Geochemistry and Geochronology of Igneous Rocks Exposed in ...mynces.org/download/2018/ProceedingMNCES2018/54_Wai Yan Phy… · granite. Metamorphic rocks are biotite muscovite schist,

The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 701

November 29-30, 2018, Hinthada University, Hinthada, Myanmar

1Demonstrator, Department of Geology, Meiktila University

2Demonstrator, Department of Geology, Meiktila University 3Lecturer, Dr, Department of Geology, Mandalay University

# Corresponding author: [email protected]

Geochemistry and Geochronology of Igneous Rocks Exposed in Tawma-

Nabebin Area, Singaing Township, Mandalay Region

Wai Yan Phyo1#

, Thae Su Htwe2, and Tin Aung Myint

3

Abstract

Tawma - Nabebin area is situated near Singaing Township, Mandalay Region. It is located between East Longitude 96° 07′ and 96° 15′ and North Latitude 21° 40′ and 21° 45′ in one

inch topographic map no. 93 C/2. Granitoid rocks in Myanmar occur in three north–south

trending linear belts over a distance of more than 1500 km from Kachin State in the north

through Mogok, Mandalay, Taungoo and Mon State to the Tanintharyi Region in the south.

The study area is located within the central granitoid belt and tectonically, it lies within the N-

S trending Mogok Metamorphic Belt (MMB). The major element data, using the construction

of variation diagrams, display the data as bivariate or trivariate plots. Field and petrochemical

data suggest that hornblendite, hornblende diorite, biotite microgranite and other granites

intruded along the highly deformed metamorphic rocks of the study area. The igneous rocks

exposed in the area consist of hornblendite, hornblende diorite, tourmaline granite, biotite

granite, biotite microgranite, leucogranite, garnet-biotite granite and porphyritic biotite

granite. Metamorphic rocks are biotite muscovite schist, biotite schist, epidote quartzite, diopside calc-silicate rock, and white marble. Hornblendite is the oldest igneous and might

have been formed before the continental epiorogenic uplift. Generally, the granitoid rocks are

classified into I-type and S type. All granitic rocks of the study area fall in S-type except

hornblendite and some hornblende diorite which are typical I-type characters. The age of the

igneous activity in the study area might have been formed during Tertiary period.

Keywords: Geochemistry, I-type granite, S-type granite, geochronology of igneous rocks

Introduction

In this paper describes a suite of granitoid rocks exposed at the Tawma - Nabebin

area, covering about 48 km2 in the near Singaing Township, Mandalay Region, Central

Myanmar (Fig. 1), described previously. The area is located within latitudes 21° 40′ N and

21° 45′ N and longitudes 96° 07′ E and 96° 15′ E and lies about 25 km south of Mandalay.

Mogok Belt including the study area lies within the western part of the Sibumasu Block (Also

known as the Shan- Thai Block, Shan –Tennasserim Block, the Sinoburmania Block or the

Burmese Malayan Block), along the northwestern margin of the Shan Plateau and southwards

between the north- south trending Sagaing fault and Shan Scarp (Searle and Haq, 1964).

Therefore, MMB is the key tectonic position in the SE Asia tectonics.

The aim of this paper is to describe the petrology, geochemistry and geochronology

of these granitoid rocks in the Tawma - Nabebin area and to discuss their petrogenesis and

tectonic setting.

Regional Framework and Local Geology

The study area lies between the western margin of Shan-Tanintharyi Belt and the

eastern margin of Central Cenozoic Belt. It is also situated within the N-S trending Mogok

Metamorphic Belt of highly deformed metamorphic units (Searl and Haq, 1964). The rock

assemblages of Mogok Metamorphic Belt not only occur at Mogok and its environs but also

are widespread throughout Thabaikkyin, Madaya, Mandalay and Kyaukse areas. The Mogok

Metamorphic Belt including the study area extends over 1500km along the western margin of

702 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)


Shan-Thai Block. The metamorphic units of the study area were regarded as Precambrian age

(La Touche, 1913 and Chhibber, 1934). Recently, various workers such as Mitchell et al.,

(1999, 2007), Bertrand et al.(2007), Bertrand and Ranging (2003). Morley et al., (2002),

Barley et al., (2003), Searle et al., (2007), Khin Zaw et al., (1990 & 2014 a&b) studied the

