# Columbia supercontinent remnants in the Tuareg Shield
(Iforas, Mali)

Delphine Bosch, Olivier Bruguier, Vincent Monchal, Heloise Pinon

► To cite this version:

Delphine Bosch, Olivier Bruguier, Vincent Monchal, Heloise Pinon. Columbia supercontinent remnants in the Tuareg Shield (Iforas, Mali). Precambrian Research, 2026, 435, pp.108019. 10.1016/j.precamres.2026.108019. hal-05497845

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Research Paper

# Columbia supercontinent remnants in the Tuareg Shield (Iforas, Mali)

Delphine Bosch<sup>a, *,</sup>, Olivier Bruguier<sup>a</sup>, Vincent Monchal<sup>a, b</sup>, Heloise Pinon<sup>a, c</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

- Columbia supercontinent
- Granulite
- Paleoproterozoic
- Iforas Granulitic Unit
- Hoggar
- LASS-ICP-MS
- U-Pb ages

## ABSTRACT

The Iforas Granulitic Unit (Tuareg Shield), east of the West African Craton, is a key place to help reconstructing the Columbia supercontinent evolution. In this area, the emplacement of magmatic protoliths is constrained at c. 2.05–2.06 Ga and the production of the magmas involved recycling of a Paleoproterozoic to Archean crustal component (up to 3.0 Ga). For the first time, this study emphasizes two distinct Paleoproterozoic granulitic events. The first one occurs at ca. 2.0 Ga and was followed by a second event of very-high temperature (up to 920°C) at ca. 1.89 Ga. Rocks from the Iforas Granulitic Unit subsequently remained at high temperature (≥500°C) for a protracted period of c. 150 Myrs and record multiple phases of accessory mineral growth, dissolution and recrystallisation. This is illustrated by U-Pb monazite ages, peaking at 1780–1760 Ma, reflecting fluid driven replacement reactions as documented by apatite / allanite coronas around monazite. Cooling of the granulites, down to c. 500°C, occurred in the range 1720–1700 Ma (apatite and rutile ages). The chronology of the magmatic/metamorphic events is consistent with a collisional setting involving a micro-continental block consisting of the Iforas / In Ouzzal granulitic units sandwiched between the West African Craton and an eastern landmass which may be the Saharan metacraton. This study has major consequences for global reconstructions of the Columbia supercontinent as it demonstrates that, at c. 1.8 Ga, this microcontinent fits as a continuation of the Khondalite Belt or Trans North China Belt of the North China Craton.

## 1. Introduction

The onset of plate tectonics in the early Earth remains highly debated (e.g. Cawood et al., 2022) with related fundamental issues such as mantle geodynamics, the formation of the continental crust, the assembly of supercontinents and the emergence of life, all these processes operating in a feedback system (Li and Zhong, 2009). Amongst these processes, the assembly and breakup of supercontinents through times appears to be cyclic and is known as a classical Wilson cycle of c. 500 Myrs of duration (Wilson, 1966). This cyclicity typically consists in a period of accretion, followed by collisions and merging of continental landmasses, then a period of stabilization and quiescence followed by fragmentation and the beginning of a new cycle. One of the earliest clearly documented supercontinents, the Columbia (Rogers and Santosh, 2002; Zhao et al., 2002), was assembled between c. 2.1 and 1.8 Ga, with a maximum of collisional belts worldwide at around 1.9 Ga, and broke-up from c. 1.6–1.5 Ga (e.g. Grenholm, 2019). During the last decades, most attention has been focused on the main continental landmasses, such as the North China craton, Amazonia, Baltica or Laurentia, which

constitute the corner-stones of most geodynamic Columbia reconstructions (Meert and Santosh, 2017). Conversely, other regions, although geographically significant, but difficult to access or poorly exposed, suffer from a dearth of information, which creates gaps in the reconstructions of the Columbia puzzle. One of the least studied areas concerns Paleoproterozoic high-grade rock remnants currently located in the northern part of Africa. In this study, we focused on the Iforas Granulitic Unit (IGU) in Mali. This Paleoproterozoic granulitic belt is located in the southwestern part of the Tuareg Shield, a multi-terranes area that has been squeezed, during the Neoproterozoic, between the West African Craton (WAC) and the poorly known Saharan metacraton (SmC) (Liégeois et al., 2013; Shelnutt, 2021). This shield includes Paleoproterozoic/Archean terranes, such as the Iforas Granulitic Unit (IGU), the In Ouzzal Granulitic Unit (IOGU) or the LATEA metacraton (Caby, 1996; 2003; Liégeois, 2019; Liégeois et al., 2003; Ouzegane et al., 2003), which are difficult to integrate in Columbia supercontinent reconstructions although their original Paleoproterozoic record, sometimes shielded from significant Neoproterozoic overprinting as demonstrated here, can potentially provide key constraints into

<footnote><sup>a</sup> Géosciences Montpellier, Université de Montpellier, UMR CNRS 5243, 2 Place E. Bataillon, 34095 Montpellier, France</footnote>

<footnote><sup>b</sup> Trinity College of Dublin, The University of Dublin, College Green, Dublin 2, Ireland</footnote>

<footnote><sup>c</sup> Université du Québec à Montréal, case postale 8888, Montréal H3C 3P8, Canada</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: delphine.bosch@umontpellier.fr (D. Bosch).</footnote>
accretionary and fragmentation processes.

The purpose of this contribution is to address a critical knowledge gap in Columbia supercontinent reconstructions: the precise role and tectonometamorphic history of the IGU. While the general framework of Columbia is increasingly well-constrained, the position and evolution of the IGU—a key granulitic belt—remain poorly defined. To resolve this, we i) provide detailed geochronological constraints on the evolution of granulite facies rocks within the IGU, pinpointing its

tectonometamorphic evolution; ii) compare the chronology of these events with the general framework and corresponding timeline of the Columbia supercontinent; and iii) propose a revised positioning of this granulitic belt within the Columbia supercontinent, thereby clarifying its significance in global reconstructions.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** 1a: Sketch map of West Africa with the main continental landmasses (Liégeois, 2019). The location of the Tuareg Shield relative to WAC (West African Craton) and Saharan metacraton is indicated. 1b: Structural sketch map of the Tran-Saharan belt in the southwestern part of the Tuareg Shield and adjacent areas (modified after Caby, 2003). Grey domain: LATEA (Laouni/Azrou N'Fad/Tefedest/Egéré-Aleksod terranes); green domain: Iskel oceanic domain; pink domain: IGU (Iforas Granulitic Unit) and IOGU (In Ouzzal granulitic Unit).</figcaption></figure>
## 2. Geological setting of the Iforas granulitic Unit (IGU)

