Open AccessArticle

Petrogenesis of the Newly Discovered Neoproterozoic Adakitic Rock in Bure Area, Western Ethiopia Shield: Implication for the Pan-African Tectonic Evolution

Junsheng Jiang

^1,2

Wenshuai Xiang

^1,*

Peng Hu

^1,*,

Yulin Li

³,

Fafu Wu

¹,

Guoping Zeng

¹,

Xinran Guo

⁴,

Zicheng Zhang

⁵ and

Yang Bai

⁶

Wuhan Center, China Geological Survey (Central South China Innovation Center for Geosciences), Wuhan 430205, China

Research Center for Petrogenesis and Mineralization of Granitoid Rocks, China Geological Survey, Wuhan 430205, China

Institute of Geological Survey, China University of Geosciences, Wuhan 430074, China

⁴

The Geological Science Education Center of Guangdong, Guangzhou 510000, China

⁵

China National Geological & Mining Corporation, Beijing 100029, China

⁶

Geophysical Exploration Brigade HuBei Geological Bureau, Wuhan 430056, China

Authors to whom correspondence should be addressed.

Minerals 2024, 14(4), 408; https://doi.org/10.3390/min14040408

Submission received: 21 February 2024 / Revised: 4 April 2024 / Accepted: 9 April 2024 / Published: 16 April 2024

(This article belongs to the Special Issue Mineral Chemistry of Granitoids: Constraints on Crystallization Conditions and Petrological Evolution)

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Abstract

The Neoproterozoic Bure adakitic rock in the western Ethiopia shield is a newly discovered magmatic rock type. However, the physicochemical conditions during its formation, and its source characteristics are still not clear, restricting a full understanding of its petrogenesis and geodynamic evolution. In this study, in order to shed light on the physicochemical conditions during rock formation and provide further constraints on the petrogenesis of the Bure adakitic rock, we conduct electron microprobe analysis on K-feldspar, plagioclase, and biotite. Additionally, we investigate the trace elements and Hf isotopes of zircon, and the Sr-Nd isotopes of the whole rock. The results show that the K-feldspar is orthoclase (Or = 89.08~96.37), the plagioclase is oligoclase (Ab = 74.63~85.99), and the biotite is magnesio-biotite. Based on the biotite analysis results, we calculate that the pressure during rock formation was 1.75~2.81 kbar (average value of 2.09 kbar), representing a depth of approximately 6.39~10.2 km (average value of 7.60 km). The zircon thermometer yields a crystallization temperature of 659~814 °C. Most of the (Ce/Ce*)_D values in the zircons plotted above the Ni-NiO oxygen buffer pair, and the calculated magmatic oxygen fugacity (logfO₂) values vary from −18.5 to −4.9, revealing a relatively high magma oxygen fugacity. The uniform contents of FeO, MgO, and K₂O in the biotite suggest a crustal magma source for the Bure adakitic rock. The relatively low (⁸⁷Sr/⁸⁶Sr)_i values of 0.70088 to 0.70275, positive ε_Nd(t) values of 3.26 to 7.28, together with the positive ε_Hf(t) values of 7.64~12.99, suggest that the magma was sourced from a Neoproterozoic juvenile crust, with no discernable involvement of a pre-Neoproterozoic continental crust, which is coeval with early magmatic stages in the Arabian Nubian Shield elsewhere. Additionally, the mean Nd model ages demonstrate an increasing trend from the northern parts (Egypt, Sudan, Afif terrane of Arabia, and Eritrea and northern Ethiopia; 0.87 Ga) to the central parts (Western Ethiopia shield; 1.03 Ga) and southern parts (Southern Ethiopia Shield, 1.13 Ga; Kenya, 1.2 Ga) of the East African Orogen, which indicate an increasing contribution of pre-Pan-African crust towards the southern part of the East African Orogen. Based on the negative correlation between MgO and Al₂O₃ in the biotite, together with the Lu/Hf-Y and Yb-Y results of the zircon, we infer that the Bure adakitic rock was formed in an arc–arc collision orogenic environment. Combining this inference with the whole rock geochemistry and U-Pb age of the Bure adakitic rock, we further propose that the rock is the product of thickened juvenile crust melting triggered by the Neoproterozoic Pan-African Orogeny.

Keywords:

Bure adakitic rock; mineralogical characteristics; Sr-Nd-Hf isotopes; Western Ethiopia Shield

1. Introduction

The East African Orogen (EAO) has recorded a complex history of intra-oceanic and continental margin magmatic and tectono–thermal events from the Neoproterozoic to the Early Cambrian. It mainly consists of the juvenile Arabian Nubian Shield (ANS) and the largely older continental crust of the Mozambique Belt (MB) from north to south [1,2,3]. The Western Ethiopian Shield (WES) is situated in a key location, relatively close to the transition between the Arabian Nubian Shield and the Mozambique Belt. It is also adjacent to and east of the ‘Eastern Saharan Meta-craton’ [4]. It is a metamorphic terrane that includes high-grade gneisses and low-grade metavolcanic and metasedimentary rocks with associated intrusions. The granitoid rocks, which have either intruded into greenschist facies volcano–sedimentary sequences or been emplaced at the contact between low- and high-grade terranes, constitute a significant proportion of plutonic rocks in the Precambrian rocks of the WES. Many researchers have focused on the granitoid rocks in the WES, significantly advancing our understanding of regional tectonic evolution [5,6,7,8,9,10,11]. However, the magma source of these granitoid rocks in the WES, especially those intruding into the low- and high-grade rock associations within the eastern part of the WES, remains unclear. Also unclear is whether the magma source derived from mixing with pre-Neoproterozoic crustal material or not.

The Bure granite in the WES formed in the Pan-African Orogeny Period (750–650 Ma; [5]). It has an LA-ICP-MS U-Pb age of 773.8 ± 8.1 Ma and is characterized by high Sr (310~401 ppm), Sr/Y (64.9~113.6), and La/Yb (25.7~51.6), and low MgO (0.27~0.41 wt%), Y (2.71~4.78 ppm), and Yb (0.20~0.31 ppm) values [7]. Based on the study by Xu et al. [12], Jiang et al. [7] defined the Bure granite as an adakitic rock. This rock is called the Bure adakitic rock in this study. It is a newly discovered magmatic rock type in this area. However, the magma source and physicochemical conditions of the Bure adakitic rock remain unknown, hindering a comprehensive understanding of its petrogenesis and geodynamic evolution during the Pan-African period. Its mineralogical and isotopic compositions vary significantly depending on the type of precursor rocks and/or igneous processes during the evolution of its parental magma. Thus, knowledge of the mode of origin of these rocks contributes to our understanding of the Neoproterozoic evolutionary history of the WES.

This study investigates the major elements of typical minerals (K-feldspar, plagioclase, and biotite), trace elements and Hf isotopes of zircon, and Sr-Nd isotopes of the whole rock from the Bure adakitic rock in the eastern part of the WES. Combined with local and regional geological, geochemical, and geochronological data, the results shed light on the degree of pre-Neoproterozoic crustal material involvement in the source magmas, and the Neoproterozoic geological evolution of the WES.

2. Geological Setting

The EAO is a Neoproterozoic to early Cambrian mobile belt that reflects the collision between Neoproterozoic India and the African Neoproterozoic continents [1,2,13,14]. Based on its lithological and metamorphic characteristics, the EAO can be broadly subdivided into two terranes, the Arabian Nubian Shield in the north and the Mozambique Belt in the south. The ANS is dominated by low-grade volcano–sedimentary rocks associated with plutons and ophiolitic remnants [4,15,16,17,18,19], and represents the juvenile terrane. However, the MB in the south part of the EAO is a tract of largely older continental crust that was extensively deformed and metamorphosed in the Neoproterozoic/Cambrian ([10,20,21,22]; Figure 1).

