C A L C -SIL IC A T E RO CK S A T T O O L A N G I, V IC T ... · S O C . V IC T . v o l. 97, n o....
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PROC. R. SOC. VICT. vol. 97, no. 1, 1-18, March 1985 CALC-SILICATE ROCKS AT TOOLANGI, VICTORIA By W. D. B ir c h Department of Mineralogy and Petrology, Museum of Victoria, 285 Russell Street, Melbourne, Victoria, 3000 A bst r a ct : Calc-silicate rocks outcrop over a small area on Blue Mount, near Toolangi, Victoria. They were originally a sequence of calcareous mudstones and argillaceous limestones of Late Silurian- Early Devonian age, which were contact metamorphosed by a series of small granodiorite intrusions in the Late Devonian. The rocks are mainly fine-grained, pale grey hornfels with localized development of grossular-rich spheroids, aggregates and spots. Some specimens show different alteration textures, where originally dark hornfelses have been altered by reaction with metamorphic fluids. Minerals present include grossular, diopside, wollastonite, prehnite, vesuvianite, calcite, quartz, sphene, scolecite, K feldspar and plagioclase, nearly always as fine-grained aggregates. Only wollastonite and grossular develop as crystals which are visible to the naked eye. The bulk chemistry of the rocks is variable and original compositions have been modified during metamorphism. Analyses of coexisting dark (metamorphic) and pale (metasomatized) hornfels suggest that Ca, Si, Al, S, Cl and Na were the main mobile elements, with the alteration resulting in the removal of Si, Al, Na and S from the original hornfels and the concentration of Ca, K, Fe, Mg, Ba, Sr and Cl in the resulting pale hornfels. The calc-silicate assemblage crystallised in a metamorphic-metasomatic event in which initial pro - grade contact metamorphism, which may have been largely isochemical, was overlapped by metasomatism which led to the partial homogenization of the metasedimentary sequence and localized development of coarser skarn-like textures. Solutions responsible for the metasomatism were probably derived mainly from the sedimentary sequence or from meteoric sources. The lack of iron-enrichment suggests that the influence of magmatic fluids was minimal. Comparison of the various assemblages observed in the Blue Mount rocks with those in calculated and experimentally determined equilibria suggests that the maximum temperature of the prograde metamor- phism was between 500° and 550°, at pressures, estimated stratigraphically, of the order of 0.5 Kb or less. The solutions at, and following, the metamorphic peak were water-rich (Xc0 <0.2). Retrogression was marked by localized crystallisation of vesuvianite, prehnite and scolecite. 2 Calc-silicate rocks from contact metamorphic aureoles are rare in Central Victoria, reflecting the scar- city of calcareous sediments in sequences intruded by Palaeozoic granitic rocks. Small scale occurrences have been reported from the aureoles of the Bulla Granite (Tattam 1925) and the Strathbogie Batholith near Bon- nie Doon (Phillips & Wall 1980). Hornfels containing calc-silicate assemblages were recorded in the Maldon goldfield (Moon 1897), while Lindner (1953) described vesuvianite and brucite from the contact of a gabbroic intrusion into the Ordovician Digger Island Limestone at W aratah Bay (recent evidence suggests this latter con- tact is an unconformity —Singleton pers. comm. 1984, Birch unpub. data). Probably the most extensive suite of calc-silicate rocks occurs on Blue Mount, about 3 km south of Toolangi (Fig. 1). Kenley (1958) briefly described the geology of the area in preparation for the construction of the seismic observatory on the Mount and subse- quently drafted a map of the observatory excavation. However, no additional petrological or mineralogical studies of the unusual rock-types exposed were carried out until specimens were collected for the National Musuem of Victoria in 1978/9. GEOLOGY Interbedded Late Silurian to Early Devonian siltstones and sandstones of the Dargile and Humevale Formations (VandenBerg 1971) form the bedrock of the area (Fig. 1). The sediments are folded with a roughly north-south axial trend. Three kilometres to the east of Blue Mount (elevation 604 m) the north-south trending Yellingbo Fault Zone brings the acid-intermediate volcanic rocks of the Acheron Cauldron Complex into contact with the basement (Dudley et al. 1971). Intru- sion of granitic rocks associated with the complex (the Black Range composite intrusion) has resulted in a belt of contact metamorphism from 1 to 4 km wide in the basement sediments. Between Toolangi and Healesville, four small outcrops of granodiorite occur in a line ap- proximating regional trends (Fig. 1). These probably represent apophyses of a larger near-surface intrusion responsible for the contact metamorphic effects. On Blue Mount, the contact metamorphic rocks con- sist mainly of light grey calc-silicate hornfels. Several decomposed feldspathic dykes were mapped in the original excavation for the observatory (Kenley 1958). The full extent of the calc-silicate rocks was not deter - mined but the outcrop appears to be restricted to Blue Mount, and probably would not exceed 1 km2 in out- crop area. PETROGRAPHY The light grey hornfels is cherty, sometimes porcellainous, in appearance. Internal casts of shelly fossils are occasionally found. Much of the pale hornfels contains thin bands or irregular patches of a dark grey, similarly fine-grained rock. Many of the macroscopic 1
Transcript of C A L C -SIL IC A T E RO CK S A T T O O L A N G I, V IC T ... · S O C . V IC T . v o l. 97, n o....
P R O C . R . S O C . V IC T . v o l . 9 7 , n o . 1, 1 -1 8 , M a r ch 1985
C A L C -S IL IC A T E ROCKS A T T O O L A N G I , V IC TO R IA
B y W . D . B i r c h
D e p a r tm e n t o f M in e ralo g y a n d P e tro lo g y , M u se u m o f V ic to r ia , 285 R u sse ll S t re e t , M e lb o u rn e ,
V ic to r ia , 3000
A b s t r a c t : C a lc -s i l ic a te ro c k s o u tc ro p o v e r a sm a ll a re a o n B lu e M o u n t , n e a r T o o la n g i , V ic to r ia .
T h e y w e re o r ig in a l ly a se q u e n c e o f c a lc a re o u s m u d s to n e s a n d a rg i l la c e o u s l im e s to n e s o f L a te S i lu r ia n -
E a r ly D e v o n ia n a g e , w h ic h w e re c o n ta c t m e ta m o rp h o s ed b y a se r ies o f sm a ll g r a n o d io r i te in t ru s io n s in
th e L a te D e v o n ia n . T h e ro c k s a re m a in ly f in e -g ra in ed , p a le g re y h o rn fe ls w ith lo c a l iz e d d e v e lo p m e n t o f
g ro s s u la r - r ic h s p h e ro id s , a g g re g a te s a n d s p o ts . S o me sp e c im e n s sh o w d if fe re n t a l te ra t io n te x tu re s , w h ere
o r ig in a l ly d a rk h o rn fe ls e s h a v e b e e n a lte re d b y re ac t io n w ith m e ta m o rp h ic f lu id s .
M in e ra ls p re s e n t in c lu d e g ro ss u la r , d io p s id e , w o l las to n i te , p re h n i te , v e su v ia n i te , c a lc i te , q u a r tz ,
sp h e n e , sc o le c ite , K fe ld s p a r a n d p la g io c la se , n e a rly a lw a y s a s f in e -g ra in e d a g g re g a te s . O n ly w o l la s ton i te
a n d g ro s s u la r d e v e lo p a s c ry s ta ls w h ic h a re v is ib le to th e n a k e d eye.
T h e b u lk c h e m is try o f th e ro c k s is v a r ia b le a n d o r ig in a l c o m p o s it io n s h a v e b e e n m o d if ie d d u r in g
m e ta m o rp h is m . A n a ly se s o f c o e x is t in g d a rk (m e ta m o rph ic ) a n d p a le (m e ta s o m a t iz e d ) h o rn fe ls su g g est
th a t C a , S i, A l , S , C l a n d N a w e re th e m a in m o b i le e le m e n ts , w ith th e a l te ra t io n re su l t in g in th e re m ov a l
o f S i, A l , N a a n d S f r o m th e o r ig in a l h o rn fe ls a n d th e c o n c e n tra t io n o f C a , K , F e , M g , B a , S r a n d C l in
th e re su lt in g p a le h o rn fe ls .
T h e c a lc -s i l ic a te a sse m b la g e c ry s ta l l ise d in a m e tam o rp h ic -m e ta s o m a t ic e v en t in w h ic h in i t ia l p r o
g ra d e c o n ta c t m e ta m o rp h is m , w h ic h m a y h a v e b e e n la rg e ly iso c h e m ic a l, w a s o v e r la p p e d by
m e ta so m a t is m w h ic h led to th e p a r t ia l h o m o g e n iz a t ion o f th e m e ta s e d im e n ta ry se q u e n c e a n d lo ca liz ed
d e v e lo p m e n t o f c o a rs e r sk a rn - l ik e te x tu re s . S o lu t ion s re sp o n s ib le f o r th e m e ta so m a t is m w e re p ro b a b ly
d e r iv e d m a in ly f r o m th e s e d im e n ta ry se q u e n c e o r f rom m e te o r ic so u rc e s . T h e lac k o f i ro n -e n r ic h m e n t
su g g e s ts th a t th e in f lu e n c e o f m a g m a t ic f lu id s w a s m in im a l .
C o m p a r is o n o f th e v a r io u s a sse m b la g e s o b se rv e d in th e B lu e M o u n t ro c k s w ith th o se in c a lc u la te d a n d
e x p e r im e n ta l ly d e te rm in e d e q u i l ib r ia su g g e s ts th a t th e m a x im u m te m p e ra tu re o f th e p ro g ra d e m e ta m o r
p h ism w as b e tw e e n 5 0 0 ° a n d 5 5 0 ° , a t p re ssu re s , e s t im a te d s t ra t ig ra p h ic a l ly , o f th e o rd e r o f 0 .5 K b o r
less. T h e so lu t io n s a t , a n d fo l lo w in g , th e m e ta m o rph ic p e a k w e re w a te r - r ic h (X c0 < 0 .2 ) . R e tro g re ss io n
w as m a rk e d b y lo c a l iz e d c ry s ta l l is a t io n o f v e su v ia nite , p re h n ite a n d sc o lec ite . 2
C a lc -s i l ic a te ro c k s f ro m c o n ta c t m e ta m o rp h ic
a u re o le s a re r a re in C e n tra l V ic to r ia , re f lec tin g th e sc a r
c ity o f c a lc a re o u s se d im e n ts in se q u e n c e s in tru d e d by
P a la e o z o ic g ra n it ic ro c k s . S m a ll sca le o c c u r re n c e s h av e
b e e n re p o r te d f ro m th e a u re o le s o f th e B u l la G ra n i te
(T a t ta m 1925) a n d th e S tra th b o g ie B a th o l i th n e a r B on
n ie D o o n (P h i l l ip s & W a ll 1980). H o rn fe ls c o n ta in ing
ca lc -s i l ica te a ssem b lag e s w ere re c o rd e d in th e M a ldo n
g o ld f ie ld (M o o n 1897), w h ile L in d n e r (1953) d e sc r ibe d
v e su v ia n ite a n d b ru c i te f ro m th e c o n ta c t o f a g a b b ro ic
in tru s io n in to th e O rd o v ic ia n D ig g e r I s la n d L im e s ton e
a t W a ra ta h B ay ( re c e n t ev id en c e su g g ests th is la t ter c o n
ta c t is a n u n c o n fo rm ity —S in g le to n p e rs . c o m m . 1984,
B irc h u n p u b . d a ta ) .