The IGU outcrops c. 200 km east of the main Pan-African suture zone (Fig. 1a and b), and is a c. 40 km wide and c. 250 km long N-S trending granulitic belt that represents a mid- to lower-crustal section (Caby,

2003). The unit is separated from surrounding units by two vertical, c. 500 m thick, mylonitic zones on its western and eastern sides that sharply cut the granulite facies rocks. On the western side, strain measurements along the Abeibara-Rarhous shear zone indicate a dextral sense of movement and a mean value of 4 km for the total displacement

<figure><img src="image_5.png" /><figcaption>Figure 2
(Bosch et al.)</figcaption></figure>

**Fig. 2.** Geological sketch map of the Iforas Granulitic Unit (Mali) after Caby and Buscail (2005). Numbers 1 to 6 refer to sample locations, with 1: IC56 / IC56C / IC57;2: IC07, MY3;3: IC08 / IC10;4: IC138A;5: IC103;6: IC115B.
along the shear zone (Boullier, 1986). The mylonitic deformation was dated between 566 and 535 Ma (Lancelot et al., 1983) and is thus related to the late Pan-African activity in this area. Granulite facies rocks, similar to the IGU also crop out to the west, in the Kidal terrane (In Bezzeg inlier), and to the east, in the Tin-Essako domain (Fig. 2). The occurrence of these granulite facies rocks raises the possibility that the basement of the terranes located west and east of the IGU is constituted by similar granulite facies rocks and thus that these units may have formed a single crustal block, before to be dissected during Pan-African events. Although this needs further investigations, this assumption is supported by Paleoproterozoic ages for granulite facies rocks at In Bezzeg and for orthogneisses in the Kidal and Tin-Essako areas (Bosch et al., 2016). In addition, based on similar metamorphic ages and lithologies, there is a consensus as to consider that both the IGU in Mali and the In Ouzzal Granulitic Unit (IOGU) in Algeria further north, once belonged to a single Archean/Paleoproterozoic microcontinent of a thousand kilometers long (Caby, 1996; 2003). Except on their edges, which were the locus of deformation along shear-zones (e.g. Lancelot et al., 1983; Ferkous and Monié, 2002), both terranes were little affected by Pan-African events. In the IOGU, the Pan-African overprint is limited to intrusion of post-collision granitoids rising along major crustal-scale faults located inside the terrane (e.g. Fezaa et al., 2019). In the IGU, gabbro to diorite and granodiorite intrusions have also been described and are mostly located within or nearby shear zones or sharply cut the granulites (Bertrand et al., 1984a; Caby, 2003; Caby and Buscail, 2005). These intrusions (c. 10–15% of the unit) have been studied by different radiometric methods (U-Pb and fission tracks on zircon, Rb-Sr on biotite) but the results were inconclusive since low temperature chronometers surprisingly provided the oldest ages (Bertrand et al., 1984b).

The lack of geochronological data not only hampered the precise knowledge of the tectonometamorphic evolution of the IGU, but nurtured the debate about its structural setting, with opposite views proposing that this unit represents either a Pan-African s.l. basement nappe of granulitic rocks thrust onto the migmatitic gneisses of the Kidal assemblage on the west (e.g. Boullier et al., 1978; Boullier and Barbey, 1988), or alternatively, a distinct terrane limited by vertical lithospheric scale faults (e.g. Black et al., 1994; Caby, 1996).

The IGU mainly consists of diversely retrogressed granulite facies rocks including paragneisses, mafic and felsic granulites of igneous origin, large discordant charnockitic batholiths and orthogneisses emplaced at different stages of the metamorphic evolution (Fig. 2). Following Boullier and Barbey (1988), these units underwent a poly-cyclic evolution related to the superposition of two metamorphic events separated by an episode of unroofing of the granulite. The first event involved a granulitic stage (T > 750°C and P in the range 7.5–10.5 kbar) and a subsequent retrogression under lower pressure and temperature conditions (550–750°C, 4–6 kbars). A second metamorphic event occurred under lower P-T conditions (620 ± 50°C, 5 ± 1 kbar) and has been considered as representative of Pan-African events within the IGU (Boullier and Barbey, 1988), although geochronological constraints are still lacking to support this assumption. The age of the oldest granulite facies event in the IGU was first proposed to occur at 2120 ± 20 Ma (Lancelot et al., 1983) but has been recently revised at 1986 ± 7 Ma (Bosch et al., 2016) on the ground of in-situ U-Pb allanite analyses. This age is similar to that obtained from U-Pb zircon analyses of a garnetite from the In Bezzeg inlier outcropping in the Kidal terrane (1988 ± 5 Ma; Bosch et al., 2016) and collectively these ages concur with the time span of 2002 ± 14 Ma to 1983 ± 15 Ma (2σ) attributed to the UHT event in the IOGU (Peucat et al., 1996).

### 3. Analytical methods

Detailed analytical techniques and standard compilation measured during the present study are presented in Supplementary Information SI-A and SI-B. U–Pb/Hf isotopes, and trace element LA-ICP-MS analyses are available in Supplementary Information SI-B. SEM images are presented

in Supplementary Information SI-C-Figures-1a-x. Concordia plots are presented in Supplementary Information SI-C-Figures-2a-o. Ages are reported at the ± 2σ level.

### 4. Sampling

Analyzed samples have been collected broadly along a N-S transect in the IGU (Fig. 2) and are briefly described below.

Rocks outcropping in the IGU can be broadly broken into three main types. The dominant lithology is constituted by banded gneisses represented by samples IC07 and MY3. Sample MY3 has been previously analyzed by Lancelot et al. (1983). It corresponds to a sub-alkaline leptynite composed of mesoperthitic feldspar, plagioclase, quartz with undulose extinction and clinopyroxene surrounded by bluish-green amphibole. Primary, brown-green amphibole and ilmenite-magnetite aggregates surrounded by green biotite also occur. Zircon, apatite, rutile and allanite occur as accessory minerals. Sample IC07 has undergone anatexis and displays thin mesoperthitic leucocratic veinlets (Fig. 3a-b). It contains quartz, orthopyroxene, amphibole and plagioclase with minor biotite. As a whole, the origin of these sub-alkaline gneisses is unclear but they are interpreted as representing either a thick metavolcanic formation or alkaline intrusives within metasediments (Lancelot et al., 1983).