Mogok Belt and expressed their opinions on the geology, metamorphism, magmatism and

tectonisim of the Mogok Metamorphic Belt. Myint Thein (1975) study that most granodiorite

and diorite rocks of the Belin area and Kyaukse area have originated metasomatically from

epidote-biotite schists and epidote calc-silicate rocks on the basis of the field and

petrographic evidences. The metamorphic units of Jurassic-Cretaceous age probably crop out

in the Thakhinma Taung and the Tawzu Area, north of Mandalay. In this area, the pelitic and

calcareous metasediments of Jurassic-Cretaceous age are exposed at the Belin and Sunye

area. Medium to high grade metamorphic rocks of the study area were regarded as

Precambrian to Paleozoic age (Bender, 1983). Than Than Nu (1990) described the petrology

of the Belin and Nwale Hill, Singaing Township, Mandalay Region. Lay Lay Khaing (2007)

stated that on the igneous processes governing the emplacement of the Belin intrusives in

Singaing Township, Mandalay Region. Thet Tun (2009) study "Petrology and petrogenesis of

the granitoid rocks exposed along the Shan boundary fault system between Belin-Kyaukse

and Yamethin area, Mandalay division". He stated that the granitoid magmatism of the Shan

scarp region formed in relation to the pre-dated collosion of India with Eurasia during Mid-

Jurassic to Cretaceous Time. Geochemistry and petrogenesis of igneous rocks exposed in

Tawma-Nabebin area, Singaing Township, Mandalay region described by Thae Su Htwe

(2016).

Figure (1). Location map of the Tawma - Nabebin area, Singaing Township, Mandalay

Division.

Study area



Petrography of the Granitoid Suite

Methods of Study

Fieldwork included sampling of representative rock units, the measurement of

geological structures and geological mapping using tape and compass traverses, assisted by

GPS positioning. Unknown minerals were more than fifty representative rock samples

collected from the study area have been selected and analyzed for the petrologic studies.

Petrography was studied using 80 thin-sections, and modal analyses of the igneous rock units

were made using a mechanical points counter in conjunction with a petrological microscope.

For petrochemistry, chemical data are obtained by the XRF analysis at MURC (Mandalay

University Research Center) in Mandalay City. The major and trace element oxides from

XRF analysis are shown in tables (1) and (2). Granitoid rocks are classified and named on the

basis of modal analyses, plotted using total alkali versus silica, (TAS) diagram of Cox et al.,

(1979) adapted by Wilsom (1989) showing the field of plutonic rocks of the study area.

Their detailed mineralogy and textures are described below. For the present study mineralogy

and major element oxides are used to describe, name and classify rocks. In most cases, the

chemistry alone is the criterion for classification. The major element data, using the

construction of variation diagrams, display the data as bivariate or trivariate plots. This type

of diagram is used to show the interrelationship between elements in the data-set and it is

from these relationships that geochemical processes may be inferred. These diagrams were

drawn using Tridaw, Corel Draw 11, SPSS and Petrograph software.

General Statement

The igneous units exposed in the Tawma – Nabebin area are mainly various granite,

leucogranite, hornblende diorite, hornblendite and pegmatite. Based on the constituents'

minerals assemblages, the metamorphic rocks of the study area are grouped into metapelites,

metacarbonates and skarns. According to the previous works, field relationship, petrography

and stratigraphical evidence, the igneous rocks sequence of the study area can be subdivided

into eight major rock types as shown in figure (2).

Hornblendite

This unit is mainly exposed in the north-eastern part of the study areas near Mogaung

village. It is coarse grains texture, hard and compact nature and consists of hornblende only,

it is hornblendite. They are constituted of mafic minerals only and very dark or very dark

green in color and show a pitted surface in many places. This unit is intruded by pegmatite

and aplite dykes in some places (Fig. 3.a). Hornblendite displays hypidiomorphic granular

texture and is mainly composed of hormblende with minor amount of biotite, sphene, apatite,

plagioclase and tourmaline (Fig. 3.b).

Hornblende Diorite

This unit is best seen in the northern parts of Shwetawya monastery, Taungdanshe

Taung and Balin prison quarry. It consists of mainly plagioclase and hornblende, coarse

grained texture, hard, compact and dark greenish color and manifestation on pitted surface

(Fig. 3.c). Microscopically, the rock is mainly composed of hornblende, plagioclase and

biotite. The accessory minerals are epidote, apatite, magnetite, sphene and quartz.

