geology and tectonic setting of nal ophiolite, district...

American Journal of Earth and Environmental Sciences

2018; 1(2): 115-123

http://www.aascit.org/journal/ees

Geology and Tectonic Setting of Nal Ophiolite, District Khuzdar, Balochistan, Pakistan

Mehrab Khan1, Muhammad Jahangir Khan

1, *, Muhammad Ishaq Kakar

2, Khalid Mehmud

3

1Department of Earth & Environmental Sciences, Bahria University (Karachi Campus), Karachi, Pakistan 2Center of Excellence in Mineralogy, University of Balochistan, Quetta, Pakistan 3Department of Earth Sciences, University of Sargodha, Sargodha, Pakistan

Email address

*Corresponding author

Citation Mehrab Khan, Muhammad Jahangir Khan, Muhammad Ishaq Kakar, Khalid Mehmud. Geology and Tectonic Setting of Nal Ophiolite,

District Khuzdar, Balochistan, Pakistan. American Journal of Earth and Environmental Sciences. Vol. 1, No. 3, 2018, pp. 115-123.

Received: February 11, 2018; Accepted: March 7, 2018; Published: May 18, 2018

Abstract: The ophiolite is exposed near Nal village (Khuzdar district) of Balochistan, Pakistan which represents the

northern part of Bela ophiolite. The Nal ophiolites are scattered outcrops and erected in an elongated N-S trend which is

relatively thin (in E-W direction) as compared to Bela ophiolites. In this study, these ophiolitic outcrops were mapped on the

detailed scale, such a detailed plan of geological units of understudy area was not available before. The Nal ophiolite along

with Bela ophiolite is emplaced on the Indian continental margin sediments in Paleocene. The Nal ophiolite consists of

serpentinized harzburgite, serpentinized dunite, gabbro, norite, troctolite, diorite and pillow lavas and dikes. A large number of

doleritic dikes and clinopyroxenite dikes are intruded in the mantle section of understudy area. In the second part of this study

we have discussed the petrography of primary minerals which are composed of the rock units of Nal ophiolite. These rocks

exhibit a variety of textures, commonly, porphyritic, glomeroporphyritic, amygdaloidal, ophitic to sub-ophitic, mylonitic, and

porphyroclastic.

Keywords: Nal Ophiolite, Peridotite, Pillow Lava, Doleritic Dike, Mantle Section

1. Introduction

The study of Nal ophiolites (Balochistan, Pakistan) reveals

several secrets of plate kinematics and paleogeography of

Indian and Afghan block. Neo-Tethys oceanic crust

development is engendered by geotectonic evolution which is

helpful to study the Earth’s Mantle section in western margin

of Indian plate. Locally, the study of Peridotite (ultramafic

rocks) and Nal ophiolite is important for geological

understanding and mapping of ophiolites in Balochistan. This

geological study will also be beneficial for hunting ore

deposits in ophiolites and to determine certain petrogenetic

processes in Baluchistan.

The Nal ophiolites were first reported by [5] named the

said rocks as “porali intrusions”. The Nal ophiolite scattered

and outcrop in an elongated N-S trend which is relatively thin

(in E-W direction) as compared to Bela ophiolites (Figure 1).

No detail work of these rocks has been done so far,

nevertheless, many geologists ([1], [2], [3], [12]) worked on

the southern part of Bela ophiolite The Bela ophiolite is

underlain by Thar formation and overlain by the Nal

Limestone. The contact between ophiolitic rocks and the Nal

limestone in unconformable. The Hinglaj group traditionally

overlies the Nal Limestone and Kech conglomerate

unconformably overlies the Hinglaj group (Figure 2).