The second main lithology is composed of granulitic banded gneisses of sedimentary origin (paragneisses and quartzites), the former mainly composed of garnet, sillimanite, orthopyroxene, cordierite, feldspars and rutile. In this study, this lithology is represented by the paragneiss samples IC56, IC56C, IC115B and IC138A (see Fig. 3c-d). Samples IC56 and IC56C are mainly composed of fresh garnet, plagioclase, perthitic K-feldspar, quartz, cordierite with numerous inclusions of green spinel, sillimanite and green spinel. Sample IC115B is adjacent to a sill/vein complex that includes olivine-spinel-phlogopite-bearing ultramafic rocks. It contains large prismatic sillimanite included in cordierite, garnet, quartz, mesoperthite and red biotite. Sample IC138A in addition contains orthopyroxene. The four samples contain rutile, zircon and monazite as accessories.

Intrusions of charnockite, monzodiorite and gabbronorite that were emplaced at different stages of the metamorphic evolution constitute the third lithology. These samples are represented by the charnockite IC10, the monzodioritic orthogneisses IC08 and IC103 and the *meta*-gabbronorite IC57 (see Fig. 3e). The charnockite IC10 contains quartz, plagioclase, orthopyroxene, red biotite and ilmenite. The monzodiorites display mesoperthitic K-feldspar, plagioclase, brown amphibole (IC08) or pyroxene (IC103), biotite, ilmenite and few (IC08) or no (IC103) quartz. Lastly, *meta*-gabbronorite IC57 is interbedded within the paragneisses IC56 and IC56C and is interpreted as a sill. The rock is composed of pink orthopyroxene, clinopyroxene, brown amphibole, plagioclase and titanite.

### 5. Results

An extended version of the Results is available in Supplementary Information SI-D. U-Pb ages are displayed in Fig. 4.

#### 5.1. Granulitic sub-alkaline orthogneisses (IC07 and MY3)

The granulitic sub-alkaline orthogneisses were analyzed for zircon (IC07 and MY3), monazite and apatite (IC07), to investigate their metamorphic and magmatic histories. Zircons in IC07 exhibit both euhedral, multifaceted shapes (SI-C-Figure-1a) or irregular, curvilinear forms (SI-C-Figure-1b), often in contact with ilmenite (SI-C-Figure-1c), suggesting a metamorphic origin (e.g. Vavra et al., 1999; Bingen et al., 2001). Concordant to subconcordant zircon analyses define an age of 2008 ± 8 Ma (SI-D-Figure-2a), interpreted as the time of peak metamorphism converting the original magmatic rock into a granulitic orthogneiss. Some older, discordant zircon ages suggest inherited grains
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Thin-sections microphotographs and field view of samples analyzed during this study: a) field view of sample IC07; b) sample IC07 showing the leucocratic veinlets; c) and d) paragneisses IC56 and IC115B, respectively; e) deformed monzodiorite IC08; f) IC56 showing rutile and zircon inclusion in garnet; g) IC56C showing the sillimanite-garnet-feldspar assemblage with rutile in contact with both garnet and sillimanite; h) IC115B showing rutile in equilibrium with garnet; i) IC138A showing the assemblage garnet-OPX-cordierite with zircon and rutile both included in garnet. Abbreviations: Crd: cordierite; Ilm: ilmenite; Fsp: feldspars; Grt: garnet; OPX: orthopyroxene; Rt: rutile; Zr: zircon (Whitney and Evans, 2010).</figcaption></figure>

from the magmatic protolith (SI-C-Figure-1d). Hf isotopic analysis reveals significantly negative εHf(t) values ranging from -14.0 to -15.8 (Fig. 5; SI-B-Table-S1). One inherited core with a minimum age of 2.53 Ga yields a higher εHf(t) value of -11.4. Monazite grains in IC07 show signs of fluid interaction such as ragged external edges (SI-C-Figure-1e-f) and yield an age of 1865 ± 11 Ma, marking a second, younger metamorphic event (SI-C-Figure-2b). Two analyses yield older dates (2008 ± 44 Ma and 1977 ± 52 Ma), suggesting that pre-existing grains have underwent recrystallisation or resorption and partial updating during the 1865 Ma event. Apatite analyses define an age of 1711 ± 18 Ma (SI-C-Figure-2c) interpreted as a cooling age.

In MY3, zircons are subhedral to rounded, and provide dates ranging from 2.0 to 2.7 Ga (SI-C-Figure-2d). A significant number of concordant analyses (7) define an age of 2049 ± 5 Ma, interpreted as the magmatic age of the protolith. The older ages likely represent inherited components from deep crustal sources or basement assimilation. Two youngest zircon analyses from rims around central domains yield an age of 2004 ± 12 Ma, identical to the age of the metamorphic zircons in IC07 and

similarly interpreted as a metamorphic overprint.

## 5.2. Charnockite (IC10), monzodiorites (IC08, IC103) and gabbronorite (IC57)

The charnockitic rock IC10 contains a homogeneous population of subhedral zircon grains rich in apatite inclusions (SI-C-Figure-1 g-h). The U-Pb system defines a crystallization age of 2007 ± 14 Ma (SI-C-Figure-2e), consistent with a magmatic origin as supported by trace element patterns (Fig. 6a) and Ti-in-zircon thermometry data (Ferry and Watson, 2007), indicating crystallization temperatures of 766 ± 17°C (Fig. 6b). Hf isotopes yield εHf(t) values from -6.8 to -4.7 (Fig. 5). Apatite from the same sample yields a cooling age of 1721 ± 28 Ma (SI-C-Figure-2f).

Zircon in monzodiorite IC08 is typically subhedral, with some grains showing core-rim structures (SI-C-Figure-1i-j). Central domains indicate a magmatic crystallization at 2060 ± 11 Ma (SI-C-Figure-2 g) with εHf(t) values between -8.9 and -12.1 (Fig. 5). Rims around central domains
<figure><img src="image_7.png" /><figcaption>Fig. 3. (continued).</figcaption></figure>

yield a younger age of 1998 ± 10 Ma attributed to a metamorphic overprint. Inherited zircon grains in IC08 are evident from ages up to 2763 Ma. Hf isotopes of these inherited grains show a wide range of εHf (t) values (from + 4.8 to -8.3). Apatite in this rock has high common lead and low uranium contents, providing an imprecise cooling age of 1648 ± 69 Ma (SI-C-Figure-2 h), which is however comparable to that of the neighboring samples IC07 and IC10.

