423 The Canadian Mine ral ogi st Vol. 34, pp.423434 (1996) RELATIVELYALUMINOUS ALKALI PYROXENE IN NEPHELINESYENITES FROM MALAWI: MINERALOGICALRESPONSE TO METAMORPHISMIN ALKALINEROCKS ALANR. WOOLLEY Departmentof Mineralngy,Natural History Museum, Cromwell Road Inndon SW SBD,U.K. R. GARTH PLATT Departnzntof Geology,Inkeheal. IJniversity, Thunder Bay, Ontario P7B sEI G. NELSONEBY Departmentof EarthSciences, University of Massachuseftsat Lawell, Itwell, Massachusens01854, U.S.A. ABSTRACT The North Nyasa Alkaline hovince (NNAP) of central and nofthern Malawi, easterncentral Africa, consistsof seven nephelinesyenite intrusions which were, to varying degrees,affected by the MozambiqueOrogenic event. We have analyzed the constituentclinopyroxene in rocks of five of theseintrusions by electronmicroprobe. Three groups of pyroxenescan be distinguished.(a) Alkati pyroxenesrange from diopsidetbrough aegirine-augite to aegirine.These have total A1 < 0.15 (apfu) uAl analaie takento be of primary magmatii origin. @j PyroxeneJcharacterizedby Al > 0.15 (aplu), with predominan!form a rim on group-(a) pyroxenes,cut acrossthem and also form, in one intrusion, tiny acicular crystals within nephelineand, |ocaily, feldspm.They comprisealuminian aegirine-augite, aluminian aegirine and omphacite.These pyroxenes are interpreted asmetamorphic in origin. (c) fne tma group comprisesAl-rich py'roxenesin which NAI is predominant.They are diopsideand aluminian diopside exhibiting no trend of alkali enrichment.They are probably of igneousorigin, but could also have been affectedby metamorphism.Although the productionofpurejadeitic pyroxenesprobably requires I minimnm pressureofabout 7 kbar, aluminianaegirine with higher contentsof Fe3+can probablybe generatedat much lower pressures.The preservationof primary igneouspyroxenes in the NNAP intrusions probably indicatesthat neither high pressuresnor particularly elevated lempetatn1eswere reached.The formation of the aluminousp'.roxenes was essentiallythe result of an isochemicalevent; in general,metamorphism of agpaiticrocks will give rise to aluminianaegirine and ajadeitic pyroxene,whereas miaskitic rocks will containaluminian aegirine-augiteaad an ornphaciticplroxene. Keywords:nepheline syenite, alkali pyroxene,aegirine, aluminian aegirine, atuminian aegirine-augite, omphacite, Malawi. SoMM,cB.E l,a province alcaline du Nyasa m6ridional, dans le secteurcenral et nord du Matawi, partie est-centralede I'Afrique, est faite de septmassifs intrusifs i sy6niten6ph6linique qui ont 6t6, b divers degr6s,affectds par I'orogenbsemozambiqu6enne. Nous avons analys6le clinopyroxbnepr6sent dans cinq des sept massifs par microsonde6lectronique. Nous distinguons trois groupesde compositions:(a) pyroxbnesalcalins allant de diopsidevers I'aegyrine,en passantpar I'augite aegyrinique. Ces pyroibnes possbdentmoins de 0.15 datomes d'Al par unit6 formulaire, et seraientdorigine primahe (magmatique)' (b) lrs plroxenes contenantplus de 0.15 alomesd'Al, surtout sous forme de uAl, forment une bordure sur les cristaux du groupe(i), les recoupen! et apparaissentaussi, dans un cas,sous forme d'aiguillesh f intdrieur de la n6ph6lineet du feldspath. Les tompositiotrs sont repr6ientativesd'augite aegyrinique alumineuse,daegyrine alumineuseet d'omphacite,et seraient dorigine mdtamorphique.lrs pyroxbnesdu groupe (c) sont aussi alumineux, mais I'aluminium y occupe surlout le site t6tra6drique.n s'agit <lediopside et de diopside alumineusesans indication d'enrichissementen alcalins. Ceux-ci seraient probablementd'origine ignde,mais pounaient bien avoir 6t€ affect6spar m6tamorphisme.Quoique la productionde la jad6ite pure requiert une pression minimale d'environ 7 kilobars, I'aegyrine alumineuseayant une teneur 6levde en Fer+ est tout probablementstable e une pressionde beaucoupinf6rieure i cette valeur. La prdservationde pyroxbnesprimaires dans ces massifst6moigpe probablement des conditions plut6t mod6r6esde t€mpdratue et de pressionattsintes lors du m6tamorphisme. La formation de pyroxbnesalumineux rdsulterait essentiellementd'une recristallisation isochimique. En g6n6ral, on peut s'attendreque le m€tamorphismede rochesagpartiques produise une aegyrinealumineuse et un p1'roxbnejad6itique,tandis que les rochesmiaskitiques dewaient produire une augiteaeglninique alumineuse ou un pyroxbneomphacitique. Ctraduitpar la R6claction) Mots-cl6s: syeniten6phdlinique, pyroxbne alcalin, aegyrine,aegyrine alumineuse, augite aegyriniquealumineuse, omphacite, Malawi. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/2/423/3446863/423.pdf by guest on 28 September 2021 424 THE CANADIAN MINERALOGIST hrrnooucnou Bloomfield (1965),who describedthe peffologyand general field relationships. The Chilwa Alkaline The alkaline igneousrocks of central and northern Province lies in soutlern Malawi, to the south of the Malawi (Fig. 1) that Bloomfield(1970) included in his NNAP Gie. 1). Nonh Nyasa Alkaline Proyince (NNAP) consist of The NNAP intrusions in Malawi have, to varying seven intrusions, namely Kasungu, Chipala, degrees,been affected by tectonic, thermal and Chikangawa,Mphomph4 Telelele,Ulindi and Ilomba. metasomaticevents that were probably related to the The dominant rock-type of these intrusions is genesis of the Mozambique orogenic belt. Geo- nephelinesyenite, with a little syeniteand pyroxenite. chronologicaldata for the province are sparse.Rblsr The only general study of the NNAP is that of whole-rockisochron ages of 685 x.62 Ma and 650 * 40 Ma have beenobtained from nephelinesyenites of Ilomba and Chikangawa(Ray 1974,Bloomfield et a/. 1981),and Pb-a datingofzircon from sodalitesyenite at Ilomba gave655 Ma (Bloomfield 1968).However, K-Ar dating of these intrusions gave 508 t 12 and 490 x. 12 Ma for Ilomba @loomfield et al. l98L) and410 + 16Ma for Chikangawa(Snelling 1965). The ?e youngerK-Ar datesled Bloomfield(1970) to conclude that the intrusions had been affected by a late uo, Mozambiquianphase of cataclasis. Geologicalevidence within the NNAP intrusionsfor o these tectonic and metamorphicevents is slight. Ray -.lelelete I o (1975) consideredthat the most northerlyintrusions, t6- Ilomba and Ulindi, comprise part of the Songwe Syenile Complex, which forms an igneousunit over 30 km long and 2-3 km wide. The Songwesyenites locally show signs of deformation,with development of a gneissosityindicating emplacementprior to or during an episode of deformation related to the MozambiqueOrogeny (Ray 1975). During field work on all seven of the NNAP 3 intrusions, little evidence for metamorphism or 6 deformation was observedbut. as described in tle e next section,there is microscopicevidence for defor- mation. However, detailed mineralogical work has revealedthe presenceof an unusualsuite of pyroxenes that are interpreted as being products of the meta- morphism. Gnolocy l,No PgrRocRApHy The pyroxenesthat are the subject of this paper occur in the Kasungu, Chipala llomba, Mphompha j and Chikangawaintrusions (Frg. l); they are absent Chilwa from the Ulindi and Telelele intrusions,which consist essentiallyof biotite-nephelinesyenite. The following brief geological and petrographic notes can be supplementedby the general account of Bloomfi.eld (1965) and, for llomba, by that of Bloomfield er a/. (1e81). Kasungu The Kasungu intrusion is approximately circular, has a diameter of 2 km and forms a prominent Ftc. l. Location of the major intrusions of the North Nyasa Alkaline Province NNAP) in central and northern mountain rising about 400 m above the surrounding Malawi. The area containing the Chilwa Alkaline plain (Peters 1969). The intrusive rocks, nepheline province in southernMalawi is also indicated. syenite,show little mineralogicalor textural variation, Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/34/2/423/3446863/423.pdf by guest on 28 September 2021 PYRO)GNE IN METAMORPHOSEDNEPHELINE SYEMTE. MATAWI 425 though the mafic minerals may develop a marked inclusions. Titanite, apatrte, zircon and magnetite banding or form irregular clots. In these coarse- or ilmenite are accessoryphases. gained, mesocraticrocfts, turbid perthite is generally Although not apparentin handspecimen, the micro- crackedand strained,and may be traversedby zonesof scopic textures and structuresof the Kasungu rocks fine-grainedrecrystallized feldspar. Nepheline usually indicatethat all havebeen subjected to somedegree of forms l0-20%o of most rocls, but otherwise may be deformation and considerable recrystallization. No minor. A sodic scapotte, which is marialite on the textures that are indubitably igneous have been basis of electron-microprobedata, has been found in recognized.Bloomfield (1965) consideredthat the two samples,the first such identffication in a NNAP nepheline o'showstypical replacementtextures" and intrusion. that it is the result of nephelinization.However, we The clots of mafic minerals are texturally complex could find no unequivocaltextural evidencefor sucha and dominatedby pyroxene, amFhibole and biotite. process. The pyroxenetypically occursin the centerof clots but may form at the margins where, usually intergtown ChipaLaand. Chipala East with mica and amphibole,it forms symplectitic inter- growths with feldspar, as well as myriads of small, The Chipalainfiusion forms a small hill 5 kn north isolated grains within feldspar; this texture occurs of Kusungu, and the smaller Chipala East intrusion tlroughout somerocks. The principal pyroxeneforms occursa kilometer to the east@loomfield 1965,Peters green, subhedralprisms or rounded crystals