Mg-Rich Dumortierite in Cordierite- Orthoamphibole-Bearing Rocks from the High- Grade Bamble Sector, South Norway

Mg-rich dumortierite in cordierite- orthoamphibole-bearing rocks from the high- grade Bamble Sector, south Norway

D1EDER1K VISSER AND ANTONY SENIOR Department of Petrology and Experimental Petrology, Institute of Earth Sciences, University of Utrecht, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands

Abstract Dumortierite is described from several occurrences of cordierite-orthoamphibole-bearing rocks in the upper-amphibolite facies area of the Bamble Sector, south Norway. Dumortierite occurs with chlorite, muscovite and quartz in late M 4 alteration zones or aggregates after M3, peak-metamorphic cordierite and garnet and early M 4 vein-cordierite, and intergrown with or replacing orthoamphibole. Late M 4 P- T conditions, which are interpreted as conditions of dumortierite formation, are estimated from the associated late M 4 kyanite-andalusite-chlorite-quartz assemblage and Mg-Fe exchange geothermo- metry to be 500 + 50 ~ and 3-4 kbar. Retrogression of M3 mineral assemblages is initiated by influx of fluids with Xco2 of 0.3-0.4 during early M4 followed by more water-rich fluids during late M4. Late M4 fluids may show local variations in alkalis and boron. The dumortierites are the most Mg-rich (2.23- 3.42 wt. % MgO) ever reported, and contain 0.002.05 wt.% TiO2, 0.00-1.08 wt.% Fe203, 29.62-32.42 wt.% SiO2 and 55.2059.71 wt.% A1203. A1 is the most likely substituent for Si, which shows a minor deficiency at the tetrahedral sites in most dumortierites. The major variations in the mineral chemistry can be described by the coupled substitutions Mg + Ti = 2AI vl, 3Mg = 2A1 vl and possibly Mg + H = AI vl. Favoured by lowfo 2 prevailing conditions a significant part of total iron in dumortierites at one locality is present as Fe z+ giving evidence for the Fe 2+ + Si ~v = AI IV + AIVI tschermakite substitution. FeMg_l substitution is considered to be limited. Ti-rich dumortierites coexist with rutile, ilmenite or titaniferous magnetite. The development of dumortierite from orthoamphibole correlates with an observed decrease of A1, Mg and Na and increase of Si and Fe in orthoamphibole towards dumortierite. KEYWORDS." Bamble Sector, south Norway, cordierite, Mg-rich dumortierite, garnet, orthoamphibole.

Introduction tolytic-hydrothermal deposits (Kerr and Jenney, 1935; Sabzehei, 1971; Black, 1973) and regional RECENT microscopic investigations of cordier- metamorphic quartzites, argillaceous gneisses ite-orthoamphibole-bearing rocks from the Bam- and granulites (e.g. Christophe-Michel-L6vy bte Sector, south Norway, revealed several new et al., 1959; Schreyer et al., 1975; Vrfina, 1979; occurrences of the relatively uncommon borosili- Takahata and Uchiyama, 1985; Beukes et aI., cate dumortierite (Fig. 1). This rare mineral has 1987). More than 75 years after its discovery by already been reported from the same area by Gonnard (1881), the space group (Pmcn), unit- Bugge (1943) and Touret (1979). The first dis- cell parameters and preliminary general formula covery however was questioned by Lamb (1981), for the orthorhombic dumortierite, (A1,Fe3+)TB - who identified the blue mineral of Bugge as Si3018, were deduced by Claringbull and Hey sapphirine. (1958). Its crystal structure was solved by Golo- Dumortierite is generally reported as a minor vastikov (1965), confirming the space group Pmcn constituent in pegmatitic, aplitic, and granitic assigned by Claringbull and Hey (1958), and rocks (e.g. Finlay, 1907; Graham and Robertson, further refined by Moore and Araki (1978). The 1951; Huijsmans et al., 1982), associated pneuma- presence of water has undoubtedly been demon- Mineralogical Magazine, December 1991, Vol. 55, pp. 563-577 ~) Copyright the Mineralogical Society 564 D. VISSER AND A. SENIOR

