Download The Precambrian Period

The Precambrian Period
Precambrian rocks in Thailand are metamorphic rocks found directly below the welldefined Cambrian sedimentary rocks containing distinctive Cambrian fossils. The rocks are
mostly regional metamorphic rocks and are easy to distinguish from other types of rocks in
Thailand. The typical grade of metamorphism is generally between medium to high
especially for that in gneiss as we can see segregation bandings in fresh rock faces. Together
with gneiss, schist, calcsilicate and marble are also the major species Precambrian rocks often
found as layers in the gneiss body. In some areas such as Hod district of Chiang Mai
province and Mae Sareang district of Mae Hong Son province, migmatite which is the
evidence of partial melting taking place during the metamorphic process is observed. Granite
can also be found associated with migmatite. The metamorphic rocks with this type of
history is common in areas from the northern region to the southern region and in some areas
in the eastern region of the country.
For the convenience of presenting the Precambrian rocks of Thailand, we classify the
rocks based on their locations and the result shows five major Precambrian rock regions.
These locations are (i.) The Northen and Upper-Western region, (ii.) The Eastern region,
and (iii.) The Lower-Western and Southern region (Figure 1.). Moreover, to avoid
repetition and confusion, we choose to discuss other issues regarding the Precambrian rocks
of Thailand as a whole. These discussions are about (iv.) Comparisons of rocks and their
compositions, (v.) Precambrian rock structures, (vi.) Relations between high-grade
Precambrian and low-grade Lower-Paleozoic metamorphic rocks, (vii.) Rocks derived
from metamorphism, (viii.) The age of the rocks and the age of metamorphism, and (ix.)
Types of rocks before metamorphism.
(I.) Precambrian Rocks in the Northern and Upper-Western
Region
Among the layers of Precambrian rocks in this region, the base of the rock unit
contains high-grade gneiss and schist. The calcareous substances such as calcsilicate and
marble are also present especially towards the top of the section. These two calcareous rocks
often contain their relict sedimentary structures, therefore they are called “paragneiss”. For
those metamorphic rocks that have evidences of partial melting, they usually contain granite
and are named “migmatite”.
Because most of the Precambrian rocks in this region has undergone tectonic
deformation, we observe the high-grade metamorphic rocks only in following limited areas.
(I.i) Mae Klang Waterfalls – Mt. Intanon – Area between Hod District and
Ob Luang National Park
The rocks found in this area are among the oldest ages in Thailand, phologopite
marble, quartzo-feldspathic gneiss, and schist. Along with these three rocks, veins of marble
and calcsilicates, porphyroblastic biotite gneiss and migmatite are sometimes found
intertwined in the rock bodies.
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Figure 1. The distribution of Precambrian rocks in various areas of Thailand.
Note: เชียงใหม - Chiang Mai, ตาก – Tak, กําแพงเพชร – Kampang Pet, อุทัยธานี – Utai Thani,
กาญจนบุรี – Kanchanaburi, กรุงเทพมหานคร – Bangkok, ชลบุรี – Chonburi, ประจวบคีรีขันธ –
Prachuabkirikan, นครศรีธรรมราช – Nakorn Sritammarat
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(I.i.i) Phologopite marble
This rock is generally 50 – 100 m. thick and placed at the topmost layer of the
Precambrian rocks. Its color is light grey to grey. The crystal grains are fine to coarse. In a
closer look, it contains quartz, phologopite and small layers of calcsilicate minerals. The
lower section of phologopite marble often contains more calcsilicate minerals. Major
constituents of this rock are calcite, dolomite, phologopite and small quantities of epidote,
quartz and actinolite.
At the quarry west of Tan Dokmai in Jomtong district, we find very distinguished
foliation and folds in the phologopite marble. Furthermore, we find rounded or ellipsoid
pegmatite or leucosome oriented parallel with phologopite grains. Leucosome contains
mostly feldspars with tiny amount of phologopite and quartz. However, leucosome from
some adjacent areas is found to contain epidote, actinolite, diopside and garnets in the rim of
leucosome adjacent to phologopite grains which are located just before the boundary between
this rock and the surrounding marble.
There is a report of black grey find-grain marble existing as thin layers along the Tak
– Mae Sot highway and at Lang Sang waterfalls (Campbell, 1975). This rock unit should be
similar to the phologopite marble as discussed above. The mineral composition is 90%
calcite and the rest consists of quartz, plagioclase-andesine, microcline, phologopite, sphene,
clinozoisite, epidote, blue hornblended, actinolite and subcalcic augite. The plagioclase often
shows oscillatory zoning. This marble contains two types of distinct rocks often found as
pods or rounded masses with diameter up to one meter. The first type is calcsilicate rock with
quartz and plagioclase making the rock look grey green to dark green. The other has the
same composition as granite with white grey color, semi-course grain and its crystal grains
have roughly similar size. There is no evidence of foliation in the granite. This type of
granite is usually found on top of the marble unit. Consequently, it can be inferred that the
top most layer of the marble was subjected to higher pressure than the lower part of the rock
in the past. The calcsilicate and pegmatite portion contains clusters of high-order foliated
area. The feldspars and pegmatite normally shows that they were experienced the force
making their bodies tilted in particular directions.
