scholarly journals Petrochemical constrains on the origin and tectonic setting of mafic to intermediate dykes from Tikar plain, Central Cameroon Shear Zone

2021 ◽  
Vol 3 (2) ◽  
Author(s):  
Benjamin Ntieche ◽  
M. Ram Mohan ◽  
Amidou Moundi ◽  
Pauline Wokwenmendam Nguet ◽  
Mahomed Aziz Mounjouohou ◽  
...  
Keyword(s):  
Shear Zone ◽  
Tectonic Setting ◽  
Potassium Feldspar ◽  
Crustal Thickening ◽  
Alkaline Basalt ◽  
Calc Alkaline ◽  
Mafic Dykes ◽  
The North ◽  
Basement Rocks ◽  
Northern Edge

AbstractThe 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 SiO2 content (42.08–61.96 wt%), low to high TiO2 (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.

Lower Palaeozoic and Precambrian igneous rocks from eastern England, and their bearing on late Ordovician closure of the Tornquist Sea: constraints from U-Pb and Nd isotopes

1993 ◽  
Vol 130 (6) ◽  
pp. 835-846 ◽  
Author(s):  
S. R. Noble ◽  
R. D. Tucker ◽  
T. C. Pharaoh
Keyword(s):  
Volcanic Rocks ◽  
Igneous Rocks ◽  
Calc Alkaline ◽  
Nd Isotopes ◽  
Nd Isotope ◽  
Magmatic Arc ◽  
The North ◽  
Basement Rocks ◽  
Continental Arc ◽  
Lower Palaeozoic

AbstractThe U-Pb isotope ages and Nd isotope characteristics of asuite of igneous rocks from the basement of eastern England show that Ordovician calc-alkaline igneous rocks are tectonically interleaved with late Precambrian volcanic rocks distinct from Precambrian rocks exposed in southern Britain. New U-Pb ages for the North Creake tuff (zircon, 449±13 Ma), Moorby Microgranite (zircon, 457 ± 20 Ma), and the Nuneaton lamprophyre (zircon and baddeleyite, 442 ± 3 Ma) confirm the presence ofan Ordovician magmatic arc. Tectonically interleaved Precambrian volcanic rocks within this arc are verified by new U-Pb zircon ages for tuffs at Glinton (612 ± 21 Ma) and Orton (616 ± 6 Ma). Initial εNd values for these basement rocks range from +4 to - 6, consistent with generation of both c. 615 Ma and c. 450 Ma groups of rocksin continental arc settings. The U-Pb and Sm-Nd isotope data support arguments for an Ordovician fold/thrust belt extending from England to Belgium, and that the Ordovician calc-alkaline rocks formed in response to subductionof Tornquist Sea oceanic crust beneath Avalonia.

An early Proterozoic volcanic arc succession in southeastern Wyoming

1984 ◽  
Vol 21 (4) ◽  
pp. 415-427 ◽  
Author(s):  
Kent C. Condie ◽  
Craig A. Shadel
Keyword(s):  
Fractional Crystallization ◽  
Tectonic Setting ◽  
Volcanic Rocks ◽  
Geochemical Data ◽  
Alkaline Basalt ◽  
Calc Alkaline ◽  
Early Proterozoic ◽  
Enriched Mantle ◽  
Sierra Madre ◽  
Felsic Volcanic

The Green Mountain Formation of early Proterozoic age in the Sierra Madre Range of southeastern Wyoming comprises a bimodal mafic and felsic volcanic assemblage. The rocks, which are chiefly breccias, agglomerates, flows, and volcaniclastic sediments, represent both subaerial and submarine eruptions and in part were redeposited in fluvial and nearshore marine environments. Volcanic rocks are clearly calc-alkaline in character and share a large number of geochemical features in common with continental-margin arcs or evolved oceanic-arc systems.The low Mg numbers and Ni contents of the basalts require 30–40% olivine fractional crystallization, and the high contents of the most incompatible elements, high (La/Sm)N ratios, and low Zr/Nb ratios require an undepleted or enriched mantle source. Geochemical data are consistent with an origin for the felsic volcanics and associated Encampment River granodiorite by shallow fractional crystallization of calc-alkaline basalt in a tectonic setting similar to modern arc systems. The near absence of andesites may reflect the retention of andesitic magma in crustal reservoirs during fractional cyrstallization.