P ro b a b ly th e m o s t ex ten s iv e s u ite o f ca lc -s i l ica te ro c k s o c c u rs o n B lu e M o u n t , a b o u t 3 k m s o u th o f
T o o la n g i (F ig . 1). K en ley (1958) b r ie f ly d e s c r ib e d th e
g eo lo g y o f th e a re a in p re p a ra t io n f o r th e c o n s tru ct io n
o f th e se ism ic o b s e rv a to ry o n th e M o u n t a n d su b se
q u e n t ly d ra f te d a m a p o f th e o b s e rv a to ry e x c a v a tio n.
H o w e v e r , n o a d d i t io n a l p e tro lo g ic a l o r m in e ra lo g ic al
s tu d ie s o f th e u n u s u a l ro c k - ty p e s e x p o se d w ere c a r rie d
o u t u n t i l sp e c im en s w ere c o l le c te d fo r th e N a t io n a l M u su e m o f V ic to r ia in 1 9 7 8 /9 .
G E O L O G Y
In te rb e d d e d L a te S i lu r ia n to E a r ly D e v o n ia n
s il ts to n e s a n d s a n d s to n e s o f th e D a rg ile a n d H u m e v ale
F o rm a t io n s (V a n d e n B e rg 1971) fo rm th e b e d ro c k o f the
a re a (F ig . 1). T h e se d im e n ts a re fo ld e d w ith a ro u gh ly
n o r th -s o u th ax ia l t r e n d . T h re e k i lo m e tre s to th e e as t o f
B lu e M o u n t (e le v a tio n 604 m ) th e n o r th -s o u th tre n d in g
Y e ll in g b o F a u l t Z o n e b r in g s th e a c id - in te rm e d ia te
v o lc a n ic ro c k s o f th e A c h e ro n C a u ld ro n C o m p le x in to
c o n ta c t w ith th e b a s e m e n t (D u d le y e t al. 1971). I n t r u
s io n o f g ra n it ic ro c k s a s so c ia te d w ith th e co m p le x ( th e
B lac k R a n g e c o m p o s ite in tru s io n ) h as re su lte d in a b e lt
o f c o n ta c t m e ta m o rp h is m f ro m 1 to 4 k m w id e in th e
b a s e m e n t se d im e n ts . B e tw een T o o la n g i a n d H ea lesv il le ,
f o u r sm a ll o u tc ro p s o f g ra n o d io r i te o c c u r in a lin e a p
p ro x im a t in g re g io n a l t re n d s (F ig . 1). T h e se p ro b a b ly
re p re se n t a p o p h y s e s o f a la rg e r n e a r -s u r fa c e in tru sio n
re sp o n s ib le fo r th e c o n ta c t m e ta m o rp h ic e ffec ts.
O n B lu e M o u n t , th e c o n ta c t m e ta m o rp h ic ro c k s c o n
sis t m a in ly o f l ig h t g rey ca lc -s i l ic a te h o rn fe ls . Sev era l
d e c o m p o se d fe ld s p a th ic d y k es w ere m a p p e d in th e
o r ig in a l e x c a v a tio n fo r th e o b s e rv a to ry (K en ley 1958).
T h e fu l l e x te n t o f th e ca lc -s i l ic a te ro c k s w as n o t d e te r
m in e d b u t th e o u tc ro p a p p e a rs to b e re s tr ic te d to Blu e
M o u n t, a n d p ro b a b ly w o u ld n o t ex ceed 1 k m 2 in o u t
c ro p a re a .
P E T R O G R A P H Y
T h e lig h t g rey h o rn fe ls is c h e r ty , so m e tim es
p o rc e l la in o u s , in a p p e a ra n c e . I n te rn a l c a s ts o f shelly
fo ss ils a re o c c a s io n a lly f o u n d . M u c h o f th e p a le h orn fe ls
c o n ta in s th in b a n d s o r ir re g u la r p a tc h e s o f a d a rk g rey ,
s im ila r ly f in e -g ra in e d ro c k . M a n y o f th e m a c ro sc o p ic
1
2 W . D . B IRC H
re p la c e m e n t- ty p e te x tu re s (F ig s 2 , 3) su g g est th a t th e
p a le -c o lo u re d h o rn fe ls h a s re su lte d f ro m a l te ra t io n o f
th e d a rk ro c k ty p e . W h a t a p p e a rs to b e o r ig in a l b e dd in g
is p re se rv e d lo c a lly (F ig . 4) in th e fo rm o f a l te rna t in g
d a rk a n d l ig h t b a n d s . I n g e n e ra l, th e p a le a n d d a r k g rey
h o rn fe ls a re to o f in e -g ra in e d to e n a b le in d iv id u a l
m in e ra ls to b e d is t in g u ish e d in th in se c tio n . H o w e ve r ,
X - ra y d i f f ra c t io n a n d e le c tro n m ic ro p ro b e in v e s t ig at io n s
rev e a l th a t K fe ld sp a r , d io p s id e a n d q u a r tz a re th e d o m i
n a n t m in e ra ls in b o th ro c k ty p e s , w ith m in o r ca lc ic
p la g io c lase , g ro s s u la r , w o l la s to n ite , c a lc ite , sp h en e ,
a p a t i te a n d o p a q u e s . T h e re is l i t t le te x tu ra l o r
m in e ra lo g ic a l d is t in c t io n b e tw e e n d a r k a n d l ig h t c ol
o u re d ro c k ty p e s in th in se c tio n . H o w e v e r , th e d a rk
h o rn fe ls h as m o re f in e ly d iss e m in a te d o p a q u e g ra in s,
m a in ly s u lp h id e s , w h ile th e p a le h o rn fe ls is m o re tu rb id
in a p p e a ra n c e .
T h e m o s t n o t ic e a b le fe a tu re s o f th e m o re m assiv e ,
p a le c o lo u re d h o rn fe ls a re d a r k g rey sp o ts a n d c lo ts ,
w h ich a re lo c a lly a b u n d a n t . T h e se ra n g e f ro m s im p le
c i rc u la r o r o v a l sh a p e s u p to 2 cm a c ro ss , o c c u r r ing
e i th e r s ing ly o r as b e a d e d g ro u p s (F ig . 5) to la rg er
c o a rse -g ra in e d p a tc h e s 10 to 12 cm w id e . T h e m a rg ins
o f th e se b le b s a n d p a tc h e s a re a lw ay s c u rv e d a n d
g e n e ra lly d is t in c t , b u t in so m e v a r ie tie s , m o re d iffu se
m a rg in s g ive th e ro c k a n o v e ra ll m o t t le d a p p e a ra n c e.
O f te n th e c lo ts re se m b le sp h e ru le s , w ith d is t in c t rim a n d
c o re se c tio n s in h a n d sp e c im en . O n w e a th e re d su r fa ce s ,
th e c lo ts s ta n d o u t as sm a ll n o d u le s . T h e m a in m in era l
fo rm in g th e c lo ts is g ro ssu la r , w ith m in o r w o l la s ton ite ,
d io p s id e a n d sp h e n e . In c lo ts w ith a d is t in c t r im an d
c o re , th e r im s a re r ic h e r in g ro s s u la r (9 5 % ) th a n th e c o re
(8 0 % ).
F in e ly la m in a te d ro c k ty p e s a re o c c a s io n a lly
d e v e lo p e d . In th e se a n d in th e m o tt le d ro c k ty p e s ,
c o a rs e r p a tc h e s , o f th e o rd e r o f sev era l m m a c ro s s , a n d
s h o r t v e in le ts , a re o f te n d e v e lo p e d . T h e se g e n e ra lly c o n
ta in assem b lag e s in v o lv in g w o l la s to n ite , g ro ssu la r , d io p
s id e , p re h n i te , c a lc ite a n d q u a r tz .
M IN E R A L O G Y
T h e m in e ra lo g y o f th e ca lc -s i l ic a te ro c k s is re la tiv e ly
re s tr ic te d , re f lec tin g th e sm a ll size o f th e o u tc rop , th e
u n i fo rm ity o f f o rm a t io n c o n d i t io n s a n d , to a n e x te nt,
th e s im ila r ity o f th e o r ig in a l s e d im e n ta ry c o m p o n e nts .
G r o s s u l a r : A s th e m a jo r c o n s t i tu e n t o f th e sp h e ru li t ic -
l ik e c lo ts , th e g ro s s u la r a p p e a rs m id to d a r k g rey , o c c a
s io n a lly w ith b ro w n is h o r p in k is h o v e r to n e s , t ra n s lu c e n t
to o p a q u e a n d w ith a g rea sy lu s tre . W h e re in d iv id u al
g ra in s c a n b e d is t in g u ish e d in th in se c t io n , th e g ro ssu la r
is n o t is o t ro p ic , b u t sh o w s lo w b ire f r in g e n c e . In so m e o f
th e c o a rs e r p a tc h e s a n d in th e m o tt le d ro c k ty p e s , in
d iv id u a l d o d e c a h e d ra l o u t l in e s c a n b e d is t in g u ish e d, a n d
th e g ro s s u la r ex h ib its s tro n g tw in n in g e ffec ts , s imi la r to
th e s ix lin g tw in n in g o b se rv e d in so m e o c c u r re n c e s of
c o rd ie r i te (F ig . 6 ). T h is s tro n g s im ila r i ty w as p rob a b ly
re sp o n s ib le fo r th e o r ig in a l d e s c r ip t io n o f th e B lue
F ig . 1 —G e n e ra l g e o lo g ic a l a n d lo c a l i ty m a p o f th e H e a le sv i l le -T o o la n g i a re a . T h e c a lc -s i l ic a te h o rn fels
o c c u rs o n a n d in th e v ic in i ty o f B lu e M o u n t .
| | S ed im en ts (Q ua t.)
B la ck Range
Grarvod io rite (L a te Dev.)
D yke Rocks
(D ev .)
r r n Donna Buang R hyodac ite
LLL1 (L a te Dev.)
R ub ic on Rh yo lite
(L a te Dev.)
S ca le km
0 1 2 3
E
Taggerty Subgroup
(L a te Dev, Volcan ics)
Contact M e tam o rph ic
Rocks
H um eva le Form ation
(E a rly Dev.)
D a rg ile Form ation
(L a te S il.)
F au lt
o o
C A L C -S I L I C A T E R O C K S
W . D . B IR C H
F ig . 2 —R epla c e m e n t te x tu re in c a lc -s i l ic a te h o rn fe ls . A l te ra tio n o f o r ig in a l ly d a rk h o rn fe ls h a s p ro c e e d e d
v ia a n e tw o rk o f f ra c tu re s (N M V sp e c im e n E 1 0 5 4 9 A , 8.0 c m lo n g ).
F ig . 3 — P e rv a s iv e a l te ra t io n te x tu re in c a lc -s i l ic ate h o rn fe ls (N M V sp e c im e n E 1 0 5 4 9 C , 5 .5 c m lo n g ).
C A L C -S I L I C A T E R O C K S 5
F ig . 4 —S p e c im e n sh o w in g re m n a n t la y e rs (o r ig in a l b ed d in g , su b se q u e n t ly fa u lte d ) o f u n a l te re d d a rk
h o rn fe ls in p a le a l te ra t io n p ro d u c t . S p e c im e n is 8 .5 cm a c ro ss .
F ig . 5 —G ro s s u la r b le b s in p a le h o rn fe ls . T h e la rg es t c lo t is 0 .5 c m a c ro ss (N M V sp e c im e n E 10 2 80 ).
6 W . D . B IR C H
F ig . 6 —P h o to m ic ro g ra p h sh o w in g la rg e n o n - is o t ro p ic g ro s s u la r c ry s ta l (1 m m a c ro ss ) in f in e -g ra in e d
c a lc -s i l ic a te h o rn fe ls (c a lc i te , d io p s id e , q u a r tz ) (N M V sp e c im e n E 1 0 2 8 3 ).
