Crystal fractionation of granitic magma during its non-transport processes： A physics-based perspective

Granitic continental crust distinguishes the Earth from other planets in the Solar System. Consequently, for understanding terrestrial continent development, it is of great significance to investigate the formation and evolution of granite.Crystal fractionation is one of principal magma evolution mechanisms. Nevertheless, it is controversial whether crystal fractionation can effectively proceed in felsic magma systems because of the high viscosity and non-Newtonian behavior associated with granitic magmas. In this paper, we focus on the physical processes and evaluate the role of crystal fractionation in the evolution of granitic magmas during non-transport processes, i.e., in magma chambers and after emplacement. Based on physical calculations and analyses, we suggest that general mineral particles can settle only at tiny speed(~10^(-9)–10^(-7) m s^(-1))in a granitic magma body due to high viscosity of the magma; however, the cumulating can be interrupted with convection in magma chambers, and the components of magma chambers will tend to be homogeneous. Magma convection ceases once the magma chamber develops into a mush(crystallinity, F>~40–50%). The interstitial melts can be extracted by hindered settling and compaction, accumulating gradually and forming a highly silicic melt layer. The high silica melts can further evolve into high-silica granite or high-silica rhyolite. At various crystallinities, multiple rejuvenation of the mush and the following magma intrusion may generate a granite complex with various components. While one special type of granites, represented by the South China lithium-and fluoride-rich granite, has lower viscosity and solidus relative to general granitic magmas, and may form vertical zonation in mineral-assemblage and composition through crystal fractionation. Similar fabrics in general intrusions that show various components on small lengthscales are not the result of gravitational settling. Rather, the flowage differentiation may play a key role. In general, granitic magma can undergo effective crystal fractionation; high-silica granite and volcanics with highly fractionated characteristics may be the products of crystal fractionation of felsic magmas, and many granitoids may be cumulates.

Source Enrichment Control on the Scale of Magmatic-Hydrothermal W-Sn Mineralization:Insights from Triassic and Jurassic Magma Reservoirs in the Continental Crust,Xitian,South China

There are two factors,source composition and magmatic differentiation,potentially controlling W-Sn mineralization.Which one is more important is widely debated and may need to be determined for each individual deposit.The Xitian granite batholith located in South China is a natural laboratory for investigating the above problem.It consists essentially of two separate components,formed in the Triassic at ca.226 Ma and Jurassic at ca.152 Ma,respectively.The Triassic and Jurassic rocks are both composed of porphyritic and fine-grained phases.The latter resulted from highlydifferentiated porphyritic ones but they have similar textural characteristics and mineral assemblages,indicating that they reached a similar degree of crystal fractionation.Although both fine-grained phases are highly differentiated with elevated rare metal contents,economic W–Sn mineralization is rare in the Triassic granitoids and this can be attributed to less fertile source materials than their Jurassic counterparts,with a slightly more enriched isotopic signature and whole-rockεNd(226 Ma)of−10.4 to−9.2(2σ=0.2)compared withεNd(152 Ma)of−9.2 to−8.2(2σ=0.2)for the Jurassic rocks.The initial W-Sn enrichment was derived from the metasedimentary rocks and strongly enhanced by reworking of the continental crust,culminating in the Jurassic.

Geochemistry of Micas from Issia Granite Complexe: A Marker of Geodynamic Evolution

The granites and pegmatites located in the southern part of the Issia region, near the columbo-tantaliferous placers, are characterized by the presence of rare metals such as beryl, lithium and Nb-Ta oxides. They mainly consist of micas, quartz, plagioclase and potassium feldspar. The work carried out on the micas of these granites and pegmatites (EPMA analyses) has provided new geochemical data contributing to the understanding of the magmatic evolution of the Issia granite complex. Mineralogically, the most evolved G3 granites are characterized by their abundance of muscovite compared to biotite and the presence of pegmatite veins. Geochemically, the muscovites of the G1 and G2 granites are more ferriferous than those of the G3 granites, however, the latter display higher Na contents than the G2 and G3. The muscovites of the granites show an evolution from the pure muscovite series to the zinnwaldite series (micas of the pegmatites) which are lithium-bearing micas. The mineralogical and chemical data of the micas show that they are S-type peraluminous granites and demonstrate the formation of granites and pegmatites through fractional crystallization of the same parental magma.

