A highly-fractionated garnet-bearing muscovite granite represents the marginal granitic facies of the Abu-Diab multiphase pluton in the Central Eastern Desert of Egypt. New electron microprobe analyses(EMPA) and laser...A highly-fractionated garnet-bearing muscovite granite represents the marginal granitic facies of the Abu-Diab multiphase pluton in the Central Eastern Desert of Egypt. New electron microprobe analyses(EMPA) and laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) data from garnets are reported, in order to constrain their origin and genesis. Garnet in the Abu-Diab host granite is euhedral to subhedral, generally homogeneous and, in rare cases, it shows weak zonation. The garnet contains appreciable amounts of MnO and FeO, with lesser amounts of MgO and CaO, yielding an end-member formula of Sps61–72Alm25–35Prp1–4Adr0–1. Moreover, it is depleted in large ion lithophile elements(LILE) with lower values of Ba, Nb and Sr relative to the primitive mantle. Additionally, it contains high concentrations of HREE and Y and their REE pattern shows strong negative Eu anomalies. The garnet was crystallized under relatively low temperature(646°C–591°C) and pressure(< 3 kbar) conditions. The textural and chemical features indicate that the garnet is magmatic in origin and is chemically similar to that from highly-fractionated A-type granite. It was probably formed at the expense of biotite in a highly-evolved MnO-rich magma and/or by hydroxyl complexing of Mn during the ascending fluid phases.展开更多
This study deals with unraveling the diagenesis-induced porosity evolution in a mixed clastic-carbonate sequence of the Middle Permian Indus Basin,Pakistan.Multiple data sets including outcrop,petrography,cathodolumin...This study deals with unraveling the diagenesis-induced porosity evolution in a mixed clastic-carbonate sequence of the Middle Permian Indus Basin,Pakistan.Multiple data sets including outcrop,petrography,cathodoluminescence,scanning electron microscopy(SEM),mineralogy,and geochemical isotopic compositions were integrated to establish a link between porosity evolution and diagenesis.The spatial thickness and facies variations of the strata at outcrop scale are inherently controlled by the underlying bathymetry of the basin with deepening westward trend.The low values ofδ^(18)O of the target strata,relative to average values of the Permian carbonate,hints to diagenetic alteration in the strata.The data sets used in this study reveal modification of the strata in four environments,that is,i)early marine diagenesis indicated by micritization,pervasive dolomitization and isopachous fibrous cements,followed by ii)meteoric dissolution,and iii)shallow burial diagenetic processes including the precipitation of blocky cement,compaction of skeletal and non-skeletal allochems,and stylolites,and iv)a deep burial environment,characterized by pressure solution,and micro-fractures.The clastic intervals host subangular to subrounded quartz grains,floating textures,and almost complete absence of deleterious clay minerals,consequently resulting in the preservation of primary porosity.The primary porosity of carbonate intervals is preserved in the form of intercrystalline and intracrystalline porosity.The secondary porosity evolved through various diagenetic phases in the form of fractures and dissolution.The diagenetic solution mediated by organic matter in carbonates may have experienced both bacterial decomposition and thermochemical sulfate reduction,precipitating sulfides within the pores.The plug porosity/permeability analyses generally suggest high porosity in the siliciclastic unit,and carbonates with wackestone fabric while lower values were observed for the inner shelf pure carbonate facies.However,both intervals show very low permeability values probably due to isolated moldic pores and intense micritization.Therefore,clastic intervals may provide an opportunity to serve as a moderate reservoir;however,the carbonate intervals possess very low permeability values and could generally be considered as low-moderate reservoir potential.展开更多
