Geology and Petrography of Peridotites (Mantle Section ... · Available on: Bahria University...

Available on: www.bahria.edu.pk/bukc/burjes2016 Bahria University Research Journal of Earth Sciences Vol. 2, Issue 1, June 2017

6 ISSN 2415-2234 © BURJES

Geology and Petrography of Peridotites (Mantle Section) from

Bela Ophiolite, Balochistan, Pakistan

Mehrab Khan, Muhammad Jahangir Khan

Department of Earth & Environmental Sciences Bahria University Karachi Campus, Pakistan

Corresponding Author Email: [email protected]

Abstract – The peridotite rocks (ultramafic rocks) well

exposed between Sunaro village and Ornach cross, Balochistan.

The present study provides substantial information about the

peridotite of Bela Ophiolite based on extensive field observation

and laboratory work. The peridotities (harzburgites and

dunites) in the study area are usually serpentinized. Relatively

fresh peridotites are exposed in the Sunaro and Lak Baran area,

where it is harzburgite in composition. The dunite bodies

(transition zone) are common below the Moho characterized

with serpentinization process. In the north of Lak Baran area,

the outcrops show a spectacular harzburgite dunite banding or

layering, varies in thickness from centimeter to meter, and

follow the foliation pattern in peridotites. The width of the

dunite bands increases toward north but no systematic

distrubtion is observed. The mantle section consists up of

harzburgite, depleted harzburgite, dunite and serpentinites.

Economical valuable mineral chromite deposits are common in

mantle section. In this present study the veins and dikes are

classified into four groups such as 1) Dunite veins 2) Pyroxenite

dikes 3) Gabbro dikes and 4) Doleritic dikes.

Keywords – Ophioloites, Peridotites, Bela, Serpentinization.

INTRODUCTION

The ophiolites are oceanic lithosphere exposed in

western and south western Pakistan at places such as Bela,

Muslim Bagh, Zhob, Khost and Waziristan. The ophiolite

signifies the closure of Tethys sea prior to Indian and

Eurasian continental lithospheric collision (70 Ma ago, Gnos

et al., 1998). This study focuses on Bela ophiolites which are

comprises of thick and segmented rocks along the

westernmost part of the Indian plate, forming a narrow belt

oriented north to south, covering an area 380 kilometer long

and 10-60 kilometer wide. The outcrops of “Bela ophiolite”

is the largest piece of oceanic lithosphere erected from

northwest of modern Karachi to Khuzdar (figure 1 & 2). The

field study witnessed that the peridotite rocks are also found

at Nal-Wad- Kanar –Darakal and Hazargangi areas of

Balochistan. In tectonic settings the “Bela ophioltes” marks

the western collision boundary (between Indian and Eurasian

continental plates). As a matter of fact the western collision

margin is relatively least studied than northern collision

margin (Himalayas) thereby needs more work.

GEOLOGICAL SETTING

The interpretation of geophysical data (magnetic field

data) exhume that the northern part of the ophiolite is offset

along sinistral fault (Ornach-Nal fault) exhibiting the tectonic

influence and the Bela ophiolite continues southward near the

coast of the Arabian Sea covered by alluvium, Zaigham

(1991). The Neothethys ocean floor was present between two

continental plates (Indain to east and Afghan block to the

west). The initial subduction of oceanic floor (Neo-Tethys

sea) was started 70 million years ago, Gnos et al. (1998). Bela

ophiolite is composed of thick rock units which are

tectonically subdivided into two units: the lower unit –

represents the piece of oceanic lithosphere placed of

subducting plate and the upper unit – represents the piece of

oceanic lithosphere of overriding plate (Gnos et al. (1998)

and Khan et al. (1998). The lower unit constitutes the

southern part of Bela ophiolite and is mainly consisting of 3–

5 km thick lava flows and sedimentary rocks (Hunting Survey

Corporation (1960), Sarwar (1992), Gnos (1998) and Khan,

(1998).The upper unit is located in the northern part of Bela

ophiolite, and consists of metamorphic sole, peridotite

(Harzburgite, dunite, pyroxenite) gabbro and a sheeted dike

complex and pillow lavas. The metamorphic sole

characterized the thrust sheet differentiating two tectonic

units (Gnos et al. (1998) and Khan et al. (1998). The mantle

rocks are well exposed between Sunaro Village and Ornach

cross. Moreover, the mantle rocks are also exposed at places

like Nal, Wad, Kanar Darakal and Hazargangi areas as shown

in figure 2. During the emplacement of ophiolite on the

continental margin sediments the sediments are highly

deformed and development of thrust faults and folding

signifies the tectonic activity. These structures and associated

tectonic activity may lead in development of Hydrocarbon

structural traps in underlying as well as overlying sediments.