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Fro. 1. Locality and simplified geological map of the central part of the Bamble Sector, south Norway. Opx-isograd after Field and Clough (1976). Mg-RICH DUMORT1ERITE 565 strated by several studies (e.g. Moore and Araki, mid-amphibolite facies conditions. Locally 1978; Alexander et al., 1986; Werding and upper-amphibolite to low-granulite facies con- Schreyer, 1988; 1990) giving the rearranged tact-metamorphic conditions were recorded general formula of dumortierite, a16.75[SJ0.25- around augengneiss bodies (Nijland and Senior, BSi3[O17.25(OH)0.75] (after Moore and Araki, 1991). 1978). Recently Werding and Schreyer (1988, 1990) were able to synthesise dumortierite in the system A1203-B203-SiO2-H20. Their experi- Petrography mental data show a variation of H20-contents. Dumortierite was found as a minor constituent In this paper, the occurrence in cordierite- at seven localities of cordierite-orthoamphibole- orthoamphibole-bearing rocks and the Mg-rich bearing rock-types and associated quartz-cor- mineral chemistry of the Bamble dumortierites dierite veins in the upper-amphibole facies area. will be presented and discussed. Some consider- At each locality, the dumortierite grains are so ations on the P-T conditions of formation will be scarce and small that they were found in only given. about one out of five to ten thin sections. In Table 1, the location and metamorphic assemblages of Geological setting the dumortierite-bearing samples are listed. Dumortierites in all samples have a secondary The dumortierite-bearing cordierite-orthoam- origin. Three types can be distinguished (see phibole rocks occur as thin layers and lenses in the Table 1): upper-amphibolite facies area of the central part i. Dumortierite intergrown with retrograde of the Bamble Sector (Fig. 1). The high-grade alteration products of cordierite (samples AK Bamble Sector was subject to two major periods 034, DV 034, DV 249, MA 822, RV 070, TN 087), of metamorphism and deformation (Starmer, II. Dumortierite intergrown with retrograde 1985). Aluminous reaction textures in orthoam- alteration products along cracks in garnet (sam- phibole-bearing rocks record a 'near-clockwise' ples AK 036, DV 030, MA 780), prograde P-T trajectory (M1-M3) for the Kongs- III. Dumortierite intergrown with orthoamphi- bergian Orogeny (1600-1500 Ma) (Visser and bole or replacing orthoamphibole (samples DV Senior, 1990). This prograde P-T path is 183, MA 780), explained as a collision scenario with magmatic addition which produced NE-SW elongated The first type of dumortierite occurs in two structures. Peak-metamorphic conditions (M3) generations of cordierite, namely a primary reached upper-amphibolite to granulite facies cordierite of the peak metamorphic M3-assem- conditions of about 800 _+ 60~ and 7.3 + 0.5 blage and a secondary cordierite in probably kbar (Lamb et al., 1986). A long period (300 Ma) early-M4 quartz-cordierite veins which cut across of near isobaric cooling occurred between the M3-assemblages. Both generations of cordierite Kongsbergian Orogeny and the second major locally show severe pinitisation and in some period of metamorphism and deformation, the grains the typical coarse-grained late-M4 assem- Sveconorwegian (ca. 1200-1000 Ma) Orogeny. blage consisting of kyanite-(_+ andalusite)-chlor- The tectonic effects of the Sveconorwegian Oro- ite-quartz developed. The euhedral to subhedral geny (M4 according to Visser and Senior, 1990) in dumortierite grains occur at the margin of the the Bamble Sector are more pronounced towards alteration aggregates composed of chlorite + its margin with the Telemark Sector. 'Near'- quartz + pinite + kyanite _+ andalusite (Figs. 2 isothermal uplift of the Bamble Sector, represent- and 3). Intergrowth of dumortierite with kyanite ing emplacement in the upper crust, is indicated or andalusite has not been observed. Type I by M4 assemblages in cordierite-orthoamphi- dumortierite occasionally is in contact with titan- bole-bearing rocks which include talc-kyanite- iferous magnetite (C. Maijer, unpublished data). quartz (early M4) and kyanite-andalusite- The dumortierite grains normally are very small chlorite-quartz (late M4) (Visser et al., 1990). (50-175 btm in length) but dumortierite in sample Domains with 'Kongsbergian' structures and ages AK 034 measures up to 350 btm in length (Fig. 3). as e.g. the Arendal granulite facies terrain, Besides the red pleochroism (X = red, Y = Z = largely seem to have survived the effects of the colourless), the type I dumortierites sometimes Sveconorwegian Orogeny (Starmer, 1985, 1990). display a faint purple-blue pleochroism in the The Sveconorwegian Orogeny, an event of exten- centre of the grains. Large euhedral, optically sive but channelled fluid activity associated with unzoned, pale yellow to light brown-green tour- the formation of pegmatites, granite, albitites and maline grains up to 700-900 ~tm accompany quartz-veins, regionally reached greenschist to quartz and cordierite in samples MA 822, AK 034 566 D. VISSER AND A. SENIOR

Table i. The location and mineral assemblage in selected samples_ Map references are from maps prepared by Norges Geografiske Oppm~ling, Series M711, sheets 1612 II-III and 1712 IV.

Locality Sample Minerals

Boylefossbru 4831-64966 DV 030 Qtz, Grt, oam, St, Crd (M3), Ap, Rt, St, (Chl, Ms, Dum II)

DV 034 Qtz, Crd (M4), Ap, (Chl, Ky, And, Ms, Tur, Dum I)

DV 249 Qtz, Oam, Crd (M3) , Bt, Rt, Ap, (Chl, Ms, Ky, Dum I)

Haugsj~ 4844-64968 TN 087 Qtz, Oam, Grt, Crd (Ms), St, Rt, Opq, (Ky, Ms. Chl, Dum I, CC)

Stutgsen 4886-65009 DV 183 Qtz, Oam, Bt, Crd (M3) , Rt, Grt, (Ms, Chl, Dum III)

Solbutjernane 4915-65059 RV 070 Qtz, Crd (M4) , St, Rt, Opq, (Tlc, Chl, Tur, Dum I, Cc, Preh)

Norum 4921-65098 AK 034 Qtz, Grt, Tur, St, Rt, Opq, Crd (M3), (Rt, Ms, Chl, Tur, Ky, And, Dum I)

AK 036 Qtz, Oam, Grt, Crd (M~), Bt, Tur, Opq, Rt, (Ky, And, Ms, Chl, Cc, Tlc, Dum If)

Rod 5030-65096 MA 780 Grt, Bt, Ord (M3) , Oam, Qtz, Ap, Tur, Krn, Gr, Opq, Rt, Ilm, (Dum II, Dum II, Chl, Ms)

Bjordammen 5245-65342 MA 822 Qtz, Bt, Crd (M4) , Ap, Tur, Mag, (Chl, Ky, And, Ms, Tur, Dum I)

