings on pediplain landscapes of Adamaoua
397 highland, persistent humid climatic conditions and efficient drainage have sustained rock
398 weathering processes with a noticeable differentiation and fractionation of trace and REE
399 elements across the composite lateritic profile.

400

401 Acknowledgements- The IRD program JEAI is thanked for funding and support to the
402 MORPHOCAM project under which this work has been done. Financial support of the APIC
403 annual programs (2019-2020) from the "Centre de Recherche et d'Enseignement des
404 Géosciences de l'Environnement" (CEREGE) is also acknowledged. IRD (Disco department)
405 is also thanked for granting MNM stays at Cerege, Aix Marseille Université, OSU Pytheas,
406 France. Remarks and suggestions from two anonymous reviewers and the editor Dr.
407 Scarciglia are also much acknowledged.

408

409

410
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551

552 **FIGURE CAPTIONS**

553 Figure. 1. a) Localization and geologic map of the study area. Structural, petrographic and
554 chronological data are adapted from Lasserre (1961); b) Geomorphological cross
555 section showing regolith landform distribution according to lithological variations in
556 Mayo-Bali area.

557 Figure. 2. Field illustration of lateritic landscape and regolith in the Eastern Adamaoua
558 plateau. a) Late Miocene volcanics morphology surrounded by alluvial plains and
559 granite-related pediments; b) Regolith structure of pediment with a nodular ferricrete
560 embedding massive and undifferentiated fragments; c) Footslope pediment below
561 erosional slope exposing bedrock (black arrow). d) Regolith structure of a footslope
562 pediment; e) Slope erosion of duricrusted lateritic regolith surface formed on basalt;
563 f) Regolith landscape on basalt with hillslope incision and formation of a younger
564 surface.

565 Figure. 3. a) Profile structure of the composite lateritic profile; b-d) Details of horizons
566 showing textural and lithological changes. The blue dotted line marks the lithological
567 unconformity between the lower and the upper part of the profile. b) transition
568 between the nodular horizon M1g and the overlying yellowish-red mottled clay M2g,
569 marked by the yellow dotted line in 3a; c) transition between the mottled clay M2g of
570 the lower section and the yellowish mottled clay M1b of the upper section; d)
571 transition between the yellowish mottled clay M1b and the surface gravelly horizon
572 M2b; evidences of prismatic basalts are shown. Numbered white squares are the
573 collected samples and correspond to samples ID in the tables.
574 Figure. 4. a) Granite sample of the Pan-African basement; b) lower part of the nodular

575 horizon with few nodules; c) Nodular horizon M1g with numerous nodules; d)

576 Undifferentiated structure of mottled clay M2g; e) Mottled clay M2g with preserved

577 granite strcutures.

578 Figure. 5. a) Aphiric basalt sample belonging to the early volcanics on the Adamaoua plateau.

579 b) Mottled clays M1b; c) Surface pebbly horizon M2b.

580 Figure. 6. Composition diagram Al-Si-Fe and index of laterization (IOL) along the composite

581 profile

582 Figure. 7. Granite and basalt normalized trace elements spider-diagrams of the different

583 horizons of the composite profile

584 Figure 8. Trace and major elements scattergrams. a) Hf vs Ga; b) Cr vs Th; c) IOL vs Cr; d)

585 IOL vs Th; e) TiO₂ vs Ni; f) Al₂O₃ vs Ni; g) Fe₂O₃ vs Ni; h) Fe₂O₃ vs Ga; i) Zr/Ti vs

586 Nb/Y

587 Figure. 9. a & b) Granite and basalt normalized REE spider-diagram of the different horizons

588 of the profile. c & d) Chondrite normalized REE spider-diagram of the different

