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Variation in mantle lithology and composition beneath the Ngao Bilta volcano,Adamawa Massif,Cameroon volcanic line,West-central Africa

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Geoscience Frontiers 2020年第2期11卷 665-677页

作者： robert temdjim Merlin Patrick Njombie Wagsong Arnold Julien Nzakou Tsepeng Stephen Foley Department of Earth Sciences Faculty of SciencesUniversity of Yaounde IP.O.Box.812 YaoundeCameroon Department of Earth and Planetary Sciences and ARC Centre of Excellence for Core to Crust Fluid Systems Macquarie UniversityNorth RydeNSW2109Australia

Mantle peridotites entrained as xenoliths in the lavas of Ngao Bilta in the eastern branch of the continental Cameroon Line were examined to constrain mantle processes and the origin and nature of melts that have modified the upper mantle beneath the Cameroon *** xenoliths consist mainly of lherzolite with subordinate harzburgite and *** commonly contain olivine,orthopyroxene,clinopyroxene and spinel although the dunite is *** is an essential constituent in the *** chemistry differs between the three types of peridotite:olivines have usual mantle-like Mg#of around 90 in lherzolites,but follow a trend of decreasing Mg#(to 82)and NiO(to 0.06 wt.%)that is continuous in the *** also contain orthopyroxenes and/or clinopyroxenes with low-Mg#,indicating a reaction that removes Opx and introduces Cpx,olivine,amphibole and *** is attributed to reaction with a silica-undersaturated silicate melt such as nephelinite or basanite,which originated as a low-degree melt from a depleted source as indicated by low Al2O3 and Na2O in Cpx and high Na2O/K2O in *** estimates place the xenoliths at pressures of 11–15 kbar(35–50 km)and temperatures of 863–957C,along a dynamic rift geotherm and shallower than the region where carbonate melts may *** melt/rock reactions exhibited by the Ngao Bilta xenoliths are consistent with their peripheral position in the eastern branch of the Cameroon Volcanic Line in an area of thinned crust and lithosphere beneath the Adamawa Uplift.

关键词： Cameroon volcanic line Adamawa volcanic massif Peridotite xenoliths Partial melting Melt-rock reactions

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Baossi–Warack monogenetic volcanoes, Adamawa Plateau,Cameroon: petrography, mineralogy and geochemistry

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Acta Geochimica 2019年第1期38卷 40-67页

作者： Anicet Feudjio Tiabou robert temdjim Pierre Wandji Jacques-Marie Bardintzeff Vivian Bih Che Edith Ekatah Bate Tibang Caroline Neh Ngwa Fran?ois Xavier Onana Mebara Remote Sensing Unit Faculty of Science Department of Geology University of Buea Department of Earth Sciences Faculty of Science University of Yaoundé 1 Laboratoire de Géologie Ecole Normale Supérieure de Yaoundé Univ.Paris-Sud Sciences de la Terre VolcanologiePlanétologie UMR CNRS 8148 GEOPS Bat.504 Université Paris-Saclay Higher Teacher Training College Department of GeologyUniversity of Bamenda Institute for Geological and Mining Research (IRGM) Institute for Geological and Mining Research (IRGM) Branch of Volcanologic and Geophysics Research (ARGV)

Three monogenetic cones in the Baossi–Warack area, Ngaoundéré, Adamawa Plateau forming part of the Cameroon Volcanic Line(CVL) are documented in this study. Basaltic lavas(<1 km^3) scattered around these vents and restricted volcaniclastic deposits were emplaced by Hawaiian and mild strombolian style eruptions. The lavas are porphyritic, mainly composed of olivine(chrysolite) and clinopyroxene(diopside and augite) phenocrysts and plagioclase(andesine) microphenocrysts. Accessory minerals include titano-magnetite and titano-hematite, nepheline,apatite and amphibole xenocrysts. Sanidine occurs in some samples and sodi-potassic albite in others. Some olivines and clinopyroxenes exhibit resorbed margins and thin reaction rims while plagioclase displays oscillatory zoning, and sieved textures as a result of magma mixing. Whole-rock geochemistry data indicates that the lavas are silica-undersaturated, composed of basanites and basalts, showing little compositional variations(SiO_2: 39.20 wt.%–48.01 wt.%,MgO: 5.29 wt.%–9.70 wt.%). Trace elements patterns of these lavas suggest they are enriched in LILE including Pb,probably due to crustal contamination. REE patterns suggest cogenetic magmas below Baossi 1 and Baossi 2 volcanoes,and distinct sources below Warack volcano and nearby *** lavas studied show affinity to high-μ(HIMU), enriched type Ⅰ(EM1) and Oceanic Island Basalt(OIB)-like mantle signatures and thus indicate a heterogeneous mantle source underneath the vents as noted at other monogenetic and polygenetic volcanoes along the CVL. Primary melts derived from low degrees of partial melting(0.5%–2%) and encountered low rates of fractionation, and crustal contamination coupled with magma mixing. These melts evolved independently through structural weaknesses in the basement.

关键词： Cameroon volcanic line Adamawa Plateau Volcanic field Monogenetic volcano Magma mixing Petrogenesis

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