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J. mt. area res., Vol. 6, 2021 

   

         Journal of Mountain Area Research 
            

 

 

GEOLOGICAL AND PETROGRAPHICAL CHARACTERISTICS OF SORAP 

MASSIF GABBROS, RAS-KOH OPHIOLITE, BALOCHISTAN, WESTERN 

PAKISTAN 
Ali Muhammad, Abdul Ghaffar*, Inayat Ullah, M. Ishaq Kakar, Fida Murad    

Centre of Excellence in Mineralogy, University of Balochistan, Quetta, Pakistan 

 

ABSTRACT 

This study discussed the Sorap Massif which is the ophiolitic fragment composed of the upper mantle 

and lower crustal section of an ophiolitic sequence. An ophiolitic sequence in Sorap Massif consists 

of harzburgite, dunite, large distant units of serpentinized wehrlite intrusion and outcrop of confined 

layered gabbro covered by the Quaternary sand dunes. The basal part of gabbroic intrusion is in 

contact with mantle rocks and the upper part is juxtaposed with the Kuchakki Volcanic Group. On 

outcrop-level and in hand specimen, the gabbros exhibit needle-like ferromagnesian minerals 

including hornblende associated with plagioclase and pyroxene. Petrographically the gabbros are 

classified into norite, gabbro norite and gabbro. The mineral constituents of norite are plagioclase, 

orthopyroxene and amphibole, gabbro norite is consists of equal constituents of orthopyroxene and 

clinopyroxene, plagioclase and amphibole and the gabbro is composed of clinopyroxene, 

plagioclase and amphibole. The high constituents of minerals such as pyroxene, amphibole and low 

constituents of olivine in the Sorap gabbros indicates that these gabbros are formed by the immature 

part of the oceanic plate with dehydration of the oceanic plate subduction. 

KEYWORDS: Sorap Massif, Ras-Koh Ophiolites, Kuchakki Volcanic, Gabbro, Ocean plate subduction 

 

*Corresponding author: (Email: ghaffar.chagai@gmail.com) 

 

1. INTRODUCTION 

An ophiolite is a distinctive rock unit of 

ultramafic-mafic rocks sequence from bottom 

to the top including serpentinized ultramafic 

rocks at the base, gabbroic rocks periodically 

indicated the cumulate texture, complex of 

mafic sheeted dikes and mafic volcanic rock 

associated with distinctive bodies of chromite, 

sodic constituents of felsic types of igneous rock 

overlying with pelagic sedimentary rocks (1). In 

1813 the French Mineralogist Alexander 

Brongniart has first introduced the Ophiolite 

regarding serpentine in the mélanges, 

according to his subsequently redefined the 

definition of an ophiolite to include a suite of 

ultramafic to mafic types of magmatic rocks 

that occurred in the Apennines (2).  Gustav 

Steinmann (1927) elevated the term ophiolite to 

a modern concept, he said that the ophiolite is 

spatially associated with kindred rocks that are 

originated in the suite of intrusion in the axial part 

of geosynclines.  The genetic model and 

definition of ophiolite have been varied. Still, it is 

Full length article 

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mainly accepted that the ophiolite indicates 

that the oceanic crust has been emplaced 

onto the continents (3). An ophiolite expresses 

the fragments of the upper mantle and 

oceanic-crust (4, 5, 6), that were integrated on 

the margins of continental crust arc-continent 

and continent-continent collision (7), ridge-

trench interaction (8, 9) and subduction 

accretion (10). An ophiolite has widely occurred 

along the suture zone in both accretionary and 

collisional types in the orogenic belt, which 

demarks the tectonic boundaries between two 

plates or accreted terranes (11). Specific 

analysis of well-exposed ophiolite last two 

decades indicated that the main ophiolite 

formed in the supra-subduction tectonic setting 

in the upper part of the convergent plate and 

not occurred in the mid-oceanic ridge 

spreading centers (12, 13, 14). An ophiolite is a 

distinctive rocks unit of ultramafic – mafic rocks 

sequence from bottom to the top including a 

serpentinized ultramafic rock, gabbroic rocks 

periodically indicated the cumulate texture, 

complex of mafic sheeted dikes and mafic 

volcanic rock associated with distinctive bodies 

of chromite, sodic constituents of felsic types of 

igneous rock overlying with sedimentary rock 

units such as chert, shale and minor bed of 

limestone (5). Gabbros are formed by the 

basaltic magma, which is raised from the 

underlying mantle and as well as formed by the 

slow cooling in the magma chamber (15). 

Gabbros are essentials parts of the crustal 

section of an ophiolitic suite from bottom to the 

top including a layered gabbro in the basal part 

of the crustal section and isotropic to foliated 

gabbros at the top of the crustal section. In the 

worldwide well-exposed gabbroic section is 

widely exposed in the Semail ophiolite in Oman 

and Bay of Island ophiolite in Newfoundland 

Canada (6, 16). Geological features of the 

crustal part of Ras-Koh ophiolite have been 

early recorded by (17, 18). Until there is no 

detailed geological study of the gabbroic 

section of Sorap Massif in the Ras-Koh ophiolite. 

In this paper, we are going to discuss the 

geological and petrological features of the 

gabbroic rock units from Sorap Massif in the Ras-

Koh ophiolite by using their field features and 

petrography results. 