Mineral abbreviations after Kretz (1983) Dumortierite type I, II or III (Dum I, Dum II and Dum III) (see text), Muscovite/pinite (Ms), Opaque (Opq). Square brackets indicate secondary minerals. (M3, M4) - Primary M3-cordierite and early-M 4 vein-cordierite. and AK 306; smaller euhedral yellow-green nantly are intergrown as irregular shaped crystals tourmaline grains (up to 400 ~tm) occur with (X = red-rosa, Y = Z = colourless to pale yellow) chlorite, quartz and Al-silicate in the centre and and fibrous masses (Fig. 5) with or developed at at the margin of the alteration aggregates of the expense of subhedral to euhedral orthoamphi- cordierite in samples DV 034, RV 070, AK 034 bole. Rarely type III dumortierites occur as and MA 822. The smaller tourmalines and subparallel and sheave-like aggregates of euhed- dumortierites never occur intergrown within the ral crystals (X = red, Y = Z = colourless to pale same alteration aggregate. yellow) intergrown with orthoamphibole along Type II dumortierite occurs along narrow the cleavages. Individual crystals measure up to retrograde zones or cracks in garnet blasts (Fig. 200 ~tm in length. Minor quartz, ilmenite and 4), which may measure up to several centimetres "rutile accompany the dumortierite grains. Euhed- in diameter in sample DV 030. Dumortierite ral type II dumortierite may be present in sample commonly forms single acicular euhedral crystals MA 780 (Table 1). (X = red, Y = Z = colourless to very pale yellow) Colourless kornerupine and yellow-brown but subparallel bundles may be present. The tourmaline occur together with dumortierite in dumortierites in DV 030 are accompanied by sample MA 780. Kornerupine is present as euhedral large chlorites (up to 600 ~tm), quartz euhedral to subhedral grains intergrown with and fine grained white mica. Type lI dumortierite garnet, quartz and graphite and with orthoamphi- grains may be up to 400 ~tm in length and 80 ~tm in bole along the cleavage planes. Although inter- width. Although rutile is an important minor growth textures with orthoamphibole are very phase no contacts with dumortierite are observed. similar no overgrowths or intergrowths of type III Type 111 dumortierite is situated in narrow (up dumortierite and kornerupine are observed. to 4 mm wide) retrograde zones or cracks, along Retrogradation of kornerupine from korneru- which severe pinitisation of cordierite, chloritisa- pine-orthoamphibole intergrowths dissected by tion of garnet, biotite and orthoamphibole alteration zones only produces chlorite and fine- occurs. Dumortierites in these zones predomi- grained white mica. Mg-RICH DUMORTIERITE 567 Subhedral to euhedral yellow-brown tourma- chains built of AlO 6 octahedra and running line occurs in minor amounts overgrowing biotite parallel to the [001] direction (see Moore and and as inclusion or intergrowth with M3-cordier- Araki (1978) for graphic representation of the ite, garnet and quartz. No textural relation with structure and site-notation). The first, A1(1)O3 kornerupine or dumortierite is observed. chain, is a disordered central infinite face-sharing Common alteration products of dumortierite chain linked in a pseudohexagonal fashion to six such as muscovite/sericite and kaolinite (Schaller, external, 3AI(2)2-AI(3):O~ and 3A1(4)O12, octa- 1905; Sabzehei, I971; Beukes et al., 1987) were hedral chains by six isolated 2Si(1)O4 + 4Si(2)O4 not observed in the investigated samples. tetrahedra. The second, Al(2)2-AI(3)2012 chain is an inversion doubled pyroxene-like edge-sharing chain. The third type is a pyroxene-like edge- shared chain doubled to an A1(4)4012 chain by Mineral chemistry face-sharing. The external chains are intercon- Mineral analyses have been obtained using a nected by BO3 triangles. X-ray site refinements of Jeol JXA-8600 Superprobe at the University of Moore and Araki (1978) and Alexander et aL Utrecht which operated at an acceleration poten- (1986) indicate that site-occupancies of the B, tial of 15 kV, and a sample current of 10 nA. Raw Al(2), AI(3) and Si(1,2) sites converge to full data were corrected with a Tracor Northern occupancy (~0.95). The AI(1) central site is only PROZA computer program. Natural minerals partially occupied (75 and 80% according to and synthetic compounds were used as standards. Moore and Araki (1978) and Alexander et al. Due to the small size of the dumortierite grains no (1986) respectively). Charge-balance retire- boron or water analyses could be obtained. ments for this site involve the 3H + + [] i = Dumortierite compositions were calculated on Al 3+vl substitutional mechanism and giving the the basis of 18 oxygens assuming one B and 0.75 general formula (A16.75D0.z5)BSi3[O17.25- OH per formula unit (after Moore and Araki, (OH)0.751. As postulated by Alexander et al. 1978). The crystal structure of dumortierite can (1986) the total amount of (OH) will not only vary be visualised to consist of three distinct types of with the number of octahedral vacancies but also

Fro. 2. Photomicrograph of an alteration aggregate in cordierite (CRD) from sample DV 249. Type I dumortierite (DUM) is intergrown with chlorite and muscovite. 568 D. VISSER AND A. SENIOR