Late Jurassic - Early Cretaceous paleoenvironmental evolution of the Transbaikal basins (SE Siberia): implications for the Mongol-Okhotsk orogeny

2017 ◽  
Vol 188 (1-2) ◽  
pp. 9 ◽  
Author(s):  
Marc Jolivet ◽  
Anastasia Arzhannikova ◽  
Andrei Frolov ◽  
Sergei Arzhannikov ◽  
Natalia Kulagina ◽  
...  
Keyword(s):  
Early Cretaceous ◽  
Middle Jurassic ◽  
Late Jurassic ◽  
Tectonic Setting ◽  
Alluvial Fan ◽  
Crustal Thickening ◽  
Compressive Deformation ◽  
Transbaikal Region ◽  
Meandering River ◽  
The North

The Late Jurassic - Early Cretaceous tectonic evolution of SE Siberia was marked by the closure of the Mongol-Okhotsk ocean. While this geodynamic event led to compressive deformation and denudation in a wide area encompassing the North-Altay, Sayan and Baikal Patom ranges, it was contemporaneous to widespread extension from the Transbaikal region situated immediately north of the suture zone to the Pacific plate, affecting eastern Mongolia and northeastern China. In this study we review the paleontological and sedimentological data available in the Russian literature and provide new macro-floral and palynological data from the Mesozoic sediments of three Transbaikal basins. These data are used to describe the paleoenvironmental and paleoclimatic evolution of the Transbaikal area in order to assess the topographic evolution of the region in relation with the closure of the Mongol-Okhotsk ocean. We establish that the Transbaikal basins evolved in a continuously extensional tectonic setting from at least the Early-Middle Jurassic to the Early Cretaceous. The associated sedimentary environments are characterized by retrogradation from alluvial fan–braided river dominated systems prevailing during the Early to Middle Jurassic initial opening of the basins to meandering river– lacustrine systems that developed during the Late Jurassic - Early Cretaceous interval. No evidence of high relief topography was found and we conclude that, while compression and denudation occurred in the North Altai, Sayan and Patom ranges, in the Transbaikal region, the docking of the Mongolia-North China continent to Siberia was a “soft collision” event, possibly involving a major strike-slip displacement that did not lead to an orogenic event implying strong compressive deformation, crustal thickening and topography building.

Geology, geochronology, and fluid inclusion studies of the Xiaorequanzi volcanogenic massive sulphide Cu–Zn deposit in the East Tianshan Terrane, China

2020 ◽  
Vol 57 (12) ◽  
pp. 1392-1410 ◽  
Author(s):  
Xi-Heng He ◽  
Xiao-Hua Deng ◽  
Leon Bagas ◽  
Jing Zhang ◽  
Chao Li ◽  
...  
Keyword(s):  
Fluid Inclusion ◽  
Fluid Inclusions ◽  
Tectonic Setting ◽  
Volcanic Rocks ◽  
Massive Sulphide ◽  
Calc Alkaline ◽  
The North ◽  
Volcanogenic Massive Sulphide ◽  
Secondary Fluid ◽  
East Tianshan

The Xiaorequanzi Cu–Zn deposit is in the westernmost part of East Tianshan Terrane in northwestern China. The deposit is unique in the region being a volcanogenic massive sulphide (VMS) deposit located near a zone (or belt) containing giant late Paleozoic porphyry Cu deposits. Aiming to better understand the genesis of the mineral deposits in the terrane and their tectonic setting, we report our findings of detailed studies on fluid inclusion microthermometry, Re–Os dating of chalcopyrite from the massive ore, and U–Pb dating of zircons from the host volcanic rocks. There are two sulphide stages with early pyrite succeeded by chalcopyrite–sphalerite, which are hydrothermally overprinted and supergene enriched. The hydrothermal overprinting is characterised by quartz–sulphide veins crossed by carbonate-rich quartz veins. Quartz from the chalcopyrite–sphalerite stage is characterised by primary fluid inclusions containing H2O–NaCl(–CO2) and homogenise at 228–392 °C with a salinity of 2.2–13.3 wt.% NaCl equiv. Secondary fluid inclusions related to the hydrothermal overprinting homogenise at 170–205 °C with a salinity of 2.7–12.1 wt.% NaCl equiv. Fluid inclusions in the quartz–sulphide stage of the hydrothermal overprinting contain H2O–NaCl with homogenisation temperatures of 164–281 °C and salinities in ranging from 2.9 to 12.4 wt.% NaCl equiv. Fluid inclusion in the quartz–calcite stage contain H2O–NaCl with homogenisation temperatures of 122–204 °C with salinities of 1.4–12.4 wt.% NaCl equiv. These characteristics are like those of the secondary fluid inclusions in the VMS mineralisation. Combining these findings with H–O isotopic data from previous studies, we propose that the primary mineralising fluid is magmatic in origin. Tuff hosting the mineralisation yields a SHRIMP U–Pb zircon age of 352 ± 5 Ma, which is interpreted as the age of the tuff, and a porphyritic felsite dyke intruding the tuff yields a SHRIMP U–Pb zircon date of 345 ± 6 Ma, interpreted as the emplacement age of the dyke. Chalcopyrite from the main orebody at Xiaorequanzi yields a Re–Os isochron age of 336 ± 13 Ma with an initial 187Os/188Os ratio of 0.25 ± 0.55 (MSWD = 12). Given that the VMS deposit is a syngenetic deposit, we regard the upper ca. 349 Ma limit of the Re–Os date as the approximate age of the chalcopyrite. The three dates are the same within error, and the upper limit of the Re–Os date of ca. 349 is taken as the age of the volcanic, dyke, and mineralisation. The volcanic rocks around the Xiaorequanzi deposit have been previously classified as calc–alkaline to high-K calc–alkaline enriched in large-ion lithophile elements and depleted in high-field-strength elements, which are characteristics indicative of a forearc setting. It is suggested that VMS mineralisation formed in a forearc setting related to the north-directed subduction of the Palaeo-Kangguer or North Tianshan oceanic plates.

