The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 1247
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
1 Assistant Lecturer, Geology Department, University of Yangon. 2Lecturer, Geology Department, University of Kyaukse. 3Professor (Ret), Geology Department, University of Mandalay.
4Professor & Head, Geology Department, University of Mandalay.
Petrochemistry and Petrogenesis of Metamorphic Rocks Exposed at
Kyaukkyi-Onbaing Area, Thabeikkyin Township, Mandalay Region
Tin Moe Zar Chi1, Khin Pyone
2, Ali Akbar Khan
3 and Than Than Nu
4
Abstract
Kyaukkyi - Onbaing area is situated at the west of Mogok and 177 km North of Mandalay. It covers approximately 198 km2. The metamorphic rocks are mainly exposed the garnet-biotite
gneiss, marbles, calc-silicate rocks and skarn. The study area was subjected to at least two
metamorphic processes: high grade regional metamorphism and contact metamorphism.
Typical index minerals such as diopside and forsterite found in varieties of marbles and calc-
silicate suggest that metamorphic facies reach upper amphibolites facies. Wollastonite
observed in skarn rocks is an indicator for pyroxene-hornfels facies. Mg/(Mg+Fe2+)] is
between 0.53 – 0.57, that mean low XMg indicating that garnet biotite gneiss might not be
orthogneiss and the gneisses in the study area genetically comes from paragneiss. The
protolith for the metacarbonate sequence can be well correlated with the Plateau Limestone
Group in both northern and southern Shan States of Paleozoic in age. But the older metapelite
(Mogok Gneiss) does not uncertain as the age could be older than metacarbonate. Therefore,
the age of the metamorphic rocks in the study area is Paleozoic with some parts of Jurassic. The time of metamorphism could be assigned as Late Eocene to Middle Miocene ages.
Keywords: garnet, isograds, metacarbonate, pelitic, upper amphibolites facies
Introduction
Kyaukkyi-Onbaing area is situated west of Mogok and about 177 km North of
Mandalay. It is bounded by the Latitude 22°54′ N to 23° 02′ N and Longitude 95° 58′ 00′′ E to
96° 06′30′′ E, in one - inch topographic maps of 93 B/1, 93 A/4, 84 M/16 and 84 N/13. It
covers approximately 198 km2 of rugged terrain. The location map of the area investigated is
shown in figure (1).
Regional Geologic Setting
The Mogok Metamorphic Belt (Searle and Haq,1964) consisting of metamorphic
rocks accompanied with various igneous emplacements, has an average width of 10-13 km
and trends ENE-WSW in the Mogok area, and N-S in the Thabeikkyin area (Myint Lwin
Thein et al., 1990). This belt as a whole lying along the western edge of the Shan Plateau is
situated between Tertiary sediments of Central lowland and Pre-Paleozoic rocks of the Shan-
Thai Block.
The Mogok Metamorphic Belt (MMB) extends over 1500 km along the western
margin of the Shan-Thai Block and southwestern Flank of Himalaya. It extends from
Andaman Sea to the eastern Himalayan Syntaxis. This belt contains intrusive igneous rocks
(mainly granitic rocks) and high-grade metamorphic rocks, such as gneisses, marbles, schists,
and quartzites. The regional geologic setting of the Kyaukkyi-Onbaing area is shown in
figure (2).
The study area is bounded by the Shwebo-Monywa Plain in the west, in which non-
marine Eocene strata consisting of Male and Pondaung Formations occur. It consists of a low,
narrow ridge in the eastern part of Central Myanmar, trending approximately N-S from
Sagaing to the vicinity of Mogaung in the north. According to Gorchakov, this is the steep
narrow anticline with a core of pre-Tertiary rocks which is flanked by Tertiary and
1248 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Quaternary rocks. The Irrawaddy Formation crops out mostly along the Ayeyarwady River,
lying unconformable over the Mogok crystalline rocks.