The WES is also called the Tuludimtu Orogenic Belt, which is understood to have formed during the amalgamation of western Gondwana before the final closure of the Mozambique Ocean [15]. It can be subdivided into five litho-tectonic domains from west to east—the Daka, Sirkole, Dengi, Kemashi, and Didesa Domains. The Daka Domain lies in the southwest corner of the WES (Figure 2) and consists of pre-Neoproterozoic basement gneisses representing the western basement margin of the Tuludimtu Belt. The Sirkole Domain, composed of gneissic and volcano–sedimentary rocks intruded by granites, is located in the northwestern portion of the WES that extends into Sudan. The Dengi Domain is characterized by a deformed and metamorphosed volcano–sedimentary sequence and the Jamoa-Ganti orthogneiss; there are several intrusive bodies in this domain. It is generally thought to be a volcanic arc sequence related to the closure of the ocean represented by the Tuludimtu Ophiolite to the east. The Kemashi Domain consists of a sequence of metasedimentary rocks and abundant mafic to ultra-mafic volcanic material that has been metamorphosed to upper greenschist/epidote-amphibolite facies. The nature of these ultra-mafic/mafic plutonic rocks within the Kemashi Domain is controversial, with some scholars holding that they represent an ophiolite sequence [4,15,23], named the Tuludimtu Ophiolite. However, others [24,25,26] hold that these ultra-mafic/mafic plutonic rocks represent Alaskan-type, concentrically zoned intrusions, which were emplaced into an extensional arc or back-arc environment. The Didesa Domain within the eastern boundary of the WES is characterized by amphibolite facies paragneiss and orthogneiss intruded by Neoproterozoic intrusive rocks. It is located in the transition between the Arabian Nubian Shield and the Mozambique Belt.

Three generations of magmatism at ca. 850–810 Ma, 780–700 Ma, and 650–550 Ma [5,8,9,10,27,28], which represent pre-, syn-, and post-tectonic environments, respectively, have been recognized by previously limited ages from elsewhere in the WES [10,21]. These intrusions are usually present as strains and dikes and are developed as ductile fault contact or intrusive contact with the surrounding rock. The main types of intrusions are granite, granodiorite, monzogranite, and tonalite. The Bure adakitic rock is located at the eastern boundary of the Didesa Domain, with the surrounding rocks comprising gneisses. This rock assemblage suggests that it not only inherited the unique rock assemblages of the Arabic-Nubian Shield but also developed the typical middle-high grade metamorphic rocks of the Mozambique Belt.

3. Samples and Analytical Methods

3.1. Petrography

The Bure adakitic rock appears light gray in the field, with a fine granitic texture. It is mainly composed of K-feldspar (45–48 wt%), plagioclase (20–23 wt%), quartz (23–25 wt%), biotite (4–5 wt%), and minor amounts of muscovite (1–2 wt%) (Figure 3a). The K-feldspar is heteromorphic granular, with a size of 0.2–1.5 mm, some of which show slight kaolinization on the surface. The plagioclase is granular and 0.1 to 1 mm in size, with characteristics of polysynthetic twins and Carlsbadal bite compound twins. The surface of the plagioclase is usually altered, displaying light sericitization. The quartz is xenomorphic-granular, with a size of 0.05–0.7 mm (Figure 3b).

3.2. Analytical Methods

Electron microprobe analysis (EMPA) was performed on the K-feldspar, plagioclase, and biotite at the Zhongnan Mineral Resources Supervision and Test Center for Geoanalysis, Wuhan Center, China Geological Survey. During the analysis, a 10-μm spot size was used for the plagioclase and K-feldspar, and a 1-μm spot size was used for the biotite, with an accelerating voltage of 20 kV and a beam current of 20 nA. The integration times for the Ti and Mn peaks were 20 s and that for the remaining elements was 10 s. The SPI and ZBA mineral standards and ZAF calibration were employed for all minerals.

Trace element analyses of zircon were conducted synchronously using LA–ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd. Laser sampling was performed using a GeolasPro laser ablation system consisting of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7700e ICP-MS instrument was used to acquire ion-signal intensities. Zircon 91,500 and glass NIST610 were used as external standards for trace element calibration. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. The spot size and frequency of the laser were set to 32 µm and 10 Hz, respectively. Each analysis incorporated a background acquisition of approximately 20–30 s followed by 50 s of data acquisition from the sample. Excel-based software ICPMSDataCal 11.8 was used to perform quantitative calibration for trace element analysis [29].

About 0.1–0.2 g of whole rock powder of each sample was dissolved in digestion bombs with a mixture of double distilled HNO₃, HF, and HClO₄. They were then placed in an electric oven and heated to 190 °C for 48 h. Columns of DoweAG50WX8 and HDEHP resin were used successively for the separation and purification of rare earth elements (REEs) and finally for the separation of Nd and Sm by HCl eluant. The Sr-Nd isotopic measurements were performed using the Triton Ti thermal ionization mass spectrometer (TIMS) at the Laboratory of Isotope Geochemistry, Wuhan Center of China Geological Survey. ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios were normalized to ¹⁴³Nd/¹⁴⁴Nd = 0.7219 and ⁸⁷Sr/⁸⁶Sr = 8.375209, respectively. Measurements of the La Jolla and SRM NBS987 standards during this course gave average ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios of 0.511847 ± 3 (2σ, n = 25) and 0.710254 ± 8 (2σ, n = 22), respectively. ¹⁴⁷Sm/¹⁴⁴Nd and ⁸⁷Rb/⁸⁶Sr ratios of the samples were calculated using Sm, Nd, Rb and Sr concentrations as measured by the ICP-MS, and their relative uncertainties are ∼0.3% and ∼1%, respectively, based on USGS standard analyses [30].

In situ Hf isotope ratio analysis was conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Dreieich, Germany) in combination with a Geolas HD excimer ArF laser ablation system (Coherent, Göttingen, Germany) at the Wuhan Sample Solution Analytical Technology Co., Ltd., Hubei, China. A single spot ablation mode at a spot size of 44 μm was used, and the energy density of the laser ablation was ~7.0 J·cm⁻². Each measurement consisted of 20 s of acquisition of the background signal followed by 50 s of ablation signal acquisition. The detailed operating conditions of the laser ablation system and the MC-ICP-MS instrument and analytical method are the same as described by [31]. The normalized ¹⁷⁹Hf/¹⁷⁷Hf = 0.7325 and ¹⁷³Yb/¹⁷¹Yb = 1.132685 were used to calculate the mass bias of Hf (βHf) and Yb (βYb), respectively [32]. The interference of ¹⁷⁶Yb on ¹⁷⁶Hf was corrected by measuring the interference-free ¹⁷³Yb isotope and using ¹⁷⁶Yb/¹⁷³Yb = 0.79639 to calculate ¹⁷⁶Yb/¹⁷⁷Hf [31]. Similarly, the relatively minor interference of ¹⁷⁶Lu on ¹⁷⁶Hf was corrected by measuring the intensity of the interference-free ¹⁷⁵Lu isotope and using the recommended ¹⁷⁶Lu/¹⁷⁵Lu = 0.02656 to calculate ¹⁷⁶Lu/¹⁷⁷Hf. Off-line selection and integration of analyte signals and mass bias calibrations were performed using ICPMSDataCal [33]. In order to ensure the reliability of the analysis data, three international zircon standards of Plešovice, 91,500, and GJ-1 were analyzed simultaneously with the actual samples. Plešovice was used for the external standard calibration to further optimize the analysis and test results. 91,500 and GJ-1 were used as the second standard to monitor the quality of data correction. The external precision (2SD) of Plešovice, 91,500, and GJ-1 was better than 0.000020. The test value is consistent with the recommended value within the error range. At the same time, we used the internationally recognized high Yb/Hf ratio standard sample, Temora 2, to monitor the test data of the high Yb/Hf ratio zircon. The Hf isotopic compositions of Plešovice, 91,500, and GJ-1 have been reported by Zhang et al. [34].

4. Results

4.1. Mineral Compositions

4.1.1. K-Feldspar

The K-feldspar crystals of the Bure adakitic rock show relatively uniform compositional variation in the major elements (Table 1), with 11.23–16.66 wt% of K₂O (average value of 15.74 wt%), 0.37–1.20 wt% of Na₂O (average value of 0.62 wt%), and 10.81–19.36 wt% of Al₂O₃ (average value of 18.51 wt%). The low contents of CaO, MgO, TiO₂, and MnO indicate that there is less isomorphism and the formation temperature of K-feldspar is low [35]. The orthoclase (Or) value is high (89.08–96.37), the albite (Ab) value is low (3.57–10.59), and the anorthite (An) value is almost negligible (0–0.38), suggesting that the K-feldspar in this area is orthoclase (Figure 4a). The Or and Al₂O₃ values in the K-feldspar crystal show the same zigzag variation trend from the core to the edge, but the content of the whole porphyry is relatively stable (Figure 5a,b). This shows that the physical and chemical conditions during the formation of the potassium feldspar did not change much.