The influence of fractionation of REE-enriched minerals on the zircon partition coefficients

Zircon is widely used to simulate melt generation,migration and evolution within the crust and mantle.The achievable performance of melt modelling generally depends on the availability of reliable trace element partition coefficients(D).However,a large range of D_(REE) values for zircon from natural samples and experimental studies has been reported,with values spanning up to 3 orders of magnitude.Unfortunately,a gap of knowledge on this variability is evident.In this study we model the crystallization processes of common REE-bearing minerals from granitic melts and show that the measured zircon D_(REE) would be elevated if there is crystallization of REE-enriched minerals subsequent to zircon.Nevertheless,compared to zircon D_(REE) values measured from experimental studies,this mechanism appears to have a less significant influence on those from natural granite samples since the quantity of crystallized REE-enriched minerals is very low in natural magmatic systems and/or most of them crystallize prior to zircon.Combined with recently published studies,this work supports that analysis of natural zircon/host groundmass pairs provides more robust D_(REE) values applicable to natural systems than those measured from experimental studies,which can be used to constrain the provenance of detrital zircons.

Late Cretaceous Adakitic Granites of the Southeastern Tibetan Plateau: Garnet Fractional Crystallization of Arc-Like Magmas at the Thickened Neo-Tethyan Continental Margin

The tectonic setting of Cretaceous granitoids in the southeastern Tibet Plateau,east of the Eastern Himalaya Syntax,is debated.Exploration and mining of the Laba Mo–Cu porphyry-type deposit in the area has revealed Late Cretaceous granites.New and previously published zircon U–Pb dating indicate that the Laba granite crystallized at 89–85 Ma.Bulk-rock geochemistry,Sr–Nd isotopic data and in situ zircon Hf isotopic data indicate that the granite is adakitic and was formed by partial melting of thickened lower crust.The Ca,Fe,and Al contents decrease with increasing SiO2 content.These and other geochemical characteristics indicate that fractional crystallization of garnet under high-pressure conditions resulted in the adakitic nature of the Laba granite.Cretaceous granitoids are widespread throughout the Tibetan Plateau including its southeastern area,forming an intact curved belt along the southern margin of Eurasia.This belt is curved due to indenting by the Indian continent during Cenozoic,but strikes parallel to both the Indus–Yarlung suture zone and the Main Frontal Thrust belt.It is therefore likely that Cretaceous granitoids in both the Gangdese and southeastern Tibetan Plateau areas resulted from subduction of Neo-Tethyan lithosphere.

FRACTIONATED CRYSTALLIZATION OF HDPE IN PS/POE/HDPE/SBS BLENDS

The fractionated crystallization behavior of the minor dispersed HDPE phase in PS/POE/HDPE/SBS quaternary blends was investigated by differential scanning calorimetry (DSC).Interestingly,we found that the fractionated crystallization behavior of HDPE was molecular weight dependent.At a fixed composition,HDPE with lower molecular weight showed more obvious fractionated crystallization behavior than HDPE with higher molecular weight.This was ascribed to a finer dispersion of HDPE with lower molecular weight,a...

