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Volume 32 Issue 6
Dec.  2021
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Nguo S. Kanouo, Arnaud P. Kouské, David R. Lentz, Rose F. Yongué. New Insights into Neoproterozoic-Cretaceous Events in the Mamfe Basin (SW Cameroon, Central Africa): Evidence from Textural Analyses, U-Th Composition, and U-Pb Zircon Geochronology from Granitic Basement. Journal of Earth Science, 2021, 32(6): 1472-1484. doi: 10.1007/s12583-020-1395-6
Citation: Nguo S. Kanouo, Arnaud P. Kouské, David R. Lentz, Rose F. Yongué. New Insights into Neoproterozoic-Cretaceous Events in the Mamfe Basin (SW Cameroon, Central Africa): Evidence from Textural Analyses, U-Th Composition, and U-Pb Zircon Geochronology from Granitic Basement. Journal of Earth Science, 2021, 32(6): 1472-1484. doi: 10.1007/s12583-020-1395-6

New Insights into Neoproterozoic-Cretaceous Events in the Mamfe Basin (SW Cameroon, Central Africa): Evidence from Textural Analyses, U-Th Composition, and U-Pb Zircon Geochronology from Granitic Basement

doi: 10.1007/s12583-020-1395-6
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  • Backscattered electron BSE-CL images, U-Th abundance, and U-Pb ages were obtained for zircons from the Nkogho granitic basement rocks cropping out in the Mamfe Basin (SW Cameroon). These data are used to characterize and classify each zircon, elucidate their geochemical traits. They were also used to formulate a model of their host composition to ascertain the source, and document any preserved post-emplacement events. These zircons mainly form long to short prisms that are pyramidal to dipyramidal in shape. They typically exhibit complex oscillatory growth zoning, as well as exhibit sector zoning. These features are mainly compatible with igneous zircons, although a few examples have metamorphic signatures. The U (30 ppm-6 380 ppm), Th (4 ppm-1 280 ppm), and Pb (12 ppm-648 ppm) contents show core to rim variations with most values fall within the range of crustal granitic zircons. The Th/U ratios (0.08-1.23), with core to rim differences mainly encompass values typical of magmatic zircons with a few values characterizing metamorphic zircons that grew in equilibrium with an anatectic melt. The U-Pb ages (108.4±1.7 to 988.4±19.0 Ma) with some core and rim age differences date Early Neoproterozoic, Cryogenian-Ediacarian, Early Cambrian-Ordovician, Devonian-Carboniferous, and Aptian-Albian events. The arc-like Nkogho I-type granitoid crystallized from granitic magma during Cryogenian to Ediacarian times and was later affected by post-Ediacarian Cambrian to Albian magmatic events. Aptian-Albian ages probably reflect opening of the Mamfe Basin.
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    Yakymchuk, C., Brown, M., 2019. Divergent Behaviour of Th and U during Anatexis: Implications for the Thermal Evolution of Orogenic Crust. Journal of Metamorphic Geology, 37(7): 899-916. https://doi.org/10.1111/jmg.12469 doi:  10.1111/jmg.12469
    Yakymchuk, C., Kirkland, C. L., Clark, C., 2018. Th/U Ratios in Metamorphic Zircon. Journal of Metamorphic Geology, 36(6): 715-737. https://doi.org/10.1111/jmg.12307 doi:  10.1111/jmg.12307
    Yuan, H. L., Gao, S., Liu, X. M., et al., 2004. Accurate U-Pb Age and Trace Element Determinations of Zircon by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry. Geostandards and Geoanalytical Research, 28(3): 353-370. https://doi.org/10.1111/j.1751-908x.2004.tb00755.x doi:  10.1111/j.1751-908X.2004.tb00755.x
    Zhang, C. L., Li, M., Wang, T., et al., 2004. U-Pb Zircon Geochronology and Geochemistry of Granitoids in the Douling Group in the Eastern Qinling. Acta Geologica Sinica (English Edition), 78(1): 83-95. https://doi.org/10.1111/j.1755-6724.2004.tb00678.x doi:  10.1111/j.1755-6724.2004.tb00678.x
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New Insights into Neoproterozoic-Cretaceous Events in the Mamfe Basin (SW Cameroon, Central Africa): Evidence from Textural Analyses, U-Th Composition, and U-Pb Zircon Geochronology from Granitic Basement

doi: 10.1007/s12583-020-1395-6

Abstract: Backscattered electron BSE-CL images, U-Th abundance, and U-Pb ages were obtained for zircons from the Nkogho granitic basement rocks cropping out in the Mamfe Basin (SW Cameroon). These data are used to characterize and classify each zircon, elucidate their geochemical traits. They were also used to formulate a model of their host composition to ascertain the source, and document any preserved post-emplacement events. These zircons mainly form long to short prisms that are pyramidal to dipyramidal in shape. They typically exhibit complex oscillatory growth zoning, as well as exhibit sector zoning. These features are mainly compatible with igneous zircons, although a few examples have metamorphic signatures. The U (30 ppm-6 380 ppm), Th (4 ppm-1 280 ppm), and Pb (12 ppm-648 ppm) contents show core to rim variations with most values fall within the range of crustal granitic zircons. The Th/U ratios (0.08-1.23), with core to rim differences mainly encompass values typical of magmatic zircons with a few values characterizing metamorphic zircons that grew in equilibrium with an anatectic melt. The U-Pb ages (108.4±1.7 to 988.4±19.0 Ma) with some core and rim age differences date Early Neoproterozoic, Cryogenian-Ediacarian, Early Cambrian-Ordovician, Devonian-Carboniferous, and Aptian-Albian events. The arc-like Nkogho I-type granitoid crystallized from granitic magma during Cryogenian to Ediacarian times and was later affected by post-Ediacarian Cambrian to Albian magmatic events. Aptian-Albian ages probably reflect opening of the Mamfe Basin.