M o u n t h o rn fe ls as sh o w in g lo c a l d e v e lo p m e n t o f co r-
d ie r i te b le b s (K en ley 1958). A n is o tro p is m a n d tw in nin g
in g a rn e ts o f th e g ro ssu la r i te g ro u p is w ell d o c u m en te d
(D e e r e t al. 1962). L ig h t g rey t ra n s lu c e n t g ro ssu la r
c ry s ta ls u p to 5 m m a c ro ss a re o c c a s io n a l ly fo u n d lin in g
c a v ities o n w e a th e re d o u tc ro p su r fa c e s . M ic ro p ro b e
an a ly s is sh o w s th e g ro s s u la r to b e q u i te u n i f o rm in c o m
p o s i t io n . P y ro p e a n d sp e ssa r t i te c o m p o n e n ts a re
m in im a l a n d a n d ra d i te is th e m a in s u b s t i tu t io n a l e nd
m e m b e r (T a b le 1). H o w e v e r , o v e ra l l i ro n c o n te n ts a re
lo w ( G r :A d > 8 5 :1 5 ) a n d a re c h a ra c te r is t ic o f g a rn e t
c o m p o s it io n s in c o n ta c t m e ta m o rp h ic ro c k s r a th e r tha n
s k a rn s p ro d u c e d b y m a g m a tic a l ly -d e r iv e d f lu id s (E ina r -
d i & B u r t 1982).
W o l l a s t o n i t e : W o l la s to n i te is d iss e m in a te d th ro u g h
m u c h o f th e p a le h o rn fe ls a n d is a m in o r c o n s t i tu e nt o f
th e g ro ssu la r sp h e ru le s . W h e n v is ib le u n d e r th e
m ic ro sc o p e , in so m e o f th e c o a rs e - te x tu re d ro c k s , an d in
p a tc h e s a n d v e in le ts in f in e -g ra in e d ro c k s , it fo rms
f ib ro u s a g g re g a te s u su a lly a s so c ia te d w ith g ro s s u lar a n d
d io p s id e (F ig . 7 ). P a tc h e s o f w h ite , c o a rse ly -c ry s ta l l in e
f ib ro u s w o l la s to n ite , u p to 6 c m a c ro ss , w ith in d ivid u a l
c ry s ta ls u p to 2 cm lo n g , m a y o c c u r in th e p a le g rey
h o rn fe ls (F ig . 8). M ic ro p ro b e an a ly se s rev e a l o n ly sm a ll
a m o u n ts o f F e a n d M n (T a b le 1).
D i o p s i d e : D io p s id e is p re s e n t in a ll ro c k ty p e s , as c lo u d s
o f t in y , e u h e d ra l , e q u id im e n s io n a l c ry s ta ls in c lu d ed in
m o s t o f th e o th e r m in e ra ls . L a rg e r d io p s id e c ry s ta ls o c
c u r in p a tc h e s o f g ro s s u la r a n d w o l la s to n ite in th e
c o a rs e r - te x tu re d ro c k s . T h in v e in le ts c o n s is t in g o f d io p
s id e a n d q u a r tz o c c u r s u r ro u n d e d b y n a r ro w b le a c h e d
z o n e s in so m e o f th e h o rn fe ls (F ig . 9). M ic ro p ro b e
T a b l e 1
E l e c t r o n M i c r o p r o b e A n a l y s e s o f G r o s s u l a r a n d
W o l l a s t o n i t e i n t h e B l u e M o u n t C a l c - S i l i c a t e R o c k s .
W t% 1 2 3
S i 0 2 3 8 .6 4 38 .8 5 52.21
T i 0 2 0 .8 0 0 .3 6 0 .1 2
A I 2 O 3 2 0 .9 9 18.48 0 .0 8
F e 20 3 2 .3 0 6 .8 8 —
F e O — — 0 .4 3
M n O 0 .1 4 0 .0 4 0 .0 9
M g O 0 .0 3 0 .0 2 -
C a O 3 7 .5 0 3 6 .5 9 4 7 .6 8
N a 20 — 0 .05 —
k 2o — — —
B aO 0 .0 8 0 .1 4 —
T o ta l 100 .4 8 101.41
S tru c tu ra l F o rm u la e
(16 c a ts , 24 ‘O ’) (4
100.61
c a ts , 6 ‘O ’)
Si 5 .84 8 5 .8 9 9 2 .0 0 4
T i 0.09 1 0.041 0 .00 3
A l 3 .74 5 3 .3 0 8 0 .0 0 4
F e 3* 0 .2 6 2 0 .7 8 6 —
F e 2* — — 0 .0 1 4
M n 0 .0 1 8 0 .00 5 0 .0 0 3
M g 0 .0 0 7 0 .0 0 5 -
C a 6 .0 8 2 5 .9 5 3 1.961
N a — — —
K — — —
B a 0 .00 5 0 .0 0 8 -
1, G ro s s u la r (E 1 0 2 8 3 ); 2 , G ro s s u la r (E 1 0 2 8 0 ); 3, W ol la s to n i te
( E l 02 8 0 ).
C A L C -S I L I C A T E R O C K S 7
I
i
f
F ig . 7 —F ib ro u s w o l la s to n i te (w o ) a sso c ia te d w ith gro s s u la r (g ) a n d sm a ll d io p s id e c ry s ta ls (N M V
sp e c im e n E 1 0 2 8 2 ). S c a le b a r = 0 .5 m m .
F ig . 8 —P a tc h o f c o a rse w o l la s to n i te 4 .5 c m a c ro ss in f in e -g ra in e d c a lc -s i l ic a te h o rn fe ls (N M V sp e c im en
M 3 48 1 2 ).
8 W . D . B IR C H
T a b l e 2
E l e c t r o n M i c r o p r o b e A n a l y s e s o f D i o p s i d e a n d S p h e n e i n
t h e B l u e M o u n t C a l c - S i l i c a t e R o c k s .
W t% 1 2 3
S i 0 2 5 1 .9 6 5 1 .8 9 3 0 .9 9T i 0 2 0 .0 8 0 .1 0 3 6 .9 0a i 2o 3 0 .1 3 0 .4 4 3 .0 9F e 20 3 0 .8 7 _ _
F e O 11.51 15 .3 4 0 .1 3M n O 0 .7 3 0 .2 0 _
M g O 9 .8 6 8 .0 0 0 .0 5C a O 2 4 .8 6 2 4 .2 4 2 8 .3 5N a 20 0 .1 0 0 .11 _
C r 20 3 0 .0 7 0.11 _
H 20 c a lc . — _ 1.18T o ta l 100 .17 100 .4 3 100 .6 9
S tru c tu ra l F o rm u la e
(12 c a ts , 20
(4 c a t io n s , 6 o x y g e n s) ‘O ’, l [ O H ,F ] )
S i iv 1.985 1.998 3 .93 7
A lvi 0 .0 0 6 0 .0 0 2 0.0 63A1 — 0 .0 1 8 0 .4 0 0T i 0 .0 0 2 0.0 03 3.5 25F e 3+ 0 .02 5 _ _
F e 2+ 0 .3 6 8 0 .4 9 4 0 .0 1 4M n 0 .0 2 4 0 .0 0 7 _
M g 0 .5 6 2 0 .4 5 9 0 .0 0 9C a 1.018 1 . 0 0 0 3 .8 5 9N a 0 .0 0 7 0 .0 0 8C r 0 .0 0 2 0 .0 0 3O H — - 1 .0 0 0
1, D io p s id e in q u a r tz -d io p s id e v e in le t (E 1 0 5 4 7 ); 2 , D io p s id e
c ry s ta l (E 1 0 2 8 2 ); 3, S p h e n e c ry s ta l (E 1 0 2 8 1 ).
an a ly s is rev e a ls a re la tiv e ly n a r ro w c o m p o s i t io n a l ra n g e ,
w ith M g / ( F e + M g) v a ry in g b e tw e e n 0 .6 6 a n d 0 .4 8 (T a b le 2).
K f e l d s p a r : K fe ld sp a r is th e m a in m in e ra l in m a n y o f
th e p a le g rey p o rc e l la in o u s ro c k ty p e s. H o w e v e r , d ue to
i ts v a r ia b le a b u n d a n c e a n d e x tre m e ly sm a ll g ra in size
( < 0 .0 5 m m ) p re c lu d in g o p t ic a l d e te c t io n , X - ra y p o wd e r
d i f f ra c t io n is th e s im p les t m e th o d o f d e te rm in in g bo th
its re la t iv e a b u n d a n c e a n d c o m p o s i t io n . T h e la t te r c a n
o n ly b e d e te rm in e d w h e re q u a r tz is v ir tu a l ly a b s e n t, as
th e 201 re f lec tio n fo r K fe ld s p a r is o v e r la p p e d b y th e
p ro p o r t io n a l ly m u c h s tro n g e r 101 re f lec tio n fo r q u ar tz .
K fe ld s p a r c o m p o s i t io n s in sp e c im en s o f th e p a le -g re y
h o rn fe ls w e re e s t im a te d to ra n g e f ro m O r 85A b i5-
O r 92A b 8, u s in g th e 201 m e th o d o f W r ig h t (1968).
M ic ro p ro b e an a ly s is w as l im ite d b y th e sm a ll g ra in size,
b u t an a ly se s c o n f irm e d th e c o m p o s i t io n e s t im a te d f ro m
th e X - ra y d i f f ra c t io n d a ta . S m a ll a m o u n ts o f b o th Ba
a n d C a w ere d e te c te d (T a b le 3). A t te m p ts to d e te rm in e
K fe ld s p a r c o m p o s i t io n s b y X -ra y d i f f ra c t io n in co ex
is t in g p a le a n d d a rk re g io n s w ere h a m p e re d b y a n a bu n
d a n c e o f q u a r tz in e i th e r o n e o r b o th p o r t io n s .
M ic ro p ro b e an a ly se s h o w e v e r in d ic a te th a t a lk a li
fe ld sp a rs in th e d a rk f ra c t io n a re v a r ia b le in c o m po s i
t io n , a n d a re less p o ta ss ic a n d m o re ca lc ic th a n tho s e in
th e p a le re g io n s (T a b le 3).
T h e s t ru c tu ra l s ta te s o f fe ld sp a rs w e re d e te rm in e d
u s in g th e th re e -p e a k m e th o d o f W r ig h t (1968). T h e 060
a n d 204 re f lec tio n s fo r K fe ld s p a r a re n o t o b s c u re d by
q u a r tz p e a k s a n d th e ir p o s i t io n s w e re m e a s u re d w h e n
ev e r K fe ld s p a r w as su ff ic ien tly a b u n d a n t . U s in g a q u a n
ti fy in g m e th o d d e v e lo p e d b y R a g la n d (1970 ), K
fe ld sp a rs in th e p a le ro c k ty p e s w ere sh o w n to h av e
s t ru c tu ra l s ta te s v e ry c lo se to th a t o f o r th o c la s e , b u t
F ig . 9 —T h in d io p s id e -q u a r tz v e in le t s u r ro u n d e d b y an a l te ra t io n z o n e 2 .5 m m w id e , in d a rk h o rn fe ls
(N M V sp e c im e n E 1 0 5 4 7 ).