Formation of anorthosite on the Moon through magma ocean fractional crystallization

Lunar anorthosite is a major rock of the lunar highlands,which formed as a result of plagioclasefloatation in the lunar magma ocean（LMO）.Constraints on the sufficient conditions that resulted in the formation of a thick pure anorthosite（mode of plagioclase 〉95 vol.%） is a key to reveal the early magmatic evolution of the terrestrial planets.To form the pure lunar anorthosite,plagioclase should have separated from the magma ocean with low crystal fraction.Crystal networks of plagioclase and mafic minerals develop when the crystal fraction in the magma（φ） is higher than ca.40-60 vol.%,which inhibit the formation of pure anorthosite.In contrast,when φ is small,the magma ocean is highly turbulent,and plagioclase is likely to become entrained in the turbulent magma rather than separated from the melt.To determine the necessary conditions in which anorthosite forms from the LMO,this study adopted the energy criterion formulated by Solomatov.The composition of melt,temperature,and pressure when plagioclase crystallizes are constrained by using MELTS/pMELTS to calculate the density and viscosity of the melt.When plagioclase starts to crystallize,the Mg~# of melt becomes 0.59 at 1291 C.The density of the melt is smaller than that of plagioclase for P 〉 2.1 kbar（ca.50 km deep）,and the critical diameter of plagioclase to separate from the melt becomes larger than the typical crystal diameter of plagioclase（1.8-3 cm）.This suggests that plagioclase is likely entrained in the LMO just after the plagioclase starts to crystallize.When the Mg~# of melt becomes 0.54 at 1263 C,the density of melt becomes larger than that of plagioclase even for 0 kbar.When the Mg~# of melt decreases down to 0.46 at 1218 C,the critical diameter of plagioclase to separate from the melt becomes 1.5-2.5 cm,which is nearly equal to the typical plagioclase of the lunar anorthosite.This suggests that plagioclase could separate from the melt.One of the differences between the Earth and the Moon is the presence of water.If the terrestrial magma ocean was saturated with H_2O,plagioclase could not crystallize,and anorthosite could not form.

Was the Panzhihua Large Fe-Ti Oxide Deposit,SW China,Formed by Silicate Immiscibility?

The Panzhihua mafic intrusion,which hosts a world-class Fe-Ti-V ore deposit,is in the western Emeishan region,SW China.The formation age(~260 Ma),and Sr and Nd isotopes indicate that the Panzhihua intrusion is part of the Emeishan large igneous province and has little crustal contamination.To assess ore genesis of the Panzhihua Fe-Ti-V ore deposit,two different models have been provided to explain the formation,namely silicate immiscibility and normal fractional crystallization.Silicate immiscibility occurring around 1,000℃at the late stage of basaltic magma evolution argues against the silicate immiscibility model.Apatite-hosted melt inclusion research indicates that silicate immiscibility occurred at the late stage of Panzhihua magma evolution,which may not have offered potential to form such large ore deposits as Panzhihua.Alternatively,continuous compositional variations of the Panzhihua intrusion and calculations using thermodynamic modelling software support the hypothesis that the Panzhihua deposit was formed by normal fractional crystallization.Reciprocal trace element patterns of the Panzhihua intrusion and nearby felsic rocks also coincide with the fractional crystallization model.Normal fractional crystallization of high-Ti basaltic magma played a key role in the formation of the Panzhihua Fe-Ti-V ore deposit.

Petrogenesis of Oligocene volcanic rocks of the Lake Tana area,Ethiopian large Igneous Province

The Lake Tana area is located within a complex volcano-tectonic basin on the northwestern Ethiopian plateau.The basin is underlain by a thick succession of Oligocene transitional basalts and sub-alkaline rhyolites overlain in places,particularly south of the lake,by Quaternary alkaline to mildly transitional basalts,and dotted with Oligo-Miocene trachyte domes and plugs.This paper presents the results of integrated field,petrographic,and major and trace element geochemical studies of the Lake Tana area volcanic rocks,with particular emphasis on the Oligocene basalts and rhyolites.The studies reveal a clear petrogenetic link between the Oligocene basalts and rhyolites.The Oligocene basalts are:(1)plagioclase,olivine,and/or pyroxene phyric;(2)show an overall decreasing trend in MgO,Fe_(2)O_(3),and CaO with silica;(3)have relatively low Mg#,Ni and Cr contents and high Nb/La and Nb/Yb ratios;and(4)show LREE enriched and generally flat HREE patterns.All these imply the origin of the Oligocene basalts by shallow-level fractional crystallization of an enriched magma sourced at the asthenospheric mantle.The Oligocene rhyolites:(1)are enriched in incompatible while depleted in compatible trace elements,P and Ti;(2)show a strong negative Eu anomaly;(3)contain appreciable amounts of plagioclase,apatite,and Fe-Ti oxides;and(4)show clear geochemical similarity with well-constrained rhyolites from the Large Igneous Province(LIP)of the northwestern Ethiopian plateau.Low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers explains the origin of these rhyolites.Our study further shows that the Oligocene basalts and rhyolites are co-genetic and the felsic rocks of the Lake Tana area are related differentiates of the flood basalt volcanism in the northwestern Ethiopian plateau.