Nguo S. Kanouo, Arnaud P. Kouské, David R. Lentz, Rose F. Yongué. New Insights into Neoproterozoic-Cretaceous Events in the Mamfe Basin (SW Cameroon, Central Africa): Evidence from Textural Analyses, U-Th Composition, and U-Pb Zircon Geochronology from Granitic Basement. Journal of Earth Science, 2021, 32(6): 1472-1484. doi: 10.1007/s12583-020-1395-6
Citation: Nguo S. Kanouo, Arnaud P. Kouské, David R. Lentz, Rose F. Yongué. New Insights into Neoproterozoic-Cretaceous Events in the Mamfe Basin (SW Cameroon, Central Africa): Evidence from Textural Analyses, U-Th Composition, and U-Pb Zircon Geochronology from Granitic Basement. Journal of Earth Science, 2021, 32(6): 1472-1484. doi: 10.1007/s12583-020-1395-6
  • The Mamfe Basin is Cretaceous in age (Martin et al., 2017; Ajonina, 2016; Njoh et al., 2015), tectono-sedimentary structure formed during the opening of the southern part of Atlantic Ocean (Dumort, 1968; Reyment, 1954). This basin is historical and genetically linked to the Benue Trough (in the southeast of Nigeria) (Ajonina et al., 2001; Dumort, 1968), a linear depression filled with up to 6 500 m of lithified marine and continental sediments (Coulon et al., 1996; Benkhelil, 1989) ranging in age from Middle Albian to Maastrichtian (Benkhelil, 1989; Ofoegbu, 1984). It is an NW-SE segment of the NE-SW trending Benue Trough, which started to form during the opening of the Gondwana supercontinent in the Triassic (Dumort, 1968). The similarity in the mode of tectonic evolution between the Mamfe Basin and the Benue Trough is supported by their presence in both rifts and fold axes that are parallel to their respective basin axis (Ajonina et al., 2001). The rift propagated along existing lines of weakness and broadened during Early Jurassic times (Dumort, 1968). Olade (1975) suggested that rifting in the Mamfe Basin aborted in the Upper Albian to Lower Cenomanian, due to the sub- crustal contraction and compression that led to the westward displacement of its depositional axis. Spreading ceased in the Middle Jurassic and as the lithosphere cooled, the shallow depression deepened (Ajonina et al., 1998). Indeed, rifting that formed the Mamfe Basin is thought to have been accompanied by rapid tectonic subsidence that was in response to thermal recovery of the lithosphere following the thermal disturbance that led to the stretching and thinning of the crust beneath the basin (Ajonina et al., 2001). Sedimentation in the Mamfe Basin started in the Albian (Dumort, 1968) as Gondwana started to break up during the Early Cretaceous (Eseme et al., 2002). Several small NW-trending anticlines are reported at the eastern end of the Mamfe Basin (Ndougsa-Mbarga et al., 2007).

    Basement rocks around the Mamfe Basin consist of gneisses, granites, syenites, and mica schists that recorded ductile and brittle cataclastic tectono-magmatic to metamorphic events (Kanouo et al., 2017a; Kanouo, 2014; Regnoult, 1986; Dumort, 1968; Wilson, 1928). The dominant strike direction for foliated rocks is E-W with occasional swings to the N and S (Regnoult, 1986; Dumort, 1968; Wilson, 1928). Gneisses with a foliation dip of N 10°–30°, belong to the central African mobile zone or Cameroon mobile belt (Dumort, 1968). Recent study of the Otu granitic pegmatite (very coarse grained to inequigranular) and Babi mica schist (slaty to weakly foliated and lepidograno-porphyroblastic) (Kanouo et al., 2017a; Kanouo, 2014) show that they are, respectively, composed of (1) microcline, orthoclase, mono to polycrystalline quartz, biotite, and clinopyroxene and (2) muscovite, biotite, and mono to polycrystalline quartz. The obtained U-Pb zircon ages (ca. 490–653 Ma, close to those of most intrusive rocks within the Cameroon mobile belt) for the Otu granitic pegmatite date an Early Precambrian to Cambrian emplacement and progressive cooling of the source magma (Kanouo et al., 2017a). For Kanouo et al. (2017a), the Babi mica schist, with it's variable U-Pb zircon ages (ca. 529–2 019 Ma), was probably formed (during Calymmian Period) from metamorphic transformation of Paleoproterozoic clastic sediments sourced from magmatic rocks that was later affected by the Pan-African tectono-metamorphic events.

    The Mamfe Basin was essentially filled with continental clastic sediments (conglomerates, sandstones, arkoses, marlstones, siltstones, mudstones, shales) with local limestones and evaporitic deposits (Eric et al., 2019a, b, c; Eyong et al., 2019, 2013; Ngueutchoua et al., 2019; Bassey et al., 2013; Eseme et al., 2006, 2002; Eyong, 2003; Regnoult, 1986; Dumort, 1968; Wilson, 1928) and enclosing organic matter with some petroleum potential (Njoh and Njie, 2016). In the west, these Mamfe Basin rocks are locally overlain by Cenozoic to Paleoproterozoic sourced corundum-bearing placers (Kanouo et al., 2016, 2015, 2012a, b; Kanouo, 2014). Sedimentary rocks in the basin are locally cut by syenitic intrusions and doleritic dykes, and locally overlain by trachytic and basaltic flows, all assumed to be Tertiary age (Kanouo et al., 2017b; Kanouo, 2014; Wilson, 1928). Basaltic exposures partly overlying sedimentary and basement rocks in the west and south of the basin, include basanites, picro-basalts, alkali basalts, and tholeiitic basalts (Kanouo et al., 2017b; Kanouo, 2014). Undated phonolites, tephri-phonolites, trachytes, and basanites are found at Mount Nda Ali in the southeastern end of the basin (Njonfang and Moreau, 1996). In this area, those extrusive rocks overlie undated gabbros, diorites, monzonites, and syenites of alkaline affinity (Njonfang and Moreau, 1996).

    Nkogho, in the southwestern part of the Mamfe sedimentary basin (Fig. 1), is built on magmatic and sedimentary terrains. Sedimentary rocks in this area, mainly sandstones (part to the Ngeme Formation; Eyong, 2003), are locally overlain by basanite flows and basalt volcanoclastic rocks (Kanouo et al., 2017b; Kanouo, 2014), and underlain by granitic basements (Kanouo, 2014). Other basement rocks found in this locality are syenites, cropping out in the east (Kanouo, 2014). The syenitic rocks are whitish in color, and mainly composed of microcline, orthoclase, and plagioclase with very minor quartz (non fractured, cataclastic, and polycrystalline), and biotite (Kanouo, 2014).