589 horizons of the profile

590 Figure. 10. Geochemical diagrams of REE fractionation across the composite profile. a)

591 Eu/Eu* vs. IOL; b) ΣREE vs. IOL; c) ΣREE vs (La/Yb)N; d) (La/YB)N vs (Gd/Yb)N;

592 e) Eu/Eu* vs (Gd/Yb)N; f) (La/YB)N vs. Ce/Ce*

593 Figure. 11. Schematic evolution of the lateritic weathering, and erosion of Eastern Adamaoua

594 landscape. a) Lateritic weathering profile of pediplains formed on the granitic

595 basement of Adamaoua highland. b) Erosion-truncated profile exposing mottled

596 clays at relatively high altitude (> 1200 m a.s.l.) at Mayo-Bali. c) Basaltic strata over
597 previously truncated profiles of pediplain formed on granite. d) Second stage of
598 landscape dynamics marked by lateritic weathering of both basalt and underlying
599 profile on granite.

600

601 TABLES CAPTION

602 Table 1. Major elements content (wt%) of each sample collected in the composite profile and
603 the fresh rocks. LOI = Lost of ignition; Detection limits: 0.01; (-) = Elements below
604 detection limits; IOL (%) = Index of Laterization; See figure 3a for the location of
605 each sample. Depths of fresh rocks samples are unknown.

606 Table 2. Trace elements content (µg/g) of each sample collected in the composite profile and
607 the fresh rocks. d.l. = Detection limits; (-) = Elements below detection limits.

608 Table 3. Rare earth elements content (µg/g) of each sample collected in the composite profile
609 and the fresh rocks. d.l. = Detection limit.

$$
610 \quad \text{Ce/Ce*}(\text{Ce}_{\text{sample}}/\text{Ce}_{\text{chondrite}})/(\text{La}_{\text{sample}}/\text{La}_{\text{chondrite}})^{1/2} (\text{Pr}_{\text{sample}}/\text{Pr}_{\text{chondrite}})^{1/2}
$$

$$
611 \quad \mathrm{Eu}/\mathrm{Eu}^* = (\mathrm{Eu}_{\text{sample}}/\mathrm{Eu}_{\text{chondrite}})/(\mathrm{Sm}_{\text{sample}}/\mathrm{Sm}_{\text{chondrite}})^{1/2} (\mathrm{Gd}_{\text{sample}}/\mathrm{Gd}_{\text{chondrite}})^{1/2}
$$

$$
612 \quad (\mathrm{La}/\mathrm{Yb})_N = (\mathrm{La}_{\text{sample}}/\mathrm{La}_{\text{chondrite}})/(\mathrm{Yb}_{\text{sample}}/\mathrm{Yb}_{\text{chondrite}})
$$

$$
613 \quad (\mathrm{Gd}/\mathrm{Yb})_N = (\mathrm{Gd}_{\mathrm{sample}}/\mathrm{Gd}_{\mathrm{chondrite}})/(\mathrm{Yb}_{\mathrm{sample}}/\mathrm{Yb}_{\mathrm{chondrite}}).
$$

614
<figure><img src="image_2.png" /><figcaption>Fig. 1</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 2</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 3</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 5</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 6</figcaption></figure>
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 8</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Figure - Upload high quality Images</figcaption></figure>

Click here to access/download:Figure - Upload high quality Images;Figure. 9.eps

<figure><img src="image_12.png" /><figcaption>Fig. 9</figcaption></figure>
<caption>Figure - Upload high quality Images</caption>

<figure><img src="image_13.png" /><figcaption>Click here to access/download; Figure - Upload high quality Images; Figure. 10.eps</figcaption></figure>

