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

: 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 (log f O 2 ) values vary from − 18.5 to − 4.9, revealing a relatively high magma oxygen fugacity. The uniform contents of FeO, MgO, and K 2 O in the biotite suggest a crustal magma source for the Bure adakitic rock. The relatively low ( 87 Sr/ 86 Sr) 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 Al 2 O 3 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.


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 (2.71~4.78ppm), 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.

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 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.
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 ultramafic/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.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.

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).

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 Kfeldspar 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).

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 0.1-0.2g 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

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 0.1-0.2g of whole rock powder of each sample was dissolved in digestion bombs with a mixture of double distilled HNO 3 , HF, and HClO 4 .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/ 144 Nd and 87 Sr/ 86 Sr ratios were normalized to 143 Nd/ 144 Nd = 0.7219 and 87 Sr/ 86 Sr = 8.375209, respectively.Measurements of the La Jolla and SRM NBS987 standards during this course gave average 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios of 0.511847 ± 3 (2σ, n = 25) and 0.710254 ± 8 (2σ, n = 22), respectively. 147Sm/ 144 Nd and 87 Rb/ 86 Sr 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 179 Hf/ 177 Hf = 0.7325 and 173 Yb/ 171 Yb = 1.132685 were used to calculate the mass bias of Hf (βHf) and Yb (βYb), respectively [32].The interference of 176 Yb on 176 Hf was corrected by measuring the interference-free 173 Yb isotope and using 176 Yb/ 173 Yb = 0.79639 to calculate 176 Yb/ 177 Hf [31].Similarly, the relatively minor interference of 176 Lu on 176 Hf was corrected by measuring the intensity of the interference-free 175 Lu isotope and using the recommended 176 Lu/ 175 Lu = 0.02656 to calculate 176 Lu/ 177 Hf.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].

K-Feldspar
The K-feldspar crystals of the Bure adakitic rock show relatively uniform compositional variation in the major elements (Table 1), with 11.23-16.66wt% of K 2 O (average value of 15.74 wt%), 0.37-1.20 wt% of Na 2 O (average value of 0.62 wt%), and 10.81-19.36wt% of Al 2 O 3 (average value of 18.51 wt%).The low contents of CaO, MgO, TiO 2 , 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 Al 2 O 3 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.

Plagioclase
The major elements in the plagioclase crystals of the Bure adakitic rock show a small range of compositions (Table 2).The SiO 2 content is relatively high, ranging from 62.57 to 67.75 wt% (average value of 65.18 wt%), with small variations of 6.71-10.02wt% of Na 2 O (average value of 8.88 wt%), 0.09-0.74wt% of K 2 O (average value of 0.16 wt%), and 2.52-4.28wt% 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 Al 2 O 3 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.

Biotite
The Fe 2+ and Fe 3+ 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 35.19-39.67wt% of SiO 2 , with an average value of 37.65 wt%.The biotite has relatively high contents of FeO (16.11-20.99wt%; average value of 19.12 wt%), Al 2 O 3 (13.96-19.13wt%; average value of 15.77 wt%), and TiO 2 (2.01-3.84wt%; average value of 2.90 wt%).In comparison, the MgO, K 2 O, Na 2 O, and CaO contents in biotite are relatively low, with values of 6.54-9.59wt% of MgO (average value of 8.33 wt%), 7.07-9.85wt% of K 2 O (average value of 8.33 wt%), 0.01-0.16wt% of Na 2 O (average value of 0.08 wt%), and 0.08-0.33wt% 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 Fe 2+ /(Mg + Fe 2+ ) 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 Mg 2+ and Al 3+ 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 Mg 2+ and Al 3+ may have occurred during the crystallization process of the calc-alkaline and peraluminous magmatic system (Figure 6; [40]).

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

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 logf O 2 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].

Zircon Lu-Hf Isotopes and Whole-Rock Sr-Nd Isotopes
Ten Lu-Hf isotopic analyses were conducted on the zircons of the Bure adakitic sample, yielding 176 Hf/ 177 Hf ratios of 0.282572~0.282734,and εHf(t) values from 7.64 to 1 (average value = 11; Table 5).On the Age-εHf(t) diagram, the corresponding two-stag model ages vary from 802-1161 Ma (Figure 8a).The Sr-Nd isotopic results of the B adakitic rock are shown in Table 6.The 87 Sr/ 86 Sr ratios ranging from 0.707381 to 0.7 (average value = 0.70741) are higher than that of the current original mantle value ( 87 Sr = 0.7045; Table 6).Correspondingly, the calculated ( 87 Sr/ 86 Sr)i ratios vary from 0.7008 0.70275 (average value = 0.70184), and the εNd(t) values have a relatively large variatio 3.26 to 7.28 (average value = 4.72; Figure 8b).Their two-stage Nd model ages range f 820 to 1210 Ma.

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 176 Hf/ 177 Hf 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 twostage 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 87 Sr/ 86 Sr ratios ranging from 0.707381 to 0.70745 (average value = 0.70741) are higher than that of the current original mantle value ( 87 Sr/ 86 Sr = 0.7045; Table 6).Correspondingly, the calculated ( 87 Sr/ 86 Sr)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.

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

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 × 10 5 to 2.81 × 10 5 Pa (mean 2.09 × 10 5 Pa) for the Bure adakitic rock.The calculated emplacement depth of the Bure adakitic rock is 6.39~10.2km (mean 7.60 km) according to the empirical formula p = ρgh (ρ =2800 kg/m 3 ; g = 9.8 m/s 2 ), which indicates that the magmatic emplacement depth was relatively deep.
Generally, the Fe 3+ , Fe 2+ , and Mg 2+ 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 Fe 2 O 3 -Fe 3 O 4 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 Ce 4+ in magmas.The similar radius of Ce 4+ and Zr 4+ leads to Ce 4+ being more likely than Ce 3+ 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).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 Ce 4+ in magmas.The similar radius of Ce 4+ and Zr 4+ leads to Ce 4+ being more likely than Ce 3+ 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).

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 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 Ce 4+ in magmas.The similar radius of Ce 4+ and Zr 4+ leads to Ce 4+ being more likely than Ce 3+ 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).

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

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-Al 2 O 3 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 ( 87 Sr/ 86 Sr)i values of 0.70088-0.70275and 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 ( 87 Sr/ 86 Sr) 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).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 εHf(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 ( 87 Sr/ 86 Sr)i values of 0.70088-0.70275and positive εNd(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 ( 87 Sr/ 86 Sr)i-εNd(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).

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 (72.26-72.78wt%), Al2O3 (14.91-15.82wt%), 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 (2.71-4.78ppm), 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].

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 SiO

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.

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

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

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

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

Figure 5 .
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 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 .
Figure 6.Diagram of the chemical variation of Al2O3 vs. MgO in the biotite.

Figure 6 .
Figure 6.Diagram of the chemical variation of Al 2 O 3 vs.MgO in the biotite.

Minerals 2024 ,
14,  x FOR PEER REVIEW 15 of 20 increasing importance of pre-Neoproterozoic crust southwards in the EAO, but detailed and systematic investigations are necessary to fully understand the issue.
2 km (average of 7.60 km).The Fe 3+ , Fe 2+ , and Mg 2+ 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.

( 1 )
The crystallization temperature of the Bure adakitic rock ranges from 659 to 814 • C, and its calculated emplacement depth was 6.39~10.2km (average of 7.60 km).The Fe 3+ , Fe 2+ , and Mg 2+ 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.

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

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

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

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

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

Table 6 .
Sr-Nd isotopic data for the Bure adakitic rock.
Sample No.