(I.i.ii) Quartzofeldspathic gneiss, biotite gneiss and schist
This rock unit is found right below the previous rock unit, the phologopite marble. Its
thickness ranges from 300 to 400 m. The top part of the unit consists of diobside marble,
calcsilicate, and garnet-diopside calcsilicate. This unit is cataclastic or has experienced
mylonitic deformation with a great magnitude. In general, the crystal grains are smaller than
the lower units. Some calcsilicate layers show tight fold to recumbent fold.
The bottom part of this unit is altered to layered schist and layered gneiss. The unit
also includes porphyroblastic quartz biotite schist, banded gneiss and augen gneiss.
Moreover, layers or lens of pegmatite rocks are sometimes present. The rock bodies are
semi-coarse at the lower part and become fine-grained at the top part. They also contain
white bands with garnet crystals. Large feldspar grains and lens of pegmatite are always
rotated to particular directions (Figure 2. – Figure 4.).
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Figure 2. Gneiss found
along the highway to mt.
Intanon, the highest peak in
Thailand.
Figure 3. The close-up
look of cataclastic gneiss in
mt. Intanon area.
Figure 4. Gneiss at Ob
Luang national park in
Chiang Mai province.
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The mineral compostion of calcsilicate is quartz, calcite, diopside, phologopite,
epidote, garnets and minute amount of other minerals. For the marble, there are only calcite
and diopside.
For gneiss rock, the major minerals are quartz, potash feldspar, plagioclase, biotite
and the minor minerals which are found in some sample only are muscovite, garnets,
sillimanite and cordierite. Other accessory minerals are apatite, monazite, zircon and dark
minerals such as magnetite, ilmenite and sulfides.
For pegmatite rock, it primarily consists of quartz, potash feldspar, plagioclase, and
biotite. In the white layer, we often find red garnet, apatite, zircon and small amount of dark
minerals.
(I.i.iii) Biotite gneiss and migmatite
The orientation of this rock unit is quite conformed with previous units. The
thickness exceeds 600 m. The rock unit is usually found at the bottom of the metamorphic
strata or found at the center of high-grade metamorphic rock body. The unit consists of
biotite gneiss, biotite-garnet-sillimanite gneiss and migmatite (Figure 5.). In gneiss, we
normally find quartz, potash feldspar, plagioclase and biotite, but sillimanite garnet and
muscovite are only found in some samples. The texture of the rock coarse and irregular or
heterogeneous. In some portion of the unit , we find orthogneiss. In migmatite especially in
its granitic part, pegmatite and aplite, we often find muscovite and/or large sheeted biotite
(diameter about 1-2 cm.). Furthermore, other minerals found in these rocks but only in small
amount are plagioclase, quartz, orthoclase, microcline, sillimanite, and the easy-to-identify
garnets.
Figure 5. Migmatite and biotite gneiss in mt. Intanon area.
(I.ii) Western Part of Chiang Mai Province
Metamorphic rocks found in this area include gneiss, schist, marble and calcsilicate.
The studies of Baum et al. (1970) and Braun (1969) indicate that this rock unit are mostly
derived from sedimentary rocks. Some portion of the rock had undergone anatexis and were
converted to granite by granitization process. They also believed that the metamorphic
process responsible for creating high-grade paragneiss and orthogneiss occurred in the
Precambrian. They argued that this rock unit was metamorphosed with a greater degree than
the rock of the lower Paleozoic which located above the Precambrian rock in great quantity.
The granitization and melting should take place in the Carboniferous.
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Moreover, they added that migmatite were normally found in the lower part of biotite
+/- garnet zone and biotite – garnet +/- sillimanite zone because their observations showed
that granite were present in larger quantity than the rest of the zone.
(I.iii) Bhumibhol Dam Area
High-grade metamorphic rocks found in Bhumibhol dam area were studied by
Natalaya (1974), Piyasin (1974) and Bunopas (1980). The finding was several rock units
from youngest to oldest as follows.
(I.iii.i) Mica Schist
Mica schist with thickness around 700 m., color grey to brown grey, are present. The
rock contains mostly quartz, plagioclase, biotite and muscovite. Other minor minerals are
garnets, andalusite, cordierite, penite and chlorite and accessory minerals are apatite, zircon,
tourmaline, and dark minerals (Figure 6.)
Figure 6. The illustration of crenalation in quartz-mica schist located at the Bhumibhol dam.
(I.iii.ii) Marble and Calcsilicate
This rock unit is about 500 m. thick. Its color varies from white to grayish green.
The texture is dense, compact, coarse and granular. Minerals consisted in the rock, mostly
calcite, are well-oriented. One may find plagioclase, biotite, phologopite, epidote, soilsite
and amphibole. Accessory minerals, tremolite, diopside, garnets, tourmaline and dark
minerals are sometimes found in this rock unit.
(I.iii.iii) Biotite-microcline Gneiss
This rock unit is rather thick, 1,200 m. It consists of mostly microcline and biotite.
