Zircon Trace Element Geochemistry and Ti-in-Zircon Thermometry of the Ngazi-Tina Pan-African Post-Collisional Granitoids, Adamawa Cameroon

Alexis Hamdja Ngoniri¹, Habib Dadjo Djomo², Timoleon Ngnotue^1*, Patrick Ayonta Kenne¹, Ghislain Ngassam Mbianya¹, Sylvestre Ganno^3*, Jean Paul Nzenti³

¹Department of Earth Sciences, University of Dschang, Dschang, Cameroon.
²Institute for Geological and Mining Researches (IGMR), Yaounde, Cameroon.
³Department of Earth Sciences, University of Yaoundé I, Yaounde, Cameroon.

DOI: 10.4236/ijg.2021.124017                

Abstract

In Cameroon, the Ngazi-Tina region belongs to the Adamawa-Yade domain of the Pan-African Central African Fold Belt (CAFB). It is composed of two petrographic types: quartz-monzonites (majority) and nepheline syenites. Two morphological types, prismatic and pyramidal, were recognized in the zircon grains samples. These zircon types display internal structures typical of magmatic zircons. Zircons separated from the Ngazi-Tina samples contain higher abundances of Hf (close to 8000 ppm) and moderate trace elements (Y, Th, U, Nb, Ta) and REE contents, suggesting a variable degree of magmatic evolution. The chondrite-normalized REE patterns of zircons are characterized by LREE depletion relative to HREE with positive Ce and negative Eu anomalies, typical of magmatic zircons. The high Hf content together with high Ce/Ce*, Th/U, Zr/Hf ratios suggest magma crystallization under variable oxidation and oxygen fugacity. The application of Ti-in-zircon thermometer reveals crystallization temperatures ranging from 678°C to 811°C and 658°C to 768°C for quartz monzonites and nepheline syenites respectively. These features indicate probably a partial melting of continental crust as the source of these zircons grains and emplacement in the magmatic-arc setting.

Keywords

Granitoids, Zircon Geochemistry, Ti-in-Zircon Thermometer, Continental Crust, Ngazi-Tina, Adamawa-Yade Cameroun

1. Introduction

Pan-African orogeny contributed to the formation and recycling of continental crust during the melting of Gondwana from 870 to 550 Ma ( [1] [2] ). This Pan-African orogeny generated granitoids with different chemical compositions and evolutionary processes, in various tectonic environments [3]. In Cameroon, three main geological domains have recorded the Pan-African orogeny imprints (Figure 1). The Adamawa-Yadé domain is characterized by widespread syn- to post-collisional granitoids of Neoproterozoic in age ( [4] [5] [6] ). During the past two decades, these granitoids have been studied in order to better understand the geodynamic evolution of this orogenic belt, which generally involve collision processes between continents ( [7] - [12] ) However, except recent works ( [13] [14] ), these studies do not integrate the trace element composition of zircon as well as the temperatures of the magma in which these zircons were crystallized. Here, we examine the morphology, internal structure, trace elements and Ti-in-zircon thermometer of zircon grains extracted from Ngazi-Tina rocks with the aim to clarify the origin and nature of magma, and to better constraint the tectonic setting.

2. Geological Setting

The Central African Fold Belt (CAFB; [15] [16] [17] ) or North Equatorial Pan-African Fold Belt [18] is a huge domain of the Pan-African orogeny [19], limited to the northwest by the Trans-Saharan belt, to the south by the Congo Craton, and extends west to northeast Brazil by the Sergipano Belt, thus forming the Pan-African-Braziliano Belt ( [20] [21] [22] ). The latter results from the collision between large continental blocks: the West African/São Francisco cratons, the Congo craton, and an intermediate Neoproterozoic age domain, which includes the polycyclic basement provinces of Hoggar, Nigeria and Borborema (Figure 1(a)).

In Cameroon, the CAFB is mostly formed of Precambrian rocks. It extends from the Yaoundé Group (YD) in the south to the Western Cameroon Domain (WCD) in the Far North of Cameroon [20]. This belt, formed between 1122 Ma (U-Pb age on zircon; [21] ) and 558 Ma (EMP age; [22] ), has been subdivided into three domains ( [14] [23] ): the West Cameroon Domain (WCD) in the north, the Adamawa-Yade Domain (AYD) in the center and the Yaounde Domain (YD) in the south (Figure 1(b)).

