701 The Canndian M ineralo g is t Vol.36, pp. 701-7ll (1998)
Mn-Fe SPINELSAND SILICATESlN MANGANESE-RICHROCKS FROM THE OSSA-MORENAZONE, SOUTHERN IBERIAN MASSIE SOUTHWESTERN SPAIN
ruANJIMENEZ-MILLANI
Depaftamcnto de Geologfu Facaltad de Ciencias Experitnentales, Universidad de Jada Campus Universitario, E-23071 Jadn, Spain
NICOLAS VELILLA
Departamento de Mineralttgta y Petolog{q Facultad de Ciencias, tlniversidad de Granada, E-18002 Granada, Spain
AssrRAcr
Manganese-rich rocks occur in a greenschist-facies volcano-sedimentary complex of the Ossa-MorenaZnte, in the Iberian Massif' southwestern Spain. Four unusually Fe-rich manganese associations containing Mn-Fe spinels and silicates have been idenffied: f) magnetite + pyroxmangite + spessartine + quartz, tr) manganian jacobsite + pyroxma-ngite + aegirine + quartz, IIf jacobsite + pyroxmangite + tephroite + spessartine + rhodochrosite, and IV) manganoan magnetite + rhodonite + ferroan tephoite + spessartine+ rhodochrosite. Major compositional variations of these minerals are complex functions of several factors. Oxygen fugacity determines the Fe content of pyroxenoids in such a way that it is very low in the pyroxmangite of associations bearing Mnk spinels (<0.4 7o FeSiO3) and reaches 307o FeSiO3 in associations with Fe2+-dominantspinels. The presence of tephroite indicates X(CO) values lower than 0.2, a condition that evolved toward higher values, as indicated by poikilslln5l5 of rhodonite. The most important effect of the whole-rock composition is the crystallization of tephroite in rocks with a Mn:Si ratio higher than l.7.Lncal Ca availability determines the crystallization of rhodonite or pyroxmangite. In rocks with a low Mn:Fe ratio, the formation of tephroite is favored only when the accompanying pyroxenoid is rhodonite. kr addition, in the absenceof tephroite, the pyroxmangite is Fe-enriched (307o FeSiO3), but in the absenceof pyroxmangite, the tephoite may contain approximately 20VoFe2SiOo. The iron content of the garnet is controlled by coexisting minerals that preferentially partition Fe.
Kewords: Mn-Fe spinels, Mn-Fe silicates, Mn-rich rocks, chemical composition,flO), X(CO),Iberian Massif, Spain.
Souvarnr
Des roches riches en manganb.seont 61@lscristalti56e5 da:rs le facibs schistes verts dans le complexe volcano-s6dimentaire de la zone de Ossa-Moren4 massiflberique, dans le sud-ouestde l'Espagne. Nous d6crivons quafte assemblagesmanganifbres anormalement enrichis en fer conterurnt un spinelle et des silicates d Mn-Fe: I) magn6tite + pyroxnangite + spessartine + quartz, II) jacobsite riche en Mn3* + pyroxmangite + aegyrine + quaftz, III) jacobsite + pyroxmangite + tephroite + spessartine + rhodochrosite, et tV) magnetite manganifere + rhodonite + tephroi'te feneuse + spessartine + rhodochrosite. Les variations importantes dans la composition de ces min6raux semblent 6tre des fonctions complexes de plusieurs variables. La fugacit6 de I'oxygbne d6termine la teneur en Fe des pyrox6noides de telle sorte qu'elle est trbs faible dans la plnoxmangite des associations contenant le spinelle riche en Mnh (<0.4 VoFeSiO) et aneint 307o de FeSiO3dans les associationsof le Fe2*est dominant dans le spinelle. La prdsence de t6phroi'te indique des valeurs de X(CO) inf6rieures d 0.2, condition qui a €vo1u6 vers des valeurs plus 6levdes, comme le t6moigne le d6veloppement de poeciloblastes de rhodonite. L-influence la plus marqu6e de Ia composition globale des roches serait la cristallisation de la tephroi'te dans les roches dont le rapport Mn:Si d€passe 1.7. La disponibilit6 locale du Ca d6terrnine la cristallisation de la rhodonite ou de la pyroxmangite. Dans les roches d faible rappon Mn:Fe, la for- mation de la tephroi'teest favoris6e ld of le pyrox6noide qui I'accompagne est Ia rhodonite. De plus, en I'absence de tephroi'te, la pyroxmangite est endchie en fer (30VoFeSiO), mais sanspyroxmangite, la tephroi'tepeut conlenir environ2}VoFe2SiOa.La teneur en fer du grenat d6pend de l'assemblage des mindraux coexistants qui pourraient capter le fer.