C A L C -S I L I C A T E R O C K S 9
T a b l e 3
E l e c t r o n M i c r o p r o b e A n a l y s e s o f F e l d s p a r s i n t h e B l u e M o u n t C a l c - s i l i c a t e R o c k s
W t% 1 2 3 4 5
S i 0 2 6 5 .3 2 6 4 .3 6 6 4 .9 6 6 4 .5 9 5 7 .0 0
a i 2o 3 18.93 19 .1 4 19 .9 4 19 .6 0 24.31
F e20 3 0 .0 8 0 .2 9 0 .2 4 0 .1 9 0 .4 6
M g O
C a O
—0 .0 6
0 .0 5
1.76
0 .0 7
3 .2 3 10.76
N a 20 0 .9 4 0 .6 2 6 .2 7 3 .1 9 2 .95
k 2o 14 .6 4 14.87 5 .65 8 .8 0 2 .3 8
B a O 0 .9 6 0 .7 0 0 .2 6 0 .2 4 0 .2 5
T o ta l 100 :87 100 .0 4 99 .1 3 99 .91 98.11
E n d m e m b e r C o m p o s i t io n s (m o l .% )14 .9
O r 89 .5 9 2 .5 33 .8 5 3 .6
A b 8 .7 5 .9 5 6 .9 2 9 .5 28.1
A n _ 0 .3 8 .8 16.5 56 .5
C e l 1.8 1.3 0 .5 0 .4 0 .5
1, K f e ld s p a r in p a le , f in e g ra in e d ro c k (E 1 0 2 8 0 ); 2, K fe ld s p a r in p a le f r a c t io n (E 1 0 5 4 4 ); 3, A lk a l i fe ld s p a r
in d a rk f r a c t io n (E 1 0 5 4 4 ); 4 , A lk a l i f e ld s p a r in d ark f r a c t io n (E 1 0 5 4 4 ); 5, P la g io c la s e in d a rk f r a c t io n
(E 1 0 5 4 7 ).
w ith b e tw e e n 5 -1 0 % o f o rd e r in g to w a rd s m a x im u m
m ic ro c lin e . K fe ld sp a rs in th e d a rk f ra c t io n w ere alw ay s
s l ig h tly m o re o rd e re d , w ith a b o u t 2 0 -2 5 % o rd e r in g
to w a rd s m a x im u m m ic ro c lin e . T h e se d a ta a re sh o w n in
T a b le 4 a n d F ig . 10. I t c a n th e re fo re b e c o n c lu d e d th a t
th e p ro c e ss w h ic h p a r t ia l ly o r c o m p le te ly a l te re d th e
d a rk h o rn fe ls , h a s re su lte d in th e d e v e lo p m e n t o f a m o re
d is o rd e re d , m o re p o ta ss ic fe ld sp a r in th e p a le a l tera t io n
zo n e s .
P l a g i o c l a s e : X -ra y d i f f ra c t io n rev e a le d th a t p la g io c lase
is p re s e n t in m o s t o f th e f in e -g ra in e d ro c k ty p e s , b u t a s a
m in o r m in e ra l o n ly . M ic ro p ro b e an a ly se s su g g e st it is
m a in ly la b ra d o r i te (T a b le 3).
Q u a r t z : T h e a b u n d a n c e o f q u a r tz v a r ie s , re a c h in g a
m a x im u m in so m e o f th e d a r k h o rn fe ls . I t is a b s e n t f ro m
th e sp h e ru le -b e a r in g p o rc e l la in o u s v a r ie ty . In th e f ine-
T a b l e 4
K F e l d s p a r S t r u c t u r a l S t a t e E x p r e s s e d a s P e r c e n t a g e
O r d e r i n g T o w a r d s M a x i m u m M i c r o c l i n e f r o m O r t h o c l a s e
S a m p le D a rk L ig h t C o m p , (e s t.)
E 1 0 71 3 _ 9 —
E 1 0 2 8 0 — 6 O r 8BA b i 2
E 1 0 5 47 21 — —
E 1 0 5 4 9 C 24 6 O r 94A b 6(l ig h t)
E 1 05 4 9B 20 7 O r 9SA b 5(lig h t)
E 1 0 5 44 20 11 O r 95A b5( l ig h t)
E 1 0 5 4 9 A 42 27 —
N o t e s o n M e t h o d : T w o s m e a r m o u n ts o f e a c h sa m p le w e re
r u n in d u p l ic a te u s in g N i- f i l te re d C u K „ ra d ia t io n , w ith a c h a r t
sp e e d o f 2 c m /m in u te a n d a s c a n n in g ra te o f 0 .2 5 ° /min u te .
K B r 0 3 w a s u se d a s a n in te rn a l s ta n d a r d f o r th e th ree p e a k
p o s i t io n s m e a su re d .
g ra in e d h o rn fe ls it fo rm s sm a ll a n h e d ra l g ra in s , p oss ib ly
re p re se n tin g o r ig in a l re c ry s ta l l ize d d e t r i ta l f ra g me n ts .
P a tc h e s o f in te rs t i t ia l q u a r tz a re a s so c ia te d w ith la rg e r
g ra in s o f g ro ssu la r , w o l la s to n ite , d io p s id e , v e su v ia n ite ,
p re h n i te a n d c a lc ite in th e c o a rs e r - te x tu re d ro c k s . S m all
q u a r tz -d io p s id e v e in le ts a re ev id en c e f o r m o b i l iz a tio n o f
Si d u r in g m e ta m o rp h is m (F ig . 9).
C a l c i t e : C a lc i te is m o s t p ro m in e n t in th e c o a rse r-
te x tu re d ro c k s , w h e re i t fo rm s in te rs t i t ia l p a tc h e s. I t
g e n e ra lly en c lo ses sm a ll d io p s id e in c lu s io n s a n d m ay
co e x is t w ith n e a r ly a ll o th e r m in e ra ls , in c lu d in g qu a r tz .
I t is a b s e n t o r a m in o r p h a s e o n ly in m a n y o f th e fin e
g ra in e d h o rn fe ls .
V e s u v i a n i t e : V esu v ian ite o c c u rs in se v e ra l ro c k ty p es,
fo rm in g e i th e r b lo c k y c ry s ta ls re se m b lin g p h e n o c ry sts o r
p a le b ro w n z o n e d p r is m a t ic c ry s ta ls (F ig . 11). In bo th
cases, th e m a x im u m c ry s ta l le n g th is a b o u t 1.5 m m .
M ic ro p ro b e an a ly se s (T a b le 5) o n th e b lo c k y c ry s ta ls
a n d th e co re s o f th e p a le b ro w n p r ism s a re s im ila r.
F e /M g ra t io s a re v a r ia b le a n d th e v e s u v ia n i te c o n tain s
a b o u t 1 .4 % T i 0 2 a n d b e tw e e n 1 a n d 2 % F . T h e r im s o f
th e p r ism a tic c ry s ta ls h a v e a n u n u s u a l c o m p o s i t io n
(T a b le 5) w ith m o re s tru c tu ra l w a te r , a n d d o n o t
re c a lc u la te to a v e su v ia n ite s tru c tu ra l f o rm u la . T he
id e n t i ty o f th is p h a s e is u n k n o w n .
P r e h n i t e : P re h n i te is w id e s p re a d in c o a rse r -g ra in e d
ro c k ty p es as ir re g u la r g ra in s sh o w in g b r ig h t in
te r fe re n c e c o lo u rs . I t a p p e a rs to b e a n in te rs t i t ial , la te -
c ry s ta l l iz in g m in e ra l. A ty p ic a l m ic ro p ro b e an a ly s is is
sh o w n in T a b le 6.
S p h e n e : S m all c ry s ta ls o f sp h e n e o c c u r in th e r im s o f th e
g ro s s u la r b le b s , in th e m a tr ix o f th e f in e -g ra in e d h o r n
fe ls a n d in th e c o a rs e r - te x tu re d ro c k ty p e s. T h e very
sm a ll c ry s ta l size (a b o u t 0 .03 m m ) m a k e s th e d e te rmin a -
2 9(060)
10 W . D . B IR C H
2 0 ( 2 0 4 )
F ig . 1 0 -2 9 1 0 4 -2 0 0 6 0 p lo t f o r K fe ld s p a rs in sp e c im en s o f th e f in e -g ra in e d c a lc -s i l ic a te ro c k s (see a lso
T a b le 4 a n d te x t f o r d isc u ss io n ) . F e ld sp a rs c o e x is tin g in d a rk ( fu l l c irc le s) a n d p a le h o rn fe ls (o p e n c irc les)
a re jo in e d b y tie - l in e s.
C A L C -S I L I C A T E R O C K S 11
T a b l e 5
E l e c t r o n M i c r o p r o b e A n a l y s e s o f V e s u v i a n i t e i n t h e B l u e
M o u n t C a l c - s i l i c a t e R o c k s
W t% 1 2 3
S i 0 2 3 5 .9 0 35 .5 8 38 .5 7
T iO z 1.55 1.40 1.81
A120 3 1 5 .1 9 16.23 14.09
F e O 3.51 4 .1 4 2 .94
M n O — 0 .0 6 0 .1 0
M g O 2.61 1.33 0 .7 2
C a O 3 5 .8 0 36 .0 7 25 .3 8
N a 20 0 .0 4 0 .0 9 0 .05
B a O — — 0 .7 9
F 0 .8 2 1.08 0 .1 6
C l _ 0.61 0 .1 6
* h 2o 2 .05 1.77 0 .1 0
- 0 = F ,C 1 0 .3 4 0 .5 9 0 .1 0
T o ta l 9 7 .1 3 9 7 .7 7
S tru c tu ra l F o rm u la e
(25 c a t io n s , 38 o x y g e n s , 4 O H )
84 .8 7
Si 8 .8 4 0 8 .7 63 -
A l 4 .4 0 9 4 .7 1 0 -
T i 0 .2 8 7 0 .2 5 9 —
F e 2* 0 .7 2 2 0 .8 5 3 -
M n — 0 .0 1 2 -
M g 0 .9 5 8 0 .4 8 7 -
C a 9 .4 4 4 9 .51 7 —
N a 0 .0 1 8 0 .0 4 5 -
B a — — —
F 0 .6 3 7 0.841 —
C l — 0 .2 5 4 —
O H 3 .3 6 3 2 .9 0 5 —
1, V e su v ia n ite c ry s ta l (E 1 0 5 4 5 ); 2 , C o re o f p a le b ro w n c ry s ta l
( E l 0 28 2 ); 3 , U n k n o w n p h a se r im m in g 2.
* C a lc u la te d
t io n o f p h y s ic a l p ro p e r t ie s d if f icu lt. A m ic ro p ro b e
a n a ly s is is sh o w n in T a b le 2.
S c o l e c i t e : T h is c a lc iu m -r ic h ze o lite is ra re , w ith o n ly
tw o sm a ll c ry s ta ls d e te c te d b y m ic ro p ro b e (T a b le 6).
O t h e r M i n e r a l s : V ery ra re g ra in s o f y t t r iu m p h o s p h a te
w ere e n c o u n te re d d u r in g m ic ro p ro b e w o rk . T h ey a re
p ro b a b ly x e n o t im e a n d m a y b e d e tr i ta l in o r ig in . S ev era l
m in u te g ra in s o f c h ro m i te m a y h av e a s im ila r o r ig in.
P y r rh o t i te a n d a rs e n o p y r i te a re th e m o s t a b u n d a n t of
th e o p a q u e m in e ra ls p re s e n t, o cc a s io n a lly fo rm in g pa t
ches u p to 2 m m ac ro ss , b u t u su a lly m ic ro sc o p ic .
G a le n a a n d sp h a le r i te h av e b e e n o b se rv e d as sm a ll bleb s
u p to 2 m m ac ro ss a n d g a le n a o c c a s io n a lly fo rm s ragg ed
g ra in s in th e c o re o f sp o ts .