FIG. 3. Photomicrograph with crossed nicols of an alteration aggregate in cordierite (CRD) from sample AK 034. Type l dumortierite (DUM) is intergrown with chlorite, muscovite and pinite. with the di- (Mg, Ca, Fe 2+) and tetravalent (Ti) intermediate TiO2 contents (0.00-2.05 wt.%). substituents for octahedral A1 and trivalent sub- The Bamble dumortierites record the highest stituents (A1) for tetrahedral Si (see also Werding MgO content to date for this mineral (1.36 wt.% and Schreyer, 1990). The assumption of 0.75 OH MgO has been reported by Vr~ina, 1979). Indi- per formula unit in our calculations, although the vidual dumortierite grains from the three local- average oxide total including boron and water is ities show no zoning, but notably TiO2, A1203 and 100.24% (n = 36, o,, = 1.30), will therefore only MgO vary from grain to grain in the same sample be a rough estimate. Recent analytical data on the and between the different textural types. Titan- boron content of natural (Kayupova and Tilepov, ium is low in type II dumortierite (0.00-0.46 wt. % 1977) and synthetic dumortierite (Werding and TiO2) and type I and III dumortierite grains not Schreyer, 1988, 1990) are generally supportive of associated with respectively titaniferous magnet- full occupation of the B-site by boron. ite and ilmenite/rutile. Moderate values (1.11- Dumortierite grains were analysed from 2.05 wt.% TiOz) are obtained in type I and III samples MA 822 (Type I, 3 grains, 5 analyses), dumortierites associated with the Ti-bearing DV 030 (Type II dumortierite, 3 grains, 8 phases. analyses) and MA 780 (Type III, 5 grains, 23 Our analyses show most of the tetrahedral sites analyses). Representative compositions of the [Si(1,2)] to be slightly deficient in silicon. AI is the dumortierites and associated minerals are pre- most likely substituent for Si at the tetrahedral sented in Table 2a-c. sites (Fig. 6a). More important substitutions are implied by the very low Al-values (6.259-6.601) of the dumortierites. A high correlation between A1vl and Ti, AI vt and Mg (r -- -0.91; r = -0.86 Dumortierite respectively) and a strong correlation between Ti All three types are magnesium varieties (2.23- and Mg (r = 0.72) indicate the predominant Mg + 3.42 wt.% MgO) with variable but sometimes Ti = 2 AIvl coupled substitution (see Fig. 6b-c). high iron (0.00-1.08 wt.% Fe203) and low to Subtracting the effects of this substitution, that is, Mg-RICH DUMORTIERITE 569 adding the total amount of Ti and an equal + AI w, tschermakite substitution, suggesting amount of the coupled Mg to AI w (labelled relatively high Fe2+/(Fe2+ + Fe 3+) ratios. A1v~*) and subtracting the total amount of Ti from Mg (labelled Mg*), reveals a strong negative Cordierite correlation (r = -0.71) between A1vl* and Mg* (Fig. 6d) and no significant correlation of Mg* The cordierite from sample MA 822, associated with other elements. This indicates the introduc- with type I dumortierites, is chemically unzoned tion of Mg at the octahedral sites by the possible and Mg-rich [XMg = Mg/(Mg + Fe 2+) = 0.95] substitutions 3Mg = 2A1vx and Mg + H = A1vl. with trace amounts of Na. Cordierite is able to Data on the oxidation state of Fe in dumortier- incorporate important gas species such as H20 ite are scarce. The only two wet chemical analyses and CO2 in its channels. The wet chemical which include FeO and Fe203 (Claringbull and analyses show relatively high H20 contents of Hey, 1958) show that both trivalent and divalent 1.96 wt.% [n(H20) = 0.67 moles H20 per Fe may be present. The weak inverse relation of formula unit] and very low CO2 contents of 0.17 Fe with A1VI in type I and II dumortierites may wt.% [n(CO2) = 0.025]. The optically unzoned suggest a low FeZ+/(Fe2+ + Fe 3+) ratio due to cordierites have optic angles between 79 and 92 ~ substitution of most Fe as Fe 3+ for AI 3+ at the with an average of 84~ 7-er ranges from 0.0075 to octahedral sites, but the substitution Fe 2+ + H + 0.009. Application of the triangular diagram = A13+ would also be possible. Type III dumor- constructed by Armbruster et al. (1982, p. 265) to tierites (excluding one iron-free grain) however explain the influence of H20 and CO2 on optical show a high positive correlation of Fe with Si (r = properties like birefringence and optic angle of 0.88, Fi~) 7a) and Mg (r = 0.60), strong negative cordierite, likewise shows that cordierite is sys- with A1 (r = -0.87) and AlVl(r = -0.69, Fig. tematically CO2-poor with an average n(H20) of 7b) and a weak correlation of Mg with Si (r = about 0,065. The content differs from the wet 0.40) and AI TM (r = --0.39). This essentially chemical determination in that much lower values involves Fe as Fe 2+ in the Fe z+, (Mg) + Si = A1TM are recorded [n(H20) = 0.33-0.38]. The H20

FIG. 4, Photomicrograph of type II dumortierite fibres (DUM) intergrown with chlorite (CHL) and muscovite along cracks in garnet (GNT) blasts of sample DV 030. 570 D. VISSER AND A. SENIOR

