01 - sodic pyroxenes - cmp 1978.pdf
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Contrib. Mineral. Petrol. 68, 7-11 (1978)
o n t r i b u t i o n s t o
ineralogy and
Petrology
9 by Springer-Verlag 1978
Sodie Pyroxen es From M etabas i tes in the as tern Medi terranean
A.I. Okay
Depart ment of Mineralogy and Petrology, Downin g Place, C ambridge, Great Britain
A bst rac t
The compositions of sodic pyroxenes from
blueschist metabasites in Northwest Turkey were in-
vestigated by electronprobe microanalysis. These are
compared with sodic pyroxenes from high-grade
blueschist terrains and eclogites with special reference
to the island of Syros in Greece. Pyroxenes from
lawsonite zone metabasites in Northwest Turkey are
chloromelanites, aegerine augites and aegerine
jadeites with the acmite contents ranging from 65%
down to 30%. Epidote-garnet bearing high-grade
blueschist metabasites of Syros on the other hand
contain omphacite with the acmite content less than
30%. All the analyzed sodic pyroxenes from eclogites
and high-grade blueschists in the literature are also
omphacites with low acmite contents. It is argued
that the acmite content of the sodic pyroxene de-
creases with ris ing temperature of metamorphism.
This is almost entirely related to the appearance of
epidote as an additional ferric ion bearing phase with
an increase in temperature.
Int roduct i on
Sodic pyroxenes are common minerals in the basic
rocks of the blueschist facies and related eclogites.
Their composi tions fall. within the ternary field of
acmite-jadeite-augite.
A considerable amount of new data on sodic py-
roxene compositions from blueschists and eclogites
has become available in the last ten years. Each batch
of new data has narrowed the miscibility gaps ,
based on the apparent absence of sodic pyroxene com-
positions over certain regions in the sodic pyroxene
ternary field (Dobretsov, 1962; Coleman and Clark,
1968), so that now almost the whole of the acmite-
jadeite-augite field is filled. The electron microprobe
analyses presented here demonstrate the widely vari-
able compositions of sodic pyroxenes. A new attempt
is made to relate the composition of sodic pyroxene
to the extensive, e.g., rock composition, and intensive
variables, e.g., pressure and temperature, of meta-
morphism.
Geol og i ca l E nvi ronment
In Northwest Turkey blueschists form a coherent, discontinuous
belt of about 20 km in width and 300 km in length. They flank
the margins of large ultramafic massifs in the Izmir- Ankara op hio-
lite zone (Brinkman, 1976). The rock types are domin antly metaba-
sites and metacherts. The eastern part of the blueschist belt in
the region o f Mihallicik was studied by Cogulu (1967) who dis-
tinguished three metamorp hic zones - lawsonite - albite, lawsonite
- crossite, epidote - crossite - with an increase in grade northwards.
The epidote zone truncates westwards. The studied areas, all
e n
t i r e l y in the lawsonite zone, lie in the central portion of the blue-
schist belt in t h e d i s t r i c t s of Tavsanli (area 1 in Fig. 1) and Gumele
(area2 in Fig. 1). In both areas metamorphism occurred during
a single event and retrogressive readjustment h as been minor. Pluri-
facial metamor phism, which complicates the inte rpretation o f min-
eral assemblages in the Alps, is absent.
M e t h o d
Sodic pyroxene compositions were determined for twelve elements
(Na, Ca, K, Fe, Mg, Mn, V, Ni, Cr, Ti~ AI, Si) using an electron-
probe microanalyzer With a Harwell Si(Li) detector and pulse
processor (Statham, 1976). The correction procedures are given
by Sweatman and Long (1969). Olivine and jadeite standards were
used throughout.
The analyses were recalculated to a four cation formu la unit.
As there is no substitution of silicon by tetrahedral aluminium,
the jadeite component equals to the aluminium in the formula
unit. Acmite component was calculated assuming Ac=fe rric ion =
(Na-AI) in the formula unit. The remainder of the cations were
included in the augite component. Transition elements such as
Ti, Mn, Cr o ccur in amounts less than 0.01 of the formula unit,
so that jadeite, acmite, dio psi de and hedenbergite components
make up 98% or more of the sodic pyroxene.
