Plutonic garnets from the Werner batholith, Lassiter Coast ...

American Mineralogist, Volume 64, pages268-273, 1979

PlutonicgarnetsfromtheWernerbatholithL, assiterCoast,AntarcticPeninsula

WeI-rsn R. VnNNutr.r

Department of Geology,SonomaState College
Rohnert Park, Cal{ornia 94928
and U.S. GeologicalSuruey
Menlo Park, California 94025

ANDCHARLEsE. Mrysn

U.S. GeologicalSuruey
Menlo Park. California 94025

Abstract

Electron microprobe analysesand physical and optical properties of six garnets from
plutonic rocks of the compositeconcentrically-zonedWerner batholith show that the garnets
are dominantly almandine(65-70 percent)with lesseramountsof spessartine( l3-29 percent),
pyrope (2-ll percent),and grossular(l-5.6 percent).Garnets from two contaminatedfacies
(diorite and granodiorite) of the batholith are interpreted as xenocrystsderived from sur-
rounding pelitic metasedimentsw, hile those from an aplite dike which intrudes the gran-
odiorite are consideredto be phenocrysts.

Introduction from an aplite dike that intrudesthe granodiorite.

Almandine-rich garnet phenocrystshave been re- Physicaland optical propertiesof threegarnet sepa-
c o r d e d i n c a l c - a l k a l i n ei g n e o u sr o c k s i n m a n y p a r t s rates from contact metamorphic rocks associated
of the world. The host rocks are commonly gran- with the Werner batholith are reported, and a dis-
o d i o r i t e ,r h y o d a c i t e ,a n d d a c i t e ,b u t a l s oi n c l u d er h y - c u s s i o no f t h e o r i g i n o f t h e i g n e o u sg a r n e t si s g i v e n .

o l i t e , g r a n i t e , a n d e s i t e ,q u a r t z d i o r i t e , n o r i t e , a n d Methods of study
felsic pegmatite.Origins other than as phenocrysts

have, however, been postulated for these crystals. Garnet concentrateswere obtained with conven-
Zeck (1970),Birch and Gleadow (1974),and Maka- tional heavy-liquid and magnetic separation tech-
rov and Suprychev (1964) have respectivelycon- n i q u e s .C o m p o s i t i o n sw e r ed e t e r m i n e dw i t h a n A n l -
c l u d e dt h a t g a r n e t si n S p a n i s h ,A u s t r a l i a n ,a n d R u s - EtvIx microprobe' with the following operating
sian igneousrocks are xenocrystsor residualsderived c o n d i t i o n s :e x c i t a t i o np o t e n t i a l o f l 5 k V , s p e c i m e n
from adjacent pelitic metamorphic rocks. Elliott current of 0.025microamperesand a beam diameter
( 1 9 6 5 , 1 9 6 6 ) a t t r i b u t e d t h e o r i g i n o f A n t a r c t i c i g - of ( I micron sweptover a l0- by l0-micron area.An
n e o u sg a r n e t st o t h e a s s i m i l a t i o no f p e l i t i cs e d i m e n t s , almandinegarnet standard analyzedby wet-chemical
a n d B a r t r u m ( 1 9 3 7 )a n d E d w a r d s( 1 9 3 6 )p o s t u l a t e d methods was used for determination of major ele-
that New Zealandand Australian garnetscrystallized ments, and minor elements were determined with
from a magma after its contamination with pelitic other appropriate mineral standards. Corrections
were made for background,atomic-numbereffects,
material.

This paper provides electron microprobe analyses absorption, characteristicfluorescence,and instru-
and physical and optical propertiesfor four garnet m e n t a l d r i f t( B e e s o n , 1 9 6 7B; e a m a na n d I s a s i ,1 9 7 0 ) .

xenocrystscollected from two contaminated facies

(diorite and granodiorite) of a composite con- ' Use of brand names in this report is for descriptive purposes
centrically-zonedbatholith in the southernAntarctic o n l y a n d i n n o w a y c o n s t i t u t e se n d o r s e m e nbt y t h e U . S . G e o l o g i c a l
Peninsulaand from two garnetphenocrystscollected Survey