  • The granitic rocks (Nkogho anatectic granite: Dumort, 1968) from which the studied zircon crystals were sampled, outcrop as small domes and fragments on hill slopes and valleys mostly found in the south and west of Nkogho (SW of the Mamfe sedimentary basin). The presented data characterizing the Nkogho granite are from Kanouo et al. (2017b) and Kanouo (2014). The Nkogho granite, a coarse-grained to inequigranular rock, is locally overlain by reddish sandstones, olivine basanite pillow lava, olivine basalt flows, and cobble to boulders size fragments or cross-cut by a vesicular olivine doleritic dyke (Kanouo et al., 2017b). The granite is an arc-like, I-type granitoid, both biotite-enriched and locally biotite-depleted. The biotite-enriched part (the source of zircon crystals in NK2) is metaluminous, belongs to high-K calc-alkaline series and alkali-calcic to calc-alkali groups (Kanouo, 2014). It is composed of quartz, plagioclase, microcline, orthoclase, biotite, and zircon (Kanouo, 2014). The biotite-depleted part of this granite (the source of zircon crystals in NK1 and NK3) is peraluminous, has shoshonitic affinities, belongs to calc-alkali groups, and is made up of quartz, plagioclase, microcline, orthoclase, biotite, pyroxene, and zircon (Kanouo, 2014). Quartz in those two parts are anhedral with some cracks and commonly polycrystalline. Some alkali feldspar crystals have perthitic exsolutions and are locally sericitized.

  • Heavy mineral concentrates from where the studied zircon crystals were separated and pre-concentrated at the Department of Earth Sciences (University of Yaoundé I, Cameroon) are from the crushed anatectic granitoid samples. Twenty kilograms of rock fragments from each sample were milled at the ALS mineral division laboratory in Mvan (Yaoundé, Cameroon). Before milling, precautions were taken to avoid any contamination. Samples were cleaned, chipped to reduce the grain size, crushed and milled (at 1 mm grain size). Milled samples were regularly washed and panned to obtain heavy mineral concentrates, which were later dried in oven (for 24 h, at 50 ℃). Dried concentrates were sent to China for zircon separation, mounting and BSE-CL imaging. Before mounting and BSE-CL imaging, zircon crystals were handpicked under a binocular microscope (ZEISS Stemi 2000-C), mounted with epoxy resin on glass slide, and polished with abrasive to a standard thickness of 30 μm at China University of Geosciences, Wuhan, China.

  • Zircon's SEM-CL images were obtained by exposing mounted-polished crystals to cathodoluminesence imaging equipment at Northwest University in Xi'an, China. The procedure used to acquire zircon's morphological features and internal texture is similar to that described in Corfu et al. (2003). Images were taken using a CL detector attached to a scanning electron microscope (SEM-CL). The SEM-CL supported SEM backscattered electron (BSE) imaging. The obtained SEM-CL imaging was used to classify zircon based on features presented in Corfu et al. (2003) and Hoskin and Schaltegger (2003).

    The procedure used for U-Pb zircon dating is similar to that of McFarlane and Luo (2012) presented in Kanouo et al. (2017a). The polished slabs were analyzed using modern 193 nm ArF excimer laser ablation (LA) and quadrupole-inductively coupled plasma-mass spectrometry (Q-ICP-MS) instrumentation. Data on Th, U, and Pb abundance and U-Pb ages were obtained from craters (33 μm of diameter and < 20 μm of depth) targeted in the core and rim of each zircon crystal and any oscillatory zones (Fig. 2). They were obtained by combining enhanced ICP-MS sensitivity, fast and efficient sample cells, and sophisticated software controls, modern 193 nm ArF excimer LA-ICPMS systems.

    Figure 2.  Zircon BSE-CL images showing internal texture of zircon crystals with analyzed spots (a) sample NK1; (b) sample NK2.

  • The grain sizes, morphological, and internal compositional variations of the studied zircons are shown in Figs. 2, 3. The grain sizes (length dimension) are generally greater than 100 µm with most crystals being greater 150 µm. The crystals are mainly euhedral (prismatic: short and long, lamellar, pyramidal, or dipyramidal), with a few of them being subspherical, subhedral, and anhedral.

    Figure 3.  Zircon BSE-CL images showing internal texture of crystals with analyzed spots from sample NK3.

    For sample NK1 (Fig. 2a), the zircon's grain size ranges of 100–200 µm, and the shape (short and long prisms, lamellar, pyramidal, and dipyramidal). The internal structures show oscillatory zoning, complex growth zoning, sector zoning, and patchy zoning with local recrystallization. Some crystals are pitted and/or twinned, and have brighter core and darker rim similar to xenocrystic zircons described in Corfu et al. (2003).

    For sample NK2 (Fig. 2b), zircon's grain size ranges of 70–250 µm. The shape is prismatic, pyramidal, dipyramidal, subspherical, subhedral, or anhedral. The internal structures show oscillatory zoning, faint broad zoning, a brighter core and darker rim, or no clear core-rim difference. Few crystals are crack, pitted or unzoned.

    For sample NK3 (Fig. 3), the zircon crystal's grain size ranges from > 100 to 250 µm, and the shapes range from lamellar, prisms, pyramidal, and dipyramidal. The internal structures show oscillatory zoning, complex growth zoning, patchy zoning, and brighter core and darker rim. Some crystals have fractured core, or are pitted.

  • Spotted areas (C: core) and (R: rim or zone) in selected zircon crystals (Figs. 2, 3), show variable U, Th, and Pb contents, and calculated Th/U ratios (Tables 1, 2, and 3). The obtained 206Pb/238U and 207Pb/235U ages in each spots are also variable with some similarities or closeness (Tables 1, 2, and 3).