In the southeastern part of the study area, the intrusive igneous rocks, meta-igneous
rocks, meta-sedimentary rocks, and paragneiss (Precambrian?) also occur (Ali Akbar Khan,
1985). In the northern part of the area, the marbles and related rocks have been cut off by the
Momeik fault that runs E-W. North of the Momeik fault, there are exposures of Orbitolina-
bearing limestones (Cretaceous) and some intrusive (Clegg, 1941).
Rock Sequences
The rock units on the eastern bank of the Ayeyarwady River, displayed in the
stratigraphic succession (see table) are recognized in this investigation area. The rock
succession was established mainly on the basis of correlation and field relation. The major
rock succession of the study area is in table (1).
Table (1) - Rock Succession of Kyaukkyi -Onbaing area
Lithologic Unit Ages
Sedimentary Units
Alluvium Recent
Irrawaddy Formation Pliocene to Miocene
~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~
Igneous Units
Pegmatite Middle Miocene
Biotite microgranite Early Miocene (15.8 + 1.1Ma)
Syenitic rocks Late Oligocene (25 Ma)
Leucogranite Early Oligocene (32 ± 1 Ma)
~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~
Metamorphic Units
Skarn
Calc-silicate rocks Paleozoic, partly Jurassic
Marbles partly Jurassic
Gneiss
Figure (1). Location map of the study area. Study area
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November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Study Area
Figure (2). Regional Geologic setting of the study area and its environs (MGS, 2014)
Rocks Distribution
Rock units exposed in the study area are:
1. Metamorphic units (garnet-biotite gneiss, marbles, calc-silicate rocks and skarn)
2. Igneous units (granitic rocks, syenite and pegmatite)
The metamorphic rocks exposed in the study area are lithological similar to those of the
Mogok Series of Banerji (1932), Iyer (1953) and Clegg (1941). These rocks invaded by the
younger intrusive (chiefly granitic in composition) are therefore referred to as the Mogok
Group. The exposed rock units in the study area may be different in lithology and also in age.
Figure (3) is the geological map of the study area.
Type of Metamorphism
Metamorphic complexes abound in carbonate, silicate-carbonate, and carbonate-
silicate rocks, hereafter considered as metacarbonate rocks. These rocks are the product of
regional and contact metamorphism as well as metasomatism. Petrological, mineralogical
studies and textural characters strongly suggest that metamorphism and magmatism
governing the Kyaukkyi-Onbaing area was subjected to at least two processes: regional
metamorphism and contact metamorphism.
The occurrence of the foliation, minerals lineation, recrystallization, segregation,
neomineralization and metamorphic differentiation indicate the evidences for regional
metamorphism. Regional metamorphism of pelitic rocks and calcareous rocks gave rise to the
formation of garnet-biotite gneiss, a variety of marbles and calc-silicate rocks. Metamorphic
minerals such as garnet in gneiss, diopside, phlogopite and forsterite in marble are
noteworthy.
After that, the study of metamorphic rocks under microscope reveals the presence of
strong preferred orientation of mica flakes in gneiss, bending and breaking of twin lamella
calcite crystals in marbles, broken twin plane in plagioclase in calc-silicate rock that the
intense deformation might have been prevailing probably due to an intrusion tectonism.
In some places, the later intrusion of biotite microgranite, syenite, and leucogranite
occurred in the present area where the regional metamorphism was superimposed by contact
metamorphism. The contact metamorphism is recognized by the occurrence of skarn rocks
4
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November 29-30, 2018, Hinthada University, Hinthada, Myanmar
such as Ca, Fe and Mg hydrous silicate minerals. It contains chiefly wollastonite associated
with diopside, calcite, plagioclase and grossularite. These mineral assemblages indicate that
the rocks were affected by contact metamorphism.
Figure (3). Geological map of the Kyaukkyi - Onbaing area (Modified after Min Nyo Oo
1993 & Myint Oo Than, 1981)
Mineral Assemblages and Metamorphic Facies of Calcareous Rocks
In the area, calcareous rocks occur as a variety of marbles, calc-silicate rock and
skarn. Marbles are widely distributed in the whole Kyaukkyi-Onbaing area. Calc-silicate
rocks are found overlying the marble units. The following diagnostic mineral assemblages are
recorded for the identification of metamorphic facies for calcareous rocks. Whereby,
accessory minerals like sphene, zircon, apatite, iron ores, etc. are excluded.
