Paragneiss

The NID-1 sample is a high-grade aluminous paragneiss exhibiting a well-developed gneissic foliation striking approximately N 45˚ and dipping 80˚ SE. The foliation is defined by alternating quartz-rich and biotite-rich layers and is locally affected by tight to isoclinal folds (Figure 3(a), Figure 3(b)). In thin section, the rock displays a medium-grained granolepidoblastic texture, except for garnet porphyroblasts, and consists predominantly of quartz, garnet, biotite and sillimanite, with subordinate feldspar (Figure 3(c)). Quartz forms elongated ribbons parallel to the foliation, commonly arranged in bands of 1 - 2 mm thick. Garnet occurs as subhedral porphyroblasts reaching up to 3 mm in diameter and commonly contains inclusions of quartz, biotite and rutile. Biotite forms reddish-brown flakes that define the rock foliation, whereas sillimanite occurs as abundant prismatic crystals preferentially associated with the biotite-rich domains. Numerous opaque minerals are also aligned parallel to the foliation. Accessory minerals include zircon, monazite and pyrite, listed in decreasing order of abundance. Zircon occurs as small prismatic to rounded grains, whereas monazite is generally associated with the biotite-rich domains. Pyrite is present as disseminated euhedral crystals throughout the rock.

Figure 3. Field photographs and photomicrographs illustrating the petrographic features of the Archean gneiss samples analyzed in this study. (a-c) Images corresponding to NID-1 and (d-f) to TRO-4. (a) Outcrop showing subvertical gneissic foliation; (b) hand specimen with garnet porphyroblast and elongated quartz crystal; (c) photomicrograph in plane-polarized light displaying the characteristic garnet-sillimanite-biotite assemblage, indicative of a high-grade protolith; (d) dome-shaped outcrop of the TRO-4 orthogneiss; (e) hand specimen with weakly developed foliation; (f) photomicrograph in crossed-polarized light showing quartz-plagioclase-orthopyroxene-myrmekite, consistent with granulite-facies conditions.

Orthogneiss

In the field, TRO-4 sample is spatially associated with the aluminous paragneiss described above, although the contact between the two lithologies was not observed. The rock forms strongly migmatized, dome-shaped outcrops and is composed predominantly of quartz and feldspar (>90 vol.%) (Figure 3(d)). Relict pockets of smoky quartz are locally preserved within the outcrop. Foliation is weakly developed owing to the scarcity of phyllosilicate minerals, whereas abundant pink K-feldspar gives the rock its characteristic grey to pink colour (Figure 3(e)). Under the microscope, the orthogneiss exhibits a predominantly granoblastic texture ranging from fine- to coarse-grained. Centimetre-scale leucocratic domains composed mainly of quartz and pink K-feldspar are readily distinguished from the surrounding matrix by their markedly coarser grain size (Figure 3(f)). Garnet is preferentially concentrated within these leucocratic domains, where crystals locally attain diameters of up to 1 cm. The surrounding matrix is fine-grained and contains biotite and hypersthene, imparting a pale green colour to the rock. Garnet commonly develops fine symplectitic intergrowths at contacts with quartz and biotite and locally contains fine sillimanite needles.

Ages of NID-1

Zircons extracted from NID-1 are approximately 200 μm long and are predominantly dark brown in transmitted light, with locally transparent central domains. Most grains display rounded terminations and contain numerous alteration zones together with small apatite inclusions. The cathodoluminescence (CL) images reveal well-developed oscillatory zoning bordered by thin luminescent rims (Figure 4). The zircon cores occupy the largest proportion of each grain and are commonly surrounded by narrow overgrowths. Sixteen in-situ LA-ICP-MS analyses are reported for the NID-1 paragneiss (Table 3); age interpretation follows the adopted 95% concordance criterion. The analysed zircons display Th/U ratios ranging from 0.05 to 2.28, whereas the measured ²⁰⁶Pb/²³⁸U ages range from 2582 to 3165 Ma. The ²⁰⁷Pb/²⁰⁶Pb ages define younger and older Archean components. The younger group is characterized by Discordia upper-intercept ages of 2895 ± 18 Ma (n = 2) and 2820 ± 30 Ma (MSWD = 0.47; n = 3). The older group yields an upper-intercept age of 3089 ± 23 Ma (MSWD = 0.88; n = 8) together with a concordant age of 3164 ± 41 Ma (MSWD = 1.7; n = 2). Spot #10, which is slightly affected by Pb loss, was excluded from further interpretation (Figure 5).

