Petrochemical constrains on the origin and tectonic setting of mafic to intermediate dykes from Tikar plain, Central Cameroon Shear Zone
Benjamin NtiécheM. Ram MohanAmidou MoundiPauline Wokwenmendam NguetMahomed Aziz MounjouohouZakari NchouwetDaouda Mfepat
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Abstract The Tikar plain is located on the Cameroon Central Shear Zone. It is also part of the North Equatorial Pan-African Belt. It is formed of granitoids intruded in places by mafic and intermediate dykes. The mafic dykes are essentially banded gabbros composed of plagioclases, pyroxenes, amphiboles, biotites and opaques. Their textures range from porphyroblastic to porphyritic. The intermediate dykes are monzonites and monzodiorites and are characterized, respectively, by cataclastic and mylonitic textures. The minerals identified are amphiboles, potassium feldspar, pyroxenes, epidotes, sphenes and opaques. Seritization reaction is mostly present on the mafic and intermediate dykes, while chloritization is much more pronounced on the intermediate dykes. The Tikar plain dykes are high-k calc-alkaline to shoshonitic. They are characterized by low to moderate SiO 2 content (42.08–61.96 wt%), low to high TiO 2 (0.47–2 wt%) and low Ni (1.48–99.18 ppm) contents. The mafic dykes show fractional trends with negative anomalies of Zr, U and P and positive Rb, Ba, Ta, Pb and Sr in multi-element diagrams, while the intermediate dykes present negative anomalies of Nb, Ta, Zr, Sr P and Ti and relative positive anomalies of Rb, Ba and Pb. The rare-earth elements (REE) patterns show positive Eu anomalies for the mafic dykes and negative anomalies for the intermediate dykes. The REE spectrum of all the dykes shows enrichment in LREE with relatively flat HREE, which can indicate arc magmatism. In the Zr–Ti/100–Sr/2 diagram, the mafic dykes plot in the island arc tholeiite and calc-alkaline basalt fields. The Th, Nb and LREE concentrations indicate that the subducted lithosphere with crustal component contributed to generation of the intermediate dykes of the Tikar plain. The geochemical characteristics of the mafic to intermediate dykes suggest their derivation from a various degree of partial melting in the garnet spinel facies, probably between depths of 80 and 100 km. The collision between the Central African Fold Belt and the northern edge of the Congo craton resulting in crustal thickening, sub-crustal lithospheric delamination and upwelling of the asthenosphere may have been the principal process in the generation of the intermediate dykes in the Tikar plain. The magma for the mafic and intermediate dyke would have migrated through the faults network of the Central Cameroon Shear Zone before crystallizing in the granito-gneissic basement rocks.Keywords:
Amphibole
Porphyritic
Cataclastic rock
Abstract The Ghansura Felsic Dome (GFD) occurring in the Bathani volcano-sedimentary sequence was intruded by mafic magma during its evolution leading to magma mixing. In addition to the mafic and felsic rocks, a porphyritic intermediate rock occurs in the GFD. The study of this rock may significantly contribute toward understanding the magmatic evolution of the Ghansura dome. The porphyritic rock preserves several textures indicating its hybrid nature, i.e. that it is a product of mafic-felsic magma mixing. Here, we aim to explain the origin of the intermediate rock with the help of textural features and mineral compositions. Monomineralic aggregates or glomerocrysts of plagioclase give the rock its characteristic porphyritic appearance. The fact that the plagioclase crystals constituting the glomerocrysts are joined along prominent euhedral crystal faces suggests the role of synneusis in the formation of the glomerocrysts. The compositions of the glomerocryst plagioclases are similar to those of plagioclases in the mafic rocks. The results from this study indicate that the porphyritic intermediate rock formed by the mixing of a crystal-rich mafic magma and a crystal-poor felsic melt.
