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Fayalite and Clinopyroxene in the Porphyries Of

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Fayalite and Clinopyroxene in the Porphyries Of Canadion Mineralogist Yol. 22, PP. zt0l-409(1984) FAYALITEAND CLINOPYROXENEIN THE PORPHYRIESOF THE TIBCHI ANOROGENIC RING-COMPLEX,NIGERIA: POSTMAGMATIC INITIATION OF A PERALKALTNETREND ECHEFUC. IKE DE)artmentof Geologt,Ahmadu Bello University,Zoria, Nigerip PETER BOWDEN Depafimentof Geology,University of St. Andrews, St.Andrews, Fife, ScottandKy16 gST ROBERT F. MARTIN Deportmentof GeologicalSciences, McGill University,3450 tlniversity Street,Montreal, QuebecH3A 2A7 ABSTRACT enAc i unephase fluide capablede s'infiltrer le long des clivages,La durdede I'interactionavec cette phase fluide The quanz and granite porphyries of the Tibchi interstitiellea 6t€la plus longuedans le casdu porphyre anorogenicring-complex, Nigeria, and their monzonitic and granitique.Le magmaa 6t€mis enplace tranqrillement le syenitic enclaves, contain fayalite and a clinopyroxene long d'une faille annulaireapr0s 6vacuation de la partie whose composition varies from ferroaugite to fer- supdrieurede la chambremagmatique par fluidisation. rohedenbergiteto sodian hedenbergite.The fayalite, which Compardeau porphyrei ph6nocristauxde quartz, qui a is unzoned, ranges from Fa92to Faeein composition; this r6sult6de cettemise en placesoudaine, le porphyregra- spread allows the ranking of the quartz porphyry and its nitiquene contientaucune trace de fayaliteet montreune enclavesin a crystallization sequence.The Fe content of r6actionplus pouss€e entre les compositions magmatiques pyroxene grains increases outward; the most evolved depyroxEne, form€es d'abord, et la phasefluide rdsiduelle. maernatic composition crystallized as hedenbergiterather A Tibchi, le caractbrehlperalcalin s'est manifestd au stade than as B-wollastonite.The proportion of the Ac compo- post-magmatiquede cristallisation. nent, which is low during most of the crystallization inter- val, increasesmarkedly at alate stage; the formation of Mot*clls: porphyred quartz,porphyre granitique, fayalite, the rim is attributed to a residual fluid phase that could h6denbergite,acmite, hyperalcalinit6, stade penetratealong cleavageplanes. Residence time ofthis pore post-magmarique,Tibchi (Nig6ria), complexe annulaire, fluid probably was longer in granite porphyry; 'etly the the anorog6nique. magma rose q along a ring fault after the evacuation of the mapatic reservoir by fluidization. Compared to the INTRoDUCTIoN quartz porph)'ry that resultedfrom this rapid emplacement, the granite porphyry containsno fayalite and showsa more The Tibchi'ring-complex forms part of the thorough reaction betweenearly-formed magmatic pyrox- well-known belt of high-level anorogenic intrusive ene and the fluid phase. At Tibchi, the appearance of a bodies that collectively define the Younger Granite peralkaline overprint occurred at the postmagmatic.stage, province of Nigeria and Niger. The emplacementof Keywords: quartz porphyry, granite porphyry, fayalite, theseepizonal complexes, and of analoguesin eastern hedenbergite,acmite, peralkalinity, posmagmatic stage, Canada and the United States, signals the inception Tibchi, Nigeria," ring complex, anorogenic. of rifting culninating in the opening of the Atlantic Ocean. The Tibchi fing-complex, ivhich is Late (a gives SoraNlarns Jurassic Rb-Sr isochron 175 t 3 Ma: M. A. Rahaman,pers. cornm. 1977),consists mainly of rocks granitic Le porphyre i phenocristaux de quartz et le porphyre of composition; in the Younger granitique du complexe annulaire anorogdnique de Tibchi Granite province as a whole, such rocks constitute (Nig6ria), ainsi que leurs enclaves monzonitique et more than 9090of the igneousrocks exposed.The sy€nitique, contiennent de la fayalite et un clinopyroxdne reader will find descriptions of the various dont la composition va de ferroaugite i ferrohddenbergite fing-complexesin Jacobsonet ol, (1958),Buchanan i h6denbergitesodique. La fayalite, qui est non-zon6e, a et al. (1911), Bowden & Turner (1974), Turner une composition comprise entre Fa92et Fa99;ce domaine (1976),Turner & Bowden(1979), Martin & Bowden de composition permet d'ordonner les dChantillons de (1981)and Bowden (1982).Whereas many of these porphyre d quartz et les enclaves dans une s6quencede complexes are well mapped, only now are cristallisation. Le pyroxdne montre un enrichissement en quantitative mineralogical data becoming fer vers la bordure; la composition magmatique la plus available. 