Download RELICT PLAGIOCLASE PHENOCRYSTS FROM AMPHIBO_ LITE

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