Precambrian Research, 25 (1984) 349--364
349
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
CO-EXISTING CORDIERITE- ALMANDINE -- A KEY TO THE
METAMORPHIC HISTORY OF SRI LANKA
L.R.K. P E R E R A
Department of Geology, University of Peradeniya, Peradeniya (Sri Lanka)
(Received January 25, 1983; revision accepted March 5, 1984)
ABSTRACT
Perera, L.R.K., 1984. Co-existing cordierite--almandine -- a key to the metamorphic history of Sri Lanka. Precambrian Res., 25: 349--364.
A co-existing c o r d i e r i t e l l m a n d i n e assemblage is described from the pelitic metasedimentary rocks of the Precambrian granulite-facies terrain of Sri Lanka. The chemical
control on this assemblage is demonstrated, as has been observed in granulite-facies terrains elsewhere. The previous view of a low-pressure origin of cordierite in pelitic rocks,
and the subdivision of the central granulite-facies belt of Sri Lanka based thereon, is
hence doubtful.
INTRODUCTION
The island of Sri Lanka, for the most part, is underlain by Precambrian
metamorphic rocks belonging to three distinct units, namely, the Highland
Group, Southwest Group, and the Vijayan Complex (Cooray, 1978). The
Highland Group--Southwest Group granulite-facies belt is flanked on either
side by the Vijayan Complex reflecting amphibolite-facies mineral assemblages (Fig. 1).
Cordierite-bearing mineral assemblages were previously recognized in pelitic rocks of the Southwest Group. They have also been increasingly recognized in certain pelitic rocks of the Highland Group (Fig. 1). Cordierite plays
a determinant role in the Precambrian metamorphic history of Sri Lanka. Its
occurrences have been variously interpreted, and the following paragraph
summarizes them. This paper suggests a different interpretation in the light
of world-wide observations on the co-existing cordierite--almandine assemblage in granulite-facies terrains.
C O R D I E R I T E AND METAMORPHIC HISTORY OF SRI LANKA
Katz (1971) suggested three episodes of granulite-facies metamorphism
for the metamorphic history of Sri Lanka. A variation of pressure from 3.5
(Southwest Group) to 6 kbars (Highland Group)during a final metamorphic
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© 1984 Elsevier Science Publishers B.V.
350
.!
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O CORDIERITE-BEARING META,OELITES
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,#"
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11 KAOUGANNAWA
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18 LAXAPANA
28 PINNAOODA
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19 RATNAPURA
29 PELAWATTE
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20 KALAWANA
30 OALLE
Fig. 1. M a j o r s u b d i v i s i o n s o f the Precambrian of Sri L a n k a ; H G - - H i g h l a n d G r o u p , S W G
S o u t h w e s t G r o u p , E V C and W V C - - Eastern and Western Vijayan C o m p l e x e s . M - - M i o c e n e limestone; .and the studied and reported occurrences of cordierite and almandine-
bearing metaPelites Of Sri Lanka.
overprint, also under the granulite-facies, is said to have determined the
stability of cordierite in pelitic metasediments of the Highland Group-Southwest Group belt (Katz, 1972). The low-pressure/high-temperature
metamorphic conditions (cordierite stable) prevailing within the Southwest
Group and the intermediate pressure/high temperature conditions (cordierite
unstable) prevailing within the Highland Group, have been attributed to two
contrasting facies series; the former reflecting the highest grade of the Abu-
351
kuma type and the latter the highest grade of the Barrovian type (Miyashiro,
1961). Thus, Katz (1972) designated the relationship between the Highland
Group and the Southwest Group as a paired metamorphic belt, and attempted to relate it to contiguous Gondwana terrains.
Hapuarachchi (1968) pointed out the necessity of erecting a cordieritegranulite facies, which he later (Hapuarachchi, 1975) named as the cordierite
division of the hornblende-granulite subfacies, to accommodate the cordierite-bearing assemblages. In his proposal of two periods of metamorphism,
the first period involving two phases and the second involving three phases,
cordierite-granulite facies conditions were thought to have prevailed during
the second phase of the second period of metamorphism, which only affected the Southwest Group.