Geochemical and Sr-Nd-Pb isotopic compositions of volcanic rocks from the Iheya Ridge,the middle Okinawa Trough：implications for petrogenesis and a mantle source

As an active back-arc basin, the Okinawa Trough is located in the southeastern region of the East China Sea shelf and is strongly influenced by the subduction of the Philippine Sea Plate. Major element, trace element and Sr-NdPb isotopic composition data are presented for volcanic rocks from the Iheya Ridge（IR）, the middle Okinawa Trough. The IR rocks record large variations in major elements and range from basalts to rhyolites. Similar trace element distribution characteristics together with small variations in ^87Sr/^86Sr（0.703 862–0.704 884）, ^144Nd/^143Nd（0.512 763–0.512 880） and Pb isotopic ratios, demonstrate that the IR rocks are derived from a similar magma source. The fractional crystallization of olivine, clinopyroxene, plagioclase, and amphibole, as well as accessory minerals, can reasonably explain the compositional variations of these IR rocks. The simulations suggest that approximately 60% and 75% fractionation of an evolved basaltic magma can produce trace element compositions similar to those of the intermediate rocks and acid rocks, respectively. The analysis of their Sr-Nd-Pb isotopic content ratios suggest that the source of the rocks from the IR is close to the depleted mantle（DM） but extends to the enriched mantle（EMII）, indicating that the mantle source of these rocks is a mixture between the DM and EMII end members. The simulations show that the source of the IR volcanic rocks can be best interpreted as the result of the mixing of approximately 0.8%–2.0% subduction sediment components and 98.0%–99.2% mantlederived melts.

Origin of the Volcanic Rocks Erupted in the Eastern Manus Basin：Basaltic Andesite-Andesite-Dacite Associations

There has been much recent interest in the origin of intermediate lava and several hypotheses are: 1) direct melting of the mantle under water-saturated conditions, 2) partial melting of altered crust, 3) fractional crystallization of parent magma, and 4) magma mixing of mafic magmas with dacitic/rhyolitic magmas. Volcanic rocks series ranging from basaltic andesite to dacite(SiO_2 ranges from 53.8 to 63.0 wt.%) from the eastern Manus Basin were detected for major and trace element compositions to understand their origin. Low H_2O contents, positive correlations of La-SiO_2 and Yb-SiO_2, oxygen isotope data and the indistinct change of trace element concentrations in oscillatory zoning of plagioclase phenocrysts rule out the models in which silicic lava results from direct melting of hydrous mantle, partial melting of altered oceanic crust or gabbros, and magma mixing, respectively. Besides, the geochemical data of whole rock and melt inclusions indicate that fractional crystallization plays a dominant role in generating the intermediate lava with subduction features.