Fig. 10
<figure><img src="image_14.png" /></figure>

Post-Miocene, 11 Ma to modern landscape

Stage 3

Pre-Miocene

Stage 1

Stage 2

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

Stage 4

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

lithological unconformity

Nodular ferricrete

Ferruginous carapace

Mottled clays &
Fe/nodular horizon

Saprolite

Granite

Fracturation

<figure><img src="image_21.png" /></figure>

Eroded

<figure><img src="image_22.png" /></figure>

In-situ Fe-nodules formation

<figure><img src="image_23.png" /></figure>

Fe-coated fractures

<figure><img src="image_24.png" /><figcaption>Fig. 11</figcaption></figure>
<table><caption>Table 1</caption><thead><tr><th>Samples ID</th><th>Horizon ID</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Fe<sub>2</sub>O<sub>3</sub></th><th>TiO<sub>2</sub></th><th>CaO</th><th>MgO</th><th>Na<sub>2</sub>O</th><th>K<sub>2</sub>O</th><th>MnO</th><th>P<sub>2</sub>O<sub>5</sub></th><th>Total</th><th>LOI</th><th>IOL</th></tr></thead><tbody><tr><td>MEI-9</td><td>M2b</td><td>23.5</td><td>26.7</td><td>30.4</td><td>2.3</td><td>0.02</td><td>0.11</td><td>0.01</td><td>0.06</td><td>0.03</td><td>0.29</td><td>100.42</td><td>11.30</td><td>70.8</td></tr><tr><td>MEI-8</td><td>M2b</td><td>26.7</td><td>28.4</td><td>23.8</td><td>3.43</td><td>0.03</td><td>0.16</td><td>0.01</td><td>0.06</td><td>0.05</td><td>0.43</td><td>99.9</td><td>13.10</td><td>66.2</td></tr><tr><td>MEI-7</td><td>M1b</td><td>28.3</td><td>29.7</td><td>19.85</td><td>4.59</td><td>0.06</td><td>0.14</td><td>0.01</td><td>0.02</td><td>0.05</td><td>0.62</td><td>99.26</td><td>12.60</td><td>63.6</td></tr><tr><td>MEI-6</td><td>M1b</td><td>28.5</td><td>26.6</td><td>25.1</td><td>3.75</td><td>0.01</td><td>0.15</td><td>0.01</td><td>0.03</td><td>0.03</td><td>0.3</td><td>99.6</td><td>12.35</td><td>64.5</td></tr><tr><td>BAS-MEI</td><td></td><td>42.1</td><td>12.7</td><td>12.15</td><td>2.6</td><td>10.75</td><td>11.7</td><td>2.61</td><td>1.59</td><td>0.19</td><td>0.95</td><td>99.54</td><td>1.9</td><td>37.2</td></tr><tr><td>MEI-5</td><td>M2g</td><td>46.7</td><td>28.7</td><td>8.82</td><td>1.87</td><td>0.02</td><td>0.04</td><td>0.01</td><td>0.05</td><td>0.01</td><td>0.09</td><td>99.56</td><td>13.20</td><td>44.5</td></tr><tr><td>MEI-4</td><td>M2g</td><td>51.4</td><td>28.2</td><td>6.54</td><td>1.63</td><td>0.01</td><td>0.03</td><td>0.01</td><td>0.04</td><td>0.01</td><td>0.08</td><td>100.34</td><td>15.05</td><td>40.3</td></tr><tr><td>MEI-3</td><td>M2g</td><td>47.2</td><td>27.5</td><td>9.81</td><td>1.56</td><td>0.12</td><td>0.03</td><td>-</td><td>0.02</td><td>0.01</td><td>0.09</td><td>98.97</td><td>15.65</td><td>44.1</td></tr><tr><td>MEI-2</td><td>M1g</td><td>31.6</td><td>23.4</td