Neoproterozoic Juvenile Crust Research Articles

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 (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 εNd(t) values of 3.26 to 7.28, together with the positive εHf(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.

Timescale of magma recharge for a crystal-rich mush: Perspectives from the compositional zoned quartz in Xiangshan caldera, SE China

Crystal-rich (>30 vol%) volcanic rocks, which are generally thought to be formed by the eruption of crystal-rich mush after its reactivation, have provided an important lens through which to view the connection of volcanic and plutonic rocks in the calderas worldwide. The crystal-rich rhyolitic porphyritic lava is widely exposed in Cretaceous calderas in the southeast China. However, the petrogenesis of crystal-rich porphyritic lava still remains controversial. Xiangshan caldera is a uranium mineralization related volcanic-plutonic complex, which is composed of first subcycle rhyodacite and second subcycle tuff, crystal-rich rhyolitic porphyritic lava, granite porphyry and quartz monzonitite porphyry. All the volcanic and plutonic rocks have consistent zircon U-Pb ages (135 to 133 Ma). Sr-Nd-Hf-O isotopic characteristics show that they all mainly originated from crust derived intermediate-silicic magma. Zircon εHf(t) values for inherited zircons from rhyolitic porphyritic lava and rhyodacite range from −6.9 to +9.5. Correspondingly, zircon Hf two-stage model ages of inherited zircons are 2.12 to 1.10 Ga, suggesting their derivation from the Neoproterozoic juvenile crust and the Archean to Paleoproterozoic ancient crust. The rhyolitic porphyritic lavas show higher SiO2 contents and Rb/Sr ratios, and more obvious negative anomalies of Eu/Eu*, Ba, Sr, P and Ti; The granite porphyry show lower SiO2 contents and Rb/Sr ratios, with weak negative anomalies of Eu/Eu*, Ba, Sr, P and Ti. Combined with similar Sr-Nd-Hf-O isotopic composition, they may be derived from the same magma reservoir. Lower εNd(t), εHf(t), δ18O values and older model ages of tuff indicate more contribution of older crustal material. The cathodoluminescence (CL) brightness is strongly correlated with Ti concentration in quartz. The CL of selected quartz grains from crystal-rich rhyolitic porphyritic lava and granite porphyry show brightness zoned mantles (Ti = 19 to 77 ppm) and bright rims (high-Ti; >70 ppm). Some grains have dark cores (low-Ti; <9 ppm). However, the Ti contents of the quartz in the tuff of the second subcycle are homogeneous. Resemble the crystal-rich rhyolitic porphyritic lavas, the rims of quartz phenocrysts in the granite porphyry have consistent CL-brightness with the quartz in groundmass and have small embayments caused by growth impediments in them, which is due to the pressure release. It indicates that the granite porphyry also migrated upward rather than in situ cumulate. The repeated rise of CL brightness and Ti contents in the mantle of quartz phenocrysts recorded the rise of temperature and/or the injection of more primitive magma, indicating several times of magma recharge in the magma reservoir. The timescale of Ti diffusion in the compositional zoned mantle of quartz documented an ephemeral reactivation and remobilization of the crystal-rich mush within several to 16 thousand years with at least 7 times of magma recharge which lasted for 17 years to >11 thousand years each time.

Geochemical Characteristics of the Granodiorite Porphyry in Dongxiang W-Cu Deposit, SE China

The Dongxiang tungsten–copper deposit is a large W–Cu deposit located in the northeast of Jiangxi province in south China. Previous studies have mainly considered the geochemical and isotopic attributes of the ore deposit, but information is still lacking on the genesis and setting of Dongxiang W–Cu mineralization-related intrusive rocks. This paper presents systematic elements and Sr–Nd–Hf isotopic data of the Dongxiang granodiorite porphyry. The Dongxiang granodiorite porphyry is intermediate–acidic in composition, with SiO2 contents of 60.00–75.16 wt.%, Al2O3 contents of 10.15–18.53 wt.%, an K2O contents of 2.95–4.28 wt.%. The REEs content ranges from 64.2 to 198.1 ppm, with LREE/HREE ratios of 7.67–17.47. It is characterized by adakitic geochemical affinities with high Sr/Y and (La/Yb)N ratios but low Y and Yb contents. A slight Eu anomaly, extreme depletion in Y and Yb, relatively low MgO content, and relatively high 207Pb/204Pb ratios together indicate that the Dongxiang granodiorite porphyry was likely derived from partial melting of the thickened lower continental crust. Based on zircon εHf(t) values (−11.8–−4.5), two-stage Hf model ages (1.33–1.94 Ga), and the regional geological setting, it can be inferred that Dongxiang granodiorite porphyry magma is mainly derived from Neoproterozoic juvenile crust with the involvement of Paleoproterozoic ancient crust.

