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Petrology of Clintonite-Bearing Marbles in the Boulder Aureole, Montana

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Petrology of Clintonite-Bearing Marbles in the Boulder Aureole, Montana American Mineralogist, Volume 64, pages 519-526, 1979 Petrologyof clintonite-bearingmarbles in theBoulder aureole, Montana Jecr M. Rlcr Departmentof Geology,Uniuersity of Oregon Eugene,Oregon97403 Abstract The trioctahedralcalcium brittle mica, clintonite,occurs locally in aluminousmarbles near the intrusivecontact of the Boulderbatholith. Assemblages include clintonite-calcite-olivine- clinopyroxene-phlogopiteand clintonite-calcite-olivine-spinel-phlogopite.Microprobe analysesof coexistingphases show clintonite to lie withjn the phasevolume calcite-olivine- clinopyroxene-spinel,close to the MgSi-rich end of the synthesizedsolid solution. Phase relationsare characterizedby both continuousand discontinuousreactions. Discontinuous reactionsbetween the observedphases result in a topology in pCOz-pHzOspace which restrictsthe stability field of clintonite to relatively low pCOz and/or high pHrO. The formation of clintonite was not relatedto metasomatism,but rather to reaction between calcite,olivine, clinopyroxene, spinel, and HzO introducedfrom the intrusivebody. In the Boulderaureole the formation of clintonitecan be restrictedto temperaturesbetween 580o and 620'C with fluid compositionsbetween XCOz of 0.05and approximately0.2. Introduction changedthrough metasomatism.Such an origin has Trioctahedralcalcium brittle micas,most appro- also been invoked to explain the observationsthat priately referredto by the generalname clintonite, clintonite-bearingrocks are typically restricted to the form a solid-solutionseries which can be represented immediatecontacts of intrusiveigneous rocks and by the generalformula oftenspatially associated with skarns(Knopf, 1953). In the contact-metamorphicaureole surrounding + + Ca [(M g, Fe'z ), *,(Al, Fe3 )2 - A\ _ 10(O H, F "f "Si"O ), the northernmostportion of the Boulderbatholith of Although suchmicas are rare, being found occasion- Montana, clintoniteis found locally in impure lime- ally in thermally-metamorphosedimpure carbonate stones near the contact with granodiorite. Knopf rocks, they are interestingfrom a crystal-chemical (1953) reported several occurrencesof clintonite point of viewbecause of their high ratio of aluminum south and southeastof Helena in rocks which he to silicon(>l) in the tetrahedralsites. Olesch (1975) called pyrometasomatictactites. This report de- has synthesizedhomogeneous clintonites over the scribesclintonite-bearing marbles in the area l0 km compositionalrange z : 0.6 to 1.4.This rangeof westof Helena(Rice, 1975;1977). For theserocks the solid solution encompassesand on the aluminum- rather specialbulk-compositional requirements were rich sideexceeds the observedvariation in naturally- met in the originalsediment, and, exceptfor changes occurring brittle micas. Olesch and Seifert (1976) in the chemicalpotentials of HzO and COz,the for- havedetermined the upper thermal stability limit of mation of clintonite was not relatedto metasomat- the solid-solutionseries, and have shown that such ism, but ratherto nearlyisochemical reactions taking micas have a rather large P-T field of stability. In placein HzO-richfluids. view of the large P-Z stability field, they conclude that the critical parametersfor clintonite formation Clintoniteparageneses are not