(Traduit par la R6daction)
Mors-cl4s: spinelle d Mn-Fe, silicates d Mn-Fe, roches manganifBres,composition shimique,flO2), X(COr), massif Ibdrique, Espagne.
I E-mail address: [email protected]
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INTRoDUcIIoN amongmetacherts and bandsof coticules,Within these lenses,a centimeter-sizedmicro-scale banding is Thereare few reportedassociations of Fe and Mn observed. spinelswith Mn silicatesand Mn carbonates.Most of Ttvo stagesof Hercynianmetamorphism have been the studieson suchassociations have been caried out identified(Vauchez 1975, Quesada & Munh61990). in the Mn-rich rocks from the SausarGroup, India The first stageis evidencedby a foliation defined (Bhattacharyaet al. 1988,Dasgupta er aI. 1990,1993) by the preferentialorientation of the phyllosilicates and Namibia (Btihn er al. 1995).The availabledata in shaly lithologies.Manganese silicates crystallized do not yet permit clarification of the influence of along this foliation during the second stage of the controls of metamorphism[R T, whole-rock Hercynian metamorphism,which did not lead to composition,/(Or),X(CO)I andthe crystalchemistry foliation andwas late- to post-tectonicin relationto the of the mineralshave on the stabilityof the Mn andFe first deformationphase (Jim6nez-Mill6n et al. 1994). phases,nor is the partitioning of elementsamong The formation of the Mn mineralsmay be relatedto the phaseswell understood.There is a particularlack of the existenceof small thermaldomes that, in this sector knowledgeconcerning the effect of bulk chemical of the Ossa-MorenaZone,are considered responsible composition on the chemistry and stability of the for assemblagespostdating the main Hercynianphase minerals,as there have beenfew attemptsto obtain ofdeformation(Quesada & Munh61990) . Thepresence individual chemical analyses of the microlayers of spessartinein the rocks studiedsuggests a lower- characteristicof theassociations of Mn andFe minerals. temperaturelimit for this metamorphic event of In this senseothe papers of Huebneret al. (1992),F1ohr approximately400'C (Hsu 1968).However, since all & Huebner(1992) andFlohr (1992)have treated the ofthe samplesstudied are located very closetogether chemical and mineral compositionsof rocks in and since igneousbodies with contactmetamorphic the systemMn-Si-O-Ca-(C). However,there are no haloesare absento the compositional differences among referencesfor studiesof this type in relativelyFe-rich minerals cannot be explained by the effect of systems.This studyis anattempt to improveknowledge temperaturegradients. of tle relationshipsbetween the mineralogyand the chemical composition in Mn-Fe-rich systems.We ANALYTICAL METHODS examinefour mineral associationsin the centralarea of the Ossa-Morenazone, in southwesternSpain, The mineral compositions were determined using characterizedby the common presenceof Mn-Fe a Camebax SX-50 automated elecffon microprobe in spinels (acobsite and magnetite)with Mn silicates the wavelength-dispersion mode operated under the (mineralsof thepyroxenoid, olivine and gamet groups) fotlowing conditions: accelerating voltage 20 kV, probe andrhodochrosite. current 5 nA, beam diameter 0.5 pm. The following compounds were used as calibration standards:albite, Geolocrcer- CoNTEXT orthoclase,periclase, wollastonite and synthetic oxides (Al2O3,FerO3 and MnTiO3). The Ossa-MorenaZoneis locatedin the southern Owing to the microbanded character of the rockso part of the Iberian Massif. During the Hercynian bands with different mineral associations were separated Orogeny,this zone was partitionedinto bandswith before the chemical analysis of the rocks was carried fairly homogeneousinternal tectonic-stratigraphic out. The bands were cut into laminae and crushed, and characteristics,although there are noticeablediffer- a stereo microscope was used to hand-pick the purest encesamong them. The rockssrudied occur in Oliva portions of the complete