The geological contributions of [3] proposed that the Bela

ophiolite consists of two units, the upper and lower units. The

upper unit represents the slice of overriding plate while the

lower unit signifies the piece of subducting plate (under

thrusting) which was adjacent to the Indian plate. The upper

unit is well exposed between Khuzdar-Nal and Sunaro

village (Figure 1). While the lower unit mainly covers the

area between Sunaro village and Uthal town. The upper unit

comprises metamorphic sole rocks, peridotites, Gabbro,

sheeted dikes and pillow lavas. The complete sequence of

Bela ophilite is exposed between Ornach cross - Sunaro

village Khanozai village (Figure 3) However, a large and

dismembered pieces of upper unit is also exposed in Nal,

116 Mehrab Khan et al.: Geology and Tectonic Setting of Nal Ophiolite, District Khuzdar, Balochistan, Pakistan

Wad, Darakolo, and Harargangi (Figure 1). Here the lower

unite is visible between Sunaro village and Uthal town

(Figure 1) which is mainly composed of pillow lavas and

sheet flows covered by sedimentary rocks (radiolarian cherts,

shales and calciturbidite) [3]. The lower unit is intruded or

capped by microgabbro intrusions, sills and alkaline volcanic

rocks related to Reunion hot spot. These Reunion related

rocks cover as much as 30% of the lower unit [3].

Figure 1. An overview of the Nal oceanic lithosphere assemblages.

American Journal of Earth and Environmental Sciences 2018; 1(3): 115-123 117

The study area lies in the Survey of Pakistan topo-sheet No. 35 1/1 and 35 ½ and is bounded by 66°7’ E to 66°12’ E

longitudes and 27°40’N to 27° 47’N latitudes. This study presents first time the field mapping, petrography and modal analysis

of Nal ophiolite. Figure 2 illustrates the detailed field map of Nal ophiolite.

Figure 2. Detailed Geological map of rock units of Nal Ophiolite. The Longitudes (along x axis) and Latitudes (along y axis) are in Degree Decimal system.


Figure 3. (a) A geologic map for the upper unit of Bela ophiolite (modified after [3]). (b) Landsat ETM+ image distinguishes the upper unit of Bela ophiolite

with lithological boundaries (modified after [3]).

2. The Tectonic Settings

The tectonic model suggested by [3] and [6] reveals that

before the emplacement of Bela ophiolite, the intraoceanic

subduction initiated in Neo-Tethys sea at the western side of

Indian plate at 70 Ma (late Cretaceous). Because of this

subduction the Neo- Tethys Sea started to close and Bela

ophiolite was emplaced on the Indian continental margin in

Paleocene ([3] and [11]). After emplacement of ophiolite

over the continental sediments the sea was still present and

shallow marine Nal Limestone (Eocene) and Hinglaj

(Oligocene) formations were deposited on Bela ophiolite.

Continued subduction closed the Neo-Tethys Sea located on

the western side of Indian plate and finally Indian plate and

Afghan micro continent collided in late Oligocene. The

overlying sediments of Bela ophiolite evidence that the Neo-

Tethys Sea located on the western side of Indian plate closed

late (Oligocene) as compared to the collision in the north

between Indian and Eurasian plates. The Indian plate at its

western side collides with Afghan block in Oligocene while

in the north the collision occurred in Eocene (50Ma) ([10]

and [9]). Because of this collision between Indian plate and

Afghan Block the Indian continental margin sediments got

highly deformed, folded and thrusted and formed Sulaiman –

Kirther range. This zone of folds and thrusts is 60-150 km

wide, with a strike length of about 1,250 km. It extends

southward from Waziristan, through Loralai-Bugti area,

around the Quetta syntaxes, down south to Karachi [8]. The

collision ceased and converted into Transform fault

movement. The Chaman Transform fault generated at (25-

20Ma) ([4]). So, the western boundary of Indian plate shows

an excellent geological laboratory in which the subduction,

followed by a collision and then followed by Transform fault

movement can be well studied. The major tectonic elements


are annotated in Table 1.

Table 1. The major tectonic subdivision of western Indian plate from West to

East.

3. Methodology

The present study emphasized on extensive field work of

Nal ophiolite for the collection of spatially distributed field

sample (nearly 100 samples) of various rock units. The

results in this paper provide substantial information about the

geology of Peridotite (ultramafic rocks) exposed in Nal area.

Thin sections of the selected field sample were prepared for

petrographical anlysis. The polarizing microscope was used

to distinguish the mineral composition of the understudy rock

units and modal analysis helped in determining the

percentage of minerals in rocks.