Varieties of Marble units include:
Calcite + phlogopite - graphite - quartz
Calcite + diopside + phlogopite - graphite ± quartz
Calcite + phlogopite + spinel + chondrodite ± quartz
Calcite + diopside + forsterite + spinel - quartz
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November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Calc-silicate Rock include:
Diopside + calcite +plagioclase + quartz
Skarn Rock includes:
Diopside+grossularite +wollastonite + plagioclase +sphene
These mineral assemblages of the various rock units are studied on the thin sections.
For the present research work, the metamorphic facies based on the diagnostic mineral
assemblages is defined using the works books of Winkler (1979), Spear and Best (1985),
Yardley (1987), Bucher and Frey (1994), Raymond (1995) Turner and Verhoogen (1961),
(1962), Hyndman (1985), Winter (2001) and Winter (2010).
The above mentioned diagnostic mineral assemblages indicate the present of two
metamorphic facies, upper amphibolites facies and pyroxene hornfels facies for calcareous
rocks in the area. Upper amphibolites facies has been recognized for calcareous rocks in the
study area. The minerals diagnostic of greenschist facies like albite, chlorite, epidote and
actinolite have not been occurred in the present study area. Diopside and forsterite minerals
have been found in calcareous rocks such as marbles and calc-silicate as the index minerals
of upper amphibolites facies.
Calcite + diopside + forsterite assemblage, as discussed by Turner (1981), has rarely
been recognized by decarbonation steps in the presence of H2O-rich fluids. The mineral
association of calcite-diopside-forsterite and calcite-diopside-phlogopite are recorded in
diopside marble. The formation of diopside in the diopside marble apparently was the product
of the following reaction:
3Tremolite+ Calcite + Quartz = 5 Diopside + 3CO2 + H2O
Tremolite + 5Calcite = 11Diopside + 2Forsterite + 5CO2 + H2O
So, the forsterite – diopside bearing marble is the characteristic assemblage of high-
grade metamorphic temperature corresponding to the upper amphibolites facies. Potassium
is present in impure limestone and the reaction of impure limestone with potash – feldspar;
the possible paths are as follows:
3Dolomite + K-feldspar + H2O = Phlogopite + 2Calcite + 3CO2
If the rock contains excess silica, phlogopite reacts with calcite to form diopside with
potash feldspar as shown in the following reaction;
6SiO2 + 3Ca + Phl = Kfs + 3Di + 3CO2
In the research area, albite, chlorite, epidote and actinolite minerals do not occur as
the index mineral of green schist facies. So, the rocks of the study area do not belong to green
schist facies. Figures (4.a & 4.b) is the ACF diagram of upper amphibolites facies.
The contact metamorphism is recognized by the occurrence of skarn rocks and
metasomatic Ca–Fe–Mg–(Mn)-silicate rocks are formed by the interaction of a carbonate
and a silicate system in mutual contact. Typical skarn minerals include, wollastonite,
diopside, grossular, zoisite, anorthite, scapolite, margarite (Ca skarns); hedenbergite,
andradite, ilvaite (Ca–Fe skarns); forsterite, humites, spinel, phlogopite, clintonite, fassaite
(Mg skarns); rhodonite, tephroite, piemontite (Mn skarns) (Bucher and Grapes, 2011).
At very high temperature and low pressure, calcite may react with any quartz present
to produce calcium silicate mineral, such as wollastonite. In Kyaukkyi-Onbaing Area,
wollastonite is occurred in skarn rocks and associated with diopside and grossularite.