The rare metals of Abu Dabbab area in the Central Eastern Desert of Egypt have been investigated for their mineralogy and conditions of precipitation using combination of EMPA and fluid inclusions studies,in order to ...The rare metals of Abu Dabbab area in the Central Eastern Desert of Egypt have been investigated for their mineralogy and conditions of precipitation using combination of EMPA and fluid inclusions studies,in order to delineate the source,mechanism of formation and evolutionary model for these economic metals.The(Ta-Nb-Sn)-bearing minerals at the Abu Dabbab area include columbite group minerals(CGMs),wodginite and cassiterite.In both granitic intrusion and its enclosed quartz veins,most of zoned CGMs and cassiterite grains are commonly characterized by a well-developed twostage texture.Hence,columbite-(Mn)(CGM-Ⅰ)represents the early formed phase of CGMs that is characterized by high Mn#values(0.64-0.92)with quite low Ta#values(0.13-0.49).It was invaded by Ta-rich phases including tantalite-(Mn)(CGM-Ⅱ;Ta#=0.13-0.49)and wodginite,which contain high Ta_(2)O_(5)and SnO_(2)(17.91 wt.%).In regard to cassiterite,there are distinct compositional differences between the early-phase cassiterite(Cst-Ⅰ)and the late-phase one(Cst-Ⅱ),where the latter is enriched in Ta_(2)O_(5),Nb_(2)O_(5)and FeO.The chemistry and textural criteria of the early stage CGM-Ⅰand Cst-Ⅰ,all are indicative of magmatic origin.While,the latter CGM-Ⅱ,wodginite and Cst-Ⅱwere influenced by the late magmatic Ta-rich fluids.Fluid inclusions microthermometry shows criteria of phase separation represented by both boiling and fluid immiscibility.The initial fluid was supposed to be of magmatic origin(magmatic CH4),that was consequently influenced by fluid mixing/dilution with post-hydrothermal/meteoric water with respect to the decompression process during uplift.Isochore construction gave rise to an estimate P-T conditions(T=330-370℃,P=22-50 MPa).The fluid inclusions’microthermometry supports a transition between magmatic and late to post-hydrothermal activities in addition to surface-derived fluid(meteoric fluid?)in a part as main source for the polymetallic deposits.展开更多
Bafia Group is part of the southernmost portion of the Central African Fold Belt (CAFB) in Cameroon. The geological feature of the group is characterized by the presence of metamorphic rocks in which tourmaline had be...Bafia Group is part of the southernmost portion of the Central African Fold Belt (CAFB) in Cameroon. The geological feature of the group is characterized by the presence of metamorphic rocks in which tourmaline had been recognized among accessory minerals. In the present study, attention is focus on the tourmaline bearing quartzite to the southeast of Kombé II. Structure refinement shows that tourmaline is a Fe-dravite with the formula X(Na<sub>0.95</sub>[]<sub>0.05</sub>)Y(Mg<sub>2.39</sub>Fe<sub>0.61</sub>)Z(Al<sub>5.10</sub>Mg<sub>0.90</sub>)(BO<sub>3</sub>)<sub>3</sub>T[Si<sub>6</sub>O<sub>18</sub>](OH)<sub>3</sub>[(O,OH)<sub>0.88</sub>F<sub>0.12</sub>]. The Fe-dravite is hosted in a Ca-poor quartzite, which is made up, in addition to quartz and tourmaline, of biotite and muscovite. The structure of the dravites shows a low vacancy at the X site, which militates for a crystallization of the tourmaline at a high temperature > 750℃. This is in agreement with previous work which shows that the metamorphic peak in the associated biotite gneiss reaches 825℃. The R1 value of 1.24% means that the crystal structure of the tourmalines is of high quality. The genetical link between gold mineralization and tourmaline should stimulate exploration interest in the study area.展开更多