Hornblende displays hypidiomorphic granular texture and size ranges from 0.01mm to

0.03mm diameter. (Fig. 3.d)



Figure (2). Geological map of the Tawma - Nabebin area Singaing Township, Mandalay

Region (after. Than Than Nu 1990)

Granite Suite

Biotite granite is well exposed near Tawma Village, Kyaukkyi Taung, Taungdanshe

Taung, western and southern part of the Nwale Taung. This rock shows hard, compact and

light grey color on weather surface and light grey to white on fresh surface. It exhibits

medium grained, homogeneous in texture and some granite rocks are showed medium to

coarse grained nature. Some places, columnar structures have six sides and occur in granite

rocks. Exfoliation is the common character of this granite (Fig. 3.e). Xenolilths of country

rocks are found in the biotite granite (Fig. 3.g). This unit is intruded by pegmatite and aplite

dykes in some places, estimating that these pegmatite and aplite are younger than biotite

granite. This biotite granite almost everywhere intruded either regionally metamorphosed

rocks or older intrusions. Porphyritic granite is found as both a core zone in the central part of

the granitoid plutons and individual plutons. It displays the light grey, coarse grained,

porphyritic texture. Quartz and feldspar occur as porphyroblast. Garnet bearing biotite

granite, tourmaline granite and biotite microgranite are well exposed northeast of monastery

and Belin quarry in the study area. It shows white to brownish on fresh surface and light grey

color when weathered. Migmatites are well exposed in the central part of the study area

which is associated with biotite granite and biotite gneiss in many places (Fig. 3.h).

Ptygmatic folds are irregular and isolated fold structures that typically occur as tightly folded

veins or thin layers of strongly contrasting lithology. Leucogranite found as small stocks at

Belin prison quarry and either small or large bodies in some places at Shwemyintin Taung in

the lower part of the mountain ranges. It is coarse-grained and essentially composed of felsic

minerals such as feldspar and quartz. Pegmatites generally occur as small dykes and vein

61 62 63 64 65 66 67 68

61 62 63 64 65 66 67 68

34

33

32

31

30

29

28

27

Q

34

33

32

31

30

29

28

27



which intruded all metamorphic and igneous units. It shows whitish color on fresh surface

and brownish yellow on weather surface, medium to very coarse-grained, leucocratic rocks.

In some places, small aplite dykes and quartzofeldspathic veins are fairly observed. Most

pegmatite dykes have NNW-SSE trend. In thin section, granite contains orthoclase, quartz,

biotite, plagioclase and accessory minerals (apatite and magnetite). The plagioclase

composition ranges from albite to oligoclase, and the simple twins are very common.

Perthitic orthoclase enclosed within the anhedral plagioclase indicates that the plagioclase is

early crystallized than the perthitic orthoclase. Biotite shows anhedral to subhedral

appearance and strong pleochroism. Some crystals occur as flaky aggregates along grain

boundaries (Fig. 3.f). It is partially altered to chlorite. Muscovite occurs as prismatic form

with one set of perfect cleavage. The size ranges from 0.5 to 2 mm in length. Garnets are

locally abundant in these rocks, and form as anhedral to subhedral grains. Tourmaline occurs

as fails small crystal. Myrmekitic texture is observed at the quartz-feldspar intergrowth. Most

orthoclase show untwined. They are altered to kaolinite and sericite.

Chemical Characteristics of Igneous Rocks

In general, granitic rocks have SiO2 (68-76%), Al2O3 (12-17%), CaO (0.2-2%), MgO

(0.1-0.7%), Fe2O3 (0.1-2%), TiO2 (0.02-0.2%) and Na2O+K2O (3-10%). Based on XRF

analytical data and Harker’s variation diagram (Fig. 4a), major element oxides (CaO, MgO,

Fe2O3, and TiO2) are negatively correlated with SiO2. Na2O, K2O and Al2O3 have a positive

correlation with SiO2.

Wilsom (1989) uses TAS diagram, adopted from Cox et al. (1979), to give a

preliminary classification of plutonic igneous rocks which is one of the most useful

classification schemes. For the present study chemical data, i.e., the sum of the Na2O+K2O

content (total alkalis, TA) and the SiO2 content (S) are taken directly from a rock analysis as

wt% oxides and plotted onto the classification diagram. This diagram, (Fig. 4.b), points out

the bulk composition of igneous rocks is granite. A further criterion is referenced to the

O’Connor (1965) normative Ab-Or-An diagram.

