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THE AMERICAN RELICT MINERALOGIST, VOL 54, MARCH APRII,, 1969 PLAGIOCLASE PHENOCRYSTS FROM AMPHIBO_ LITE GRADE METAVOLCANIC ROCKS Tuonas A. Vocnr-, Departmentof Geology,Michigan State Universil'y, East Lansing, Michigon 48823 l.Nn Wtrrrell H. SreNco, I{orth Carolina Stale Llnit:ersily, Departmentof Geosciences, Raleigh, North Carolino 27600' Alstx.,tcr Relict plagioclase phenocrysts have changed composition and have ordered by solid state diffusion without recr)'stallization. Original twin planes are preserved, and the orientation of the optical indicatrix with respect to the twin plane has readjusted to reflect the new composition and structural state. INrnooucrroN Metavolcanic rocks with primary features such as pillow-lavas, agglomeratic textures and relict phenocrl'sts occur in the Grenville Province near Plevna Lake in southeasternOntario (Meen, 1944; Smith, 1958). Many of these flows retain recognizableporphyritic textures and the original unrecrystallized plagioclasegrains are preserved in a few of these rocks. These metavolcanic rocks are interlayered with metaconglomerateand paragneiss(Smith, et,al'.,1969). The relict phenocrystsnear Plevna Lake occur in the center of a lenselike metavolcanic flow which has a maximum outcrop width of 1900 feet, and length of 9100 feet. The preservationof these phenocrystsis probably due to their physical size (up to one and one-halfinchesin length), and lack of deformation; and possibly to a lack of excessvolatiles in the center of the flow during metamorphism. The metamorphic grade of the metasedimentaryand metavolcanic sequenceis interpreted to be in the lower to middle amphibolite facies, partly on the basis of plagioclase with a composition of Anzo-Anqo in the amphibolite; and also on the presenceof kyanite in the associated metasedimentary rocks (Smith et al., t969). Detailed studies of the amphiboles are in progress,and preliminary microprobe analysesindicate that the amphibole is low in AlzOaand high in MgO. X-ray diffraction patterns of the amphiboles are almost identical in detail with actinolite. If the amphibole present in the amphibolite is in the actinolite series,the mineral assemblagewould indicate conditions at or near the greenschist-amphibolitefaciesboundary. MBrnons The compositions of relict plagioclase phenocrysts and secondary groundmassplagioclasegrains were studied by means of electron micro522 Rh:,LICT PLAGIOCLASr, IIROM A.VL?LIIBOLITL s2J Rosur,rs or. fNVESTTGATToN Whole rock chemical analysesfor SiO2,CaO, and KzO were determined bv X-ra.v fluorescenceon the three sampres. The analyses fail in the TaeLE 1. Penrrar, Carurcel 7-38 5 2. 8 8.8.5 0.61 ANar_vsrs ol Meravor,centcs 7+A 11-8N A+ B** .5.i 8 9.97 o.41 s 8. 2 8.48 0.51 .54.3 8.30 0.38 54.2 7. 9 2 1.11 7-38,7-4A and l1-8N plevna Lake, Ontario * Quartz diorite, Hawaii (Kuno, et ol, 1957t. ** "Average,, andesite (Nockolds, 19.54). general range of quartz diorite to andesite (Table 1), and it is reasonabre to assumethat volcanics in the generarcompositional range from basartic to andesitic were the antecedentsto these iocks. According to williams, Turner, and Gilbert (19s4) the averagepragioclasecompos-ition of andesites-is approximately Anas, that in basalts is approximately An55and porphyritic plagioclasein most andesitesis more calcic than Anru. The plagioclase in each of the metavolcanic samples show different ranges of_plagioclasecomposition: An2a-2s,Anao_asand An3e_a1, and there is no clifferencein composition between the recrystallized plagioclase of the groundmass and the primary plagioclasephenocrysts. AII three samples may represent disequilibrium assemblagesras indi_ cated-bv relict amphibole phenocrysts which have been puitiully .orrverted to a secondaryamphibole. sample 7-38, which has ihe mosi sodic plagioclase(Anrr-rr), has more secondary amphibole developed than the other two samples and representsa closlr approach to equiiibrium. The metavolcanic rocks that contain no rerict plagioclase and amphibole phenocrysts (and are therefore completely recrysta[ized) contain the 524 THOMASA. VOGELAND WILLIAM H' SPENCE to Angoand this range most sodic plagioclasewith a composition of Anzo sampleswhich contain *uy ,"pr.r.nt"the equilibrium composition' Those plagioclasewith the the greatest amount of relict urnphibole also contain highest anorthite content. igneous grains by ihe plagioclasephenocrysts are shown to be relict present (Carlsbad their lath-like euhedral ,hup., by the type of twinning which are sometimes and albite-Carlsbad), and also by re-entrant angles 1965; Turner' 1951; observed (Gorai, 1951; Ross, tbST; Sarbadhikari' similarity of the priVance, 1961) (Figures 1 and 2). The compositional plagioclase grains mary plagioclase phenocrysts and the recrystallized plagioclase phenocryst (right side) and Frc. 1. Photomicrograph of a partly recrystallized (left side)' phenocryst a completely recrystallized and changed demonstrates that these phenocrysts have homogenized this composition by solid state dinusion without recrystallization. _In were 2) (Fig' angles p.o*., the oiiginal twin planes and re-entrant