However, increasing recognition of cordierite-bearing assemblages within
the Highland Group makes the above interpretations doubtful. Katz (1972)
attributed the cordierite-bearing rocks of the Highland Group to local overlap of the two facies series mentioned earlier. Hapuarachchi (1975) considered that cordierite-granulite facies conditions prevailed locally within the
Highland Group, to explain these occurrences. Hence, Cooray (1978) also
pointed out the necessity to revise the 'cordierite problem' in Sri Lanka.
Further, the occurrence of cordierite has been one of the main points that
stimulated a subdivision of the central granulite belt of Sri Lanka into the
Highland and Southwest Groups.
C O R D I E R I T E AND ALMANDINE-BEARING METAPELITES OF SRI LANKA
Mineralogy and subdivision
Pelitic metasedimentary rocks of the Highland Group--Southwest Group
granulite-facies belt of Sri Lanka can be subdivided on the basis of stability
of cordierite and almandine. Four categories are recognized:
(1) cordierite stable (almandine absent) rocks;
(2) rocks with stable co-existing cordierite--almandine;
(3) almandine stable (cordierite absent) rocks;
(4) transitional category between (2) and (3) (upper) and (1) and (2)
(lower) i.e., rocks showing instability of the co-existing cordierite--almandine pair at its upper and lower limits, respectively.
Rocks belonging to the four categories and their respective mineral assemblages are summarized in Table I. A'FM diagrams in Fig. 2a, b represent the
mineral assemblages graphically. The close association and interlayering of
rocks of the categories themselves is worth noting. Also interlayered and
associated with them are the other granulite-facies metasediments and charnockites of both the Highland and Southwest Groups. The studied occurrences of cordierite- and almandine-bearing metapelites of Sri Lanka are
shown in Fig. 1. A bluish colour is characteristic of the cordierite-bearing
rocks. Some of the cordierite- and almandine-bearing rocks are migmatitic.
352
Intermingling of cordierite, cordierite--almandine, and almandine-bearing
rocks has b e e n n o t e d in m a n y o t h e r granulite-facies terrains ( H i e t a n e n , 1 9 4 7 ;
Reinhardt, 1968).
TABLE I
Mineralogy and subdivision of cordierite and almandine-bearing metapelites of Sri Lanka a
Category
Rock types
1
crd sm gneiss
4 Transitional
crd--sm--kfp _+pl + q
~B) crd--bt--sm--kfp _+ pl + q
crd bt gneiss
@
crd bt hyp gneiss
~lD~f
~ crd--bt--hyp--kfp ± pl ÷ q
A transitional rock
between 2B and 1D.
2
crd alto sm gneiss
crd--bt--kfp ± pl + q
crd--hyp intergrowths with magnetite
around alm in 2B of category 2
crd--alm--sm--kfp ± pl + q
crd alm bt gneiss
~)
crd alm hyp gneiss ?
(2c) crd--alm--hyp--kfp ± pl + q ?
A transitional rock
between 3B and 2A or 2B
crd--alm--bt--kfp ± pl + q
crd--kfp intergrowths with magnetite
(± spinel) around alm in 3B of category
3
alm sm gneiss
alm---sm--kfp--pl + q
alm sm bt gneiss
alm--sm--bt--kfp ± pl + q
alm bt gneiss
alm bt hyp gneiss
Accessories:
(~
crd bt sm gneiss
(Lower)
4 Transitional
(Upper)
Mineral assemblages
) alm--bt--kfp--pl + q
[3D~ alm--bt--hyp--kfp ± pl + q
Magnetite, ilmenite, pyrite, apatite, muscovite, zircon, and calcite
are commonly associated with many of the mineral assemblages
mentioned above. Graphite and green spinel are restricted to rocks
in category 3 and categories 1 and 2, respectively. Pleochroic halos
around zircon inclusions in cordierite are characteristic
acrd -- cordierite; alm -- almandine; sm -- sillimanite; bt -- biotite; hyp -- hypersthene;
kfp -- K feldspar; pl -- plagioclase; q -- quartz. Numbers within the circles are for ease
of comparison with the A'FM diagrams of Fig. 2a, b. Broken circles indicate rare assemblages.