In this diagram, (Fig. 4.c), the majority fall in the granite field and a few in the

Trondhjemite field. In addition, the fractionation trend is well fitted to the Bowen’s reaction

series, i.e., calcic to sodic composition chemical designation in most common use is based on

silica percentage. For the study area SiO2 content ranges from 68 to 74 % in leucogranite,

biotite microgranite, biotite microgranite, tourmaline granite, garnet biotite granite,

porphyritic biotite granite and up to 75% in pegmatite. Based on the above criterion, the

magma responsible for the igneous rocks of the study area is acidic in composition.

In the Al2O3-CaO-(Na2O+K2O) diagram (Fig. 4.d), the igneous rocks of the study area

fall in the field of peraluminous. It is substantiated by NK/A versus A/CNK diagram. In this

diagram, (Fig. 4.e), all of the igneous rocks fall within the peraluminous field except

hornblendite and some hornblende diorite fall in metaluminous. Concerning the mineral

chemistry, the igneous rocks of the study area have the mole percent alumina is greater than

the sum of lime, soda and potash (Al2O3 > CaO+Na2O+K2O) and the norm contains

corundum. Shand (1949) classified four groups of rocks in terms of alumina saturation.

Following the Shand’classification, these rocks belong to peraluminous suite.

Based on the silica and alkali content, igneous rocks have been classified into two

major series: alkaline and subalkaline. In SiO2 versus total alkali diagram (Fig. 4.f) all of the

igneous rocks of the study area fall in the field of subalkaline. The dividing line was

following the work of Irvine and Baragar (1971).



Figure (3). (a). Photograph showing pegmatite vein (dyke) intruded in massive nature of

hornblendite: (3.b). Photomicrographs showing biotite (Bt), feldspar (Fel) and

hornblende (Hbl) found in hornblendite, under XN: (3.c). Photograph showing

close up view of hornblende diorite at Taungdanshe Taung: (3.d)

Photomicrographs showing biotite (Bt), feldspar (Fel) and hornblende (Hbl)

found in hornblende diorite, under XN: (3.e) Exfoliation of biotite microgranite

exposed at Taungkantlant area:(3.f) Photomicrographs showing biotite (Bt),

quartz (Qtz) and feldspar (Fel) found in biotite granite, under XN: (3.g)

Xenolilths of different country rocks found in the biotite granite:(3.h) Ptygmatic

folds in the Belin quarry (hammer for scale). Note the wavelength variation as a

function of layer thickness.

Hbl

Bt

Fel

(b)

(c)

(e) (f)

Bt

Qtz

Fel

(d)

Hbl

Bt Fel

(h)

Ptygmatic folds are irregular and

isolated fold structures

Granite

(g)

(a)



The subalkaline series was further subdivided, (Tilley, 1950), into tholeiitic and calc-

alkaline by using AFM diagram. Figure (4.g) indicates almost all of the igneous rocks of the

study area fall in the calc-alkaline field and single fractionation trend. It is substantiated by

Si2O versus alkalinity index diagram. According to this diagram, leucogranite, biotite

microgranite and pegmatite belong to the calc-alkaline series (Fig. 4.h). Based on the above

criterion, the igneous rocks of the study area are genetically related to the subduction related

plate tectonic processes.

The plots of Na2O+K2O-Fe2O3-MgO diagram (Fig.4.g) show that alkali is enriched

during the late stage of crystallization. In addition, Na2O-K2O-CaO diagram (Fig. 4.i) shows

the later stage of magmatic evolution trend showing an increase in K2O and Na2O with

depletion of CaO, i.e., fractionation trend follows the Bowen’s reaction series. Therefore, it

can be concluded from figures (4.c), (4.g), (4.h) and (4.i) that igneous rocks of the study area

were derived from single parental magma.

The above-mentioned points indicate the granitic rocks of the study area belong to

acid clan, peraluminous nature and calc-alkaline (subalkaline) series but hornblendite fall

within alkaline series (Fig. 4.f). For the type of granitoids, various plots are used to

distinguish I- and S-type. The figure (4.j), ACF diagram, indicates that the majority of

igneous rocks from the study area belong to S-type. In addition, some distinctive chemical

properties such as the molar ratio of Al2O3/ Na2O+CaO+K2O is greater than 1% and CIPW

normative corundum content is mostly >1% in igneous of the study area point out S-type

nature. According to the CIPW Norm calculation, hornblendite and hornblende diorite fall in

the field of I- type but some hornblende diorite can be seen at the boundary of I-type and S-

type because ilmenite and magnetite have equal amount of percent.. It is also found that all

granite fall in the field of S- type because ilmenite is typical present in norm calculation (Fig.