    Crystal and spot No. Analysis No. Ages (Ma) Atomic ratios (%) Concentrations (ppm) Th/U
    206Pb/238U (Prop_2SE) 207Pb/235U (Prop_2SE) 206Pb/238U (Prop_2SE) 207Pb/235U (Prop_2SE) 207Pb/206Pb (Prop_2SE) 208Pb/232Th (Prop_2SE) U Th Pb
    NK1-9C Output_1_50 988.4 (19.0) 982.7 (54.0) 0.165 7 (0.003 4) 1.633 (0.270) 0.071 5 (0.007 2) 0.049 9 (0.006 1) 462.0 566.0 252.4 1.225
    NK1-9R Output_1_51 464.4 (6.8) 477.9 (5.8) 0.074 7 (0.001 1) 0.602 (0.009) 0.059 0 (0.000 5) 0.018 2 (0.001 0) 1 645.0 772.0 153.3 0.470
    NK1-8C Output_1_48 613.1 (8.1) 626.0 (4.9) 0.099 8 (0.001 4) 0.853 (0.009) 0.062 3 (0.000 4) 0.032 1 (0.000 9) 676.0 452.0 137.7 0.669
    NK1-8R Output_1_49 551.2 (9.6) 535.0 (10.0) 0.089 3 (0.001 6) 0.694 (0.017) 0.056 8 (0.001 0) 0.027 6 (0.000 7) 445.0 283.3 97.6 0.637
    NK1-7C Output_1_46 620.3 (9.0) 617.3 (9.9) 0.101 0 (0.001 5) 0.837 (0.018) 0.061 2 (0.000 8) 0.031 4 (0.000 8) 111.7 116.4 35.4 1.042
    NK1-7R Output_1_47 466.9 (7.0) 491.9 (5.7) 0.075 1 (0.001 2) 0.624 (0.009) 0.060 4 (0.000 4) 0.022 2 (0.001 1) 3 410.0 564.0 126.0 0.165
    NK1-6C Output_1_44 619.9 (7.9) 566.0 (14.0.) 0.101 0 (0.001 3) 0.746 (0.024) 0.053 9 (0.001 5) 0.031 7 (0.001 1) 294.0 239.0 93.4 0.813
    NK1-6R Output_1_45 390.0 (75.0) 404.0 (71.0) 0.063 0 (0.013 0) 0.52 (0.300) 0.056 9 (0.006 4) 0.040 0 (0.033 0) 30.4 4.9 14.9 0.161
    NK1-5C Output_1_41 660.0 (8.3) 679.0 (17.0) 0.107 8 (0.001 4) 0.962 (0.031) 0.064 5 (0.001 7) 0.048 3 (0.002 4) 232.3 144.4 312.0 0.622
    NK1-5M Output_1_42 596.5 (8.2) 593.0 (8.0) 0.097 0 (0.001 4) 0.795 (0.014) 0.060 0 (0.000 9) 0.030 1 (0.000 8) 259.3 141.2 47.3 0.545
    NK1-5R Output_1_43 373.0. (8.4) 374.0 (8.1) 0.059 6 (0.001 4) 0.447 (0.011) 0.055 2 (0.000 4) 0.017 4 (0.000 7) 2 300.0 1 017.0 204.8 0.442
    NK1-4C Output_1_39 650.0 (130) 632.0 (14.0) 0.106 2 (0.002 3) 0.865 (0.024) 0.061 0 (0.001 3) 0.034 2 (0.001 2) 2 060 2 230 76.5 1.083
    NK1-4R Output_1_40 615.9 (8.0) 617.4 (4.6) 0.100 3 (0.001 4) 0.837 (0.008) 0.061 0 (0.000 3) 0.031 9 (0.000 9) 904.0 562.0. 165.5 0.622
    NK1-3C Output_1_37 772.8 (11.0) 783.0 (12.0) 0.127 4 (0.002 0) 1.162 (0.025) 0.067 1 (0.001 2) 0.039 0 (0.001 3) 156.0 75.5 32.29 0.484
    NK1-3R Output_1_38 549.0 (13.0) 571.0 (11.0) 0.089 0 (0.002 1) 0.758 (0.019) 0.062 3 (0.000 8) 0.024 8 (0.002 5) 1 171.0 313.0 94.3 0.267
    NK-1-3R2 Output_1_59 599.6 (8.6) 604.0 (6.4) 0.097 4 (0.001 5) 0.814 (0.012) 0.061 0 (0.000 9) 0.029 5 (0.000 8) 269.0 172.9 49.8 0.643
    NK1-2R Output_1_36 461.0 (29.0) 485.0 (31.0) 0.074 4 (0.004 8) 0.625 (0.050) 0.059 2 (0.001 5) 0.038 0 (0.007 7) 234.0 111.0 78.0 0.474
    NK-4R1 Output_1_61 520.8 (10.0) 526.0 (10.0) 0.084 2 (0.001 7) 0.680 (0.016) 0.058 5 (0.001 0) 0.025 8 (0.000 6) 720.0 420.8 133.1 0.584
    NK-1-R1 Output_1_53 580.2 (9.4) 588.9 (8.1) 0.094 2 (0.001 6) 0.788 (0.014) 0.061 8 (0.000 8) 0.028 2 (0.000 9) 268.0 100.8 33.3 0.376
    Prop_2SE. Propagated 2 standard error.

    Table 1.  Trace element and U-Pb isotopic results (zircon from sample NK1) with C, R and M the spotted areas

    Crystal and spot No. Analysis No. Ages (Ma) Atomic ratios (%) Concentrations (ppm) Th/U
    206Pb/238U (Prop_2SE) 207Pb/235U (Prop_2SE) 206Pb/238U (Prop_2SE) 207Pb/235U (Prop_2SE) 207Pb/206Pb (Prop_2SE) 208Pb/232Th (Prop_2SE) U Th Pb
    NK2-5C Output_1_94 122.8 (7.0) 115.7 (6.0) 0.019 2 (0.001 1) 0.121 (0.007) 0.046 1 (0.000 1) 0.000 5 (0.000 5) 6 1 648.0 0.201
    380.0 280.0
    NK2-4C Output_1_93 611.0 (11.0) 600.0 (11.0) 0.099 4 (0.001 8) 0.807 (0.020) 0.059 5 (0.001 1) 0.029 7 (0.001 0) 57.3 48.4 13.8 0.845
    NK2-3C Output_1_92 611.2 (8.9) 597.0 (9.8) 0.099 5 (0.001 5) 0.803 (0.017) 0.059 0 (0.000 8) 0.030 8 (0.000 9) 76.8 61.6 19.5 0.802
    NK2-2C Output_1_90 596.3 (10.0) 592.1 (9.9) 0.096 9 (0.001 7) 0.792 (0.018) 0.058 5 (0.001 1) 0.029 5 (0.001 0) 75.1 82.9 23.7 1.104
    NK2-2R Output_1_91 606.1 (11.0) 601.0 (10.0) 0.098 6 (0.001 8) 0.802 (0.018) 0.059 5 (0.000 9) 0.030 4 (0.001 2) 59.0 48.8 14.5 0.827
    NK2-1C Output_1_88 619.3 (9.3) 616.0 (9.0) 0.100 9 (0.001 6) 0.835 (0.016) 0.060 5 (0.000 7) 0.031 1 (0.001 0) 79.3 73.7 22.5 0.929
    NK2-1R Output_1_89 610.8 (1.10) 598.0 (13.0) 0.099 4 (0.002 0) 0.803 (0.023) 0.059 3 (0.001 3) 0.030 6 (0.001 1) 65.7 45.8 14.6 0.697