The AYD is composed mainly of metamorphic basement rocks intruded by granitoids ( [4] [20] [24] [25] [26] [27] ). The metamorphic rocks comprised schists, amphibolites and gneisses. Their U-Pb zircon dating has revealed late Archean to Paleoproterozoic protolith ages and Pan-African metamorphic age ( [26] [28] [29] [30] ). The granitic rocks (granites, granodiorites, quartz monzonites, monzodiorites) are metaluminous to weakly peraluminous. Most of these granitoids are hyper-potassic with calc-alkaline to shoshonitic affinity. They show signatures of I-type granites ( [5] [6] [31] [32] [33] [34] ). The granitoids of the

Figure 1. (a) Geological sketch map of West-Central Africa and Northeast Brazil with cratonic masses and the Pan-African-Brasiliano belts in West Gondwana (modified from [4] ); (b) Geological map of Cameroon showing the major lithotectonic domains. WCD: West Cameroon Domain, AYD: Adamawa-Yadé Domain, YD: Yaoundé Domain, TBSZ: Tcholliré-Banyo Shear Zone, ASZ: Adamawa Shear Zone, SSZ: Sanaga Shear Zone. Location of study area is marked by a blue square.

Adamawa-Yade Domain are Neoproterozoic in age ( [11] [35] [36] [37] [38] ).

The Ngazi-Tina area belongs to the eastern part of the AYD at the border with the Central African Republic (CAR). This area comprises both granitoids and metamorphic rocks ( [27] [39] ). The granitoids are composed of granites, syenites and diorites, both showing metaluminous and I-type signatures. However the diorites are ferroan and high-K calc-alkaline while the granites and syenites are magnesian and shoshonitic. LA-ICP-MS U-Pb zircon analyses yield emplacement age of 576.4 ± 1.9 Ma (granites) and 585.9 ± 2.1 Ma (syenites) [39] ). The metamorphic rocks are meta-igneous (gneisses, amphibolites) and meta-sedimentary (schists) rocks (Figure 2). These basement rocks are locally covered by Cenozoic volcanic rocks of the Cameroon Volcanic Line [5] and by Cretaceous deposits.

3. Analytical Methods

Samples were collected from fresh outcrops in Mount Tina. Two samples (quartz-monzonite, NGT 28 and nepheline syenite, NGT 23) were selected for zircon analyses. Zircon grains were extracted from each sample using conventional gravimetric and magnetic techniques, following by handpicking under a binocular microscope at Langfang Rock Detection Technology Services Ltd (Hebei, China). Then, all subsequent zircon treatment was conducted at Beijing Kehui Testing International Co. Ltd (Beijing, China) ( [40] [41] ). The representative zircon grains were mounted on an epoxy resin and polished to mid-section to expose their cores for analyses ( [9] [40] [41] [42] ). The scanning electron microscope JSM 6510, attached to a Gatan CL detector with a voltage of 10 kv, was used to produce microphotographs in transmitted and reflected light, and cathodoluminescence (CL) images. Zircon trace elements and U-Th-Pb analyses on zircon were performed using an ESI NWR 193 nm laser ablation system and an ICP-MS Anlyitik Jena PQMS Elite instrument. These analyses were performed with a beam diameter of 30 μm, a repetition frequency of 10 Hz, and an

Figure 2. Geological map of the Ngazi-Tina area [27].

energy of 4 J/cm. The standards used for each series of 5 to 10 analyses are 91500 zirconia, GJ-1 glass, Plesovice, Qinhu and NIST610 ( [9] [41] [43] [44] ).

4. Results

Fifty-five spots were obtained on zircon grains, including 29 for quartz monzonites and 26 for nepheline syenites. Cathodoluminescence (CL) images were used for spot selection during trace element analyses, and to describe the external shape (morphology) and their internal structure.

4.1. Zircon Morphology

Quartz-monzonite zircons display both prismatic and pyramidal shapes. The prismatic zircons are euhedral with a size ranging from 50 × 40 to 180 × 60 µm and axial ratios between 2:1 and 3:1 (Figure 3(a)). The pyramidal zircons are subeuhedral in shape. They are quite smaller compare to prismatic zircons, ranging from 30 to 120 µm in length (Figure 3(a)). The length/width ratio gives values of 1:1 and 2:1. Zircons from nepheline syenite sample also exhibit pyramidal and prismatic shapes, similar to those of quartz-monzonite. The prismatic shape is the more abundant type, displays size ranging from 40 × 30 to 105 × 50 µm with an average axial ratio of 2:1 (Figure 3(b)). Pyramidal-shape zircons show a variable size with average length to width ratio of 2:1 (Figure 3(b)). Both nepheline syenite and quartz-monzonite shared fine grains zircons and sometimes truncated zircons (Figure 3(a) and Figure 3(b)).