4.j).

Table (1 & 2 ) Major- and minor- element analyses ( in Wt.% ) and norms of the

plutonic rocks from the study area.

Table (1). Major and minor element analyses (in Wt.%) and norms of the study area.

.



Table (2). Major and minor element analyses (in Wt.%) and norms of the study area.

Figure (4). (a). Variation diagram of selected major oxides vs

SiO2 in granitic rocks of the

study area. Arrows indicate the fractionation trend with increasing silica content.

Figure (4). (b) Chemical classification of plutonic

rocks using total alkali versus silica,

(TAS) diagram of Cox et al. (1979)

adapted by Wilsom (1989) showing the

field of plutonic rocks of the study

area.

Figure (4). (c) Normative albite-orthoclase-

anorthite diagram for the granitic

rocks of the study area, with

dividing lines of O’connor (1965).



Figure (4). (d) Al2O3-CaO-(Na2O+K2O)

diagram for the granitic rocks of

the study area.

Figure (4). (e) Aluminous saturation indices

(Shand, 1949) of the granitic

rocks from the study area.

(Source: Winter, 2001)

Figure (4). (f) Diagram showing the

subalkaline nature of the igneous

rocks of the study area. (Source:

Irvine and Barager, 1971)

Figure (4). (g) Fe2O3(T) -(Na2O+K2O) –MgO,

AFM diagram in terms of alkalis,

total Fe and Mg for granitic rocks

of the study area. The dividing

line is based on the work of

Irvine and Baragar (1970).

Figure (4). (h) Alkalinity index of the

igneous rocks of the study area.

(field names are after Wright, and

the dashed line separates alkaline

from calc-alkaline, after Khin

Zaw 1986)

Figure (4). (i) Na2O-K2O-CaO diagram for

granitic rocks of the study area



Figure (4). (j) ACF diagram for the granitic

rocks of the study area. Molar

ratios: A- Al2O3 +Na2O+K2O, C-

CaO, F- Fe2O3+MgO (after

Hyndman, 1985)

Condition of the Crystallization of the Granites

Almost all of the analysed rocks of the study area possess >40% normative

Ab+Or+Qtz and >40% normative Or+An+Qtz. These ratios are plotted in ternary diagrams

(Fig. 5 & 6).

In figure (5), the quartz-feldspar boundaries at water pressure of 2kbar and 10kbar are

shown projected onto the anhydrous base of the tetrahedron, after Tuttle and Bowen (1958).

The blue line represents the Ab/An piercing points projected onto the plane Ab-Or-Qtz at 5kb

water pressure (Wiebe, 1974). With reference to figure (6), almost all the points of biotite

microgranite and leucogranite lie within the ternary minimum of 2kb and 10kb, and one point

of biotite microgranite lies at the 5kb curve. Assuming PH2O = Ptotal , the minimum PH2O

allowed by the scattered points are 2kb and 5kb respectively.

In addition in the normative Or-An-Qtz diagram (Fig. 6), almost all of the biotite

microgranite and some leucogranite lie between the ternary minimum of 1kb and 5kb. If the

igneous rocks were assumed as crystallization at minimum pressure of 2kb, their liquidus

temperatures could have been cited from the diagram that shows the relationship between

differentiation index and temperature at 2kb water pressure (Fig. 7). According to this

diagram, the liquidus temperatures for biotite microgranite and leucogranite are 690°C and

650°C respectively.