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8 A.I. Okay: Sodic Pyroxenes From Metabasites in Turkey
- . -
Fig. 1. Map of Aegean and surrounding areas showing
major ophiolite occurrences black) and blueschist
outcrops in Northwest Turkey ruled). The ophiolite
outcrops in Western Turkey after Bingol 1976)
Mineral ssemblage and the Composition of the Sodic
Pyroxene
Blueschist Metabasites From Northwest Turkey
This group comprises thoroughly recrystallized, often
foliated, lawsonite bearing blueschist metabasites cor-
responding both in mineralogy and texture to Typ e
II I blueschists from California (Coleman and Lee,
1963).
The mineral assemblage in the metabasites is so-
dic amphibole (often crossite or glaucophane) + law-
sonite _+ sodic pyroxene + chlorite +_ quartz + phen-
gite + sphene . Sodic pyroxene is a common part of
the metabasic mineral assemblage in Northwest
Turkey making up approximately 5 to 25 modal per-
cent of the rock. Out of 86 thin sections of the metaba-
sites examined 50 contained sodic pyroxene that usually
occurred as small subhedral grains, occasionally
showing a reaction relationship to sodic amphiboles.
Sodic pyroxene is, along with the lawsonite, one
of the first metamorphic minerals to form in North-
west Turkish metabasites. As observed in the less
well recrystallized parts of the blueschist terrain in
Northwest Turkey, initialize spilitic dolerites are
transformed to an assemblage o f lawsonite, chlorite,
acmitic sodic pyroxene and quartz with incipient cros-
site. At this stage the original igneous texture is still
intact with plagioclase pseudomorphed by an assem-
blage of lawsonite, quartz and chlorite while augite
is largely replaced by an acmite rich sodic pyroxene.
With further recrystallization crossite forms at the
expense of sodic pyroxene, quartz and chlorite and
the rock loses its ghost texture with the development
of a distinct schistosity.
Sodic pyroxenes from ten blueschist metabasites
were analyzed by electlon-microprobe and plotted on
the jadeite-acmite-augite ternary diagram in Fig. 2.
Specimens with numbers starting with 93 come from
the area Gumele (area 2 in Fig. 1). Blueschists from
Gumele were studied, especially with regard to their
whole rock geochemistry, by Lunel (1967). The re-
mainder of the specimens with prefixed K come from
the districf north of Tavsanli (area 1 in Fig. 1). All
the analyzed rocks contain the assemblage lawso-
nite + crossite + phengite + sphene . Addit ionally
specimens K47, K75, K54, K29, 93163 contain pri-
mary chlorite, K47, K54 and 93180 contain quartz
and K33, K29, K114 contain calcite. However these
latter minerals occur in very small quantities, crossite,
lawsonite, and sodic pyroxene making up 90% or
more of the metabasites.
Compositions plotted in Fig. 2 demonstrate that
sodic pyroxenes with similar parageneses show a wide
range of compositions both within and between differ-
ent metabasites. Zoning in sodic pyroxenes, like most
other minerals with variable compositions from blue-
schist, is very common. The sodic pyroxenes from
Northwest Turkey using the terminology ado.pted for
sodic pyroxenes by Essene and Fyfe (1967), plot in
the fields of chloromelanite, aegerine augite and
aegerine jadeite with the acmite contents ranging from
65% down to 30%. Compositions of sodic pyroxenes
are apparently rather rare in this range, such that
a miscibility gap was proposed (Dobretsov, 1962;
Coleman and Clark, 1968), which the new analyses
fill at least in the ferric-rich portion.
High Grade Blueschist Metabasites
From Other Terrains
This group includes thoroughly recrystallized epidote-
garnet bearing blueschist metabasites and eclogites.