Glass and Mineral Chemistry of Northern Central Indian Ridge Basalts:Compositional Diversity and Petrogenetic Significance

The glass and mineral chemistry of basaits examined from the northern central Indian ridge （NCIR） provides an insight into magma genesis around the vicinity of two transform faults： Vityaz （VT） and Vema （VM）. The studied mid-ocean ridge basalts （MORBs） from the outer ridge flank （VT area） and a near-ridge seamount （VM area） reveal that they are moderately phyric plagioclase basalts composed of plagioclase （phenocryst [An60-90] and groundmass [An35-79]）, olivine （Fo81-88）, diopside （Wo45-51, En25-37, Fs14-24）, and titanomagnetite （FeOt -63.75 wt% and TiO2 -22.69 wt%）. The wholerock composition of these basalts has similar Mg# [mole Mg/mole（Mg＋Fe2＋）] （VT basalt： -0.56-0.58; VM basalt： -0.57）, but differ in their total alkali content （VT basalt： -2.65; VM basalt： -3.24）. The bulk composition of the magma was gradually depleted in MgO and enriched in FeOt, TiO2, P2O5, and Na2O with progressive fractionation, the basalts were gradually enriched in Y and Zr and depleted in Ni and Cr. In addition, the ∑REE of magma also increased with fractionation, without any change in the （La/ Yb）N value. Glass from the VM seamount shows more fractionated characters （Mg#： 0.56-0.57） compared to the outer ridge flank lava of the VT area （Mg#： 0.63-0.65）. This study concludes that present basalts experienced low-pressure crystallization at a relatively shallow depth. The geochemical changes in the NCIR magmas resulted from fractional crystallization at a shallow depth. As a consequence, spinel was the first mineral to crystallize at a pressure 〉10 kbar, followed by Fe-rich olivine at 〈10 kbar pressure.

Geochemical Characteristics of a Carbonatite Dyke Rich in Rare Earths from Bayan Obo, China

The whole-rock geochemistry of a rare earths rich carbonatite dykes that locates at Dulahala and lies 3 km north-east to the East Ore body of the giant Bayan Obo RE-Nb-Fe deposit was analysed. The dyke cuts cross H1 coarse quartz sandstone and H2 fine quartzite of the Proterozoic Bayan Obo group. RE content in the dyke varies greatly up to 20% (mass fraction), which comprises rich RE ores. Light RE in carbonatites are extremely enriched and strongly fractionated relative to heavy RE, but no Eu anomaly. The carbonatite may be produced by mechanisms as follows: the carbonatite mana is directly formed by very low degree (F <1%) partial melting of enriched lithospheric mantle, leaving residual minerals characterized by abundant garnet; then the magma arises into a chamber within the crust where they will undergo fractional crystallization, which makes RE further concentrated in carbonatite. The RE patterns and spider diagrams of the carbonatite are identical to those fine-grained dolomite marble that is the ore-host rock for the Bayan Obo deposit. However, the carbonatite is calcic, which is different from the fine-grained dolomite marble in major element geochemistry. The difference is suggested to be resulted from that the carbonatite dyke is not affected by a large scale dolomitization, while the fine-grained dolomite marble might be the product of dolomitized carbonatite intrusive body that might set up a hydrothermal system in the region, which transported Mg from the Bayan Obo sediments, especially form the shales to the carbonatite intrusion.

Accretion Processes of Oceanic Crust in Slow-spreading Ridges:Plagiogranite Perspective of the Xigaze Ophiolite,South Tibet

Structural and petrological data suggest that the Xigaze ophiolite from the Yarlung Zangbo Suture Zone(YZSZ)in south Tibet was a typical slow-spreading ridge.A new field,geochemical,mineral,and U-Pb zircon dataset of plagiogranite intrusions were used to constrain the dynamic processes of oceanic accretion in this slow-spreading ridge.Plagiogranites mainly occur as dykes or intrusions intruded into the whole sequence of the ophiolite and have a similar orientation to the dolerite dykes developed in the late stage of detachment faulting.U-Pb zircon ages of 122–123 Ma were obtained for two types of plagiogranites and associated dolerite dykes.Detailed geochemical and mineralogical examinations suggest that the plagiogranites are the product of low-pressure(2–3 kbar)fractional crystallization of midocean ridge basalt-like magma and unlikely to have been derived from the partial melting of hydrous gabbroic rocks.The complex cross-cut relationship between the plagiogranites and ophiolite sequence reflects that they are controlled by small discontinued melt lenses rather than a big magma chamber under the ridge axis and reveals multiple injections during the oceanic crust accretion.The formation of plagiogranites possibly reflects the complex characteristic of oceanic accretion at slow-spreading ridges,time-dependent on structural(external)and magmatic(internal)processes.