Monzodiorite IC103 also contains rounded to irregular zircons (SI-C-Figure-1 k-l). Central domains give a magmatic age of 2051 ± 10 Ma (SI-C-Figure-2i), while rims yield a younger age of 1993 ± 10 Ma. There is no clear chemical distinction between these two clusters, though the younger zircons tend to be slightly depleted in U and Th, which is consistent with zircon recrystallization during metamorphism (Pidgeon, 1992). In thin section, monazite displays anhedral shapes with saw-tooth edges and apatite (inner) / allanite (outer) coronas (SI-C-Figure-1 m-n). The saw-tooth edges of the monazite, along with the contact with biotite, suggests fluid-driven dissolution processes. These coronas are strikingly identical to those described by Finger et al. (1998) and Hentschel et al. (2020) for amphibolite- and greenschist-facies metamorphic rocks respectively and attributed to dissolution and reprecipitation processes under retrograde metamorphic conditions. The main monazite population crystallized at 1760 ± 6 Ma, while older and younger populations at 1870 ± 13 Ma and 1694 ± 19 Ma (SI-D-Figure-2j) suggest a protracted metamorphic history with multiple events.

The gabbronorite IC57, observed as a sill within the granulitic paragneisses IC56 and IC56C, contains zircon grains of two main types. The dominant group consists of small, multifaceted grains (SI-C-Figure-1o-p) typical of high-grade metamorphic rocks (e.g. Vavra et al., 1999). These grains yield an age of 1892 ± 7 Ma (SI-C-Figure-2 k) regarded as dating the metamorphic crystallization of zircon during high-grade metamorphism. A second group comprises larger, rounded grains with ages in the range 1975 ± 20 Ma to 2121 ± 20 Ma. Given the low Zr and Si activity in mafic magmas (e.g. Palme and O'Neill, 2003), these older zircons are unlikely to be inherited from the magma source and are instead interpreted as acquired from surrounding rocks during ascent and emplacement of the sill into the sediments.

### 5.3. Granulitic paragneisses (IC56, IC56C, IC115B and IC138A)

Granulitic paragneisses were also studied to investigate sedimentary provenance and metamorphic overprint. Zircon grains in all samples show a variety of shapes, dominated by rounded forms consistent with a detrital origin (SI-C-Figure-1r-t). However, many also exhibit features such as unzoned rims and embayments, indicating high-grade metamorphic modifications (SI-C-Figure-1q-s-u-v-w-x). Some small multifaceted zircons are also present and resemble typical « soccer-ball » metamorphic grains (SI-C-Figure-1r).

Age spectra for detrital zircon grains range from the late Archean to Paleoproterozoic (Fig. 7), with the youngest analyses clustering around 1890 Ma in all samples. The later were obtained from rims and the correspondence between these c. 1.89 Ga dates and the age of the metamorphic zircon in the gabbronorite supports a regional metamorphic episode affecting the entire sedimentary sequence. These metamorphic ages set apart the maximum depositional ages of the sediments is constrained by the youngest concordant detrital zircon grains analyzed, with dates at 1928 ± 12 Ma (IC56), 1997 ± 34 Ma (IC56C), 1921 ± 30 Ma (IC115B) and 1944 ± 32 Ma (IC138A). Overall, the age spectrum of the detrital zircons points to two dominant Paleoproterozoic age groups (1.92–2.21 Ga and 2.27–2.49 Ga) and a subordinate component of Archean grains, with ages at 2.57–2.65 Ga (n = 3) and 3.1 Ga (n = 1). These zircon populations suggest that the sediments originated from a mixture of late Archean and early to mid-Paleoproterozoic sources. Hf isotopic data from samples IC56 and IC115B reveal a wide range of negative εHf(t) values (from −18.2 to −1.8). Only one Archean grain from sample IC56 yields a positive εHf(t) (+2.6), indicating a possible juvenile crustal component. Zircon rims analyzed for Hf isotopes have more uniformly negative values (from −13.6 to −15.9 in sample IC115B and at −19.8 in IC56).

Monazite grains from IC56 exhibit a complex age range between 1760 and 1930 Ma, with clusters at 1915–1930 Ma (n = 5), 1870 Ma (n = 2), 1830 Ma (n = 1), and 1760–1790 Ma (n = 3) (SI-C-Figure-2 l). It is worth noting that the oldest group aligns with the youngest detrital zircon age (1928 ± 12 Ma). Younger dates, postdating the 1890 Ma metamorphic event, are attributed to sub-solidus metamorphic growth.
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Compilation of concordant U-Pb ages obtained for zircon, monazite, rutile and apatite from the different samples studied during this work. Magmatic (blue dot) and metamorphic (crossed red dot) zircons have been distinguished. The pink areas highlight the time interval determined for each high-grade metamorphic events recorded at ca. 2.0 Ga and ca. 1.89–1.87 Ga. The error bar for absolute age is indicated at 2 sigma level.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Zircon crystallisation ages versus εHf(t) values. The evolution lines DM (bold orange) and CHUR (bold black) are after Chauvel and Blichert-Toft (2001) and Bouvier et al. (2008), respectively. The grey area is delimited by lines with ¹⁷⁶Lu/¹⁷⁷Hf slopes = 0 indicating Pb losses during high-grade metamorphic events. The orange and white stars represent the timing of hypothetical mantle-crust separation events with age estimates at c. 2.7 – 3.0 Ga (extraction from a Depleted Mantle reservoir) or c. 2.4–2.7 Ga (extraction from a chondritic mantle reservoir). Error bars are at 2σ.</figcaption></figure>

²⁰⁷Pb/²⁰⁶Pb weighted mean ages are 1920 ± 10 Ma, 1872 ± 17 Ma, and 1771 ± 39 Ma.

Monazite from IC138A shows a rather similar, although distinct, age

distribution, with main peaks at 2002 ± 34 Ma (n = 5), 1872 ± 16 Ma (n = 21), and 1781 ± 17 Ma (n = 7), plus one concordant point at 2131 ± 12 Ma (SI-C-Figure-2 m). Common age peaks around 1870 and
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Chondrite normalized rare earth element (REE) patterns for zircons analyzed in the charnockite IC10. Normalization values from McDonough and Sun (1995). Inset shows the results of Ti-in-zircon thermometry calculated after Ferry and Watson (2007).</figcaption></figure>

1770–1780 Ma suggest a shared metamorphic history from ~ 1.9 Ga onward. The older grains (~1.92 Ga in IC56, ~2.0 Ga and 2131 ± 12 Ma in IC138A) are considered as detrital in origin.

Rutile from IC115B and IC138A provides a simpler geochronological record (SI-C-Figure-2n-o). Both define discordia lines with similar upper intercept ages of 1701 ± 9 Ma and 1709 ± 5 Ma, and concordant $^{207}$Pb/$^{206}$Pb weighted mean ages of 1701 ± 9 Ma and 1708 ± 5 Ma, respectively. These are consistent with apatite ages from nearby igneous and metamorphic rocks IC07, IC08 and IC10. Trace element analysis of rutile shows Zr contents ranging from 3254 to 5066 ppm (IC115B) and from 3909 to 4719 ppm (IC138A). Using the Ferry and Watson (2007) thermometer, these concentrations translate into mean temperatures that are indistinguishable within the margin of error, respectively 913 ± 21°C and 923 ± 16°C.

