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Some Metasomatic Calc-Magnesian Silicate Rocks from Connemara' Western Lreland: Mineralogical Control of Rock Composition

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Some Metasomatic Calc-Magnesian Silicate Rocks from Connemara' Western Lreland: Mineralogical Control of Rock Composition American Mineralogist, Volume 65, pages 26-36, 1980 Some metasomaticcalc-magnesian silicate rocks from Connemara'western lreland: mineralogicalcontrol of rock composition BBnNeRn E. LsarP Department of Geology, University of Glasgow Glasgow, Gl2 8QQ, Scotland Abstract Chemical analysesfor 33 major oxidesand minor elementsare reported for a seriesof 26 associatedmetasediments that range from staurolite-garnet schists through similar pelites with graphiteto schistswith varying contentsof actinolitic amphiboleand diopside-richcalc- magnesiansilicate rocks. The calc-magnesiansilicate rocks cannot be matched with any likely original sedimentand have probably formed by metasomatismof pelitesby processes similar to thosethat producerodingites. The presenceof graphite,gradation into pettes, and the positive correlationof Ti and Niggli al-alk support a part sedimentaryparentage for the calc-magnesianrocks. ca, Mg, cr, and Ni (maxima l3.7voCaO,24.2Vo MgO,247O ppm ct atd 620ppm Ni) were added from solutions derived from nearby ultramafic rocks serpentin- ized during static regional metamorphism, which caused andalusite and cordierite growth in someof the associatedpelites. Although the original sedimentarycomposition and the compositionof the metasomattzitg fluids were important in determining the chemical composition of the metasomatic rocks, the stableamphibole and pyroxenemineralogy in the calc-magnesiansilicate rocks was no less important. Theserocks are an exampleof the partial mineralogicalcontrol of the composi- tion of metamorphicrocks. It is proposed that a characteristic feature found in many sedimentary rocks, but not in ig- neousrocks, is a marked positive correlation of Ti and Niggli al-alk. Introduction in addition to calc-magnesianschists rich in acti- nolitic amphibole and diopsidic pyroxene.This suc' In the Dalradian succession south of Cornamona, cession(Fig. l) lies north of a thin staurolite-garnet- Co. Galway, Ireland (Fig. l), is a series of actinolitic two mica pelite that itself lies north of a coarsepeb- amphibole-diopside-biotite or phlogopite-plagio- bly grit. The whole area has been mapped in detail clase-quartz-graphite schists that grade into graph- on the scaleof I : 10,560with every outcrop exam- itic pelites with staurolite and garnet. This paper tab- ined and recorded;Figure I is a simplified geological ulates the chemistry of these rocks and tries to map. The calc-magnesianrocks are interlayed with deduce the origin of the amphibole- and pyroxene- graphitic schists and have transitional boundaries bearing rocks. The study was initially undertaken to both acrossstrike and along strike. The schistosityis determine whether the present rocks were derived by poorer in the calc-magnesianrocks, due to late rather metamorphism of dolomitic pelites or whether their random growth of actinolitic amphibole and clinopy- origin involved basic or ultrabasic igneous material roxene.Some ofthe rocks have a bandedappearanoe mixed with sedimentary matter. The results obtained due to differential weathering,but the contactsbe- point to the operation of metasomatic as well as orig- tween the bands are almost always gradational (e.g. inal sedimentary processes,the rocks being produced samplesBL36l l-7). Clinopyroxene-richclots some- by the interaction of solutions, similar to those that times occur in amphibole-rich schist (e'9. BL36l7), produce rodingites, with graphitic pelites. and patchesof garnet-rich rock (e.9. BL3623) occur The samples come from a graphitic schist series occasionally. Although ultramafic lensesoccur in the that contains some serpentine and ultramafic lenses, area, these are always distinct with sharp contacts. rare thin calcite marbles, and thin banded quartzites, There is no gradation into the studied rocks, nor do 0003-004x/80/0l 02-0026$02.00 26 LEAKE: CALC-MAGNESIAN SILICATE ROCKS n AREA STUOIED o"w Fig' l' Geological map of part of the area south of Cornamona, Co. Galway, Ireland showing the sample locations. Mappcd by the author. the calc-magnesianrocks occur in flattenedor unflat_ of pelites (defined by over 100 Connemara pelite tened tectonic lenses. analysesgiven in Senior and Leake, 1978),in which representativeConnemara pelites plot towards a composition that lies between magnesiteand dolo- mite. These plots show that neither pure limg5ge1. (Niggli c 100)nor pure magnesite(c zero,Niggfi mg 1.0) was involved as a component of the original kyanite pre-date a strong static metamorphism that sedimentary mixture. If carbonate-pelite mixtures producedcm-sized cordierite and andalusiteporphy_ were involved, then from the c-mg plot of Figure 2 roblastsin a patchy distribution in the pelites.It was they must have been roughly equal proportions of during this post-F. metamorphismthat the amphi_ dolomite and magnesite mixed with variable bole- and pyroxene-bearing rocks crystallized, for amountsof pelite. Such a sedimentwould have been theseminerals have random arrangement. a most curious one, in that the proportion of dolo- Geochemistry mite to magnesitewas rather invariable and also be- causeprimary magnesiteis Carbonate -pelite mix tures not at all usual in sedi- mentary rocks; indeed, Johannes (1969) supposes Figures 2 and 3 show that the analyzed rocks (Ta_ that magnesitealrnost always originates by metaso- bles I and 2) plot in a cluster extending from the field matism. 