Felsic
Porphyritic
Igneous differentiation
Magma chamber
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Cataclastic rock
Mylonite
Dharwar Craton
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Mylonite
Cataclastic rock
Greenschist
Shearing (physics)
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Porphyritic
Igneous differentiation
Hornblende
Amphibole
Magma chamber
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Cataclastic rock
Brittleness
Pure shear
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Abstract Southwest of the Middle Proterozoic Åland rapakivi batholith (1575 Ma) there occur dolerites closely associated with quartz-feldspar porphyries. The dolerites and the quartz-feldspar porphyries were formed during a bimodal igneous event characterizing the initial stages of the rapakivi intrusion. A coarse-porphyritic porphyry carries abundant mafic and hybrid enclaves interpreted as resulting from simultaneous intrusion and mixing of basaltic (dolerite) and granitic (porphyry) melts. The fine-grained mafic enclaves are a Fe-enriched differentiation product of the doleritic melts that intruded the country rock. Mixing tests indicate that the hybrid enclaves are a result of magma mixing between melts now represented by the mafic enclaves and the coarse-porphyritic porphyries. Discrepancies resulting from mixing tests at three localities within the porphyry are explained by variations in the degree of homogenization that are due to the incorporation into the hybrids of early phenocrysts formed from the two end member melts. The coarse-porphyritic porphyry also has petrographical features such as tiny mafic enclaves, labradorite xenocrysts and quartz ocelli, which indicate that the composition of the porphyry itself had been to some extent influenced by mixing. It is suggested that the incorporation of the tiny mafic enclaves and labradorite xenocrysts into the porphyry occurred during the very earliest stage of the bimodal intrusion event, when the mafic melt interacted with a porphyry melt newly formed by anatexis. New surges of mafic melt chilling against acid melt generated the fine-grained mafic enclaves. The hybrid magma, now represented by the hybrid enclaves, was formed by mixing of the contrasting magmas.
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Igneous differentiation
Phenocryst
Magma chamber
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The Hercynian, post-collisional Karkonosze pluton contains several lithologies: equigranular and porphyritic granites, hybrid quartz diorites and granodiorites, microgranular magmatic enclaves, and composite and lamprophyre dykes. Field relationships, mineralogy and major- and trace-element geochemistry show that: (1) the equigranular granite is differentiated and evolved by small degrees of fractional crystallization and that it is free of contamination by mafic magma; (2) all other components are affected by mixing. The end-members of the mixing process were a porphyritic granite and a mafic lamprophyre. The degree of mixing varied widely depending on both place and time. All of the processes involved are assessed quantitatively with the following conclusions. Most of the pluton was affected by mixing, implying that huge volumes (>75 km3) of mafic magma were available. This mafic magma probably supplied the additional heat necessary to initiate crustal melting; part of this heat could have also been released as latent heat of crystallization. Only a very small part of the Karkonosze granite escaped interaction with mafic magma, specifically the equigranular granite and a subordinate part of the porphyritic granite. Minerals from these facies are compositionally homogeneous and/or normally zoned, which, together with geochemical modelling, indicates that they evolved by small degrees of fractional crystallization (<20%). Accessory minerals played an important role during magmatic differentiation and, thus, the fractional crystallization history is better recorded by trace rather than by major elements. The interactions between mafic and felsic magmas reflect their viscosity contrast. With increasing viscosity contrast, the magmatic relationships change from homogeneous, hybrid quartz diorites–granodiorites, to rounded magmatic enclaves, to composite dykes and finally to dykes with chilled margins. These relationships indicate that injection of mafic magma into the granite took place over the whole crystallization history. Consequently, a long-lived mafic source coexisted together with the granite magma. Mafic magmas were derived either directly from the mantle or via one or more crustal storage reservoirs. Compatible element abundances (e.g. Ni) show that the mafic magmas that interacted with the granite were progressively poorer in Ni in the order hybrid quartz diorites—granodiorites—enclaves—composite dykes. This indicates that the felsic and mafic magmas evolved independently, which, in the case of the Karkonosze granite, favours a deep-seated magma chamber rather than a continuous flux from mantle. Two magma sources (mantle and crust) coexisted, and melted almost contemporaneously; the two reservoirs evolved independently by fractional crystallization. However, mafic magma was continuously being intruded into the crystallizing granite, with more or less complete mixing. Several lines of evidence (e.g. magmatic flux structures, incorporation of granite feldspars into mafic magma, feldspar zoning with fluctuating trace element patterns reflecting rapid changes in magma composition) indicate that, during its emplacement and crystallization, the granite body was affected by strong internal movements. These would favour more complete and efficient mixing. The systematic spatial–temporal association of lamprophyres with crustal magmas is interpreted as indicating that their mantle source is a fertile peridotite, possibly enriched (metasomatized) by earlier subduction processes.