The less abundant minerals in this rock are quartz, and muscovite. Minute quantities of
sillimanite, epidote and chlorite are also expected. Apatite, zircon, tourmaline and dark
minerals are rarely present.
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(I.iv) Lansang Area and Along the Tak – Mae Sot Highway
Metamorphic rocks in Lansang area are gneiss, schist, calcsilicate and marble. These
rocks can be seen at several waterfalls in Lansang national park and along the highway
between Tak city and Mae Sot district around the kilometer post 12 to the kilometer post
21.5. In the studies of Campbell (1975) and Bunopas (1976), they discussed about this rock
unit a follows.
The metamorphic rock, quartz-feldspar-biotite gneiss, covers about 60 -80 % of all the
rocks found in this area. The gneiss is often found associated with lime silicate (covers about
10 – 20%) and igneous rocks with some degree of metamorphism (covers about 10 – 2-%).
Moreover, they reported gneiss that had undergone some erosion processes becoming
cataclastic gneiss in the easternmost part of the national park (Figure 7. and Figure 8.).
For some part of this rock unit that is leucocratic granite, it is suspected that the rock’s
origin is melting or anatexis. The granite is found penetrating along the mineral grain. Some
portion developed to pegmatite and the rest was cutting through mineral grains of gneiss and
granite gneiss.
Figure 7. Illustration of folding and recrystallization of calcsilicate rocks of Precambrian age
observed in Ban Tak district, Tak Province.
Figure 8. Ptymatic fold and foliation patterns observed in gneiss in Tak province.
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(I.v) Ban Rai District Area
High-grade metamorphic rock unit located in the western part can be seen in the
geological map of Thailand with 1:250,000 scale that the unit extends south of Tak province,
Kampang Pet province and ends at Ban Rai district, Utai Thani province. This rock unit
contains biotite schist, calcsilicate and biotite marble. Silimanite mineral can be found in
schist as well (Campbell, 1975).
(I.vi) Kanchanaburi Province and Western Supanburi Province Area
The metamorphic rocks found at Tapsila village, River Kaew Yai and mt. Chon Kai
belong to the same rock unit. They are significantly different from the mt. Chong Insee
metamorphic rock unit which is located in Bo Ploy district, Kanchanaburi province. These
two rock units contain slightly similar rock, quartz-feldspar-biotite gneiss. The major
difference is that gneiss of mt. Chong Insee rock unit consists of silimanite mineral and also
has some amount of calcsilicate rocks (Figure 9.) (Bunopas, 1976).
Figure 9. The close-up look of quartz-feldspar-biotite gneiss in Saiyoke district,
Kanchanaburi province.
Metamorphic rocks found in Tamaka district, Kanchanaburi province (Figure 10.), is
essentially the extended portion of the River Kaew Noi – Lin Tin village in Tongpapoom
district metamorphic rock unit. This rock unit consists of sillimanite schist, paragneiss,
calcsilicate, granite gneiss containing fibrolitic sillimanite crystals that are not well-oriented.
This feature is perhaps the result of granitic intrusion (Dheeradilok et al., 1985). Sillimanite
minerals are also limited in the confined area parallel to the boundary of granite rocks.
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Figure 10. Schist with calcsilicate-compositioned passive flows which contain welldeveloped fractures along the fold axes in Tamaka district, Kanchanaburi province.
(II.) Precambrian Rocks in the Eastern Region
Rocks of regional metamorphism origin found in the eastern part of Thailand are
prominent in Chachengsao, Chonburi, and Rayong provinces. These areas were named
“Chonburi massif” after Campbell (1975) and Areesiri (1982). The massif is oriented
northwest – southeast. Reports on metamorphic rocks in this area are limited to studies of
Brown et al. (1951) and Nakinbordi et al. (1985). The detailed geology of mt. Chao, Nong
Yai district, Chonburi province reported by Areesiri (1982) is as follows.
(II.i) Mt. Chao Area, Panasnikom District – Nong Yai District, Chonburi
Province
High-grade metamorphic rocks in the area ordering from the youngest to the oldest
are;
-
Biotite-diopside gneiss
Biotite-sillimanite gneiss
Hornblend-diopside gneiss
Biotite gneiss
Biotite-hornblend gneiss
Biotite-feldspar-quartz gneiss.
(II.i.i) Biotite-diopside gneiss
This rock unit is usually fine-grained. In some areas, there are intrusions of biotitehornblend-diopside gneiss, amphibolite and biotite gneiss. The main constituents of the
biotite-diopside gneiss are diopside, biotite, plagioclase, potash feldspar, quarts and minute
quantities of apatite and zircon.
(II.i.ii) Biotite-sillimanite gneiss
The upper part of this rock unit is sillimanite-biotite-potash feldspar augen gneiss,
while the lower part contains garnet-biotite-sillimanite-quartz schist. Cordierite minerals are
expected to be in the lower part of the rock unit as well.
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(II.i.iii) Hornblend-diopside gneiss
The texture of this rock unit is fine to semi-coarse. Typical minerals found in the rock
are hornblend and diopside in a large percentage. Potash feldspar, apatite and sphene are
minor minerals in this rock. In some areas, biotite-hornblend-diopside gneiss, amphibolite
and biotite-sillimanite schist are found accompanying the hornblend-diopside gneiss rock
unit.