4.1.2. Plagioclase

The major elements in the plagioclase crystals of the Bure adakitic rock show a small range of compositions (Table 2). The SiO₂ content is relatively high, ranging from 62.57 to 67.75 wt% (average value of 65.18 wt%), with small variations of 6.71–10.02 wt% of Na₂O (average value of 8.88 wt%), 0.09–0.74 wt% of K₂O (average value of 0.16 wt%), and 2.52–4.28 wt% of CaO (average value of 3.51 wt%). In addition, the contents of FeO, MnO, and MgO in the plagioclase are below the detection limits. The Ab has high values of 74.63–85.99 (average value = 81.14), while the Or values are almost negligible (0.56–4.87, with an average of 1.00). The An values range from 12.31–24.15, with an average of 17.86. Thus, all the plagioclases of the Bure adakitic rock are macro-feldspar (Figure 4a). In the plagioclase porphyry of the Bure adakitic rock, the content of An and Al₂O₃ has a relatively coupled synchronous change trend (Figure 5c,d). The contents of An are higher in the core and mantle, with an increasing trend from the core to the mantle, and a decreasing trend from the mantle to the edge.

4.1.3. Biotite

The Fe²⁺ and Fe³⁺ in the biotite of the Bure adakitic rock were adjusted using the method proposed by [38], and the number of cations and related parameters of the biotite were calculated using 22 oxygen atoms as the unit. In the major element content of the Bure adakitic rock, there is 35.19–39.67 wt% of SiO₂, with an average value of 37.65 wt%. The biotite has relatively high contents of FeO (16.11–20.99 wt%; average value of 19.12 wt%), Al₂O₃ (13.96–19.13 wt%; average value of 15.77 wt%), and TiO₂ (2.01–3.84 wt%; average value of 2.90 wt%). In comparison, the MgO, K₂O, Na₂O, and CaO contents in biotite are relatively low, with values of 6.54–9.59 wt% of MgO (average value of 8.33 wt%), 7.07–9.85 wt% of K₂O (average value of 8.33 wt%), 0.01–0.16 wt% of Na₂O (average value of 0.08 wt%), and 0.08–0.33 wt% of CaO (average value of 0.16 wt%) (Table 3; Figure 4b).

The low Ca content of the biotite indicates that it was not, or only rarely, affected by chlorite and sericite alteration caused by primary metamorphism after the magmatic stage [39]. In addition, the Ti atomic numbers of the biotite in this study range from 0.13 to 0.22 (mean of 0.17), which is consistent with the fact that the Ti atomic number in the magmatic biotite is less than 0.55. The Fe²⁺/(Mg + Fe²⁺) ratio in the biotite presents a small variation (0.26–0.52, with an average value of 0.38), also suggesting that the biotite is of magmatic origin. The FeO, MgO, and K₂O contents from the core to the edge of the biotite fluctuate slightly, showing a gentle trend and indicating that there was no mixing of basic magmatic components during crystallization (Figure 5e,f). Generally, the substitution modes of Mg²⁺ and Al³⁺ are crucial in calc-alkaline and peraluminous magmatic systems. The obvious negative correlation of MgO and Al₂O₃ in the biotite implies that the displacement reaction of Mg²⁺ and Al³⁺ may have occurred during the crystallization process of the calc-alkaline and peraluminous magmatic system (Figure 6; [40]).

4.2. Trace Element Compositions of Zircon

The zircon trace elements and calculated oxygen fugacity parameters from the Bure adakitic rock are shown in Table 4, respectively. They are depleted in LREEs and enriched in HREEs, with significant positive Ce anomalies and weak negative Eu anomalies in the chondrite-normalized REE patterns (Figure 7), indicating that they are magmatic zircons [41]. The magmatic crystallization temperatures of the Bure adakitic rock calculated based on Ti-in-zircon thermometry [42] vary from 659 to 814 °C (mean of 705 °C). The corresponding logfO₂ values of the zircons from the Bure adakitic rock range from −11.5 to −5.2, with a median of −8.6 [43,44,45].

4.3. Zircon Lu-Hf Isotopes and Whole-Rock Sr-Nd Isotopes

Ten Lu-Hf isotopic analyses were conducted on the zircons of the Bure adakitic rock sample, yielding ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.282572~0.282734, and ε_Hf(t) values from 7.64 to 12.99 (average value = 11; Table 5). On the Age-ε_Hf(t) diagram, the corresponding two-stage Hf model ages vary from 802–1161 Ma (Figure 8a). The Sr-Nd isotopic results of the Bure adakitic rock are shown in Table 6. The ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.707381 to 0.70745 (average value = 0.70741) are higher than that of the current original mantle value (⁸⁷Sr/⁸⁶Sr = 0.7045; Table 6). Correspondingly, the calculated (⁸⁷Sr/⁸⁶Sr)i ratios vary from 0.70088 to 0.70275 (average value = 0.70184), and the ε_Nd(t) values have a relatively large variation of 3.26 to 7.28 (average value = 4.72; Figure 8b). Their two-stage Nd model ages range from 820 to 1210 Ma.

Table 5. Zircon Hf isotopic data for the Bure adakitic rock.

No.	¹⁷⁶Hf/¹⁷⁷Hf	1σ	¹⁷⁶Lu/¹⁷⁷Hf	1σ	¹⁷⁶Yb/¹⁷⁷Hf	1σ	Age (Ma) [7]	ε_Hf(t) [47]	TDM₂ (Ma)
BR0101Grt1-02	0.282734	0.000020	0.001766	0.000051	0.064510	0.001820	660	12.44	803
BR0101Grt1-05	0.282623	0.000013	0.000892	0.000018	0.029611	0.000544	753	10.91	974
BR0101Grt1-07	0.282680	0.000021	0.001743	0.000019	0.055813	0.000457	743	12.29	877
BR0101Grt1-12	0.282702	0.000017	0.002429	0.000022	0.070191	0.000389	743	12.73	849
BR0101Grt1-17	0.282599	0.000020	0.003626	0.000051	0.129672	0.002161	744	8.50	1122
BR0101Grt1-21	0.282689	0.000022	0.002049	0.000063	0.067411	0.002036	696	11.48	893
BR0101Grt1-26	0.282718	0.000021	0.003151	0.000071	0.109087	0.002564	746	12.99	834
BR0101GRT1-10	0.282631	0.000019	0.002470	0.000074	0.056882	0.001625	715	9.63	1027
BR0101GRT1-15	0.282645	0.000020	0.002279	0.000038	0.060088	0.001038	728	10.46	983
BR0101GRT1-22	0.282667	0.000017	0.000706	0.000009	0.017882	0.000247	729	12.02	883
BR0101GRT1-28	0.282657	0.000022	0.001282	0.000020	0.033778	0.000571	735	11.54	919
BR0101GRT1-31	0.282644	0.000021	0.001278	0.000002	0.034658	0.000082	713	10.61	962
BR0101GRT1-32	0.282620	0.000023	0.000766	0.000016	0.020989	0.000557	731	10.38	991
BR0101GRT1-35	0.282644	0.000020	0.001313	0.000004	0.033838	0.000049	753	11.44	940
BR0101GRT1-39	0.282572	0.000029	0.002650	0.000025	0.072997	0.000613	724	7.64	1161

Table 6. Sr–Nd isotopic data for the Bure adakitic rock.

Sample No.	⁸⁷Rb/⁸⁶Sr	⁸⁷Sr/⁸⁶Sr	±2σ	(⁸⁷Sr/⁸⁶Sr)_i	¹⁴⁷Sm/¹⁴⁴Nd	¹⁴³Nd/¹⁴⁴Nd	±2σ	ε_Nd(t)	TDM₂ (Ma)
BR0101Grt1	0.523	0.707381	0.000006	0.701898	0.122	0.512463	0.000009	3.62	1140
BR0101Grt2	0.623	0.707409	0.000006	0.700880	0.115	0.512618	0.000007	7.28	820
BR0101Grt3	0.449	0.707450	0.000010	0.702746	0.130	0.512485	0.000007	3.26	1210

Note: ε_Nd(t) = [(¹⁴³Nd/¹⁴⁴Nd)_sample(t)/(¹⁴³Nd/¹⁴⁴Nd)_CHUR(t) − 1] × 10⁻⁴; T_DM2 = 1/λ × {1 + [(¹⁴³Nd/¹⁴⁴Nd)_sample − ((¹⁴⁷Sm/¹⁴⁴Nd)_sample − (¹⁴⁷Sm/¹⁴⁴Nd)_crust) × (e^λt−1) − (¹⁴³Nd/¹⁴⁴Nd)_DM]/((¹⁴⁷Sm/¹⁴⁴Nd)_crust − (¹⁴⁷Sm/¹⁴⁴Nd)_DM)}. (¹⁴⁷Sm/¹⁴⁴Nd)_CHUR = 0.1967, and (¹⁴³Nd/¹⁴⁴Nd)_CHUR = 0.512638 [48]; (¹⁴⁷Sm/¹⁴⁴Nd)_DM = 0.2136, and (¹⁴³Nd/¹⁴⁴Nd)_DM = 0.51315 [49]; (¹⁴⁷Sm/¹⁴⁴Nd)_crust = 0.118 [50].