Petrogenesis of the Alubaogeshan intrusion in the Maodeng–Xiaogushan area, southern great xing'an range, NE China: Implications for magma evolution and tin–polymetallic mineralization

• The Alubaogeshan intrusion formed at ca. 134 Ma and belongs to transitional I–A 2 -type granite . • The Alubaogeshan intrusion was formed by partial melting of a Neoproterozoic juvenile crust. • The syenogranite porphyry evolved from the porphyritic monzogranite. • Fluid–melt interaction is crucial for W mineralization. • The magma evolutionary degree influences the size of the Sn–polymetallic deposits in the SGXR. The Alubaogeshan intrusion, consisting of porphyritic monzogranite in the central part and syenogranite porphyry in the edge, is genetically related to the Maodeng–Xiaogushan Sn–polymetallic deposit. This paper aims to investigate the genetic link between these two granite phases and the relationship between magma evolution and Sn mineralization. LA-ICP-MS zircon U–Pb dating of the porphyritic monzogranite and syenogranite porphyry yielded ages of 134 ± 1.0 Ma and 133 ± 0.5 Ma, respectively. The Alubaogeshan intrusion has high SiO 2 and total alkali (Na 2 O + K 2 O) contents, with low Al 2 O 3 , Fe 2 O 3 , FeO, and MgO contents, belonging to high-K calc-alkaline metaluminous–weakly peraluminous granites. Chondrite-normalized rare earth element (REE) patterns are characterized by light REE enrichment and heavy REE depletion, with extremely negative Eu anomalies. In addition, the intrusion was also enriched in Rb, Th, U, K, Zr, Hf, and light REE and strongly depleted in Ba, Sr, P, and Ti. Mineralogical and geochemical characteristics, together with high zircon saturation temperature, indicate that the Alubaogeshan intrusion belongs to transitional I–A 2 -type granites. Compared with the porphyritic monzogranite, the syenogranite porphyry reveals higher SiO 2 content and lower TiO 2 , Al 2 O 3 , Fe 2 O 3 , FeO, CaO, MgO, and P 2 O 5 contents, with more obvious negative anomalies of Eu, Ba, Sr, P, and Ti. The temporal and spatial relationship and geochemical characteristics of the two rocks indicate that the syenogranite porphyry evolved from the porphyritic monzogranite. The modeling results show that the fractional crystallization of amphibole, plagioclase, K-feldspar, biotite, apatite, monazite, and allanite occurred during the magma evolution. The granitic magma differentiation plays a key role in the formation of the Maodeng–Xiaogushan Sn–polymetallic deposit. The ε Hf ( t ) values of zircon range from 2.58 to 7.08, and the two-stage Hf model ages range from 619 Ma to 850 Ma, implying that the Alubaogeshan intrusion was predominantly derived from partial melting of a Neoproterozoic juvenile crust. The TE 1,3 values of the ore-related granites from W–(Sn) deposits are generally higher than 1.1, suggesting that fluid–melt interaction is crucial for W mineralization. The degree of magmatic evolution is one of the controlling factors for the size of Sn–polymetallic deposits in the southern Great Xing′an Range.

Petrogenesis of the Late Jurassic to Early Cretaceous granites in the Taiping–Huangshan area, north‐eastern Yangtze Block, China

Yanshanian magmatisms are intensive in the eastern Jiangnan orogen and can be divided into early (150–136 Ma) and late (136–120 Ma) stages. About these granitic rocks, the petrogenesis and associated tectonic setting are still hotly debated. The Taiping–Huangshan complex, a representative granitic rock in this region, is selected to perform geochemical studies. Three lithologic units have been identified in the Taiping–Huangshan complex: granodiorite, monzogranite, and alkali‐feldspar granite with zircon U–Pb dating results of 149–144 Ma, 148 Ma, and 135–131 Ma, respectively, belonging to the early stage for the granodiorite and monzogranite, and late stage for the alkali‐feldspar granite. The granodiorite and monzogranite have similar major and trace element characteristics with intermediate‐acidic SiO2 contents ranging from 65.9 to 69.7 wt.%, low Mg# values of 35.8–38.4, enriched LREE, Pb, Th, and U contents, and weak Eu anomalies (δEu = 0.71–0.79). The alkali‐feldspar granite is characterized by high SiO2 contents (73.3–76.6 wt.%), low Mg# (2.6–10.0), enriched Th, U, Ta, Pb, and Nd, depleted high‐field‐strength elements, and pronounced negative Eu anomalies (δEu = 0.01–0.19). The granodiorite and monzogranite have consistent εHf(t) (−5.38 to −9.41) and δ18O (7.18‰ to 11.00‰) values, while the alkali‐feldspar granite has elevated εHf(t) values (−0.96 to −6.48) and δ18O values (6.27‰ to 8.34‰). Integrated geochemical characteristics and batch partial melting modelling constrain that the early stage granitic rocks derived from ca. 8% batch melting of the Neoproterozoic juvenile crust, and the late‐stage granites derived from a similar crust source with a little contribution from mantle. From early to late stage, the tectonic setting translated from post‐orogenic to anorogenic and the later corresponded to a back‐arc extensional setting as increasing of the slab subducted angle of the Palaeo‐Pacific Plate.