METHODOLOGY

The present study emphasized on extensive field work of

Bela ophiolite for collection of spatially distributed field

samples (nearly 200 samples) of various rock units of Bela

ophiolite. The results in this paper provide substantial

information about the Peridotite (Ultramafic rocks) of Bela

Ophiolite. Thin sections of the selected field samples were

prepared for petro-graphical analysis. The polarizing

microscope was used to distinguish the mineral composition

of the understudy rocks. The Bela ophiolite is sandwiched

between thick sedimentary rock units. The underlying and

overlying rocks of the Bela ophiolite are summarized in

Table-1.

RESULTS

The results are based on field work, mineralogical and

textural study, and study of dikes and veins in the mantle

section. The petrographical results reveals that the rocks of

the peridotite (ultramafic rocks) can be divided into

harzburgite, depleted harzburgite, dunite, serpentinite, dikes

and veins. The findings are sum-up in the following.


Page 17 ISSN 2415-2234 © BURJES

Fig. 1 Location of Bela ophiolite shown on Landsat image. The northern white box on Bela ophiolite

shows the location of the local geologic maps prepared by

Gnos et al. (1998) given in figure 2.



Fig. 2 Overview map of the Bela oceanic lithosphere assemblage based onHunting Survey

Corporation (1960) Ahsan et al. (1980) andGnos et al. (1998).



(a) (b)

Fig. 3 (a) A geologic map for the upper unit of Bela ophiolite (after Gnos et al. 1998).

(b) Landsat image distinguishes the upper unit with lithological boundaries.

Fig. 4 Field photograph showing bands of dunite (light) harzburgite (dark)

near Ornach cross, Bela, Balochistan

Peridotite

(Ultramafic rocks)



Table 1. Rock under lying and overlying rock unit of

Bela ophiolite.

A) Mineralogical Study

Harzburgite

The harzburgite are mainly exposed in the upper unit

(northern unit) of Bela ophiolite between Sunaro village and

Ornach cross (Balochistan) shows a transition towards dunite

(further north). It forms massive bodies the peridotite

(mantle section) in the study area (Figure 5). The

morphology of the harzburgite in the study area resembles

“hob-nail” structure. The thin section study shows that the

harzburgite are highly serpentinized. The weathered color of

the harzburgite is greenish black but the fresh sample color

varies from dark brown to brownish, covered by sparse

seasonal vegetation. They are medium to coarse grained and

prophyroclastic to mylonitic in texture. The mineralogical

composition of harzburgite consists up of orthopyroxene,

spinel, and olivine with clinopyroxene as a minor constituent

and does not exceeds more than 2%. The percentage of the

mineral constituents are mentioned in table-2.

Fig. 5 Field photograph showing close-up view of the

normal Harzburgite in the Baran Lak area, Khuzdar District

Depleted Harzburgite

The term depleted harzburgite characterized those

harzburgite enriched in dunite. During field work of Bela

ophiolite, a section before reaching the dunite rich zone

(transition zone) we observed the depleted harzburgite in

north of the main massif of harzburgite. Figure 6 depicts the

out crop view of depleted harzburgite in the study area.

Depleted harzburgite mainly consist up of orthopyroxene,

spinel, and olivine. Sometimes few grains of clinopyroxenes

were also observed. The percentage of mineralogical

assemblages are given in table-2

Fig. 6 Field photograph of the mantle section showing

depleted harzburgite (dunite rich)

Dunites

Further moving north, 1 kilometer thick massive dunite

bodies lying below the Moho boundary in the study area. The

dunite bodies represents a transition between crust and

mantle. Figure 7 illustrates the dunite in the transition zone

north of Lak Baran area. Many outcrops of dunites have the

concentration of chromites which are being mined locally in

the areas. The structures in these mineable chromite bodies

are concordant to discordant with the foliation of host rock

(dunite) such as chromite deposits in semail ophiolite, Oman

(Ali & Khan, (2014) and Muslim Bagh ophiolite of Pakistan.