M IN E R A L E Q U IL IB R IA
D a r k H o r n f e l s w i t h P a l e A l t e r a t i o n : X -ray d if frac
t io n p a tte rn s w ere u sed to d e te rm in e th e m in e ra lo g y o f
five sam p le s o f f in e-g ra in ed h o rn fe ls in w h ich d a rk a n d
pa le f ra c tio n s co ex is ted . T h e resu lts a re su m m ar ized in
T a b le 7. In th re e sam p les (E10544, E 10549B & Q th e
q u a r tz c o n te n t in th e in it ia l assem b lag e (q u a r tz , dio p s id e ,
K fe ld sp a r , p lag io c lase , m in o r g ro ssu la r) w as red u ced by
th e a lte ra t io n . T h e o n ly sam p le c o n ta in in g d e tec table
ca lc ite ( E l 0549A ) a p p e a re d la rge ly una f fec ted .T h e re is n o ev id en c e re m a in in g fo r th e re a c t io n s p ro
d u c in g K fe ld sp a r a n d d io p s id e in th e d a rk h o rn fe ls, as
th e re a c ta n ts w ere c o n s u m e d d u r in g p r o g ra d e m e ta m o r
p h ism . H o w e v e r , th e m o s t c o m m o n ly o b se rv e d re a c
t io n s in s il iceo u s c a rb o n a te ro c k s c o n ta in in g sm all
a m o u n ts o f, M g a n d F e (d e f in ed a p p ro x im a te ly b y th e
sy stem C a 0 - M g 0 - S i0 2- H 20 - C 0 2, e .g . K e rr ick 1974)
a re :1. d o lo m i te + q u a r t z —d io p s id e + 2 C 0 2 (R ice 1977)
2. t re m o li te + 3 c a lc i te + 2 q u a r tz — 5 d io p s id e + 3 C 02
+ h 2o
3. m u sc o v ite + 6 q u a r t z + 2 K +— 3 K fe ld sp a r + 2 H 20
+ 2 H +In th e d a rk h o rn fe ls , th e f o rm a t io n o f sm a ll a m o u n ts
o f g ro s s u la r w as p ro b a b ly c o n tro l le d b y th e eq u i l ibr iu m :
4 . 2 ca lc ite + a n o r th i te + q u a r tz —g ro ssu la r + 2 C 0 2 (G o r
d o n & G re e n w o o d 1971).
T a b l e 6
E l e c t r o n M i c r o p r o b e A n a l y s e s o f P r e h n i t e a n d S c o l e c i t e
i n t h e B l u e M o u n t C a l c - S i l i c a t e R o c k s
Wt«7o 1 2
S i 0 2 4 4 .3 6 5 9 .6 4
T i 0 2 0 .03 —
A I 2 O 3 2 3 .6 7 17.23
F e O 0 .2 3 0 .07
C a O 2 6 .8 4 8 .7 7
N a 20 0 .0 6 0 .4 6
k 2o — 0 .2 8
B aO 0 .1 4 0 .0 6
T o ta l 95 .3 3 86.51
S tru c tu ra l F o rm u la e
(22 ‘O ’) (72 ‘O ’)
Si 6 .1 1 0 2 6 .855
A l 3 .8 4 3 9 .1 4 7
T i 0 .0 0 3 —
Fe 0 .0 2 6 0 .0 2 7
C a 3.961 4 .2 3 2
N a 0 .0 0 8 0 .4 0 0
K — 0 .1 6 2
B a 0 .0 0 7 0.011
1, P re h n i te (N M V E 1 0 2 8 2 ); 2, S c o lec ite (N M V E 1 0 2 8 2 ).
T h e a m o u n t o f g ro ssu la r fo rm e d w o u ld be d e te rm in
e d b y th e a m o u n t o f c a lc ite p re se n t in it ia l ly , as th is a p
p e a rs to h a v e b e e n th e le as t a b u n d a n t re a c ta n t in th e
h o rn fe ls . T h e m o s t o b v io u s c h a n g e in th e f in e -g ra ine d h o rn fe ls , th a t o f c o lo u r , is d u e p r im a r i ly to th e ox id a
t io n o f su lp h id e s , a n d p o ss ib ly c a rb o n a c e o u s m a tte r.
12 W . D . B IR C H
F ig . 11 - P r is m a t ic v e su v ia n i te c ry s ta ls (v ) in c a lc i te p a tc h e s a ss o c ia te d w i th g ro s s u la r (g ) a n d d io ps id e (d )
(N M V sp e c im e n E 1 0 2 8 2 ). S c a le b a r = 0 .5 m m .
C o a r s e - g r a i n e d R o c k s a n d P a t c h e s : T h e m in e ra lo g y
o f c o a rs e - te x tu re d ro c k s , a n d c o a rse ly -c ry s ta l l iz e d p a t
ch es in so m e o f th e f in e -g ra in e d ro c k s , is re la tiv ely
u n i fo rm . T h e fo l lo w in g asse m b la g e s h a v e b e e n o b se rve d in th in se c tio n :
g ro s s u la r + c a lc ite + q u a r tz + p re h n i te
g ro s s u la r + w o lla s to n ite
d io p s id e + sp h e n e
w o l la s to n ite + q u a r tz + ca lc ite
v e su v ia n ite + q u a r tz + g ro s s u la r
v e su v ia n i te + ca lc ite
ca lc ite + p re h n i te + w o l la s to n ite + g ro s s u la r + sc o lec ite + su lp h id e s
In so m e cases it is d if f icu lt to d e c id e e x a c t ly w h at is
a n “ a s se m b la g e ” as te x tu ra l r e la t io n s h ip s a re n o t alw ay s
c le a r. T h u s , n e a r ly a ll p h a s e s in th e a b o v e assem b lag e s
c o n ta in c lo u d s o f m in u te d io p s id e c ry s ta ls , b u t th ese a re
c o n s id e re d to b e e a r ly - fo rm e d a n d n o t n ec essa r i ly pa r t
o f a n y e q u i l ib r ia c lo se to th e p e a k o f m e ta m o rp h is m. K
fe ld s p a r a n d p la g io c la se (ss) co e x is t w ith th e se asse m b la g e s in th e en c lo s in g m a tr ix .
A s w ell as re a c t io n 4 , th e re a re a n u m b e r o f o th e r
e q u i l ib r ia in v o lv in g th e o b se rv e d p h a s e s a n d w h ic h m a y
h a v e b e e n re s p o n s ib le fo r th e s e a ssem b lag e s. In a p
p ro x im a te o r d e r o f in c re a s in g te m p e ra tu re , th e se a re :
5. v e su v ia n i te + 4 q u a r t z — 5 g r o s s u la r + 3 d io p s id e +
w o l la s to n ite + 4 H 20 (H o c h e l la e t al. 1982)
6. p r e h n i t e — a n o r t h i t e + w o l la s to n i t e + H 20 ( L io u 1971)
7. c a lc ite + q u a r t z + r u t i le — s p h e n e + C 0 2 (J a c o b s & K e r r ic k 1981)
8. c a lc i te + q u a r t z — w o l la s to n i te + C 0 2 (G re e n w o o d 1967)
9. g ro ssu la r + q u a r tz —2 w o lla s to n ite + a n o r th i te (New to n 1966)
10. c a lc i te + a n o r th i te + w o l la s to n i te —g ro s s u la r + C 0 2 (G o rd o n & G re e n w o o d 1971)
T a b l e 7
M i n e r a l o g y o f F i n e - G r a i n e d H o r n f e l s w i t h D a r k a n d P a l e F r a c t i o n s
S a m p le D a rk
E l 0547 Q + D i + P + K
E l 0 5 4 9A C c + K + D i + P + Q + G r ( t r )
E 1 0 5 4 9 B Q + K + P + D i + G r
E 1 0 5 4 9 C K + D i + P + Q + G r
E l 0 5 44 Q + K + D i + P -t- G r ( t r )
A sse m b la g e s (m a in p h a se s ) L ig h t
Q + D i + P + K (tr )
K + C c + D i + P + Q + G r ( t r )
K + D i + P + Q + G r
K + D i + P + G r
K + D i + Q + P + G r ( t r )
M a in c h a n g e
K re d u c e d
L a rg e ly u n a f fe c te d
Q re d u c e d
Q re m o v e d
Q re d u c e d
Q , Q u a r tz ; D i, D io p s id e ; P , P la g io c la s e ; K , K fe ld sp a r ; G r , G ro s s u la r ; C c , C a lc i te .
C A L C -S I L I C A T E R O C K S13
F ig . 12 —T -X C02 p lo t o f th e v a r io u s e q u i l ib r ia c o n tro l l in g th e m in e ra lo g y o f th e B lu e M o u n t c a lc -s i l ica te
ro c k s . R e a c t io n s a re n u m b e re d a s th e y a p p e a r in th e te x t . T h e se c tio n a s sh o w n is n o t i so b a r ic . E q u i l ib r ia
l a , 4 , 5 , 7 , 8 , 9 a n d 10 a re a t 2 K b ; e q u i l ib r ia 1, 2 a n d 8 a a t 1K b a n d e q u i l ib r ia 11 a n d 8b a t 0 .5 K b. P o s i
t io n s o f th e re a c t io n s 8 a n d 8 a a re f r o m J a c o b s a n d K e rr ic k (1981 ) a n d o f 8 b f r o m W il l ia m s -Jo n e s (19 8 1).
T h e y a re p lo t te d to i l lu s t ra te th e m a rk e d e ffec t o f p re ssu re o n th e c a lc i te + q u a r tz — w o l la s to n i te + C O2
e q u i l ib r iu m . T h e sh a d e d a r e a re p re se n ts th e re g io n in w h ic h th e m a in p h a se s c ry s ta l l ise d .
11. 5 g r o s s u la r + 4 d io p s id e + 6 c a lc ite + 4 H 2 O — 6 w o l
la s to n i te + v e s u v ia n i te + 6 C 0 2 (W i l l ia m s -J o n e s
1981)12. 2 g ro s s u la r + 2 d io p s id e + w o l la s to n ite + c a lc ite + 2
H 20 —v e su v ia n ite + 2 q u a r t z + C O2 ( I to & A re m
1970).E q u i l ib r iu m cu rv e s fo r m o s t o f th e se re a c t io n s are
re a s o n a b ly w ell k n o w n , e i th e r f ro m th e rm o d y n a m ic
c a lc u la t io n s o r e x p e r im e n ta l d a ta . T h e y a re sh o w n in th e
T -X co p lo t o f F ig . 12. O n th is d ia g ra m , p re ssu re s fo r
e a c h e q u i l ib r iu m v a ry f ro m 0 .5 K b ( re a c t io n 11),
th ro u g h 1 K b ( re a c t io n s 1 & 2) to 2 K b ( re a c t io n s 4, 7 , 8,
9 , 10). T h e in v a r ia n t p o in t f o rm e d b y th e in te rse c tio n o f
re a c t io n s 4 , 8, 9 a n d 10 a n d m a rk e d b y th e assem b lag e
g r o s s u la r + a n o r t h i t e + w o l la s to n i t e + c a lc i te + q ua r tz
( + C O 2 ) lies c lo se to 600° C a t 2 K b . T h is a sse m b la g e is
p re s e n t o n th e sc a le o f a th in se c tio n in m a n y o f th e B lue
M o u n t ro c k s a n d a p p e a rs to re p re s e n t c o n d i t io n s c lose
to th e p e a k o f m e ta m o rp h is m . T h e m a rk e d effec t o f
p re ssu re o n th e to p o lo g y o f re a c t io n 8, sh o w n in F ig 12,
su g g ests th a t th e in v a r ia n t p o in t m a y b e so m e w h a t
lo w er , a ro u n d 5 5 0 °C , fo r p re ssu re s as lo w as 0 .5 K b.