Located southeast of the city of Ngaoundere(Adamawa Plateau)within the continental section of the Cameroon Volcanic Line(CVL),the village of Dibi has been the scene of numerous volcanic manifestations,dominated by thr...Located southeast of the city of Ngaoundere(Adamawa Plateau)within the continental section of the Cameroon Volcanic Line(CVL),the village of Dibi has been the scene of numerous volcanic manifestations,dominated by three types of dynamism(explosive phreatomagmatic,effusive,and Strombolian eruptions);whose rocks,however,are poorly studied.Here,the lavas emitted during those eruptions are analyzed to constrain their origin.Whole rock geochemistry revealed that these rocks consist of basanites and foidites with Mg#varying from 67.3%to 71.2%.They are porphyritic and mainly composed of olivine[(Fo77.28-87.28 in foidites)and(Fo78.92-87.04 in basanites)],diopside[(Wo51En35Fs14 in foidites)and(Wo50-46.18En34.8-41.29Fs10.41-15.92 in basanites)],plagioclase[(labradorite:Ab37.31-39.67An57.43-60.70Or1.99-2.28)in basanites],and alkali feldspar[(anorthoclase:Ab64.45-65.13An6.81-9.15Or26.87-27.08 in foidites and Ab60.92-64.11An8-8.47Or28.45-32.27 in basanite)].Accessory minerals include Al-chromite,Ferrian chromite,titanomagnetite,and apatite.The lavas are silica-undersaturated(SiO_(2):38.9-40.3 wt.%in foidites and SiO_(2):42.5-43.6 wt.%in basanites).They are within-plate basalts with similar features to Oceanic Island Basalt(OIB)magmas,affinity to High-μ(HIMU),Enriched type I(EM1),and recycled component.Their primary melts derived from low degrees of partial melting(3%-5%)of a garnet peridotite mantle source,as shown by the Nb/La(1.30-1.51),Zr/La(4.91-5.85),and Zr/Ce(2.49-2.88)ratios that are consistent with an OIB-like asthenospheric mantle source.Equilibration temperatures have been estimated from the clinopyroxene-liquid thermometer and range between 981.6˚C and 1051.6˚C,at pressures of 0.9-1.6 kbar.展开更多
The magmatic arcs in the north-west region of Pakistan comprises of numerous volcanic and plutonic bodies of different ages and compositions evolved during the subduction of the Neo- Tethys Ocean under the Eurasian su...The magmatic arcs in the north-west region of Pakistan comprises of numerous volcanic and plutonic bodies of different ages and compositions evolved during the subduction of the Neo- Tethys Ocean under the Eurasian supercontinent. This study focusses on the examination of the granitoids of the Kohistan batholith (a part of Kohistan-Ladakh Island Arc;KLIA) and the Khunjerab pluton, concentrating on their petrological traits, mineral chemistry, in-situ zircon U-Pb geochronology, and whole-rock major and trace element geochemistry. According to zircon U-Pb dating, the Kohistan batholith granitoid was emplaced around 91.7 ± 0.3 Ma, while zircons of the Khunjerab pluton yield ages of 106.4 ± 0.4 Ma and 106.4 ± 1.0 Ma. All the samples from both magmatic units have calcic to calc-alkaline (Na_(2)O + K_(2)O: 3.6–10.6 wt.% and SiO_(2): 60–73 wt.%), metaluminous to peraluminous properties (Aluminum Saturation Index (ASI): 0.9–1.2). Notably, Nb, Ta, and Ti show depletion, while large ion lithophile elements like Cs, Rb, and K have been enriched. Additionally, we find that SiO_(2) and P_(2)O_(5) have a negative correlation while Rb and Th have a positive correlation, which confirm an I-type arc magmatism. Together with the published literature, TEM analysis, and thermal modelling, our zircon U-Pb results point to a period of continuous magmatic activity from the Late Jurassic to the Late Cretaceous (between 150 Ma and 91 Ma) in the Kohistan Island arc region while the Khunjerab pluton (part of Karakorum block/Eurasian plate) experienced widespread magmatism around 120 Ma to 106 Ma. With SiO_(2) concentrations ranging from 67.5–73.3 wt.% and 60–71.4 wt.% and relatively low alkali (Na_(2)O + K_(2)O) contents between 3.6–10.6 wt.% and 5.1–7.4 wt.% in the Kohistan batholith and Khunjerab pluton respectively, showing clear signs of acidity. The whole rock as well as the mineral geochemical analysis and the elevated water contents (8–10 wt.% and 3.1–3.5 wt.%) inferred from amphibole and biotite chemistry respectively, indicates that the Kohistan batholith was most likely formed through partial melting of a (hydrous) magma originating from a more or less altered metasomatized mantle wedge. Likewise, the Khunjerab pluton whole rock geochemistry also indicates its origin through partial melting of magma originating from an altered metasomatized mantle wedge. This study also shows that both units are not only different in terms of the nature of magmatism but also in terms of their ages i.e., continental arc magmatism occurred in the Khunjerab (Karakoram) block in the middle Cretaceous (106 Ma) while island arc magmatism occurred on the Kohistan side in the late Cretaceous (91 Ma). Further, this study also investigate why multi-grain U-Pb zircon dating is necessary for studying magmatic rocks by using transmission electron microscopy.展开更多