><td>30.3</td><td>1.1</td><td>0.02</td><td>0.03</td><td>-</td><td>0.06</td><td>0.01</td><td>0.14</td><td>99.83</td><td>16.65</td><td>63.0</td></tr><tr><td>MEI-1</td><td>M1g</td><td>56.8</td><td>26.0</td><td>5.33</td><td>0.81</td><td>0.01</td><td>0.03</td><td>0.01</td><td>0.08</td><td>0.01</td><td>0.06</td><td>100.47</td><td>16.85</td><td>35.5</td></tr><tr><td>GRA-MEI</td><td></td><td>71.5</td><td>14.05</td><td>2.26</td><td>0.31</td><td>1.32</td><td>0.44</td><td>3.38</td><td>5.43</td><td>0.04</td><td>0.08</td><td>99.88</td><td>0.93</td><td>18.6</td></tr></tbody></table>
<table><caption>Table 2</caption><thead><tr><th></th><th></th><th>Nb</th><th>Zr</th><th>Y</th><th>Sr</th><th>Rb</th><th>Ba</th><th>Ga</th><th>Zn</th><th>Cu</th><th>Ni</th><th>Co</th><th>Cr</th><th>V</th><th>As</th><th>Sc</th><th>Pb</th><th>Th</th><th>Hf</th><th>U</th><th>Ta</th></tr><tr><th>Samples</th><th>Horizon ID</th><th>dl: 0.1</th><th>2</th><th>0.1</th><th>0.1</th><th>0.2</th><th>0.5</th><th>0.1</th><th>2</th><th>1</th><th>1</th><th>1</th><th>10</th><th>5</th><th>5</th><th>1</th><th>2</th><th>0.05</th><th>0.1</th><th>0.05</th><th>0.1</th></tr></thead><tbody><tr><td>MEI-9</td><td>M2b</td><td>68.8</td><td>489</td><td>14.2</td><td>93.6</td><td>4.5</td><td>162</td><td>40.7</td><td>29</td><td>28</td><td>72</td><td>5</td><td>930</td><td>460</td><td>18</td><td>10</td><td>33</td><td>63.1</td><td>11.8</td><td>6.45</td><td>3.8</td></tr><tr><td>MEI-8</td><td>M2b</td><td>99.4</td><td>509</td><td>22</td><td>288</td><td>4.3</td><td>443</td><td>42.3</td><td>75</td><td>42</td><td>99</td><td>13</td><td>730</td><td>359</td><td>6</td><td>31</td><td>38</td><td>30.8</td><td>12.3</td><td>11.2</td><td>5.5</td></tr><tr><td>MEI-7</td><td>M1b</td><td>134</td><td>491</td><td>48.3</td><td>817</td><td>2.4</td><td>645</td><td>39.7</td><td>76</td><td>41</td><td>152</td><td>15</td><td>680</td><td>377</td><td>-</td><td>28</td><td>66</td><td>20.6</td><td>11</td><td>10.9</td><td>7.4</td></tr><tr><td>MEI-6</td><td>M1b</td><td>103</td><td>546</td><td>12.1</td><td>38.9</td><td>2</td><td>44.7</td><td>42.8</td><td>61</td><td>34</td><td>86</td><td>15</td><td>460</td><td>369</td><td>8</td><td>34</td><td>27</td><td>17.5</td><td>11.5</td><td>14.1</td><td>5.8</td></tr><tr><td>BAS-MEI</td><td></td><td>90.6</td><td>282</td><td>26.6</td><td>923</td><td>47.5</td><td>715</td><td>20</td><td>108</td><td>46</td><td>258</td><td>57</td><td>430</td><td>216</td><td>-</td><td>22</td><td>5</td><td>9.58</td><td>5.8</td><td>2.47</td><td>3.6</td></tr><tr><td>MEI-5</td><td>M2g</td><td>68.3</td><td>882</td><td>15.8</td><td>92.7</td><td>4.9</td><td>93</td><td>50.3</td><td>29</td><td>9</td><td>31</td><td>5</td><td>260</td><td>185</td><td>5</td><td>14</td><td>33</td><