Relative abundance of various categories
A m o n g t h e c o r d i e r i t e - b e a r i n g r o c k s , t h o s e b e l o n g i n g t o t h e c a t e g o r y 2 are
more abundant in the Southwest Group. Most of the cordierite-bearing rocks
o b s e r v e d w i t h i n t h e H i g h l a n d G r o u p also b e l o n g t o t h i s c a t e g o r y . T h r e e
353
c o r d i e r i t e - - a l m a n d i n e - b e a r i n g parageneses m a y possibly o c c u r in m e t a p e l i t e s
o f c a t e g o r y 2. T h e y are, c r d - - a l m - - b t , c r d - - a l m - - s m , and c r d - - a l m - - h y p
(Table I, Fig. 2b). In Sri L a n k a , c r d - - a l m - - b t is t h e m o s t f r e q u e n t and c r d - alm--sm has also b e e n observed. S o m e t i m e s t h e y even o c c u r in the same
t h i n - s e c t i o n , b u t c r d - - a l m - - s m - - b t n e v e r o c c u r t o g e t h e r . All t h r e e parageneses were observed in a m e t a p e l i t e f r o m L a x a p a n a . T h e paragenesis c r d - a l m - - h y p has n o t b e e n r e c o r d e d in any o t h e r instance. E q u a l l y a b u n d a n t
w i t h i n b o t h Highland and S o u t h w e s t G r o u p s are the r o c k s o f c a t e g o r y 3
with t h e assemblages 3A and 3B (Table I), 3C being t h e rarest o f this category. Less a b u n d a n t are the r o c k s o f c a t e g o r y 1, 1D being the rarest assemblage. R o c k s o f t h e transitional c a t e g o r y are v e r y rare.
A ~'
/~rrt
+ quartz
÷ K- feldspar
+ pla~ioclose
+ magnetite
i itmeoite
zircon
F
I
+ quartz
÷ K- feldspar
÷ plogioclas e
~ sITI
+ magnetite
A
/I \\
/ I
\\
/i,,\\
MF
Fig. 2o
÷ ,m..,e
+ zircon
..oo.,
M
Fig. 2b
Fig. 2. Graphical representation of mineral assemblages of the cordierite and almandinebearing metapelites of Sri Lanka in A'FM diagrams; crd -- cordierite, alto -- almandine,
sm -- sillimanite, bt -- biotite, hyp -- hypersthene. (a) categories 1 and 3, (b) category 2
and the rare assemblages (broken circles). Dashed lines in Fig. 2a, b indicate the formation of the new assemblages owing to the breakdown of the co-existing cordierite-almandine pair at its upper and lower stability limits, respectively, during the low pressure
metamorphic overprint described in the t e x t . ( A l s o see Table I, Figs. 5 and 6.)
Accessory minerals
Magnetite, ilmenite, zircon, p y r i t e , apatite, m u s c o v i t e , and calcite are t h e
c o m m o n l y o b s e r v e d accessory minerals in all m e t a p e l i t e s o f t h e f o u r categories. Muscovite, w h e r e p r e s e n t , is associated with biotite, and a t r a n s f o r m a t i o n is always seen b e t w e e n t h e m . T w o o t h e r accessories, graphite and green
spinel, seem t o have r e s t r i c t e d and c o n t r a s t i n g o c c u r r e n c e s . G r a p h i t e o c c u r s
o n l y in r o c k s o f c a t e g o r y 3, m o r e a b u n d a n t l y in a l u m i n o u s rocks. Dissemin a t e d flakes o f graphite m a y c o n s t i t u t e a trace u p t o 10% o f t h e rocks.