文摘A highly-fractionated garnet-bearing muscovite granite represents the marginal granitic facies of the Abu-Diab multiphase pluton in the Central Eastern Desert of Egypt. New electron microprobe analyses(EMPA) and laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) data from garnets are reported, in order to constrain their origin and genesis. Garnet in the Abu-Diab host granite is euhedral to subhedral, generally homogeneous and, in rare cases, it shows weak zonation. The garnet contains appreciable amounts of MnO and FeO, with lesser amounts of MgO and CaO, yielding an end-member formula of Sps61–72Alm25–35Prp1–4Adr0–1. Moreover, it is depleted in large ion lithophile elements(LILE) with lower values of Ba, Nb and Sr relative to the primitive mantle. Additionally, it contains high concentrations of HREE and Y and their REE pattern shows strong negative Eu anomalies. The garnet was crystallized under relatively low temperature(646°C–591°C) and pressure(< 3 kbar) conditions. The textural and chemical features indicate that the garnet is magmatic in origin and is chemically similar to that from highly-fractionated A-type granite. It was probably formed at the expense of biotite in a highly-evolved MnO-rich magma and/or by hydroxyl complexing of Mn during the ascending fluid phases.
基金funded by the National Natural Science Foundation of China(Grant/Award Numbers:41272115,41572086)OeAD(Austria's Agency for Education and Internationalization).
文摘This study deals with unraveling the diagenesis-induced porosity evolution in a mixed clastic-carbonate sequence of the Middle Permian Indus Basin,Pakistan.Multiple data sets including outcrop,petrography,cathodoluminescence,scanning electron microscopy(SEM),mineralogy,and geochemical isotopic compositions were integrated to establish a link between porosity evolution and diagenesis.The spatial thickness and facies variations of the strata at outcrop scale are inherently controlled by the underlying bathymetry of the basin with deepening westward trend.The low values ofδ^(18)O of the target strata,relative to average values of the Permian carbonate,hints to diagenetic alteration in the strata.The data sets used in this study reveal modification of the strata in four environments,that is,i)early marine diagenesis indicated by micritization,pervasive dolomitization and isopachous fibrous cements,followed by ii)meteoric dissolution,and iii)shallow burial diagenetic processes including the precipitation of blocky cement,compaction of skeletal and non-skeletal allochems,and stylolites,and iv)a deep burial environment,characterized by pressure solution,and micro-fractures.The clastic intervals host subangular to subrounded quartz grains,floating textures,and almost complete absence of deleterious clay minerals,consequently resulting in the preservation of primary porosity.The primary porosity of carbonate intervals is preserved in the form of intercrystalline and intracrystalline porosity.The secondary porosity evolved through various diagenetic phases in the form of fractures and dissolution.The diagenetic solution mediated by organic matter in carbonates may have experienced both bacterial decomposition and thermochemical sulfate reduction,precipitating sulfides within the pores.The plug porosity/permeability analyses generally suggest high porosity in the siliciclastic unit,and carbonates with wackestone fabric while lower values were observed for the inner shelf pure carbonate facies.However,both intervals show very low permeability values probably due to isolated moldic pores and intense micritization.Therefore,clastic intervals may provide an opportunity to serve as a moderate reservoir;however,the carbonate intervals possess very low permeability values and could generally be considered as low-moderate reservoir potential.