Wollastonite in nature does not form in regional metamorphic rocks under closed system
1252 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
conditions. Even under granulite facies condition, the assemblage is calcite+quartz remain
stable (Bucher, 1994). The reaction to form wollastonite provides one of the most common
types of reaction,
Calcite + Quartz = Wollastonite
CaCO3 SiO2 → CaSiO8 + CO2 (Yardley, 1987)
Other skarn mineral assemblages are calcite, diopside and grossularite found at the
contact between biotite microgranite and marble. These mineral assemblages might have
been formed by the following reactions:
Anorthite + 2Calcite + Quartz → Grossularite
These mineral assemblages in reactions indicate that the grade of metamorphism had
reached up to the level of Pyroxene hornfels Facies. Figure (4.c) is the ACF diagram of
Pyroxene hornfels Facies. Table (2) is mineral assemblages of Amphibolite and Pyroxene-
hornfels facies for calcareous rocks and pelitic rock. Figure (5) is the P-T diagram showing
the Pyroxene-hornfels and amphibolite facies for contact and regional metamorphism of the
study area.
Mineral Assemblages and Metamorphic Facies of Pelitic Rocks
Metamorphic facies for the pelitic rocks in garnet - biotite gneiss of the study area is
interpreted on the basis of the occurrence of index minerals and equilibrium mineral
paragenesis. The characteristic mineral assemblages of pelitic rocks are as follows:
Quartz + almandine + plagioclase + biotite + K- feldspar
The above occurrences of mineral assemblage of quartz + almandine + plagioclase +
biotite + K-feldspar in garnet-biotite gneiss represent the amphibolite facies. These diagnostic
mineral assemblages were used to define characteristic metamorphic zone in a Burrovian-
style regional metamorphic terrain. Garnet zone is approximately similar to amphibolite
facies in terms of grade as well as pressure (Yardley, 1989). Mineral assemblages and
metamorphic facies of the Kyaukkyi-Onbaing area are systematically described in Table (2).
(a) Peletic mineral assemblages
(b) calcareous mineral assemblages
(c) Pyroxene-hornfels facies
Figure (4). Mineral assemblages in metamorphic facies: (a) and (b) Amphibolite facies, and
(c) Pyroxene-hornfels facies. See also Table (2).
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Table (2). mineral assemblages and metamorphic facies of Kyaukkyi – Onbaing Area
Figure (5). P-T diagram showing the Pyroxene-hornfels facies for contact metamorphism
and amphibolite facies for regional metamorphism of the study area. (Source:
Winter, 2010)
Mineral Chemistry of the pelitic Rock
The pelitic gneiss in Kyaukkyi-Onbaing area belongs to upper-amphibolite and/or
granulite facies grades; they are overlain by various types of marbles and calc-silicate. In part
in the sampling area marble are emplaced by granitoids. The common assemblages of gneiss
sample are garnet, biotite, plagioclase, quartz, and K-feldspar with minor amount of graphite
and ilmenite.
Chemical analysis of the gneiss sample was examined using JEOL JXA-8800R (WDS
+ EDS) electron-probe microanalyzer (EPMA) housed at the Nagoya University, under
conditions of 15 kV accelerating voltage and 12 nA on the Faraday cup. 5 µm beans in
diameter were utilized for garnet, biotite and feldspar analyses (Figs. 6 & 7). Representative
analyses of major phases are also listed. Iron content in garnet was assumed to be ferrous.
The proportions of the end-members were estimated to be as those of divalent cations in the
8-coordinated sites.
1254 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
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Garnet grains are subhedral and reach to 0.3 mm in diameter. Although garnet is highly
weathered and were not measured from rim to rim, random analyses in the core and rim show
normally a solid solution of almandine-pyrope series with almandine -rich core and
almandine-poor towards the rim. The representative chemical composition of the sample is
Alm58-64 Prp31-37 Sps2-3 Grs2-3 (See Fig. 8 & Table 3)
Figure (6). (a & b). X–ray mapping of the sample and complex garnet, plagioclase and
biotite in garnet - biotite gneiss in the study area.
Based on the compositional table (3), the result indicates that all garnets in the area
have high Al2O3, FeO indicating the garnet comes from sedimentary origin and peraluminous
in nature. Fe content is 3 times higher than that of Mg. according to compositional range of
garnet in figure (8); the garnets in the study area are typically of almandine which is formed
in regional metamorphism.