The Silurian(?) Passamaquoddy Bay mafic dyke swarm, New Brunswick: petrogenesis and tectonic implications

2001 ◽  
Vol 38 (11) ◽  
pp. 1565-1578 ◽  
Author(s):  
Nancy A Van Wagoner ◽  
Matthew I Leybourne ◽  
Kelsie A Dadd ◽  
Miranda LA Huskins
Keyword(s):  
New Brunswick ◽  
Incompatible Element ◽  
Tectonic Setting ◽  
Crustal Contamination ◽  
Dyke Swarm ◽  
Mafic Dyke ◽  
Mafic Dykes ◽  
The North ◽  
Passamaquoddy Bay ◽  
Mafic Dyke Swarm

The volcanic and sedimentary rocks of the Passamaquoddy Bay (PB) area of southeastern New Brunswick are part of the Silurian–Devonian Coastal Volcanic Belt (CVB), an extensive belt of bimodal volcanic rocks. The PB sequence is 4 km thick, has four cycles of mafic and felsic volcanism, and is intruded by mafic dykes at all levels. There are two ages of dykes, those related to the Late Silurian PB volcanism (PB dykes) and Mesozoic dykes (the Minister Island Dyke) related to the opening of the North Atlantic. The PB mafic dykes are subalkalic basalt to basaltic andesite, within-plate tholeiites. The dykes are moderately to highly evolved (Mg# = 66.6 to 26.6), with trends of major and trace elements typical of the fractionation of olivine, pyroxene, plagioclase, and ilmenite. The PB mafic dyke swarm comprises over 155 dykes which represent a greater range of compositions than the associated flows, suggesting that they give a more complete representation of the Late Silurian PB mafic magmas. They exhibit incompatible element characteristics best accounted for by crustal contamination. The dykes plot on a linear array away from mantle mixing lines between depleted and enriched mantle sources and toward the composition of the PB felsic units, suggesting that these felsic units are representative of partial melts and fractionates of the source contaminate. The variable TiO2 contents (1.2–4.3 wt.%) and incompatible element ratio trends plotted against a fractionation index suggest that mantle metasomatism, either fluid or melt derived, may also have influenced the mantle source of the dykes. The dykes dip steeply and have a relatively consistent strike to the north. Most dykes range in thickness from 0.5 to 2 m, but range up to 9 m. The single orientation of the dykes, along with their chemical characteristics and volume, and association with a bimodal intraplate volcanic sequence, are consistent with an extensional tectonic setting. Constraints of the regional geology suggest that this extension was associated with convergence, perhaps in a back-arc setting.