(II.i.iv) Biotite gneiss
The texture of this rock unit is also fine to semi-coarse and usually found right below
the hornblend-diopside gneiss unit. Major minerals in this rock unit are quartz, potash
feldspar, plagioclase and biotite. Minute quantities of zircon, magnetite and apatite are also
present.
(II.i.v) Biotite-hornblend gneiss
The texture of this rock unit is semi-coarse to coarse. There are small amount of
amphibolite, calcsilicate and biotite-sillimanite schist present with this biotite-hornblend
gneiss rock unit. Minerals with great quantities in this rock unit are biotite, hornblend,
plagioclase, quartz, potash feldspar and garnets. Minerals with lesser amount present are
apatite, zircon and magnetite.
(II.i.vi) Biotite-feldspar-quartz gneiss
This type of gneiss are found in layers sandwiched between semi-coarse and coarse
grained rocks. We often find thin amphibolite rock layers associated with this rock unit. In
some areas, this rock was partially converted to migmatite, and veins of amphibolite were left
in the rock in a discontinuous fashion (Figure 11).
Figure 11. Biotite-feldspar-quart gneiss in the quarry in Nong Yai district, Chonburi
province. Notice the presence of migmatite and the isoclinal fold with NE-direction axial
plane.
Beside these six high-grade metamorphic rocks, Areesiri (1982) also distinguished
marble-calcsilicate unit and amphibolite unit from them. Descriptions of both rock units are
as follows.
Marble-calcsilicate: Marble is often formed into lens accompanying biotitehornblend gneiss. Major minerals in this marble are calcite, dolomite, and phologopite. For
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calcsilicate, the nature of origin is similar. Its major minerals are diopside, calcite, and
sphene.
Amphibolite rock unit is found in layers whose thickness ranges from 2 cm. to 10 m.
It is commonly found intertwined with all types of gneiss. Classification of amphibolite is
based on its composition such that amphibolite contains hornblend, plagioclase, quartz,
apatite, and sphene. Others are called using this classification rule as biotite amphibolite,
diopside amphibolite, garnets amphibolite, and hornblend amphibolite.
(II.ii) Area North of Chamao Mountains in Glang District, Rayong
Province
Rocks in this area are also high-grade Precambrian metamorphic rocks. This rock
unit is located in the easternmost part of the country and is controlled by faulting making the
outcrop narrow and long. Several rocks such as mica schist (Figure 12.), hornblend schist,
calcsilicate, biotite-muscovite granite, pegmatite, aplite with large garnet crystals, belong to
this rock unit.
We often find gneiss and mica schist in small hills between mt. Chamao and the
municipal center of Glang district.
Figure 12. Porphyroblastic garnet schist in the mt. Chamao area, Glang district, Rayong
province.
(III.) Precambrian Rocks in the Lower-Western and Southern
Region
High-grade metamorphic rocks found in this region of Thai peninsula were reported
in Hua Hin district and Pran Buri district of Prachuabkirikan province (Campbell, 1975, and
Pongsapich et al., 1980), in Tab Sakae district of Prachuabkirikan province, and in Ranong
province (Campbell, 1975). Another study (Lamjuan, 1977) also indicated their presence in
Kanom district and Sichon district of Nakorn Sritammarat province. Classification of rock
units and their major features are discussed in the following sections.
(III.i) Area in Pran Buri District of Prachuabkirikan Province
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The study by Pongsapich et al. (1980) showed metamorphic rock units in a
stratigraphic sequence from youngest to oldest as;
- Marble
- Quartzite
- Calcsilicate, quartzofeldsparthic and marble
- Metapelite and gneiss.
(III.i.i) Marble
The typical color of this marble rock unit is light grey to white. Chert with diameter
from 2-3 cm. up to 35 cm. can be occasionally found on top of quartzite, but there is no
evidence for the boundary. This occurrence of rocks can be seen at mt. Karok and north of
mt. Huatam. Major minerals in this marble rock unit are calcite and dolomite.
(III.i.ii) Quartzite
The quartzite rock unit of this area has fine-grained to semi-coarse-grained texture.
The unit contains calcsilicate and quartzite. Major minerals composed in this unit are green
hornblend, glossularite, sphene, and plagioclase. In some samples, small amount of zoizite
and clinozoizite is expected.
(III.i.iii) Calcsilicate, quartzofeldsparthic and marble
These rocks generally occur as strata with semi-coarse textures. The calcsilicate is
observed to gradually transform its compostion to be quartzofeldsparthic. Major minerals in
this rock unit are diopside, plagioclase, microline, quartz, calcite, tremolite, hornblend, and
sphene. Minute quantities of scapolite and biotite are also present.
(III.i.i) Metapelite and gneiss
This rock unit consists of mica-sillimanite schist and gneiss. Mineral bands showing
the evidence of metamorphism are clearly observed. Schist contains quartz, biotite,
sillimanite, and garnets. In some schist samples, there are orthoclase and/or cordierite. For
gneiss, it usually contains quartz, biotite, microcline, garnets, sillimanite, orthoclase, and
plagioclase (An20 to An30). Orthoclase feldspar grains are generally big with ellipsoid shapes.