Figure 8. Diagrams of Hf (a) and Sr-Nd isotopes (b) for the Bure adakitic rock. Zircon Hf isotope-age data obtained from the Arabian Nubian Shield [51]; Mozambique Belt [52]; ranges for depleted mantle (DM), chondritic uniform reservoir (CHUR), and juvenile crust from Griffin et al. [53]. Sr-Nd isotopic data of the Depleted Mantle [54] and the Arabian Nubian Shield [10,28,55].

5. Discussion

5.1. Physicochemical Condition of Magma Crystallization

Zircon, a mineral that typically crystallizes early in acidic magma, usually at temperatures close to the magma formation temperature, serves as an indicator of initial crystallization in granitoids. Thus, the magmatic crystallization temperature of the Bure adakitic rock calculated based on Ti-in-zircon thermometry varies from 659 to 814 °C, with a mean of 705 °C. In conclusion, we propose that the crystallization temperature of the Bure adakitic rock was concentrated between 659 to 814 °C.

Emplacement pressure can be estimated from biotite compositions using the empirical formula of the biotite all-aluminum manometer in granitoids based on the hornblende manometer: p × 100 = 3.03 × T^Al − 6.53 (±0.33) [56]. The estimated pressures show a range from 1.75 × 10⁵ to 2.81 × 10⁵ Pa (mean 2.09 × 10⁵ Pa) for the Bure adakitic rock. The calculated emplacement depth of the Bure adakitic rock is 6.39~10.2 km (mean 7.60 km) according to the empirical formula p = ρgh (ρ =2800 kg/m³; g = 9.8 m/s²), which indicates that the magmatic emplacement depth was relatively deep.

Generally, the Fe³⁺, Fe²⁺, and Mg²⁺ values in biotite can be used to estimate the oxygen fugacity during crystallization. The electron probe data of the biotite in the Bure adakitic rock projected into the correlation diagram of biotite composition and oxygen buffer pairs show that all the data fall between the Ni-NiO and Fe₂O₃-Fe₃O₄ buffer lines and all are close to the Ni-NiO buffer lines, implying that the biotite in the Bure adakitic rock crystallized in a high oxygen fugacity environment (Figure 9). The presence of the variable valence elements of Ce and Eu in zircon makes it an ideal candidate for calculating the oxygen fugacity in coexisting magmas [42]. Unlike most rare earth elements, which exist in the +3 valence, the Ce element can exist in the form of Ce⁴⁺ in magmas. The similar radius of Ce⁴⁺ and Zr⁴⁺ leads to Ce⁴⁺ being more likely than Ce³⁺ to enter the zircon lattice due to isomorphism. Thus, Ballard et al. [43] proposed that the positive Ce anomaly of zircon can reflect the oxidation state in magma. Most of the points of the Bure adakitic rock are in the FMQ–HM range, and nearly half of the calculated zircon points reach the magmatic oxygen fugacity level of MH, suggesting a high oxygen fugacity of the magma (Figure 10a,b).

5.2. Magma Source and Genesis

The relationship between the MB and ANS, collectively referred to as the EAO by Stern [4], is not well understood. The inherited zircons of Mesoproterozoic age reported from the different granitic populations in the contrasting low- and high-grade terranes by Kebede et al. [8,9] indicate a contribution of pre-Neoproterozoic crustal material to the source magmas of these rocks. In eastern Ethiopia, Teklay et al. [60] suggested pre-Neoproterozoic crustal reworking based on Paleoproterozoic zircon inheritance and Mesoproterozoic to Archean crust residence ages for the granitoids. Kröner and Sassi [61] also reported a Mesoproterozoic to Paleoproterozoic crystalline basement intruded by Neoproterozoic granitoids in northern Somalia. Farther north in the ANS, studies [62,63,64] rule out the involvement of pre-Neoproterozoic crust. These studies seem to indicate the increasing importance of pre-Neoproterozoic crust southwards in the EAO, but detailed and systematic investigations are necessary to fully understand the issue.

As mentioned above, the biotite in the Bure adakitic rock enriched in iron and aluminum [7], together with the major elements plotted onto the MgO–FeO–Al₂O₃ and TFeO/(TFeO + MgO)–MgO diagrams, suggest that the rock is a calc-alkaline orogenic granite (Figure 11a), with a crustal magmatic source affinity (Figure 11b). The positive ε_Hf(t) values > 7 (ranging from 7.64 to 12.99) of the Bure adakitic rock fall above the Hf isotope evolution line of the chondrites, and completely fall into the ANS area [51], implying generation from a juvenile source. The Sr-Nd isotope results show that the Bure adakitic rock has low (⁸⁷Sr/⁸⁶Sr)i values of 0.70088–0.70275 and positive ε_Nd(t) values of 3.26 to 7.28, suggesting that the rock was sourced from a juvenile crust rather than lithospheric mantle material [54]. The (⁸⁷Sr/⁸⁶Sr)_i-ε_Nd(t) map shows that the Bure adakitic rock is consistent with the magmatic rocks in the ANS [10,28,55], which further indicates that the magma was derived from a juvenile crust. Although the Nd isotope depleted mantle model age of 820 Ma to 1210 Ma (average age = 1060 Ma) of the Bure adakitic rock is older than that of the crystallization age of 733.8 Ma [7], it is obviously younger than the Mesoproterozoic and Archaean ancient crust. This result further demonstrates that the Arab-Nubian Shield in the Neoproterozoic was characterized by a juvenile crust. The mean Nd model age for the WES is 1.03 Ga, which is between those calculated by Stern [22] based on existing Nd isotopic data from northern Ethiopia and Eritrea (mean value of 0.87 Ga; [22,55,65]) and the Southern Ethiopia Shield (1.13 Ga), respectively. This indicates that the transition between northern and southern Ethiopia lies in the Western Ethiopia Shield, reflecting a gradual transition between the northern ANS and the southern MB of the EAO. Additionally, the mean Nd model ages from the northern parts (Egypt, Sudan, Arabia Shield, and Eritrea and northern Ethiopia) to the central parts (western Ethiopia shield) and southern parts (southern Ethiopia shield, Kenya) of the EAO show an increasing trend, which indicates an increasing contribution of pre-Pan-African crust towards the southern part of the EAO (Figure 12).

5.3. Tectonic Environment

The plagioclase in the Bure adakitic rock shows no distinct zonal structure, indicating that the magma chamber was almost undisturbed, and the original molten slurry was in a balanced crystalline environment. In general, the crystallized minerals from the molten slurry easily reacted with the melt to form a uniform composition of minerals, leading to no zonal characteristics in the crystallized minerals. In the Lu/Hf-Y and Yb-Y diagrams of zircon, the trace elements of zircon from the Bure adakitic rock fall into the volcanic arc environment (VAB) and the area towards the within plate environment (WPB; Figure 13a,b). As mentioned above, the zircon U-Pb age of 750~710 Ma from the Bure adakitic rock [7] corresponds to the tectono–thermal event of approximately 780–700 Ma measured in previous studies of other locations in the ANS. This suggests a syn-tectonic environment [5,8,9,10,22]. In addition, the high SiO₂ (72.26–72.78 wt%), Al₂O₃ (14.91–15.82 wt%), Sr (310–401 ppm), Sr/Y (64.9–113.6), and La/Yb (25.7–51.6), low MgO (0.27–0.41 wt%), Y (2.71–4.78 ppm), and Yb (0.20–0.31 ppm), and Na₂O/K₂O values of 1.13–1.38 [7] of the Bure adakitic rock suggest that it was mainly formed by the partial melting of a thickened juvenile lower crust. Consequently, we propose that the Bure adakitic rock is the product of thickened juvenile crust melting triggered by the Pan-African Orogeny during the Neoproterozoic [68].

6. Conclusions

The petrological, mineralogical, and geochemical features of the Bure adakitic rock lead to the following conclusions:

(1): The crystallization temperature of the Bure adakitic rock ranges from 659 to 814 °C, and its calculated emplacement depth was 6.39~10.2 km (average of 7.60 km). The Fe³⁺, Fe²⁺, and Mg²⁺ values of biotite, and the positive Ce anomaly and calculated magmatic oxygen fugacity values of zircon reveal a high oxygen fugacity of the magma.
(2): The major elements of biotite and the Sr-Nd-Hf isotopes indicate that the Bure adakitic rock was derived from juvenile crustal materials. Additionally, the mean Nd model ages progressively increase from the northern to the central and southern parts of the EAO, which indicates an increasing contribution of the pre-Pan-African crust towards the southern part of the EAO.
(3): The Bure adakitic rock is the product of thickened juvenile crust melting triggered by the Pan-African Orogeny during the Neoproterozoic.