Contributions of trans-magmatic fluid in the formation of porphyry copper deposits: A case study from the Baoshan deposit, South China

The classical mechanism “source-transport-storage” of the formation of porphyry copper deposit has been advanced in recent studies, as the “source” is not the main factor for the mineralization in some Cu deposits, and the metallogeny may be affected by other variables factors during the magma-fluid transportation or storage. We recommend the essential role of trans-magmatic fluid in the ore-forming process, this fluid is released from the melting of the sedimentary overlying the subducted plate with high water and volatiles concentrations and high oxygen fugacity. The Baoshan granodioritic cryptoexplosion breccia representing the influence of hydrothermal events as well as the unaltered Baoshan granodiorite porphyry are conducted by LA-MC-ICP-MS analysis, to identify the contribution of trans-magmatic fluid. In case of the εNd(t) of whole rock do not increase with the MgO increasing and SiO2 decreasing, the large variations of zircons εHf(t) values in Baoshan granodiorite porphyry (BGP, −14.24 to −6.38) and Baoshan granodioritic crypto-explosion breccia (BGCB, −25.24 to −6.62) were considered to be the interaction of partial melting of ancient mafic lower crust and Neoproterozoic juvenile crust. However, the copper mineralization requires high oxygen fugacity and a large amount of water, according to the tectonic settings of Baoshan, we recommend that it is the trans-magmatic fluid trapping and concentrating Cu from the whole pluton during the upwelling driven by magma convection. The initial magma was stalled by the ductile-brittle transition at shallow depths of upper-crust. The trans-magmatic fluid leads to the pressure increases at the top of the initial magma, then the overlying rock is ruptured and cryptoexplosion produced. Moreover, after the cryptoexplosion, the sudden reduction of circumference temperature and pressure leads to the decrease in the oxygen fugacity of the ore-forming system, which will change the valence state of sulfur from S6+ to S2−. Finally, sulfur precipitates with chalcophile elements like copper in the metallogenic system and forms porphyry copper deposits with the low Sr/Y ratio. This study highlights the use of trans-magmatic fluid and ductile-brittle transition in the formation of the Baoshan porphyry copper deposits.

Crust-derived felsic magmatism in the Emeishan large igneous Province: New evidence from zircon U-Pb-Hf-O isotope from the Yangtze Block, China

Numerous intrusive bodies of mafic–ultramafic to felsic compositions are exposed in association with volcanic rocks in the Late Permian Emeishan large igneous province (ELIP), southwestern China. Most of the granitic rocks in the ELIP were derived by differentiation of basaltic magmas with a mantle connection, and crustal magmas have rarely been studied. Here we investigate a suite of mafic dykes and I-type granites that yield zircon U-Pb emplacement ages of 259.9 ± 1.2 Ma and 259.3 ± 1.3 Ma, respectively. The εHf(t) values of zircon from the DZ mafic dyke are –0.3 to 9.4, and their corresponding TDM1 values are in the range of 919–523 Ma. The εHf(t) values of zircon from the DSC I-type granite are between –1 and 3, with TDM1 values showing a range of 938–782 Ma. We also present zircon O isotope data on crust-derived felsic intrusions from the ELIP for the first time. The δ18O values of zircon from the DSC I-type granite ranges from 4.87‰ to 7.5‰. The field, petrologic, geochemical and isotopic data from our study lead to the following salient findings. (i) The geochronological study of mafic and felsic intrusive rocks in the ELIP shows that the ages of mafic and felsic magmatism are similar. (ii) The DZ mafic dyke and high-Ti basalts have the same source, i.e., the Emeishan mantle plume. The mafic dyke formed from magmas sourced at the transitional depth between from garnet-lherzolite and spinel-lherzolite, with low degree partial melting (<10%). (iii) The Hf-O isotope data suggest that the DSC I-type granite was formed by partial melting of Neoproterozoic juvenile crust and was contaminated by minor volumes of chemically weathered ancient crustal material. (iv) The heat source leading to the formation of the crust-derived felsic rocks in of the ELIP is considered to be mafic–ultramafic magmas generated by a mantle plume, which partially melted the overlying crust, generating the felsic magma.

Magmatic controls on the mineralization potential of a porphyry Cu system: The case of Jurassic Tongshan skarn Cu deposit in the Qin–Hang Belt, South China