Figure 4.Cathodoluminescence (CL) images of zircon grains from the NID-1 paragneiss sample. Black circles mark the LA-ICP-MS analytical spots used for U-Pb isotope measurements. The scale bar corresponds to 200 µm.

Table 3. U-Pb laser ablation data on zircon from paragneiss NID-1 sample.

Figure 5. Concordia diagram for zircon U-Pb analyses from the NID-1 paragneiss sample. Ellipses represent individual spot analyses, with colors distinguishing analytical reliability and age groups (white = excluded/unreliable analyse, dark blue = younger Archean group, yellow = older Archean group, orange = oldest concordant analyses).

Ages of TRO-4

Zircons separated from the TRO-4 anatectic orthogneiss are predominantly rounded to blunt and generally exhibit a dark appearance in transmitted light. Cathodoluminescence images systematically reveal the presence of inherited cores surrounded by overgrowths with diffuse boundaries (Figure 6). In most grains, the inherited cores account for more than 75% of the zircon surface area. Oscillatory zoning is locally preserved within the overgrowths and is best observed in a few relatively transparent zircon grains. Seventeen in situ LA-ICP-MS analyses performed on sixteen zircon grains are reported in Table 4 and age interpretation follows the adopted 95% concordance criterion. The analysed zircons display Th/U ratios ranging from 0.11 to 2.02, with most ²⁰⁷Pb/²⁰⁶Pb ages older than 3000 Ma. The oldest individual analysis yielded a ²⁰⁷Pb/²⁰⁶Pb age of 3342 ± 18 Ma. The Concordia diagram shows principal Archean age components at ca. 3082 ± 20 Ma, 3204 ± 21 Ma, and 3.32 Ga. Seven analyses yield a mean age of 3320 ± 47 Ma (MSWD = 6.11). The relatively high MSWD indicates significant dispersion among these analyses, suggesting that they do not define a statistically homogeneous zircon population (Figure 7).

Figure 6. Cathodoluminescence (CL) images of zircon grains from the TRO-4 orthogneiss sample. Black circles mark the LA-ICP-MS analytical spots used for U-Pb isotope measurements. The scale bar corresponds to 200 µm.

Table 4. U-Pb laser ablation data on zircon from anatectic orthogneiss TRO-4.

Figure 7. Concordia diagram of zircon U-Pb analyses from the TRO-4 orthogneiss. Ellipses represent individual spot analyses, with yellow indicating Archean zircon and black marking unreliable spots. Red circles show the locations of LA-ICP-MS analyses on representative grains. Zircon images are captured from transmitted light.

The zircon populations identified in the NID-1 paragneiss and TRO-4 orthogneiss record a complex Archean history involving several episodes of zircon crystallization and subsequent crustal reworking. Cathodoluminescence images reveal large cores surrounded by narrow overgrowth rims in many grains, indicating that several generations of zircon growth may be preserved within individual crystals. In the NID-1 paragneiss, the rounded to subrounded morphology of many grains and the sedimentary origin of the protolith require the U-Pb ages to be interpreted primarily as inherited and/or detrital-source ages unless independent evidence demonstrates a single magmatic crystallization event [31].

The analysed zircons display Th/U ratios ranging from 0.05 to 2.28 in the NID-1 paragneiss and from 0.11 to 1.74 in the TRO-4 orthogneiss. Most analyses yield Th/U values greater than 0.1, a range commonly associated with magmatic zircon, whereas lower values may reflect zircon growth or recrystallization under metamorphic conditions. These criteria indicate a predominantly magmatic origin for many zircon domains, although later high-grade metamorphic modification is also recorded [32]-[36] (Figure 8). In NID-1, ages close to 2.82 Ga occur partly on zircon rims, which is compatible with late Archean zircon growth or recrystallization, but similar ages also occur in zircon cores. The ca. 2.82 Ga component is therefore interpreted cautiously as a composite record that may include metamorphic overgrowth/recrystallization together with inherited or detrital zircon components.

Figure 8. Plot of Th/U ratios versus ²⁰⁷Pb/²⁰⁶Pb ages for zircon grains from the NID-1 paragneiss (blue squares) and the TRO-4 orthogneiss (red diamonds). Magmatic and metamorphic zircon origins are distinguished by a Th/U ratio threshold of 0.1. Two age clusters are highlighted by dashed ellipses: ~2700 - 2900 Ma and ~3100 - 3320 Ma, reflecting distinct zircon growth episodes recorded in the studied samples.