Eocene Arc-related Magmatism within the Neotethys Subduction System in NW Iran; Geochemistry and SHRIMP zircon U-Pb Dating on Magmatic Phases in the Shahjahan Batholith

2021 ◽  
Author(s):  
Zahra Ahmadi ◽  
Ahmad Jahangiri ◽  
Mohssen Moazzen ◽  
Chang Whan oh
Keyword(s):  
Tectonic Setting ◽  
Crustal Thickening ◽  
Spinel Lherzolite ◽  
Calc Alkaline ◽  
Nw Iran ◽  
Neotethys Ocean ◽  
High K ◽  
Felsic Dikes ◽  
Spider Diagrams ◽  
Shrimp Zircon

Abstract Granitoids of the composite Shahjahan batholith in the northernmost part of the Urmia-Dokhtar magmatic arc of Iran, and southernmost of the Lesser Caucasus (South Armenia) show SHRIMP zircon ages of 37.1±1.2 to 47.1±4.5 Ma. Dioritic rocks of the pluton with an age of 46.6 ± 4.6 to 47.1 ± 4.5 Ma are calk-alkaline to high-K calc-alkaline, metaluminous and I-type. They show arc-related affinities, characterized by LREE and LILE enrichment and HREE and HFSE depletion, especially negative Ti, Nb and Ta anomalies (TNT effect) in the normalized spider diagrams. low Ce/Pb, Nb/La and high Ba/Nb, U/Th and Hf/Zr ratios along with positive Pb, K, Th and Sr anomalies in the normalized spider diagrams for the studied samples are compatible with magma contamination with crustal materials during ascend to the lower crustal levels. Felsic dikes with granodiorite and syenite compositions and 37.1 ± 1.2 to 38.57 ± 0.41 Ma old, are characterized by high-K calc-alkaline to shoshonitic, metaluminous, and A2- type affinities which show post-collision tectonic setting geochemical features. The REE patterns for all studied samples and the composition of the trace element ratios indicate a geochemically enriched spinel-lherzolite lithospheric mantle source for the magmas, which underwent a low degree of partial melting. Dating arc-related dioritic samples and post collision felsic dikes put constrain on timing of Neotethys Ocean closure in NW Iran. Based on the present study, Middle to Upper Eocene is suggested as closure time of the Neotethys Ocean, Arabia and Central Iran plates’ collision and crustal thickening in Northwest Iran.

Neoproterozoic extensional detachment in central Madagascar: implications for the collapse of the East African Orogen

2000 ◽  
Vol 137 (1) ◽  
pp. 39-51 ◽  
Author(s):  
ALAN S. COLLINS ◽  
THEODORE RAZAKAMANANA ◽  
BRIAN F. WINDLEY
Keyword(s):  
Shear Zone ◽  
Tectonic Evolution ◽  
Structural Phase ◽  
East African ◽  
Metasedimentary Rocks ◽  
The North ◽  
Basement Rocks ◽  
Granitic Magmatism ◽  
Shear Sense ◽  
East African Orogen

A laterally extensive, Neoproterozoic extensional detachment (the Betsileo shear zone) is recognized in central Madagascar separating the Itremo sheet (consisting of Palaeoproterozoic to Mesoproterozoic sediments and underlying basement rocks) from the Antananarivo block (Archaean/Palaeoproterozoic crust re-metamorphosed in the Neoproterozoic). Non-coaxial deformation gradually increases to a maximum at a lithological contrast between the granitoids and gneisses of the footwall and the metasedimentary rocks of the hangingwall. Ultramylonites at this highest-strained zone show mineral-elongation lineations that plunge to the southwest.σ-, δ- and C/S-type fabrics imply top-to-the-southwest extensional shear sense. Contrasting metamorphic grades are found either side of the shear zone. In the north, where this contrast is greatest, amphibolite-grade footwall rocks are juxtaposed with lower-greenschist-grade hangingwall rocks. The metamorphic grade in the hangingwall increases to the south, suggesting that a crustal section is preserved.The Betsileo shear zone facilitated crustal-scale extensional collapse of the East African Orogeny, and thus represents a previously poorly recognized structural phase in the story of Gondwanan amalgamation. Granitic magmatism and granulite/amphibolite-grade metamorphism in the footwall are all associated with formation of the Betsileo shear zone, making recognition of this detachment important in any attempt to understand the tectonic evolution of central Gondwana.

Field relations, geochemistry, origin and emplacement of the Baltoro granite, Central Karakoram

1992 ◽  
Vol 83 (3) ◽  
pp. 519-538 ◽  
Author(s):  
M. P. Searle ◽  
M. B. Crawford ◽  
A. J. Rex
Keyword(s):  
Country Rock ◽  
Hot Water ◽  
Crustal Thickening ◽  
Dyke Swarm ◽  
Contact Aureole ◽  
Calc Alkaline ◽  
Stress Regime ◽  
Metasedimentary Rocks ◽  
The North ◽  
Southern Margin