Assuming that they were emplaced at PH2O of 10kb, the temperature of crystallization

of the rocks would become considerably lower. The depth of emplacement of these granitic

rocks is considered to be epizonal to mesozonal. Points 1 suggest the epizonal intrusion

(<1Km), Points 2-5 show the mesozonal intrusion (5-20 Km) and point 6 indicates the

catazonal intrusion. According to the depth-temperature relation diagram of (Marmo, 1956),

leucogranite will crystallize at 23.5 km, tourmaline granite, biotite microgranite, garnet

biotitegranite and porphyritic granite would be at 24-26 km, biotite granite might probably

be at 24.5 km, hornblende diorite would be at 29 km and hornblendite might be probably be

at 33 km. Based on these facts, the granitic rocks of the area might intrude at the deeper

crustal level according to the depth-temperature relation diagram and then, they evolved and

intruded into the shallow crustal level because of the presence of skarn minerals, xenoliths

Tourmaline granite

Leucogranite

Garnet biotite granite

Porphyric biotite granite

Biotite microgranite

Biotite granite

Hornblende diorite

Hornblendite



and destruction of the country rocks.The depth of crystallization of the igneous rocks for the

study area can be interpreted from the schematic depth-temperature relation diagram (Fig. 8).

Figure (5). Normative Qtz-Ab-Or-H2O

diagram for the granitic rocks

of the study area.

Figure (6). Weight percent of normative quartz-

orthoclase-anorthite variation for

granitic rocks of the study area. The

boundary curves and minima at 0.5 to

5kb. (after Tuttle and Bowen, 1958)

Figure (7). Temperature- differentiation

index diagram for the

igneous rocks of the study

area, at PH2O = 2kb. ( after

Piwinskii and Wyllie, 1970)

Figure (8). Schematic depth-temperature relation

diagram showing the position of the

igneous rocks of the study area. (After

Marmo, 1958)

Emplacement and Depth of Intrusions

Field and petrochemical data suggest that hornblende diorite, porphyritic biotite

granite, leucogranite, biotite microgranite and other granites intruded along the highly

deformed metamorphic rocks of the study area. The emplacements of these igneous bodies

were accomplished by forceful intrusion. Pegmatite and aplite dykes are later intruded into

the biotite microgranite bodies. Hornblendite is the oldest igneous and might have been

formed before the continental epiorogenic uplift. The relationships of the igneous and



metamorphic rocks of the area imply that the latest metamorphism immediately preceded the

emplacement of the microgranite.

The field evidences and petrochemical criteria point out the depth of emplacement of

igneous intrusions from the study area. The following facts are used in estimating the depth

of emplacement.

(1) Temperature of country rock is greater than 450oC. The country rock belongs to

amphibolite facies.

(2) Migmatites are observed in the country rock in Belin Quarry.

(3) Mostly discordant structural relations to the country rocks.

(4) No graditional contact between country rock and intrusions.

(5) Abundance of associated pegmatite and aplite containing tourmaline, beryl, rubellite,

garnet and others.

(6) Contact metamorphic zones are few and small.

(7) The chilled border zones are present.

(8) Xenoliths of the country rocks are found.

These points collectively suggest that the depth of igneous emplacement of the study

area may be estimated at catazone through mesozone to epizone.

Geochronology

Based on the K/Ar dating method, Geological Survey and Exploration Project (1978)

stated that the bluish microgranite near Yesin yields 24 ± 2.7Ma (Upper Oligocene). (in Ma

Oo, 1995).

Numerous analyses by the Ar39

/ Ar40

method (Bertrand et al., 2001) indicate that the

undeformed Kabaing Granite (Mogok area) that intrudes the Mogok Series yields 39

Ar/40

Ar

age of 15.8 1.1 Ma that postdates the ductile stretching along the belt.

Barley et al., (2003) stated that the sensitive high-resolution ion microprobe U-Pb in

zircon geochronology for the Mogok Metamorphic Belt shows that strongly deformed

granitic orthogneisses near Mandalay contain Jurassic (~170Ma ) zircons that have partly

recrystallized during (~43Ma) high-grade metamorphism. Metamorphic overgrowths to

zircon in the orthogneisses near Mandalay date a period of Eocene (~43Ma) high-grade

metamorphism possibly during crustal thickening related to the initial collision between India

and Eurasia (at 65 to 55 Ma). This was followed by emplacement of syntectonic hornblende

syenites and leucogranites between 35 and 23 Ma. In addition, they point out the zircon cores

of augen gneiss from Kyanigan hills have a mean age of 170.1 ± 1.1Ma, suggesting the

magmatic age of the orthogneiss protolith.