    Table 2.  Trace element and U-Pb isotopic results (zircon from sample NK2) with C and R the spotted areas

    Crystal and spot No. Analysis No. Ages (Ma) Atomic ratios (%) Concentrations (ppm) Th/U
    206Pb/238U (Prop_2SE) 207Pb/235U (Prop_2SE) 206Pb/238U (Prop_2SE) 207Pb/235U (Prop_2SE) 207Pb/206Pb (Prop_2SE) 208Pb/232Th (Prop_2SE) U Th Pb
    NK3-4R2 Output_1_62 336.0 (15.0) 329.7 (26.0) 0.053 5 (0.002 8) 0.384 (0.130) 0.052 2 (0.003 3) 0.013 0 (0.009) 1 391.0 576 159.5 0.414
    NK3-C1 Output_1_52 596.3 (8.3) 604.7 (5.4) 0.096 8 (0.001 4) 0.815 (0.010) 0.061 7 (0.000 5) 0.033 3 (0.002) 361.2 35.9 12.2 0.100
    NK3-9C-CRACK Output_1_73 960.0 (22.0) 934.0 (19.0) 0.160 7 (0.004 0) 1.508 (0.047) 0.068 1 (0.001 0) 0.048 4 (0.002) 71.5 55.4 26.4 0.774
    NK3-8C Output_1_71 933.0 (18.0) 954.0 (14.0) 0.155 7 (0.003 2) 1.557 (0.035) 0.072 6 (0.000 5) 0.069 8 (0.013) 573 77.4 94.8 0.135
    NK3-8R Output_1_72 308.0 (16.0) 315.0 (16.0) 0.049 0 (0.002 7) 0.365 (0.022) 0.053 5 (0.000 4) 0.038 4 (0.005) 990 87.1 85.1 0.088
    NK3-7C Output_1_69 609.6 (13.0) 615.8 (16.0) 0.099 2 (0.002 2) 0.835 (0.031) 0.061 6 (0.001 4) 0.029 3 (0.002) 118.7 88.5 24.5 0.746
    NK3-7R Output_1_70 625.8 (8.5) 623.6 (6.6) 0.101 9 (0.001 5) 0.846 (0.012) 0.059 9 (0.000 7) 0.031 5 (0.001) 240 168.4 49.9 0.702
    NK3-6R1 Output_1_68 602.3 (16.0) 612.0 (16.0) 0.097 9 (0.002 6) 0.828 (0.026) 0.061 7 (0.001 0) 0.029 9 (0.001) 196.6 92.2 30.5 0.469
    NK3-6R2 Output_1_67 594.0 (15.0) 587.0 (16.0) 0.096 6 (0.002 6) 0.788 (0.028) 0.059 4 (0.001 4) 0.029 5 (0.001) 91 68.3 21.9 0.751
    NK3-5C Output_1_63 469.6 (16.0) 449.0 (16.0) 0.075 6 (0.002 6) 0.558 (0.022) 0.053 8 (0.000 9) 0.023 4 (0.001) 803 665.0 173.3 0.828
    NK3-5R2 Output_1_65 112.7 (2.7) 108.4 (1.7) 0.017 6 (0.000 4) 0.113 (0.002) 0.046 2 (0.000 1) 0.005 4 (0.000 4) 4 700 1 464.0 353.9 0.311
    NK3-4-C Output_1_60 569.5 (10.0) 560.0 (11.0) 0.092 4 (0.001 7) 0.738 (0.019) 0.059 0 (0.001 3) 0.026 6 (0.001) 70.2 89.3 26.3 1.272
    NK3-3C Output_1_57 597.0 (15.0) 622.0 (17.0) 0.097 1 (0.002 5) 0.852 (0.029) 0.062 5 (0.001 0) 0.031 5 (0.002) 124.1 111.5 156.5 0.898
    NK3-3R1 Output_1_58 633.2 (9.0) 635.7 (6.5) 0.103 2 (0.001 5) 0.868 (0.012) 0.061 3 (0.000 7) 0.031 7 (0.001) 214 148.4 46.5 0.693
    NK3-2C Output_1_54 620.6 (9.9) 632.4 (7.1) 0.101 1 (0.001 7) 0.864 (0.013) 0.062 5 (0.000 5) 0.032 0 (0.001) 325 169.0 54.1 0.520
    NK3-2R1 Output_1_56 663.6 (11.0) 663.0 (11.0) 0.108 4 (0.001 9) 0.923 (0.020) 0.063 0 (0.001 0) 0.034 9 (0.001) 187 115.0 44.9 0.615
    NK3-2R2 Output_1_55 596.1 (8.0) 611.0 (3.5) 0.096 9 (0.001 4) 0.826 (0.006) 0.061 6 (0.000 2) 0.032 1 (0.001) 1 513 251.0 78.5 0.166
    NK3-6C Output_1_66 535.2 (14.0) 496.0 (13.0) 0.086 6 (0.002 3) 0.632 (0.020) 0.052 8 (0.001 0) 0.028 9 (0.001) 115.9 112.0 38.4 0.966

    Table 3.  Trace element and U-Pb isotopic results (zircon from sample NK3) with C and R the spotted areas

  • Zircon crystals from NK1 (Table 1), the U, Th, and Pb contents (in C spots) range from 111 ppm to 679 ppm, 75.0 ppm to 566 ppm, and 32.0 ppm to 253 ppm, respectively. The lowest Pb contents is quantified in spots with lowest U-Th contents (due to low U and Th concentration and limited radioactive nuclei decaying of these two elements to give daughter Pb). The Th/U ratios vary from 0.48 to 1.23 with most values > 0.5 (within the range in magmatic zircon; cf. Kirkland et al., 2015; Hoskin and Schaltegger, 2003). In R spots, the U, Th, and Pb contents range from 30.0 ppm to 2 300 ppm, 4.80 ppm to 1 017 ppm, and 14.9 ppm to 204.5 ppm, respectively. The Th/U ratios range from 0.16 to 0.65 with most values being less than 0.50.

    Zircon crystals from NK2 (Table 2) are composed of C spots, with U (57 ppm–6 380 ppm), Th (48 ppm–1 280 ppm), Pb (13.7 ppm–648 ppm), and Th/U ratios (0.2–1.11) and R spots, with U (up to 65.7 ppm), Th (up to 45.8 ppm), Pb (up to 14.56 ppm), and Th/U ratios (up to 0.827). The U and Th contents are generally less than 80 ppm with exception of NK2-5C where U (6 380 ppm) and Th (1 280 ppm) values are the highest.