The mineral assemblage is glaucoph ane+epid ote
+ sodic pyroxene + garnet + white mica _+ quartz + ru-
tile . They grade into eclogites with the dominance
of sodic pyroxene and garnet in the mineral paragen-
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A.I. Okay: Sodic Pyroxenes From Metabasites n Turkey 9
Fig. 2. Compositionsof sodic pyroxenes rom
lawsonite-bearing metabasites, Northwest Turkey
A c A c
O O o o . . 2
~ 1 7 6
o o
9 9 o o
9
ss
o 1(47
9 K~.I
AU Au Jd
Ac
097503
o @
A 97506
o o
I)97542 9 9
V V V V \
A u J d
Fig. 3. Compositionsof sodic pyroxenesfrom epidote-garnet bear-
ing high-grade blueschists, Syros
with numbers 97503 and 97667 have recrystallized
in plagioclase rich portions of the original coarse
grained protolith (Dixon, 1968). They represent an
unusually aluminous environment for a metabasic
rock.
Sodic pyroxenes from analyzed Syros metabasites
fall largely in the omphacite field with the acmite
contents ranging from 30% down to 2%. Omphacitic
pyroxenes of these compositions are characteristic of
eclogites and high-grade blueschists.
esis. These differ from the blueschist metabasites of
Northwest Turkey in the coarser grain size, the re-
placement of lawsonite and sphene by epidote and
rutile respectively and the occurrence of almandine
garnet. The eclogitic exotic blocks from the Francis-
can terrain in California, Type IV of Coleman and
Lee (1963), are a good example for these types of
rocks.
The blueschists from the island of Syros in the
Aegean provide another good example of high grade
blueschists. They were studied in detail by Dixon
(1967). The island consists of an alternating sequence
of pelitic blueschists z~nd marbles with rare intercala-
tions o f metabasic gneisses, metagabbros and serpen-
tinite (Dixon, 1976). The sodic pyroxene analyses
have been performed on the metagabbros and
gneisses. These sodic pyroxenes are currently under
study for their microstructure (Carpenter, 1978).
Sodic pyroxenes from four metabasic rocks from
Syros are shown in the acmite-jadeite-augite ternary
diagram in Fig. 3. All four rocks have the mineral
assemblage glaucophane + epidote + sodic pyroxene-
+ga rne t + white mica + qua rtz and all except 97506
contain rutile. Specimen 97542 also contains some
actinolite. The jadeite rich pyroxenes from specimens
i s c u s s i o n
Sodic pyroxenes from both terrains show a wide range
of compositions within a relatively uniform bulk rock
chemistry. In the metabasites the absence of albite
is reflected in the highly variable jadeite contents of
pyroxenes, even within a single rock, preventing any
geobarometric deductions. Although the jadeite con-
tents of pyroxenes from metabasites in Syros and
Northwest Turkey are similar the pressure has been
considerably higher in Syros as indicated by the pres-
ence of jade ite+ quar tz assemblages in rocks of ap-
propriate chemistry (Dixon, 1976).
However there is a distinct difference in the acmite
contents of the sodic pyroxenes between lawsonite-
bearing metabasites from Northwest Turkey and
other high-grade epidote-garnet bearing blueschists
and eclogites, including those from Syros. Such a
difference between lawsonite-zone metabasites on the
one hand and epidote-zone metabasites on the other
hand seems to be a general case (see Fig. 4). This
difference must lie in the intensive variables of meta-
morphism, e.g., pressure and temperature, as the bulk
rock chemistry is approximately the same in both
cases. Both oxygen isotope studies (Black, 1974; Tay-
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1 0 A . I . O k a y : S o d i c P y r o x e n e s F r o m M e t a b a s i t e s i n T u r k e y
l o r a n d C o l e m a n , 1 96 8) a n d e x p e r i m e n t a l w o rk o n
the s tab i l i ty o f lawso n i te (Ni t sch , 1974) show tha t
ep ido te-garne t - ru t i l e bear ing b luesch i s t s have recrys -
t a l li z e d a t c o n s i d e ra b l y h i g h e r t e m p e ra t u r e s (38 0 ~
5 5 0 ~ t h a n l a w s o n i t e - s p h e n e b e a r i n g b l u e s c hi s ts
(2 7 0 0 -4 0 0 0 C ) . Th e l a w s o n i t e / e p i d o t e t r a n s fo rm a -
t ion , be ing a deh ydr a t ion reac t ion , ha s a s t eep slope
o n t h e P - T d i a g ra m , m a k i n g i t a g o o d g e o t h e rm o m e -
ter.