T h e s ta b i l i ty re la t io n s o f v e su v ia n ite ( id o c ra se ) were
in v e s tig a te d b y I to a n d A re m (1970 ), S h o j i (1971) an d
H o c h e l la e t al. (1982). H o w e v e r , a g re e m e n t b e tw e en
th e se s tu d ie s is l im ite d , d u e to th e c o m p le x ity o f th e
su b s t i tu t io n a l c h e m is try o f v e su v ia n ite a n d th e se ns it iv i
ty o f s ta b i l i ty f ie lds to v a ry in g s ta r t in g c o m p o s itio n s .
T h e lo w e r te m p e ra tu re s ta b i l i ty l im its o f (M g -r ic h ) v esu
v ia n ite is s ig n if ic an t ly re d u c e d b y th e p re se n c e o f q u a r tz ,
to a ro u n d 380° C a t P H O = 1 K b > w ith re sp e c t to th e a s se m b la g e g ro s s u la r + c l io p s id e + w o l la s to n i te ( H o
ch e lla e t al. 1982). T h e u p p e r s ta b i l i ty l im it , a b o v e w h ich
m e lil i te a n d m o n tic e l l i te a re re a c t io n p ro d u c ts , w as
d e te rm in e d to b e a ro u n d 7 0 0 °C ( I to & A re m 1970) o r
800° C (H o c h e l la e t a l. 1982). T h e a b s e n c e o f th e se
m in e ra ls a t B lue M o u n t se ts a n u p p e r te m p e ra tu re l im it
o f 7 0 0 °C . T h e p re h n i te d e c o m p o s i t io n e q u i l ib r iu m
( re a c t io n 6) h a s b e e n d e te rm in e d to lie a t 4 4 0 ° C a t 1 K b
(L io u 1971). T h u s th e p re h n i te o b se rv e d is m o s t l ike ly to
re p re se n t r e tro g ra d e c ry s ta l l iz a t io n . T h is c o n c lu s io n is
su p p o r te d b y th e co e x is ten ce o f p re h n i te a n d sc o lecite in
o n e c o a rse g ra in e d p a tc h .
S p h e r u l e - b e a r i n g R o c k s : T h e a s se m b la g e in th e
s p h e ro id a l b le b s (essen tia lly g ro s s u la r -w o l la s to n ite -
d io p s id e -sp h e n e ) in d ic a te s th a t re a c t io n s 4 , 7 a n d 8 w ere
in s t ru m e n ta l in th e ir f o rm a t io n . T h e c o n t ra s t in g
m in e ra lo g y b e tw e e n sp h e ru le s a n d m a tr ix su g g ests tha t
th e f o r m e r w e re o r ig in a l l y c a l c i t e - q u a r t z - c a l c i c
p la g io c la se -ru t i le in c lu s io n s in a m o re m ic a c e o u s , i .e . ,
K -r ic h , h o s t. H o w e v e r , th e s p h e ru l i t ic fo rm s a p p e a r to
re p re s e n t n u c le a t io n a n d g ro w th r a th e r th a n m e re ly
iso c h em ica l re a c t io n s w ith in in c lu s io n s .
C H E M IS T R Y O F T H E B U L K R O C K S
T h re e re p re se n ta t iv e B lu e M o u n t ro c k ty p e s a n d th e
co e x is t in g p a le a n d d a rk f ra c t io n s f ro m sa m p le E 1 0 54 4
w ere a n a ly se d b y c o n v e n tio n a l X R F a n d w e t ch e m ic a l
( fo r F e 2+) te c h n iq u e s . In a d d i t io n , a f in e -g ra in e d sa m p le
W . D . B IR C H
T a b l e 8
C h e m i c a l A n a l y s e s o f C a l c - S i l i c a t e R o c k s f r o m B l u e M o u n t ( 1 - 5 ) a n d B o n n i e D o o n ( 6 )
w t % 1 2 3 4 5 6
S i 0 2 6 5 .1 2 5 9 .1 2 51 .12 4 6 .0 4 4 8 .8 2 49 .4 5T i 0 2 0 .2 4 0 .6 7 0 .5 7 0 .4 8 0 .5 8 0 .4 4a i 2o 3 14 .3 0 11.59 11 .9 9 10 .95 12 .7 6 8 .95F e20 3 0 .6 8 0 .5 6 0 .1 0 1 .1 2 1.74 __
F e O 2 .9 4 4 .9 0 3 .8 4 3 .43 3 .7 0 3 .43M n O 0.0 3 0 .0 8 0 .0 7 0 .0 9 0 .0 9 0 .0 7M g O 2.3 5 2 .8 5 2 .5 2 2.91 2 .6 9 3 .1 5C a O 4 .2 9 10.42 2 2 .7 2 3 0 .2 4 2 0 .8 5 2 9 .4 8N a 20 2 .3 3 1.71 1.30 0 .0 5 0.51 4 .9 7K 2Q 4 .3 5 5 .4 7 2 .4 4 0 .0 7 4 .5 0 0 .9 4P 2O 5 0 .15 0 .1 7 0 .0 9 0 .1 0 0 .1 0 0 .0 8Ig . L o ss n d n d n d 4.01 n d n dT o ta l 9 7 .2 8 9 7 .5 9 9 6 .7 6 9 9 .4 9 9 6 .3 4 100.96p p m
V 99 82 85 75 100 62C r 120 106 113 219 249 121B a 1696 2464 97 0 31 3724 329Sc 11 23 11 10 11 8C u 21 173 38 142 45 122C o 10 15 18 19 16 16Z r 273 352 235 149 208 167Y 28 34 32 21 29 23Z n 70 114 117 81 93 84N i 48 53 77 69 67 57S r 814 1960 2022 213 1492 781R b 20 9 301 136 12 20 6 58C l 127 195 242 128 184 172S 2277 147 97 41 37 9 136P b 18 19 13 8 89 12
C IP W N o rm s
Q 2 1 .9 9 7 .9 3 0 .3 9O r 25 .71 32 .3 3 1 4 .4 2 0.41 18 .1 0 __
A b 19 .7 2 14.47 9 .2 4 0 .4 2 _ __
N e — — 0 .9 5 — 2 .3 4 2 2 .1 0N s - - — — — 0 .2 9L c — — — — 6 .6 6 4 .3 6A n 15.71 7 .7 9 19.67 2 9 .4 5 19 .2 4 __
D i 3 .7 4 2 9 .5 6 2 5 .1 2 2 4 .4 6 2 3 .0 4 2 7 .6 4H y 7 .6 5 — _ _ _ __
W o - 2 .9 8 2 5 .9 2 37 .5 5 23 .11 4 2 .0 9M t 0 .9 9 0.81 0 .1 4 1.62 2 .5 2 __
I lm 1.41 1 .27 1.08 0.91 1.10 0 .8 4A p 0 .3 7 0 .4 8 0.21 0 .2 4 0 .2 4 0 .1 9
R a tio sA n 58 19 28 32 29D i 14 73 36 27 35 40H y 28 — — _ __
W o - 7 37 41 35 60
S a m p le s : 1, C a lc -s i l ic a te h o rn fe ls (d a rk f ra c t io n ) (N M V E 1 05 4 4 ) B lu e M t . ; 2 , C a lc -s i l ic a te h o rn fe ls (pa le
f ra c t io n ) (N M V E 1 05 4 4 ) B lu e M t . ; 3 , C a lc -s i l ic a te ho rn fe ls (N M V E 1 02 8 0 ) B lu e M t. ; 4 , C a lc -s i l ic a te
h o rn fe ls (N M V E 1 02 8 3 ) B lu e M t . ; 5 , C a lc -s i l ic a te h orn fe ls (s p o t te d ) (N M V E 1 0 7 1 3 ) B lu e M t . ; 6 , C a lc - s i l ic a te h o rn fe ls B o n n ie D o o n .
n d : n o t d e te rm in e d .
C A L C -S I L I C A T E R O C K S 15
o f ca lc -s i l ic a te h o rn fe ls f ro m th e B o n n ie D o o n lo c at io n
(P h i l l ip s & W a ll 1980) w as an a ly se d . T h is sp e c im en c o n
s is ted m a in ly o f w o l la s to n ite , d io p s id e a n d K fe ld sp a r
w ith sm a ll len ses o f c a lc ite , w o l la s to n ite a n d q u a rtz .
M a jo r a n d tra c e e lem e n t d a ta a re sh o w n in T a b le 8 .
T h e b u lk ro c k s h a v e v a r ia b le c o m p o s i t io n , ch ie f ly in
v o lv in g C a a n d S i, a n d to a le sser e x te n t K . O f th e tra c e
e le m e n ts , B a , S r a n d S sh o w th e g re a te s t v a r ia t io n . I t is
re a s o n a b le to a ssu m e th a t m u c h o f th e ch e m ic a l v a r ia
t io n re f lec ts d if fe ren ces in o r ig in a l s e d im e n ta ry co m
p o n e n ts , p a r t ic u la r ly in te rm s o f th e i l l ite (c lay -m ica )
( i .e . K ), ca lc iu m c a rb o n a te (C a ) a n d q u a r tz (S i) f ra c
t io n s . H o w e v e r , an a ly se s o f th e d a rk a n d p a le p o r t io n s
o f sa m p le E 1 0 5 4 4 in d ic a te th a t a l te ra t io n o f th e o rig in a l
d a rk h o rn fe ls re su lte d in th e re m o v a l o f S i, A l, N a a n d S
a n d th e c o n c e n t ra t io n o f C a , K , F e , M g , B a , S r a n d C l
in th e p a le a l te ra t io n zo n e s . I f th is an a ly s is p a i r is
re p re se n ta t iv e , th e n th e m e ta m o rp h is m w as n o t
iso c h e m ic a l, a t le as t in its f in a l s tag es, b u t w as a c c o m
p a n ie d b y e x c h an g e o f c o m p o n e n ts f ro m f lu id s in w h ich
C a , S i, N a , S , A l a n d K w ere th e m o s t m o b i le e lem e nts .