Flakes are generally a few millimetres in size, and are o r i e n t e d parallel t o the
f o l i a t i o n o f t h e r o c k . In s o m e almandine--sillimanite rocks, graphite consti-
354
tutes an essential component, and these are called graphite schists. Uneconomic, disseminated graphite is c o m m o n in many of the charnockites and
metasediments of the granulite-facies belt, but most of the economic deposits are restricted to the Southwest Group. Green spinel occurs in more aluminous rocks of the categories 1 and 2. It is always associated with sillimanite,
and is c o m m o n l y intergrown with magnetite. Thus, patchy-aggregates of the
three minerals are c o m m o n (Fig. 3).
rfDm
Fig. 3. M i c r o g r a p h s h o w i n g s t a b l y c o - e x i s t i n g c o r d i e r i t e - - a l m a n d i n e in a c o r d i e r i t e - a l m a n d i n e - - - s i l l i m a n i t e g n e i s s o f c a t e g o r y 2; c r d - - c o r d i e r i t e , a l m - - a l m a n d i n e , s m - - sillimanite, bt -- biotite, q -- quartz, mg -- magnetite, sp -- green spinel. Note deformed cordierite and almandine aligned parallel to the foliation of the rock.
Textural features
During the study emphasis was placed on the textural relationships of cordierite and almandine. From this point of view, textural features of rocks of
the categories 2 and 4 received attention.
Category 2: in rocks of this category cordierite and almandine co-exist in
perfect equilibrium. Many of the rocks contained deformed almandine and
355
cordierite aligned parallel to the foliation of the rock (Fig. 3). This indicates
the formation of the cordierite--almandine assemblages before, or syntectonically with, D2 (Berger and Jayasinghe, 1976), which caused the development of the planar fabric in these Sri Lankan rocks.
Category 4: two kinds of vermicular intergrowths of minerals were recognized around almandine in rocks of this category, related to the upper and
lower limits of stability of the co-existing cordierite--almandine assemblage.
(1) cordierite--K feldspar intergrowths with magnetite (+ spinel), Fig. 4.
(2) cordierite--hypersthene intergrowths with magnetite (Fig. 5). Intergrowth (1) was recognized only in certain rocks with the assemblage 3B, and
(2) in certain rocks with 2B (Table I, Fig. 2b).
Categories 1 and 3: cordierite and almandine in rocks of categories 1 and
3, respectively, are in equilibrium with their common associates. Cordierite
in category 1 is xenoblastic. Porphyro- to poikiloblastic habit of almandine
is characteristic in category 3.
Chemistry
Available major element data of the categories mentioned above are presented in Table II. FeO/(FeO + MgO) ratio, which determines the width of
Fig. 4. Photomicrograph showing vermicular intergrowths of cordierite and K feldspar
rimming almandine in a almandine sillimanite biotite gneiss, crd -- cordierite, alto -almandine, kfp -- K feldspar. The intergrowths mark the transformation o f category 3 to
category 2 during a low pressure metamorphic overprint, which caused a second phase o f
cordierite generation (see Table I and the text).
356
NM4~dM~oNNdd
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dN4~ddNN4o
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357
the divariant field of stable co-existence of cordierite and almandine (Currie,
1971; Hensen and Green,1971) in metapelites, has been c o m p u t e d for each
of the rocks in Table II. A characteristic variation of the FeO/(FeO + MgO)
ratio among the rocks of the categories is observed in accordance with the
stability of the co-existing cordierite--almandine assemblage (Table III).
Fig. 5. Photomicrograph showing vermicular intergrowths of cordierite and hypersthene
with magnetite (black dots) rimming almandine in a cordierite--almandine--biotite gneiss.
crd -- cordierite, alm -- almandine, hyp -- hypersthene. Clearly note later formed hypersthene and cordierite (phase 2) vermicules protruding early formed cordierite (phase 1)
indicating the instability of the co-existing cordierite--almandine pair. It is a transformation of category 2 to category 1 owing to the same low pressure metamorphic overprint
mentioned under Fig. 4 (see Table I and the text).
Where cordierite is the only stable (Fe, Mg)A1 silicate phase (category 1) the
ratio is < 0 . 4 2 . For rocks with stably co-existing cordierite--almandine (category 2) the FeO/(FeO + MgO) ratio lies between 0.42 and 0.74. Where almandine is the only stable (Fe, Mg)A1 silicate phase (category 3) the ratio
exceeds the value of 0.74. Assuming a temperature of 700°C, after the calculations of metamorphic temperature for the Highland Group--Southwest
Group belt by Hapuarachchi (1975), the pressure range of stability satisfying the observed cordierite--almandine relationship in each of the rocks, has
been determined according to Fig. 6 (Table III). A pressure of 5.5--6
kbars at a temperature of 700°C satisfies the observed relationship between
cordierite and almandine in all the rocks studied. Thus, these values may be
taken as the equilibrium PT conditions of all cordierite and almandine-
358
bearing metapelites. A f t e r a s t udy of co-existing ferromagnesian minerals of
charnockites, and p y r o x e n e and hornblende-bearing granulites, J a y a w a r d e n a
and Carswell (1976) obtained a similar result (5--7 kbars, 700 + 50°C).