Th e e f f e c t o f p r e s s u re o n s o d i c p y ro x e n e c o m p o s i -
t ions in metabasa l t s i s more d i f f i cu l t to as sess . The
a b s e n c e o f p r i m a ry a l b i te a n d t h e p r e s e n c e o f j a d e i t e -
+ quar tz as semblages in Syros (Dixon , 1968) ind ica te
cons iderab ly h igher p ressu res (12 to 15 kbar ) fo r
b l u e s c h is t s f r o m S y ro s t h a n t h o s e f ro m e p i d o t e z o n e
b l u e s c h i s t s i n N e w C a l e d o n i a , w h i c h s h o w a v e ry
s i m il a r m i n e ra l p a r a g e n e s is w i t h t h e e x c e p t i o n o f t h e
s tab i l i ty o f a lb i t e (Bro the rs , 1974) , ind ica t ing lower
p re s s u re s o f r e c ry s ta l l iz a t io n . H o w e v e r t h e r a n g e o f
o b s e rv e d a c m i t e c o n t e n t i n t h e e p i d o t e - z o n e s o d i c
p y ro x e n e s f ro m t h e b l u e s c h i s t m e t a b a s i t e s i n N e w
C a l e d o n i a i s n e a r l y t h e s a m e a s t h o s e f ro m S y ro s ,
30 dow n to 5 (Black , 1974) , sugges t ing tha t t em-
p e ra t u r e h a s t h e d o m i n a n t e f f e c t o n t h e a c m i t e c o n -
t e n t o f t h e s o d i c p y ro x e n e .
Th i s t r e n d o f d e c re a s i n g a c m i t e c o n t e n t i n t h e
s o d i c p y ro x e n e w i t h r is in g t e m p e ra t u r e m a y b e r e l a t e d
e i t h er t o t h e fo rm a t i o n o f a n a d d i t i o n a l f e r r ic b e a r i n g
p h a s e o r t o r e d u c t i o n d u r i n g t h e p ro g re s s i v e m e t a -
m o rp h i s m . O m p h a c i t i c p y ro x e n e is n o t t h o u g h t t o
b e i n h e re n t l y u n s t a b l e i n t h e l a w s o n i t e - z o n e c o n d i -
t i o n s a s i n d i c a t e d b y i ts p r e s e n c e i n s o m e p a r t i a ll y
r e c ry s ta l l iz e d b a s i c r o c k s (C a rp e n t e r a n d O k a y , i n
prep.) .
B l u e s c h i s t m e t a b a s i t e s g e n e ra l l y d o n o t c o n t a i n
any i ron ox ides , as fe r r i c ion has a l t e rna t ive s i tes
i n t h e s o d i c a m p h i b o l e s a n d s o d i c p y ro x e n e s . Th e re -
fo r e t h e m a j o r f e r r i c i r o n b e a r i n g m i n e ra l s i n l a w s o -
n i t e - z o n e b l u e s c h i s t s a r e s o d i c p y ro x e n e s a n d s o d i c
a m p h i b o l e s . I n t h e e p i d o t e - z o n e b l u e s c h i s t s , e p i d o t e
i t se l f i s an add i t iona l F e 3+ bear ing ph ase . Th us the
fo rm a t i o n o f e p i d o t e in th e t e m p e ra t u r e i n t e rv a l o f
3 5 5 -4 0 0 C w o u l d b e e x p e c t e d t o a f f e c t t h e f e r r i c i r o n
c o n t e n t s o f t h e a s s o c i a t e d s o d ic p y ro x e n e a n d s o d i c
a m p h i b o l e .