Author Contributions

J.J., W.X. and P.H. conceived this contribution and conducted all field and analytical work, assisted by Y.L., F.W., G.Z., X.G., Z.Z. and Y.B. The manuscript was written by J.J., W.X. and P.H., with contribution from Y.L., F.W., G.Z., X.G., Z.Z. and Y.B. All authors have read and agreed to the published version of the manuscript.

Funding

J.S.J. acknowledges support from the Natural Science Foundation of Hubei Province of China (2022CFB850), National Natural Science Foundation of China (42202092), Open Fund of the Research Center for Petrogenesis and Mineralization of Granitoid Rocks, China Geological Survey (No. PMGR202018), and the China Geological Survey (DD20221802, DD20230575).

Data Availability Statement

Data are contained within the article.

Acknowledgments

Staff at Wuhan Sample Solution Analytical Technology Co., Ltd., Zhongnan Mineral Resources Supervision and Test Center for Geoanalysis, Wuhan Center, China Geological Survey are gratefully acknowledged for assistance with instrument operation. We thank the reviewers for the journal Minerals.

Conflicts of Interest

Zicheng Zhang is employee of China National Geological & Mining Corporation. The paper reflects the views of the scientists and not the company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Geological map of the Arabian-Nubian Shield, northeast Africa (after [14]).

Figure 2. Sketch of the regional geology of the western Ethiopian terrain (after [15]).

Figure 3. Hand specimen photograph (a) and microphotograph (b) for the Bure adakitic rock. Qtz—quartz; Bt—biotite; Kfs—K-feldspar.

Figure 4. Ternary classification diagram for feldspar (a), [36]); Mg–(Al^Ⅵ + Fe³⁺ + Ti)–(Fe²⁺ + Mn) classification diagram for biotite (b), [37]).

Figure 5. Electron microprobe line profile analysis of K-feldspar (a,b), plagioclase (c,d) and biotite (e,f) for the Bure adakitic rock.

Figure 6. Diagram of the chemical variation of Al₂O₃ vs. MgO in the biotite.

Figure 7. Chondrite-normalized REE patterns (a) and Ce/Ce* vs. Sm_N/La_N (b); [41]). The Chondrite data for the normalization and plotting are from [46].

Figure 9. Correlative diagram between biotite composition and oxygen buffer-reagents [57].

Figure 10. (Ce/Ce*)_D of the zircons vs. 10,000/T (a); [58]) and logfO₂ vs. T (b); [59]) diagrams for the Bure adakitic rock.

Figure 11. MgO–FeO–Al₂O₃ discrimination diagram of the tectonic setting (a); [66]) and TFeO/(TFeO + MgO) vs. MgO diagram (b); [67]) of biotite. A: anorogenic alkaline suites; C: calc-alkaline orogenic suites; P: peraluminous suites; C: crustal source; M: mixing source between crust and mantle; M: mantle source.

Figure 12. The mean Nd-model ages of the EAO in Africa [22]. Eg—Egypt; Su—Sudan; As—Arabian Shield; En—Eritrea and northern Ethiopia; SES—Southern Ethiopia Shield; K—Kenya.

Figure 13. Lu/Hf vs. Y (a) and Yb vs. Y (b) diagrams of zircons [69] for the Bure adakitic rock. N-MORB: normal mid-ocean ridge basalt; VAB: volcanic arc basalt; WPB: within-plate basalt.

Table 1. Electron microprobe composition of K-felspar (wt%) for the Bure adakitic rock.

Ele.	Grt8-3 -fs01	Grt8-3 -fs02	Grt8-3 -fs04	Grt8-5 -fs01	Grt8-5 -fs02	Grt8-5 -fs04	Grt8-5 -fs05	Grt8-5 -fs06	Grt8-5 -fs07	Grt8-5 -fs08	Grt8-5 -fs09	Grt3-1 -fs01	Grt3-1 -fs02	Grt3-1 -fs03	Grt3-1 -fs04	Grt3-1 -fs05	Grt3-1 -fs06	Grt3-5 -fs07	Grt3-5 -fs08
CaO		0.042	0.022	0.016	0.041		0.042	0.035	0.012	0.011		0.01	0.02	0.03	0.03		0.03	0.03	0.01
Na₂O	0.648	0.629	0.858	0.555	0.462	0.658	0.586	0.537	0.607	0.387	0.507	0.70	0.74	0.68	0.60	0.79	0.84	0.66	0.75
K₂O	16.085	15.769	15.465	16.152	16.66	16.136	15.93	16.026	14.227	15.874	15.892	16.28	16.07	15.74	16.34	15.91	15.61	16.10	15.02
SrO
TFeO
MgO
SiO₂	64.565	63.241	64.184	65.094	63.424	63.986	63.244	63.48	67.125	63.714	66.579	64.10	64.08	63.72	63.11	64.49	65.81	65.34	64.98
MnO
Al₂O₃	19.002	18.788	18.486	19.02	19.106	19.144	18.528	19.086	19.045	19.023	17.944	18.65	17.95	18.16	18.24	18.43	18.65	18.83	18.55
BaO
Total	100.30	98.47	99.02	100.84	99.69	99.92	98.33	99.16	101.02	99.01	100.92	99.74	98.87	98.34	98.32	99.61	100.94	100.95	99.30
Number of cation on basis of 8 oxygens
Si	5.950	5.938	5.979	5.963	5.909	5.926	5.952	5.923	6.047	5.942	6.070	5.954	6.000	5.987	5.957	5.983	6.003	5.977	6.008
Al	1.548	1.559	1.522	1.540	1.573	1.567	1.541	1.574	1.517	1.568	1.446	1.531	1.486	1.509	1.522	1.511	1.504	1.523	1.516
Mg	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Fe	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Mn	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Ca	0.000	0.002	0.001	0.001	0.002	0.000	0.002	0.002	0.001	0.001	0.000	0.000	0.001	0.002	0.001	0.000	0.001	0.001	0.000
Na	0.029	0.029	0.039	0.025	0.021	0.030	0.027	0.024	0.027	0.017	0.022	0.031	0.034	0.031	0.028	0.035	0.037	0.029	0.034
K	0.473	0.472	0.459	0.472	0.495	0.477	0.478	0.477	0.409	0.472	0.462	0.482	0.480	0.472	0.492	0.471	0.454	0.470	0.443
Sr	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Ba	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
An	0.0	0.2	0.1	0.1	0.2	0.0	0.2	0.2	0.1	0.1	0.0	0.0	0.1	0.2	0.1	0.0	0.1	0.1	0.0
Ab	5.8	5.7	7.8	5.0	4.0	5.8	5.3	4.8	6.1	3.6	4.6	6.1	6.5	6.2	5.3	7.0	7.5	5.8	7.0
Or	94.2	94.1	92.1	95.0	95.8	94.2	94.5	95.0	93.8	96.4	95.4	93.9	93.4	93.7	94.6	93.0	92.3	94.0	92.9
Ele.	Grt3-4 -fs01	Grt3-4 -fs02	Grt3-4 -fs03	Grt3-4 -fs04	Grt3-4 -fs05	Grt3-4 -fs06	Grt3-4 -fs07	Grt3-3 -fs05	Grt3-3 -fs06	Grt3-3 -fs07	Grt3-3 -fs08	Grt3-3 -fs09	Grt3-5 -fs01	Grt3-5 -fs02	Grt3-5 -fs03	Grt3-5 -fs04	Grt3-5 -fs05	Grt3-5 -fs06	Grt3-5 -fs07	Grt3-5 -fs08
CaO	0.01	0.03	0.01	0.01	0.02	0.02		0.02	0.02	0.03	0.01	0.03	0.07	0.03	0.01	0.03	0.03	0.07	0.03	0.01
Na₂O	0.55	0.51	0.58	0.57	0.61	0.60	0.55	0.52	0.61	0.68	0.51	0.46	1.20	0.70	0.56	0.62	0.84	0.70	0.66	0.75
K₂O	16.19	15.61	16.46	16.29	16.16	16.55	16.19	15.96	16.44	15.78	16.43	16.35	15.37	15.88	16.02	15.91	15.44	15.75	16.10	15.02
SrO
TFeO
MgO
SiO₂	64.79	62.78	64.62	65.35	66.01	65.06	65.05	65.06	65.78	64.51	64.68	64.55	62.67	64.95	64.78	65.80	63.77	62.82	65.34	64.98
MnO
Al₂O₃	18.80	18.78	18.75	18.91	18.53	18.53	19.25	18.81	18.81	18.31	18.43	18.55	18.99	19.05	19.08	19.07	18.33	18.93	18.83	18.55
BaO
Total	100.34	97.71	100.40	101.13	101.33	100.76	101.04	100.36	101.67	99.31	100.06	99.93	98.30	100.61	100.44	101.43	98.40	98.27	100.95	99.30
Number of cation on basis of 8 oxygens
Si	5.969	5.935	5.961	5.972	6.011	5.981	5.948	5.980	5.983	5.996	5.985	5.977	5.900	5.958	5.955	5.979	5.980	5.916	5.977	6.008
Al	1.531	1.570	1.529	1.527	1.492	1.506	1.556	1.528	1.513	1.504	1.507	1.518	1.580	1.545	1.550	1.531	1.519	1.575	1.523	1.516
Mg	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Fe	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Mn	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Ca	0.000	0.002	0.000	0.001	0.001	0.001	0.000	0.001	0.001	0.001	0.001	0.001	0.003	0.001	0.000	0.001	0.001	0.004	0.001	0.000
Na	0.025	0.024	0.026	0.025	0.027	0.027	0.024	0.023	0.027	0.031	0.023	0.020	0.055	0.031	0.025	0.027	0.038	0.032	0.029	0.034
K	0.476	0.470	0.484	0.475	0.469	0.485	0.472	0.468	0.477	0.468	0.485	0.483	0.461	0.465	0.470	0.461	0.462	0.473	0.470	0.443
Sr	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Ba	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
An	0.0	0.2	0.0	0.1	0.1	0.1	0.0	0.1	0.1	0.1	0.1	0.1	0.3	0.1	0.0	0.1	0.1	0.3	0.1	0.0
Ab	4.9	4.8	5.0	5.0	5.4	5.2	4.9	4.7	5.3	6.2	4.5	4.1	10.6	6.3	5.0	5.6	7.6	6.3	5.8	7.0
Or	95.0	95.1	94.9	94.9	94.5	94.7	95.1	95.2	94.6	93.7	95.4	95.8	89.1	93.6	95.0	94.3	92.3	93.3	94.0	92.9