4. Results and Discussion

The results are based on field work, mineralogical and

textural learning along with the study of dikes and veins in

the mantle section

The petrographical results communicate that the rocks of

the peridotite (ultramapfic rocks) can be divided into five

groups known as serpentinized harzburgite, serpentinized

dunite, gabbro, norite, troctolite, diorite and pillow lavas and

dikes. The findings are sum-up in the following.

A) Ultramafic Rocks (Peridodites)

Peridotites consist of Harzburgite and Dunite which are

usually serpentinized, that covers about 75% of the studied

area.

Serpentinized Harzburgite

Harzburgite is mainly composed of olivine and

orthopyroxene where as other constituents include

clinopyroxenes, chromite, magnetite, ilmenite, serpentinite

and calcite. Following types of alteration are found in the

harzburgite.

a) The olivine is altered into serpentinite

b) The orthopyroxene is altered into basalt

c) The clinopyroxenes are slightly altered into actinolite

and green hornblende.

Serpentinized Dunite

The dunites are found at the top level of harzburgite and

are rooted into the harzburgite. The contact between dunite

and harzburgite is gradational. They are mainly composed of

more than 90% olivine and spinels (1-2%) from which about

30% of olivine is altered to serpentine.

Serpentinite

In hand specimens, serpentinites are light green, dark

green or brown in color and are extremely fine grained. They

show numerous slickensides in the field and give a soapy

feeling of touch. They are mainly composed of antigorite but

in some thin sections chrysotile is also found. Unaltered

remnants of olivine and pyroxene and Opaque minerals are

also present in minor amounts. These rocks have been

derived by the alteration of olivine and pyroxene being

parent rocks as is indicated by the unaltered grains present in

the cores surrounded by serpentine.

B) Mafic Rocks

The mafic rocks in the study area are Gabbro, Norite, and

Troctolite.

Gabbro

The gabbros are coarse to medium grained rocks

composed essentially of plagioclase and clinopyroxenes

(augite). Clinopyroxenes is the most characteristic

ferromagnesian mineral of these rocks. Inclusions of the

opaque minerals are present in clinopyroxenes and altered

into tremolite as well as hornblende. Orthopyroxne is also

present but forms not more 2% of the rocks. Biotite, Zoisite

and Apatite are also found in a very less amount and the

opaque iron oxide which is magnetite, found less than one

percent. The alteration in these rocks is slight to moderate,

but in some cases it is intense. The alteration products are

sericite, hornblende, tremolite and chlorite. Plagioclase is

altered to finely grained sericite and the pyroxene

phenocrysts show alteration to tremolite as well as chlorite.

Hornblende is an alteration product of pyroxene whereas

Biotite shows slight alteration to chlorite.

Norite

These rocks are coarse grained, consisting mainly of calcic

plagiolclase and orthopyroxene. In the studied area these

rocks found in the form of Layers. Their weathered color is

brownish black to black and a fresh surface is mottled white

and black to grey black and some are greenish black. The

mineralogical composition in these rocks is, Plagioclase

(28%), orthopyroxene (31%), clinopyroxenes (10%)

Hornblende (9%). The plagioclase found in these rocks is

labradorite to bytownite in composition.

These rocks show moderate to intense alteration.

Orthopyroxene and clinopyroxene show alteration to

hornblende. Reaction rims are present around the pyroxene,

which are composed of hornblende. The grains of

clinopyroxenes show alteration to chlorite and tremolite.

Troctolite

These are essentially composed of olivine and plagioclase.

Olivine is the major constituent of these rocks. Smidgens of

olivine are surrounded by very fine-grained chlorite and

serpentine. In some rocks olivine is completely altered to

serpentine. Chlorite is very fine-grained fibrous and

alteration product of biotite. Picotite and magnetite are

present in very small amounts, not even exceeding one

percent.

These rocks show moderate to intense alteration. The

alteration products are serpentine, chlorite and sericite.

Amygdaloidal Pillow Lavas

Pillow lavas form the upper most unit of the ophiolite.


They are amygdaloidal in texture and amygdules are filled by

zeolites and calcite. The pillows are beautifully zoned and

have radial fractures. Pillow lavas of Nal area are up to meter

across. Basalts are composed of plagioclase (Labradorite to

anorthite) and pyroxene. Opaque minerals are present in

fewer amounts and in some cases olivine is also present. The

amygdaloidal basalts are present in the north of Nal and

Karku village. (Figure 2).