AbstractThe Miocene Baltoro granite forms a massive plutonic unit within the Karakoram batholith, and is composed of comagmatic monzogranites and leucogranites with a mineralogy consisting of quartz-K-feldspar-plagioclase-biotite ± muscovite ± garnet, with accessory sphene, zircon, monazite and opaques. Geochemically the Baltoro granites are mildly peraluminous, and show a calc-alkaline trend on trace-element normalised diagrams with high LIL/HFS element ratios and negative Nb, P and Ti anomalies. REE are strongly fractionated with little or no Eu anomaly. Leucogranites are depleted in most elements compared to monzogranites with notable exceptions being Rb, K and the HREEs. Initial 87Sr/86Sr ratios are 0·7072-0·7128, considerably lower than High Himalayan leucogranites (0·74-0·79), and are indicative of a lower continental crust source. The probable petrogenesis of the Baltoro granite involves dehydration melting of a biotite-rich pelite to produce a voluminous, hot, water-undersaturated magma which could then separate from its source and intrude through an already thickened and still hot crust. Fractional crystallisation of the monzogranites produced the leucogranites and a pegmatite dyke swarm. A suite of lamprophyre dykes including amphibolerich vogesites and biotite-rich minettes intrude the country rock, dominantly to the north, around the Baltoro granite. These calc-alkaline shoshonitic lamprophyres are volatile-rich mantle-derived melts intruded around the same time as the granite, indicating simultaneous melting of the mantle and lower crust beneath the Karakoram during the Miocene, approximately 30 Ma after the India-Asia collision which initially caused the crustal thickening. Intrusion of mantle melts provided heat to promote crustal melting and may have selectively contaminated the granite magma.The Baltoro granite intrudes sillimanite gneisses with melt pods along the southern margin indicating temperatures above 700°C at the time of intrusion. Locally, internal fabrics and numerous aligned xenoliths along the southern margin in the Biafo glacier region indicate steep, southward-directed thrusting during emplacement. Along the northern contact, the Baltoro granite intrudes anchimetamorphic to greenschist facies metasedimentary rocks with an andalusite-bearing contact aureole. Northward-directed culmination collapse normal faulting during Miocene emplacement is inferred, in order to explain the P-T differences either side of the pluton. This also provided an extensional stress regime in the upper crust to accommodate the rising magma.

scholarly journals Mineralogical features and petrogenetic significance of the clinopyroxene and hornblende of the Wuhaolai mafic complex in northern North China Craton, Inner Mongolia

2019 ◽  
Vol 23 (2) ◽  
pp. 133-146
Author(s):  
Chen Wang ◽  
Liu Jianchao ◽  
Zhang Haidong ◽  
Ge Jiakun ◽  
Xi Zhixuan ◽  
...  
Keyword(s):  
North China Craton ◽  
Lithospheric Mantle ◽  
Inner Mongolia ◽  
North China ◽  
Tectonic Setting ◽  
Parental Magma ◽  
Magma Source ◽  
Alkaline Basalt ◽  
The North ◽  
Mafic Complex

The Wuhaolai mafic complex is located in the north margin of the North China Craton (NCC), Inner Mongolia. To discuss the mineralogical features, magma evolution process, and tectonic setting of the complex, we analyzed the geochemical compositions of clinopyroxene and hornblende using an electron probe. The results revealed that the parental magma of this complex belonged to the intraplate alkaline basalt series. The normal zoning texture and the relation between Mg# and FeO, Al2O3, CaO, Na2O, SiO2 and Cr2O3 suggested that the clinopyroxenes of pyroxenite and gabbro crystallized from the same parental magma. The similar CaO content of clinopyroxenes indicated that the parental magma of the Wuhaolai complex may have suffered crustal contamination. Furthermore, the characteristics of hornblende demonstrated that the magma source was modified by fluids derived from subducted slab. Based on the value of Kdcpx (0.23–0.27), the equilibrium melt with clinopyroxene exhibited a relatively low Mg# (43–53), indicating that the parental magma was derived from the lithospheric mantle and underwent crystal fractionation. The gabbro crystallization temperature and pressure was found to be lower than that of pyroxenite, indicating that gabbro was formed at a lower depth than that of pyroxenite. Combining the tectonic setting discrimination diagram of clinopyroxene with the results of previous studies on the late Paleozoic intrusions near the research area, we proposed that the Wuhaolai complex was formed in an intraplate environment. The magma source was modified by fluids derived from the subducted slab during the subduction of the Paleo-Asian Ocean (PAO). After the PAO closure, the parental magma of the Wuhaolai complex was produced by the partial melting of the enriched lithospheric mantle

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