(III.i) Area in Kanom District and Sichon District of Nakorn Sritammarat
Province
High-grade metamorphic rocks in the Thai peninsula were also reported in Nakorn
Sritammarat province (Nakinbordi et al., 1985). The late study of Kosuwan (1996) added
more information about the rock unit of this area. These studies divided the rocks in to two
rock units locating at the top and bottom;
- Kao Yoi schist unit
- Ni Plaw beach gneiss unit (Figure 13.).
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Figure 13. Some features indicating that this gneiss rock belongs to the Ni Plaw beach gneiss
rock group found in Kanom district, Nakon Sritammarat province.
(III.ii.i) Kao Yoi schist unit
This rock unit consists of schist, quartzite, small lens of calcsilicate and marble. The
schist unit actually contains mica schist, muscovite-garnet schist, and quartz-mica schist. The
quartzite unit also contains fine-grained quartzite, and mica quartzite.
Minerals found in the schist unit are quartz, muscovite, orthoclase, biotite, garnets and
stibnite. For calcsilicate rock, diopside, quartz, plagioclase, actinolite, tremolite, calcite,
epidote and hematite are present.
(III.ii.ii) Ni Plaw beach gneiss unit
This gneiss rock unit actually contains intertwined layers of biotite gneiss and
sillimanite gneiss. The texture of the rock unit is uneven. In the rock body, the coarsegrained portion is porphyroblastic while the fine-grained portion contains uniform mineral
crystals with intrusions of calcsilicate. The latter aggregate causes white, green and brownish
purple bands of colors. In gneiss, there are potash feldspar (orthoclase), and microcline
minerals that often engulf mica, quartz, and sillimanite. Other minerals found in gneiss are
quartz, biotite, plagioclase, muscovite, sillimanite and garnets. For sillimanite, we find that it
is fibrolite with needle-like shapes embedded in quartz and biotite crystals. We also observed
the rotation of garnet crystals which drag quartz and biotite crystals along.
In calcsilicate, we find that it was altered from gneiss. Minerals contained in
calcsilicate are diopside, quartz, plagioclase, calcite, sphene and actinolite-tremolite.
There was a report on finding pegmatite and aplite with clear foliation occurring as
dikes penetrating this Ni Plaw beach gneiss unit. These rocks consist of quartz, potash
feldspar, muscovite and garnets.
Kosuwan (1996) also observed that there are Tong Yang gneiss rock unit which
composes of gneissic granite or orthogneiss rather than paragneiss, Mt. Datfah granite unit,
and finally Mt. Pret granite unit intruding Kao Yoi schist and Ni Plaw beach gneiss rock
units.
(IV.) Comparison of Rocks and Their Compositions
Pongsapich et al. (1983) reported that all high-grade metamorphic rocks in Thailand
have the roughly the same features. The lowermost layer consists of orthogneiss and
paragneiss and the subsequent layers all the way to the top are schist, calcsilicate, quartzite
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and marble respectively. Quartzite and marble are only present in some particular portion of
the rocks.
Outcrops in the Northern and Upper-Western region contain very similar stratigraphic
sequences of Percambrian rocks. The lowermost part is coarse-grained gneiss with
embedded lens of marble. Granite, migmatite or orthogneiss are found in this same
lowermost part of some outcrops. In the middle of the Precambrian strata, gneiss and schist
with smaller mineral crystals are present. The upper strata contains higher quantities of
calcsilicate and marble than the rest. The rocks of this section are cataclastic or show distinct
evidences of mylonization. The feldspar crystals are often rotated, and in some portion are
crushed forming mylonite rocks. Near the upper Precambrian boundary, they observed grey
marble with phologopite and feldspar crystals emerging in clusters. Quartzite is only present
in small amount usually in Hod and Jom Thong districts.
In the Eastern region, areas around mt. Chao in Panasnikom district and Nong Yai
district, Chonburi province have paragneiss outcrops. The composition of paragneiss is
toward mafic because the contained minerals are mostly hornblend, diopside, biotite, red
granet, and calcic plagioclase. This area contains amphibolite outcrops in the largest amount
in the country (Areesiri, 1982). The lower portion of paragneiss often consists of migmatite.
There are schist outcrops with sillimanite in the area south-west of mt. Chao. The schist is
normally located right on top of paragneiss (Salyapongse et al., 1997). There was also a
report of biotite-muscovite granite showing clear foliation of minerals in the same area.
In the Lower-Western and Southern region specially the areas in Hua Hin and
Pranburi districts of Prachuabkirikan province. Dheeradilok et al. (1985) reported quartzite
beneath marble and calcsilicate rock starta. The lower strata contain schist and gneiss. For
areas in Nakorn Sritammarat province, Lamjuan (1979), Sananseang et al. (1985) and
Kosuwan (1996) agreeably reported that areas in Kanom and Sichon districts contain an
outcrop of gneiss in the east, and an outcrop of schist in the west. Biotite-muscovite gneissic
granite was found occasionally and in small quantity.