The chromite deposits in Bela ophiolite are common in the

Lak Baran area and Sunaro area. Dunites are mainly

composed of olivine (More than 90%) and spinel minerals.

The percentage of mineralogical assemblages are given in

table-2

Fig. 7 Field photograph showing the dunite in the

transition zone north of Lak Baran area



The transition zone is well exposed in the north of Lak

Baran and Ornach cross as shown in figure 7. In the study

area the width of transition zone is one kilometer thick. The

transition zone separates the depleted harzburgite and the

base of the continuous layered gabbro. Field study witnessed

that the transition zone is rich in dunites, however,

southwards, the dunite bodies are penetrated into the

depleted harzburgite whereas northward, alternate bands of

harzburgite and dunites are found.

Serpentinites

The study area is characterized with 50 – 80% of the

peridotites (ultramafic rocks) which are highly serpentinized.

Sepentanization signifies the process of hydrothermal

metamorphism in which serpentine minerals replaced the

pre-existing minerals of harzburgite and dunite and thus form

serpentinite rocks. The serpentinite rocks are massive and

vary in color from light green to brownish green in the study

area. The serpentines are primarily composed of serpentine

minerals (chrysotile and lizardite) whereas accessary

minerals are opaque (spinel and magnetite) and olivine. The

massive serpentinization had affected principally the dunites.

Table 2. Mineralogical composition

Minerals Harzburgite Depleted

Harzburgite

Dunite

orthopyroxene 15 %, 5 % -

clinpyroxene, 2 – 3% -

spinel 2 % 2 % 1-2 %

olivine 75 % >90 % 90 %

B) Textural Study

Observations have been carried out on more than 50 thin

sections. By considering the behavior of the olivine,

orthopyroxene and spinel the following textural types have

recognized.

Mylonitic Texture

The geological conditions associated with movement

along the fault planes engender grinding, crushing and

recrystallization of minerals in host rocks and form mylonitic

texture. The said texture is unique characteristic of basal

peridotites which are present just above the metamorphic

sole in Sunaro area (figure1 & 2). During field work, the

mylonitic rocks found in few meters above the amphibolite

which are fine grained, very hard and compacted. It is very

difficult to distinguish the grains with the help of hand lens

because average grain size is smaller than 0.1 mm. In thin

section the olivine grains are largely recrystallized (figure 8).

Porphyroclastic Texture

Porphyroclastic texture is typically developed in the

basal peridotites and is frequent in the massif of Sunaro area.

In study area the porphyroclastic texture is identified with

bimodal grain size distribution of orthopyroxene mineral

(millimeter-sized). Porphyroclasts are fine grained and

dominated with olivine matrix (figure 9)

Fig. 9 Photomicrograph of basal peridotite illustrating

mylonitic texture

Equigranular or Coarse Grained Texture

Figure 10 provides a record of equigranular texture

studied under microscope. The equigranular texture is

characterized with a unimodal grain size distribution about

4-5 mm. Its boundaries are sharp, often slightly curved, and

meeting in triple points at 120 degree angle in XY-plane. The

grains are slightly flattened and elongated. Most of them lack

optically distinguishable substructures. This texture is

common in the upper level of the mantle section in the

studied area.

Fig. 10 Photomicrograph of harzburgite illustrating

equigranular texture

C) Study of Dikes And Veins In The Mantle Section

During field work dikes and vein are found ranging from

few centimeters to decameters in the mantle section of Bela

ophiolite. In present study we classified the veins and dikes

into four groups such as 1) Dunite veins, 2) Pyroxenite dikes,

3) Gabbro dikes, 4) Doleritic dikes.

Dunite “Veins”

The dunite veins are made up of forsterite and spinel

minerals and the spinel minerals are parallel to wall rock

reveals the plastic flow of the mantle in the study area. The

veins are forming either sharp or diffuse contact with the host

Fig. 8 Photomicrograph of basal peridotite illustrating

mylonitic texture



rocks (dunite, harzburgite). Most of the veins are less than 20

cm thick, however narrow vein (2 to 5 cm) are common.

Pyroxenite Dikes

The pyroxenite dikes are enriched with orthopyroxenites

and clinopyroxenites minerals. They are centimeter to

decimeter thick and abdundant in the upper part of the mantle

section in study area. The said veins are forming symmetrical

screens of depleted dunites on both sides of the dikes. The

orthopyroxenite occur in the lower level whereas the

clinopyroxenites have observed in the upper level of the

mantle section, showing general orientation 1300 N and 350

N.

