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Magnetite in the Carbonatites from the Jacupiranga Complex, Brazil

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Magnetite in the Carbonatites from the Jacupiranga Complex, Brazil American Mineralogist, Volume 6E, pages 195-213, 1983 Magnetite in the carbonatitesfrom the JacupirangaComplex, Brazil JosB C. Gespan Instituto de GeosciAncias Universidade de Sao Paulo Sao Paulo, Brazil CEP 05508 eNo PBren J. Wyrue Department of Geophysical Sciences University of Chicago Chicago, Illinois 60637 Abstract Electron microprobe analysesof magnetitesfrom five carbonatiteintrusions (Cr, oldest, to C5, loungest) constituting the carbonatite plug in the Jacupirangacomplex confirm' previous results from Jacupirangagiving compositions in the magnesioferrite-magnetite series very close to Fe3Oa.Magnetites from other carbonatitesare similar with somewhat more Ti and less Mg. MgO in Jacupirangamagnetites reaches no more than l0 wt.%. All analyzedgrains are zoned, with Fe3Oaincreasing toward the rim. In magnetitesfrom C2 to C5, Fe2O3 replacement is mainly by Al2O3 and less by TiOz; in C1 magnetites TiO2 replacementis more important. Despite their limited range of compositions, the cores of magnetitesin each of the five intrusions are chemically distinct and distinguishablefrom each other as indicated by projections from within the Cr-free spinel prism, MgFe2Oa- MgzTiOa-MgAI2Oa-Fe3Oa-Fe2TiOa-FeAl2Oa,and a plot of Mn ys. Mg. Magnetites from special locations such as dikes, bandedreaction zonesbetween carbonatite andjacupiran- gite, and in intergrowths with pyrite, are chemically related but distinct from the carbonatite magnetites.The systematicchemical variation and zoning of magnetitesin the five carbonatite intrusions indicate magmaticorigin. Magnetite crystals nucleatedthrough- out the crystallization interval of the carbonatites, but most of them show evidence of marginal resorption. The oldest carbonatite, C1, was probably derived from a magma somewhat different chemically from those producing carbonatites C2 through C5. The precipitation of carbonatite C2 probably went to completion independentlyof C3 through C5, whereas carbonatites C3 through C5 probably were precipitated from successive batchesof magmarepresenting a continuum in time and magmaticevolution. Introduction from carbonatites. Mitchell (1978)and Boctor and Svisero (1978)presented analyses of the magnetites Magnetite is a characteristic mineral of nearly all from Jacupiranga.Other analyseshave been report- carbonatites,usually as an accessory,but locally as ed from Oka by Gold (1966) and McMahon and an essentialmineral (Heinrich, 1966,p. 182).The Haggerty (1976, 1979), for zoned magnetites from magnetite occurs as disseminated euhedra, com- South African carbonatites by Prins (1972), from monly in flow bands, or as lenses and irregular Alno by von Eckermann (1974), and from Sarfar- aggregates.Magnetite is also concentrated in ore tdq, Greenland, by Secher and Larsen (1980). rocks which occur as integral parts of the ring Mitchell (1979)summarized the chemical character- complexes. The magnetite crystals are homoge- istics of the magnetitesfrom carbonatitesas "com- neous, but exsolution of lamellar ilmenites may positionally very close to pure magnetite" and as occur (Prins,1972;Mitchell, 1978;Boctor and Svi- membersof the magnesioferrite-ulv<ispinel-magne- sero, 1978). tite series". According to Mitchell: "the principal There are few published analyses of magnetites characteristicsofcarbonatite spinels are the lack of 0003-09)vE3/0I 02--0 I 95$02.00 195 I% GASPAR AND WYLLIE: MAGNETITE IN CARBONATITES FROM JACUPIRANGA Cr2O3,the low TiO2 and MgO relativeto kimberlite spinel,and commonly high MnO". The presence of five carbonatite intrusions in a single complex at Jacupiranga,and the continuous outcrop and freshness of the rocks in the quarry, make this an unusual opportunity to samplein detail the mineralogy of a carbonatitecomplex. In this paper, we present detailed chemical analyses of zoned magnetites from the five carbonatites, and explore the applications of their chemistry for pet- rogenesis. The Jacupiranga Complex The JacupirangaComplex is one of many alkalic complexes, several containing carbonatites, that occur in the borderof the ParanaBasin, in the south * (Amaral,1978). of Brazil. It is 130 5 m.y. old Fig. l. Simplified geological map of the Jacupiranga Accordingto Ulbrich and Gomes(1981) the Jacupir- carbonatites.C1-sovite ; C2Jolomitic sovite; C3-sovite; Ca- angaComplex and other nearby complexesof simi- sovite; C5-rauhaugite and JAC-jacupirangite. Solid lines- lar agesform the JacupirangaProvince. This prov- observed geological contacts; dashed lines-inferred contacts; ince was interpreted by Herz (1977)as the site of a F-fault. The contacts on the map are incomplete for lack of secureevidence, becausethe quarry is on a hill. hot spot