td>72.8</td><td>22.4</td><td>9.43</td><td>3.8</td></tr><tr><td>MEI-4</td><td>M2g</td><td>63.2</td><td>855</td><td>15</td><td>82.6</td><td>3.3</td><td>84.9</td><td>46.1</td><td>28</td><td>3</td><td>27</td><td>3</td><td>110</td><td>126</td><td>-</td><td>10</td><td>27</td><td>76.5</td><td>21.1</td><td>6.91</td><td>3.7</td></tr><tr><td>MEI-3</td><td>M2g</td><td>59.8</td><td>747</td><td>12.6</td><td>66.4</td><td>2.5</td><td>76.7</td><td>45.8</td><td>29</td><td>2</td><td>25</td><td>4</td><td>130</td><td>129</td><td>-</td><td>10</td><td>29</td><td>74.3</td><td>18.5</td><td>6.43</td><td>3.5</td></tr><tr><td>MEI-2</td><td>M1g</td><td>40.4</td><td>522</td><td>8.1</td><td>75.8</td><td>5.4</td><td>94.7</td><td>54.9</td><td>23</td><td>3</td><td>21</td><td>1</td><td>400</td><td>370</td><td>14</td><td>14</td><td>69</td><td>125</td><td>13.1</td><td>12.6</td><td>2.2</td></tr><tr><td>MEI-1</td><td>M1g</td><td>32.5</td><td>376</td><td>8.3</td><td>54.4</td><td>6.8</td><td>83.5</td><td>34.7</td><td>20</td><td>1</td><td>24</td><td>2</td><td>170</td><td>75</td><td>-</td><td>6</td><td>17</td><td>57.7</td><td>9.5</td><td>5.48</td><td>1.7</td></tr><tr><td>GRA-MEI</td><td></td><td>21.7</td><td>240</td><td>14.1</td><td>208</td><td>277</td><td>1035</td><td>23.2</td><td>50</td><td>1</td><td>3</td><td>2</td><td>20</td><td>21</td><td>-</td><td>2</td><td>32</td><td>42</td><td>6.4</td><td>3.3</td><td>1.1</td></tr></tbody></table>
<table><caption>Table 3</caption><thead><tr><th></th><th></th><th></th><th>La</th><th>Ce</th><th>Pr</th><th>Nd</th><th>Sm</th><th>Eu</th><th>Gd</th><th>Tb</th><th>Dy</th><th>Ho</th><th>Er</th><th>Tm</th><th>Yb</th><th>Lu</th><th>ΣREE</th><th>La/Y</th><th>Ce/Ce*</th><th>Eu/Eu*</th><th>(La/Yb)<sub>N</sub></th><th>(Gd/Yb)<sub>N</sub></th></tr><tr><th>Samples</th><th>Horizon ID</th><th>dl :</th><th>0.1</th><th>0.1</th><th>0.02</th><th>0.1</th><th>0.03</th><th>0.02</th><th>0.05</th><th>0.01</th><th>0.05</th><th>0.01</th><th>0.03</th><th>0.01</th><th>0.03</th><th>0.01</th><th></th><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td>MEI-09</td><td>M2b</td><td></td><td>80.6</td><td>135</td><td>14.5</td><td>47.9</td><td>8.37</td><td>2.12</td><td>6.27</td><td>0.82</td><td>4.04</td><td>0.68</td><td>1.56</td><td>0.23</td><td>1.35</td><td>0.17</td><td>303.66</td><td>5.68</td><td>0.94</td><td>0.84</td><td>6.49</td><td>3.72</td></tr><tr><td>MEI-08</td><td>M2b</td><td></td><td>219</td><td>248</td><td>37.5</td><td>107.5</td><td>16.3</td><td>3.82</td><td>10.35</td><td>1.36</td><td>7.19</td><td>1.23</td><td>2.78</td><td>0.36</td><td>2.04</td><td>0.27</td><td>657.70</td><td>9.95</td><td>0.65</td><td>0.85</td><td>11.67</td><td>4.06</td></tr><tr><td>MEI-07</td><td>M1b</td><td></td><td>420</td><td