Figure (7). (a, b & c) Back - scattered electrons images of garnet, plagioclase and biotite
showing chemical zoning. Horizontal line shows appropriate positions of points.
Analyzed (1 to 49) points.
Figure (8). Compositional range of garnet in pelitic gneiss in the Kyaukkyi-Onbaing area
a b
c a b
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Biotite shows reddish brown Z-axial color, and occurs as isolated grain in the matrix,
and irregular biotite crystals filling fracture and/or vein of garnet. It has been analyzed from
rim to rim and show homogeneous composition. It composition is as follow Si = 2.72-2.82
apfu (apfu for O = 11), Ti = 0.12-0.20, and XMg [= Mg/(Mg + Fe2+
)] = 0.53 –0.57 (See Fig 9).
The diagram in figure (9) shows that Ti and XMg have the negative correlation in
biotite.
Mg/(Mg + Fe2+
)] is between 0.53 – 0.57, that mean low XMg indicating that garnet
biotite gneiss might not be orthogneiss and the gneisses in the study area genetically comes
from paragneiss. According to table (3), FeO content is higher than MgO showing the felsic
and continental nature.
Matrix plagioclase generally displays poor retrograde zoning and its average
composition is An49±1Ab48±1Or1±1. The barium content is low and under detection limit of 0.1.
K-feldspar also occurs in the matrix and its average composition is Or95±1Ab3±1 An0.2±1. Based
on the Ab-An content, the plagioclase is in the range from Labradorite to Andesine in
composition. Representative analyzed of plagioclase mineral in pelitic gneiss show that the
plagioclase in the study area is rich in high aluminous (peraluminous) and falls in calc-
alkaline series (Table 5). According to Ab-An-Or pressure-temperature diagram (figure 10,
a), both fine and coarse -grained plagioclase falls < 5kbar and between 700-800 °C.
Table (3). Representative analyses of garnet mineral in pelitic gneiss from the study area.
Figure (9) Compositional variation of biotite in pelitic gneiss. Relationship between XMg
equal Mg/(Mg + Fe) value and Ti (pfu) content of biotite mineral according to
their mode of occurrence.
SiO2 38.17 38.78 37.49 38.88 39.06 38.84 38.78 38.43 38.68 38.63 39.04 38.44 35.88
TiO2 0.06 0.05 0.06 0.06 0.04 0.04 0.03 0.04 0.03 0.07 0.05 0.04 2.63
Al2O3 21.02 21.07 20.68 21.35 21.62 21.48 21.28 21.05 21.39 21.17 21.12 21.02 17.66
Cr2O3 0.03 0.03 0.02 0.03 0.02 0.01 0.00 0.01 0.00 0.07 0.03 0.00 0.05
FeO 28.13 28.00 28.91 26.72 26.49 26.75 28.70 28.39 29.21 28.83 28.42 27.73 16.71
MnO 1.06 1.14 1.20 0.68 0.68 0.70 1.20 1.22 1.22 1.14 1.02 0.84 0.13
MgO 7.92 7.59 7.90 9.23 9.44 9.15 7.85 7.88 7.78 8.28 8.30 8.48 11.52
BaO 3.05 3.18 2.34 2.93 2.89 2.94 2.93 2.75 2.39 2.18 2.63 2.95 0.11
CaO 0.03
Na2O 0.16
K2O 9.77
Total 99.44 99.84 98.60 99.88 100.24 99.91 100.77 99.77 100.70 100.37 100.61 99.50 94.65
Garnet Biotite
1256 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
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Table (4). Representative analyses of biotite mineral in pelitic gneiss from the study area.
(a)
(b)
Figure (10). (a) Ab-An-Or Diagram showing the P-T condition of coarse and fine - grained
plagioclase and (b) Anorthite content of plagioclase in pelitic gneiss from the
study area.
Table (5). Representative analyses of plagioclase in pelitic gneiss from the study area.