Using U-Th-Pb dating method, Searle et al., (2007) point out two Tertiary

metamorphic events affected the Mogok Metamorphic Belt. The first was the Paleocene

event that ended with intrusion of crosscutting postkinematic biotite granite dykes at

(~ 59Ma). A second metamorphic event spanned late Eocene to Oligocene (at least from 37,

possibly 47, to 29Ma). This resulted in synmetamorphic melting producing garnet and

tourmaline bearing leucogranites at 45.5 ± 0.6 Ma and 24.5 ± 0.7 Ma. The later metamorphic

event is older than 24.5 ± 0.7 Ma - the age of leucogranites that crosscut all earlier fabrics.



The age of metamorphism along the MMB has been the source of numerous

discussions: Bertrand et al., (1999) and Barley et al., (2003) have considered it to be

Oligocene to middle Miocene age according to Ar40

/ Ar39

method. Besides almost all of the

igneous rocks of the study area intruded and crosscut the metamorphic units indicating that

the age of metamorphism was earlier than the emplacement of igneous rocks.

Thus, it is reasonable to conclude that the age of the igneous activity in the study area

might have been formed during Tertiary period. But the hornblendite in the study area could

be Mesozoic time.

Granitic Type and Tectonic Environment

Generally, the granitoid rocks are classificated into four main types, namely: I-type

and S - type (Chappell and White, 1974), A - type (Colling et al., 1982 in Winter, 2010) and

M - type (White, 1979). I - type granites are generated in cordilleran subduction and post-

orogenic uplift regimes while S - type granites are the products of continental collision

(Beckinsale, 1979 in Pitcher, 1987). A-type and M-type granites are generated in anorogenic

and oceanic environments, respectively (White, 1979; Collision et al., 1982) (in El -sayed

M.M. et al.,2002).

On the basis of chemical analyses, i.e, major, minor and trace element concentration

by X - ray fluorescence spectrometry (EDXRF) method, the granitic rocks were classified as

various characters by various authors. In terms of major elements, all the granitic samples in

the present study area plot as sub-alkaline and calc-alkaline fields by Irvine and Baragar

(1971), high-K calc-alkaline series by Le Maitre et al., (1989), peraluminous nature by Shand

(1972) (in Winter, 2010) and calcic series by El . Sayed et al., (2002) respectively.

The distribution of the granitic rocks as discordant character, their calc-alkaline

affinity and peraluminous character, relatively high K2O content, felsic type having CIPW

corundum content > 1% strongly suggest that the granitic rocks in the present study area are

S-type which may be derived by partial melting of already peraluminous sedimentary source

rocks ( Winter, 2010).

Several schemes have been proposed for the classification of granites on the basis of

tectonic setting (Pitcher 1983, Pearce et al. 1984, Brown et al., 1984, Maniar and Piccoli

1989, Pearce 1996 in El. Sayed et al., 2002). Among these, the tectonic setting of granitic

rocks from the study area was studied by using major elements and trace elements with

reference to Mariar and Piccoli, 1989 and Pearce et al., 1984).

Maniar and Piccoli, 1989 classified the granitic rocks on the basis of tectonic setting

as follows:

(1) Island arc granitoids (IAG)

(2) Continental arc granitoids (CAG)

(3) Continental collision granitoids (CCG)

(4) Post orogenic granitoids (POG)

(5) Rift related granitoids (RRG)

(6) Continental epiorogenic uplift granitoids (CEUG)

(7) Oceanic Plagiogranites (OP)



According to Maniarand Piccoli (1989), tectonic discrimination diagrams based on

major oxides for the studied granitic rocks are shown in figures (9. a-d).

In (Fig. 9.a), K2O versus SiO2 diagram shows the distinction between oceanic

plagiogranite (OP) and other granitic rocks types. (IAG + CAG + CEUG + RRG) and all the

studied igneous rocks fall within the field of IAG + CAG + CCG + CEUG + RRG. In Al2O3

versus SiO2 and MgO and SiO2 diagrams (Fig. 9.b), the igneous rocks can be subdivided into

three groups (IAG + CAG + CEUG; POG and RRG + CEUG) and the studied igneous rocks

plot within the field of IAG + CAG + CCG (Fig. 9.c) but hornblende diorite and hornblendite

fall in CEUG. Shand’s diagram shows the distinction between CCG, CAG and IAG. In the

diagram (Fig. 9.d) all granitic rocks from the studied area occupy the field of CCG.

Figure (9). (a) K2O versus SiO2 diagram

showing the distinction between

OP and IAG + CAG+ CCG +

CEUG + RRG.