associatedwith the first triple junction formed in the area during the initial opening of the South Atlantic ocean. group. Although the field evidence is not conclu- Geology of the complex sive, it suggeststhat the southern intrusions are Melcher (1966)presented a geologicalmap of the older than the northern intrusions. Given this con- Jacupirangaalkalic complex. The complex has an clusion, the sequenceof carbonatite intrusions from oval shape with an area of about 65 km2, and is oldest to youngestis Cr, Cz, Ct, Co and Cs. The intrusive into granodiorites and mica schists which main petrographical and mineralogical features of are fenitized for some distancefrom the complex. It each carbonatite intrusion are summarizedin Table is composed mainly of peridotites, pyroxenites, 1. jacupirangites and irjolites, surrounded by fenites The intrusive contact of Cr into C2 is shown in and nepheline syenites. According to Melcher Figure 1 as a dashed line, becausethe banding (1966),a peridotitic body was first emplacedin the which is obvious within each intrusion becomes northern half of the intrusion, and then surrounded diffuse in the region of this contact. In addition, by a ring intrusionof pyroxenite. South of this, and there is an increase in the amount of dolomite partly cutting it, an almost circular jacupirangite present in the rocks near the contact. In some plug was intruded, and partly differentiated into a regions, there appear to be a swarm of beforsite concentric, crescent-shapeijolite body. The car- dikes intruding the sovites,but in other regionsthe bonatitesoccupy a centralposition in thejacupiran- increasein dolomite could be attributed to a meta- gite. somaticprocess of dolomitizationof the sovite.The S and B samples in Table I are respectively the The carbonatites calcite and dolomite rock of this region. The carbonatite body has steeply outward-dip- Alvikite and beforsite dikes are widely distribut- ping contacts with the jacupirangite and, according ed. They usually contain fewer non-carbonated to Melcher (1966),it was emplacedin two plugs, minerals than the larger carbonatite intrusions. The one partly cutting the other. Detailed mapping by Badike in Table I is a beforsitecutting the Cr sovite. one ofus (JCG)has revealedthe occurrenceoffive The beforsite dike 85 is an offshoot from the C5 distinct intrusions of different ages, as shown in rauhaugite,which also cuts the Cn sovite. Figure 1. The intrusions Ct, Cz and C3 comprisea At contacts between carbonatite and jacupiran- southerngroup, and Ca and C5 comprise a northern gite (xenoliths and country rock), a banded rock is GASPAR AND WYLLIE: MAGNETITE IN CARBONATITES FROM JACUPIRANGA 197 Table l. Petrographyof carbonatitesand jacupirangite Rocktype* l,lain mineral ogy** Grain size Otherfeatures Calcite,apatite, magnetite, c1 Sovite olivine, phogopite,dolomite, Coarse Coarselybanded sulfi des. Calcite, dolomite,apatite, Fine to Finely banded SouthernIntrusi ons c? Doloni ti c magnetite,phl ogopite, medium sovite sulfi des. Calcite, apatite, magnetite, libdium to Non-carbonatemi neral s ca Sovite phlogopite,dolomite, coarse fewer than in other olivine, sulfides. sovi tes. Banding l ess evi dent. Calcite, apatite, rnagnetite, C4 Sovi te ol i vi ne, phlogopi te, doloni te , ti,ledium sulfides. Northern Intrusi ons Dolomite, apatite, phlogopite, lledi um to Veryfew si I icates, c5 Rauhaugite magnetite, sulfides, calcite. coarse oxidesand sulfides phlogopite, Fi in the B4 Beforsi te Dolomite, ne Intruded C4 magnetite, calcite sovite UI KCS Dolomite, calcite, apatite, l'4edi um to 0ffshoot of the Cq B5 Beforsi te magnetite, phlogopite, fi ne rauhaugite in the' sul fi des. C4 sovite calcite, dolomrte, apatlte, I{edr um to Rocksfrom the Sovite phlogopite, magnetite, coarse Diffuse banding contact between sul fi des. carbonati te Dolomite, calcite, apatite l',ledi um Diffuse banding intrusions C2 Befors i te and Ca magnetite, sulfi des Heactlon zone 51| rcate rrcn bands between the Phlogopite, alkal i amphibole, Fineto alternate with 'intrusions (Cr,Ca) - Reacti on calcite, magnetite, apatite, coarse carbonate rich bands and Ca) and tfie rocK su1fides, ilmenite, cl inohumite (magnetite occurs in jacupirangite the silicate bands only Host rock JAC Jacupirangite Titanoaugite, titanomagnetite ltledium Oxher rock tgyes cited in the text: aJ.vikite (cafcite, apatite, dolonite, phLogopite) and ijoTite ( cI ircpgroxene, nepLel ine ) . The mineraTs are cited in an estimated decreasing otdet of abvndance TIre ctnracteristics cited for these two dikes refer specifical,Tg to them ard. not to tle dikes in gereraJ- tlat occur in the carbo@tite bodies. developedvarying in width from l0 cm to more than In carbonatite C1, the magnetite crystals vary in 2 m. Centimeter-wide bands of carbonate alternate size, but they are most commonly coarse grained. with silicate-rich
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