associationsfor analysis. de la Frontera(province of Badajoz) and belong to Whole-rock analyses were carried out at the X-Ray the South{entral belt, one of the southernmost Assay Laboratories in Don Mills, Onfario using X-ray bandsof the Ossa-MorenaZone ffig. l). Theserocks fluorescence for major elements. occur in a tectonostratigraphicunit referred to as the Cumbres-Hinojalesunit by Apalategui & Pernocnepnv oF TrD, MANGANESEAssocIATroNs S6nchez-Carretero(1991). This unit containsa Cambrian to earlyDevonian stratigraphic sequence metamorphosed In the study area, four mineral associations to the greenschistfacies. An UpperCambrian - Lower were distinguished in which minerals of the spinel Ordovicianvolcanosedimentary complex occurs within and pyroxenoid groups coexist: I) magnetite -r this unit and is characterizedby silicic crystal pyroxmangite + spessartine + quartz, II) manganian metatuffs, slates and Mn- and Fe-rich lithologies: jacobsite + pyroxmangite + aegirine + quartz, III) manganiferousmetatuffs, Mn-bearing slates, coticules, jacobsite + pyroxmangite + tephroite + spessartine + braunite beds, hematite beds and rocks containing rhodochrosile,and IV) manganoanmagnetite + rhodonite Mn-pyroxenoidsand tephroite(Jim6nez-Mill6n et al. + ferroan tephroite + spessartine+ rhodochrosite. 1992),which arethe subjects ofthis study.These rocks The pyroxenoids pyroxmangite and rhodonite were occur in lensesless than I meter thick interbedded optically distinguished according to the values of the
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FIc. 1. A. Geological map of the Oliva de la Frontera area; SP: South Portuguese Zone; OM: Ossa-Morena Zone: CI: Cenual-IberiaaZone: GTM: GaliciaTras os Montes Zone; WAL: Western Astur-Leonese Zone; Ca: CantabianZone. B. Stratigraphic column of the Cumbres-Hinojales Unit. C. Stratigraphic column of the Zahinos volcano-sedimentary complex. Based on Ruiz de Almodovar (1983) and Apalategui & Sdnchez-Canetero (1991).
2V angle,which is close to 90" in the caseof the The rocks containingthe type-Il associationhave rhodoniteand to 40ofor the pyroxmangite.A signifi- a matrix mainly composedof small (70 ttm) anhedral cant fact is the presenceof quartz in the two first crystals of quartz and pyroxmangite.This matrix associations,whereas in assemblagesIII and IV, an includeslarger (200 pm) isolatedcrystals of manganian olivine-groupmineral and rhodochrosite occur instead. jacobsite,pyroxmangite and aegirine.The jacobsite The rockscontaining the fype-I association develop appearsas skeletalcrystals, whereas pyroxmangite and a characteristicbanding, centimeters thick, defined aegirineare anhedral and contain abundant inclusions by layers very rich in spessartine(coticules) and ofquartz. In associationtr, a diffusebanding defined discontinuouslayers formed by a quartzitic matrix by variationsin the modalconcentration ofjacobsite includingthe minerals of associationI. The spessartine canbe observed. of the coticuleshas medium grain-size (<300 pm) and Thetype-Itr associations contain a matrixcomposed a mosaictexture. In thelayers with a quartziticmatrix, of small subhedral crystals of pyroxmangite and thereare also spessartine crystals, but theyare euhedral, spessartine(< 80 pm). In this matrix,the rhodochrosite small (50 pm) and dispersed.The pyroxmangite occupiesan interstitialposition of apparentlyresidual appearsas prismaticcrystals up to 300 pm in size in character.Dispersed in the matrix arenodules several contactwith quartzor magnetite.The latter, in general, millimeters in size composedof anhedralcrystals forms skeletal crystals ranging in size from 30 to (150-200pm) of tephroiteandjacobsite, or ofjacobsite 250 pm that may be replacedby hematire.Locally, alone.Exceptionally, in certainnodules, somejacobsite thereare somemillimeter-sized lenticular aggregates crystalsare transformedto hematiteparallel to various very rich in magnetite. crystallographicplanes.