6voludea cristallisd sous forme de h€denbergiteplutOt que Here, we focus on the microphenocrystic mafic de wollastonite-B.La proportion du terme Ac, qui demeure minerals of the Tibchi eranitic porphyries and their faible pendant la majeure partie de I'intervalle de genetically related enclaves. cristallisation, augmeote de fagon frappante i un stade Past investigators of the ferromagnesianminerals tardif; on attribue la formation de la surcroissanceenrichie in theseanorogenic rocks lBain (1934)on fayalite 401 402 TITE CANADIAN MINERALOGIST from Kudaru, Borley (1963) on ferroaugite from of granite porphyry, a central mass dominated by Pankshin, Jacobson et al, (1958) on fayalite and biotite granite, a plug of quartz porphyry and a suite hedenbergite from Ririwail were hampered by the of related ignimbritic rocks (Fig. 1). The narrow textural complexity of the multiphase clusters. The rine-dyke was formed after cauldron subsidenceand only valid tool in studies of this type is the electron partial evacuation through fluidization of the microprobe.This overcomesthe problemsposed by subvolcanicreservoir (Ike 1983).The complex co- the small modal proportion of the mafic minerals vers approximately 16 x 27 k'm; most of its area, in theserocks (between0. 5 and 20/oby volume) and at the presentlevel of erosion, is occupiedby the the intergrown Fe-Ti oxide minerals. basementmetamgrphic complex, mainly consisting of gneissicgranite of Pan-African age. DEScRTPTIoNoF THECoNapLsx Directly relevant to this mineralogical study is a suite of enclaves that are encountered in the The Tibchi complex consistsof an annular dyke porphyritic units. These enclaves are closely ItrE T105 T108 NIGERIA YOUNGER ffiANlTE PrcUNCE YARDA Q .40rm GUNGLT\4E' t00E TIBCHI Tll T ffiffi "n'pu,q,4 lF,@ erorm GRANTTE N onmnE PoRPHYRY ffiloulnrz PoRPHYRIEs l==1 EXTRUSIVEVOLCANIC L:J ROCKS l-l onmmc BAsEMENT or..l-.3,i-.f*' pto. 1. Simplified geological map of the Tibchi complex, showing the major rock-units. The ring dyke of granite porphyry overlaps the southernmost portion of the Ningi-Burra complex. The basementcomplex consists mainly of gneissic granite emplacedduring the Pan-African orogeny. The complex was mapped by E.C. Ike. Middle inset is a detailed sketch of the area south and east of the village of Tibchi, showing the following samplelocations: black star T60, white star T135' black squareT167, white squareT115, black diamondT170, white diamondT169' dotted circle T133, crossedcircle T168. Width of middle insetrepresents 3.3 km. FAYAINE AND CLINOPYRO)GNE OF THE TIBCHI RING.COMPLEX, NIGERIA 403 interrelated petrographically; also, they are con- sidered cognatewith respectto the host porphyries. The contact between host and enclave typically is diffuse and irregular. Two lithologies are found as enclaves:monzonite and syenite. These more basic rocks may have formed from melts that were precursorsof those that gave the porphyr.ies. In the Tibchi complex,fayalite is restrictedto the quartz porphyry and to enclaves of monzonite. Clinopyroxene, which belongs to the series ferroaugite - ferrohedenbergite, occurs in quartz porphyry, in the granite porphyry ring-dyke, and in the enclaves. The quortz porphyry Ftc.2.Typical specimen of quartzporphyry from Tibchi. Notetle fragmentalappearance of thephenocrysts, an The pristine samplesof quaru porphyry Gig. 2) ignimbritictendency. Specimen T105. Width of label are dark green; the phenocrysts of quartz I cm. (paramorphic after B-quartz), clear green alkali feldspar (originally sanidine, now orthoclase perthite), ferrohedenbergitic clinopyroxene and with respectto ferrohedenbergite, which commonly fayalite, which commonly are fragmented, account forms a rim of small anhedral to subhedral crystals. for 40 to 5090of the rock by volume. The unit of The microcrystalline groundmasslocally displays quartz porphyry gradesinto crystal-rich ignimbrites, striking devitrification textures, mainly spherulites and reprments afrozen feederto the latter (Ike 1983). in various stagesof ssarsening.It consistsof alkali Xenoliths of earlier ignimbrite and related felsic feldspar (typically a mixture of orthoclase + intrusive rocks are common in addition to the cos- intermediate microcline + albite), quartz, nate plutonic enclaves already mentioned. ferrohedenbergite, ferrian ilmenite (Ike & Bowden, Based principally on the nature of the in press),monazite, apatite, zircon and fluorite. The clinopyroxene, two variants ofthe quartz porphyry opaquemineral occludedin the microphenocrystsof - may be recognized: l) a fayalite ferroaugite - pyroxene and fayalite @igs. 3A, B) is ilmenite (Ike ferrohedenbergite type, and 2) a fayalite - & Bowden, in press).Aenigmatite was sought but ferrohedenbergitetype. In type 1, the clinopyroxene not found. varies continuously from ferroaugite to Representative bulk compositions of quartz ferrohedenbergite. Pleochroism increasesfrom the porphyry are listed in Table l. ferroaugite core to the sodian ferrohedenbergiterim; the rim has a larger extinction angle (y.rI C = 50o) The gronite porphyry than the core by up to 5o. The ferroaugite core, although prismatic, commonly displays a skeletal The rock tiat
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