4.2. Zircons Microstructure

In CL images, the studied zircon grains are light to light brown in color. They

Figure 3. Cathodoluminescence (CL) images of zircon grains from the Ngazi-Tina granitoid (Laser ablation spots and number indicated in red. Ages indicated in black).

show regular concentric zoning, characterized by light and dark bands (Figure 3(a)). This feature is typical of zircon grew in magmatic environment ( [41] [45] [46] ). In a few grains, the internal parts display many inclusions which were avoided during analysis. No relic’s core and metamictisation have been found within the studied samples. Fractures are visible in some nepheline syenite zircon grains, whereas they are rare in quartz-monzonite zircons. Overall, CL images of the zircons from Ngazi-Tina granitoids indicate a single crystallization episode with no evidence for either later metamorphism or inheritance of earlier grains.

4.3. Zircon Geochemistry

Trace elements and rare earth elements (REE) composition of fifty-five individual zircon grains are listed in Table 1 and Table 2. The REE were normalized to chondrite, with the normalization values for chondrite being taken from [47].

Trace elements of the two samples vary significantly between grains in the same sample. The U and Th content vary from 72.02 to 215.76 ppm (average 125.92 ppm) and 95.32 to 297.33 ppm (average 193.79 ppm), respectively for quartz monzonites, and from 76.84 to 185.9 ppm (average 134.19 ppm) and 100.82 to 323.5 ppm (average 214.34 ppm), respectively for nepheline syenites. Y values are relatively higher and range from 467.86 to 1276.19 ppm (average 797.33 ppm) in nepheline syenite zircons, and from 433 to 1137.57 ppm (average 788.39 ppm) in quartz monzonite zircons. Nb and Ta show low content within the two granitoids samples. Their abundances range from 1.6 to 8.36 ppm (average 2.82 ppm) and 0.58 to 2.03 ppm (average 0.96), respectively for quartz monzonite while in nepheline syenite zircon grains, their contents range from 1.56 to 5.99 ppm (average 3.10 ppm) and 0.58 to 1.76 ppm (average 1.07 ppm), respectively. However, Hf concentrations for all the rocks are close to 8000 ppm (average = 7840 ppm and 8133 ppm, respectively for quartz monzonites and nepheline syenites). In general, most of the highest concentrations are found in nepheline syenites and the lowest values in quartz monzonites. This demonstrates the enrichment of Hf in zircons from nepheline syenite sample relative to quartz-monzonite samples and consequently mirrors their crystallization at a higher degree of magmatic evolution [46].

The chondrite-normalized REE patterns of the zircons grains (Figure 4(a) and Figure 4(b)) show an enrichment of MREE to HREE over LREE from the two samples (quartz monzonites and nepheline syenites). In addition, REE patterns for all zircon types are nearly parallel and show little variation in overall concentrations, reflecting their similar crystallization environment ( [48] [49] ). Zircons in our samples are characterized by positive Ce and negative Eu anomalies in chondrite-normalized patterns. The positive Ce anomaly (Ce/Ce*) range from 2.27 to 360 and 1.92 to 667 respectively for nepheline syenites and quartz monzonites zircons, which is typical of magmatic zircons ( [46] [50] ). The Eu anomaly (Eu/Eu*) are weakly negative and range from 0.58 to 0.84 (nepheline

Figure 4. Chondrite-normalized REE patterns for (a) quartz monzonite and (b) nepheline syenite zircons.

syenites) and from 0.71 to 0.89 (quartz monzonites). According to [46], the positive Eu anomaly is related to oxidation processes or an oxidizing environment, while the negative Eu anomaly is indicative of plagioclase fractionation.

4.4. Ti-in-Zircon Thermometry

The Ti-in-zircon thermometer using [51] formula’s, indicates a crystallization temperatures ranging from 678˚C to 811˚C in quartz monzonites (Table 3). The minimum is observed on the spot NGT28-11 while the maximum is recorded by the spot NGT28-08. The average temperature calculated is ca. 726˚C (Figure 5(a)). In nepheline syenites, the crystallization temperatures vary from 658˚C (NGT23-04) to 768˚C (NGT23-05) (Table 3), with an average of ca. 720˚C (Figure 5(b)), slightly lower than that of quartz monzonite zircons.