The coexistence of several Archean age components within the studied zircon populations indicates incorporation and reworking of crustal material of different ages. In the paragneiss, these ages primarily constrain the ages of zircon source components rather than a unique crystallization age of the sedimentary protolith. In the orthogneiss, the age spectrum likewise records inheritance and subsequent crustal reworking. These relationships are discussed below in the context of the Archean development of the Kenema-Man Domain.

The zircon U-Pb data provide new constraints on the Archean evolution of the Kenema-Man Domain in western Côte d’Ivoire. The oldest individual analysis from TRO-4 yields a Paleoarchean ²⁰⁷Pb/²⁰⁶Pb age of 3342 ± 18 Ma. The seven analyses yielding a mean age of 3320 ± 47 Ma are characterized by a relatively high MSWD of 6.11, indicating that they do not constitute a statistically homogeneous zircon population. Rather than representing a single discrete crystallization event, this dispersion may record zircon crystallization during a protracted period of Paleoarchean crustal accretion and growth. The ca. 3.32 Ga age is therefore considered to represent an ancient crustal component preserved within the TRO-4 orthogneiss, rather than a precisely defined single magmatic event. This interpretation is consistent with previous isotopic evidence suggesting the presence of ancient crust beneath the Man Domain. Whole-rock Sm-Nd model ages between 3300 and 3200 Ma reported for granulitic grey gneisses and related lithologies have long suggested the existence of older crustal components, although direct zircon evidence has remained limited. [11] interpreted these Nd model ages as recording an early stage of continental crustal growth in the Man Rise. Additional support comes from the Balmer tonalitic gneiss, for which [26] obtained an in-situ LA-ICP-MS zircon age of 3207 ± 7 Ma, interpreted as the crystallization age of its magmatic protolith. That study proposed that the Balmer gneiss represents one of the oldest juvenile crustal units currently recognized in Côte d’Ivoire and suggested that even older rocks might still be discovered within the Man Domain. The Paleoarchean zircon components documented here extend the recognized record of ancient crustal material in western Côte d’Ivoire.

At the scale of the southern West African Craton, these new data are consistent with the regional chronological framework summarized by [7], who recognized crust-forming episodes between approximately 3200 and 3000 Ma, together with evidence for older Paleoarchean rocks locally preserved in Guinea and neighbouring regions. Although ages approaching 3.5 Ga have not been identified in the present dataset, unlike the older continental crust reported from Guinea [12][18], the zircon populations described in this study confirm that the Archean basement of western Côte d’Ivoire preserves a long history of crustal evolution, marked both by the retention and subsequent reworking of ancient continental material.

The zircon age spectra obtained from the paragneiss and anatectic orthogneiss indicate that the Archean basement of western Côte d’Ivoire preserves a record of multiple crustal events spanning more than 500 Ma. Rather than reflecting a single episode of crust formation, the occurrence of Paleoarchean zircon components together with younger ca. 3.32, 3.20, 3.08 and 2.82 Ga ages suggests a long-lived history involving magmatic activity, inheritance, crustal reworking and high-grade metamorphism.

The oldest zircon group identified in this study indicates that Paleoarchean crustal material was already present in the western part of the Man region Domain by at least ca. 3.34 Ga. Younger zircon age components document additional episodes affecting this crust during the Meso- to Neoarchean [5][19][37]. The ca. 2.82 Ga ages recorded in NID-1 occur in both rims and cores and therefore cannot be attributed unequivocally to a single metamorphic event; they may represent a composite record of late Archean zircon growth or recrystallization and inherited/detrital input. Although the precise tectonic significance of each age component cannot be established from U-Pb geochronology alone, the overall age distribution is consistent with repeated growth and reworking of pre-existing continental material. These results agree with regional models proposed for the Kenema-Man Domain, in which Archean crust developed through successive magmatic and metamorphic events prior to the Paleoproterozoic assembly of the West African Craton. Similar age ranges have been reported from neighbouring regions of Liberia, Sierra Leone and Guinea, indicating that the Archean terranes of the southern West African Craton shared a broadly comparable geological evolution. The occurrence of Paleoarchean zircon inheritance in western Côte d’Ivoire therefore strengthens the regional continuity of this ancient crustal domain.

Although the present study is based on two representative samples, it provides new geochronological evidence that complements previous isotopic and geochronological investigations in western Côte d’Ivoire. Additional U-Pb analyses combined with Lu-Hf isotopic data from zircon and broader sampling across the Man Domain will be essential to distinguish juvenile crustal growth from crustal reworking and to refine the spatial distribution and evolution of the Paleoarchean basement.