Chlorite
35.88 36.43 36.25 35.77 36.49 35.77 35.67 36.60 37.59 36.31 36.13 36.22 25.91
2.63 2.79 2.76 2.57 2.59 1.66 2.91 2.98 2.71 2.94 2.17 3.52 0.07
17.66 17.71 17.68 17.71 18.01 17.86 17.02 16.86 17.80 17.19 17.69 16.86 20.66
0.05 0.06 0.07 0.10 0.05 0.02 0.04 0.09 0.03 0.07 0.04 0.09 0.01
16.71 16.85 16.72 17.16 16.75 17.02 16.72 16.38 15.45 16.71 17.09 17.01 24.41
0.13 0.17 0.17 0.14 0.17 0.18 0.17 0.19 0.11 0.17 0.17 0.19 0.37
11.52 11.62 11.34 11.28 11.17 11.95 11.77 12.31 11.32 12.10 11.74 11.42 14.93
0.11 0.07 0.11 0.11 0.11 0.02 0.08 0.10 0.06 0.10 0.07 0.13 0.00
0.03 0.03 0.02 0.04 0.02 0.06 0.01 0.01 0.06 0.01 0.04 0.04 0.04
0.16 0.13 0.12 0.15 0.15 0.10 0.16 0.17 0.16 0.14 0.12 0.13 0.00
9.77 9.77 9.83 9.43 9.72 9.78 9.82 9.76 8.52 9.81 9.49 9.79 0.01
94.65 95.63 95.07 94.46 95.23 94.42 94.37 95.45 93.81 95.55 94.75 95.40 86.41
Biotite
SiO2 38.17
TiO2 0.06
Al2O3 21.02
Cr2O3 0.03
FeO 28.13
MnO 1.06
MgO 7.92
BaO 3.05
CaO
Na2O
K2O
Total 99.44
Garnet
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Protolith
In the present study area, marbles and calc-silicate rocks are found overlying the
gneiss units. Marbles and calc-silicate rocks are widely distributed in the whole area and a
few of gneiss unit exposure in the western part of the Kyaukkyi-Onbaing area. On the basis
of textural criteria and mineral paragenesis, it can be concluded that the metamorphic rocks
of the study area belong to two major types of protolith: pelitic and calcareous.
The dominant rock types of the study area are a variety of marbles that are mostly
medium to coarse - grained and vary in color from white to grey. Their surface exposures are
rough and pitted. In calcareous rocks, calcite + spinel + chondrodite + phlogopite, calcite +
diopside + phlogopite + graphite + quartz, calcite + diopside + forstesite + phologipite +
quartz, plagioclase + diopside + quartz + calcite mineral assemblages indicate sedimentary
carbonate protoliths. Besides, diopside and forsterite minerals point out the precursor rock
may have more than 18% of magnesium content. This fact indicated that the precursor rocks
may be derived from dolomitic limestone and siliceous limestone.
Likewise, in pelitic rock, quartz + K-feldspar + plagioclase + biotite + almandine
mineral assemblages and other accessory minerals in gneiss are considered to be indicator
minerals to the pelitic protoliths. Myint Lwin Thein et.al., (1990) considered that the marble
and calc-silicate rock in the Mogok area as metamorphosed equivalent of the lower Paleozoic
carbonate rock of southern and northern Shan State according to their lithlogic similarity and
the presence of galena as indicator minerals. The presence of siliceous nodules in marble, east
of Wabyutaung (Maung Thein, 1979), east of Kyetsaungtaung (Maung Maung, 1986) and
north east of Chaunggyi (Myint Naing, 1987) indicates that these marbles are the
metamorphosed Plateau limestone.
Based on the mineral assemblages, lithologic characters, field observation of outcrop
nature and the correlation of the surrounding areas, the metamorphic sequence of the present
study area is well correlated to the Plateau Limestone Group in both northern and southern
Shan States of Paleozoic in age.