基金Ministry of Higher Education of Egypt for supporting his research stay at University of Vienna(Austria)as a postdoctoral fellow。
文摘The rare metals of Abu Dabbab area in the Central Eastern Desert of Egypt have been investigated for their mineralogy and conditions of precipitation using combination of EMPA and fluid inclusions studies,in order to delineate the source,mechanism of formation and evolutionary model for these economic metals.The(Ta-Nb-Sn)-bearing minerals at the Abu Dabbab area include columbite group minerals(CGMs),wodginite and cassiterite.In both granitic intrusion and its enclosed quartz veins,most of zoned CGMs and cassiterite grains are commonly characterized by a well-developed twostage texture.Hence,columbite-(Mn)(CGM-Ⅰ)represents the early formed phase of CGMs that is characterized by high Mn#values(0.64-0.92)with quite low Ta#values(0.13-0.49).It was invaded by Ta-rich phases including tantalite-(Mn)(CGM-Ⅱ;Ta#=0.13-0.49)and wodginite,which contain high Ta_(2)O_(5)and SnO_(2)(17.91 wt.%).In regard to cassiterite,there are distinct compositional differences between the early-phase cassiterite(Cst-Ⅰ)and the late-phase one(Cst-Ⅱ),where the latter is enriched in Ta_(2)O_(5),Nb_(2)O_(5)and FeO.The chemistry and textural criteria of the early stage CGM-Ⅰand Cst-Ⅰ,all are indicative of magmatic origin.While,the latter CGM-Ⅱ,wodginite and Cst-Ⅱwere influenced by the late magmatic Ta-rich fluids.Fluid inclusions microthermometry shows criteria of phase separation represented by both boiling and fluid immiscibility.The initial fluid was supposed to be of magmatic origin(magmatic CH4),that was consequently influenced by fluid mixing/dilution with post-hydrothermal/meteoric water with respect to the decompression process during uplift.Isochore construction gave rise to an estimate P-T conditions(T=330-370℃,P=22-50 MPa).The fluid inclusions’microthermometry supports a transition between magmatic and late to post-hydrothermal activities in addition to surface-derived fluid(meteoric fluid?)in a part as main source for the polymetallic deposits.
文摘Bafia Group is part of the southernmost portion of the Central African Fold Belt (CAFB) in Cameroon. The geological feature of the group is characterized by the presence of metamorphic rocks in which tourmaline had been recognized among accessory minerals. In the present study, attention is focus on the tourmaline bearing quartzite to the southeast of Kombé II. Structure refinement shows that tourmaline is a Fe-dravite with the formula X(Na<sub>0.95</sub>[]<sub>0.05</sub>)Y(Mg<sub>2.39</sub>Fe<sub>0.61</sub>)Z(Al<sub>5.10</sub>Mg<sub>0.90</sub>)(BO<sub>3</sub>)<sub>3</sub>T[Si<sub>6</sub>O<sub>18</sub>](OH)<sub>3</sub>[(O,OH)<sub>0.88</sub>F<sub>0.12</sub>]. The Fe-dravite is hosted in a Ca-poor quartzite, which is made up, in addition to quartz and tourmaline, of biotite and muscovite. The structure of the dravites shows a low vacancy at the X site, which militates for a crystallization of the tourmaline at a high temperature > 750℃. This is in agreement with previous work which shows that the metamorphic peak in the associated biotite gneiss reaches 825℃. The R1 value of 1.24% means that the crystal structure of the tourmalines is of high quality. The genetical link between gold mineralization and tourmaline should stimulate exploration interest in the study area.
文摘Located southeast of the city of Ngaoundere(Adamawa Plateau)within the continental section of the Cameroon Volcanic Line(CVL),the village of Dibi has been the scene of numerous volcanic manifestations,dominated by three types of dynamism(explosive phreatomagmatic,effusive,and Strombolian eruptions);whose rocks,however,are poorly studied.Here,the lavas emitted during those eruptions are analyzed to constrain their origin.Whole rock geochemistry revealed that these rocks consist of basanites and foidites with Mg#varying from 67.3%to 71.2%.They are porphyritic and mainly composed of olivine[(Fo77.28-87.28 in foidites)and(Fo78.92-87.04 in basanites)],diopside[(Wo51En35Fs14 in foidites)and(Wo50-46.18En34.8-41.29Fs10.41-15.92 in basanites)],plagioclase[(labradorite:Ab37.31-39.67An57.43-60.70Or1.99-2.28)in basanites],and alkali feldspar[(anorthoclase:Ab64.45-65.13An6.81-9.15Or26.87-27.08 in foidites and Ab60.92-64.11An8-8.47Or28.45-32.27 in basanite)].Accessory minerals include Al-chromite,Ferrian chromite,titanomagnetite,and apatite.The lavas are silica-undersaturated(SiO_(2):38.9-40.3 wt.%in foidites and SiO_(2):42.5-43.6 wt.%in basanites).They are within-plate basalts with similar features to Oceanic Island Basalt(OIB)magmas,affinity to High-μ(HIMU),Enriched type I(EM1),and recycled component.Their primary melts derived from low degrees of partial melting(3%-5%)of a garnet peridotite mantle source,as shown by the Nb/La(1.30-1.51),Zr/La(4.91-5.85),and Zr/Ce(2.49-2.88)ratios that are consistent with an OIB-like asthenospheric mantle source.Equilibration temperatures have been estimated from the clinopyroxene-liquid thermometer and range between 981.6˚C and 1051.6˚C,at pressures of 0.9-1.6 kbar.