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Associationsof type IV containtextures similar FeSiO,end-member is a compositionalparameter that to those of type III. The matrix is also fine-grained clearly differentiatesthe pyroxmangitein various (about50 pm) and is composedof ferroantephroite, associationsin the following order:Pxm II (
TABLE l. REPRESENTATM COMPOSHONSOF \In-SILICATES, OSSA-MORENAZONE, SPAIN Mineral Pyroxenoids Association I IIIIII Si02 46.36 46.86 46.17 46.78 35.93 35.86 299t 29.81 Ti02 0.52 0.42 0.28 Al203 l7.57 t6.12 r7.53 FeO* I5.09 0.r5 3.83 2.57 t.o I 7.05 3.72 4.35 t3.62 MnO )o.lz 50.08 48.23 4t.7s 36.t6 36.80 40.74 &.98 s5.65 Mgo t.z5 2.49 0.99 0.50 0.ls 0.16 0.05 0.69 0.68 '7.97 CaO 0.88 0.45 0.56 t.47 1.3I l.4l 0.M o.l7 Total 99.68 100.03 99.78 99.57 99.47 97.30 99.59 99.97 99.93
Si r.004 1.001 r.00r 1.000 2.997 2.980 2.9)2 t.001 0.999 Ti 0.033 0.027 0.018 AI | .728 t.597 | .722 FeJ+8* 0.2'72 0.403 0.260 Fe2+*e o.273 0.003 0.064 0.046 0.263 0.093 - 0.122 0.382 Mn2* 0.663 0.906 0.883 0.756 2.556 2.762 2.877 1.841 1.579 Mg 0.400 0.079 0.03s 0.016 0.019 0.020 0.005 0.034 0.034 Ca 0.m0 0.01r 0.017 0.182 o.r32 0.n8 0.126 0.002 0.006 Resultrof elecfm-microprrobemalyses re qudedin wt"%oddes. Thestrucunal formulae re basedon 2 catios for the pyo: Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/36/3/701/4006414/701_36_3_cm.pdf by guest on 29 September 2021 Mn-Fe SPINELS AND SILICATES. OSSA*MORENA ZONE 705 TABLE 2. REPRESENTATIVECOMPOSMONS OF O)ilDES. OSSA-MORENAZONE, SPAIN ulu Analysisl2345678 SiOr 0.16 0.27 0.| 3 0.05 0.03 0.05 0.04 O.0Z Ti02 0.50 0.23 0.57 0.58 0.40 0.65 0.s5 0.39 AlzOr 0.10 0.t2 0.26 0.24 0.16 0.08 0.07 0.M Fe2O3 l0l.l I l0l.l8 48.34 48.89 75.04 82.10 96.42 98.19 MnO t.42 1.07 46.78 47.15 24.29 18.39 3.98 2.79 MgO 0.03 0.04 0.l7 0. 14 O.O2 0.06 O.O2 0.Oz CaO 0.04 0.01 0.0| Total t03.32 t02.9t 96.29 97.M 99.94 101.33 r0r.09 101.45 si 0.006 0.01I 0.005 0.002 0.001 0.002 0.002 0.001 Ti 0.0t4 0.007 0.017 0.0t7 0.012 0.0t9 0.016 0.011 Al 0.005 0.005 0.012 0.0il 0.007 0.003 0.003 0.002 Fe3+n* t.954 1.960 t.4t4 t.420 1.96i 1.956 1.961 t.974 Fe2+* * 0.973 0.980 o.zt6 o.4t9 0.884 0.918 lilnlrxx 0.530 0.531 Mn2+d* 0.046 0.035 I.010 1.0t0 0.i96 0.599 o.t3z 0.093 Mg 0.002 0.m2 0.010 0.008 0.001 0.003 0.001 0.001 Ca 0.002 0.000 0.001 Resultxof eleckon-microprobcmalyses 6e quotedin wt% oxids. Colrms: I nd 2: magnetito,3 and4: manganianjacobsite, 5 md 6:jacobsite,7 and8: mang@ommagD€tite. Thestructnal formulaeme based on 3 catios. * Total Feis cxpresscdas FqO3, and Mn, as MnO. ** Inferredvalues based on cbarge.balmcecosiderations. A Pyromangtte I J ryroxmangtte tr @ Tephrotte III a III niromanglte Bl Ferroan tephrolte [V a Rhodonlte IV Mn Me Ftc. 2. Ca-Fe-Mn plot of pyroxenoidcompositions from Ftc. 3. Compositionalvariation of olivine minerals from Ossa-Morena. Ossa-Morena. Flohr 1992), although they are somewhat lower than in be differentiated according to their octahedral site garnet of the coticules of Venn Stavelot (Belgium), occupancy. Thus, in the associations with magnetite which were considered by Schreyer et al. (1992) as (IV), the garnet has a higher Al content than that formed from a rhodochrosite-rich precursor. The garnet associated with jacobsite (Itr), which is more Fe-rich of the associationswith rhodochrosite (Itr and W) can and contain over 0.31 apfu Fe3*. This implies the Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/36/3/701/4006414/701_36_3_cm.pdf by guest on 29 September 2021 706 THE CANADIAN MINERALOGIST "Blythite" v#flra"3jsisotz M -1-- I //l_-l- Calderite ,nan3jsirol; vn2jrdjsiroz "Ca3Qa2SiqQ2 '\ l\ Crossularite .a_ rdraditE - Ca3Ai2Si3O12 isi?o1? I I | -----i -_---'- - J- -----7 J---- "AhA12si3012 Almandine Ftc. 4. Vector diagram showing compositional variation of garnet from Ossa-Morena. presence of.4.5Voto l87o of the calderite end-member QMagnetite I Manganian jacobsite II in the garnet of the associationswith jacobsite, as I ! JacobsiteIIIa compared to O-9Va in the garnet associated with JacobsiteIIIb magnetite. [n any case,tlese values are comparatively Manganoan magnetite IV low compared to those of garnet in associations with Mnfe-t rhodochrosite andjacobsite described by Dasgupta et = al. (1987), where 52Voof the calderite end-member is reached. FeqOa MnFe2Oa Mn304 An unusual feature of the oxide compositions is the Magnetite Jacobsite Hausmannite high proportion of the hausmannits,end-member(28Vo) in spinel of association II, classified as manganian jacobsite, but spinel in the remaining associationsdoes Frc. 5. Linear diagramshowing the compositionof the Fe-Mn spinel. not contain Mn3*. In the latter, the composition can be expressedwith i n the series Mn2*Fe3*zO o - F e2*Fe3* 20 4 (Fig. 5); the highest Mn content corresponds to jacobsite of associationIII. Within Mn3*-freespinel, two compositional fields can be distinguishedaccording Fe-rich jacobsite (53-59Vo MnFerOo, symbol ! in to the proportion of the jacobsite end-member in solid Fig. 5) such lamellae are absent.The magnetitein asso- solution. The Mn2*-rich jacobsite contains between ciation IV shows a moderate compositional interval 77 andT9Vo MnFe"Oo (symbol Itra in Fig. 5), and it is (5-l.6Vo of jacobsite), whereasmagnetite in association lexturaly characteized, by the development of hematile I is Mn-poor, and the proportion of jacobsite falls to lamellae along {111} and {100}, whereasin more below 2Vo. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/36/3/701/4006414/701_36_3_cm.pdf by guest on 29 September 2021 Mn-Fe SPINEIS AND SILICATES. OSSA-MORENA ZONE 707 From a chemical point of view, the rocks are rich in TABT.E 3. REPRESENTATIVL o/o), Mn(3448Vo MnO) and Fe(12-28Vo FezOs),and poor WHOL&ROCK COMPOSHONS (wt OSSA_MORENAZONE SPAIN in Al (<8VaAl2O3), Ca ( Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/36/3/701/4006414/701_36_3_cm.pdf by guest on 29 September 2021 708 with Fe2*-spinels(associations I and [V), crystaltized and evolved toward higher X(CO) values after the at oxygen fugacity conditions below those of the crlstallizatl6nof theferroan tephroite. This evolutionis hematite-magnetitebuffer. corlmon in themetamorphism of rockscontaining Mn On the other hand, the lamellar intergrowthsof silicates (Winter et al. 1981.,Dasgupta et al. 1988, hematitein someoccrurences ofjacobsite in association Jim6nez-Milldn& Velilla 1994).Nevertheless, other Itr mark a very local changein thefiO) conditionsthat studieshave documentedstable X(COr) conditions favored development of oxidation reactions and during metamorphism.Thus, Dasguptaet al. (1993) affectedcomposition of this spinel.Thus, oxidation of proposed that X(COt) was buffered by mineral the initial jacobsite gave way to the crystallization reactionsto form Mn silicatesin therocks of Parseoni. of hematiteand moreMn-rich jacobsite,according to India. the following reactionproposed by Essene& Peacor (1983):MnFe,Oo + 02 = (Fe,Mn)rO,+ (Mn,Fe)rOo. Effectof bulk composirton Conditionsof X(CO,) Biihn et al. (1995), among others, showedthat compositional differences of Fe-Mn minelals in The experimentalstudies of the systemMn-Si-O-(C) different types of rocks dependon the paragenesis, (Muan 1959,Candia et al. 1975,Abs-Wurmbach er aL which is strongly controlledby initial differencesin 1983)reveal that the associationpyroxenoid + Mn bulk compositionand oxygen