# 6. Discussion

## 6.1. A proposal for the age of basement rocks in the IGU

The basement rocks of the IGU record a protracted history of magmatic, metamorphic, and sedimentary processes occurring during the Paleoproterozoic. New geochronological and isotopic data from diverse lithologies—including granulitic orthogneisses, charnockites, monzodiorites, gabbronorites, and granulitic paragneisses—reveal that the oldest magmatic events are preserved in orthogneisses and monzodiorites, with protolith ages clustering at ca. 2.05–2.06 Ga. These ages are consistent with that obtained for the crystallization of the sub-alkaline orthogneiss MY3 (2049 ± 5 Ma), the dominant lithology in the IGU, supporting a late Rhyacian component in the basement of the IGU.

Other constraints on deep seated basement rocks can be obtained

from inherited components detected in these intrusives, assuming that the magmas tapped the crustal column during their upward travel path. Inherited zircon cores from two granulitic orthogneiss and a monzodiorite, with ages spanning 2.1–2.5 Ga, and late Archean components in the range 2.6–2.8 Ga, suggest that basement rocks from the IGU include Eburnean (2.1–2.3 Ga) and Early Paleoproterozoic to late Archean rocks. It is noteworthy that Early Paleoproterozoic activity (2.3–2.5 Ga) is lacking in the West African Craton (WAC), which is characterized by magmatism, metamorphism and sedimentation mainly in the period between 2.25 Ga to 2.0 Ga (Tshibududze et al., 2013; Parra-Avila et al., 2019) related to accretionary orogenic activity. This contrasts with the WAC reinforcing the hypothesis that the IGU represents a distinct terrane (Bosch et al., 2016).

In Fig. 5, almost all zircons analyzed for Lu-Hf isotopes (detrital zircons from IC56 and IC115B and cores from orthogneisses IC07 and IC08) define a spread along a line with a slope corresponding to $^{176}$Lu/$^{177}$Hf = 0. This distribution is interpreted as due to Pb losses occurring during the high-grade events identified in this study. It follows that most detrital zircons from the metasediments and cores from the orthogneisses IC07 and IC08 are consistent with derivation from protoliths formed between c. 2.7 Ga and 3.0 Ga or between c. 2.40 and 2.75 Ga depending on whether magmas were extracted from a depleted mantle reservoir (orange stars in Fig. 5) or from a chondritic mantle reservoir (white stars in Fig. 5). Igneous zircons from the orthogneisses IC07 and IC08, dated at c. 2.05 Ga, plot in the grey area indicating that production of the Rhyacian magmas at c. 2.05 Ga mainly involved reworking of Archean crust. On the other hand, magmatic zircons from charnockite IC10 plot above this array suggesting either reworking of a younger component in the source of the magmas, evolution along a slope close to that defined for the continental crust (0.015 after Griffin et al., 2000), or the input of juvenile material at 2.0 Ga. It can thus be
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Relative probability distribution diagrams for concordant zircons (< 5% discordance) from the metasediments IC56, IC56C, IC115B and IC138A (a-d) of the Iforas Granulitic Unit. Panel e is a compilation of panels a) to d). The green area in the c. 2.0 Ga range corresponds to the first metamorphic event (from 2008 to 1993 Ma), whereas the orange area at c. 1.9 Ga corresponds to the second metamorphic event (from 1897 to 1870 Ma). In each panel, YD corresponds to the youngest detrital grain analyzed and its corresponding age (±2σ). M1 and M2 are the first and second metamorphic event respectively.</figcaption></figure>
concluded that magmatism in the IGU at c. 2.05 Ga was typified mainly by reworking of an older Archean crust. This information indicates that Archean rocks constitute part of the deep-seated basement of the IGU for which we have no evidence till now except, perhaps, for two detrital grains with ages close to 3.1 Ga in the paragneiss IC56. Archean to Early Archean Nd model ages are known further north (Peucat et al., 1996) in the IOGU and provide further support to the hypothesis that the IGU and the IOGU once belonged to a single terrane or micro-continent.

## 6.2. Timing of metamorphic events in the Iforas granulitic Unit

The schematic metamorphic evolution is summarized in Fig. 8 and detailed below.

As seen above, the magmatic samples (orthogneiss and charnockite) are witnesses of a Rhyacian magmatic component. All these samples have registered a metamorphic overprint dated between 2008 ± 8 Ma and 1993 ± 10 Ma. This event was responsible for anatexis of the sub-alkaline orthogneiss IC07 (2008 ± 8 Ma), and for the metamorphic ages determined on zircon rims from samples MY3 (2004 ± 12 Ma), IC08 (1998 ± 10 Ma) and IC103 (1993 ± 10 Ma). Intrusion of char-nockitic magmas also occurred during this time interval, as revealed by the U-Pb zircon analyses of sample IC10 (2007 ± 14 Ma). The time

interval between 2008 ± 8 Ma and 1993 ± 10 Ma is attributed to the high grade, granulite facies metamorphism affecting the IGU. The pop-ulation of detrital monazites dated at 2002 ± 34 Ma in the paragneiss IC138A can be sourced from metamorphic areas in the IGU that were subjected to uplift and erosion. This time interval is strikingly similar to the age range of the high- to very high-temperature metamorphism affecting the In Ouzzal granulitic terrane (between 2002 ± 7 Ma and 1983 ± 15 Ma after Peucat et al., 1996) reinforcing the view that the IGU-IOGU tandem once formed a single granulitic belt.

Following the c. 2.0 Ga-old metamorphic event, the belt was exhumed and eroded as demonstrated by detrital material in the para-gneisses. These sediments were deposited after 1920–1930 Ma, the age of the youngest detrital zircons and monazites in sample IC56, but before 1892 ± 7 Ma, the age of metamorphic zircons in the gabbronorite IC57 intruding the sediments. The later age is attributed to a second high-grade metamorphic event, the occurrence of which is also sup-ported by 1890 Ma-old “soccer-ball” zircons or unzoned rims around detrital zircon grains in the four studied metasediments. Following the thermometer of Ferry and Watson (2007), rutile from paragneisses IC115B and IC138A yields identical Zr-in-rutile temperatures of 910-920°C which we attribute to formation of rutile under very-high to ultra-high metamorphic temperature prevailing during the 1.89 Ga old

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Schematic metamorphic evolution of the Iforas Granulitic Unit (IGU) with a comparison with the In Ouzzal Granulitic Unit (IOGU) and the global evolution of the West African Craton (WAC). For IGU, M1 corresponds to the first high-grade metamorphic event at ca. 2.0 Ga and, M2 to the second metamorphic event at ca. 1.89–1.87 Ga. For IOGU, Archean protolith ages and the very high temperature event (VHT) are from Peucat et al. (1996) and cooling ages are from Maluski et al. (1990).</figcaption></figure>
granulitic event. The occurrence of two metamorphic events in the IGU is consistent with the observations of Boullier and Barbey (1988), who concluded that metamorphic assemblages in rocks from the IGU resulted from the superposition of two metamorphic events separated by decompression and unroofing of the first-formed, granulite-facies, metamorphic units. However, these authors suggested a Pan-African age for the second metamorphic event. Our study reveals on the contrary, for the first time, that both events are Paleoproterozoic in age.