Hence, the estimated PT conditions could be regarded as the conditions prevailing during the peak m e t a m o r p h i s m of all Sri Lankan rocks.
TABLE III
A comparison of FeO/(FeO + MgO) ratios of cordierite and almandine-bearing metapelites of Sri Lanka, presented in Table II a
Category
1
No.
FeO
FeO + MgO
Stable
(Fe, Mg)A1 silicate
phase
1
2
0.36
0.42
crd
crd
3
4
5
6
7
8
0.42
0.48
0.55
0.67
0.72
0.74
crd--alm
crd--alm
crd--alm
crd--alm
crd--alm
crd--alm
9
10
11
12
0.75
0.81
0.85
0.90
alm
alm
alm
alm
P range of stability
(kbars)
< 5.5
< 5.8
5.5--7.5
5.1--7.2
4.8--7.0
4.4--6.5
4.3--6.1
4.2--6.0
> 5.7
>5.5
>5.3
>5.0
aThe observed (Fe, Mg) Al silicate phase in each rock and its pressure range of stability
according to Fig. 6, are also given, crd -- cordierite, alto -- almandine.
DISCUSSION
A co,existing cordierite--almandine assemblage has been n o t e d in metapelites o f m a n y granulite-facies terrains t h r o u g h o u t the world. Examples are
the Broken Hill region of Australia (Binns, 1964) and Lapland (Eskola,
1952). However, it is n o t restricted t o granulite-facies terrains. WynneEdwards and Hay (1963) summarized the worid-wide occurrences of cordierite--almandine and described the assemblage f r o m the amphibolite--granulite transition terrain o f southern Ontario, Canada. Hence, Winkler (1976)
p oin ted to the presence of c o e x i s t i n g cordierite--almandine from terrains
o f medium-grade, high-grade and granulite-grade metamorphism. Whatever
the terrain, the co,existence of cordierite and almandine is solely d e p e n d e n t
on the bulk chemistry of the pelitic rocks. Experimental studies by Hensen
and Green (1971) and Currie (1971) conf i rm this.
Th e variation o f F e O / ( F e O + MgO) ratio o f the metapelites o f Sri Lanka
and the variation of the stability o f co-existing cordierite--almandine also
359
point to the chemical control on this assemblage in the Highland Group-Southwest Group granulite belt. Metapelites with varying FeO/(FeO + MgO)
ratio appear to have undergone essentially the same set of conditions estimated above, during the metamorphic peak; the consequence being the wide
variety of assemblages presented in Table I. There is hardly any evidence for
a systematic spatial distribution of the assemblages in the field to suggest a
variation in physical conditions during metamorphism. Intermingling of cordierite, cordierite--almandine, and almandine-bearing assemblages in the
field, without argument, favour the chemically controlled origin under a
unique set of conditions. The metamorphic peak is either pre- or syn-D2
(Berger and Jayasinghe, 1976) as indicated by the deformed almandine and
cordierite in the gneissic fabric. Deformation D2 caused the foliation in Sri
Lankan rocks.
As suggested by De Waard (1966), the following two reactions would relate the observed parageneses in rocks of the category 2. Exact reactions pro-
XFeO
0.0
0.2
I
Kb
0.4
I
i
I
0.6
I
I
0.~
I
I
1.0
I
700°C
7
....
aim
crd
6
. . . . . . . . . . . .
P?
5
crd
4
cotegory I ---4
I-cotegory 2-~
tronsltionol
(lower)
I--- co tegory 3
tronsitionol
(upper)
Fig. 6. P -- X [= FeO/(FeO + MgO)] diagram for co-existing cordierite and almandine in
the presence of sillimanite and quartz at 700°C (after Currie, 1971), and its application
to the cordierite and almandine-bearing metapelites of Sri Lanka. P-effective load pressure
during the main metamorphic peak of Sri Lanka. p -- probable effective pressure during
the late metamorphic overprint.