0003-004x/7/90304-0268$02.00 268

VENNUMAND MEYER: PLUTONIC GARNETS 269

Geologic setting 65" 700 300

Williams et c/. (1972), Williams and Rowley WEDDELL SEA
(1971),and Rowley (1973) summarizedthe geology
of the entire LassiterCoast and southernBlack Coast Xfo
(Fig. l). The Upper JurassicLatady Formation, a
sequenceof intensely-foldedblack and gray shale, it
siltstone,and minor sandstonei,s the oldestand most 500
widespreadunit on the Lassiter Coast. It contains
abundant volcaniclasticmaterial(Williams and Row- Fig. l. Index map showing location of the Black and Lassiter
ley, 1972),and is overlain by and intertonguedwith C o a s t s ,A n t a r c t i c P e n i n s u l a .B l a c k r e c t a n g l eo u t l i n e so u t c r o p a r e a
silicic ash-flow tuff, andesiticto dacitic lava flows, o f W e r n e r b a t h o l i t h( V e n n u m , 1 9 7 8 ) .
and air-fall tuff (Williams et al., 1972).These rocks
are consideredto be Upper Jurassicon the basisof locally contaminatedby assimilationof stopedblocks
apparent intertonguing with the Latady Formation and inclusionsof Latady Formation, and that com-
and on the basisof age relationsof similar volcanic position of the granodiorite has been further modi-
rocks exposedelsewhereon the Antarctic Peninsula fied by assimilationof stoped blocks and inclusions
(Adie, 1972;Dalziel and Elliott, 1973). of diorite and gabbro. Contacts betweenthe plutonic
rocks and the Latady Formation are extremelyirreg-
The Latady Formation and overlying volcanic u l a r , a n d m a n y o f t h e a b u n d a n tx e n o l i t h sa n d s t o p e d
rocks were intruded and contact-metamorphosedto blocks found in both faciesof the batholith appearto
hornbfende-hornfels facies(Plummer, 1974) by more be partially digestedby the magma. Flat-lying peg-
t h a n 5 0 s t o c k sa n d b a t h o l i t h st h a t a r e l a r g e l yq u a r t z m a t i t e b o d i e s a n d a b u n d a n t i n c l u s i o n s ,r o o f p e n -
monzonite, granodiorite, and quartz diorite, but dants, and septaof both metasedimentaryrocks and
range in composition from gabbro to granite. Field gabbro-diorite suggesthat the granodioriteliesnear
relations, petrography,and chemistry (Rowley and the roof of the batholith.
W i l l i a m s , 1 9 7 4 )i n d i c a t et h a t t h e p l u t o n i c r o c k s a r e
correlativewith the Andean intrusivesuite,a seriesof Chemical analyses of igneous rocks from the
calc-alkaline plutonic rocks exposed extensively southwesternDana Mountains which have identical
t h r o u g h o u tt h e A n t a r c t i c P e n i n s u l aa n d C h i l e a na n d silica percentagesdiffer from each other chemically
P a t a g o n i a nA n d e s ( A d i e , 1 9 5 5 ) .K - A r d a t e so n l 5 and mineralogicallyby as much as 6.7 percentAl2Os,
samplesfrom elevenplutons yield Upper Cretaceous 4 . 8 p e r c e n tM g O , l 3 p e r c e n tm o d a l p l a g i o c l a s el,l
ages(Mehnert et al., 1975;Rowley et al., 1976). percentmodal biotite, l5 percentmodal hornblende,
and l0 percentmodal potassiumfeldspar.
The largest pluton in the Lassiter Coast is the
Werner batholith, which is named for extensiveex- In addition to the variation in chemistryand min-
posuresof this body in the Werner Mountains (Ven- eralogy, several textural features often reported in
n u m , 1 9 7 8 ) .T h i s b a t h o l i t h a v e r a g e sa b o u t 2 0 k m i n contaminatedplutonic rocks havebeennoted in sam-
width and extendsfor nearly 150km southwardfrom ples from the southwesternDana Mountains. These
the southern Black Coast to the central Lassiter include strongly poikilitic or sieve-texturedmafic
Coast. Most of this concentrically-zonedbatholith minerals, clots of mafic minerals, and plagioclase
consistsof quartz diorite and granodiorite which lo- grains with moderately-zonedcalciccoressharplyde-
cally intrude a thin marginal facies (generallyless
than 2 km wide) of heterogeneousdiorite and minor
gabbro (Rowley, unpublisheddaIa,1976).Five K-Ar
dates that range from 98.2 to 114.4m.y. have been
obtained from the Werner batholith (Rowley et c/.,
1 9 7 6 ) .T h e m a i n f e l s i cf a c i e si s a b o u t 1 0 0m . y . o l d ,
and the mafic marginal faciesis probably about ll4
m.y.old.