fluid pressureor temperaturebut a rather In the areawest of Helena(Fig. I ), granodioriteof specialbulk compositioncharacterized by high AlzO, the CretaceousBoulder batholith and Paleozoicsedi- and low SiO, contents.The rarity of suchcomposi- ments are in subverticalcontact over a distanceof tions has led them to further suggestthat clintonite about 20 km. The metamorphosedcarbonate rocks willnot form unlessthe originalbulk compositionis are impure dolomitic limestones,typically silica-un- 0003404x/ 79 /0506-05 I 9$02.00 519 520 RICE: CLINTONITE-BEARING MARBLES Clintonite-bearinglimestones have been found at four localitieswithin 100meters of the intrusivecon- tact.Knopf (1953)reported an additionaloccurrence at the contactof the smallsatellite stock in the north- ern portion of Figure l. The assemblagesin these samplesare eithercalcite-clintonite-olivine-clinopy- roxene-phlogopiteor calcite-clintonite-olivine-col- orlessspinel-phlogopite. As illustratedin Figure 3, clintonitetypically occurs as randomly-oriented elon- gate blades I to 2 mm in length. When the brittle mica is abundant(ca. 20 modal percent)the rock takeson a microscopicjackstraw texture. The clinto- nite is characterizedby a smalloptic angle(2V" : 5"; and pleochroismranging from colorlessalong a to very pale brown alongB and 7. Olivine and clinopy- roxeneoccur as prismaticidiomorphic poikiloblasts composedof numeroussubhedral grains with com- mon extinction.The observedtextural relationships indicate that clintonite formed by prograde meta- morphismin equilibriumwith its coexistingphases. Basedon the calculationsof Jaeger(1957), the temperatureat the immediatecontact of the gran- odiorite is believedto have been between600 and 700"C.Structural and stratigraphicrelations summa- Fig. l. Outline geologic map of the northernmost portion of the rizedby Rice(1975) suggest that the total lithostatic Boulder batholith showing the location of clintonite-bearing pressureduring metamorphismwas approximately samples. Numbered isograds described in detail by Rice (1977) include: (l) first appearance of tremolite and phlogopite; (2) first 1000bars. The absenceof eitherpericlase or brucite appearance of diopside in silica-saturated bulk compositions; (3) appearanceof olivine * spinel and simultaneous disappearanceof calcic amphibole * dolomite; (4) appearanceof diopside * calcite * spinel. dersaturatedand highly aluminous.In the central and outer portionsof the contactaureole, rocks of this bulk compositioncontain the assemblagecalcite- dolomite-calcicamphibole-chlorite-phlogopite (see Fig. 2 for chemographicrelations). Isograd 3 marks the first appearanceof the assemblageolivine-cal- cite-spineland the simultaneousdisappearance of calcicamphibole * dolomite.Between isograd 3 and the intrusivecontact, the predominantmineral as- semblageis calcite-dolomite-olivine-spinel-phlogo- pite. Less common bulk compositionsare slightly more silica-richand containthe assemblagecalcic amphibole(pargasite) * calciteinward from iso- grad 3. Rocks of this bulk compositionreact along Fig. 2. Compositionof modelphases in the systemCaO-MgO- isograd 4 to produce clinopyroxene,resulting in SiOr-AlrOr-COr-HrOprojected from calcite,COz, and HrO onto the assemblagecalcite-olivine-clinopyroxene-spinel- the plane dolomite-quartz-corundum. Abbreviations and phlogopiteinward from isograd4. Overmost of the compositions:Qtz = quartz;Co = corundum;Do : dolomite;Fo : : : : area, the assemblagecalcite-olivine-clinopyroxene- forsterite;Tr tremolite; Di diopside;Cats calcium Tschermak'spyroxene; Sp = MgAl-spinel; Cte : chlorite spinelcontinues inward to the contact;locally, how- [Mg"ALSLO,.