American Mineralogist, Volume 62, pages 1038-1041, 1977

Anorthoclase-calciterodding within a kaersutitexenocryst from the KakanuiMineral Breccia.

R. Cr-sleNo Wa.LLACE'

Geology D epartment, U niuersity Dunedin. New Zealand

Abstract

Unusualwhite rods of anorthoclaseand calciteare described from a kaersutitexenocryst fromthe Kakanuimineral breccia, New Zealand. The calcite and anorthoclase are interpreted as secondaryinfillings of paralleltubicles in the kaersuitite.These tubicles are thought to be largeelongate fluid inclusionsthat resultedfrom the growth of gaseous"bubbles" on the crystalcontemporaneously with crystalgrowth.

Introduction ual rods occasionallychange direction. The rods re- parallel changing direction, and in the The constituentsof the Mineral Breccia main after they narrow significantly.The na- have been investigated by many authors (Mason, region of bending specimendoes not allow observationsas 1966,1968; Dickey, 1968a,1968b; White et al., 1972), ture of the whether all the rods are continuous at the bend: who described xenoliths (lherzolite, eclogite horn- to however, at least three are continuous. The rods blendite,pyroxenite, granulite, and schist)and xeno- direction through an angle of 28o to the Z crysts (pyrope, augite, anorthoclase, hornblende, change the new direction being at 13oto and ilmenite) set in a tuffaceousmatrix rich in car- vibration direction, (l plane(Fig. bonates, montmorillonite, anorthoclase,and phil- the l0) 2). The rods usuallyhave a rounded crosssection, but lipsite. Melanephelinite,which mantles many of the of rod is con- inclusions,is thought to be the magma that brought in some instancesthe morphology the by cleavage,producing a rhombic cross the xenoliths to the surface.During a recent visit to trolled the section. The rods are composed of fibrous anortho- the Kakanui Mineral BrecciaI found a large horn- clase, often radiating from the kaersutitehost (Fig. blende xenocryst with coarse white rods passing 3), with magnesiancalcite infilling the intersticesbe- through it (Fig. I ). tweenthe anorthoclase.Optical zoning in the kaersu- Mineralogy tite adjacent to the rods can occasionally be dis- The xenocrystis a rounded,relatively well-polished cerned. the kaersutite(table I show that it is kaersutitecrystal measuring l5 X l5 X l0 cm, and is Analysesof ) similar composition to the other amphibole conspicuousin havingwhite rods passingthrough it. of xenocrystsdescribed from the breccia(Mason, 1966, The rods are up to 3 mm in diameter, with a max- 1968; Dickey, 1968a;White et al., 1972). The opti- imum length of 5 cm, and often have blind bulbous cally-zonedkaersutite adjacent to the rods is richer in tips. They commonly pinch and swelland occasion- magnesium and titanium and depletedin iron com- ally flatten into broad spoon-shapedregions. No ex- pared to the rest of the crystal. ample of a rod bifurcating was observed. The rods are not uniformly distributed throughout Discussion the grain, but form zonesof parallel rods lying on the nature the anorthoclaseand calciteforming (l l0) cleavageplane at an angleof49o to the cleavage The of rods, and occurrence of anorthoclase and intersection(Fig. 2). Although this direction is the the the calcitein the tuffs associatedwith the mineral breccia, dominant one, othersdo occur (Fig. I ), and individ- suggestthat these minerals are of secondaryorigin. The areasnow filled by the calcite and anorthoclase t Presentaddress: Geolosical Survev.Pretoria. South Africa may have been produced by: (l) exsolutionof a sec-

l 038 WALLACE ANORTHOCLASE-CALCITE RODDING I039

Fig. 3. Cross-sectionof a rod in the kaersutite,showing the Fig. l. White rodding in the kaersutitexenocryst; note the slight radially-orientatedanorthoclase around the perimeter of the rod change in direction of the rodding and the smooth furrows where and calcite, also fibrous-radiating,toward the core The ratio the rods have been removed. Photograph is 7 cm wide. calcite:anorthoclaseis quite variable The photomicrographis 0.6 mm wide. ond phaseout of the kaersutiteand the laterremoval of thisphase; (2) eutecticcrystallization or subsolidus The regular nature of the rodding presentsa strong reequilibrationand removalof oneof the phases;(3) case for formation by exsolution. Descriptions of the formation of tubular fluid inclusionsin the phasesexsolving from actinolite and common horn- kaersutite. blende are common (see Rcisset al., 1969;Cooper z and Lovering, 1970, and referencestherein), and poorly developed examples of ilmenite apparently

rabrer''"":;:L;"il::Til',":::;:"'''" containins

(110) si02 40.39 5.959 39.79 5.779 13.9r 2.4r8 14.41 2.464 Al203 Ti02 4.98 0.553 5.87 0.641