Discovery of Enclaves from Cenozoic PuluVolcanic Rocks in West Kunlun Mountains and ItsGeological Implications

In this paper, we present the occurrence and mineral components of the enclaves firstly discovered in the Cenozoic Pulu volcanic rocks in west Kunlun Mountains, and propose that the enclave is accumulated by fractional crystallization within high-level magma chamber. In addition, the chemical compositions of its primary magma are calculated. The calculated compositions are similar to those of the Kangxiwa volcanic rocks that belong to the same volcanic belt in the Pulu volcanic region, suggesting their origin from the same source region. However, the temperatures and oxygen fugacity of magmas at high-level magma chamber decreased along with fractional crystallization.

Mineralogy and Magmatic Processes of Cenozoic Intraplate Alkaline Volcanic Rocks of Bafang and Its Environs (Cameroon Volcanic Line, West Africa)

Alkaline basalts of Bafang and its environs are consisted of feldspars, olivines, pyroxenes and oxides which appear as phenocrysts, microphenocrysts and microcrysts. Feldspars are plagioclases (An67.97-15.84Ab69.19-30.43Or20.59-1.51) and anorthoclases (Ab68.11-61.20Or33.87-20.91An10.98-4.93). Plagioclases are the most abundant amount these feldspars. Anorthoclases appear only in mugearite (BAF 3 and BAF 37) the most differentiated of the studied alkaline-basalts. In High Magnesian basalt, (HMg-B) plagioclases are labradorites (An67.97-59.30Ab38.74-30.43Or2.75-1.60) and sanidine (An45.44-31.82Ab62.66-51.79Or5.52-2.77), whereas in Low Magnesian basalt (LMg-B) there are labrador (An67.4.75-51.96Ab44.98-33.72Or3.06-1.51), andesine (An45.44-31.82Ab62.66-51.79Or5.52-2.77), oligoclase (An26.65-15.84Ab69.19-63.57Or20.59-8.55) and anarthoclase (Ab68.11-61.20Or33.87-20.91An10.98-4.93). Olivines are magnesian (Fo86.7-50.1) and ferriferous (Fo48.8-37.8). In HMg-B, olivine are only magnesian. These olivines are chrysolites and hyalositerite. In LMg-B, olivines are magnesian and ferriferous with the predominance of ferriferous. They are chrysolites, hyalositerite and hortonolite. Pyroxenes are Ca, Mg and Fe clinopyroxenes. There are diopsides (Wo51.94-45.02En44.41-33.16Fs16.42-10.70) and augites (Wo44.88-43.64En41.03-37.04Fs18.25-14.43). Oxides are magnetites represented by ulvospinel (Usp90-75Sp2-7Mt5-23). Fractionation of ferromagnesian minerals (opaque oxide, olivine and pyroxene) is the main differentiation process. Two stages of fractional crystallization can be distinguished: the first stage comes with basanites and the second with hawaiites to mugearites. Chemical compositions of phenocrystals in studied basaltics lavas record signatures of magma recharge by pulsatory intrusions of new magma into the existing magma reservoir before the eruptions.

Geochemistry and petrogenesis of Rajahmundry trap basalts of Krishna-Godavari Basin,India