Petrography,mineralogy, chemistry,and detailed
field studiesin the southwesternDana Mountains of
the northern LassiterCoast (Vennum, 1978)indicate
that both faciesof the Werner batholith have been

270 VENNUM AND MEYER PLUTONICGARNETS

marcated from a weakly-zonedor nonzoned albitic good crystalform, none appearsstronglyresorbedor

rim that is not in optical continuity with the core. embayed,nor do they appearto have reactedwith

Megascopic(2 mm) red-brown garnetsare irregu- any of the primary igneousminerals. Euhedral iso-

larly distributed throughout both faciesof the Wer- tropic pale-brown garnets as much as 4 mm long

ner batholith in the southwesternDana Mountains, occur in the Latady Formation. X-ray data and addi-
and sub-megascopicgarnetsappear in 15 percentof tional optical and physical properties are listed in

the thin sectionsexamined frorn the same area. No T a b l e l .

significanttextural or mineralogicaldifferencesexist Composition
betweenthe garnet-bearingand garnet-freeplutonic

rocks. Garnets are apparently absent elsewhere Significant chemical differencesexist between the

within the batholith. garnetscollectedfrom the diorite, granodiorite,and

Garnets from the heterogeneousdiorite faciesare aplite (Table 2). All contain 65-70 percent of the

anhedral to subhedral,occasionallydisplay weak bi- almandine component. Those from the diorite con-

refringence (d : 0.000-0.002),are pale chocolate- tain 26-29 percent spessartine,2 percent pyrope, and

brown in plane-polarizedlight, or are irregularly 1.5percentgrossular;thosefrom the granodiorite l3-

zoned from pale chocolate-browncores to colorless l5 percentspessartinel,0-l I percentpyrope,and 5-6

rims. Those from the granodioriteare anhedral,col- percent grossular; and those from the aplite l7-22
orless, and weakly birefringent (6 : 0.001-0.002). percent spessartine,9-ll percent pyrope, and 3-5

Although none of the garnetsin either faciesdisplays percent grossular. Measurement of the FeLa and

T a b l e l . X - r a y p o w d e r d a t a , o p t i c a l , a n d p h y s i c a lp r o p e r t i e so f g a r n e t sf r o m t h e W e r n e r b a t h o l i t h a n d a s s o c i a t e dc o n t a c t - m e t a m o r p h i c
rocks. Lassiter Coast, Antarctica

v36 Y42 v77 KOZSZe Ro290c Bo30b
d (obs) d(obs) d (obs )
hkl-------- rl a l ( o b s) r d(obs) I d (obs)
2I 1-------- 4 . 72 8
220-------- 2.867 4 2.886 4 2.867 4 2.860 5 2.985 7
32t-------- 10 4.100
400-------- 2.601 r0 2.583 2.567 10 2.565 L0 2.669 IO
2.454 I I 3.086 2.548 I
420-------- 2.3r9 4 2.360 2.344 I 2 . 8 75 2.340 4 2.438 6
332-------- 2.254 2 I 2.254 r 2.25L 2 2.343
422-------- 2.103 4 2.rO5 2.097 2 2.587 10 2.105 2 2.185 5
5r0-------- I 2.456
5 2I - - - - - - - - 1.867 4 r.876 I 2.356 I 3
I .663 4 r.668 2.265
440-------- r. 598 7 r.600 5 2.108 4
6I l-------- 1.539
444-------- r.542 9 2 r.867 3 2.O43 I 1.867 2 1.890 J
640-------- L. 4 4 2 2 1.662 2 I .875 5 1.658 2 L.726
1 .5 9 8 4 1.667 4 r.596 4 1.658 5
642-------- 1.603 5
800-------- 1.539 6
840-------- r.540 6 | .445 7 r.442 I r.598 9
842-------- t.442 r t.293 2 r.287 I 1.495
664-------- r.290 I t.262 r.259 r I.337 +
1 .2 5 8 2 r.233 2 1.305
'(il'------ r.229 I 3 J
I L.ZtO
o3 ---------
c.4 -------- 1 1 .5 4 11.55 I1.53 11.57 1t.56 1 1. 9 5
1 . 8 1 5 + 0 .0 0 5 I . 815+0.005 I .805+0.005 I .795+0.005 I . 795+0.005 I . 825+0.005
4.20+0.05 4.20+0.05 4. I5+0.05 4. I0+0.05 4. l0+0. 05 3 . 7 0 + 0 .0 5

v36 Garnet-bearing blotit.e diorite, 63010' l"t., 73024' s.

v42 carnet-bearing hornblende biotlte granodiorite, 63022' w., 73o25'5.

v77 Muscovire garnet aplite, 63oL2' w., 73o20' s.