fugacity conditions spinel+ tephroite+ rhodochrositecan be formeda) by among the protoliths.Thus, the crystallizationof reductionreactions frorn a precursorrich in Mn-oxides, tephroiteis restrictedto very limited bulk composi- b) or by oxidationand decarbonation reactions from a tions, for which the valuesof both Mn/Si and Mn/Fe carbonate-richprotolith. areparticularly important.The experimentalstudies of The role of reductionprocesses has been illustrated Muan(1959) and Abs-Wurmach et al. (1983),as well by the petrographicstudies of Ashley (1989) and as its formula, show that progenitormaterials with a Jim6nez-Mill6n& Velilla (1994)on the crystallization Mr/Si ratio higherthan 1 arerequired for the appearance of a tephroite-bearingparagenesis from a hausmannite- of tephroite.This fact was also verified in various rich precursor,and also Jim6nez-Mi1l6n& Velilla associadonsfrom theHoskins mine, Australia (Ashley (L993),who describedthe appearanceof rhodoniteas a 1989),Sierra Nevada, California (Flohr 1992)and resultof the reductionof braunite.On the otherhand, Atacena,Spain (Jim6nez-Mil16n& Velilla 1994),in the studiesof Peterset aI. (1973)and Bhattacharya er which thetephroite only formedin rockswith a Mn/Si al. (1988)suggest the crystallizationof pyroxenoid, valuevery muchhigher than 1. In agreementwith these olivine and spinel by successiveprograde reactions data,the tephrcite-bearingassociations from Oliva de involving decarbonationor oxidation(or both). la Frontera(Itr andIV) havebulk Mn/Si > 1.7,whereas In tle associationsfrom the Oliva de la Frontera thosein which tephroiteis absent(I andII) havebulk area,there is no textural evidencethat tephroitewas Mn/Si lower than 0.6. However,the Mn/Si valuesof formed by reductionreactions involving Mn oxides, associationsI and II are higher than thoseof other Mn silicates or quartz. The residual texture of the rocks from this area characterizedby ttre absence rhodochrositein associationsIII and fV suggests of pyroxenoidsand the presenceof spessartineor that theseparageneses are related to decarbonation piemontite(or both) (Jim6nez-Mill6n& Velilla 1993). processesof a precursor rich in carbonate and, Dasgupta et al. (1993) have shown that the probably,in Fe-Mn oxides. bulk Mn/Fe ratio also determinesthe stability of In thecase of decarbonation,the X(CO) conditions tephroite,since this phaseadmits Fe moreeasily than exercisean important control on the stability and, pyroxmangiteand, therefore,is stabilizedin Fe-rich therefore,on tle order of crystallizationof olivine bulk compositions.However, the data provided by and pyroxenoid.At fixed P andT, rhodochrositeand the Oliva de la Fronteraassociations allow us to make rhodonitereact to form olivine whenX(CO) is lower additionalobservations.The range of Mn/Fe valuesin than0.2; at higherX(CO2) conditions, a pyroxenoidis thetephroite-bearing associations (1.56-2.72) to a large the stablephase, whereas olivine is unstable(Candia et extentoverlaps the rangeencountered in tephroite-free aI. 1975).This finding suggestslocal differencesof ,rssociations(2.1+3,03), Furthermore, the Krm-F" values X(CO, conditionsamong the studiedassociations. betweenpyroxenoid-olivine and mineral-rock show Thus, the olivine in associationsIII and fV indicates that Fe is not stronglypartitioned by tephroitewhen the lower X(CO) conditions(<0.2) than those of associa- coexistingpyroxenoid is pyroxmangite(Kr**-o,m Fe = tionsI andII, whereolivine is absent.Both thetextural 0.89, Kppr-."1Mn-Fe= 5.07 and Kpol-*1h-p" = 5.69). drangement of the ferroan tephroite, included in However, this tendency changeswhen the pyroxenoid poikiloblastsof rhodonite,as well asthe