Note: Blank space is below the detection limit.

Table 2. Electron microprobe composition of plagioclase (wt%) for the Bure adakitic rock.

	Grt8-1 -fs02	Grt8-1 -fs03	Grt8-1 -fs04	Grt8-1 -fs05	Grt8-1 -fs06	Grt8-1 -fs07	Grt8-1 -fs08	Grt8-1 -fs09	Grt8-2 -fs01	Grt8-2 -fs02	Grt8-2 -fs03	Grt8-2 -fs04	Grt8-2 -fs06	Grt3-2 -fs01	Grt3-2 -fs02	Grt3-2 -fs03	Grt3-2 -fs04	Grt3-2 -fs05	Grt3-2 -fs06	Grt3-2 -fs07	Grt3-2 -fs08
CaO	3.13	3.07	3.28	3.39	3.65	2.68	2.99	2.92	3.65	3.65	3.61	3.26	3.62	2.52	3.54	3.93	4.00	3.75	3.94	3.42	2.78
Na₂O	9.68	9.50	10.02	9.80	7.11	9.69	9.49	9.55	9.52	9.44	9.66	8.40	8.60	9.69	9.72	6.71	8.37	9.63	7.98	8.56	7.96
K₂O	0.12	0.18	0.19	0.10	0.16	0.15	0.17	0.14	0.16	0.13	0.14	0.13	0.09	0.33	0.13	0.17	0.15	0.17	0.18	0.13	0.74
SrO
TFeO
MgO
SiO₂	64.99	64.61	65.70	64.23	65.06	65.05	65.12	64.05	64.36	64.72	64.10	65.01	65.56	64.62	65.70	66.41	66.00	65.07	66.07	65.81	64.70
MnO
Al₂O₃	22.68	22.89	23.32	23.55	24.11	22.59	23.24	22.46	23.25	23.66	22.67	23.58	24.13	21.95	23.07	23.74	23.50	23.11	22.97	23.41	23.11
BaO
Total	100.59	100.25	102.52	101.07	100.10	100.16	101.00	99.13	100.94	101.62	100.17	100.38	102.01	99.10	102.15	100.95	102.01	101.72	101.13	101.33	99.29
Number of cation on basis of 8 oxygens
Si	5.686	5.671	5.652	5.607	5.668	5.706	5.668	5.684	5.625	5.615	5.648	5.671	5.638	5.733	5.668	5.725	5.676	5.645	5.720	5.690	5.706
Al	1.754	1.776	1.774	1.817	1.857	1.752	1.788	1.762	1.796	1.815	1.765	1.818	1.834	1.722	1.759	1.809	1.787	1.772	1.758	1.789	1.802
Mg	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Fe	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Mn	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Ca	0.147	0.144	0.151	0.158	0.170	0.126	0.139	0.139	0.171	0.170	0.170	0.152	0.167	0.120	0.163	0.181	0.184	0.174	0.182	0.159	0.132
Na	0.410	0.404	0.418	0.415	0.300	0.412	0.400	0.411	0.403	0.397	0.412	0.355	0.358	0.417	0.406	0.280	0.349	0.405	0.335	0.359	0.340
K	0.003	0.005	0.005	0.003	0.005	0.004	0.005	0.004	0.004	0.004	0.004	0.004	0.003	0.009	0.003	0.005	0.004	0.005	0.005	0.003	0.021
Sr	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Ba	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
An	15.0	15.0	15.2	15.9	21.9	13.1	14.7	14.3	17.3	17.5	17.0	17.5	18.8	12.3	16.6	24.1	20.7	17.5	21.2	18.0	15.4
Ab	84.2	84.0	83.8	83.5	77.0	86.0	84.3	84.8	81.8	81.8	82.3	81.6	80.6	85.8	82.7	74.6	78.4	81.5	77.7	81.3	79.7
Or	0.7	1.0	1.0	0.6	1.2	0.9	1.0	0.8	0.9	0.8	0.8	0.9	0.6	1.9	0.7	1.2	0.9	0.9	1.1	0.8	4.9
	Grt3-3 -fs01	Grt3-3 -fs02	Grt3-3 -fs03	Grt3-3 -fs04	Grt3-6 -fs01	Grt3-6 -fs02	Grt3-6 -fs03	Grt3-6 -fs04	Grt3-6 -fs06	Grt3-8 -fs01	Grt3-8 -fs02	Grt3-8 -fs03		Grt3-8 -fs04		Grt3-8 -fs05		Grt3-8 -fs06		Grt3-8 -fs07
CaO	3.58	3.54	3.33	3.17	3.69	3.52	3.60	3.62	3.59	4.00	3.79	3.88		4.28		3.97		3.87		3.91
Na₂O	8.47	7.73	8.91	9.19	8.83	9.85	7.93	8.26	8.90	8.16	8.89	7.54		9.31		8.90		8.83		9.52
K₂O	0.15	0.12	0.12	0.12	0.18	0.10	0.14	0.16	0.13	0.18	0.16	0.18		0.15		0.12		0.16		0.20
SrO
TFeO
MgO
SiO₂	66.55	66.89	67.75	66.29	65.18	64.62	65.97	65.88	66.04	65.32	64.39	65.27		63.68		63.52		63.94		63.39
MnO
Al₂O₃	23.09	23.42	22.32	22.36	22.92	23.03	23.15	23.14	23.20	23.31	23.22	23.57		23.63		23.21		23.13		23.37
BaO
Total	101.84	101.70	102.42	101.14	100.79	101.12	100.78	101.07	101.84	100.97	100.46	100.44		101.05		99.72		99.92		100.39
Number of cation on basis of 8 oxygens
Si	5.723	5.737	5.789	5.749	5.683	5.639	5.721	5.708	5.692	5.675	5.642	5.682		5.573		5.614		5.634		5.584
Al	1.755	1.776	1.686	1.714	1.766	1.777	1.774	1.772	1.768	1.790	1.798	1.814		1.828		1.814		1.802		1.820
Mg	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000		0.000		0.000		0.000		0.000
Fe	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000		0.000		0.000		0.000		0.000
Mn	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000		0.000		0.000		0.000		0.000
Ca	0.165	0.162	0.152	0.147	0.172	0.164	0.167	0.168	0.166	0.186	0.178	0.181		0.200		0.188		0.182		0.185
Na	0.353	0.321	0.369	0.386	0.373	0.417	0.333	0.347	0.372	0.344	0.378	0.318		0.395		0.381		0.377		0.406
K	0.004	0.003	0.003	0.003	0.005	0.003	0.004	0.005	0.003	0.005	0.004	0.005		0.004		0.003		0.004		0.005
Sr	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000		0.000		0.000		0.000		0.000
Ba	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000		0.000		0.000		0.000		0.000
An	18.8	20.0	17.0	15.9	18.6	16.4	19.9	19.3	18.1	21.1	18.9	21.9		20.1		19.7		19.3		18.3
Ab	80.3	79.2	82.3	83.4	80.4	83.1	79.2	79.7	81.2	77.8	80.2	77.0		79.1		79.7		79.8		80.6
Or	0.9	0.8	0.8	0.7	1.1	0.6	0.9	1.0	0.8	1.1	0.9	1.2		0.8		0.7		0.9		1.1

Note: Blank space is below the detection limit.