Although consistent with the 1.89 Ga metamorphic zircon age, monazite from two orthogneisses as well as from two paragneisses yields slightly but significantly younger ages (1865–1872 Ma). In the case of the orthogneiss IC07, monazite grains display a ragged appearance (SI-C-Figure-1e-f) indicating a modification of the original shape of the crystals. This is interpreted as reflecting corrosion by fluids (SI-C-Figure-2b). This is not consistent with cooling by volume diffusion and the time interval 1865–1872 Ma is best interpreted as corresponding to an episode of dissolution and recrystallisation of pre-existing monazite. This event, that occurred shortly after the peak of metamorphism dated by the metamorphic zircons at 1890 Ma, is interpreted as reflecting fluid activity during the early retrograde path.

A second cluster of younger monazite ages is observed in the range 1760–1780 Ma and is particularly well constrained in the orthogneiss IC103 (1760 ± 6 Ma). From SI-C-Figure-1 m-n, it can be seen that the external shape of the original monazite has been deeply modified and that the monazite crystals are surrounded by a corona made of apatite (inner corona) and allanite (external corona). This texture is similar to those described by Finger et al. (1988) or Hentschel et al. (2020) and has been interpreted as documenting fluid driven replacement reactions by dissolution/reprecipitation processes. These processes occur during the decompression stages of the metamorphic evolution, from amphibolite facies (550-600°C, Finger et al., 1998; 510 ± 35°C, Lo Po et al., 2016), down to greenschist facies conditions (between 300 and 450°C, Hentschel et al., 2020). Given that these monazites are significantly older than the apatite (with ages around 1700 Ma, see below), it is more likely that the replacement reactions at 1760–1780 Ma occurred during retrograde amphibolite facies metamorphism rather than under greenschist facies conditions. In monazite from sample IC103, the preservation of intra-crystalline domains with different ages (c. 1870 and 1760 Ma) indicates that the replacement reactions were inefficient at completely resetting the U-Pb system of the corona-bearing monazites. This is consistent with experimental studies (e.g. Williams et al., 2011) indicating that fluid related coupled dissolution-reprecipitation processes, such as those observed in this study, can lead to resetting of the U-Pb systems in some intra-crystalline domains, but unaltered domains can be preserved.

Lastly, low retentivity minerals such as apatite (450-550°C for cooling rates of 1-10°C/Ma and crystal radii of 100–500 µm, after Cherniak et al., 1991) and rutile (490-640°C for cooling rates less than 4°C/Ma and crystal radii of 120–280 µm, after Kooijman et al., 2010) were dated in order to pinpoint the last steps of the metamorphic evolution followed by the granulitic units. It is striking to note that both minerals yielded identical ages at around 1700–1720 Ma (see SI-C-Figure-2c-f-h-n-o). The consistency of U-Pb systems for minerals with similar closure temperatures and between different samples suggests a common control for the apatite and rutile ages. Since these ages are younger by c. 150–200 Myrs than the age of the second granulite facies metamorphism, we conclude that the apatite and rutile ages record the point in time when the area has cooled to a temperature where diffusive loss of radiogenic Pb from both minerals ceased. These cooling ages are consistent with the Statherian period of rifting that affected NW Hoggar (Caby and Andreopoulos-Renaud, 1983) and suggest exhumation of the granulites during this event. Interestingly, at the scale of the Columbia supercontinent, this period is also known as corresponding to an extensional regime responsible for intraplate rifting and interpreted as reflecting the early stages of Columbia fragmentation (e.g. Danderfer Filho et al., 2015; Huang et al., 2024).

Moreover, the Zr thermometry in rutile from the paragneiss IC138A and IC115B yielded values of c. 900°C, which are well above the closure temperature for Pb diffusion in this mineral (see above). The U-Pb rutile cooling ages of c. 1700 Ma are thus clearly disconnected from the UHT event. This study therefore is another example of the high retentiveness for Zr in rutile, as already demonstrated by previous studies (e.g. Kooijman et al., 2012), which is more robust than the U-Pb system. In slow cooling terranes, there is therefore a risk to relate U-Pb cooling ages to metamorphic temperatures reflecting earlier metamorphic stages.

In addition, this study is evidence that from 1890 Ma to c. 1700 Ma, rocks from the IGU remained at temperatures close to or above 500°C. Assuming a peak metamorphic temperature of 900°C at 1890 Ma and a cooling down to c. 500°C at 1700 Ma, a mean cooling rate of about 2°C/ Ma can be calculated. This is likely to be an oversimplification as monazite from the paragneiss IC138A and from the orthogneiss IC103 indicates fluid flow events at c. 1870 Ma and 1760 Ma as well as close to 1700 Ma (1694 ± 19 Ma), thus substantiating that heat has been added to the system at different times of the post-peak history and that cooling was likely not steady. The conclusion that rocks from the IGU remained hot for a period of time of about 150–200 Myrs remains however valid. Long residence times (> 100 Myrs) at high temperatures (> 500°C) and associated slow cooling rates are the hallmark of long-lived hot orogens (e.g. Mezger et al., 1990; Jiao et al., 2020a; Zhang et al., 2024). These characteristics are consistent with our observations and the constraints we obtained for the IGU.

The preservation of c. 1700–1720 Ma old apatite and rutile ages has another indirect implication pertaining to the tectonometamorphic evolution of the IGU. Indeed, since apatite and rutile U-Pb systems were not reset after the Statherian period, it implies that no thermal event (i.e. no Pan-African event) significantly affected this area after the Paleoproterozoic. In the IGU, only small plutons, possibly of Pan-African age (Bertrand et al., 1984b), intruded the granulitic units. Therefore, similarly to the IOGU, which is only affected by brittle faults, local greenschist facies metamorphism and intrusion of c. 600 Ma-old post-collisional high-level granitoids (e.g., Fezaa et al., 2019), the IGU represents a fragment of a Paleoproterozoic granulitic belt shielded from significant later deformation and metamorphism during the Pan-African orogeny.