C) Intermediate Rocks

Diorite

In hand specimen these rocks are medium to course

grained. The weathered surface is black grey, the fresh

surface is dark grey. These rocks are mainly composed of

andesine with anorthite of 46 -48%, hornblende 25-30%,

secondary chlorite, 17-20% present, pyroxene 2-3%, quartz

1-2%, and the rest accessories are apatite, epidote, sphene,

and opaque minerals. Diorite show intense to moderate

alteration. The alteration products are sericite, chlorite, and

hornblende. Sericite is the alteration product of plagioclase

whereas Hornblende has been altered from pyroxene.

D) Felsic Rocks

Granite Intrusion

Granite rocks are exposed north of Karku village (Figure

2). It is coarse to medium and mainly composed of quartz,

alkali feldspar (orthoclase), plagioclase, biotite as major

constituents whereas muscovite, clinopyroxenes (augite),

zircon, apatite are found as accessory minerals. About 1%

iron oxide is also found in the granite. Apatite occurs in a

very less amount and zircon is associated with biotite and

also occurs in association with quartz. Opaque iron oxide

includes magnetite and ilmenite and does not exceed more

than 1% by volume.

E) Dikes and Veins

Five groups of dikes and veins have been distinguished in

the Nal ophiolite. These are Pyroxenite dikes, Anorthosite

lenses, magnesite veins, chromite deposits and doleritic

dikes.

Clinopyroxenite Dikes

The Clinopyroxenites are found in the form of dikes. Their

thickness varies from 10cm-30m. These are found in the

harzburgite as well as in the dunite. They mainly consist of

diopside (81-91%) with minor orthopyroxene (2-4%). The

altered minerals are 2-5% and opaque 4%. Some samples

contain a small amount of plagioclase.

Anorthosite Lenses

The Anorthosites are found in the form of lenses. Their

thickness varies from 1-2 meter. These are found in

harzburgite. In hand specimen the rock is phaneritic in

texture. The weathered surface is light grey to brownish and

fresh surface is greenish white. They are mainly composed of

plagioclase (Bytownite to Anorthite) while the other

constituents are augite and hornblende. Calcite is present as

secondary mineral here.

Magnesite Veins

On certain locations magnesite veins run through the

peridotites. The magnesite occurs mostly as cryptocrystalline,

massive form, whereas some of them show well developed

acicular transparent crystals which magnesite forms lenses

and veins.

Chromite Deposits.

Podiform chromites are found in dunite and are a few cm

to m thick and extend several kilometers in lengths. The

veins are often discontinued due to the displacement of

faults. They are concordant and discordant with the host

rock. Two types of chromite deposits are more common:

nodular and massive. The chromite deposits are being mined

on large scale.

Doleritic Dikes

A large number of dikes are intruded in the mantle section.

They give chilled margins. Younger calcite veins occur cross

cutting the doleritic dikes which vary in thickness from

30cm-10meters. These rocks are mainly composed of

plagiolclase and clinopyroxenes (augite) with the opaque

minerals present as an accessory. In some cases, quartz is

present up to 2% by volume. There are a few quartz dolerites

consist of 67% plagioclase, 9% clinopyroxenes, 8% quartz as

essential minerals while the hornblende dolerites contain

plagioclase, hornblende and clinopyroxenes as their vital

minerals. Plagioclase and pyroxene are also highly altered

products.

Table 2. Modal Analysis of Field Samples (SERPENTINITES).

Sr.

No.