preserved. show that Optical examination by five-axis universal stage techniques original any that and ,h" ph.rro.rysts are in itt" low structural state' X from angles ' .Y ' and Z compositionul ,onirrg has been obliterated' The deformation albite to i(OtO) of the primary twin planes and secondary curve (Fig-ure3)' twin planes fall on o. ,r..y .t"uitht ordered migration Measurementoftheo.ientationoftheopticalindicatrixto-L(010)of scatter in samples adjacent twin lamella show no optical-crystallographic 7-4Aand7-3B.Thecompositionaldataobtainedbythefive-axi]smethod in the microprobe is in agreement with the compositional data obtained the orienphenocrysts' the of and Tsuboi studies. The fact that in some RELIC'J'PLAGIOCLASEFROM AMPHIBOLITE 525 tation of the optical indicatrix to r(010) plot on the ordered curve without any internal optical-crystailographic icatter would indicate that the twin plane and opticar indicatrir have concomitantly reoriented themselves to a new composition and structural state (Vogel, 1964), for the position of the twin plane is controlled by the composition of the crystal as well as the structural state. A small but consisient internal scatter in sample r1-gN may indicate that the reorientation in this sample is not comprete. Zoning is present in a thin area adjacent to the composition pranes of primarf twins and may be induced by strain near the twin boundary. F-rc' 2. Relict plagioclase phenocryst with a weil-developed re-entrant angre. The smalr needles are amphibole. Noble (1966) found relict, calcium-rich, disordered plagiocrase pheno'wh.r. crysts preserved in metavolcanic rocks of the greensclisi facies. the volcanic rocks had been intruded earlier by a granodiorite pruton, the metavolcanics developedminerar assemblagesof the hornblenie hornfels facies; ordered relict plagioclase, complete with oscillatory zoning are preserved in some of these hornfelsed rocks. Noble observed that in the regionally metamorphosed rocks that he studied, relict disordered, calcium-bearing, plagiocLse phenocrysts were stable up to approximately middle greenschistfacies conditions. At this point, they inverted to an ordered structural state while still maintaining the delicate composition zoning. This structural inversion without compositional changeby diffusion is in agreementwith the work of Goldsmith (1952) who pointed out that local rearrangement of aluminum and silicon atoms will occur much more readily than the long range diffusion of !'llAMAS A' l'OGItL AND WILLIAM 11 SPIiNCI' >2(t needed to change aluminum and silicon (as weII as sodium and calcium) showed that the the composition and homogenize the crystal' Noble also that no eviand relict phenocrysts apparenlly acted as closed systems was noted phases dence for an isochemical breakdown into two or more on a microscopic scale. a mineral At Plevna Lake, the relict phenocrysts have developed the pres Under assemblase indicative of the lower amphibolite facies' 50 /4. YrL(OlO) I t 4t , t 6! t t , 70 9 o -{ x 90 oo'--i- 5! 4' a - - --:- -llfj t t u J"' -")i' 70. (dashed line) and disordered Ilrc. 3. PIot of X to I (010) versus I to 'L (010) for ordered open circles are 7-4A 7-3B' are circles Solid 1965)' (after Noble. (solid line) plagioclase that are discussed in the text ITELICT PLAGIOCLASE F'ROM A],TPI] I BOLI TE s2i AcxNowttnclrnNro Rsr.rREl{cns trilnom, v' B. (1944) Geologl' of the Grimsthorpe-Barrie area. ont Dept. Mines, Ann. Rep.5r, (pt. 4) t-50. Monsr, s A. (1968) Revised dispersion method for lorv plagiocrase. Amer. Mineror., s3, 1 0 5 - 115 . I{oss, J' v (1957) combination trvinning in plagioclase ferdspars Amer. J.sci.,255, 6.5o 655. sanraourxarr, T' R. (1965) on the difierence in twinning between 1>henocrystand grounrrmass plagioclaseof basalts. A mer. M ineral, SO,7466_1+69. snrrn, B' L. (1958) Georogy of the crarendon-Dalhousie area. ont. Depr. Mines., Ann. R e p t . , 6 5 , p t . 7, p . l - 4 6 -, Troues A' vocnr,, eNo w*r.reu H. sprNcr (1969). precambrian metaco'glomer ate mantling a granite dome in the Grenville province of southeastern Ontarlo. Geot. Soc. Amer. Bu!|,. 8O, 2974A2. sPEN-cE, wrl.uu H. (1968) Rerict plogi.ocrase phenocrysts From Metawrcanic Rocks Mantl'ing a Granite Dome and the pitrorogy of the Associated Rocks in the Grendile Protince of SoutheasternOntayio. ph.D. Thesis, Rutgers Univ. TEOMAS A. VOGEL AND WILLIAM 528 E. SPENCE in cleevageflakes' Tsuror, snnuro (1923)A dispersionmethodof determiningplagioclase M ineraJ.M og., 20, 108-122, metamorphic rocks. TunNnn, F. J. (ig51) Observationson twinning of plagioclasein Amer.M ineral',36' 581-589. ' Amu' Minuol''46' 1097-lll9' plagioclase VaNcn, -r. J. A. (1961)Polysynthetictwinning in A. (1964) optical-crystallographicscatter in plagioclase.Amer. Minerol.,49, voc'", 6r+633. (1968) Tsuboi method of plagioclase determination; an evaluation and comparison (abstr.). Geotr.Soc.Amer', Spu. PaP. 115,299' to C. M. Grlsnnr (1954) Petrography: An Introd'uct'ion WrrrtAus, II., F. J.Tunrrn,'ert 406 p' Francisco' San and Company' Freeman W'H' the Stud.yoJ Rocksin Thin Secti'ozs. _- AugustT, 196g; acce4teitfor publicahion, December18 ' 1968' M anuscri,pl recei.zteil