### 6.3. Possible tectonic settings

Development of UHT metamorphic conditions requires a strong anomalous thermal regime (for a review, see Harley, 2016) with likely geodynamic environments related to: (1) arc, back-arc and extensional-accretionary settings (e.g., Collins, 2002; Brown, 2006); (2) large-scale collisional belts with thickened continental crust and high heat-producing elements (e.g., Jamieson and Beaumont, 2011; Harley, 2016); (3) ultra-hot orogen developed on a thin lithospheric mantle and characterized by high inputs of juvenile magmas (e.g., Chardon et al., 2009); (4) post-collisional slab break-off and asthenospheric upwelling; (5) syn-orogenic underplating of mafic magmas (e.g., Lobjoe et al., 2018). In the IOGU, where a 2.0 Ga-old UHT event was identified, Peucat et al. (1996) concluded that the granulite facies metamorphism could be either attributed to collisional processes between continental landmasses (including the WAC in the west), or to mantle plume underplating or even to a major extensional event. Although it is difficult to decipher between these different hypotheses, it is interesting to note that, in the WAC, the 2.25–2.05 Ga time interval is characterized by accretionary-collisional orogenic processes, during which multiple outboard terranes (now known as the Baoulé-Mossi domain in the southeastern part of the WAC) have been docked to the Archean nucleus of the craton (Baratoux et al., 2011; Block et al., 2016; Parra-Avila et al., 2017; Grenholm et al., 2019; Grenholm 2019). This accretionary setting substantiates closure of oceanic domains (along the WAC active margin) during this period that culminated in the collisional phase of the Eburuan orogeny (i.e., 2.15–2.05 Ga). The 2.0 Ga-old metamorphic event
identified in the IGU/IOGU is younger, but can be fitted into this geodynamic scenario assuming it registers a major collisional event between the WAC (and already accreted terranes) and the IGU/IOGU continental microblock. In this hypothesis, the combined IGU / IOGU domain represents a granulitic belt underlining the 800 km long Paleoproterozoic continent-continent collisional zone where crustal shortening and thickening are responsible for the first granulitic event.

This first event was followed by erosion of the thickened crust and
deposition of detritus by 1930–1920 Ma or later. The age spectrum of
detrital zircon grains (see Fig. 7) is characterized by the lack of a sig-
nificant 2.0 Ga-old peak, suggesting that the deep granulitic crust was
not significantly exhumed and subjected to erosion at this time. In
addition, the detrital zircon age spectrum includes grains with ages close
to the inferred depositional age of the detritus as well as grains from
older sources (early Paleoproterozoic to Archean). This detrital zircon
age pattern is consistent with a collisional setting and deposition of the
sediments in a foreland basin (Cawood et al., 2012).

The second event occurred at 1890–1870 Ma under UHT conditions (c. 920°C) in the IGU. Rocks affected by this event were subsequently subjected to long residence times (about 150–200 Myrs) under temperatures of 500°C or higher, therefore suggesting a very slow cooling period and negligible exhumation.

Following Harley (2016) and Brown (2006), most UHT granulite
belts are likely to have developed in the orogenic hinterlands of large hot
orogens for long-lived events (30–100 Myrs), or in arcs and back-arcs
settings for short-lived events (< 10 Myrs). In the IOGU, UHT meta-
morphism occurred between 2002 and 1983 Ma (Peucat et al., 1996),
that represents a duration of c. 20 Myrs, consistent with the 2008–1993
Ma time interval in the IGU. The duration of the younger UHT meta-
morphism in the IGU is also estimated to be at least 20 Myrs (from 1890
to 1865 Ma). Although these durations are in the low end for long-lived
events they are nevertheless consistent with a geodynamic setting cor-
responding to large hot orogens, which we favor here for the IGU/IOGU
microcontinent. In the model of large hot orogens of Jamieson and
Beaumont (2011) applied to the Grenville orogen, (U)HT temperatures
are attained c. 40 Myrs after the onset of collision and are accompanied
by the formation of a ductile channel flow resulting from conductive
self-heating and thermal maturation of a high-heat production thick-
ened crust. Therefore, the 2.0–1.98 Ma age range may be regarded as a
minimum age for collisional processes and the 2.05 Ga Rhyacian mag-
matism as magmatic products emplaced during the onset of the collision
that enabled heat production and softening of the crust. In the IGU, the
two metamorphic events were separated by deposition of sediments
(now represented by the granulitic paragneisses IC56, IC56C, IC115B
and IC138A). This sequence of event precludes a metamorphic contin-
uum between 2.0 Ga and 1.89 Ga and is evidence for polymeta-
morphism. Deposition of sediments between 1.99 and 1.92 Ga indicates
exhumation of crustal levels, which may reflect gravitational spreading
along the ductile channel flow accommodating a reduction of the
convergence rate after the first orogenic phase. The second event iden-
tified in the IGU at 1.89–1.87 Ga is evidence for a renewed tectono-
metamorphic activity. The reason for this is puzzling and although
speculative can be attributed to different processes such as far-field ef-
fects from ongoing WAC collision (see for example Kelsey et al., 2009 or
Lahtinen and Huhma, 2019 for other orogenic systems), a renewed
convergence due to a switch in the subduction obliquity or even to
arrival of a new indentor which might be the Saharan metacraton at the
time. The latter hypothesis is consistent with conceptual models
describing the outward growth of orogens by successive collisions in
which the incoming continental landmasses act as indentors (e.g.
Beaumont et al., 2006). Whatever the cause, this event must have driven
rapid burial in order to bring the sediments under granulite facies con-
ditions. In the IGU, Boullier et al. (1978) described nappe structures
consistent with burial of the units by underthrusting and thrust stacking.
These authors attributed these features to the Pan-African cycle, but our
new results clearly indicate that the nappe structure of the IGU cannot be

Pan-African in age. By inference, we attribute it to the 1.89 Ga-old event.