基金funded by the Higher Education Commission of Pakistan under the Faculty Development Program for the Khushal Khan Khattak University,Karak,Pakistan with grant No.386-388/PC/PAB/KKKUK/21Partial funds are provided by the University of Vienna,Austria.
文摘The magmatic arcs in the north-west region of Pakistan comprises of numerous volcanic and plutonic bodies of different ages and compositions evolved during the subduction of the Neo- Tethys Ocean under the Eurasian supercontinent. This study focusses on the examination of the granitoids of the Kohistan batholith (a part of Kohistan-Ladakh Island Arc;KLIA) and the Khunjerab pluton, concentrating on their petrological traits, mineral chemistry, in-situ zircon U-Pb geochronology, and whole-rock major and trace element geochemistry. According to zircon U-Pb dating, the Kohistan batholith granitoid was emplaced around 91.7 ± 0.3 Ma, while zircons of the Khunjerab pluton yield ages of 106.4 ± 0.4 Ma and 106.4 ± 1.0 Ma. All the samples from both magmatic units have calcic to calc-alkaline (Na_(2)O + K_(2)O: 3.6–10.6 wt.% and SiO_(2): 60–73 wt.%), metaluminous to peraluminous properties (Aluminum Saturation Index (ASI): 0.9–1.2). Notably, Nb, Ta, and Ti show depletion, while large ion lithophile elements like Cs, Rb, and K have been enriched. Additionally, we find that SiO_(2) and P_(2)O_(5) have a negative correlation while Rb and Th have a positive correlation, which confirm an I-type arc magmatism. Together with the published literature, TEM analysis, and thermal modelling, our zircon U-Pb results point to a period of continuous magmatic activity from the Late Jurassic to the Late Cretaceous (between 150 Ma and 91 Ma) in the Kohistan Island arc region while the Khunjerab pluton (part of Karakorum block/Eurasian plate) experienced widespread magmatism around 120 Ma to 106 Ma. With SiO_(2) concentrations ranging from 67.5–73.3 wt.% and 60–71.4 wt.% and relatively low alkali (Na_(2)O + K_(2)O) contents between 3.6–10.6 wt.% and 5.1–7.4 wt.% in the Kohistan batholith and Khunjerab pluton respectively, showing clear signs of acidity. The whole rock as well as the mineral geochemical analysis and the elevated water contents (8–10 wt.% and 3.1–3.5 wt.%) inferred from amphibole and biotite chemistry respectively, indicates that the Kohistan batholith was most likely formed through partial melting of a (hydrous) magma originating from a more or less altered metasomatized mantle wedge. Likewise, the Khunjerab pluton whole rock geochemistry also indicates its origin through partial melting of magma originating from an altered metasomatized mantle wedge. This study also shows that both units are not only different in terms of the nature of magmatism but also in terms of their ages i.e., continental arc magmatism occurred in the Khunjerab (Karakoram) block in the middle Cretaceous (106 Ma) while island arc magmatism occurred on the Kohistan side in the late Cretaceous (91 Ma). Further, this study also investigate why multi-grain U-Pb zircon dating is necessary for studying magmatic rocks by using transmission electron microscopy.