FIc. 5. Photomicrograph of irregular shaped type III dumortierite (DUM) patches replacing anthophyllite (ANTH) in sample MA 780. content is overestimated by the wet chemical range 5-7 tool.%, showing no relation to the methods, mostly likely due to intergrowth of zoning in Mg, Fe and Mn. chlorite/muscovite in the analysed cordierite frac- tion. The optically determined H20 and COz contents correspond to a molar ratio Xcoz [CO2/ Orthoamphibole (CO2 + H20)] of 0.155. The same molar ratio in The structural formula of the orthoamphiboles cordierites is, according to Johannes and Schreyer in sample MA 780, which are partly consumed by (1981), indicative of the ratio of these species in type llI dumortierite, have been normalised to 15 the coexisting fluid system. Utilising their experi- cations. K and Na are excluded and assigned to mental data in the system Mg-cordierite-H20- the A-site. Negligible amounts of Fe~+ are COz at Pnuid = 5 kbar and T = 600~ Xco2 of suggested by calculations from stoichiometry 0.155 in the analysed Mg-rich early M4 vein- (Robinson et al., 1982). The anthophyllites are cordierite of sample MA 822 implies an Xco~ for slightly zoned with A1TM (0.18 to 0.60), A1vl the fluid phase of 30-40%. (0.25-0.47), Mg (4.43-4.69) and Na(A) (0.06- 0.15) decreasing and Si and Fe (1.90-2.22) increasing towards the orthoamphibole-dumor- Garnet tierite contact. Typically the compositional variation of ortho- The large garnet blasts in sample DV 030 show amphiboles shows three main types of substitu- homogeneous pyrope-rich cores with an average tion: FeMg_l, Na(A) + A1TM = [] + Si TM, andMg XMg of 0.470. Towards the retrograde cracks + Si TM = A1vl + AI TM. The anthophyllites of garnet shows a sharp but continuous decrease of sample MA 780 deviate from this scheme, to the Mg and an increase of Fe and Mn. XMg varies effect that Fe 2+ shows a moderate strong negative from 0.470 to 0.293; the spessartine component correlation with A1TM, AI vl and Na(A). This ranges from 1 tool. % in the core to 2 tool.% along suggests that the tschermakite coupled substitu- the cracks. The grossular component is in the tion essentially involves Fe 2+ instead of Mg. The Mg-RICH DUMORTIERITE 571 orthoamphiboles further contain low to negligible Type I dumortierites occur intergrown with amounts of TiO2, MnO, CaO and K20. alteration products of cordierite such as chlorite, quartz and pinite. In these textures dumortierite occurs instead of kyanite and/or andalusite Kornerupine suggesting The kornerupine formula was calculated on the cordierite + vapour (B20 3 + H20 ) --- basis of 21.5 oxygens or 210 + (OH, F, Cl) after chlorite + dumortierite + quartz. (1) Moore and Araki (1979). The unzoned korneru- pines with XMg ranging from 0.75 to 0.82 are Reaction 1 and the development of the late M4 relatively Al-poor with compositions close to mineral phases andalusite and kyanite both may 4(Mg,Fe)O : 3A1203 : 4SIO2 (4 : 3 : 4) and slightly occur within the same decomposed Mg-rich more siliceous than the join between the 1 : 1 : 1 cordierite grains in most samples indicating simi- and 4 : 3 : 4 compositions. The low electron-probe lar P-T conditions for both reactions. P-T totals of 95.2 to 97.6 wt.% indicate high water and conditions are estimated at pressures of 3-4 kbar boron contents. Na, Ca, Mn and Ti occur in trace and temperatures of about 500--550~ when amounts which together total less than 0.3 wt.%. based on the Al-silicate polymorph transition of Holdaway (1971) and the low-temperature stability field of Mg-cordierite (Seifert and Schreyer, Tourmaline 1970). The absence of clear major optical hetero- The small yellow-green tourmalines inter- geneities or zoning towards alteration aggregates grown with chlorite, kyanite and quartz and the and grain boundaries in unaltered parts of the large brown-green tourmaline intergrown with early M4 vein-cordierite in sample MA 822 vein-quartz in sample MA 822 show very similar suggests that the exchange of cordierite with late compositional ranges. Both types of tourmaline M4 fluids was limited to these alteration aggre- and the large yellow tourmalines from sample gates. From this it may be concluded that the MA 780 are unzoned dravites with XMg of 0.75- Xco2 of 30-40% deduced from the optical data 0.76 and 0.80-0.85 respectively. The Na/(Ca + represents the Xco2 of the fluid phase during the Na) ratios are variable from grain to grain and early M4 event. Late M 4 fluids probably have within a grain in both samples and range 0.77- lower Xco2 as indicated by the hydrous signature 0.89 in MA 822 and 0.33-0.63 in MA 780. The of the replacement zones and channels in cordier- high TiO 2 contents (0.50-1.68 wt.% in MA 780 ite. Type II dumortierites occur together with and 0.49-0.69 wt.% in MA 822) are comparable chlorite and quartz along retrograde cracks within to those reported from rock types containing a Ti- garnet. The development of these dumortierites saturating phase (Abraham et al., 1972; Henry probably took place through the reaction of the and Guidotti, 1985). form garnet (core) + vapour (B203 + H20) = Chlorite garnet (rim) + chlorite + dumortierite + quartz. (2) The chlorites associated with type I (MA 822) and type II (DV 030) dumortierites are Al-rich Temperature estimates for the rim of the garnet clinochlores with XMg of 0.91 and 0.81 and coexisting cordierite and biotite in sample respectively. DV 030 are 420-510~ and 460-530~ respectively at a pressure of 4 kbar (Visser and Senior, 1990). Temperature estimates based upon the Fe- Kyanite Mg exchange reaction between garnet-rim and Analyses of kyanite, which accompanies chlor- chlorite (Grambling, 1990) are somewhat lower ite and quartz in cordierite replacement aggre- and in the range 400--470 ~ at 4 kbar. gates, show moderate to high amounts of Fe203 Type III dumortierites developed only by (0.59-1.20 wt.%). intergrowth with or decomposition of orthoamphibole. The compositional zoned orthoamphiboles essentially show a decrease of the ferro- Discussion tschermakite substitution, (A12Fe_ 1Si- 1) towards The textures in the investigated samples indi- the newly formed dumortierite grains implying: cate that the Bamble dumortierites exclusively orthoamphibole + Fe-tschermakite (Oam) + occur as and together with retrograde alteration vapour (B20 3 • H20) = dumortierite. (3) products of M3 and early M4 assemblages in orthoamphibole-cordierite-bearing rocks. The P-T conditions during the development of 572 D. VISSER AND A. SENIOR Table 2a. Representative analyses of dumortierite, tourmaline, cordierite, chlorite and kyanite from sample MA 822.

Mineral Dumortierite Trm Crd Chl Ky Spot 45 48 MA 1 51 50 53

SiO z 30.90 30.31 36.92 49.73 28.05 36.83 TiO 2 i.ii 0.46 0.59 0.00 0.00 0.00 AI203 57.24 58.42 30.88 33.42 23.72 62.54 Fe203(T ) 0.44 0.00 - - - 1.00 FeO(T) - - 5.64 1.18 4.99 - MgO 2.92 2.55 9.58 12.86 29.35 0.00 CaO 0.00 0.00 0.77 0.00 0.00 0.00 Na20 0.00 0.00 2.65 0.26 0.00 0.00 K20 0.00 0.00 0.08 0.00 0.00 0.00

B203 6.09" 6.05" I0.77" m H20 1.18" 1.17" 3.71" (1.96) m CO 2 - _ _ (0.17) m

Cl 0.00 0.00 0.00 0.00 0.04 0.00

Total 99.88 98.96 101.59 99.58 86.16 100.37

O-Basis 18 18 31 18 28 5

Si 2.939 2.902 5.960 5.002 5.381 0.993 Ti 0.079 0.033 0.072 0.000 0.000 0.000 A1 TM 0.061 0.098 0.040 0.998 2.619 0.000 A1 v! 6.357 6.496 5.836 2.964 2.744 1.988 Fe 3§ 0.031 0.000 - - - 0.020 Fe 2§ - - 0.761 0.099 0.801 - Mg 0.414 0.364 2.305 1.928 8.393 0.000 Ca 0.000 0.000 0.133 0.000 0.000 0.000 Na 0.000 0.000 0.829 0.025 0.000 0.000 K 0.000 0.000 0.016 0.000 0.000 0.000 B 1.000 1.000 3.000 - - - H 0.750 0.750 4.000 - - -