Gabbro Dikes

The gabbro dikes are commonly just below the Moho

boundary in the study area which abundant in upper most

part of the peridotite (ultramafic rocks). The thickness of

gabbroic dikes vary from few centimeter to meter in the

study area. The average orientation of these dikes is 250 N.

Doleritic Dikes

Few doleritic dikes are observed in the mantle section of

the studied area which are discordant the peridotite

structures. The dolerites dikes are identified in field by dark

grey to greenish grey color. The serpentinization along the

contact of dike with the host rock signifies the contact

metamorphism in the study area. The thickness of doleritic

dikes vary from meter to decameters. The general trend of

these dikes is 1200 N and 400 N. In the field these dikes can

be recognized as fine and coarse-grained doleritic dikes.

These dikes are more resistant to weathering than the

harzburgite.The plagioclase phenocrysts and laths, and

crystals of pyroxene can be easily seen with the help of hands

lens embedded in the aphanitic ground-mass. These dikes are

mainly composed of plagioclase 55-60%, clinopyroxene

(augite) 15-28%, orthopyroxene (hypersthenes) 3-5% as

essential minerals, and the rest as alteration products.

Epidote is found as accessory mineral in these rocks. The

alteration minerals are hornblende, epidote etc. Under the

microscope they show doleritic texture.

CONCLUSIONS

The peridotite rocks are well exposed between Sunaro

village and Ornach cross.

The peridotites are mainly consist of harzburgite and

dunite.

The lherzolitic rocks are not found in the Bela ophiolite.

The peridotite of Bela opholite are mainly

serpenitinized.

Four group of dikes and veins have been distinguished

in the area.

Podiforms choromite deposits are found in the mantle

section in Bela ophiolite and being mined on large scale.

REFERENCES

[1] Ahsan, S.N., Akhtar, T. and Ali, Khan, Z., (1988). Petrology

of the Bela-Khuzdar ophiolites, Baluchistan, Pakistan.

Geological Survey of Pakistan Inf. Rel., 307, 24.

[2] Ali M. and Khan M. J. (2013). Gephysical hunt for chromite

in ophiolites. Int. j. of econ and env. Geology. Volume 4 (2)

pp. 22-28

[3] DeJong & A.M Subhani (1979). Notes on Bela ophiolites with

special reference to Kanar area. In: Farah, A; & Dejong, K.A;

(Eds) Geodynamics of Pakistan. Geol. Surv. Pak; Quetta, 263-

269.

[4] Gnos E, Khan M, Mahmood K., Khan A.S., Shafique N.A, and

Villa I.M, (1998). Bela oceanic lithosphere assemblage and its

reation to the Reunion hotspot. Terra Nova 10, 90-95. Hunting

Survey Corporation (HSC), 1960. Reconnaissance geology of

part of West Pakistan: report published for Government of

Pakistan by the Government of Canada, Toronto, 550 p.

[5] Khan M, Gnos E., Khan A.S., and Mahmood K., (1998).

Genetically two different types of basaltic rocks from Bela

ophiolite balochistan, Pakistan. Acta. Mineralogica

Pakistanica 9, 27-36.

[6] Sarwar G., (1992). Tectonic setting of the Bela ohiolites,

southern Pakistan. Tectonophysics 207, 359-381.

[7] Zaigham N.A, and Malik K.A (1992). Upward continuity

filtering of the Uthal gravity data. Acta Miner. Pakistan;6, 149-

158.

[8] Zaigham N. A. (1991). Bela Ophiolite and associated

mineralization in southern part of Axial- belt of Pakistan

Unpublished doctoral dissertation, University of Karachi

Pakistan, 370p

[9] Xiong, Y, Khan D.S, Muhmood, K; Sisson V.B, (2011).

Lithological mapping of Bela ophiolite with remote-sensing

data. International journal of remote sensing, 32,16, 4641-

4658.

_____________________________________________________

Received : 5th May, 2017

Revised : 25th May, 2017

Accepted : 5th June, 2017

_____________________________________________________

Geology and Petrography of Peridotites (Mantle Section ... · Available on: Bahria University...

Documents

Transcript of Geology and Petrography of Peridotites (Mantle Section ... · Available on: Bahria University...