>516</td><td>77.2</td><td>256</td><td>42.8</td><td>9.91</td><td>28.8</td><td>3.41</td><td>17.1</td><td>2.47</td><td>5.07</td><td>0.5</td><td>2.23</td><td>0.26</td><td>1381.75</td><td>8.70</td><td>0.68</td><td>0.81</td><td>20.47</td><td>10.33</td></tr><tr><td>MEI-06</td><td>M1b</td><td></td><td>128.5</td><td>147.5</td><td>21.1</td><td>61.7</td><td>8.96</td><td>1.72</td><td>5.05</td><td>0.67</td><td>3.47</td><td>0.56</td><td>1.39</td><td>0.19</td><td>1.27</td><td>0.17</td><td>382.25</td><td>10.62</td><td>0.67</td><td>0.74</td><td>11.00</td><td>3.18</td></tr><tr><td>BAS-MEI</td><td></td><td></td><td>69.8</td><td>126</td><td>14.3</td><td>53.2</td><td>10.25</td><td>2.95</td><td>8.57</td><td>1.11</td><td>6.09</td><td>1.05</td><td>2.65</td><td>0.36</td><td>1.98</td><td>0.28</td><td>298.59</td><td>2.62</td><td>0.95</td><td>0.91</td><td>3.83</td><td>3.46</td></tr><tr><td>MEI-05</td><td>M2g</td><td></td><td>88.1</td><td>157.5</td><td>15.7</td><td>50.3</td><td>8.01</td><td>1.69</td><td>5.58</td><td>0.73</td><td>3.74</td><td>0.72</td><td>2.05</td><td>0.28</td><td>2.00</td><td>0.30</td><td>336.70</td><td>5.58</td><td>1.01</td><td>0.73</td><td>4.79</td><td>2.23</td></tr><tr><td>MEI-04</td><td>M2g</td><td></td><td>91.2</td><td>140</td><td>15.5</td><td>50.5</td><td>8.32</td><td>1.75</td><td>6.69</td><td>0.8</td><td>4.04</td><td>0.73</td><td>1.95</td><td>0.28</td><td>1.72</td><td>0.27</td><td>323.75</td><td>6.08</td><td>0.89</td><td>0.68</td><td>5.76</td><td>3.11</td></tr><tr><td>MEI-03</td><td>M2g</td><td></td><td>86</td><td>115.5</td><td>14.8</td><td>50.1</td><td>8.44</td><td>1.57</td><td>5.77</td><td>0.71</td><td>3.51</td><td>0.59</td><td>1.54</td><td>0.23</td><td>1.56</td><td>0.25</td><td>290.62</td><td>6.83</td><td>0.77</td><td>0.65</td><td>5.99</td><td>2.96</td></tr><tr><td>MEI-02</td><td>M1g</td><td></td><td>217</td><td>152</td><td>36.2</td><td>98.7</td><td>11.05</td><td>1.6</td><td>4.26</td><td>0.47</td><td>2.45</td><td>0.43</td><td>1.07</td><td>0.15</td><td>0.99</td><td>0.16</td><td>526.53</td><td>26.79</td><td>0.41</td><td>0.67</td><td>23.82</td><td>3.44</td></tr><tr><td>MEI-01</td><td>M1g</td><td></td><td>85.1</td><td>98.9</td><td>15.7</td><td>53.9</td><td>8.37</td><td>1.48</td><td>4.83</td><td>0.58</td><td>2.67</td><td>0.43</td><td>1.05</td><td>0.15</td><td>0.87</td><td>0.13</td><td>274.16</td><td>10.25</td><td>0.64</td><td>0.67</td><td>10.63</td><td>4.44</td></tr><tr><td>GRA-MEI</td><td></td><td></td><td>103.5</td><td>181</td><td>18.15</td><td>55.5</td><td>7.8</td><td>1.17</td><td>5.13</td><td>0.56</td><td>3.2</td><td>0.55</td><td>1.36</td><td>0.19</td><td>1.15</td><td>0.16</td><td>379.42</td><td>7.34</td><td>0.99</td><td>0.53</td><td>9.78</td><td>3.57</td></tr></tbody></table>