The final picture of the Precambrian strata is as follows.
- The topmost stratum: Marble and calcsilicate and sometimes quartzite are present.
Major minerals of marble and calcsilicate of this layer are calcite, dolomite,
phologopite, epidote, actinolite-tremolite, plagioclase, potash feldspar, zoizite,
diopside, and garnets (Puttaphiban et al., 1987).
- The middle stratum: This layer is generally called “the paragneiss stratum.” It
consists of schist, calcsilicate, marble and mylonite. In paragneiss and schists,
major minerals are quartz, potash feldspar, plagioclase, biotite, sillimanite,
garnets, andalusite and cordierite. In calcsilicate and marble, the contained
minerals are quartz, calcite, diopside, phologopite, epidote and garnets. There is a
remark for the presence of cordierite in paragneiss. This mineral is always found
embedded in paragneiss’ layers with many biotite crystals (Macdonald et al.,
1992). Other minerals that are often observed with this type aggregate are
muscovite, garnets, and sillimanite. Moreover, there was a report of discovering
schist with andalusite and cordierite coexisting at Bhumibhol dam (Natalaya,
1973). This schist was suspected to be at the top of this stratum.
- The lowermost stratum: Anatexite, migmatite or orthogneiss are terms used to
describe the same high-grade metamorphic rock unit found in this section. This
rock was undergone so severe metamorphism that it melted back to granite
composition. However, paragneiss is commonly present. In orthogneiss, the
contained minerals are quartz, plagioclase, microcline, orthoclase, biotite,
muscovite and sillimanite (Macdonald, et al., 1992). For migmatite and mixtures
between anatexite and paragneiss, it was found that quartz, potash feldspar,
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plagioclase, biotite, muscovite, garnets, and sillimanite are normally present
(Mantajit, 1975; Puttaphiban, et al., 1987). Pegmatite and aplite are coexisting
and contain quartz, potash feldspar, muscovite, and biotite. These minerals are
usually oriented in veins parallel with the strata.
Another important feature of the Precambrian rock is the clear difference between
dark mineral lens and light-colored mineral lens. Moreover, the difference in sizes of these
two types of mineral lens is also observed.
Precambrian rocks in areas beyond the above discussion rather have acid composition
while those of the Northern region have mafic composition. This conclusion is reached after
comparing contents of minerals. This rock has smaller amount of potash feldspar, quartz, and
muscovite than the paragneiss unit does.
Granite, actually biotite-muscovite granite, is also present in the Precambrian rock
units. This granite usually contains different-sized minerals and often shows foliation.
Tourmaline, muscovite, garnets, pegmatite and aplite generally coexist in this granite.
Furthermore, contact metamorphic rocks are found because the intrusion of granite in
the Precambrian rock layers. This contact metamorphism occurred after the great regional
metamorphism of the area. Mineral composition of the rocks provides explanations of this
conclusion. Some special minerals required pressures and temperatures caused only by
contact metamorphism to be made such as orthopyroxene (enstatite), diopside, anorthite,
hornblend, olivine, and potash feldspar (Mantajit, 1975). Other minerals with this special
origin also are wollastonite, grossularite, plagioclase, calcite, and diopside (Macdonald et al.,
1992).
(V.) Precambrian Rock Structures
Puttaphiban et al. (1987) concluded the nature of metamorphism and structures of the
high-grade metamorphic Precambrian rocks in Jomtong and Hod districts of Chiang Mai
province in chronological orders as the followings.
1.) The first regional metamorphism generated several high-grade metamorphic rocks
at the same time as the fist schistosity. In the lowermost stratum, there was
melting which generated granite with intrusions of pegmatite and aplite. Minerals
in these intruded rocks are aligned with the direction of schistosity.
2.) The first cataclastic deformation was found mostly in the east of the area and it
occurred together with the first schistosity. Evidences supporting this event were
the rotation of feldspar crystals and the presence of overturned isoclinal fold.
3.) The second regional metamorphism also occurred because there were large garnet
crystals penetrating the rock fabric resulting from the first schistosity. This can be
found in feldsparthic schist.
4.) The second cataclastic deformation was found in small zones that have high dip
angles. This second-time deformation is not parallel with the first one.
5.) Thrust faults also exist which can be seen from the westward movement of
Ordovician limestone and this limestone is found covering the youngest
Precambrian rock unit, the phologopite marble, at mt. Mokalan. They further
discovered a similar configuration of thrust fault west of Pae village. In this area,
the Ordovician limestone covers the red rock which is believed to be of MesozoicTriassic age. Therefore, the age of this latter observed thrust fault must be less
than that of the red rock.
For the high-grade metamorphic rocks of Eastern region,Areesiri (1982) reported at
least three events of rock transformations and two events of metamorphism. The first rock
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transformation coincided with the first metamorphism in the late Carboniferous. This
combined event was responsible for the formation of Hercynian orogeny. The second and
last rock transformations also coincided with the second metamorphism during the Permian
and Triassic generating Permo-Triassic orogeny. The mineral evidences of the first
metamorphis were usually obscured by the following events. However, because veins of
amphibolite found as penetrated veins are parallel with axial plane cleavage of isoclinal fold
which was clearly derived from the second metamorphism, it can be inferred that the first
metamorphism had previously occurred in the rock (Figure 14.). The second metamorphism
caused the melting to crystallize granite and generations of pegmatite and aplite.