La w s o n i t e a n d f e r r i c i r o n r i c h e p i d o t e a r e e x p e c t -
e d t o c o e x i s t o v e r a c e r t a in r a n g e o f t e m p e ra t u r e a n d
p re s su re . S o d i c p y ro x e n e s c o e x i s t in g s t a b l y w i t h l a w -
s o n i t e a n d f e r r i c i r o n r i c h e p i d o t e s h o u l d h a v e a n
a c m i t e c o n t e n t o f a ro u n d 3 0 a n d p l o t i n t h e a c m i t e -
j a d e i t e - a u g i t e t e rn a ry d i a g ra m i n t h e i n t e rm e d i a t e r e -
g i o n b e t w e e n l a w s o n i t e a n d e p i d o t e z o n e m e t a b a s i c
p y ro x e n e s (F i g . 4 ). U n fo r t u n a t e l y t h e s c a r c i t y o f p y -
ro x e n e a n a l y s e s o v e r s u c h t r a n s i t i o n a l t e r r a i n s p ro -
h ib i t s the t es t ing o f th i s hypo thes i s .
A
A u _ _ I d
Fi g . 4 . C o m p o s i t i o n s o f me t a m o rp h i c so d i c p y ro x e n e s ; f i l le d t r i a n -
g l e s : P y r o x e n e s f r o m i r o n - r i c h m e t a s e d i m e n t s a n d m e t a c h e r t s ;
f i l l e d c i rc l e s : p y ro x e n e s f ro m l a w so n i t e b e a r i n g me t a b a s i t e s ; o p e n
c i rc l e s : p y ro x e n e s f ro m e p i d o t e -g a rn e t b e a r i n g b l u e sc h i s t me t a b a -
s i t es a n d e c l o g it e s , o p e n t r i a n g l e s : p y ro x e n e s f ro m a l u m i n u o u s
b l u e sc h i s t s ; f i l le d sq u a re s : p y ro x e n e s f ro m m e t a g re y w a c k e s t h i s
p a p e r , M u i r W o o d 1 9 7 7 ) ; E rn s t 1 9 76 ) , B l a c k 1 9 7 4 ) ; A b ra h a m
e t a l . 1 9 7 4 ) ; K e r r i c k a n d C o t t o n 1 9 7 1 ) ; E rn s t e t a l. 1 9 7 8 ) ; O n u k i
a n d Ern s t 1 9 69 )
I t i s k n o w n t h a t t h e e p i d o t e c o m p o s i t i o n a l f i e l d
ex tends towards lower fe r r i c con ten t s wi th r i s ing t em-
p e ra t u r e d u r i n g p ro g re s s i v e m e t a m o rp h i s m (M i y a s h -
i ro and Sek i , 1958) , such tha t ep ido tes f rom ec log i tes
u s u a l l y h a v e l o w p is t a c it e c o m p o n e n t s . Th i s i m p l i e s
a c e r ta i n a m o u n t o f r e d u c t i o n d u r i n g m e t a m o rp h i s m ,
l e a d i n g t o t h e h i g h l y r e d u c e d t w o p h a s e - a l m a n d i n e -
o m p h a c i t e - e c lo g i t e . S u c h a p ro g re s s i v e r e d u c t i o n w i t h
i n c r ea s i n g g r a d e h a s b e e n o b s e rv e d i n s o m e h i g h p r e s -
s u re m e t a m o rp h i c t e r r a in s , e . g ., in H o h e T a u e rn w i n -
dow ( Ab rah am e t a l . , 1974) .