The Qin–Hang suture belt in South China formed during the Neoproterozoic amalgamation between the Cathaysia and Yangtze blocks. There are several Jurassic porphyry–skarn Cu deposits in this region, and the factors controlling the mineralization potential remain unclear. This study reports geochemical data for the Tongshan skarn deposit and zircon trace element data for five porphyry- or skarn-type Cu deposits in this region, in order to identify geochemical indexes for the mineralization potential of the ore-related intrusions. A garnet U–Pb age (170 Ma) for the Tongshan skarn indicates that Cu mineralization occurred in the Middle Jurassic, which is consistent with the zircon U–Pb ages (173 Ma–171 Ma) of the host granodiorite porphyries. Granodiorite porphyry samples from the Tongshan deposit have moderately high SiO2 contents (64.4–65.0 wt%), low MgO contents (1.39–1.47 wt%), and high (La/Yb)N ratios (14.7–16.8), typical of low-Mg adakitic rocks. The whole-rock geochemistry, bulk Earth-like εNd (t) values (–1.5 to –1.6) and positive zircon εHf(t) values (+2.7 to +6.0), corresponding to two-stage Hf model ages of 1014–832 Ma, suggest the porphyries were derived from thickened juvenile lower crust initially generated during Neoproterozoic amalgamation. In addition, the porphyry samples have arc-like geochemical characteristics (Nb–Ta–Ti depletion), low Th/Yb ratios (average = 4.7), high Ba/Th ratios (average = 346), zircon Eu/Eu* values (average = 0.71) and 10000×(Eu/Eu*)/Y values (average = 6.80), and moderate zircon Ce/Ce* values (average = 705) and Ce/Nd ratios (average = 17.1). These features demonstrate that the parental magmas of the porphyries were formed by melting of a mantle wedge.That had been metasomatized by moderately oxidized slab-derived fluid near a Neoproterozoic subduction zone. Previous studies suggest porphyries from large to giant deposit (e.g., Dexing and Yinshan) have similar two-stage Hf model ages and were derived from Neoproterozoic juvenile crust. But these porphyries have higher whole–rock Th/Yb ratios and zircon Ce/Ce* and Ce/Nd values than those of intrusions from medium-sized deposits (e.g., Tongshan, Chuankeng and Jiande). This indicates that the magma source of the former may have been distant from the trench in the Neoproterozoic, and formed by melting of a mantle wedge that had been metasomatized by highly oxidized slab-derived melt. Our results and previously published data indicate that the Neoproterozoic two-stage Hf model ages of the intrusions are positive indicators of Jurassic Cu mineralization, and granitoids with relatively high Th/Yb ratios and zircon Ce/Ce* and Ce/Nd values likely have more potential to form large to giant Cu deposits.

Geochronological and geochemical investigations of the granites from the giant Shihuiyao Rb-(Nb-Ta-Be-Li) deposit, Inner Mongolia: Implications for magma source, magmatic evolution, and rare metal mineralization

The Shihuiyao deposit is a recently discovered giant Rb deposit in association with other rare metals (Nb-Ta-Be-Li) in Inner Mongolia of northeastern China. Six ore-bearing plutons were emplaced at the Late Jurassic (ca. 145 Ma) in the Shihuiyao mining area and composed of siderophyllite monzogranite, muscovite granite, muscovite–albite granite, zinnwaldite–albite granite, and zinnwaldite–amazonite granite. Micas in these granites include siderophyllite, muscovite and zinnwaldite, and they show an increase of Rb2O and F contents and decrease of K2O/Rb2O ratios, reflecting the magmatic fractionation evolution trend. Field relationships, whole-rock data, and mineralogical features do not show a continuous progression of these granites in the Shihuiyao deposit, which can be interpreted as that they experienced different fractional processes in the magma source. Hence, combined with field evidence and Nd isotopic composition, a model was established for the petrogenesis of these granites. The oceanic slab tear led to rapid asthenosphere upwelling, which causes the partial melting of a Neoproterozoic juvenile crust, i.e., a mixed lithology composed of a major mantle-derived component with less amount of ancient crust component. Initial magmas ascended and accumulated to form a magma chamber in the upper crust. Magmas in the chamber underwent a series of fractional processes and were then discharged by a series of intrusive magmatic activities, thus forming the ore-bearing granites in the Shihuiyao mining area. In the late stages of magmatic fractionation, co-existing aqueous hydrothermal fluids most likely controlled the trace element distribution in these ore-bearing granites.In the Shihuiyao deposit, not only the fractional crystallization of magma, but also the hydrothermal fluids, exsolved from the magma and extracting elements from the wall rocks or primary granite phases, played an important role in Rb mineralization. Also, the content of F in the melt may affect the enrichment of Rb. The presence of a deep magma chamber suggests that the Shihuiyao and its surrounding areas have good prospecting potentials for rare metals mineralization.

Geochronology, geochemistry, and Hf–Sr-Nd isotopes of the Hamisana Shear Zone granitoids in northeastern Sudan: Petrogenesis and tectonic evolution of neoproterozoic juvenile crust in the Nubian Shield