Cl 0.000 0.000 0.000 0.000 0.013 0.000

Total 11.632 11.643 22.952 11.016 19.951 3.001

X~ 0.752 0.951 0.913

Mn, F, Cr, Sc, Ba and Zn not detected

* - H.O and BzO 3 calculated assuming one B and 0.75 (OH) per formula unit for dumortierite, after Moore & Araki (1978), and 3 B and 4 (OH) per formula unit for tourmaline. H~O and CO 2 in cordierite determined by wet chemical methods on mlneral separates. Mg-RICH DUMORTIERITE 573

~or~176 .~176176 II ,~ , 0 ~0000~0~000 ~ 0 t- -,-I

0-~ ~0~ ~0~0~00 ~ O ~0~ OO~O00 ~ ~0~0 ~ 0 v ..I,..,.. ." ~ ~0~000 ~ ~E~ .... I,,,,0,00 , -- ~00~ 00~000~ 0

~0~ ~0~000 g ~ON ~0~000 ~0 ~ ~00000 0 "~ ,.I...... II O0 ~ ~0~000 ~ ~ hr-- O0~ ~0~000 ~ ~ I ~ I PI .,-t

0~0 0~000 ~ ~oo o~ooooo ~0 0~000 ~H ~0 0~0000~ >= ~ ,0 ~0~0 0~0000~ ,4 -,-I HH~O 0~000 ~H 0 .,~ ..... ~ I 0 h~ ~00~0 00000~0 ~J

h ~0~0 ~o~ ~ooo~ mo 0 ~ I~ ~ "~ OO~O00 e404 t~ ~ 000 I ~0~0 0~0000~ ~L~ ~'-I H 1"- O 0 e'r 000 %0,-4 ,-4 H .,..,I,..,,*. I ,--4 ~ ~ g Q~ o~ tO 0 ~00~0 OO000HO ,-4

A %- ~) v

~0 - I

g ~0~ ~0 e9 ".1 .... |1 1.4 ~0 ~O~ ~0~ o " " ~ I I I o .P

0 h %o 0 co r..) ~o ".1 .... II ooo~ ~o~ o ~ '~0 o~ ~ON ~0~ I II -- o h 0 Q oo~

,-~oo "~ II ~o~ ~0~ 00~0 ~0~ ~0~0 ~ 0

O~ b" ~0 l~- OC~O ~ 00000 o~ o~oo~ ~

-~1 CO ~ I ~oo~o oo0~o ~

~0 "~ PI ~r OC~O ~-IH r~ 0~0o~ I ~00~0 000~o ~ s ,-I

t~ ~0 ? ~U ~m 0 574 D. VISSER AND A. SENIOR 7.0- 0.5 A \ \ B \ \ 1:1 t 6.8 \\ \\ \\ ~ \\ 0.4 , "-&e':~ \ + ~-~ 6.6 +6 -----_ , F--- i _!:1.` ** ',, < 6.4 ', Az~ \\ \ \ 0.3 \ \ \\ \ 6.2 2:1 \ 2:1 \~ \ \ \ \ \ \ \ \ , "l , 6.0 0.2 i 2.8 2.9 3 0 3.1 6.0 6.2 6.4 6.6 6.8 7.0 Si AI vl 0.20 \\ \ D 1:1 ' :1 \\2:1 , \ 2:1 \ \ 0.4 0.15- "~ \ \ \ A & \\ ~\\ \\ \ iz O.lO- \ \ 0.3 \ \ i

0"05 1 \\\\ ~ \\\\ ',\ \ 0 , O0 1 i i 0.2 6.0 ~6.2 6.4 6.6 6.8 7.0 6.2 6.4 6.6 6.8 AI w AI vl* Fro. 6a-d. Plots of (a) Si versus Al(Tot), (b) A1vl versus Mg, (c) AIvj versus Ti, and (d) A1vl* vcrsus Mg* in type I (sample MA 822, triangles), type II (DV 030, squares) and type lII (MA 780, aslerics) dumortierites. Dashed lines show I : 1 or 2 : 1 ratios. Error bars are typical lo eotinting-statistics errors. type Ill dumortierite cannot be estimated accu- 1987; Vr~na, 1979), while dumortierites inter- rately, but the associated pinitisation of cordier- grown with Ti-poor phases as e.g. corundum ire, chloritisation of biotite and growth of musco- (Schreyer et al., 1975) or plagioclase and quartz vite in garnet-cracks in the same alteration zones (Takahata and Uchiyama, 1985), show low may suggest that hydrothermal conditions are titanium concentrations. comparable with those during the formation of Oxygen fugacity levels must have been low in type I and II dumortierites. sample MA 780 during the dumortierite forma- Titanium concentration in type I and III tion as suggested by the presence of Fe 2+ in type dumortierites depends on the presence of phases III dumortierites. This is consistent with a very like rutile, ilmenite and titaniferous magnetite. high Fe2+/(Fe 2+ + Fe 3+) ratio of the whole rock These minerals must have acted as the major and the persistence of primary graphite in the source of TiO2 for dumortierite in reactions 1 and alteration zones. Relatively higher oxygen fuga- 3. Minor TiO2 could have been supplied by city levels are suggested during the formation of orthoamphibole. Ti-rich dumortierites reported type I dumortierites from sample MA 822 on the from the literature generally occur likewise asso- basis of the moderately high Fe203 contents ciated with Ti-bearing phases (e.g. Beukes et al., (0.69-1.20 wt.%) of kyanite grains not associated Mg-RICH DUMORTIERITE 575

0.08 0.08 A A A

0.06 0.06 + [] A**** A LL 0.04 L~ 0.04 ~ []