A l t h o u g h r e d u c t i o n m a y b e o f im p o r t a n c e i n h ig h -
g rade b luesch i s t s and ec log i tes , the regu lar and sharp
b re a k i n s o d i c p y ro x e n e c o m p o s i t i o n s b e t w e e n t w o
t y p e s : o f m e t a b a s a l t s , i n d i c a te t h a t t h e t e m p e ra t u r e
r e l a t e d la w s o n i t e / e p i d o t e t r a n s fo rm a t i o n i s m a i n l y r e -
s p o n s i b l e fo r t h e o b s e rv e d d i f f e r e n c e b e t w e e n l a w s o -
n i te a n d e p i d o t e -g a rn e t b e a r i n g m e t a b a s i t es .
9 Compositional Range of Sodic Pyroxenes
From Blueschists
S o d i c p y ro x e n e a n a l y s e s f ro m b l u e sc h i s ts a n d r e l a t e d
ec log i tes a re show n in acm i te - jade i te -aug i te t e rnary
d i a g ra m i n F i g. 4. Th e r a n g e o f s o d i c p y ro x e n e c o m -
pos i t ions ' fo r d if fe ren t majo r ro ck type s , i ron -r ich
ro c k s , m e t a b a s a l t s a n d m e t a g re y w a c k e s , a r e a l s o i n d i-
ca ted .
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A.I . Ok a y : So d i c Py ro x e n e s Fro m M e t a b a s i t e s i n Tu rk e y 1 1
Acmite and aegerine have a very wide pressure-
temperature stability field. They are found in iron-rich
rocks and metacherts from both lawsonite and epi-
dote zone blueschists in Northwest Turkey, California
and other blueschist terrains. The occurrence of aeger-
ine in igneous rocks is a further testimony of its large
P T
stability field. Most of the analyses of sodic py-
roxenes from iron-rich rocks come from Muir-Wood
1977) who also reports Mn as a major component
in some of the aergerines.
Metabasites on account of their calcium and ferro-
magnesium rich character do not contain any jadeite
or jadeite-rich pyroxenes either in lawsonite or epi-
dote zones. However lawsonite is stably associated
with jadeite rich pyroxenes in some metagreywackes
from the Franciscan terrain in California Ernst et al.,
1970). The occurrence o f jadeite in appropriate bulk
rock composi tions depends on the prevailing pressure
of metamorphism.
There is an apparent lack of sodic pyroxene com-
positions in a small region between jadeite and om-
phacite. A much larger area extending towards more
ferric-rich compositions and including this region was
interpreted by several workers as a miscibility gap
in the sodic pyroxene system Dobretsov, 1962; Cole-
man and Clark, 1968). However from the extensive
range of compositions plotted in Fig. 4, it is likely
that the apparent lack of sodic pyroxene compositions
in this region is due to the absence of suitable rock
compositions. Immiscibility, as demonst rated in exso-
lution, in such low temperature minerals as sodic py-
roxenes, is likely to be on a submicroscopic scale.
As a result, microprobe analyses should not be
expected to reveal the limits of immiscibility in the
sodic pyroxene ternary system.
Acknowledgements. Th i s wo rk wa s c a r r i e d o u t wh i l e i n re c e i p t
o f a g r a n t f r o m t h e M i n e r a l R e s e a r c h a n d E x p l o r a t i o n I n s t i t u t e
o f Tu rk e y (M T A) w h i c h is g ra t e fu l l y a c k n o wl e d g e d . Th e a n a l y se s
o f so d i c p y ro x e n e s f ro m t h e h i g h -g ra d e b l u e sc h i s t s o f Sy ro s a n d
f r o m i r o n r i c h m e t a s e d i m e n t s w e r e k in d l y p r o v i d e d b y M . A . C a r -
p e n t e r a n d R . M u i r W o o d r e s pe c ti v el y . I t h a n k R . M u i r W o o d ,
M .A. C a rp e n t e r , a n d S .O. Ag re l l fo r c r i t i c a l c o mme n t s o n t h i s p a -
per.
e f e r e n c e s
A b r a h a m , K . , H o r m a n n , P . K . , R a i t h , M . : P r o g r e s s iv e m e t a m o r -
p h i s m o f b a s i c r o c k s f r o m t h e s o u t h e r n H o h e T a u e r n a r e a ,
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