We have investigated the petrology, U-Pb zircon ages, whole-rock geochemistry, and Hf–Sr-Nd isotopic compositions of the Hamisana Shear Zone granitoids (HSZG) in the Arabian-Nubian Shield (ANS) in northern Sudan in order to constrain their petrogenesis and tectonic setting of formation. The HSZG rocks consist mainly of granite, quartz syenite, and quartz monzonite that collectively make up two major groups based on their ages and geochemical compositions. The older group (average U-Pb zircon age of ~663 Ma) is represented by calc-alkaline and I-type peraluminous granites, whereas the younger group (average U-Pb zircon age 623.5 Ma) is characterized by A-type and metaluminous monzonitic intrusions. All rocks show high Al2O3, La/Nb, Th/Ta, Ba/Nb ratios, and similar REE patterns indicating enrichment of LILEs (Ba, K, Pb, and Sr) and depletion of HFSEs (Nb, Ta, Sm, P, and Ti), consistent with the characteristics of subduction-related magmas. La/Yb, La/Sm, Th/Nb, Rb/Sr, and Rb/Ba ratios suggest that magmas of the HSZG formed by partial melting of a subduction-modified mantle beneath the ANS. Sr-Nd isotopic compositions of both groups with Nd model ages of 794–877 Ma have εNd(t) values of +5.24 to +6.10. In-situ Hf isotope analyses indicate εHf(t) values of +10.36 to +10.90 for the older granites and +9.9 to +10.73 for the younger quartz monzonites. These isotopic features point to a depleted mantle source of their magmas with no involvement of Pre-Neoproterozoic crust in their melt evolution. The older HSZG granite suites represent pre-collision arc magmatism, whereas the younger HSZG quartz monzonite suites mark post-collisional magmatic units. Both magmatic events contributed significantly to juvenile crust construction during the Neoproterozoic assembly of the ANS.

Petrogenesis and oxidation state of granodiorite porphyry in the Jurassic Chuankeng skarn Cu deposit, South China: Implications for the Cu fertility and mineralization potential

Large-scale Jurassic porphyry–skarn Cu mineralization in the Qin-Hang metallogenic belt of South China has attracted much attention, but comparative studies of porphyries in large- and small-scale deposits are lacking. In this paper, we present new zircon U–Pb ages, trace element composition and Hf isotopic composition as well as whole-rock geochemical and Sr–Nd isotopic compositions for the porphyries associated with mineralization in the small-scale Chuankeng Cu deposit. Drawing on our data for the Chuankeng deposit and existing data for the nearby giant Dexing Cu deposit, we aim to identify factors controlling the mineralization potential of both deposits. Granodiorite porphyries of Chuankeng deposit were emplaced at ca. 161–158 Ma. They have moderate SiO2 contents of 61.9–66.1 wt% and high Sr/Y (59–135) and (La/Yb)N (4–14) ratios. Their high MgO contents (0.2–2.9 wt%), lack of negative Eu anomaly, depletion in Y and Yb, relatively high initial 86Sr/87Sr ratios (0.7076–0.7078), and low εNd(t) values (−4.5 to −4.9), indicating that the Chuankeng porphyries were derived from partial melting of delaminated lower crust. The Middle–Late Jurassic Chuankeng porphyries are considered to have been formed in a localized intra-continental extension environment along the Jiangshao Fault in response to far-field stress from Paleo-Pacific Plate subduction. Although no Middle–Late Jurassic magmatic arc rocks have been found in the study area, the Chuankeng porphyries display similar characteristics to arc rocks. On the basis of their zircon εHf(t) values (−2.6 to +0.9), two-stage Hf model ages (1.4–1.1 Ga), and regional geological history, we infer that magma source of Chuankeng porphyries were mainly Neoproterozoic juvenile crust with the involvement of Paleoproterozoic ancient crust. Thus their arc-like features were inherited from the Neoproterozoic juvenile crust that formed by subduction of oceanic crust during the Jiangnan orogeny. Chuankeng porphyries have lower zircon Ce4+/Ce3+ and higher Ti-in-zircon temperatures and lower whole rock εNd and zircon εHf values than the Dexing porphyries, indicating they are less oxidized and less hydrous and with more involvement of the ancient crustal material in the magma source, which could explain the relatively small-scale Cu mineralization in Chuankeng. This study highlights the values of integrated studies of whole rock Nd and zircon trace element and Hf isotopic compositions in assessing the potential for Cu mineralization.

Geochemistry and zircon U–Pb–Hf isotopes of the Mante Aobao granite porphyry at East Ujimqin Banner, Inner Mongolia: implications for petrogenesis and tectonic setting

AbstractWe present detailed petrography, geochemistry and zircon U–Pb–Hf isotopes of the Mante Aobao granite porphyry in East Ujimqin Banner, Inner Mongolia, with the aim of determining its age and petrogenesis, important for understanding the early Palaeozoic tectonic evolution of the Xing’an–Mongolian Orogenic Belt. The Mante Aobao granite porphyry consists of plagioclase, quartz and minor biotite, but without amphibole. Zircon U–Pb analyses yield ages of 450 ± 1 Ma and 445 ± 2 Ma for the granite porphyry, indicating that it formed during Late Ordovician time. The granite porphyry is metaluminous to slightly peraluminous (aluminous saturation index A/CNK = 0.98–1.11) with high SiO2, K2O and Na2O concentrations and differentiation index (DI = 85–90). Chondrite-normalized rare earth element (REE) patterns display enrichment of light REEs (LREEs) with high ratios of (La/Yb)N and negative Eu anomalies. In the mantle-normalized multi-element variation diagrams, all samples are characterized by depletions of high-field-strength elements (HFSEs; Nb, Ta and Ti) and enrichments of large-ion lithophiles (LILEs; Rb, Th, U and K). These geochemical features indicate that the granite porphyry is a highly fractionated I-type granite and formed in a subduction-related setting. Zircon grains have positive εHf(t) values of +9.2 to +11.2, and TDM2(Hf) ages of 691–821 Ma, suggesting that the granite porphyry was generated by partial melting of Neoproterozoic juvenile crust with involvement of fractional crystallization during magmatic evolution. It is likely that underplating of mantle-derived magmas during Late Ordovician time provided the necessary heat to partially melt this juvenile crust. Combined with the regional geological data, we infer that the Mante Aobao granite porphyry was emplaced in an active continental margin setting that was probably related to the northwards subduction of the Paleo-Asian Plate beneath the South Mongolian Terrane along the Sonid Zuoqi–Xilinhot axis.