existence of a is rhodonite (Ko"*-o,"o-o"= 3.98, Kr*^-*n*-t" = 10.53 pyroxmangite-richmatrix that includestephroite and and K^,-*om-e' = 2.65). Therefore, we can conclude jacobsitecrystals, show that the X(COr) conditionsin that in rocks with a low Mn/Fe value, the crystallization associationIV were unbufferedduring metamorphism, of tephroite is only favored when other physical and Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/36/3/701/4006414/701_36_3_cm.pdf by guest on 29 September 2021 Mn-Fe SPINELS AND SILICATES. OSSA-MORENA ZONE 709 chemical conditions provoke the crystallization of a strong Fe enrichment in the other mineral that rhodoniteinstead of pyroxmangite. crystallizes.Thus, in associationI, wheretephroite is The crystallizationof a specifictype of pyroxenoid absent,probably due to high X(CO2)and a low bulk dependsmainly on the availability of Ca in the Mn/Si ratio, the FeSiO, componentin pyroxmangite metamorphicsystem. Abrecht & Peters(1975) and reaches307o. However, in associationIV, the X(CO) Abrecht (1988) concludedthat at given pressure conditionsand the late availability of Ca determined and temperatureconditions, rhodonite is stabilized that tephroitewould be the first-crystallizingsilicate over pyroxmangite where significant quantities and that the stablepyroxenoid would be rhodonite, of Ca are available, whereashigh contents of Fe which madepossible the Fe enrichmentin tephroite and Mg increasethe stability of pyroxmangite.In from this association. the manganese-richrocks from the Aracena area In addition,the bulk-rockcomposition determines (Jim6nez-Mill6n& Velilla 1993,1994),this hypothesis the crystallizationand compositionof otherminerals is confirmed by the crystallization of rhodonite in that affectthe compositionof the garnet.In the rocks rockswith manganoancalcite and Ca/(Ca+ Mn + Fe) studiedin this paper,the Fe3*content of garnetfrom higherthan 0.7. However,this controlover rhodonite- rhodochrosite-bearingrocks (associationsItr andfV) pyroxmangitecrystallization is not directly reflected varies according to the mineral association that in associationfV from the Oliva de la Fronteraarea, accompaniesthe garnet.Thus, the rocks with jacobsite where tle pyroxenoid is rhodonite despitethe low (associationsIII), in spiteof havinga bulk AL/Feratio Ca content of this association.Moreover, this Ca (about0.25) higher than the rocks with manganoan contentis similal to tlose of associationsI, II andIII magnetite(association tV) (about 0.10), contain ICal(Ca+ Mn + Fe) = 0.021containing pyroxmangite. garnetwith a greaterproportion of Fe3*.This opposing This apparentanomaly could be explainedby taking trend betweenthe garnetcomposition and the bulk into account the petrographic position of the compositionof the systemcan be explainedby taking pyroxenoid and the local availability of divalent into accountthe effect exercisedby the bulk Mn/Fe elementsat tlre momentof crystallization.Thus, the value on the compositionof the oxidesand tephroite presenceof largepoikiloblasts of rhodoniteindicates coexistingwith the garnet.The high Mn/Fe value of that its crystallizationwas subsequent to that of garnet the rocks with jacobsite (abott2.75) in comparison andtephroite, which preferentiallypartitioned Mn, Fe with the associationswith magnetite (about 1.60) andMg, andprovoked a laterelative enrichment in Ca causes the crystallization of Fe-poor oxides and that favoredthe stabilizationof rhodoniteinstead of tephroitein thejacobsite-bearing associations, which pyroxmangite. permitsa greaterFe entry into the coexistinggarnet. Finally, the absenceof silicatesable to incorporateFe3+ Influenceof themineral association to a