Sample

No. OL OPX CPS Srp Cal OP

1. K-M 702 - 8 - 90 - 2

2. K-M 703 9 4 2 78 - 7

3. K-M 707 3 6 2 85 - 4

4. K-M 717 1 4 1 86 - 8

5. K-M 725 4 5 2 77 - 12

6. K-M 727 2 - 1 92 - 5

7. K-M 731 1 - 1 84 13 1

8. K-M 733 - 1 - 73 25 1

9. K-M 737 1 - 1 96 - 2

10 K-M 744 - 12 2 74 2 10

11. K-M 749 1 2 - 74 - 23

12. K-M 755 7 1 - 87 - 5

13. K-M 771 - 3 9 65 - 23

14. K-M 772 1 - 1 61 21 16

15. K-M 779 - - - 73 18 9

16. K-M 787 - - 1 91 - 8

17. K-M - - - 80 - 20

18. K-M 796 - - - 70 - 30

19. K-M 805 - - - 81 - 19

20. K-M 816 - - - 95 - 5

21. K-M 818 - - 1 80 - 20

22. K-M 838 - - - 65 - 35

23. K-M 850 - 7 1 74 - 18


Table 3. Modal Analysis of Field Samples (PYROXENITES).

Sr. No. Sample No. OPX CPX Srp Hbl P1 Chl op

1. K-M 719 3 89 5 - - - 3

2. K-M 720 2 84 11 - - - 3

3. K-M 801 6 75 - - 11 6 3

4. K-M 810 5 59 2 3 - 30 1

5. K-M 815 18 29 36 2 9 4 2

Table 4. Modal Analysis of Field Samples (HARZBURGITES).

Sr. No. Sample No. 01 Opx Cpx Srp Op

1. K-M 701 32 8 4 54 2

2. K-M 704 34 9 6 51 -

Table 5. Modal Analysis of Field Samples (ANORTHOSITES).

Sr. No. Sample No. Pl Cpx Hbl Cal Op

1. K-M 857 89 2 - 9 -

2. K-M 892 90 2 3 4 1

Table 6. Modal Analysis of Field Samples (GABBROS).

Sr. No. Sample No. Pl Or Cpx Opx Bl Hbl Chl Tre Ep Op

1. K-M 756 54 - 31 2 - 2 - 8 2 1

2. K-M 809 40 - 26 3 - 3 9 18 - 1

3. K-M 8 58 3 20 - 10 3 2 3 1 -

4. K-M 867 53 - 36 7 - 3 1 - - -

5. K-M 858 62 - 20 1 - 15 2 - -

Table 7. Modal Analysis of Field Samples (NORITES).

Sr. No. Sample No. Pl Cpx Opx Srp Hbl Chl Tre

1. K-M 880 35 11 29 9 8 5 3

2. K-M 884 32 9 34 8 10 4 3

Table 8. Modal Analysis of Field Samples (TROCTOLITES).

Sr. No. Sample No. P1 01 Srp Bt Chl Cal Op

1. K-M 827 12 38 36 9 10 - 1

2. K-M 829 11 37 31 8 11 1 1

Table 9. Modal Analysis of Field Samples (DIORITES).

Sr. No. Sample No. Pl Cpx Opx Srp Hbl Chl Ep Op

1. K-M 866 60 3 - - 25 8 - 4

2. K-M 842 52 2 - 4 23 17 2 -

3. K-M 843 50 - 1 2 32 14 1

4. K-M 841 52 - - 3 25 18 1 1

5. K-M 832 47 - 2 4 28 18 - 1

6. K-M 766 55 2 - - 23 17 1 2

Table 10. Modal Analysis of Field Samples (BASALT).

Sr. No. Sample No. Pl Cpx Opx Nat Qtz Chl Cal Op

1. K-M 761 56 5 2 - 4 - 3 30

2. K-M 753 58 9 1 - - 24 - 8

3. K-M 752 51 11 - - - 21 - 7

4. K-M 763 56 2 - 5 2 - 3 32

5. K-M 768 38 31 - 2 - 2 18 9

Table 11. Modal Analysis of Field Samples (GRANITES).

Sr. No. Sample No. Pl Or Bt Ms Qtz Op

1. K-M 874 21 24 13 1 39 2

2. K-M 855 26 22 12 1 35 2


Table 12. Modal Analysis of Field Samples (DOLERITES).

Sr. No. Sample No. Pl Cpx Opx Chl Qtz Bt Ep Cal Op Spn

1. K-M 705 59 20 1 10 - 2 - - 8 -

2. K-M 708 52 14 1 24 1 4 - - 4 -

3. K-M 723 61 28 - 6 2 2 - - 1 -

4. K-M 710 56 26 - 5 - 3 1 6 3 -

5. K-M 747 62 16 - 11 - - 1 2 5 3

Table 13. Modal Analysis of Field Samples (QUARTZ DOLERITES).