Tectonic Evolution of the Polydeformed Urf Al-Mahib Belt, South-Eastern Desert, Egypt

The Urf Al-Mahib area, located to the southern part of the Eastern Desert (SED) of Egypt, is covered mainly by juvenile Neoproterozoic crust and Nubian sandstones. Field investigation and structural analyses give evidence that the area of Urf Al-Mahib developed through four successive phases of deformation (D1, D2, D3 and D4). D1 was an attenuated phase represented by tight to isoclinal folds (F1), tightly appressed fold closures and sheared-out hinges, and axial plane foliation (S1), as well as mineral and stretching lineations (L1). Structural fabrics formed during the D2 phase embrace minor- and map-scale prominent F2 overturned folds with NW (to NNW)-dipping long upper limbs and short lower overturned limbs and axial planes striking NE (to ENE)‒SW (to WSW) and dipping to the NW at moderate angles. F2 folds are geometrically- and kinematically-related to thrust propagation, and often have SE (to SSE) vergence. Thrust faults, that striking NE (to ENE)‒SW (to WSW) and dipping NW to NNW, are also common in this stage. D3 structures contain pervasive vertical to inclined mesoscopic open to very open folds (F3), whose axes plunge NW (to NNW) and SE (to SSE) at moderate to steep angles. The latest D4 deformation phase is represented by abundant small- and large-scales WNW‒ESE trending dextral semi-ductile-semi-brittle-shear zones.

The mineral chemistry of gahnite, garnet and columbite-group minerals (CGM): Implications for genesis and evolution of the Kenticha Rare-element granite-pegmatite, Adola, Ethiopia

Fracture-controlled barren to rare-element pegmatites were emplaced into the Adola belt of the Southern Ethiopian Precambrian crystalline basement during Neoproterozoic (530 Ma). The Adola belt occupies an important transitional position between the juvenile Neoproterozoic crust of the Arabian Nubian Shield and high-grade metamorphic Pan-African Mozambique belt. The Adola belt is made up of narrow N–S trending synclinal greenstone belts and the granite-gneiss tectonostratigraphic terranes. These greenstone belts are renowned for the occurrence of abundant mineral resources including rare-metals, precious metal, gemstones and industrial minerals. The Kenticha rare-element granite-pegmatite is the largest repository of rare-elements (Ta, Nb, Li, Be), known to date in the Horn of Africa. Ubiquitous accessory minerals occur in association with this pegmatite. The presence of garnet, gahnite, muscovite, tourmaline and aluminosilicates indicates the granite-pegmatite is typical of the peraluminous nature. The present study involves the mineral chemistry of gahnite, garnet and columbite-tantalite phases using electron probe micro-analyzer to constrain the genesis of rare-element granite-pegmatite and evolution of the pegmatite-forming melt. Accordingly, crystal chemistry of garnet, gahnite and tantalite-columbite are very much consistent. The Zn/Fe and (Fe+Mg)/Al ratios in gahnite range from 6.99 to 14.14 and 0.075–0.065, respectively. Garnet compositions vary only in narrow range within the pegmatite zones and also in the individual grains. The calculated structural formula is Sps53-64Alm33-46Prp0-3Grs0-2Adr0-2, showing typical of the spessartine-almandine variety. The characteristic normal zoning and high mol (Zn+Mn)/Al and low mol (Fe+Mg)/Al in gahnite, and the high MnO/(FeOt+MgO) ratios in garnet unequivocally indicate the pegmatite is crystallized from a highly evolved residual melt, enriched with a fluxing component (Li, F, P, B), via fractional crystallization processes. The textural characteristics of garnet grains, nature of zoning, high spessartine content and lack of reaction rim indicate that the granite-pegmatite was crystallized at low-pressure from a Li–H2O–F rich melt. The Fe/Mn ratio both in garnet and columbite-tantalite decreases from the wall to the core zone of the pegmatite. The positive correlation between SiO2 and MnO and the high MnO/(FeOt+MgO) ratios in garnet suggest crystallization of the granite-pegmatite from highly evolved MnO-rich melt. A systematic fractionation trend is also recognized from the quadrilateral compositional plot of tantalite-columbite minerals. It shows ferrocolumbite - manganocolumbite - manganotantalite evolutional trend from lower to the upper unit. The Mn/(Mn+Fe) ratio in columbite group of mineral progressively increases from the lower to the upper unit of the pegmatite. In general, from textural and compositional data, it could be interpreted that the columbite-tantalite mineral and other rare-elements were developed from magmatic source, however, later on, overprinted by hydrothermal replacements. The compositions of all the accessory phases consistently suggest the rare-element granite-pegmatite is crystallized from a magma rich in SiO2 and Li–F–H2O fluids derived at low pressure and shows a progressive evolutional trend from wall to the core zone of the pegmatite.