0.02 >1< ~<>k 0.02 >k ~g

0,00 [] ){( [] )~r 0.00 --, "~" '~ 2_.8 2.9 ,3.0 ,3.1 6.0 6]2 ' ~6.4 Ajv16.6' ~ ' 6.'8 7~.0 Si Fro. 7a-b. Plots of (a) Si versus Fe and (b) AIvl versus Fe, Symbols as in Fig, 6a-d. with titaniferous magnetite. According to Grew cordierite grain indicate, however, that B203/ (1980) and Feenstra (1985), these values are H20-ratios in the fluid are not uniform and vary characteristic of moderately to highly oxidised on thin section scale. As the chemical environ- rocktypes. ment for the components (FeO,MgO), SiO2 and Dumortierite-producing reactions (1-3) and AI2Os is very similar for both dumortierite and associated formation of M4 assemblages require late M4 tourmaline, the development of late M4 the addition of fluid components. In 60% of the tourmaline instead of dumortierite will mainly investigated samples dumortierite is accompanied depend on the local availability of alkalis, notably by tourmaline or kornerupine, which might have higher Na + and Ca 2+ (see mineral chemistry), acted as a source of boron for the dumortierite higher B203 and high H20 in the host rock and formation. However, textural relationships the fluid phase. between the three minerals are lacking. Korneru- It should be realised that late M4 overprint pine breakdown only produces chlorite and fine- mainly is restricted to rock types which re- grained micas, while tourmaline grains are not equilibrated under local rather than pervasive observed to decompose. In fact, tourmalines from hydrothermal conditions. Pressure-temperature some tourmaline-bearing samples are likewise conditions for the formation of type I, II and associated with the breakdown products of M3 possibly type III dumortierites in most and and early M4 assemblages. It is therefore con- probably all localities are thought, nevertheless, sidered very likely that the boron oxide compo- to be very similar (500 _+ 50~ 3-4 kbar, see nent in the fluid was generated outside the above), showing no large deviation on a regional immediate rock volume rather than from inside. scale. This implies that metamorphic conditions Only in sample MA 780, the breakdown of boron- during retrogradation of M s and early M 4 assem- bearing precursors like kornerupine might have blages and development of late M 4 assemblages supplied the boron foi the dumortierite forma- in cordierite-orthoamphibole-bearing rocks, tion. Possible sources of boron are the late-stage despite the local signature of the dumortierite K-feldspar-quartz veins and often tourmaline- development, are of regional importance in the bearing granitic pegmatites, which are present Bamble Sector. The development of dumortierite and sometimes abundant at most localities. The in late M4 assemblages seems to be restricted to ubiquitous presence of pinite replacements in the upper-amphibolite facies area (Fig. 1). This is cordierite and fine grained muscovite in cracks of probably the result of the limited metamorphic the garnet suggests that K, and probably other overprint during the Sveconorwegian orogeny alkalis, were also available in the fluid phase. (M4) in the Arendal granulite facies area. Higher B203/H20 ratios in the alteration fluid stabilise dumortierite relative to kyanite/andalusite together with chlorite and quartz. The Acknowledgements presence of both dumortierite and the alumino- We are indebted to Rent Poorter for assistance with the silicates in alteration aggregates of the same microprobe analyses. Critical comments on earlier 576 D. VISSER AND A. SENIOR drafts of this manuscript were provided by Cees Mailer, Henry, D. J. and Guidotti, C. V. (1985) Tourmaline as Timo Nijland and Theo Kloprogge. Gunnar Raade and a petrogenetic indicator mineral: an example from an anonymous referee are thanked for their reviews of the staurolite-grade metapelites of NW Maine. Am. the manuscript. Financial support for fieldwork from Mineral., 70, 1-15. the Dr. Sch~irmann and the Molengraaf Foundations (to Holdaway, M. I. (1971) Stability of andalusite and the D. V.) is gratefully acknowledged. aluminium silicate phase diagram. Am. J. Sci., 271, 9%131. Huijsmans, J. P. P., Barton, M., and van Bergen, M. J. References (1982) A pegmatite containing Fe-rich grandidierite, Abraham, K., Mielke, H., and Povondra, P. (1972) On Ti-rich dumortierite and tourmaline from the Pre- the enrichment of tourmaline in metamorphic sedi- cambrian, high-grade metamorphic complex of Roga- ments of the Arzberg Series, W. Germany (NE- land, S.W. Norway. Neues Jahrb. Mineral., Abh., Bavaria). Neues Jahrb. Mineral., Mh., 209-19. 