(OH)'];Mi"" : rangeof clintonitesolid solution; ever,rocks of this bulk compositioncontain clinto- Mi' : 6u14tz.Alo6Si,.Al2"O.(OH)r;ML : CaMgrAlSiAl, nite. O'o(OH>. RICE: CLINTONITE.BEARING MARBLES 521 Table 2. Microprobe analysesof clintonite and phlogopite CLINTONIIE PHLOCOPIIE 2 SiO2 79.2 18.8 18.4 39.8 40.0 TiO2 0.40 o.47 0.46 0.61 0.62 A1203 39.6 39.8 41.01 17.0 t6.4 FeO* L.69 1.38 L.46 1.60 r .55 Mco 2L.6 2t.6 20.9 25.5 25.6 CaO la ) 13.0 r2.9 <0.02 <0.02 Kzo <0.01 <0.01 0.01 10.7 LO.7 Na2O 0.72 0.18 0.16 0.26 0.22 F 0.1 0.3 0.2 0.5 0.4 95.86 95.4s 95.42 95.76 95.32 ForrruLa mtts per I eations Fig 3. Photomicrograph of the assemblage calcite (Cc)- si L.345 r.321 L.296 2.796 2-823 olivine (Ol )-diopside (Di)-clintonite (Mi) Alrv 2.655 2.679 2.704 I.204 L.rt1 pseudomorphsafter periclaseimplies a temperature Atvr 0.615 0.626 0.699 0.207 0.190 of lessthan 650oC(Greenwood, 1967' 1975). This Ti 0.021 0.025 0.024 0.032 0.033 3* rangeof contacttemperatures (600-650"C) is sup- Fe 0.018 0.046 0.002 0.001 0.001 ported by calcite-dolomitegeothermometry, which Fe2* 0.081 0.035 0.084 0.094 0.092 2.256 2.264 2.L94 2.671 2.692 yields temperaturesslightly greaterthan 600oC for Ca 0.991 0.979 0.973 <0.001 <0.001 samplesnear the contact(Rice, 1977). K <0.001 <0.001 0.001 0. 959 0.953 Na 0.016 0.024 0.022 0.035 0.031 Table L Microprobe analysesof clinopyroxene and olivine crrnoplFdVEfrE x Total iron as FeO, x* Totals less oxygen equivalency of fluorine. 723 si02 52.I 50.3 48.8 41.0 40.9 Mineral chemistry Tio2 0.80 1.93 2.s8 0.01 <0.01 A12O3 3.39 4.63 6.00 <0.01 <0.01 Electronmicroprobe analyses of coexistingphases FeO* 2.30 2.02 L.96 4 . s8 4.88 in three samplescontaining the assemblagecalcite- 16.5 16.0 15.7 s5.1 54.r olivine-clinopyroxene-clintonite-phlogopiteare Ca0 25.0 25.5 24.7 0.01 0.10 givenin TablesI and2. Standardsincluded a suiteof 100.1 100.4 99. 8 100.7 99.98 naturaland syntheticolivine, clinopyroxene, phlogo- pite, and anorthite, as well as a seriesof synthetic Cations per 6 otAgens 4 orAgens oxides.Data reductionwas performed with a Bence- Si 1.898 1. 834 r.790 0.976 0.982 Albee-typecorrection procedure. Ti o.022 0.053 0.071 0.0002 < .0002 Olivineis typicalof that in metamorphosedimpure A1 0.r45 0.198 0.259 <. 0001 < ,0001 dolomitesin beingclose to its idealend-member com- Fe 0.070 0.062 0.060 0.091 0.o41 position (mole percentforsterite >95). In contrast, Mg 0.896 0.871 0.858 1.955 1.936 clinopyroxenecontains significant amounts of alumi- o.976 0.996 0.969 0.0002 0.002 num and titanium.Below the analysesof clinopyrox- Fomula units per 4 cations eneare listedcalculated cations per six oxygens.All S1 r.894 7.828 1.785 three analysesyield a cation sum which is slightly .-IV 0.106 0.r72 0.2r5 greater than 4.0, suggestingthe presenceof ferric ..vI 0.039 0.026 0.o44 iron. The data havetherefore been recalculated into Ti 0.o22 0.053 0.071 4.0 In pro- 1+ formula units normalizedto cations. this te 0. 014 0.034 0.027 1L cedure,additional oxygen is addedto bring its sum !e 0.056 0.o27 0.033 to 6.0, and for each amount of oxygenadded twice 0.894 0.867 0.856 that amount of Fe2+is convertedto Fe3+(Finger, 0.97 4 0.993 0.968 1972).The resultingformulae have ferrousto ferric ratiosranging from 4.0 to 1.0.Titanium is considered * Total iron as FeO to be entirelyTio+.
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