Precambrian terrains of Central Asian orogenic belt: comparative characteristics, types and peculiarities of the tectonic evolution

The structure and peculiarities of the tectonic evolution of Precambrian terraines included into the structure of Paleozoids in different parts of the Central Asian orogenic belt are reviewed, types and comparative characteristics of Precambrian terraines are provided. We throw light on two types of Precambrian terrains structure: essentially juvenile Neoproterozoic crust (1); Mezo- and Early Neoproterozoic crust formed due to reworking of Early Precambrian formations (2). Terrains with juvenile Neoproterozioc crust, located in the Central and Eastern parts of the Central Asian orogenic belt, were generated in the oceanic sector of the Earth. Their formation was connected to the Early- and Late Neoproterozoic cycles of tectogenesis up to 200 Ma each cycle. Terrains with Mezo- and Early Neoproterozoic crust, found mainly in the West of the Central Asian orogenic belt, generated in the continental sector of the Earth during the Neoproterozoic, their evolution occurred mainly in the intracontinental environments. In the evolution all of considered terrains in the interval 800–700 Ma, an event associated with rift zones formation and intraplate magmatism was revealed, it coincided with the supercontinent Rodinia split. The conducted research allow to connect formation history of the Precambrian terrains of the Central Asian orogenic belt with the processes took place in the edge of the Syberia-Tarim part of the supercontinent Rodinia and the adjacent sector of the paleo-ocean.

From the Libyan border to the Nile – Neoproterozoic magmatism and basement evolution of southern Egypt

ABSTRACTIn southern Egypt, Neoproterozoic granitoids intrude the crust from Libya in the west to the Nile River in the east. Best exposed at Gebel Uweinat, Gebel Kamil, Bir Safsaf, Gebel El Asr and Gebel Umm Shaghir, these rocks represent the transition between the rejuvenated Archean to Paleoproterozoic crust of the east Saharan craton and the juvenile Neoproterozoic crust of the Arabian–Nubian Shield to the east. We present U-Pb zircon geochronology and whole rock geochemistry from 16 Neoproterozoic (c. 628–580 Ma) granitoids across southern Egypt which document the synchronous production of diverse compositions indicative of an intraplate or intracontinental tectonic setting. Our data better constrain the age relationships, petrogenesis, and Pan-African crustal growth associated with the amalgamation of Gondwana and contribute to understanding the crust-modifying processes that led to the development of the Saharan metacraton in the Ediacaran.

Detrital zircon U–Pb–Hf systematics of Ediacaran metasediments from the French Massif Central: Consequences for the crustal evolution of the north Gondwana margin

Combining U–Pb and Lu-Hf isotopic data of detrital zircon grains has proven a powerful tool to unravel the provenance of sediments and address continental crust evolution. In this study, we explore the origin of thick siliciclastic metasedimentary units from the high-grade internal domains of the Variscan belt of Europe and examine their significance for Neoproterozoic crust formation and evolution along the North Gondwana margin. We present data and U–Pb/Lu–Hf systematics of detrital zircons from five amphibolite-facies metasediments sampled in the eastern French Massif Central, measured in situ by LA–(MC)–ICP–MS. The sedimentary protoliths were deposited in the Ediacaran as evidenced by field relationships and maximum depositional ages ranging between 592.4 ± 5.5 and 556.8 ± 5.1 Ma. All samples contain three main zircon populations in terms of age distribution and Hf isotopes: (i) abundant 0.55–0.65 Ga zircons with considerably scattered εHf(t) from −19 to +14; (ii) varied amounts of 0.65–1.0 Ga zircons showing dominantly positive εHf(t) with the exception of the c. 1.0 Ga zircons; and (iii) ≥1.8 Ga zircons (mostly between 1.9–2.1 and 2.5–2.8, up to 3.2 Ga) with εHf(t) ranging between +5 and −7. Multidimensional scaling of our U–Pb dataset and a data compilation of Ediacaran to Lower Cambrian (meta)sediments from the North Gondwana margin reveals that the relative proportions of the three age components carry discriminant provenance information. Geological data indicate that the Ediacaran basins of the eastern French Massif Central collected the erosion products of two main source regions: (i) the Neoproterozoic Cadomian magmatic arc; and (ii) the cratonic hinterland, mostly from the Saharan Metacraton and the Arabian-Nubian Shield. Our dataset attests to the reworking of: (i) juvenile Neoproterozoic crust and, (ii) old Paleoproterozoic to Neoarchean crustal components, either as part of the detritus or incorporated in Neoproterozoic arc magmas. First-order estimates derived from the isotopic signature of S-type granitic magmas sourced in the Ediacaran metasediments suggest that 60–75% of the detritus would correspond to young Neoproterozoic crust and thus represent net additions to the continental crust volume.