Fe0*r )2.41 L 530 8.76 L 063

Itln0 0.'l0 0.012 0.03 0. 003 'l lulg0 1.88 2.614 r4.55 3.148

Ca0 t0.tl 1.597 1l.?2 1.746

Naro 2.91 0.831 2.41 0.679 'I Kzo .94 0.365 1.74 0.323 98.63 98.78

'I : Kaersutite meqacryst 2: Leachedzone in kaersutite adjacent to rodding * Total iron as Feo ** Structural forrnulaebased on 23 oxygens Fig 2 Diagrammatic representation of the rodding in the Analyzedusing l5 kV acceleratingvoltage, 2 micronbeam kaersutitexenocryst The major part ofthe rods is in the (lI0) diameter,0.03um amps specimen current and following data plane, at 49' to the intersection of the cleavages.After bending reductionmethods of Benceand Albee (1968). through 28o the rods are at 13' to the (li0) face. 1040 WALLACE: ANORTHOCLASE-CALCITE RODDING exsolvingfrom kaersutitehave been reported by Wal- the rods) is difficultto explain,but if the "bubbles" lace(1973, unpublished M.Sc. thesis, University of developedat right anglesto a faceof the grain, then Otago,New Zeland)and observedby the authorin any alterationin the orientationof this face(due to xenolithsin a maficphonolite in Otago,New Zealand variationsin the rate of crystalgrowth) would pro- (Priceand Wallace,1976). However, in theseoccur- duce a changein directionin the growth of the rencesof exsolutionfrom an amphibole,the ex- "bubble." The zoning of the megacrystabout the solutionhas been planar and the lamellaeless than a tubiclesis probablythe resultof secondaryleaching micron wide. Although coarseexsolution has not prior to or during eruption,but it may be primary beenreported in an amphibole,Ringwood and Love- and be the resultof the bubblerestricting the access ring (1970)have described coarse ilmenite exsolution of materialto the growingcrystal surface. Blackerly in a pyroxenehost. If, however,the rods represent (1968)suggested a similar mechanismof a viscous replacementof an exsolvedphase it is difficult to globuleretarding the movementof materialtoward a explainhow that phasecould have been so com- crystal surfaceto explain "convolute zoning" in pletelyremoved. Williams (1932) has reported a sili- plagioclase. catelilmenite intergrowth from the Monastery (SouthAfrica) Mine wherethe originalsilicate has Conclusion beencompletely altered but the ilmenitewas not. Roddingin a kaersutitemegacryst from the Ka- Also, wherea chromite/orthopyroxenesymplectite kanui mineralbreccia is thoughtto be a secondary from theMoeraki River diatreme (Wallace, 1975) has infillingof tubiclesproduced by themasking effect on comein contactwith the COr-HrO fluidsassociated the megacrystby a gaseousphase nucleating on it. with this intrusion,the silicatehas been altered, leav- The gas tubiclehas grown at a similar rate to the ing skeletalchromite. Therefore, environments do growth of the megacryst,thereby developing hollow existwhere one phaseof an intimatelyassociated set pipesin thegrain. During eruption the kaersutitewas of phaseshas been completely altered without appar- brokenup and polishedand the pipeswere filled by ently the other phasebeing affected.However, in fluidsfrom whichanorthoclase and carbonatescrvs- theseenvironments the silicatehas been altered. tallized. Eutecticcrystallization or subsolidusreequilibra- tion wouldnot readilyexplain the textures described, Acknowledgments as this usuallyproduces irregular symplectic inter- I wish to thank A Reay, B. R. Watters, and C. Frick for growths(Boyd, l97l; Griffin, l97l; McBirneyand discussionsconcerning the unusual texture. R. P. Stapleton as- Aoki, 1973;Whitney and Mclelland,1973;Dawson sistedwith the photography. and Smith,1975; Wallace, 1975). There is alsothe References problemof replacingone of the phaseswithout alter- ing the other. Bence,A. E. and A. L. Albee (1968) Empirical correction factors If the calcite-anorthoclaserods for the electron micro-analysis of silicates and oxides. J. Geol., are secondary, 76,382-403. thenthe texture in the kaersutiteresembles the blind Blackerly, B. A. ( 1968) Convolute zoning of plagioclase subparalleltubular voids with bulbousends in lab- phenocrysts in Miocene volcanics from the western Santa Mon- radoritephenocrysts described by Gutmann(1974). ica Mountains, California. Am. Mineral., 53,954-962. Gutmann suggestedthat thesevoids resultedfrom Boyd, F. R. (1971) Pargasite-spinelperidotite xenolith from the nucleationof a gas phaseon the crystals.The gas Wesselton Mine. CarnegieInst Wash. Year Book.,70,138-142. (1970) "bubble" prevented Cooper, A. F. and J. F. Lovering Greenschistamphiboles crystalgrowth, and both bubble from Haast River, New Zealand. Contib. Mineral. Petrol., 27, and crystalgrew at a similarrate, producing a tubular n-24. void within the crystal.This may explainthe texture Dawson,J. B. and J. V. Smith (1975)Chromite-silicate inter- in the Kakanui kaersutite.The Kakanui mineral growths in upper-mantleperidotites. Phys. Chem.Earth., 9, brecciais the resultof a gas-richexplosive eruption, 339-350, Dickey,J. S. (1968a)Eclogite and and gases otherinclusions in the mineral may havebeen exsolving while the kaersu- brecciamember of the Deborah Volcanic Formation at Ka- tite crystallized.The apparentinfluence of the cleav- kanui,New Zealand.Am. Mineral.,53,1304-1319. ageover the locationof the tubiclesmay be due to - (1968b)Observations on theDeborah Volcanic Formation someform of variationin surfaceenergy effects along near Kakanui, New Zealand. N.Z. J. Geol. Geophys.,ll, the cleavageallowing preferential nucleation of fluid |L59-n62. Griffin,W. L. (1971)Genesis "bubbles" of coronasin anorthositesof the at thesepoints. The uniform changein Upper Jotun Nappe, Indre Sogn, Norway. J. Petol., 12, directionof "bubble" Arowth(reflected in thebend of 2t9-243. I'YALLAC E.' A N ORT H OCLA SE-C A LC ITE ROD D I NG l04l