Ro232e Garne!-bearing slate, Latady ForMtion, 62054'I{., 73031' S.

Ro290c carne!-bearing hornfels, Latady tr'ormtion, 63003' i.t., 73007' S.

Bo30b Garnet-epidote inclusion in granodj.orixe, 62038' w., 73o26' S.

X-ray analyses were mde wit.h a 114.6 m powder camera f,sing Ni-filtered CuKo radiatton operated at 35 kv and
least-square's fit to
18 na. LIF was used as an internal standard.

lvisual estimation. rlz (ffi+ ) ^ , L^ o g -,e^ )\ vs a(A) for each reflection and projecting a
2Derermined by graphlng

these data back to the a(A) axls'(o = 90o).

3v"h." deEermined with sodiun l1ght source.

4Determined by suspenslon in Clerici's solution.

VENNUM AND MEYER: PLUTONIC GARNETS 2'71

Table2. Electronmicroprobeanalyseosf garnetsfrom theWernerbatholithL, assiteCr oast,Antarctica

v36 v36 v42 v42 v77 v77
Grain I Grain 2 Graln I Grain 2 Grain I Grain 2
cent.er Edge center Edge cent.er Edge center Edge center Edge center Edge

Si O o- - - - - - - - 36.3 35.9 36.5 36.1 37.2 37.2 37.0 36.7 JT.J 5I.L 31.6 36.8
28.9 29.3 29.4 28.3 29.O 29.8 31.0 30.7 30.2 30.0
FeO*-------- 0.5 0.6 2.6 29.8 27.7
MoO--------- 0.6 0.5 0.6 0.5 2.7 2.1 2.6 2.4 2.7 2.1 2.9 2.5
0.6 0.I 0.I 0.5 2.O 0.1 2.I 1.6 t. I I.3 r.7 l.l
CaO--------- 0.I 0.1 0.I 0.1 0.I 0.t 0.0 0.0 0.0
Tio2 -------- 20.4 20.6 2r.r
20.7 ll.r ll.4 20.5 zt.L 6.5 20.8 20.7 2I.I 2I.3 2r.7 2r.O
A12 03 ------- II.2 0.r 0.t r1.9 6.3 0.1 5.7 6.3 1.3 9.6 6.5 7. 5
MnO**------- 0.1 9 7. 9 99.3 0.0 0.1 99.5 0.1 0.I 0.1 0.r 0.1 0.1
Cr2 O3------- 98.5 9 7. 9 99.4
.98 .98 98.5 .92 98.6 99.5 99.2 100.1 99.1
Total----- .98 .99 .92 .92 .93 .91 .92
Psl(Fe+Mg)-- ot o)
6.00
x Total Fe ** Total Mn 4.09
0.6r
Atonic Reductions (for 24 oxygen) 0.20

si---------- 6.03 6.0r 6.03 6.03 6.05 6.01 6.00 6.01 6.03 6.02 6.O4 0.00
4.l0 4.06 3.96 3.94 4.03 4.21 4.2r 4.03 3.76 4.06 4.03
Fe---------- 4.02 0.13 0.14 0.11 0.64 0.63 o.64 0.59 0.65 0.51 0.69 r.04
0.09 0.10 0.10 0.35 0.36 0.36 0.28 0.19 0.23 o.29 0.0r
Ms---------- 0.14 r5.98
0.0I 0.00 0.00
Ca---------- 0.10 4.O2 4.08 3.99 68.9
1.00 1.32 0.88 17.4
Ti---------- 0.0r 0.0r 0.01 0.01 0.01 0.01 0.0r 0.0r 0.01 0.01 0.0t r0.3
Al---------- 4.05 4.03 4.01 4.O4 4.O4 4.O2 3.98 4.00 rs.94 1 5 .9 3 15.96 3.4
I'1o---------- l. s8 r.58 r.60 r.68 0.87 0.89 0.78 0.87
Cr---------- 0.01 0.01 0.01 0.00 0.0r 0.01 0.0r 0.0r 68.6 64.7 68.6
15.94 15.96 15.96 r5.93 rs.9l 15.96 15.99 r5.98 L7.2 22.7 14.8
TotaI----- 1r.0 rL.7
3.2 8.7 4.9
Alm***______ 69.0 69.5 69.0 6 7. 6 68.0 68.2 10.2 70.6 3.9
Sp---------- 27.0 26.7 27.0 28.8 r4.9 15.0 r3.2 r4.7
Py---------- ll. I 10.7 10.6
Gr---------- 2.3 2.3 2.3 2.0 6.0 6.1 6.0 9.9
r.6 4.8
L.t l.) t.t

t(*t' calculation of end member molecules follows the procedure of Rickwood (1968).