Figure (9). (b) Al2O3 versus SiO2 diagram


IAG + CAG + CCG, POG and

RRG + CEUG.

Figure (9). (c) MgO versus SiO2 diagram


IAG + CAG + CCG, POG and

RRG + CEUG.

Figure (9). (d) Shand’s index diagram


CCG, CAG and IAG.

Conclusion Remarks

The study area is located between the eastern Shan Highland in the east and central

Myanmar lowland in the west. It is also situated within the N-S trending Mogok

Metamorphic Belt of highly deformed metamorphic units. The Mogok Metamorphic Belt

including the study area extends over 1500km along the western margin of Shan-Thai Block.

The highest peak in the area is 440 m and all the ranges in the mountainous part have a

general N-S trend.



Petrogenesis of these rock units are described on the bases of lithology, mineral

chemistry, field relationships and correlation to those of other areas. Biotite granite suite

exposed in the central and western part of the terrane is the only felsic plutonic suite. It is

dominated by the biotite bearing granite rocks: (1) biotite granite, (2) porphyritic biotite

granite, and (3) medium to fine-grained granites such as garnet biotite granite, biotite

microgranite, tourmaline granite, leucogranite and pegmatite. There is slightly compositional

and structural variation in all of these rocks. Two former units form in continuum in overall

composition, and hence, intermixed on varied scale. The biotite granites, often, grade to the

K-feldspar megacrystic and pegmatitic. They contain slightly foliation along the country rock

interface, suggesting a timing of emplacement during the warming stages of prograde

metamorphic kinematic effects. The following field and petrographic evidences lead to a

conclusion that the granite rocks in the present area are believed to be mainly formed by the

partial melting (anatexis) of the metasedimentary rocks rather than magmatic intrusions. But

hornbledite could come directly from the magma before orogeny. The mineralogical,

petrological and geochemical characteristic features of the granitic rocks in the area suggest

that they possess the S-type granites. The peraluminous S-type granites (mainly ilmenite

series) were formed by the partial melting (anatexis) of the metasedimentary rocks.

According to the CIPW Norm calculation, hornblendite and hornblende diorite fall in the

field of I- type but some hornblende diorite can be seen at the boundary of I-type and S-type

because ilmenite and magnetite have equal amount of percent.. It is also found that all granite

fall in the field of S- type. The depth of emplacement of these granitic rocks is considered to

be epizonal to mesozonal. Points 1 suggest the epizonal intrusion (<1Km), Based on these

facts, the granitic rocks of the area might intrude at the deeper crustal level according to the

depth-temperature relation diagram and then, they evolved and intruded into the shallow

crustal level because of the presence of skarn minerals, xenoliths and destruction of the

country rocks.

The age of igneous activity in this area is mainly based on field evidence and a few

radiometric age dating. East of the quarry the metamorphic rocks are intruded by

hornblendites and later granitic dykes and by the MEC granite. The MEC granite and Belin A

are similar. Mitchell et al., 2012; also obtained a zircon U–Pb age of 44.6 ± 0.5 Ma on the

planar diorite dyke. Searle et al. 2007; reported a U–Th–Pb zircon age of 59.5 ± 0.9 Ma on

the flow-banded granite dyke which cuts banded pegmatitic granite dykes and which from

field relationships is the youngest intrusion at Belin. Thus, it is reasonable to conclude that

the age of the igneous activity in the study area might have been formed during Tertiary

period. But the hornblendite in the study area could be Mesozoic time.

Generally, the granitoid rocks are classified into I-type and S type. All granitic rocks

of the study area fall in S-type except hornblendite and some hornblende diorite which are

typical I-type characters. On the basis of tectonic setting, granitic rocks of the study area are

in continental collision granitoids (CCG). But on the other hand, hornblendite and hornblende

diorite fall in continental epiorogenic uplift granitoids (CEUG) field.

Acknowledgements

We would like to express our gratitude to Dr. Ba Han, Rector of the University of Meiktila University,

Dr.Kay Thi Thin, Pro-Rector of the University of Meiktila University for their encouragement. We acknowledge

Professor Dr Zaw Min Thein, Head of Geology Department, Meiktila University for their encouragement. We

am deeply indebted to Professor Dr Than Than Nu, Head of Geology Department and University of Mandalay,

for her kind permission and advice to carry out this research work.

This research work could not be completed without getting helpful hands and advices from all teachers

in Geology Department, University of Meiktila.



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