360
ducing the parageneses of categories 1 and 3 are unknown. (Abbreviated
mineral names are as in Table I.)
bt + sm + q ~ crd + aim + kfp + H20
bt + q ~ crd + alm + h y p + kfp + H20
At a particular temperature the bulk chemical range of co-existence of
cordierite and almandine is pressure dependent (Fig. 6). A fall of effective
load pressure (at constant T), during any subsequent event, shifts the above
range laterally. Consequently, (1) co-existence of the two minerals is made
possible in some almandine-bearing rocks of category 3 with a higher FeO/
(FeO + MgO), where cordierite was originally absent during peak metamorphism. (2) In a few rocks with a smaller F e O / ( F e O + MgO), almandine
co-existing with cordierite during peak metamorphism (category 2) breaks
down to form assemblages bearing only cordierite (category 1). Therefore,
t w o types o f transformations take place at the upper and lower limits of
stable co-existence of cordierite and almandine, owing to a fall of effective
load pressure from that prevailing during peak metamorphism. F e O / ( F e O +
MgO) ranging between 0.42 and 0.74 for rocks with co-existing cordierite-almandine during the peak metamorphism of Sri Lanka, pelitic rocks of the
transitional category exemplify the transformations described above.
Transformations (abbreviated mineral names as in Table I)
(1) Category 3 to category 2 may proceed by the reaction, Fe-rich alm +
bt + sm + q + 02 ~- Fe-poor alm + crd + kfp + mg + H20 in rocks with FeO/
(FeO + MgO) just above 0.74, producing crd + kfp intergrowths around almandine, as shown in Fig. 4. Garnet will be progressively enriched in Mg, Fe
removed forming magnetite (mg) (and spinel in aluminous rocks), during the
reaction.
(2) Category 2 to category 1 may proceed by the reaction, alm + bt + 02
crd + hyp + kfp + mg + H20, in rocks with F e O / ( F e O + MgO) just above
0.42, producing crd + hyp intergrowths with kfp and mg around almandine,
as shown in Fig. 5.
As a consequence of these transformations, there is a second generation of
cordierite and hypersthene in metapelites. In some Finnish metapelites,
Hietanen (1947) also noted a second generation of cordierite, and a reaction
texture between K feldspar and cordierite, having formed biotite and sillimanite. Reinhardt (1968) also noted vermicular hypersthene occurring at
the contacts between cordierite and almandine, and some reactions of biotite
with cordierite and hypersthene, in a metapelite from the Gananoque area,
Ontario.
Such transformations are n o t restricted to the pelitic rocks of Sri Lanka.
Zoned minerals, symplectic intergrowths, overgrowths in minerals, corona
structures, and other types of reaction and replacement phenomena, indicating incipient stages of mineral reactions under the influence of a later retro-
361
gressive event, have been recorded in many other metamorphic rocks (Perera,
1983). Similar events can be recognized in granulite-facies terrains elsewhere;
e.g., Adirondack Highlands, Gore Mountains, Uganda (De Waard, 1967),
Minnesota (Himmelberg and Phinney, 1967), Tanzania (Coolen, 1980), and
Antarctica (Yoshida, 1979). Many such terrains exhibit responses to a declining load pressure, whereas few are indicative of increased load pressure,
following peak metamorphism. The later event in Sri Lanka is known as a
metamorphic overprint (Katz, 1971, 1972), which probably took place
under cordierite-granulite facies conditions. A low-pressure origin of all cordierite in pelitic rocks during this metamorphic overprint, as suggested by
Katz (1971, 1972) and Hapuarachchi (1968, 1975), is doubtful, since there
had not been complete re-crystallization of rocks during this event. The
known effects owing to this overprint are limited to intergrowths and reaction rims, as described above, in rocks of appropriate critical mineralogical
and chemical compositions.
Geochronology
Crawford and Oliver (1969), based on poorly defined isochrons, suggested
that the Highland Group formed under granulite-facies at 2000 Ma and has
been retrogressed to the amphibolite-facies Vijayan Complex at 1150 Ma.