greaterextent than garnet produces an enrichment in this ion in thegarnet of associationI. Ashley(1989) In previous paragraphs,we have describedthe similarly noted the preferenceof Fe for garnetover influenceof the bulk composition,/(O)and X(CO) coexistingminerals in the rocksof the Hoskinmine. on the stability and compositionof the phasesof the association.However, some of the compositional CoNcr-usIoNs differencesobserved cannot directly be explainedby the effect of thesevariables. This is the caseof the The most important factors in controlling the differencesin Fe contentsofpyroxenoids from associa- crystallizationof tephroitein carbonate-richprotoliths tionsI, Itr andIV, andofolivine from associationsIII were the low X(CO) (<0.2) and the high Mr/Si ratio in and IV. Thus, the Fe contentof the pyroxenoidfrom the bulk composition(>1.7). The bulk Mn/Fe value association| (>22VoFeSiO3) is notably higher than favored the crystallizationof tephroite insteadof a thoseof the pyroxenoidfrom associationsIII and IV pyroxenoidonly wherethe pyroxenoid was rhodonite. (<107aFeSiOr). The high proportion of the fayalite UndergivenflOr) conditionsand bulk Mn/(Mn + Fe) componentin tephroitefrom associationIV (between value,olivine wasenriched in Fe whererhodonite was 18and24Vo) in comparisonwith tephroiteof association stable. In G6Vo)is also noteworthy.However, in all these A low bulk Mn/Si valueallowed the crystallization associationsthe bulk Mn/(Mn + Fe) is very similar of pyroxenoids.In the rocks studiedhere, this ratio (about0.7) and, furthermore,in all casesthe;(O2) generallyhad a valueof around0.5. Significant Ca was conditionsguarantee a high availabilityfor Fe2*in the neededfor rhodonitecrystallization, either due to high system.Therefore, neither of theseparameters can be original contentoor becausethere was Ca enrichment said to be responsibleof the Fe enrichment.Data on at the time of crystallization.Oxygen fugacity was the the elementdistribution revealthat betweencoexisting most important determiningfactor of the Fe contentof pyroxmangiteand tephroite,the partitioning of Fe the pyroxenoids,because high.r(O) conditionsprevented is very similar (Kr*"*-*o*-t") approximately5). the existenceof Fe2*and, therefore, its incorporation Thereforeothe absenceof one of theseminerals permis into the pyroxenoids.Where the associationwas Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/36/3/701/4006414/701_36_3_cm.pdf by guest on 29 September 2021 7to THE CANADIAN MINERALOGIST formed at lowerflOr) conditions, bur with a similar Buil{, 8., OKRUSCH,M., WornuanN, E., Lnrn'nnr, K. bulk Mn/(Mn + Fe) value, the presence of tephroite & HoanNrs, S. (1995): Metamorphicevolution of produced Fe-depletion of the coexisting pyroxenoid, neoproterozoicmanganese formations and their country rocksat Otjosondu,Namibia. ./. Petrol.36, 463496. whereas its absenceproduced Fe-enrichment. With regard to the garnet, the presence of phases Carun, M.A.F., Prrsns, T. & VALARELLI,J.V. (1975):The that preferentially partitioned Fe (tephroite and oxides) experimentalinvestigation of thereactions MnCO3 + SiO2 caused Fe-impoverishment of the garnet, whereas the = MnSiO: + CO, and MnSiO3+ MnCO3= MnzSiO++ absenceof silicates able to incorporate Fe3*provoked CO2ir, CO2|H2Ogas mixtures at a total pressureof 500 an increase in the proportions of andradite and calderite bars.Contrib. Mineral. Petrol. 52,261-266. componenE. Dascupra, S., Bnr'nnree,H., BHerrecHanva,P., Furuore, S. (1990):Petrogenesis of metamorphosed AcKNowLEDGENGI.ITS M. & Rov, manganesedeposits and the natureof precursorsediments. Ore Geol.Rev. 5,359-384. 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