143, 249~61. Alexander, V. D. Griffen, D. T., and Martin, T. J. Johannes, W. and Schreyer, W. (1981) Experimental (1986) Crystal chemistry of some Fe- and Ti-poor introduction of CO2 and HzO into Mg-cordierite. dumortierites. Am. Mineral., 71,786-94. Am. J. Sci., 281,299-317. Armbruster, Th., Schreyer, W., and Hoers, J. (1982) Kayupova, M. M. and Tilepov, Z. T. (1977) First find of Very high COz-cordierite from Norwegian Lapland: dumortierite concretions. Acad. Sci. USSR, Dokl., Mineralogy, petrology, and carbon isotopes. Contrib. Earth Sci., Sect., 235, 121-4. Mineral. Petrol., 81,262-7. Kerr, P. F. and Jenney, P. (1935) The dumortierite- Beukes, G. J., Slabbert, M. J., de Bruiyn, H., Botha, andalusite mineralisation at Oreana, Nevada. Econ. B. J. V., Schoch, A. E., and van der Westhuizem Geol., 30, 287-300. W. A. (1987) Ti-dumortierite from the Keimocs area, Kretz, R. (1983) Symbols for rock-forming minerals. Namaqua mobile belt, South Africa. Neues Jahrb. Am. Mineral., 68, 227-79. Mineral., Abh., 157,303-18. Lamb, R. C. (1981) Geochemical studies in Proterozoic Black, P. M. (1973) Dumortierite from Karikari penin- high-grade gneiss terrain, Bamble Sector, south Nor- sula: a first record in New Zealand. Mineral. Mag., way. Unpubl. Ph.D. Thesis, University of 39, 245. Nottingham. Bugge, J. A. W. (1943) Geological and petrographical Smalley, P. C., and Field, D. (1986) P-T con- investigations in the Kongsberg-Bamble Formation. ditions for the Arendal granulites, southern Norway: Norges Geol. Unders., 160, 150 pp. implications for the roles of P, T and COz in deep Claringbull, G. F. and Hey, M. H. (1958) New data for crustal LILE-depletion. J. Metam. Geol., 4, 143-60. dumortierite. Mineral. Mag., 31, 901-7. Moore, P. B. and Araki, T. (1978) Dumortierite, Christophe-Michel-Ldvy, M., Enberger, A. and San- Si3B[AI6.75E]0.25OIT.:z5(OM)0 751: a detailed structure drda, A. (1959) Matdriaux pour la mindralogie de analysis. Neues Jahrb. Mineral., Abh., 132,231-41. Madgascar. ll.--La Dumortidrite de Riampotsy. ----(1979) Kornerupine: a detailed crystal- Bull. Soc. fr. Mindral. Crystallogr., 82, 77-9. chemical study. Ibid. 134, 317-36. Feenstra, A. (1985) Metamorphism of bauxites on Nijland, T. G. and Senior, A. (1991) Sveconorwegian Naxos, Greece. Geol. Ultraiectina 39, 206 pp. granulite facies metamorphism of polyphase migma- Field, D. and Clough, P. W. L. (1976) K/Rb ratios and tites and basic dikes, south Norway. J. Geol., 99, metasomatism in metabasites from a Precambrian 515-25. amphibolite-granulite transition zone. J. Geol. Soc. Robinson, P., Spear, F. S., Schumacher, J. C., Laird, London, 132, 277-88. Jo, Klein, C., Evans, B. W., and Doolan, B. L. Finlay, G. I. (19(t7) On an occurrence of corundum and (1982) Phase relations of metamorphic amphiboles: dumortierite in pegmatite in Colorado. J. Geol., 15, Natural occurrence and theory. In Amphiboles: 479-84. Petrology and experimental phase relations. (Veblen, Golovastikov, N. I. (1965) The crystal structure of D. R. and Ribbe, P. H., eds.) Rev. Mineral., 9b, dumortierite. Soy. Phys.-Dokl., 10, 493-5. 1-227. Gonnard, M. F. (1881) Note sur l'existence d'une Sabzehei, M. (1971) Dumortierite from Iran: a first esp6ce min6rale nouvellc, la dumorti6rite dans le record. Mineral. Mag., 38, 526-7. gneiss de Beaunan, au-dessus des anciens aqueducs Schaller, W. T. (1905) Dumortierite. Am. J. Sci., 19, gallo-romains de la vallde de l'Izeron (Rhone). Bull. 211-24. Soc. Mindral. Fr., 4, 2-8. Schreyer, W., Abraham, K. and Behr, H. J. (1975) Graham, C. E. and Robertson, F. (1951) A new Sapphirine and associated minerals from the korneru- dumortierite locality from Montana. Am. Mineral., pine rock of Waldheim, Saxony. Neues Jahrb. 36, 916-7. Mineral., Abh., 126, 1-27. Grambling, J. A. (1990) Internally-consistent geother- Seifert, F. and Schreyer, W. (1970) Lower temperature mometry and H20 barometry in metamorphic rocks: stability limit of Mg-cordierite in the range l-7 kb the example garnet-chlorite-quartz. Contrib. water pressure: A redetermination. Contrib. Mineral. Mineral. Petrol., 105,617-28. Petrol., 27,225-38. Grew, E. S. (1980) Sillimanite and ilmenite from high- Starmer, I. C. (1985) The evolution of the south grade metamorphic rocks of Antarctica and other Norwegian Proterozoic as revealed by the major and areas. J. Petrol., 21, 39-68. mega-tectonics of the Kongsberg and Bamble Mg-RICH DUMORTIERITE 577 Sectors. In The Deep Proterozoic Crust in the North Maijer, C., and Senior, A. (1990) Retrograde talc- Atlantic Provinces (Tobi, A. C. and Touret, J., eds.) kyanite~luartz assemblage from Bjordammen, Bam- NATO ASI series C 158 Reidel, Dordrecht, 259-91. ble Sector, south Norway. Abs. 19th Nordic Geol. -- (1990) Rb-Sr systematics of a Gardar-age layered Winter Meet., Geonytt, 17, 119. alkaline monzonite suite in southern Norway: a Vrfina, S. (1979) A polymetamorphic assemblage of discussion. J. Geol., 98, 119-23. grandidierite, kornerupine, Ti-rich dumortierite, Takahata, H. and Uchiyama, K. (1985) Dumortierite- tourmaline, sillimanite and garnet. Neues Jahrb. bearing argillaceous gneisses from the Abukuma Mineral., Mh., 22-33. metamorphic terrain, northeast Japan. J. Fac. Sci., Werding, G. and Schreyer, W. (1988) Synthetic dumor- Hokkaido Univ., Ser. IV, 21,465-81. tierites: Their PTX-dependent compositional varia- Touret, J. (1979) Les roches h tourmaline-cordi6rite- tions in the system AI203-SiO2-B203-H20. disth6ne de Bjordammen (Norv6ge m6ridionale) Fortschr. Mineral., 66, Beih. 1, 168. sont-elles li6es a d'anciennes 6vaporites? Sci. Terre, ----(1990) Synthetic dumortierite: its PTX- 23, 95-7. dependent compositional variations in the system Visser, D. and Senior, A. (1990) Aluminous reaction AlzO3-BzO3-SiOz-H20. Contrib. Mineral. Petrol., textures in orthoamphibole-bearing rocks: the 105, 11-24. pressure-temperature evolution of the high-grade Proterozoic of the Bamble Sector, south Norway. [Manuscript received 15 June 1990; J. Metam. Geol., 8, 231-46. revised 4 January 1991]