T h u s a n y o r ig in a l c o m p o s it io n a l d if fe ren ces b e tw e e n in
d iv id u a l ro c k ty p e s h a v e b e e n o v e rp r in te d d u r in g
m e ta m o rp h is m . T h e a m o u n t o f ch e m ic a l m o d if ic a t io n is
d if f icu lt to q u a n t i f y in th e a b s e n c e o f c le a r z o n a tio n in
a ssem b lag e s. H o w e v e r , i t is p o ss ib le to d e r iv e a
h y p o th e t ic a l re a c t io n fo r th e a l te ra t io n o f sa m p le
E 1 0544 (n eg le c tin g o x id a t io n o f su lp h id e s) , b a s e d on th e
b u lk ro c k c o m p o s it io n s , th e re la tiv e a b u n d a n c e s o f m a
j o r m in e ra ls (T a b le 7 ), th e av e rag e a lk a li fe ld sp a r c o m
p o s i t io n in th e d a r k f ra c t io n (K o.5 N ao .4 C ao .!A I1 . 0 5
S i2 .9 5 0 g — o b ta in e d b y m ic ro p ro b e ) a n d th e av e rag e
K -fe ld sp a r in th e p a le f ra c t io n (Ko.9 N a0 . iA lS i 3 0 8 — o b
ta in e d b y X - ra y d i f f ra c t io n a n d m ic ro p ro b e ) :
13. 10 (K o sN ao 4 C a0 i )A l i o sS i2 9 5 O 8 + 5 . 5 S i0 2 +
2 (M g F e) C l j + 4KC1 + C a C l2 + H 20 -
1 0 (K 0 9N a 0 1) A l S i 30 8 + 2 C a ( M g , F e ) S i 20 6
+ 0.5A1C13 + S iC l4 + 3 N a C l + 2H C 1.T h is o v e ra ll re a c t io n d o e s n o t q u i te b a la n c e , in tha t ,
as sh o w n , it p ro d u c e s a n e x tra 0 .5 m o le C l", b u t this ca n
b e a c c o u n te d fo r b y th e s im p lif ic a tio n o f th e fe ld sp a r
c o m p o s it io n s . T h e re a c t io n re su lts in th e c o n s u m p t io n
o f q u a r tz a n d th e p ro d u c t io n o f d io p s id e , in a g re e me n t
w ith o b se rv e d m in e ra lo g ic a l c h a n g e s d u r in g a l te ra t io n
(T a b le 7). A ll m o b ile c a t io n s (K , C a , M g , F e , S i, Al, N a ,
H ) a re assu m ed to b e p re s e n t as c h lo r id e s , w h ic h is a f u r
th e r s im p lif ica tio n , a l th o u g h c h lo r id e w o u ld h a v e be e n a
m a jo r c o m p o n e n t in th e s o lu t io n s (V id a le & H e w itt
1973). N o te th a t th e u se o f th is e q u a t io n im p l ies th a t
fa c to rs su c h as p H a n d th e ac tiv i t ie s o f s il ica , C a2+ a n d
M g 2+ m a y b e o f s ig n if ican ce . A s im ila r re a c t io n co uld
p o ss ib ly b e w r i t te n in v o lv in g p la g io c lase fe ld sp a rs i f
c o m p o s it io n s o f re a c ta n ts a n d p ro d u c ts w e re k n o w n .
R e a c tio n 13 c a n n o t a c c o u n t f o r th e m a g n i tu d e o f th e
ch a n g e s in b u lk ro c k c o m p o s it io n as a l te ra t io n p ro ce e d
ed . H o w e v e r , i f A l, S i a n d N a w ere re m o v e d in s o lu tio n ,
a n d K a n d C a a d d e d , th e c h a n g e s w o u ld b e in th e
o b se rv e d d ire c t io n . T h e c o m p o s i t io n o f e a c h ro c k typ e
in th e (s i l ic a -sa tu ra te d ) A l2 S i0 5 —(M g , F e ) S i0 3 —
C a S i0 3 sy stem is p lo t te d in F ig . 13. T h e m in e ra lo g y
p re d ic te d f ro m th e p lo t te d p o s i t io n s ag re es w ell w ith
th a t o b se rv e d .T h e w id e s p re a d o c c u r re n c e o f g ro s s u la r in th e B lue
M o u n t ro c k s , a n d its a b s e n c e f ro m th e B o n n ie D o o n
c a lc -s i l i c a te s (w h e re th e e q u i v a le n t a s s e m b la g e p la g io c lase + w o l la s to n ite + c a lc ite o c c u rs —P h il l ip s &
W a ll 1980) m a y a r ise f ro m d if fe ren t N a c o n te n ts in th e
b u lk ro c k s . T h e p ro g re ss iv e a d d i t io n o f a lb i te to
p la g io c la se so lid s o lu t io n re su lts in a s ig n if ic an t s h i f t o f
th e e q u i l ib r iu m ( re p re se n te d b y re a c t io n 10) w ith
g ro ssu la r f o rm a t io n b e in g fa v o u re d b y lo w e r N a c o n
te n ts (K e rr ic k 1974).
C O N D IT IO N S O F M E T A M O R P H IS M
P ressu re a n d T em pera tu re
T h e re a re n o in d e p e n d e n t m in e ra lo g ic a l ly -b a se d
m e an s a v a i la b le to d e te rm in e th e p re ssu re a t w h ic h th e
B lue M o u n t ro c k s fo rm e d . H o w e v e r , so m e c o n c lu s io n s
m a y b e d ra w n f ro m g eo lo g ica l re la t io n s h ip s . T h e
g ra n o d io r i te h a s in tru d e d th e m a in b o u n d in g f a u l t of
th e A c h e ro n c a u ld ro n c o m p lex , a n d h e n c e la rg e ly
p o s td a te s su b s id e n c e a lo n g th e fa u l t . F ro m th e n a ture o f
th e e m p la c e m e n t m e c h a n ism o f th e v o lc an ic s , i .e . the
f il ling o f a c o l la p se s tru c tu re , it m a y re a s o n a b ly b e
a ssu m e d th a t th e leve ls o f th e to p o f th e v o lc an ic p ile
a n d th e s u r ro u n d in g la n d su r fa c e o u ts id e th e b o u n d ary
fa u l t w e re ro u g h ly c o n c o r d a n t a t th e t im e o f in tru sio n .
T h e p re s e n t a l t i tu d e d if fe ren ce b e tw e e n th e h ig h e s t
p o in ts o n th e e ro d e d v o lc a n ic p i le a n d B lu e M o u n t is
a b o u t 400 m e tre s . T h u s , i f n o m o re th a n 100 m e tre s
h av e b e e n e ro d e d f ro m th e v e r t ic a l th ic k n e ss o f th e
v o lc an ic s , th e n th e c o n ta c t m e ta m o rp h is m c o u ld re p re
se n t in tru s io n to d e p th s as sh a llo w a s 500 m e tre s . E v en
a l lo w in g fo r g re a te r e ro s io n ra te s , th e re is l i t t le d o u b t
th a t th e B lue M o u n t ro c k s c ry s ta l l ize d a t p re ssu re s o f
th e o rd e r o f 0 .5 K b o r less. A s im ila r c o n c lu s io n was
re a c h e d fo r th e B o n n ie D o o n ca lc -s i l ic a te ro c k s (P hi l l ip s
& W a ll 1980).E s t im a te s o f te m p e ra tu re m a y o n ly b e lo o se ly b a s e d
o n th e e x p e r im e n ta l ly d e te rm in e d e q u i l ib r ia m a tc h ing
th e n a tu ra l assem b lag e s (F ig . 12). T h e b e s t re a c t ion to
c o n s t r a i n t e m p e r a t u r e is g r o s s u l a r + q u a r t z — 2
w o l la s to n ite + a n o r th i te (9) s in ce it is f lu id -a b sen t. G iv en
th a t th e ca lc ic fe ld sp a r in th e ro c k s is n o t p u re an o r th i te ,
th e e s t im a te d te m p e ra tu re f ro m th is e q u i l ib r iu m is a
m a x im u m . A m a x im u m te m p e ra tu re o f b e tw e e n 500°
a n d 5 5 0 °C is su g g e sted f o r th e B lu e M o u n t ro c k s , in
g e n e ra l a g re e m e n t w ith e s t im a te s f o r th e B o n n ie D o on
o c c u r re n c e (P h i l l ip s & W a ll 1980). T e m p e ra tu re s o f
a ro u n d 4 0 0 °C f o r e q u i l ib ra t io n in th e re tro g re ss iv e
s ta g es a re su g g e sted b y th e p re se n c e o f v e su v ia n ite,
p re h n i te a n d sco lec ite .
So lu tio n C hem is try
B in a ry s o lu t io n s c o n s is tin g o f H 20 a n d C 0 2 a re th e
m a in a g e n ts in v o lv e d in f o rm a t io n o f c a lc -s i l ic a te
assem b lag e s u n d e r a w id e ra n g e o f c o n d i t io n s (K e rr ic k
1974, R ice & F e r ry 1982). T h e se so lu t io n s m a y b e d er iv
ed b y e i th e r d e v o la t i l iz a t io n re a c t io n s w ith in th e ro c k s
d u r in g p ro g ra d e m e ta m o rp h is m , o r b y in f i l t ra t io n o f
B
16
a n d a l u s i t e
W . D . B IR C H
w o l l a s t o n i t e d i o p s i d e h y p e r s t h e n e
F ig . 13 —B u lk c o m p o s it io n s o f th e B lu e M o u n t a n d B on n ie D o o n c a lc -s i l ic a te ro c k s p lo t te d in te rm s o f
n o rm a t iv e c o m p o n e n ts . T h e a r ro w e d t ie - l in e jo in s th e d a rk a n d p a le h o rn fels in sa m p le E 1 0 5 4 4 .
s o lu t io n s f ro m e x te rn a l so u rc e s , su c h a s la te -s ta g e
m a g m a t ic o r m e te o r ic f lu id s (R u m b le e t a l. 1982). A t
B lu e M o u n t , th e m a in re a c t io n s (F ig . 12) o c c u r r in g c lo se
to th e p e a k o f p ro g ra d e m e ta m o rp h is m a n d re sp o n s ib le
f o r th e a l te ra t io n o f th e d a rk h o rn fe ls a re c o n s is te n t
w ith e q u i l ib ra t io n in th e p re se n c e o f so lu t io n s w ith X co
less th a n 0 .2 . In so m e ca se s, p a r t ic u la r ly d u r in g
c ry s ta l l iz a t io n o f v e su v ia n i te a n d p re h n i te d u r in g
re tro g re s s io n , e x tre m e ly w a te r - r ic h f lu id s (X co < 0 .0 5 )
a re in d ic a te d (H o c h e l la e t a l. 1982, W il l ia m s-Jo n e s
1981). N e a r ly a l l th e d e v o la t i l iz a t io n re a c t io n s o cc u r r in g
d u r in g p ro g re ss iv e m e ta m o rp h is m o f th e B lu e M o u n t
ro c k s p ro d u c e d C 0 2. I n o r d e r to m a in ta in lo w X c o , th e
C 0 2 p ro d u c e d m u s t h av e b e e n d i lu te d b y a la rg e re s er
v o ir o f w a te r - r ic h f lu id , d e r iv e d f ro m e i th e r th e in t r u
s io n , f ro m th e w et se d im e n ts th e m se lv es o r f ro m a
m e te o r ic so u rc e a n d t r a n s p o r te d a lo n g g ra in b o u n d a rie s
o r o n f ra c tu re sy stem s. I t is u n l ik e ly th a t th e m ixin g p r o
cess re su lte d in a to ta l ly h o m o g e n o u s f lu id p h a s e . T h e
ex is te n ce o f c o a rse -g ra in e d p a tc h e s , v e in le ts , c lo ts , a n d
re m n a n t a l te ra t io n te x tu re s in f in e -g ra in e d ro c k s sug g est
v a r ia b le in f i l t ra t io n ra te s a n d h e n c e v a r ia b le f lu id c o m
p o s i t io n s . A s a n e x tre m e ex a m p le , th e p re se n c e o f bo th d io p s id e -q u a r tz a n d g ro ss u la r -w o l la s to n i te v e in le ts,
a l th o u g h n o t in th e sa m e ro c k ty p e , re p re s e n t th e
lo c a lize d e ffec ts o f C 0 2- r ic h a n d H 20 - r i c h s o lu t io ns
resp e c tiv e ly . P h i l l ip s a n d W a ll (1971) p re d ic te d v ar ia b i l i
ty o f C 0 2 a c tiv i ty o n th e sc a le o f a h a n d s p e c im e n at
B o n n ie D o o n , b a s e d o n f lu c tu a t io n s in th e p ro p o r t ion s
o f c o e x is t in g w o l la s to n ite , c a lc ite a n d q u a r tz . S u ch a n
e ffec t is lik e ly in th e B lu e M o u n t ro c k s . In su c h a sm a ll- sca le , u n z o n e d d e p o s i t , a c ro ss w h ic h P -T g ra d ie n ts m a y
b e u n d e te c ta b le , it is d if f icu lt to d e te rm in e an y
m in e ra lo g ic a l ev id en c e f o r c h a n g e in s o lu t io n c o m p os i
t io n w ith tim e . H o w e v e r , it is lik e ly th a t so lu t io ns
b e c a m e in c re a s in g ly w a te r - r ic h , re su lt in g in lo c a lize d
c ry s ta l l iz a t io n o f v e s u v ia n i te , p re h n i te a n d sc o lecite d u r
in g re tro g re s s io n .T h e f lu id s re s p o n s ib le f o r th e a l te ra t io n o f th e d ark
h o rn fe ls w ere a b le to m o v e s ig n if ic an t a m o u n ts o f Si,
A l, C a , N a , K , C l a n d S th ro u g h th e B lu e M o u n t ro c ks .