FeLB peaks(Albee and Chodos, 1970)indicatesthat a n d m i n o r s p e s s a r t i n(e0 . 5 - 2 . 5p e r c e n t )i s a l i q u i d u s
(10 weight percent of the total iron is present as p h a s ei n c a l c - a l k a l i n em a g m a sa t p r e s s u r etsy p i c a lo f
FerOr. This is consistentwith the atomic reductions the lower crust or upper mantle (9-27 kbar). Garnet
in Table 2. Yttrium, potassium,and sodium were phenocrystspresumablywould become unstableat
looked for but were not detected in significant lower pressuresand would then begin to be resorbed
amounts (<0.01 percent) above background in- and react with the melt to form different phasesin
tensity. e q u i l i b r i u m a t l o w e r p r e s s u r eT. h i s w o u l d a c c o u n t
for the common presenceof overgrowthsof cordier-
Although notable compositional differencesexist i t e a n d h y p e r s t h e n e( M i y a s h i r o , 1 9 5 5 ; G r e e n a n d
betweenthe coresand rims of the six analyzedsam- Ringwood, 1968,1972),biotite (Lyons et al., 1973
ples,no consistent rendsare apparent.Both samples Edwards, 1936),feldspar(Edwards, 1936;Oliver,
from the granodiorite (V42) have coresenrichedin 1956),or chlorite and magnetite(Fitton, 1972)on
pyrope (0.4 and 0.7 percent) relative to their rims. garnet phenocrysts. Neither overgrowths nor the
Almandine and spessartinevary antitheticallyin sam- graphic intergrowths of garnet-quartz and garnet-
ples from the diorite (V36). Garnets from the aplite fefdspar cited by Wood (1974) and Oliver (1956)
(Y77) have rims enriched in spessartine(2.6 and 5.5 respectivelyas evidenceof igneouscrystallizationare
percent) relativeto the core, while the cores of the associatedwith the garnets from the southwestern
samecrystalscontain more pyrope (1.4 and 2.3 per-
cent respectively)than the rims. Dana Mountains.

Origin Edwards(1936)and Bartrum (1937)both postu-
lated crystallizationof garnet from a magma after its

Experimental work with natural garnet-bearing contaminationwith pelitic material.In both cases

rhyodacite (Green and Ringwood, 1968, 1972) has garnets are concentratedalong the rims of horn-

shown that almandinegarnetcontainingsubordinate blende-biotite-rich inclusions which were partially

pyrope (16-30 percent)and grossular(7-20 percent) assimilatedby granodiorite and diorite magmas re-

272 VENNUM AND MEYER. PLUTONICGARNETS

spectively,and away from the xenolith margins the laceous sedimentary rocks of the Carboniferous

garnets became unstable and redissolved in the Trinity PeninsulaSeries,a unit lithologically very

magma. Similar relationshipsbetweengarnets and similarto the Latady Formation.This pluton crops

xenoliths were not observedin plutonic rocks of the o u t o n t h e n o r t h w e s t e r nT r i n i t y P e n i n s u l a ,l l 0 0 k m

Wernerbatholith. north of the southwesternDana Mountains.

If the plutonic garnets are xenocrysts,only roof ( 7 ) M i y a s h i r o ( 1 9 5 5 )c o n c l u d e dt h a t g a r n e t p h e -

rocks containing one type of garnet were incorpo- nocrysts in calc-alkalineplutonic rocks (excluding

rated by the magmas.This conclusionis supportedby pegmatitesand aplites) are generally rich in alman-

the following linesof evidence: dine, contain somepyrope, and are poor in spessar-

(I ) Outcropsof roof rocksof theWerner batholith tine.

in the southwesternDana Mountains are uniformly The following evidenceindicatesthat the garnetsin

pelitic. the aplitedike are phenocrysts:

(2) The close similarity of physical and optical ( l ) G a r n e t - b e a r i n ga p l i t e , p e g m a t i t e ,a n d c o m -

properties of the plutonic garnets and two of the posite aplite-pegmatitedikes occur throughout the

metamorphicgarnets(Table I ) suggeststhesecrystals W e r n e r b a t h o l i t h .

havesimilarcompositions. (2) The garnets in the aplite dike are large (3-4

( 3 ) L i m e s t o n ei s a m i n o r c o n s t i t u e not f t h e L a t a d y m m ) a n d e u h e d r a l .

