|
|||||
|
|
Mineral Chemistry and Crystallization Conditions of the Mboutou Layered Gabbro-Syenite-Granite Complex, North Cameroon |
|
| Authors: | PARSONS IAN; BROWN WILLIAM L; JACQUEMIN HERVE |
| Institution: | 1Department of Geology and Mineralogy, The University Aberdeen AB9 IAS Scotland 2CRPG, B.P.20, 54501 Vandoeuvre-les-Nancy Cedex, France 3Laboratoire de P?trologie-G?ochimie, Universit? de Nancy 1 B.P.239, 54506 Vandoeuvre-les-Nancy Cedex, France |
| Abstract: | The Mboutou complex is one of a line of early Tertiary ringcomplexes which runs from Lake Chad to the Gulf of Guinea, noneof which has hitherto been described in detail. The main rocktypes are layered gabbros and gabbronorites, with minor bodiesof quartz-syenodiorite, quartz-syenite and hypersolvus granite.Feldspars form a continuum with exceptional compositional range,from An85Ab13Or2 to around An1Ab46Or55, and form an entirelyhypersolvus sequence with very strong zoning in the syenodiorites.Ca-rich clinopyroxenes (salite and calcic augite) and olivines(Fo78–62) have restricted range. Orthopyroxene-bearingleucogabbros and syenodiorites contain minor orthopyroxene (En62Fs35Wo3)and quartz; olivine and orthopyroxene never coexist. In moreevolved rocks amphibole (magnesio-hornblende to ferroedenite)and minor biotite, showing progressive Fe-enrichment, are theonly mafic silicates.Major-element rock chemistry, minor elementsin clinopyroxenes and biotite chemistry show that, notwithstandingits thoroughly anorogenic setting, Mboutou was, at the outset,only very mildly alkaline. Its more evolved members embarkedon a line of evolution with some calc-alkaline characteristics,probably because of ingress of water into residual batches ofmagma, a possibility supported by stable isotope data. Thischange in behaviour corresponded with the sudden appearanceof quartz and orthopyroxene, which was not in equilibrium withclinopyroxene on the two-pyroxene surface. Amphibole then becamethe main mafic silicate with further increase in  . The more evolved rocks are relatively highly altered,but Fe-Ti oxide pairs suggest that  was maintained near to and above the QFM buffer and the rangeof biotite compositions further suggests crystallization undera regime of decreasing . Biotites maintain alkaline characteristics throughout the sequence. Zoningpatterns in the ternary feldspars in the syenodiorites, andthe hypersolvus character of the final granite, limit maximumvalues of to < 1 kb, and suggest minimum temperatures for the end of crystallizationin the syenodiorites of{small tilde} 850 ?C. |
| Keywords: | |
| 本文献已被 Oxford 等数据库收录！ | |