5. Discussion

Several authors(e.g. [52] [53] [54] ) have clearly demonstrated that, zircons generally persist as refractory relics in felsic magma (due to their low solubility in siliceous cast irons) and can potentially retain the chemical and isotopic composition

Table 3. Temperature calculation using Ti-in-zircon [51].

of the deep crust. According to [53], zircon can survive various episodes of sedimentary and magmatic recycling, metamorphism and even subduction. In such, trace element concentrations and ratios are widely used to constrain the source of magmatic zircons ( [55] [56] [57] ). For example, high contents of Y, Th, U, Nb and Ta, as well as enrichment in Ce and depletion in Eu are characteristics of zircons from magmatic melt [58]. In addition, zircons from mantle

Figure 5. Diagrams showing distribution of temperature obtained from Ti-in-zircon thermometry for each zircon grains (the blue line indicates the average temperature).

magmas show negative correlations of Hf with Th, Y and U, in contrast to zircons derived from crust, which show positive correlations [59]. In this study, the Hf vs. Th and U show a positive correlation, suggesting crust derived zircon grains (Figure 6(a) and Figure 6(b)).

In addition, the U/Yb ratio can significantly reflect the environments in which the zircons crystallized. Indeed zircons from kimberlites have an average U/Yb ratio of 2.1 while the same ratio is relatively low for continental and oceanic crust zircons, 1.07 and 0.8 respectively [60] [61]. To discriminate the source of magma from zircon grains, the U/Yb vs. Hf and U/Yb vs. Y diagrams have been proposed [60]. In these diagrams, all zircon grains of the Ngazi-Tina granitoids are plotted in the field of the continental crust zircons (Figure 6(c) and Figure 6(d)). This interpretation is consistent with the Ti-in-zircon thermometry [51] indicating a crystallization temperature ranging from 678˚ to 811˚C (quartz monzonite) and 658˚C to 768˚C (nepheline syenite).

Several authors (e.g. [14] [49] [57] [62] [63] ) have used the distribution of trace and REE in zircons as petrogenetic guides. For example, high contents of U

Figure 6. (a) Th vs. Hf; (b) Y vs. Hf bivariate diagrams; (c) U/Yb vs. Hf; (d) U/Yb vs. Yb diagrams between continental and ocean crust zircons [60].

Figure 7. (a) Ce/ce* vs. Hf; (b) Eu/Eu* vs. Ce/Ce* diagrams.

relative to Th (low Th/U ratios) are features of zircons crystallized from magma at low temperature ( [64] [65] ). Generally, during magmatic differentiation, Hf concentrations increase in zircons, hence the low Zr/Hf ratio ( [61] [66] ). To determine the conditions of crystallization of magma, some authors ( [49] [54] [57] [61] ) have proposed the use of Ce/Ce* vs. Hf diagram. In this plot (Figure 7(a)), the studied zircon grains follow the trend of oxygen fugacity (ƒO₂). In addition, the Eu/Eu* vs. Ce/Ce* diagram (Figure 7(b)) reveals that these zircons may have crystallized under varying oxidation conditions. The characteristics of zircons described above are also observed in Ekoumedion [13] and Linté [14] granitoids with the AYD. However, Ngazi-Tina zircons are distinguished by the absence of hydrothermal zircons.

The Ngazi-Tina granitoid zircons suggest a compressional-magmatic arc context in the Nb/Hf vs. Th/U and Hf/Th vs. Th/Nb (Figure 8(a) and Figure 8(b)) discrimination diagrams [54] [67] where all zircon grains fall within the field of orogenic zircons. Such tectonic environment was recently proposed in the Yoro-Yangben area [68]. This result is consistent with the delamination of the subcrustal lithospheric mantle (SCLM) and asthenospheric uplift ( [6] [17] [69] ) and

Figure 8. Tectonic discrimination diagrams (a) Hf/Th vs. Th/Nb [54]; (b) Nb/Hf vs. Th/U [67]. The blue and green circles indicated the field of orogenic and anorogenic related rocks, respectively.

resulting in crustal thickening.

6. Conclusion

The Ngazi-Tina granitoids zircons display prismatic and pyramidal shapes with light and dark bands, typical of magmatic zircons. The Ti-in-zircon thermometry indicates a crystallization temperature ranging from 678˚C to 811˚C (quartz monzonite) and 658˚C to 768˚C (nepheline syenite) which is consistent with zircons grew in the continental crust. The chondrite-normalized REE patterns of both zircon samples are similar and display a steeply-rising slope due to serious HREE enrichment and LREE depletion. The depletion in Eu relative to Ce coupled with the different chemical ratios, indicate that these rocks were probably originated from the partial melting of the continental crust. Hf values and Ce/Ce*, Th/U, Zr/Hf ratios suggest that magma crystallization took place under variable oxidation and oxygen fugacity conditions.

Acknowledgements

The data presented here form part of the first author’s PhD Thesis at the Department of Earth Sciences of the University of Dschang. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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