    The U-Th-Pb contents (in ppm) and Th/U ratios in C spots in zircon crystals from sample (NK3) (Table 3) are U (70.0 ppm –803 ppm); Th (35.0 ppm–665 ppm); Pb (12.0 ppm–173.3 ppm); and Th/U ratios (0.10–1.23). The Pb content (156.5 ppm) in NK3-3C is higher than that of U (124.1 ppm) and Th (148.4 ppm). In R spots, U contents range from 91.0 ppm to 4 700 ppm, whereas Th and Pb contents range from 55.0 ppm to 1 464 ppm and 21.0 ppm to 354 ppm, respectively. The Th/U ratios range of 0.08–0.76 with most values greater than 0.40. The only analyzed crack (NK3-9C-CRACK) is composed of U (71.5 ppm), Th (55.4 ppm), Pb (26.35 ppm), and Th/U ratio (0.774).

  • The U-Pb ages (206Pb/238U and 207Pb/235U in Tables 1, 2 and 3) are heterogeneous with concordant and discordant plots (see Figs. 46). For zircon crystals from sample NK1, 206Pb/238U and 207Pb/235U ages in C spots range from 613.1±8.1 to 988.4±19.0 Ma, and 617.3±9.9 to 982.7±54.0 Ma, respectively. Figure 4 shows concordant to discordant age ranges. The youngest core ages (566–626 Ma) date Ediacaran events and the oldest ages (632–679 Ma) and (982–988 Ma) date Cryogenian–Tonian events, respectively. Ages in R spots range from 373.0±8.4 to 615.9±8.0 Ma, and 374.0±8.1 to 617.4±4.6 Ma with youngest ages (374–485 Ma) dating Early Ordovician to Late Devonian events, and oldest ages (520–618 Ma) date Ediacaran to Early Cambrian events.

    Figure 4.  U-Pb zircon concordia plot and age determination for sample NK1.

    Figure 5.  U-Pb zircon concordia plot and age determination for sample NK2.

    Figure 6.  U-Pb zircon concordia plot and age determination for sample NK3.

    Zircon crystals from sample NK2 (Table 2, Fig. 2b) yielded 206Pb/238U and 207Pb/235U ages in C-spots range from 122.8±7.0 to 619.3±9.3 Ma, and 115.7±6.0 to 616±9 Ma, respectively, while ages in R-spots are up to 610±11 and up to 601±10 Ma. The youngest age in C-spots (up to 122.8 Ma) are the only discordant plot in Fig. 5, dating an Aptian to Albian event. The oldest ages in C-spots (592–620 Ma) are very close to those of R-spots dates of an Ediacaran event.

    The 206Pb/238U and 207Pb/235U ages in C-spots in zircon crystals from sample NK3 (Table 3, Fig. 3) range from 469.6±16.0 to 933±18 Ma, and 449±16 to 954±14 Ma, respectively. The youngest ages (up to 469.6 Ma) date an Early to Late Ordovician event, and the oldest ages (560–954 Ma) date Neoproterozoic events. Ages R-spots range from 112.7±2.7 to 663.6±11.0 Ma, and 108.4±1.7 to 663±11 Ma, with the youngest age (108–336 Ma) dating Aptian to Albian and Carboniferous events, and oldest ages (560–664 Ma) record a Middle to Late Neoproterozoic event. The only dated crack (NK3-9C-CRACK) gives ages of 960±22 (206Pb/238U) and 934±19 Ma (207Pb/235U), typical of an Early Neoproterozoic event.

  • Morphological and textural features, U-Th-Pb compositions, Th/U ratios, and U-Pb ages are used to characterize and classify each zircon crystal. They are also used to elucidate their crystallization and growth history and registered parameters. Their host-rock formation conditions are developed in correlation with local geology. Registered post-crystallization events are also presented.

  • The grain size of the studied zircon crystals greater than 75 µm are within the range limit 20 to 250 µm common for magmatic zircon (see Hoskin and Schaltegger, 2003). Zircon external morphology and internal texture reflects its geological history, especially the relevant episode(s) of magmatic-metamorphic crystallization, strain imposed both by external forces and by internal volume expansion caused by metamictization and degrees of chemical alteration (Corfu et al., 2003). The studied crystals are prismatic, lamellar, pyramidal, dipyramidal, sub-spherical, subhedral or anhedral (Figs. 2, 3), which are distinctive features differentiating them. These morphological differences may characterize different crystallization histories. Igneous zircons are usually subhedral to euhedral (Hoskin and Schaltegger, 2003). The presence of subhedral and euhedral faces in most of the studied zircons (Fig. 2b) clearly shows dominantly magmatic crystallization features. Hoskin and Schaltegger (2003) noted zircon grain size and form development depend upon when zircon saturated in the crystallization history of its protosource rocks with: (1) small and acicular crystals probably formed due to local saturation at the edge of an early crystallizing phase (Bacon, 1989); (2) large and euhedral crystals mainly formed in early zircon-saturated melts (Hoskin and Schaltegger, 2003); and (3) and anhedral crystals or those with partially developed faces, representing late crystallized zircon grew in the interstices between early formed crystals (Scoates and Chamberlain, 1995). If based on the above classification, the studied zircon crystals are dominantly crystals formed in early zircon-saturated melts with very few from late stage crystallization.

    The studied zircons present the following internal textures: oscillatory zoning; sector zoning; patchy zoning; faint broad zoning; complex growth zoning or overgrowth rims and a preserved core, and few unzoned crystals. Those with clear core-rim difference and local recrystallization are similar to some presented in Corfu et al. (2003). Oscillatory zoning is a feature probably due to the degree of zircon-saturation or the incorporation of trace elements (Vavra, 1994; Halden et al., 1993). The development of sector zoning in the Nkogho zircons could be due to kinetic factors and rapid changes in the growth environment during crystal development (cf. Paterson and Stephens, 1992) or to rapidly fluctuating and unequal growth rates related to the roughness of the growth surface and degree of saturation of the growth medium (cf. Vavra et al., 1996). Patchy zoning in some of the zircon crystals may reflect strain experienced by zircon during final magmatic emplacement (cf. Corfu et al., 2003) or could be the result of heterogeneous trace element distribution (cf. Hoskin and Schaltegger, 2003). Zircons with faint broad zoning could be crystals recrystallized along microfractures (cf. Vavra and Hansen, 1991). Zircons with complex growth zoning and local intermediate resorption from an anatectic granite are presented in Corfu et al. (2003). Their presence within the Nkogho's zircon suites could show crystallization in an anatectic granitic melt. The studied granitic rocks were named anatectic granitoids by Dumort (1968), which could be supported by the presence of complex growth zoning texture in some of it's host zircons. The absence of zoning in part of the studied zircons is difficult to interpret with just internal textural analysis. Zircons with preserved brighter core and much darker rim (e.g., Figs. 2b, 3) could be xenocrystic zircon, based on Corfu et al. (2003) zircon classification. They could be inherited crystals from older rocks, as found in many granitoid magmas (Belousova et al., 2006). Belousova et al. (2006) and Paterson et al. (1992) noted that inherited grains commonly have rounded cores overgrown by euhedral magmatic zircon. This feature is found in part of the studied zircons, and therefore support zircon inheritance.