S. No. Sample No. Pl Cpx Opx Hbl Chl Qtz Bt Cal Op

1. K-M 704 69 9 1 2 13 6 - - -

2. K-M 751 66 5 - - - 10 - 2 17

3. K-M 716 63 10 - - 17 8 1 - 1

4. K-M 765 72 11 - 1 3 6 - - 7

Table 14. Modal Analysis of Field Samples (HORNBLENDE DOLERITES).

SR. No. Sample No. Pl Hbl Cpx Qtz Op

1. K-M 718 51 37 11 1 -

2. K-M 724 50 38 5 2 5

3. K-M 769 49 40 4 - 7

4. K-M 802 51 39 9 - 1

5. Conclusion

Nal ophiolite is found in the northern part of Bela ophiolite.

The Nal ophiolite comprises serpentinized harzburgite,

serpentinized dunite, clinopyroxenite dikes, gabbro, norite,

troctoctolite, diorite and pillow lavas. A large number of

doleritic dikes are intruded in the mantle section. Nal

ophiolite is harzburgitic in composition so it is harzburgite

type ophiolite. The Nal ophiolite along the Bela ophiolite

emplaced in Paleocene on Indian continental margin

sediments.

Acknowledgements

The authors acknowledge the efforts of field volunteers

and students for collecting the field samples and mapping of

rock units.

Nomenclature

Sr Symbol Mineral

1. Ol Olivine

2. Opx Orthopyroxene

3. Cpx Clinopyroxene

4. Srp Serpentine

5. Hbl Hornblende

6. Pl Plagioclase

7. Qtz Quartz

8. Chl Chlorite

9. Cal Calcium

10. Ep Epidote

11. Tre Termolite

12. Bt Biotite

13. Or Orthoclase

Sr Symbol Mineral

14. Ms Muscovite

15 Nate Natrolite

16 Op Opaque Minerals

References

[1] Ahmed, Z., (1991). A supra-subduction origin of the Bela ophiolite indicated by acidic rocks, Khuzdar District, Pakistan. Acta, Miner, Pakistan 5, pp. 9-24.

[2] 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, pp. 24.

[3] 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, pp. 90-95.

[4] Hadi S., Khan S. D., Owen L. A., Khan S. A., Hedrick K. A., Caffee M. W., (2013). Slip rate of Chaman fault; Implication for transient strain accumulation and strain partitioning along the western indian plate margin. Tectonophysics 608, pp. 389-400.

[5] Hunting Survey Corporation (HSC), (1960). Reconnaissance geology of part of West Pakistan: a report published for Government of Pakistan by the Government of Canada, Toronto, pp. 550.

[6] Khan, M (1999). Petrological and structural studies of the ophiolites of the Baran Lak area, Bela- Khuzdar, District, Baluchistan, Pakistan. Ph.D. Thesis (unpublished) University of Balochistan, Quetta, Pakistan, pp. 147.

[7] 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 Pakistani ca, 9, pp. 27-36.


[8] Kazmi, A. H; and Jan, M. Q (1997). Geology and tectonics of Pakistan. pp. 120.

[9] Patriat, P; & Achache, J (1984). India-Eurasia collision chronology and its implication for crustal shortening and driving mechanisms of plates. Nature, 311, pp. 615-621.

[10] Powell, C, Mc A (1979). A speculative tectonic history of Pakistan and surroundings: Some constraints from the Indian Ocean. In: Farah, A; & DeJong, K. A: (eds.) Geodynamics of Pakistan. Geol. Surv. Pak; Quetta, pp. 5-24.

[11] Zaigham, N. A, and Malik, K. A (1992). Upward continuity filtering of the Uthal gravity data. Acta Miner. Pakistan; 6, pp. 149-158.

[12] Zaigham, N. A (1991). Bela Ophiolite and associated mineralization in the southern part of Axial- belt of Pakistan Unpublished doctoral dissertation, University of Karachi Pakistan, pp. 370.

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