The Rajahmundry Trap Basalts （RTB） are erupted through fault-controlled fissures in the Krishna-Godavari Basin （K-G Basin） of Godavari Triple Junction, occurring as a unique outcrop sandwiched between Cretaceous and Tertiary sediments along the east coast of India. Detailed geochemical studies have revealed that RTB are mid-Ti （1.74-1.92） to high-Ti （2.04-2.81） basalts with a distinct quartz tholeiitic parentage. MgO （6.2-13.12 wt.%）, Mg# （29-50） and Zr （109-202 ppm） suggest that these basalts evolved by fractional crystallization during the ascent of the parent magma along deep-seated fractures. Mod- erate to high fractionation of HREE, as indicated by （Gd/Yb）N ratios （1.71-2.31） of RTB, suggest their generation through 3-5% melting of a Fe-rich mantle corresponding to the stability fields of spinel and garnet peridotite at depths of 60-100 km. Low K2O/P2O5 （0.26-1.26）, high TiO2/P2O5 （6.74-16.79）, La/Nb （0.89-1.45）, Nb/Th 〉 8 （8.35-13）, negative anomalies at Rb reflect minimum contamination by granitic continental crust. （Nb/La）PM ratios （0.66-1.1） of RTB are attributed to endogenic contamination resulted through recycling of subducted oceanic slab into the mantle. Pronounced Ba enrichment with relative depletion in Rb indicates assimilation of Infra- and Inter-trappean sediments of estuarine to shallow marine character. Geochemical compositions such as A1203/TiO2 （3.88-6.83）, medium to high TiO2 （1.74 -2.81 wt.%）, positive Nb anomalies and LREE enrichment of these RTB attest to their mantle plume origin and indicate the generation of parent magma from a plume-related enriched mantle source with EM 1 signature. Ba/Th （46-247）, Ba/La （3.96-28.51） and Th/Nb （0.08-0.13） ratios suggest that the source enrichment process was marked by recycling of subduction-processed oceanic crust and lithospheric components into the mantle. Zr/Hf （37-41） and Zr/Ba （0.51-3.24） indicate involvement of an asthenospheric mantle source. The Rajahmundry basalts show affinity towards FOZO （focal zone mantle） and PSCL （post-Archaean subcontinental lithosphere） which reflect mixing between asthenospheric and lithospheric mantle components in their source. Origin of RTB magma is attributed to plume-lithosphere interaction and the upward movement of melt is facilitated by intrabasinal deep-seated faults in the K-G Basin.

Geochemistry of Volcanic Rocks of Beka,North East of Ngaoundéré(Ad­amawa Plateau,Cameroon):Petrogenesis and Geodynamic Context

Beka area is situated in the Adamaoua Plateau of Cameroon in central Arica.Lavas in this area has not been studied before the present work.The volcanism of Beka is characterized by basalt,trachyte and phonolite domes and flows.The petrographic study shows that basaltic lavas have porphyritic microlitic textures.The felsic lavas indicate trachytic textures.The rocks are composed of olivine,clinopyroxene,plagioclase and iron-titanium oxide minerals for the basalts;clinopyroxene,alkali feldspar(including foids),sphene and titanomagnetite for the felsic lavas.Chemical analyses show that basaltic lavas are basanites.Felsic lavas contain modal feldspathoid(nepheline in phonolites).All these lavas belong to the same series,because the felsic lavas are derived from the differentiation of basaltic lavas by fractional crystallization.They show an alkaline nature according to their geochemistry.Trace elements including Rare Earth Elements characteristics show that rocks emplaced in the Winthin Plate volcanic zone.They derived from an evolved parent magma showing a low degree of partial melting and characteristics closer to a modified and evolved primitive spinel lherzolite.

Petrogenesis, LA-ICP-MS zircon U-Pb geochronology and geodynamic implications of the Kribi metavolcanic rocks,Nyong Group, Congo craton