Precambrian Terranes of the Central Asian Orogenic Belt: Comparative Characteristics, Types, and Peculiarities of Tectonic Evolution

The structure and peculiarities of the tectonic evolution of Precambrian terraines included into the structure of Paleozoids in different parts of the Central Asian orogenic belt are reviewed, types and comparative characteristics of Precambrian terraines are provided. We throw light on two types of Precambrian terrains structure: essentially juvenile Neoproterozoic crust (1); Mezo- and Early Neoproterozoic crust formed due to reworking of Early Precambrian formations (2). Terrains with juvenile Neoproterozioc crust, located in the Central and Eastern parts of the Central Asian orogenic belt, were generated in the oceanic sector of the Earth. Their formation was connected to the Early- and Late Neoproterozoic cycles of tectogenesis up to 200 Ma each cycle. Terrains with Mezo- and Early Neoproterozoic crust, found mainly in the West of the Central Asian orogenic belt, generated in the continental sector of the Earth during the Neoproterozoic, their evolution occurred mainly in the intracontinental environments. In the evolution all of considered terrains in the interval 800–700 Ma, an event associated with rift zones formation and intraplate magmatism was revealed, it coincided with the supercontinent Rodinia split. The conducted research allow to connect formation history of the Precambrian terrains of the Central Asian orogenic belt with the processes took place in the edge of the Syberia-Tarim part of the supercontinent Rodinia and the adjacent sector of the paleo-ocean.

Early Cretaceous diorites in the Kenbale Cu mineralization occurrence, Tibet, China, and its geological significance

The Kenbale Cu mineralization occurrence related to the diorite is a newly discovered Cu mineralization event in the Bangong-Nujiang metallogenic belt, Tibet, China. The Cu mineralization is hosted in the contact between the monzogranite or biotite quartz diorite and the fine-grained diorite, which is the mineralization related intrusion. In order to constrain the Kenbale mineralization age, petrogenesis and tectonic setting, we conducted LA-ICP-MS zircon U-Pb dating and Hf isotopic analyses of the biotite quartz diorite and fine-grained diorite and also the whole-rock geochemical study of the biotite quartz diorite. Zircon UPb dating show that weighted mean 206Pb/238U ages of the biotite quartz diorite and fine-grained diorite are 123.5 ± 1.9 Ma (MSWD = 2.3, n = 16) and 118.9 ± 1.3 Ma (MSWD = 2.5, n = 18), respectively. The biotite quartz diorite is a high-K calc-alkaline I-type magma rock and was controlled mainly by partial melting process during the magma formation and evolution. This intrusion is characterized by positive eHf(t) values (2.6 to 5.8) and old Hf crustal model ages (813 to 1016 Ma), indicating that the magma was sourced from partial melting of the Mesoproterozoic to Neoproterozoic juvenile crust of the northern Lhasa Terrane. Compared with the biotite quartz diorite, the mineralization associated fine-grained diorite has much higher zircon eHf(t) values (8.2 to 11.4) and younger Hf crustal model ages (450 to 650 Ma). These characteristics are similar with those of the coeval magmatic rocks induced by slab break-off of the southward subducted Bangong-Nujiang Ocean. The geochronology and geochemical results show that the Kenbale Cu mineralization was controlled by the slab break-off of the southward subducted Bangong-Nujiang Ocean.

Old Continental Crust Underlying Juvenile Oceanic Arc: Evidence From Northern Arabian‐Nubian Shield, Egypt

AbstractThe Neoproterozoic Arabian‐Nubian Shield (ANS) is the best preserved and the largest exposed Neoproterozoic juvenile crust on Earth. While the lithology and early Sr and Nd isotopic data demonstrate that the ANS crust is overwhelmingly juvenile, pre‐ANS old zircon crystals have been increasingly recognized in the ANS igneous and sedimentary rocks, casting doubt on the “juvenility” of the ANS crust. In order to understand the origin of the old continental materials in the ANS and its roles in generation of juvenile oceanic arcs, we carry out for the first time an integrated in situ analysis of zircon U‐Pb age and Hf‐O isotopes for greywacke and felsic volcanic cobble samples from the Atud Formation in the Eastern Desert of northwestern part of the ANS. Our data indicate that the Atud Formation was deposited between ca. 720 and 700 Ma, concurrent with the production of oceanic arcs in the ANS. The Atud greywacke was derived from the erosion of a proximal arc terrane that contains numerous old continental crust materials. We identify for the first time a 755‐Ma felsic volcanic cobble from the Atud Formation that is derived from old continental materials during juvenile crust production, suggesting presence of an old continental crust substrate that underlies the ANS. Our work demonstrates that reworking of old continental crust played important roles in generation of oceanic arcs in the northwestern ANS that is likely much less juvenile than previously thought. Thus, the crustal growth rates calculated based on estimates of temporal island arc development need to be revised.