Age of Metamorphic Rocks
Various geologists’ researchers proposed about the age of metamorphic rocks of the
Mogok Metamorphic Belt (including the present study area) that they are as follows:
La Touche (1913), Fermor (1932), Chhibber (1934), Pascoe (1950), Iyer (1953), and
Coggin Brown (1953) considered the Mogok metamorphic rocks and related intrusive
granitoid as Precambrian in age.
Clegg (1941) suggested that age of the metasediments of Mogok marbles are from
Precambrian to Cretaceous after discovering the Orbitolina bearing limestones at the first and
second defiles of the Ayeyarwaddy River situated northwest of Mogok.
Searle and Haq (1964) stated that the host rock in the migmatite zone of the Mogk
area is the representative of the Chaungmagyi Series which is accepted as pre-Paleozoic in
age.
Maung Thein and Soe Win (1969) proposed that the marble units are probably the
metamorphic equivalents of the Plateau limestone of Permo-Carboniferious age, since Upper
Paleozoic fossils bearing marbles which are apparently part of Mogok Series that were
discovered from Thandawmywet in Kyaukse District.
Myint Lwin Thein et al., (1990) stated that the pre-metamorphosed sedimentary
sequence, succession, stratigraphic position and relic sedimentary structures, bedding
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characteristics and weathering patterns, all clearly indicate that the rocks of the Mogok Group
are the metamorphosed sedimentary sequence of the lower Paleozoic of the Shan Plateau
region.
On the basis of the above mention, the age of metamorphic rocks of Kyaukkyi-
Onbaing area that including in Mogok Metamorphic Belt is Paleozoic with some part of
Jurassic.
Time of Metamorphism
In Mogok Metamorphic Belt, the time of metamorphism has been controversial
among various authors since the 1930s. The time of metamorphism was estimated by La
Touche (1913) and Chhibber (1934a) as early Precambrian and as either late Paleozoic or
post-mid-Cretaceous by Clegg (1941).
In recently controversy, the equivalence of Mogok marble and Shan States Paleozoic
rocks had been proposed by Myint Lwin Thein (1973) after that had been correlated with
Himalayan metamorphism by Searle and Ba Than Haq (1964).
Some authors have considered it to be Precambrian to Paleozoic (eg. Bender, 1983;
Wolfort et al., 1984), while the others have suggested that it could be as young as Cenozoic
(eg., Searle and Haq, 1964, United Nations, 1978 a, b; Mitchell, 1989).
Bertrand et al., (1999, 2001) obtained 40
Ar/39
Ar and 40
K/40
Ar ages on biotite,
muscovite or phlogopite and explained ages for the Mogok Metamorphic Belt ranging from
Oligocene to Middle Miocene metamorphism and cooling related to ductile extension.
GIAC project (1999), contributed by the Ar/Ar dating on phlogopite, biotite and
muscovite ages of 19-22 Ma from Mogok-Thabeikkyin area. This indicates that the latest
major phase of regional metamorphism took place during Early Miocene.
Barley et al., (2003), zircon ages U-Pb geochronology for the Mogok Metamorphic
Belt shows that strongly deformed granitic orthogneisses near Mandalay. These authors
reported zircon ages of Jurassic, mid-Cretaceous and early Eocene time, confirming that
Andean-type granite magmatism was widespread along the Burma margin throughout the
pre-collisional period (Mitchell, 1993). Zircon rim ages of 43-30.9 Ma suggest that new
zircon growth occurred during a post collisional high-grade metamorphic event in the late
Eocene-Oligocene (Barley et al., 2003).
Garnier et al., (2006) obtained 40
Ar/39
Ar ages of 18.7-17.1 Ma on phlogopite in ruby-
bearing marble near Mogok.
Searle et al., (2007) obtained U-Th-Pb ages on monazite, xenotime, zircon and
thorite. From these they inferred an early Paleocene and a late lower Tertiary metamorphic
event, both related to the India collision.
Khin Zaw et al., (2010) established zircon U-Pb ages from metasomatic rubies at
Mogok of 31-32 Ma and considered these to be syngenetic.
