Mineral chemistry and crystallization conditions of the Mboutou layered gabbro-syenite-granite complex, North Cameroon

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Authors:Parsons, Ian; Brown, William L.; Jacquemin, Herve
Author Affiliations:Primary:
Univ. Aberdeen, Dep. Geol. and Mineral., Aberdeen, United Kingdom
Cent. Rech. Petrogr. et Geochim., France
Volume Title:Journal of Petrology
Source:Journal of Petrology, 27(6), p.1305-1329. Publisher: Clarendon Press, Oxford, United Kingdom. ISSN: 0022-3530
Publication Date:1986
Note:In English. 56 refs.; illus. incl. 2 tables, sketch map
Summary:The Mboutou complex is one of a line of early Tertiary ring complexes which runs from Lake Chad to the Gulf of Guinea. The main rock types are layered gabbros and gabbronorites, with minor quartz syenodiorite, quartz syenite and hypersolvus granite. Feldspars range from An85Ab13Or2 to around An1Ab46Or55 and form an entirely hypersolvus sequence with very strong zoning in the syenodiorites. Ca-rich clinopyroxenes and olivines have a restricted range. Orthopyroxene-bearing leucogabbros and syenodiorites contain minor orthopyroxene (En62Fe35Wo3) and quartz; olivine and orthopyroxene never coexist. In more evolved rocks amphibole and minor biotite are the only mafic silicates. The chemistry of the rocks and of biotite, and the trace elements in clinopyroxene, show that the complex was only very mildly alkaline; its more evolved members developed some calc-alkaline characteristics, possibly due to ingress of water in residual batches of magma. This change corresponded with the sudden appearance of quartz and orthopyroxene (the latter not in equilibrium with the clinopyroxene). Zoning in the ternary feldspars in the syenodiorites, and the hypersolvus character of the final granite, limit PH2O to <1 kbar and suggest minimum T for the end of crystallization in the syenodiorites of approx 850°C. [R.A.H.]
Subjects:Biotite; Cenozoic; Chain silicates; Clinopyroxene; Composition; Crystallization; Differentiation; Fugacity; Gabbros; Granites; Igneous rocks; Intrusions; Layered intrusions; Lower Tertiary; Magmas; Mica group; Mineral composition; P-T conditions; Petrology; Plutonic rocks; Pyroxene group; Ring complexes; Ring structures; Sheet silicates; Silicates; Syenites; Tertiary; Africa; Cameroon; West Africa; Mboutou Complex
Coordinates:N000000 N150000 E0150000 E0050000
Abstract Numbers:87M/4902
Record ID:1988019127
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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