T h e se e le m e n ts a re lik e ly to b e a m o n g s t th e m o s t c om
C A L C -S I L I C A T E R O C K S 17
m o n a n d m o b i le in m e ta m o rp h ic f lu id s (V id a le & H e w itt
1973, C ra w fo rd e t al. 1979, F e r ry & B u r t 1982, F e r ry
1983). A s w ell as c a t io n s , o r c a t io n ic co m p lex es,
m e ta m o rp h ic f lu id s c o m m o n ly c o n ta in s ig n if ican t H 2S,
C H 4 (F e rry & B u r t 1982), c h lo r id e s (S isso n e t al. 1982,
C ra w fo rd e t al. 1979) a n d f lu o r id e s . A l th o u g h th e
o v e ra ll ef fec t o f u p to 10 w t% N a C l o n C 0 2 a c tiv i ty in
H 20 - C 0 2 f lu id s m a y n o t b e o f s ig n if ican ce a t lo w
p re ssu re s (J a c o b s & K e rr ic k 1981), lo c a lize d c o n c e nt ra
t io n s m a y in f lu e n ce th e s ta b le a ssem b lag e . F o r ex a mp le ,
sc a p o l i te c ry s ta l l iz a t io n m a y o c c u r in th e p re se n c e o f
N a C l-r ic h f lu id s . A l th o u g h f lu o r id e s a re o f le sser
s ig n if ican ce in m e ta m o rp h ic s o lu t io n s , b e in g s tro n gly
p a r t i t io n e d in to c ry s ta l l iz in g s il ica tes , it is p o ss ib le th a t
lo c a lize d F c o n c e n tra t io n s m a y f a v o u r f o rm a t io n o f
v e su v ia n ite r a th e r th a n g ro ssu la r .
A p a r t f ro m th e ir a b i l i ty to c h a n g e th e b u lk c h e m is try
o f th e ro c k s , th e s o lu t io n s in f lu e n ced th e s t ru c tu ra l s ta te
o f th e K fe ld sp a rs . T h e s l ig h tly lo w e r d e g re e o f o rd e r in
th e K fe ld sp a rs f ro m th e p a le a l te re d z o n e s c o m p a red to
th e d a rk h o rn fe ls su g g ests a h ig h e r c ry s ta l l iz a t io n
te m p e ra tu re . H o w e v e r , i t is u n lik e ly th a t a s ig n if ic an t
te m p e ra tu re g ra d ie n t c o u ld b e m a in ta in e d o v e r su c h
re la tiv e ly s h o r t d is ta n c e s . A s th e c o n t in u e d p re se nc e o f
m e ta m o rp h ic s o lu t io n s in th e a l te ra t io n zo n e s w o u ld
fa c i l i ta te th e K fe ld sp a r o rd e r in g p ro c e ss d u r in g co o l in g ,
th e o b se rv e d d if fe ren ce is th e o p p o s i te to w h a t m igh t be
ex p e c te d . T h e e x p la n a t io n m a y lie in re c ry s ta l l iz a tio n
d u r in g a l te ra t io n w h ich ef fec tive ly re m o v e d p o re f lu id s
f ro m th e a l te re d z o n e s , so th a t th e fe ld s p a r o rd e r in g
p ro c e ss w as n o t ass is te d . A l te rn a t iv e ly c o m p o s it io na l
d i f fe re n c e s , p a r t ic u la r ly th e h ig h e r levels o f B a in th e
a l te re d z o n e K fe ld sp a rs (sm a ll a m o u n ts o f B a m a y
h in d e r o r d e r in g —M a k a g o n & S c h m a k in 1971) m a y be
su ff ic ie n t to a c c o u n t f o r th e s t ru c tu ra l s ta te d if fe ren ces.
D IS C U S S IO N A N D C O N C L U S IO N S
T a y lo r a n d O ’N e il l (1977) d e f in e d s k a rn a s re fe r r ing
to c o a rse -g ra in e d ro c k s w h ich a re th e p ro d u c ts o f
m e ta s o m a tis m , w h e rea s ca lc -s i l ic a te h o rn fe ls b e s t
d e s c r ib e d ro c k s w h ich a re th e re su lt o f iso c h em ica l
m e ta m o rp h is m . T h is s im p le d is t in c t io n h a s su b s e q u e nt
ly b e e n rec o g n ise d as in a d e q u a te to c o v e r th e d iv e rs i ty o f
s k a rn a n d sk a rn - l ik e d e p o s its (E in a rd i & B u r t 1982). T h e
c o n ta c t m e ta m o rp h is m o f th e B lu e M o u n t ro c k s w as
c lea rly n o t iso c h em ica l, a t le as t o n th e sc a le o f in
d iv id u a l b e d s . T h e o r ig in a l se d im e n ts w ere a l im ited se
q u e n c e o f a rg i l la c e o u s l im e s to n e s a n d c a lc a re o u s
m u d s to n e s . M e ta m o rp h is m a p p e a rs to h a v e p ro c e e d e d
in tw o s tag es, p a r t ic u la r ly in th e le ss re a c t iv e roc k ty p es
re p re s e n te d b y th e d a rk h o rn fe ls . A n e a r ly , la rg e ly
iso c h e m ic a l ( a p a r t f ro m d e v o la t i l iz a t io n ) re c ry s ta ll iz a
t io n w as fo l lo w e d b y a la te r m e ta s o m a tic a l te ra t io n, th e
ef fec ts o f w h ich c a n o n ly b e m o n ito re d w h e re th e roc k s
w ere p a r t ia l ly a l te re d . In c o n t ra s t to th e d a rk h o rn fe ls ,
th e m assiv e p a le h o rn fe ls ic a n d m o tt le d ro c k ty p e s sh o w
n o ev id en ce f o r a m u lt i -s ta g e p ro c e ss . L o c a l e x c h ang e o f
c o m p o n e n ts b e tw e e n b e d s a n d p a r t ia l h o m o g e n iz a t io n ,
w ith th e f o rm a t io n o f ‘s k a rn o id s ’, h a v e n o w b e e n
rec o g n ise d in in te r la y e re d se q u en c es su c h as th a t on B lu e
M o u n t (E in a rd i & B u r t 1982, H a r r is & E in a rd i 1982).
T h e f lu id s re sp o n s ib le fo r th e m e ta s o m a tis m w ere
e i th e r in te rn a l ly -d e r iv e d f ro m th e p re s u m a b ly o r ig in a l ly
w et se d im e n ts th e m se lv es ( i .e . th e y w ere a p ro d u c t o f
b o th p o re f lu id s a n d d e v o la t i l iz a t io n re a c t io n s ) o r else
w ere o f m e te o r ic o r ig in . A l th o u g h th e lo w -p re ssu re ,
h ig h -lev e l e n v i ro n m e n t m a y a p p e a r to f a v o u r th e in flu x
o f m a g m a t ic f lu id s , t h e i r o n - r i c h a s s e m b la g e s
c h a ra c te r is t ic o f m a g m a tic m e ta s o m a t is m a re la c k in g in
th e c a rb o n a te ro c k s . S o m e m a g m a tic in f lu e n ce c a n n o t
b e ru le d o u t e n t ire ly h o w e v e r, p a r t ic u la r ly i f th e o v e ra ll
sy stem ( in tru s io n p lu s se d im e n ts) w as re la tiv e ly d ep le te d in i ro n .
T h e b u lk c h e m is try o f th e ro c k s w as v a r ia b le , b u t
w ith in su ff ic ien tly n a r ro w l im its to e n a b le th e c rys ta l l iz a
t io n o f g ro s s u la r in m o s t ro c k ty p e s . T h e o v e ra l l lo w N a
c o n te n t w as p ro b a b ly th e c r it ic a l f a c to r in f lu e n c ing
g ro ssu la r c ry s ta l l iz a t io n .
B a sed o n e x p e r im e n ta l a n d c a lc u la te d d a ta f o r re le
v a n t p h a s e e q u i l ib r ia , th e e s tim a te s f o r P -T c o n d i tio n s
d u r in g m e ta m o rp h is m o f th e B lu e M o u n t ro c k s ( P < 0 .5
K b , T max = 5 0 0 -5 5 0 °C ) a re in g e n e ra l a g re e m e n t w ith
th o s e d e d u c e d f o r th e o n ly o th e r s tra t ig ra p h ic a l ly
s im ila r o c c u r re n c e in th e re g io n (B o n n ie D o o n ) . T h e
m a jo r a ssem b lag e s e q u i l ib ra te d in th e p re se n c e o f wa te r-
r ic h f lu id s (X CO2< 0 .2 ) .
A C K N O W L E D G E M E N T S
T h e a u th o r ac k n o w le d g e s th e a s s is ta n c e g iv en b y M r
P e te r K en ley , G e o lo g ic a l S u rv e y o f V ic to r ia , in in fo rm
in g h im o f th e ex is ten ce o f th e o c c u r re n c e . A ll ch em ic a l
an a ly se s (ex c ep t fo r d e te rm in t io n o f fe r ro u s iro n ) w ere
o b ta in e d in th e D e p a r tm e n t o f G e o lo g y , U n iv e rs i ty of
M e lb o u rn e . T h e ass is ta n c e o f M r M . H a u k a in c a r ry ing
o u t th e X R F an a ly se s is a c k n o w le d g e d . D r G . N .
P h il l ip s k in d ly p ro v id e d th e sp e c im e n o f th e B o n n ie D o o n ca lc -s i l ic a te ro c k . D r T . K w a k o f fe re d v a lu a b le a d
v ic e o n th e m a n u s c r ip t .
R E F E R E N C E S
C r a w f o r d , M . L ., K r a u s , D . W . & H o l l i s t e r , L . J . 1979.
P e t ro lo g ic a n d f lu id in c lu s io n s tu d y o f c a lc -s i l ic ate
ro c k s , P r in c e R u p e r t , B r i t ish C o lu m b ia . A m . J . Sc i.
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D e e r , W . A . , H o w i e , R . A . & Z u s s m a n , J . , 1962. R o c k -
fo r m in g M ine ra ls , Vo l. 5 : O r th o - a n d R in g S ilica tes.
L o n g m a n s , G re e n & C o . , L td . , L o n d o n .
D u d l e y , R . J . , D u d l e y , J . & R o s s i t e r , A . , 1971. G e o lo g y o f
th e A c h e ro n C a u ld ro n , C e n tra l V ic to r ia . B .S c .(H o n s )
th e s is , U n iv e rs i ty o f M e lb o u rn e , M e lb o u rn e .
E i n a r d i , M . T . & B u r t , D . M ., 1982. I n t r o d u c t io n —T e r
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