Formation in the southern LassiterCoast (Williams (3) The lower pyrope content of the rims of the

et al., 1972), but does not crop out in the south- crystalsis consistentwith igneouszonation.

westernDana Mountains. Physicaland optical prop- (4) Miyashiro (1955) noted that garnet pheno-

erties of garnet collected from a garnet-epidote crysts in pegmatiteassociatedwith calc-alkalineplu-

i n c l u s i o n( p r e s u m a b l ya n i n c l u s i o no f c o n t a c t - m e t a - tonic rocks are rich in almandine-snessartineand

m o r p h o s e dL a t a d y I i m e s t o n ei)n g r a n o d i o r i t e( T a b l e poor in pyrope.

l) are considerablydifferent from the above-men- Acknowledgments
tioned samples.
W e t h a n k B a r r y F l e i s c h l ,a s t u d e n ta t S o n o m a S t a t eC o l l e g e ,f o r
Severaladditional linesofevidencesuggesthat the his help with the X-ray work and Barbara Vennum for her help
garnets from the diorite and granodiorite are w i t h t h e m a n u s c r i p t a n d c a l c u l a t i o n s .J o h n C . E i c h e l b e r g e rr e -
xenocrystsderivedfrom the surroundingpelitic meta- viewed the manuscriptand offeredvaluablecommentstoward its
sedimentsof the Latady Formation: r e v i s i o n T h e s e n i o ra u t h o r a l s o e x p r e s s ehsi s a p p r e c i a t i o nt o P e t e r

(l) They occur only in rocks of the southwestern D . R o w l e y f o r h i s e n c o u r a g e m e ndt u r i n g t h i s s t u d y . G a r n e t s e p a -
D a n a M o u n t a i n s a n d a r e a b s e n ti n r o c k s o f s i m i l a r r a t i o n s w e r ed o n e b y E z e k i e lR i v e r a a n d D w i g h t E . R o b e r t so f t h e
composition elsewhere within the concentrically- U.S. Geological Survey. Ted E. Bunch (NASA Ames Research
z o n e dW e r n e r b a t h o l i t h . Center,Moffett Field,California) provided the computer-derived
atomic reductionsin Table 2. This study was financed by NSF
(2) They are irregularly distributed within that grantAG-187.

part of the batholith in which they occur. References
( 3 ) E x p e r i m e n t a ld a t a ( H e n s e na n d G r e e n , 1 9 7 3 )

suggestthat garnet containing less than 2 percent Adie, R J (1955) The Petrology ofGraham Land lI, the Andean

CaO and occurring in calc-alkalineplutonic rocks Granite-GabbroIntrusiue Suite. Falkland Islands Deoendencies
representsrelic or refractory phases from partial Surv.Sci.Rep. 12.
meltingof minor admixedpeliticsediments. - (1972)Evolution of volcanismin the Antarctic Peninsula.
In R. J. Adie, Ed., Anr.arcticGeologyand Geophysics,p. 137-141.
(4) Garnets from the granodiorite contain five
times as much of the pyrope moleculeas do those U n i v e r s i t e t s f o r l a g e tO, s l o
from the diorite. This is the reverseof what would be A l b e e , A L a n d A . A . C h o d o s ( 1 9 7 0 )S e m i - q u a n t i t a t i v ee l e c t r o n
expectedif these crystals resulted from fractional
crystallizationin a differentiatingmagma. m i c r o p r o b ed e t e r m i n a t i o n6 f p ' s 2 + / p s 3 +a n d M n ' + / M n 3 + i n o x -
i d e sa n d s i l i c a t e sa n d i t s a p p l i c a t i o nt o p e t r o l o g i cp r o b l e m s . , 4 r n .
(5) Their physical,optical, and petrographicchar- Mineral , J5, 491-501.
acteristics(refractiveindices,cell edge,specificgrav- Bartrum, J A ( 1937) Interesting xenoliths from Whangarei
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