28 LEAKE: CALC-MAGNESIAN SILICATE ROCKS Moreover, most of the samplescontain as much SiOr, or even more, than typical Connemarapelites [!v ^ Published Connemoro which have less than 5Vototal CaO + MgO (Fig. a). v pelite dnolyses. Thus averageConnemara pelite tlas 55.39VoSiO, and + llo omphibole or Pyroxene -- Actrnolitic or tremolrtrc v omphibole wilh diopsrde . Actrnolile tremolite rich [-?v ^ Publishcd Connemoro " pelite omlyses. + No omphibote or pyroxere E ra Aciinotitic or ir.molilic dmphibole wilh dioDsidc . Actinolite tremotiie rich a 6.p. ++ ? o a!) o 6)^-R -9 Ql -Tremolrte 0 c Fig. 3. Plot of Niggli al-alk against c for the analyzed rocks plus published Connemara pelites. 4.26VoCaO + MgO (Senior and Leake' 1978), o whereas the present samples (Table l) average +! E 6O.08VoSiOr, or if only thosesamples with more than l}Vo CaO + MgO are considered,then the SiO, aver- ages5495Vo and the CaO + MgO 22-34Vo.This is an insignificant silica change despite lSVomorc CaO + + 4, T pelite. Even if the SiOuhad tp oHJ MgO than in an average " cr, +,+ to complete decarbonation,no + been increaseddue mixture of carbonate with pelite can be as rich or richer in SiO, either before or after decarbonation than the same pelite alone, because of the residual CaO * MgO content in the mixture. the CaOlMgO ratio againstSiO, (Fig. 4) suggestsvariation between pelite, tremolite, and diopside,not pelite and carbon- ate. Complete decarbonation is not an attractive the- ory becausenearby stable calcite marble is common in the area (Fig. 1) and decarbonationis not signifi- Actinolite 9.'6 cant. with 70'l.Mg against the carbonate-pelite mix- 30'/. Fe #: lrremot,te Further evidence a the trace elements ..O a ture hypothesis is obtained from Ni and Cr, both of which are present in sedimentary + carbonatedeposits in very low amountswhereas pe- +, lites comnonly contain much more Ni and Cr. Mix- ,oB di -o g f progres- l+o tures of pelite and carbonate should have 0t sively lower contents of Cr and Ni as the carbonate 0 content increases.Figure 2 showsthat this does not m9 occur. The averageNi and Cr contentsof the eleven : + MnO Fig. 2. Plots of Niggli mB Mol MgO/(MgO * FeO samplesthat are amphibole- and pyroxene-freeare 2 FerOr) against Ni and Cr in ppm and Niggli c. Published + ppm comparedwith 166ppm Connemarapelite analysesare taken from Leake (1958), Evans 88 ppm Ni and 128 Cr, (1964),and Cruse and Leake (1968)in this and subsequentplots. Ni and 688 ppm Cr for the averageof the 16 amphi- LEAKE: CALC-MAGNESIAN SILICATE ROCKS 29 Table l. Chemical analyses of amphibole- and pyroxene-free Explanation to Tables I and 2 metasediments BIJ511 eraphitic qurtz schist with albite dd Kf po4)hyroblasts ad - phlogopite, 3r t612L t61t J615 36j6 3621 1622 1621 1625 j612 1631 a little sulphide ed actiaolitic mphibole. 100 yeds SSW of Sench ylatk 14.4 that is on a bridge on the Co@orclerLusk bomdry. BL1612A E@ttz-tuscovite-biotite graphite schist uith rre soalI ganets md relic biotite crystalloids lying acloss the sclristosity. Iocalion ae 1511, BLt612B As 3612L b[t with ephibole ed pJEoxene clots, 8L1611 Albite, with q@rtz irclBions in a relict S., @covite- biotite-q@rtz-rde stauolite ed to@1in6 schist. Location as ,611. BLt61 4 Ewtz-actinolitic @phibole-diopeide-ph-loeopite-g?apllite- eufphide schi6t. As J611. BLr515 Biotite-nuscovlte schist with qrJatz folia, oligoclase over- growing puckeretl graphite trails, ecl rae gamet mal tomaline. l,ocation as ,611 . 81,1616 Phlogopite-qErtz-feldspd-gTaphite schist {ith a little prehlite in biotite ad rde K-Cveins. As ,511. 31,)61J Diopside-rich rock with coopletely sericj.tised feldspd, a clppo 117 110 224 176 1|14 fa pa 94 165 150 little act-inof itic @phibole, qMrtz, leucoxeLised sphene, 348 graphite md su-lphide. As J5'11 . 96 8L161 A qDil tu -tich-@phibole-sericiti sed f eldspar sclris t with a 41 little sulphide ed biotite. 70 tBrds SSWof the first o of Con@ona (tomlmd) as @itten on sheet ,9, Co. Galway 147 1 t1A,55O. BLJ619 Diopside-actlnolitic @phibole phlogopite-q@rtz-Kf-selicite schist with graplrite, sulpldde ed sphene. 140 yads SSV of 117 the last a Cofreom (tomlad) 12 of as ritten on sheet J9, Co. Galway 1,10,160. 91 Nb151a12i1161|71|7:|4 16 BI1620 Diopside-actinofitic @phibof e ph-logopite-Icf-sericitised cs 14 15 16 14 15 16 io 14 16 17 feldspar schist uith graphite, su-lphide, qrEtz dd sphene. Ba 1BB5 1850 940 B2B 1180 814 6A1 TOa 2175 As 1619. r,a 49 51 36 27 i2 4t 41 41 4t 42 BLJ621 Biotite-nGcovite-qwtz-p1ag'ioclase schist vith a little ce 110 123 89 76 129 105 150 102 1|o4 91 game!, su]ptdde ad eraphite.
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