Metavolcanic rocks are well-exposed in the Kribi area within the Nyong Group, Congo craton, but their origin, age, and tectonic significance are poorly known.Here we report integrated field mapping and petrography,geochemistry, and LA-ICP-MS zircon U-Pb ages of these metavolcanic rocks to constrain their petrogenesis and geodynamic implications. The studied rocks consist of mafic granulite, garnet-amphibole gneiss, and garnet-biotite gneiss, and occur interbanded with sharp contact and intruded by syenite dyke. These metavolcanic rocks are classified as MORB-like tholeiitic to calc-alkaline basalts,basaltic andesite, and rhyodacite rocks with within-plate setting geochemical signatures. The metabasite rocks(basalt to basaltic andesite protolith) are likely the equivalent of a spinel peridotite product representing ~ 2–5 %partial melting of metasomatized mantle source, while the metarhyodacite rocks are derived from the fractional crystallization of the same parental magma. Zircon U-Pb data revealed that the rhyodacite rocks initially formed at2671 ± 51 Ma and underwent later metamorphism at2065 ± 55 Ma. The Neoarchean protolith age is comparable to the ca. 2628 Ma tholeiitic magmatism and ca.2666 Ma high-K granites, suggesting bimodal Neoarchean magmatic event within the Ntem Complex, while the metamorphic ages fall within the ca. 2100–2000 Ma highgrade tectono-metamorphic event attributed to Eburnean/Trans-Amazonian orogeny. At the regional scale,metavolcanic rocks with similar origins and ages are documented in the Sa o Francisco Craton in Brazil, suggesting comparable geodynamic evolution on both sides of the south Atlantic during the Paleoproterozoic.

On the cratonization of the Arabian-Nubian Shield: Constraints from gneissic granitoids in south Eastern Desert, Egypt

The Shaitian granite complex(SGC)spans more than 80 Ma of crustal growth in the Arabian–Nubian Shield in southeast Egypt.It is a voluminous composite intrusion(60 km2)comprising a host tonalite massif intruded by subordinate dyke-like masses of trondhjemite,granodiorite and monzogranite.The host tonalite,in turn,encloses several,fine-grained amphibolite enclaves.U-Pb zircon dating indicates a wide range of crystallization ages within the SGC(800±18Ma for tonalites;754±3.9 Ma for trondhjemite;738±3.8 Ma for granodiorite;and 717±3.2 Ma for monzogranite),suggesting crystallization of independent magma pulses.The high positiveεNdi(+6–+8)indicate that the melting sources were dominated by juvenile material without any significant input from older crust.Application of zircon saturation geothermometry indicates increasing temperatures during the generation of melts from 745±31℃ for tonalite to 810±25℃ for trondhjemite;840±10℃ for granodiorite;and 868±10℃ for monzogranite.The pressure of partial melting is loosely constrained to be below the stability of residual garnet(<10 kbar)as inferred fromthe almost flat HREE pattern((Gd/Lu)N=0.9–1.1),but>3 kbar for the stability of residual amphibole as inferred from the significantly lower NbN and TaN compared with LREEN and the sub-chondrite Nb/Ta ratios exhibited by the granitic phases.The inverse relation between the generation temperatures and the ages estimates of the granitoid lithologies argue against a significant role of fractional crystallization.The major and trace element contents indicate the emplacement of the SGC within a subduction zone setting.It lacks distinctive features for melt derived from a subducted slab(e.g.high Sr/Y and high(La/Yb)N ratios),and the relatively low MgO and Ni contents in all granite phases within the SGC suggest melting within the lower crust of an island arc overlying a mantlewedge.Comparisonwith melts produced during melting experiments indicates an amphibolite of basaltic composition is the best candidate as source for the tonalite,trondhjemite and granodiorite magmas whereas the monzogranite magma is most consistent with fusion of a tonalite protolith.Given the overlapping Sm-Nd isotope ratios as well as several trace element ratios between monzogranite and tonalite samples,it is reasonable to suggest that the renewed basaltic underplating may have caused partialmelting of tonalite and the emplacement ofmonzogranite melt within the SGC.The emplacement of potassic granite(monzogranite)melts subsequent to the emplacement of Na-rich granites(tonalitetrondhjemite-granodiorite)most likely suggests major crustal thickening prior arc collision and amalgamation into the over thickened proto-crust of the Arabian-Nubian shield.Eventually,after complete consolidation,the whole SGC was subjected to regional deformation,most probably during accretion to the Saharan Metacraton(arc–continent collisions)in the late Cryogenian-Ediacaran times(650–542 Ma).