Figure 14. The growth of sillimanite crystals penetrating through folded biotite and
quartzitic-mica schist.
Kosuwan (1996) reported the structure of high-grade metamorphic rocks in Kanom
district of Nakorn Sritammarat province that there were foliation and schistosity with inclined
angles to the beddings remaining which can be concluded from differences in composition
between quartz-mica schist and mica schist. In the adjacent areas of foliation, slip cleavage
or crenulation cleavage are present.
In paragneiss, he observed overturned isoclinal fold, while in pegmatite-aplite, he
found ptygmatic fold. Interesting structures of this Precambrian rock unit are the followings.
1.) The first regional metamorphism coincided with the formation of several foliated
metamorphic rocks, foliated granite, and intrusions of pegmatite, and aplite.
These rocks share the same direction of foliation that is parallel to the original
sedimentary strata. Overturned isoclinal fold and cataclastic deformation were
also observed to follow this regional metamorphic event.
2.) The existence of the second regional metamorphism is controversial. Evidences
are unclear such that they only indicate small metamorphism events causing lowgrade metamorphic rocks. Moreover, mineral veins in amphibolite observed as
intrusions in mica can not be used to support the second regional metamorphic
event (Areesiri, 1982).
3.) The second cataclastic deformation clearly occurred.
4.) Thrust faults are suspected to form after the Triassic.
(VI.) Relations between High-grade Precambrian and Low-grade
Lower-Paleozoic Metamorphic Rocks
16
The reasons behind the uncertainty of the relations between these rocks are that the
rocks are usually covered by Quarternary sediments, there are some faults cutting through the
rock strata or the contact between these rock units is replaced totally by granite. With these,
the interpretation finally leads to the presence of an unconformity (Braun, 1969; Baum et al.,
1970; Natalaya, 1974; Campbell, 1975; Bunapas, 1976; Dheeradilok and Lamjuan, 1978;
Puttaphiban et al., 1987)
After investigating types of rocks in the high-grade Precambrian rock unit and the
low-grade Paleozoic rock unit from reports of Baum et al. (1970) and Mantajit (1975), we
find that the upper Precambrian rock consists of calcsilicate, marble, and chert-like
quartezite. In the mean time, lower-Paleozoic rock units (Cambrian unit and Ordovician unit)
contain siliciclastic-pelitic-carbonate rocks with mineral content that looks just like that of the
upper-Precambrian rocks.
There was a report of a suspected unconformity between the high-grade Precambrian
metamorphic unit and the lower-Paleozoic unit located west of Chiang Mai province because
there was a layer of pebble sediments separating the two strata (Braun, 1969). Later,
Putthapiban et al. (1987) argued that there was a movement of pebbles caused by block
faulting mechanism instead.
Order of metamorphism relation between these two rock units was discussed in the
study by Macdonald et at. (1992). This study states that there is the lower-Paleozoic rock
unit found in mt. Inthanon area contains biotite schist covered by fine-grained marble. The
marble layer is continuous and extends to the limestone unit with known Ordovician fossils.
In lower-Paleozoic rocks, garnets often coexist with andalusite crystals. In the marble layer,
typical minerals found in amphibolite are absent. The biotite schist is derived from high
temperature greenschist facies. In the mean time, there were many reports on this
Precambrian metamorphic rock unit that overwhelmingly argued for the low-grade
metamorphism occurring only in a limited amphibolite stratum (Mantajit, 1975; Putthapiban
et al. 1987; and Macdonald et al., 1992). These studies extensively state that the calcsilicate
unit, the topmost layer, contain calcite, dolomite, actinolite-tremolite, phologopite, epidote,
plagioclase, potash feldspar, and diopside. In the Bhumibhol dam area, Tak province, there
was a report of andalusite and cordierite in the upper-Precambrian rocks (Natalaya, 1974).
From these data, the orientation of the metamorphosed layers must be parallel to the
original strata. The supporting evidences are the gradual increase of rock age from the
Precambrian to the lower Paleozoic, the uniform types of metamorphic rocks, and the
sameness in grades of metamorphism observed in nation-wide areas. The scenario where
high and medium-grade metamorphic rocks created in the Precambrian extending to the
lower Paleozoic strata is concluded. In these rock strata, there was perhaps a single event of
regional metamorphism (Dheeradilok et al., 1985; Chanmee et al., 1981; Macdonald et al.,
1992 and Salayaphong, 1996).
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(VII.) Rocks derived from metamorphism
อาจารยครับ section นี้ยากมากสําหรับคนที่มี Petrology background ไมคอยดีอยางผม
ผมตองขออนุญาตเวนไวนะครับ หัวขอ 3.7 หินแปรที่เกิดจากกระบวนการแปรสภาพ เริ่มตั้งแตหนา 48
หมด 51
17
สุทัศชา
วันจันทรที่ 19 กุมภาพันธ พ.ศ. 2550 เวลา 11:00 น.