Table 3. Electron microprobe composition of biotite (wt%) for the Bure adakitic rock.

	Grt3-1 -ms01	Grt3-1 -ms02	Grt3-1 -ms03	Grt3-1 -ms04	Grt3-1 -ms05	Grt3-1 -ms06	Grt3-1 -ms07	Grt3-3 -ms01	Grt3-3 -ms02	Grt3-3 -ms04	Grt3-3 -ms06	Grt3-3 -ms07	Grt3-3 -ms08	Grt3-3 -ms09	Grt3-3 -ms10	Grt3-3 -ms11	Grt3-3 -ms12	Grt3-3 -ms13	Grt3-6 -ms01	Grt3-6 -ms02	Grt3-6 -ms03	Grt3-6 -ms04
SiO₂	37.512	36.417	37.188	36.342	37.424	37.667	36.166	36.344	38.646	37.178	37.884	37.184	38.548	35.189	37.295	36.984	35.891	37.385	38.104	38.663	37.11	37.509
TiO₂	2.547	2.994	2.657	2.731	2.663	2.574	2.866	2.399	2.314	2.645	3.06	3.024	2.596	2.058	2.673	2.73	2.919	2.736	3.586	3.817	3.479	3.536
Al₂O₃	15.299	15.548	15.843	15.789	13.959	15.272	15.474	15.725	15.99	15.1	15.565	15.06	16.669	19.128	16.605	15.403	14.672	15.376	16.037	15.856	15.883	16.038
FeO	20.427	19.903	19.169	20.682	20.507	19.56	20.307	20.457	18.94	18.25	19.449	20.196	18.327	16.111	19.864	20.587	19.835	20.987	19.921	18.693	20.427	19.627
MnO	0.226	0.218	0.214	0.213	0.28	0.222	0.252	0.224	0.233	0.19	0.251	0.267	0.218	0.181	0.27	0.249	0.283	0.262	0.252	0.211	0.198	0.162
MgO	8.765	8.47	8.752	9.126	8.945	8.639	8.734	9.49	9.04	9.43	8.488	8.277	8.045	6.535	8.341	9.045	8.568	8.675	8.546	8.262	8.993	8.829
CaO	0.118	0.18	0.182	0.143	0.139	0.265	0.1	0.193	0.31	0.332	0.129	0.316	0.273	0.18	0.082	0.142	0.29	0.202	0.276	0.081	0.135	0.081
Na₂O	0.097	0.054	0.085	0.096	0.089	0.065	0.089	0.065	0.078	0.089	0.095	0.081	0.108	0.033	0.076	0.11	0.159	0.121	0.104	0.073	0.078	0.078
K₂O	9.25	9.239	8.879	9.583	9.627	8.713	9.516	9.699	9.025	8.992	9.404	9.195	8.297	7.696	9.059	9.231	8.835	9.448	9.141	9.302	9.852	9.425
Total	94.241	93.023	92.969	94.705	93.633	92.977	93.504	94.596	94.576	92.206	94.325	93.6	93.081	87.111	94.265	94.481	91.452	95.192	95.967	94.958	96.155	95.285
Number of cation on basis of 22 oxygens
Si	2.833	3.458	2.898	2.853	2.889	2.812	2.927	2.927	2.832	2.815	2.937	2.907	2.909	2.895	2.952	2.851	2.865	2.858	2.865	2.873	2.875	2.927
Al^Ⅳ	1.167	0.542	1.102	1.147	1.111	1.188	1.073	1.073	1.168	1.185	1.063	1.093	1.091	1.105	1.048	1.149	1.135	1.142	1.135	1.127	1.125	1.073
Al^Ⅵ	0.028	1.648	0.292	0.288	0.340	0.252	0.214	0.326	0.260	0.251	0.370	0.298	0.318	0.277	0.456	0.677	0.369	0.260	0.245	0.266	0.301	0.341
Ti	0.129	0.000	0.148	0.176	0.155	0.159	0.157	0.151	0.169	0.140	0.132	0.156	0.177	0.177	0.150	0.125	0.154	0.159	0.175	0.158	0.204	0.217
Fe³⁺	0.129	0.124	0.280	0.281	0.323	0.211	0.240	0.341	0.233	0.189	0.343	0.303	0.322	0.300	0.440	0.489	0.322	0.254	0.268	0.255	0.344	0.397
Fe²⁺	0.638	0.000	1.040	1.023	0.922	1.127	1.101	0.931	1.097	1.136	0.861	0.890	0.927	1.015	0.734	0.603	0.954	1.076	1.056	1.093	0.913	0.787
Mn	0.003	0.000	0.015	0.014	0.014	0.014	0.019	0.015	0.017	0.015	0.015	0.013	0.016	0.018	0.014	0.012	0.018	0.016	0.019	0.017	0.016	0.014
Mg	1.841	0.211	1.010	0.989	1.014	1.053	1.043	1.001	1.020	1.096	1.024	1.099	0.972	0.961	0.918	0.789	0.955	1.042	1.020	0.994	0.961	0.932
Ca	0.025	0.000	0.010	0.015	0.015	0.012	0.012	0.022	0.008	0.016	0.025	0.028	0.011	0.026	0.022	0.016	0.007	0.012	0.025	0.017	0.022	0.007
Na	0.032	0.039	0.015	0.008	0.013	0.014	0.013	0.010	0.014	0.010	0.011	0.013	0.014	0.012	0.016	0.005	0.011	0.016	0.025	0.018	0.015	0.011
K	0.913	0.888	0.912	0.923	0.880	0.946	0.961	0.864	0.951	0.959	0.875	0.897	0.921	0.913	0.810	0.795	0.888	0.910	0.900	0.926	0.880	0.898
	Grt8-1 -ms01	Grt8-1 -ms02	Grt8-1 -ms03	Grt8-1 -ms04	Grt8-1 -ms05	Grt8-1 -ms06	Grt8-1 -ms07	Grt8-1 -ms08	Grt8-3 -ms01	Grt8-3 -ms02	Grt8-3 -ms03	Grt8-3 -ms04	Grt8-3 -ms05	Grt8-3 -ms06	Grt8-5 -ms01	Grt8-5 -ms02	Grt8-5 -ms03	Grt8-5 -ms04	Grt8-5 -ms05	Grt8-5 -ms06	Grt8-5 -ms07	Grt8-5 -ms08
SiO₂	46.568	47.913	45.736	46.328	45.586	46.436	47.622	47.476	36.007	37.312	38.717	39.669	38.79	39.312	38.683	37.77	37.651	39.147	38.832	38.474	38.011	38.31
TiO₂	1.106	1.021	1.089	0.988	1.444	0.712	0.435	0.41	2.567	2.603	2.652	2.43	2.555	2.708	2.49	3.608	2.887	3.377	3.615	2.806	3.821	3.836
Al₂O₃	29.682	29.113	29.725	29.342	29.037	29.447	30.081	30.942	14.599	15.062	15.274	16.682	15.338	16.827	17.12	15.606	15.604	15.778	15.419	16	16.315	15.844
FeO	4.451	5.048	4.95	5.009	4.875	5.623	5.254	5.515	18.37	18.689	18.977	17.017	17.374	17.353	18.587	19.489	20.461	17.878	18.553	17.535	17.189	18.617
MnO	0.046	0.016	0.072	0.036	0.038	0.048	0.019	0.064	0.299	0.314	0.234	0.228	0.305	0.223	0.301	0.306	0.283	0.266	0.287	0.236	0.2	0.217
MgO	1.586	1.746	1.411	1.418	1.519	1.659	1.582	1.662	8.473	8.096	8.276	7.325	7.537	7.104	7.598	8.116	8.108	8.382	8.021	7.928	7.576	7.522
CaO	0.005	0.008	0.023	0.019				0.005	0.128	0.13	0.081	0.105	0.076	0.133	0.118	0.086	0.168	0.096	0.123	0.102	0.134	0.141
Na₂O	0.189	0.13	0.173	0.171	0.168	0.145	0.083	0.121	0.088	0.082	0.079	0.061	0.054	0.042	0.056	0.04	0.07	0.082	0.072	0.053	0.014	0.139
K₂O	11.513	11.665	11.654	11.521	11.346	11.47	11.491	11.355	8.591	9.037	8.939	8.219	8.931	7.072	8.933	9.389	9.161	9.474	8.885	8.665	8.48	9.057
Total	95.146	96.66	94.833	94.832	94.013	95.54	96.567	97.55	89.122	91.325	93.229	91.736	90.96	90.774	93.886	94.41	94.393	94.48	93.807	91.799	91.74	93.683
Number of cation on basis of 22 oxygens
Si	3.183	3.229	3.152	3.187	3.165	3.179	3.209	3.169	2.920	2.950	2.985	3.046	3.039	3.036	2.947	2.899	2.901	2.966	2.967	2.984	2.944	2.938
Al^Ⅳ	0.817	0.771	0.848	0.813	0.835	0.821	0.791	0.831	1.080	1.050	1.015	0.954	0.961	0.964	1.053	1.101	1.099	1.034	1.033	1.016	1.056	1.062
Al^Ⅵ	1.574	1.541	1.567	1.567	1.540	1.555	1.597	1.604	0.316	0.354	0.372	0.556	0.456	0.568	0.484	0.310	0.318	0.375	0.356	0.447	0.433	0.370
Ti	0.057	0.052	0.056	0.051	0.075	0.037	0.022	0.021	0.157	0.155	0.154	0.140	0.151	0.157	0.143	0.208	0.167	0.193	0.208	0.164	0.223	0.221
Fe3+	0.254	0.284	0.285	0.288	0.283	0.322	0.296	0.308	0.323	0.345	0.387	0.534	0.448	0.608	0.420	0.350	0.321	0.399	0.431	0.446	0.491	0.422
Fe2+	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.923	0.891	0.837	0.558	0.691	0.513	0.764	0.901	0.997	0.733	0.755	0.691	0.622	0.772
Mn	0.003	0.001	0.004	0.002	0.002	0.003	0.001	0.004	0.021	0.021	0.015	0.015	0.020	0.015	0.019	0.020	0.018	0.017	0.019	0.016	0.013	0.014
Mg	0.162	0.175	0.145	0.145	0.157	0.169	0.159	0.165	1.024	0.954	0.951	0.839	0.880	0.818	0.863	0.929	0.931	0.947	0.914	0.917	0.875	0.860
Ca	0.000	0.001	0.002	0.001	0.000	0.000	0.000	0.000	0.011	0.011	0.007	0.009	0.006	0.011	0.010	0.007	0.014	0.008	0.010	0.008	0.011	0.012
Na	0.025	0.017	0.023	0.023	0.023	0.019	0.011	0.016	0.014	0.013	0.012	0.009	0.008	0.006	0.008	0.006	0.010	0.012	0.011	0.008	0.002	0.021
K	1.004	1.003	1.025	1.011	1.005	1.002	0.988	0.967	0.889	0.912	0.879	0.805	0.893	0.697	0.868	0.919	0.900	0.916	0.866	0.857	0.838	0.886