  • The U, Th, and Th/U ratios have been used to characterize zircons crystals and understand their crystallization and growth history (e.g., Yakymchuk et al., 2018; Kirkland et al., 2015; Belousova et al., 2006, 1998; Corfu et al., 2003; Ahrens et al., 1967), as this mineral strongly influences the behavior of these two elements and others (e.g., Nb, Ta, Y, and REE) during magmatism (Hoskin and Schaltegger, 2003; Belousova et al., 2002; Heaman et al., 1990). The Th/U ratios generally ranging from 0.2–1.3 are within the range limit of crustal igneous zircons published in Kanouo et al. (2017a), Kirkland et al. (2015), Hoskin and Schaltegger (2003), Belousova et al. (2002), and Konzett et al. (1998), this shows that the studied zircons were mainly crystallized within crustal magmas. The U-Th-Pb abundances for samples NK1 and NK3 with some core and rim difference distinguish two groups of zircons (Tables 1 and 3): (1) those with rim U-Th-Pb abundance higher than that of the core, and (2) those with core U-Th-Pb abundance higher than that of the rim. The variability of U and Th contents in the analyzed spots can be caused by the availability these two elements in their cooling magmas, with some of these magmas being considerably U-Th enriched than others. It can be associated to a progressive crystallization of each zircon crystal during different stages of their growth within a cooling, crystallizing, and fractionating magma (unzoned zircons); or to core crystallization in original or source melt and later growth of a zone and/or rim in U-Th depleted or U-Th enriched melts of the same zircon crystal (for zoned zircons and those with preserved core and overgrowth rim). The variation of Pb content in the same zircon crystal is influenced by those of U and Th and the radioactive decay of these two elements to liberate the concentrated daughter Pb (Hoskin and Schaltegger, 2003). This can be the same for the studied zircons, as zircons with higher U-Th contents have relatively high Pb content, whereas those with lower U-Th contents have low Pb content.

    The U (30 ppm–6 380 ppm) and Th (4 ppm–1 280 ppm) contents in the studied zircons largely show crustal origins, as mantle zircons typically show very low U (< 30 ppm) and Th (< 10 ppm) abundances (Kanouo et al., 2018, 2017a, 2016, 2015, 2012a; Belousova et al., 1998; Heaman et al., 1990; Ahrens et al., 1967). The obtained U values (> 110 ppm) in the cores of zircon crystals from samples NK1 and NK2 and most of their rims are generally within the range limit (154 ppm–4 116 ppm) in Ahrens et al. (1967) granitic zircons. The calculated Th/U ratios for the above zircons are generally greater than 0.4, exceeding the average value (0.4) in Ahrens et al. (1967) granitic zircons. This confirms their crystallization in mainly granitic melts. The very low U and Th contents recorded in some portions in zircons from samples NK1 and NK2 can relate their growing in U-Th depleted melts. Some zones exhibit very low Th/U ratios (0.08 to < 0.17), compatible with values published for metamorphic zircons grown in an anatectic melt by Kanouo et al.(2018, 2017a), Heiss et al. (2008), and Hoskin and Schaltegger (2003). This could relate the crystallization of these very low Th/U ratios in zircons in anatectic melts. The U (59 ppm–6 380 ppm) and Th (45 ppm–1 280 ppm) in zircons from NK2 dominantly less than 80 ppm, are out of the range limit in Ahrens et al. (1967) granitic zircons. Their Th/U ratios (mostly > 0.6) are generally more than the average value in Ahrens et al. (1967) granitic zircons. They could be zircons crystallized in low U-Th granitic melts. One zircon (NK2-5C) with the highest U-Th contents probably crystallized from U-Th enriched melts.

  • The obtained zircon U-Pb ages for samples NK1, NK2, and NK3 are heterogeneous with some core, zone, and rim differences, which could show different crystallization and growing histories. The crystallization ages (613–989 and 617–983 Ma) recorded in the core of zircons from sample NK1 yield: Late Neoproterozoic (Ediacaran), Middle Neoproterozoic (Cryogenian), and Early Neoproterozoic (Tonian) times, which could date different magmatic events. The oldest core age (Early Neoproterozoic) could be that of an inherited granitic zircon carried by the cooling crustal granitic magma. The youngest zircon core ages (Cryogenian to Ediacaran) could date a progressive crystallization of their crustal granitic parent magma. This cooling probably began at Middle Neoproterozoic with the peak in the Late Neoproterozoic. The recorded ages (373–616 and 374–618 Ma) in the rim and zone on those zircons show different growth periods: Early Cambrian, Ordovician, and Devonian that could date different post-formation crustal magmatic events. The rims of two groups of zircons (NK1-6R: Early Devonian, and, NK1-7R: Middle Ordovician) with their Th/U ratios (≤0.165) (compatible with values of zircon in an anatectic melt) can date Early Devonian and Middle Ordovician anatexis.

    The crystallization ages (122–619 and 115–616 Ma) recorded in the core of zircons from sample NK2 show two peak periods: Aptian and Ediacaran. Ediacaran can be the crystallization period of their U-Th depleted, crustally sourced granitic parent magma. These ages are within the range of Ediacaran zircons from sample NK1. This similarity shows that part of zircons from NK1 and NK2 were crystallized during the same period, probably, in relatively U-Th depleted and U-Th fairly enriched melts, respectively. The main U-Th enrichments of their source magma(s) was obtained in Early Cretaceous time (Fig. 7), although other enrichments were noted in the Late Devonian and Middle Ordovician (Tables 13). The recorded Aptian core age for NK2-5C show that its host rock was probably affected by a post-Late Neoproterozoic magmatic event during Aptian times.