Figure 15. Graph showing the melting curves of several mixtures of minerals that are major
constituents of the Precambrian metamorphic rocks.
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(VIII.) The Age of the Rocks and The Age of Metamorphism
The age of the Precambrian rocks is defined as the age of the original sedimentary
rocks before metamorphism occurred. To determine the age, grade of metamorphism,
stratigraphic sequence of the rocks and radioactive isotope age data are considered. With the
fact that Precambrian metamorphic rocks have the highest grade of metamorphism, it
becomes easier to distinguish the Precambrian rocks from the adjacent rocks that are also
metamorphic but at a much lower grade of metamorphism. Moreover these adjacent rocks
often contain lower Plaeozoic fossils because the mild metamorphis did not destroy all of
them. The lothology of these two types of rocks such as the contact between the younger and
older rocks also helps. Occasionally, we observed granite dikes right at the contact. With
these evidences and logical considerations, we conclude that the high-grade metamorphic
rocks are older and are the original materials which later became sediments during the lower
Paleozoic (Baum et al., 1970; Natalaya, 1974; Bunopas, 1983; Pongsapich et al., 1983 and
Puttaphiban et al., 1987). The studies of Dheeradilok (1975) and Janmee (1981) on
Precambrian rocks in Tamaka and Muang districts of Kanchanaburi province respectively
conclude in agreement that the lithology observed in green schist indicates that this schist had
continued eroding and making sedimentary rocks up to the Cambrian and Ordovician.
The radioactive isotope age data and the mineral composition of the regional
metamorphic rocks really help us understand the history and evolution of the rocks.
Macdonald et al. (1992) and Dunning et al. (1995) reported the preliminary age of
orthogneiss, the age of granite that evolved during metamorphism and the age of cataclastic
deformation. The conclusion is that the regional metamorphism in Thailand only occurred
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once in the late Cretaceous. They further hypothesized that the age metamorphic rocks in
Thailand was perhaps late Precambrian – early Cambrian.
The geology section of the Department of Mineral Resources of Thailand has
supported Institute for Geology and Dynamic Lithosphere of Gottingen, Germany to perform
research on age determination of minerals contained in the previously described metamorphic
rocks, and of minerals contained in the low-grade metamorphic cover units in various areas
since 1992. The method used in the research is the measurement of radioactive isotope ratios
of U/Pb, Sm/Nd, Rb/Sr and K/Ar for crystalline samples. For sediments or low-grade
metamorphic rocks, only K/Ar is used to determine the age of the rock. Results (Mickein,
1997) indicate that there were two events of regional metamorphism occurred in the rock
history. The first event was right at the Triassic/Jurassic boundary (approximately 200 Ma)
which was very severe regional metamorphism event resulting as the gneiss forming event.
This conclusion is drastically different from the previous results obtained by Mcdonald et al.
(1992) and Dunning et al. (1995) which confirmed the single regional metamorphic event
scenario only during the Cretaceous. The second regional metamorphic event took place
during the Cretaceous (approximately 117-72 Ma) from the U/Pb data of monazite minerals.
This second event was a milder one that did not alter the pre-existing rock fabric. There was
also an uplift event in the recent Oligocene/Miocene which indicates the existence of several
large thrust faults in the region. Up to the present, there has been no evidence indicating the
metamorphic event in the Precambrian.
The age of minerals composed in original sedimentary rocks before undergoing highgrade regional metamorphism in the Triassic and Jurassic is found to be between 1,000 –
2,000 Ma which marks the lowermost part of the sediments. The age of the topmost
sediments is perhaps Cambrian and Ordovician (approximately 500 Ma). These topmost
sediments belong to Tarutao and Tung Song rock units which occur adjacent to the highgrade metamorphic rocks. Therefore, the age of the Precambrian rocks falls between 1,000 –
500 Ma.
(IX.) Types of Rocks before Metamorphism
Baum et al. (1970) argued that the original rocks before becoming the Precambrian
metamorphic rocks in Thailand were sedimentary rocks such as limestone. Moreover, with
the discovery of quartz crystals in pebbles with gneiss composition, he concluded that the
original rocks must be very old and contain a lot of quartz for example gneissic granite.
Mantajit (1975) proposed that paragneiss were derived from impure arenaceous
sediments such as subgraywacke sandstone or subarkosic sandstone with shale. For schist, it
was metamorphosed from shale with sandstone. For quartzite, it was derived from sandstone
with high percentage of quartz minerals. Chert, calcsilicate and marble were transformed
from limestone and limestone with small impurities.
Puttaphiban et al. (1987) reported that the Precambrian rocks were derived from
subgraywacke sandstone, shale, and pyroclastic rocks. The original rocks at the top of the
sediment stratra were expected to be calcareous sediments and limestone with some degree of
impurity.
Macdonald et al. (1992) discussed about the Thai Precambrian rocks that because the
major rock component of the strata is orthogneiss, the original rock type should be granite.
He did not provide the origin of these granite rocks whether they came from melting or from
the crystallization of magma in the upper mantle.
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