Note: Blank space is below the detection limit.

Table 4. Trace element compositions of zircon (ppm) for the Bure adakitic rock.

No.	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y	Ti	t/℃ [42]	(Ce/Ce*)_D [43]	logfO₂ [45]	△FMQ [45]
BR0101Grt1-05	0.01	10.39	0.04	0.87	2.66	0.24	15.51	6.08	77.30	32.03	150.74	32.78	307.81	63.94	957.16	4.49	675.1	383.1	−9.6	7.9
BR0101Grt1-06	0.22	49.94	0.21	2.13	5.36	1.27	32.86	11.94	146.11	55.28	240.64	46.74	413.50	84.28	1513.41	3.63	659.0	560.7	−9.1	8.9
BR0101Grt1-07	0.13	27.75	0.29	3.99	7.62	3.49	40.70	13.41	163.41	64.89	299.34	64.48	615.10	132.45	1890.88	21.77	814.2	190.1	−5.2	9.0
BR0101Grt1-15	0.63	88.37	0.94	10.03	13.87	4.47	62.97	20.56	239.48	89.35	382.39	74.31	658.28	135.13	2380.80	8.13	723.0	177.5	−9.8	6.4
BR0101Grt1-17	0.26	161.18	0.39	4.73	9.97	2.61	71.27	26.37	336.42	132.64	588.23	115.67	1005.19	199.90	3637.32	3.06	646.3	971.1	−7.8	10.5
BR0101Grt1-26	0.41	184.47	0.69	6.19	11.69	3.74	66.89	25.61	325.23	134.75	609.07	121.47	1079.09	217.81	3838.12	5.38	689.1	874.7	−5.7	11.5
BR0101Grt1-32	0.20	17.83	0.27	2.20	3.99	0.80	20.21	7.06	86.07	34.79	161.39	33.70	315.24	68.03	1017.32	8.03	721.8	224.5	−9.0	7.3
BR0101Grt1-35	0.00	7.49	0.09	1.79	3.76	0.53	25.91	8.91	110.30	42.79	192.76	39.88	359.63	74.73	1224.30	7.90	720.5	116.6	−11.5	4.8
BR0101Grt1-39	0.80	93.02	0.58	6.05	13.51	3.77	77.29	26.39	298.01	108.37	448.54	84.63	703.25	137.54	2797.50	5.60	692.3	284.1	−9.7	7.3

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Jiang, J.; Xiang, W.; Hu, P.; Li, Y.; Wu, F.; Zeng, G.; Guo, X.; Zhang, Z.; Bai, Y. Petrogenesis of the Newly Discovered Neoproterozoic Adakitic Rock in Bure Area, Western Ethiopia Shield: Implication for the Pan-African Tectonic Evolution. Minerals 2024, 14, 408. https://doi.org/10.3390/min14040408

AMA Style

Jiang J, Xiang W, Hu P, Li Y, Wu F, Zeng G, Guo X, Zhang Z, Bai Y. Petrogenesis of the Newly Discovered Neoproterozoic Adakitic Rock in Bure Area, Western Ethiopia Shield: Implication for the Pan-African Tectonic Evolution. Minerals. 2024; 14(4):408. https://doi.org/10.3390/min14040408

Chicago/Turabian Style

Jiang, Junsheng, Wenshuai Xiang, Peng Hu, Yulin Li, Fafu Wu, Guoping Zeng, Xinran Guo, Zicheng Zhang, and Yang Bai. 2024. "Petrogenesis of the Newly Discovered Neoproterozoic Adakitic Rock in Bure Area, Western Ethiopia Shield: Implication for the Pan-African Tectonic Evolution" Minerals 14, no. 4: 408. https://doi.org/10.3390/min14040408

APA Style

Jiang, J., Xiang, W., Hu, P., Li, Y., Wu, F., Zeng, G., Guo, X., Zhang, Z., & Bai, Y. (2024). Petrogenesis of the Newly Discovered Neoproterozoic Adakitic Rock in Bure Area, Western Ethiopia Shield: Implication for the Pan-African Tectonic Evolution. Minerals, 14(4), 408. https://doi.org/10.3390/min14040408

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Petrogenesis of the Newly Discovered Neoproterozoic Adakitic Rock in Bure Area, Western Ethiopia Shield: Implication for the Pan-African Tectonic Evolution

Abstract

1. Introduction

2. Geological Setting

3. Samples and Analytical Methods

3.1. Petrography

3.2. Analytical Methods

4. Results

4.1. Mineral Compositions

4.1.1. K-Feldspar

4.1.2. Plagioclase

4.1.3. Biotite

4.2. Trace Element Compositions of Zircon

4.3. Zircon Lu-Hf Isotopes and Whole-Rock Sr-Nd Isotopes

5. Discussion

5.1. Physicochemical Condition of Magma Crystallization

5.2. Magma Source and Genesis

5.3. Tectonic Environment

6. Conclusions

Author Contributions

Funding

Data Availability Statement

Acknowledgments

Conflicts of Interest

References

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