Based on the above-mentioned factors, the time of metamorphism in the Kyaukkyi-
Onbaing area could be assigned as Late Eocene to Middle Miocene ages.
Result and Conclusion
Kyaukkyi - Onbaing area is situated west of Mogok and 177 km North of Mandalay.
It is bounded by the Latitude 22°54′ N to 23° 02′ N and Longitude 95° 58′ 00′′ E to 96°
06′30′′ E. It covers approximately 198 km2 of rugged terrain - moderate relief. The Mogok
The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 1259
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Metamorphic Belt (Searle and Haq,1964) consisting of metamorphic rocks accompanied with
various igneous emplacements, has an average width of 10-13 km and trends ENE-WSW in
the Mogok area, and N-S in the Thabeikkyin area (Myint Lwin Thein et al., 1990).
Structurally, the study area is bounded by N-S trending Sagaing fault in the west and
by E-W trending Momeik fault in the north-east. The three types of rocks (metamorphic
rocks, sedimentary rocks and its related igneous intrusion) exposed in this area. But mainly
two types of rocks (metamorphic rocks and its related igneous intrusion) have been studied.
The metamorphic rocks are garnet-biotite gneiss, marbles, calc-silicate rocks and skarn.
Petrological and mineralogical studies, and textural characters strongly suggest that
metamorphism and magmatism governing was subjected to at least two metamorphic
processes: high grade regional metamorphism and contact metamorphism. Diopside and
forsterite minerals have been found in varieties of marbles and calc-silicate as they are typical
index minerals of upper amphibolites facies. Wollastonite is observed in skarn rocks and is an
indicator for pyroxene-hornfels facies. The first appearance of important mineral
assemblages, such as diopside and forsterite combined with their petrological criteria reveals
two well-defined isograds; diopside and forsterite isograds which can be defined as zones for
metacarbonate rocks. Similarly, the occurrences of mineral assemblages such as quartz +
almandine + plagioclase + biotite + K-feldspar in garnet-biotite gneiss represent the
amphibolite facies as well as garnet zone because of the well-known index mineral garnet.
Mg/(Mg + Fe2+
)] is between 0.53 – 0.57, that mean low XMg indicating that garnet biotite
gneiss might not be orthogneiss and the gneisses in the study area genetically comes from
paragneiss.
The protolith for the metacarbonate sequence can be well correlated with the Plateau
Limestone Group in both northern and southern Shan States of Paleozoic in age. But the older
metapelite (Mogok Gneiss) does not uncertain as the age could be older than metacarbonate.
Therefore, the ages of the metamorphic rocks of the Mogok Metamorphic Belt including the
study area are ranging from Precambrian to Paleozoic with some part of Jurassic. The time of
metamorphism could be assigned as Late Eocene to Middle Miocene ages.
Acknowledgments
We are deeply indebted to Professor of Dr Day Wa Aung, Head of Geology Department in University
of Yangon for his kind permission and guidance to carry out this research work. We are greatly indebted to Dr
Myint Thein (Retd. Rector), valuable advice and fruitful suggestion, Dr Thuya Oo, Rector of Monywa, Dr
Maung Maung Naing, Rector of Yadanabon University, Dr Min Aung, Pro-Rector of Maubin University for
their valuable advice and fruitful suggestion. Thanks are also due to Dr Zaw Win Ko, Past Lecturer, Dr Tin
Aung Myint, Lecturer of Geology Department in Mandalay University, Masaki Enami, Professor, Department
of Earth and Planetary Sciences, Nagoya University, Japan, Dr Tin Myo Myo Htwe, Professor, Department of
Geology, Lashio University, Dr Maw Maw Win, Lecturer of Geology Department in Yadanabon University, Dr
Tint Swe Myint, Lecturer, Department of Geology, Kalay University, U Ye Kyaw Thu, Assistant Lecturer of Geology Department in Magwe University for their helpful hands, various suggestions and supporting.
Finally, we would like to extend our gratitude to the responsible personnel and Ondagu villagers of the
study area for their hospitality and willing help during field trip and all our colleagues in Geology Department,
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