Petrochemistry and Petrogenesis of Metamorphic Rocks ...mynces.org/download/2018/ProceedingMNCES2018/102_Tin Moe Za… · Petrochemistry and Petrogenesis of Metamorphic Rocks Exposed

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)


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



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



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



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



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).



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.



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



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



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



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



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



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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