6.4. *The Iforas connection to Columbia*

The timing of events defined above enables establishing a compari-
son with similar aged granulitic belts worldwide and to propose con-
nections of the IGU/IOGU tandem in the general framework of the
Paleoproterozoic Columbia supercontinent. In the Columbia supercon-
tinent realm, the occurrence of high-grade metamorphic belts in the
range 2.05–1.80 Ga is widespread and has been recognized in almost all
cratonic landmasses constituting this supercontinent. These events are
generally associated with accretionary orogenic systems transitioning to
collisional activity and suturing of continental blocks that marked final
amalgamation of the supercontinent. In the southern part of Columbia
(Atlantica reconstruction of Grenholm, 2019, Fig. 9), UHT granulites
formed in the range 2.0–1.9 Ga occur only in few places, in the
Amazonian craton (Roever et al., 2003), east of the West African Craton,
in the neighbouring Tuareg Shield (Peucat et al., 1996; Bendaoud et al.,
2008), and in the North China Craton (NCC), namely in the Khondalite
Belt (KB; Kusky and Santosh, 2009; Lobjoie et al., 2018; Jiao et al.,
2020a; 2020b; Shi et al., 2025; Zou et al., 2022) and Trans-North China
Belt (TNCB; Wan et al., 2006; Wu et al., 2017). In the reconstruction of
Grenholm (2019), both the KB and TNCB correlate with the Reguibat
Shield (Yeti-Eglab domain) and the Baoulé-Mossi domain on the WAC
and thus, also with the displaced terranes of the Tuareg Shield, including
the Paleoproterozoic LATEA metacraton (Liégeois et al., 2003; Bend-
aoud et al., 2008) and the IGU/IOGU granulites. Both the KB and TNCB
experienced a protracted and complex magmatic-metamorphic history
from 1.96 to 1.85 Ga. Recent studies indicate that these two belts
recorded an early HP metamorphism at c. 1.96–1.94 Ga followed by a
long-lived Paleoproterozoic UHT metamorphism in the range 1.93–1.85
Ga (e.g., Jiao et al., 2020a; 2020b; Yang et al., 2014). It is striking to note
that both the NCC belts and the IGU/IOGU share a similar and fully
consistent evolution. In particular, we emphasize key similarities such as
the sequence of an early event (1.96 – 1.94 Ga HP event in NCC versus c.
2.0 Ga in the IGU/IOGU) followed by a UHT event (c. 1.93 – 1.85 Ga in
NCC versus c. 1.89 – 1.87 Ga in IGU/IOGU). This extends the network of
c. 1.9 Ga-old UHT granulitic belts of the Columbia supercontinent to the
western margin of the Tuareg Shield, east of the Baoulé-Mossi domain of
the WAC. This assumption is further supported by the fact that the
northern margin of the NCC recorded a 2.2–2.0 Ga-old arc magmatism
(Wu et al., 2022), which may be seen as a temporal equivalent of
accretionary magmatism in the Baoulé-Mossi domain at the same
period. In any cases, UHT granulites of the Tuareg Shield should be
included in the Columbia reconstructions, which gives further support to
the 1.8 Ga Columbia configuration of Grenholm (2019), specifically the
positioning of the Tuareg Shield elements east of the WAC and south/
southwest of the NCC.

7. Conclusions

The present study defines a period of magmatic emplacement and
metamorphic events followed by cooling which took place during about
300 Myrs, from c. 2.06 Ga to 1.70 Ga. The chain of events is the
following:

* The Iforas granulitic Unit (IGU) contains basement rocks with ages around 2.05-2.06 Ga which formed by recycling of older crustal components with Paleoproterozoic to Archean ages (up to 3.0 Ga). Along with the IOGU terrane further north, it is proposed that the IGU once formed a single Archean to Paleoproterozoic micro-continental block;

* These units were affected by two different Paleoproterozoic high-grade metamorphic events separated by about 100 Myrs and occurring between 2008 ± 8 Ma and 1993 ± 10 Ma and between 1892 ± 7 Ma and 1865 ± 11 Ma respectively;
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Global reconstruction of the Columbia supercontinent at 1.8 Ga after Grenholm (2019) with the proposed location of the IGU / IOGU microcontinent (green dashed domain, this study). AC: Amazonian Craton; CC: Congo Craton; EEC: East European Craton; KC: Kalahari Craton; Lau: Laurentia; NAC: North Atlantic Craton; NCC: North China Craton; RPC: Rio de la Plata Craton; SC: Siberian Craton; SFC: Sao Francisco Craton; SLC: Sao Luis Craton; SmC: Saharan *meta*-Craton; WAC: West African Craton.</figcaption></figure>

* The first event, dated at c. 2.0 Ga, is coeval with the UHT event recognized in the IOGU further North (Peucat et al., 1996), and is attributed to collision between the IGU-IOGU microcontinental block and terranes accreted along the eastern margin of the WAC. UHT metamorphic conditions, which were reached in the IOGU at this time, were likely accompanied by development of a ductile crustal channel flow as hypothesized in the models of large hot orogens (e.g. Jamieson and Beaumont, 2011);

* The second event at c. 1.89–1.86 Ga, heretofore only recorded in the IGU, reached UHT conditions of c. 900°C and is best interpreted as reflecting ongoing collisional processes related to arrival of a new indentor, such as the Saharan metacraton for example, or to a change in the subduction geometry. The nappe structures described in rocks from the IGU (e.g. Boullier et al., 1978; Boullier, 1991) may be related to underthrusting and nappe stacking of the granulitic units occurring during this event;

* The IGU-IOGU granulitic belt records a sequence of tectonometamorphic episodes (from 2.05 to 1.7 Ga) similar to that recorded in the Khondalite belt or Trans-North China belt of the North China Craton. This is consistent with recent reconstructions of the main cratonic landmasses of the Columbia supercontinent (e.g. Grenholm, 2019) which position the WAC south of the NCC. The IGU-IOGU, set in the prolongation of the Khondalite belt or Trans-North China belt,

represents a major granulitic belt formed in the hinterland of a large hot orogen resulting from collisional processes;

* The preservation of Paleoproterozoic ages in low U-Pb retentive minerals (such as apatite or rutile) further demonstrates that, with the exception of late intrusions, possibly of Pan-African age (Bertrand et al., 1984b), the IGU, similarly to the IOGU, has remained thermally undisturbed since c. 1.7 Ga and that the main metamorphic features are related to assembly of the Columbia supercontinent.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful to D. Delmas and C. Nevado (Service Lith-
olamellage, Géosciences Montpellier, Université de Montpellier) for
performing thin and thick sections and to F. Fernandez (Service de
Microscopie Electronique, Université de Montpellier) for SEM images.

Special thanks to Jacques Russel (aka “Ernest”) for his help and
encouragement throughout this study. This work was supported by the
Tellus program of the CNRS-INSU (call 2021-2022 – PI D. Bosch, call 2023-2024 – PI O. Bruguier). Special thanks to R. Caby for fruitful discussions and for providing the studied samples which were collected during several field trips carried out from the mid-1970s to the mid 1980s in the Iforas. The authors are warmly grateful to two anonymous reviewers and to the Editor M. Babinski for their thorough reviews which greatly improved the manuscript.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.precamres.2026.108019.

## Data availability

All data acquired during the present work are available in Supplementary Materials

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