Though attributed to two distinct events, the rocks dated during this study
show a wide range of ages between 700 and 3000 Ma. Five Rb/Sr whole-rock
age groups have been recognized; namely, (1) older than 2000 Ma, (2) between 1200 and 2000 Ma, and three groups apparently around (3) 1150 Ma,
(4) 900 Ma, (5) 650 Ma. Rocks older and younger than the isochron age, lying between 1150 and 3000 Ma, prevail within the Highland Group--Southwest Group granulite belt. Similarly, rocks younger, and older than the isochron age (1150 Ma) have been noted within the Vijayan Complex. The ten
pelitic rocks dated also give widely different ages like the other granulite
facies rocks. They include alm--bt and alm--bt--sm gneisses, and two cordierite-bearing gneisses. A less prominent event at 650 Ma, and a widespread
event at 450 Ma, have been recognized throughout the island in K--Ar hornblende and biotite ages. These ages presumably reflect the "metamorphic
overprint" described in the preceding section, with a time span of 200 Ma.
As suggested earlier by Cooray (1969), it could be a metamorphic--orogenic
episode which affected the contiguous parts of Gondwanaland. Though less
pronounced, its effects on the whole-rock ages of peak metamorphism are
not understood. The widely different ages of Sri Lankan rocks may sometimes be a consequence of radiogenic isotope losses during this event. However, since the appearance of Crawford and Oliver's (1969) data, attribution
of different whole-rock age groups mentioned above, to distinctive events,
based on the observed mineral assemblages and transformations of rocks, has
been the common practice of all workers on Sri Lankan Precambrian geology. For more details the reader is referred to Crawford (1969), Katz (1971,
362
1972, 1978), Hapuarachchi (1975, 1983), Cooray (1978), Munasinghe and
Dissanayake (1982) and Dahanayake and Jayasena.(1983). As described in
this paper, bulk chemical control on (1) the mineral assemblages during peak
metamorphism, (2) the mineral transformations during a subsequent less pronounced 'metamorphic overprint', in Sri Lankan rocks, has been increasingly
recognized. Thus, the long-standing confusion between the metamorphic history and geochronology would be an interesting topic for research.
CONCLUSION
The foregoing discussion suggests that there has been two phases of cordierite and hypersthene generation in Sri Lankan metapelites.
Phase 1: generation of cordierite-bearing assemblages represented by the
A'FM diagrams in Fig. 2a, b during the metamorphic peak of Sri Lanka,
under a pressure of 5.5--6 kbars at 700°C in rocks with an appropriate bulk
chemical composition.
Phase 2: formation of cordierite--K feldspar and cordierite--hypersthene
intergrowths around almandine, owing to an instability of the latter in some
rocks with a critical mineralogical and chemical composition, caused by a fall
of effective load pressure during a later metamorphic overprint.
The presence of cordierite in Sri Lankan metapelites does not necessarily
mean that the island has undergone the conditions of the cordierite-granulite
facies of Hietanen (1967) partly, as suggested by Hapuarachchi (1968, 1975)
and Katz (1971, 1972), to account a low-pressure origin for cordierite and
its distribution. Intermingling of cordierite, cordierite--almandine, and almandine-bearing assemblages in the field clearly demonstrates the chemically
controlled origin of cordierite and almandine. The comparatively more rare
occurrence of cordierite-bearing rocks within the Highland Group than in
the Southwest Group, is owing to lack of appropriate compositions. Hence,
the subdivision of the central granulite facies belt of Sri Lanka into the Highland Group and the Southwest Group, and the designated 'paired metamorphic belt' relationship between them, based on the presence of cordierite-bearing assemblages is doubtful. Thus, the presence of co-existing cordierite--almandine assemblage in the metapelites of the central granulite belt
of Sri Lanka is a key to the problems in her metamorphic history.
ACKNOWLEDGEMENTS
The author thanks Professor Th.G. Sahama, Department of Geology,
University of Helsinki, Finland, for kindly reading the manuscript, Mr.
K.R. Bogoda for his kind co-operation in the field, and Mr. S.M.B. Amunugama for drafting the figures.
363
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