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Felsic
Fractional crystallization (geology)
Igneous differentiation
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New insights into the role of amphibole in arc magma petrogenesis are provided by the mineral chemistry and U–Pb geochronology of Cretaceous amphibole-rich mafic rocks and associated granitoids from Shikanoshima Island (Kyushu, Japan). In the northeastern part of Shikanoshima Island a relatively large body (about 600 m in length) of amphibole-rich mafic rocks is found within granodiorite host-rocks. The core of the mafic body consists of amphibole-rich gabbrodiorite with a porphyritic texture. Towards the host granodiorite the porphyritic texture is progressively lost and a band of relatively homogeneous medium- to fine-grained mafic rock marks the boundary with the granitoid rocks. The amphibole-rich porphyritic gabbrodiorite consists of large amphibole grains (up to 60 vol. %) characterized by brown cores, occasional inclusions of clinopyroxene, and green rims. These large amphibole grains are dispersed in a fine-grained matrix consisting of green amphibole, clinopyroxene and plagioclase. Literature whole-rock data on the mafic rocks from Shikanoshima Island suggest that they are the intrusive counterparts of high-Mg andesite (HMA). Major and trace element mineral compositions reveal a marked chemical contrast between the brown amphibole (and its inclusions) and the matrix minerals, suggesting that they are not on the same liquid line of descent. The brown amphibole and its clinopyroxene inclusions were inherited from amphibole-rich ultramafic intrusive crustal rocks (e.g. hornblendites) crystallized from a melt with a chemical composition close to that of continental arc basalts. U–Pb geochronological data suggest that the xenocrystic material is about 20 Myr older than the matrix minerals. The matrix mineral crystallized from a parental liquid similar to sanukite-type HMA and with a trace element signature characterized by strong enrichment in elements with high crustal affinity and depletion in heavy rare earth elements. Green amphibole is a common mineral in all the studied lithologies; this allowed us to monitor the compositional variations in the liquid from which it crystallized moving from the core of the mafic complex to the host granodiorite. The data reveal that the interstitial melt had interacted with a melt enriched in elements with a high crustal affinity that, given the close association in the field, is inferred to be the host granitoid. These results favour an origin for sanukite-type HMA not from primary mantle melts but from mantle melts that have been affected by crustal processes and have been contaminated by crustal material. The major and trace element composition of the brown amphibole from the Shikanoshima Island mafic rocks is compared with that of brown amphibole from other amphibolite-rich intrusive rocks in orogenic settings worldwide (Alpine chain and Ross Orogen). The observed similarities suggest that the amphibole-rich mafic rocks are the expression of a magmatic process with a common geochemical affinity that is independent of the age and local geodynamic setting and thus related to a specific petrogenetic process. Amphibole-rich mafic and ultramafic intrusive rocks could be a common feature of all collisional systems and thus represent a ‘hidden’ amphibole reservoir in the arc crust. We show that amphibole plays a major role in the petrogenesis of sanukite-type HMA but is also expected to play a major role in the differentiation of many other arc magmas.
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Ultramafic rock
Petrogenesis
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The Qianlishan complex, located in Hunan Province of South China, is closely associated with intense W-dominated polymetallic mineralization. The Qianlishan complex is composed of three phases: the main-phase porphyritic and equigranular granites, granite porphyry, and mafic dykes. Geochronologically, the zircon U-Pb dating results show that the porphyritic and equigranular granites have ages of approximately 159 and 158 Ma, respectively, similar to those of mafic dykes (approximately 158 Ma), while the granite porphyry was formed later at approximately 145 Ma. Geochemically, the mafic dykes are characterized by calc-alkaline high-Mg andesite (HMA) with high MgO, TiO2, Mg#, and CA/TH index. They exhibit significantly depleted εNd(t) and εHf(t) with high Ba/La, La/Nb, and (La/Yb)N, indicating that they formed from mixing melts of depleted asthenospheric mantle and metasomatized subcontinental lithospheric mantle (SCLM). The main-phase granites are peraluminous and are characterized by high SiO2, low (La/Yb)N ratios, and relative depletion in Ba, Sr, Ti, and Eu. They also display negative correlations between La, Ce, Y, and Rb contents, suggesting that they are highly fractionated S-type granites. Furthermore, they show high εNd(t) and εHf(t), CaO/Na2O ratios, HREE, and Y contents, indicating that they were produced by parental melting of ancient basement mixed with mantle-derived components. In contrast, the granite porphyry shows A-type signature granites, with higher εNd(t) and εHf(t) and CaO/Na2O ratios than the main-phase granites but similar Zr/Nb and Zr/Hf ratios to the mafic dykes, suggesting that they are the products of partial melting of a hybrid source with ancient basement and the mafic dykes. We thus infer that the slab roll-back led to generation of Qianlishan back-arc basalt and HMA and further triggered the formation of the Qianlishan granite.
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