Gutmann, J. T. (1974) Tubular voids within labradorite in amphiboles as indicators of subsolidus thermal histories.Min- phenocrystsfrom Sonora,Mexico. Am. Mineral, 59, 666-672 eral. Soc Am Spec.Pap.,2,275-299 Mason,B. (1966)Pyrope, augite, and hornblende from Kakanui, Wallace, R. C. (1975) Mineralogy and petrology of xenolithsin a New Zealand.,N.Z. J. Geol.Geophys.,9,474-480. diatreme from South Westland, New Zealand. Contrib. Mineral. - (1968)Eclogite xenoliths from volcanicbreccia at Kakanui, Petrol..49,l9l-199. New Zealand. Contrib. Mineral. Petrol., ],9, 316-327 White, A. J. R., B. W. Chappel and P. Jakes (1972) Coexisting McBirney,A. R. andK. Aoki (1973)Factors governing the stabil- clinopyroxene,garnet, and amphibole from an "eclogite", Ka- ity of plagioclaseat high pressuresas shownby spinel-gabbro kanui, New Zealand Contrib. Mineral. Petrol., 34, 185-191. xenolithsfrom the KerguelenArchipelago., Am. Mineral., 58, whitney, P. R. and J. M. Mclelland (1973) Origin of coronasin 271-2't6. metagabbros of the Adirondack Mountains, N.Y . Contrib. Min- Price,R. C. andR. C. Wallace(1976) The significanceof corona eral Petrol.. J9. 81-98. texturedinclusions from a high pressurefractionated alkalic Williams, A. F. (1932) The Genesisof the Diamond Ernest Benn, lava,,New Zealand.Lithos, 9, 319-329. London. Ringwood,A. E. andJ. F. Lovering(1970) Significance of pyrox- ene-ilmeniteintergrowths among kimberlite xenoliths.Earlft Planet.Sci. Lett., 7, 37l-37 5. Manuscript receiued, October 20, 1976; accepted Ross,M., J. J Papikeand K. W. Shaw(1969) Exsolution textures for publication, March 24, 1977.