    Figure 7.  Sketch diagrams showing correlation between U-Th abundance and and 206Pb/238U age. (a) Zircon from sample NK1; (b) zircon from sample NK2; (3) zircon from sample NK3.

    The crystallization ages (469–933 and 449–954 Ma) recorded in the core of zircons from sample NK3 are Ordovician, Early Cambrian, Late Neoproterozoic, and Early Neoproterozoic. The Early Neoproterozoic age (933 and 954 Ma: NK3-8C), less than that of the oldest zircon (NK1-9C) from NK1, could be that of an inherited zircon from an anatectic melt, as it's Th/U ratio (< 0.2) is compatible with values of zircons grew in this type of melt. Anatexis was also recorded by the rim of the Early Neoproterozoic zircon during Middle Carboniferous and the core of NK3-C1 and rim of NK3-2R2 during Late Neoproterozoic time. The Late Neoproterozoic zircons in rocks with anatectic features could be syngenetically formed from partial fusion of the preexisting rock. The recorded ages (112.7±2.7 and 108.4±1.7 Ma: Aptian to Albian) in the rim of one zircon (NK3-5R2) are close to the core age of NK2-5C, suggesting that their host rock was affected by an Early Cretaceous magmatic event.

  • The combination of interpretations on zircon morphological and internal textural data, U-Th and Th/U features, and U-Pb ages presented in the above paragraphs will help to develop a formation model for the arc-like Nkogho (I-type) granite and present registered post-crystallization events within the local and regional settings.

  • The plotted data in Figs. 46 point on three mains concordia ages: Tonian, Cryogenian, and Ediacaran. From these concordia ages, it is suggested that the crystallization of the source magma of the arc-like Nkogho anatectic granite started during the Cryogenian and ended, during Ediacaran time. This magma probably originated by partial melting of a preexisting igneous arc-like granitic protolith during the Cryogenian time, with the incorporation of Early Neoproterozoic xenocrystic zircons. The Middle to Late Neoproterozoic core ages recorded for most zircon from the Nkogho granite are generally more than Late Neoproterozoic to Early Cambrian ages obtained for Otu granitic pegmatite (cropping out to the west of Mamfe sedimentary basin) and Late Neoproterozoic metamorphic and granitic zircons found in Babi mica schist outcropping in the SW part of this basin (Kanouo et al., 2017a). This age difference shows that the Nkogho granite is older than the Otu granitic pegmatite. The Late Neoproterozoic to Early Cambrian ages recorded in the core zones and rims of some zircons from the Nkogho granite (Tables 1 and 3) are close to those of zircons from Otu granitic pegmatite and Babi mica schist. This shows that the magmatic event source of the Otu granitic pegmatite was also recorded in part of the Nkogho granite, which probably underwent partial fusion (with crystallization zircons during the Late Neoproterozoic); the formation of granitic overgrowth zones on some early formed-zircon crystals occurred during Early Cambrian. The age similarity between the Nkogho anatectic granite and Babi mica schist, which was affected by Late Neoproterozoic tectono-metamorphic and magmatic Pan-African events (Kanouo et al., 2017a) source of the Cameroon mobile belt (Ngako et al., 2008; Toteu et al., 2004; Abbelsalam et al., 2002), show that this studied rock was also affected the Pan-African events.

  • The Nkogho granite was later affected by post emplacement partial fusion and magmatism including that of Early Cambrian; Ordovician, Devonian, Carboniferous, and Early Cretaceous. The most prominent post-crystallization magmatic events are: anatexis (registered in Middle Ordovician, Early Devonian, and Middle Carboniferous) and Aptian–Albian partial fusion and cooling of a granitic melt. It is therefore possible that the studied granite underwent partial fusions with cooling during Cambrian, Ordovician to Middle Carboniferous times, and further a partial fusion and crystallization of a granitic melt during Aptian– Albian times.

    Ordovician to Middle Carboniferous partial fusions and granitization are exclusively recorded by Nkogho anatectic granite and are difficult to correlate locally, as these events have not been studied in other basement rocks in the Mamfe Basin. The Aptian–Albian ages recorded by some zircon crystals from the studied granite shows that this rock probably underwent partial fusion with U-Th enrichment via selective partial melting and fractionation during Aptian–Albian times. The existence of Aptian–Albian partial fusion and crystallization of a granitic melt is not yet mentioned within the Cameroon mobile belt. Its existence in the Mamfe Basin (the southernmost Cameroon branch of the Benue Trough) could date the opening of this basin (a mega-tectono-sedimentary structure filled with Albian to Maastrichtian lithified marine and continental sediments; Benkhelil, 1989; Ofoegbu, 1984). According to Benkhelil (1987), the Mamfe Basin is an Early Cretaceous structure stretching in a southeasterly direction. The obtained ages are older than Late Albian to Early Cenomanian, suggesting constraints on the end of rifting in the Mamfe Basin (Olade, 1975). Dumort (1968) proposed Albian to Cenomanian ages for sedimentary sequences in the Mamfe Basin. Palynological analyses of syn-rift deposit of the Manyu Group shows that these rocks are younger than Early Barremian and older than Middle Albian (Ajonian, 2016). Njoh et al. (2015) proposed Late Cenomanian to Turonian palynomorph ages for some sedimentary rocks found in the Nfaitok Formation. All these micropaleontogical ages are younger than the obtained U-Pb zircon Aptian–Albian ages, hence, could support the suggestion that the Mamfe sedimentary basin opened during Aptian–Albian time.

  • Zircon crystals from the arc-like Nkogho (I-type) granite, with their morphogical, textural, U-Th-Pb, and U-Pb age variations (Early Neoproterozic to Early Cretaceous), are mainly igneous crustal granitic zircons with minor metamorphic zircon crystals that grew in equilibrium with an anatectic melt.

    The arc-like Nkogho I-type granitoid crystallized from a granitic source rock during Cryogenian to Ediacarian, which was later affected by post-Ediacarian tectono-magmatic events, dating Cambrian to Albian with the Aptian–Albian ages probably reflecting opening of the Mamfe Basin.

  • The authors thank Prof. Zhenbing She from China University of Geosciences in Wuhan for funding of the BSE-CL imaging of the zircon crystals. Our gratitude to the laboratory personnel at the University of New Brunswick who carried out the U-Th analysis and dating of the zircon crystals. These analyses supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant to Prof. David R. Lentz (UNB). The authors thank Prof. Roger Mason and an anonymous reviewer whose useful comments helped to improve the original manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1395-6.

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