2. GEOLOGICAL SETTING OF RAS-KOH 

ARC 

Ras-Koh Arc is the southern part of Chagai-Ras-

Koh double chain arc system (19), trending NE 

to SW direction and about 250 km long in E-W 

direction and 40 km wide in N-S direction (20, 

21). The northern margin of the Ras-Koh range is 

border by the Dalbandin Trough, from the east 

it is terminated by the Noshki-Chaman Fault, 

from the west it is ended by Hari Rud fault while 

it is covered by the Quaternary alluvial and sand 

dune of the Kharan desert in the south (Fig.1) 

(22). The NE to SW trending direction of the Ras-

Koh volcanic arc is disputed to the E to W 

trending direction of the Chagai-Magmatic arc 

which is considered the frontal arc of the 

Chagai-Ras-Koh arc system (23, 24). The 

southern part of Ras-Koh Range is bounded by 

the fore arc basin to Makran coast area mainly 

comprised up flysch types of sediments called 

Makran accretionary wedge. Makran 

accretionary wedge deformed thick zone 



 
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developed by the prolonged northward 

deposition of marine sediments during the 

Eocene to Miocene periods. Combine the 

accretionary prism and fore arc basin is one of 

the widest trench arc gaps on the earth (25, 21).  

The Ras-Koh ophiolite is located in the western 

part of the Ras-Koh range, it can be 

distinguished by the two main localities in this 

range, i.e., Bunap intrusion and Sorap Massif. 

These ophiolites are intensely serpentinized 

harzburgite outcrops in the Bunap area and 

moderately weathered in the Sorap Massif at 

the western margin in the Ras-Koh range (Fig.1). 

Mainly the outcrops are faulted and thrusted 

occurred along the parallel fault in the main 

trend of the range. The red conglomerate 

mainly consists of basaltic fragments and cherty 

pebbles associated with serpentines at different 

localities that indicate the Late Cretaceous 

weathering and sub-areal exposure of the 

ophiolites, it may be formed before the 

deposition of the Late Cretaceous to 

Paleocene Rakhshani Formation (17). 

 

 

Figure.1. Simplified Tectonic and geological map of Ras-Koh 

Arc, showing the distribution of lithological units and Ras-Koh 

ophiolite in the region (20). 

Stratigraphic sequence of the Ras-Koh Range 

comprised of Early to Late Jurassic Ras-Koh to sub-

recent-recent alluvial deposits. Early to Late Jurassic 

accretionary complex mainly consists of Bunap 

sedimentary complex, Charkohan radiolarian chert 

and Ras-Koh ophiolitic mélanges. Late Cretaceous 

Kuchakki Volcanic Group mainly composed of 

basaltic to the andesitic lava flow and volcanic clast 

including agglomerate, tuff, lapilli tuff, breccia and 

volcanic conglomerate associated with sandstone, 

limestone, mudstone siltstone and radiolarian chert 

(20, 18).  Paleocene Rakhshani Formation is 

composed of volcanic flysch subordinated with 

sandstone intercalation of shale, mudstone and 

limestone indicating a turbidities sequence (26, 20, 

18). Eocene Kharan Limestone comprised of medium 

to thick-bedded of fossiliferous to argillaceous 

limestone, Eocene to Oligocene Nauroze Formation 

dominantly shale interbedded with sandstone and 

minor thin-bedded of limestone and conglomerate 

and Miocene to Pliocene Dalbandin Formation 

composed of shale subordinate with sandstone, 

shale and conglomerate capped by the sub recent 

to recent gravel, sand, silt and clay (Fig.2) (20, 18, 22).     



 

 

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Figure.2. Generalized stratigraphic column of Ras-Koh 

Arc (18).  

3. GEOLOGY OF SORAP MASSIF AND 

GABBROS 

Ophiolitic outcrops in Sorap Massif at the 

western part of Ras-Koh range covered by the 

transition from the upper mantle to the lower 

crustal section of an ophiolitic sequence. 

Ophiolite sequence is mainly composed of 

harzburgite, dunite, large distinct units of 

serpentinized wehrlite intrusion and an outcrop 

of confined layered gabbro covered by the 

sand dune in the Sorap Massif (Fig.3). The crust-

mantle transition zone (Moho) is somewhere 

exposed in the study area. Low grade 

disseminated to nodular chromite ore is 

currently mined at three localities in the Sorap 

Massif (Fig.3) (17). Since neither a metamorphic 

sole nor the upper part of a classic ophiolite 

series has been discovered (1), we can only 

conclude that the outcrops of the Sorap region 

are tectonically less affected equivalents of the 

Bunap ophiolite based on their similar tectonic 

location above the Kuchakki Volcanic Group. 

Outcrop of gabbroic intrusion of Sorap Massif is 

located in the southwest, approximately 40 to 

60 Km from Dalbandin. 

 

Figure.3.  The geological map illustrates the study 

area in the Sorap Massif in Ras-Koh Ophiolite (17).  

In fieldwork, the geological mapping and 

lithological characteristics have defined the 

rock composition and length of gabbroic 

intrusion of Sorap Massif. The field features of 

Sorap Massif mainly composed up basic types 

of gabbroic rock units. The gabbro intrusion is 

trending from the north-east to south-east 

direction from eastern to the western part of 

Sorap Massif area. In the eastern part of the 

study area in Sorap Kaur (Nala) intrusion of 

gabbroic rock units’ basal part of gabbroic 

intrusion mainly composed of mantle rock units 

harzburgite, mylonitic peridotite, dunite with 

close jointing of gabbro, contact with the 

Cretaceous Kuchakki Volcanic Group and 

covered by the Quaternary sand dune (Fig.4a-

b). In the central part of the study area, the 

outcrop of gabbroic rock units mainly 



 
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composed of needle-like dark ferromagnesian 

minerals i.e., hornblende associated with light 

minerals plagioclase (Fig.4c). This part of the 

gabbro mainly consists of a foliated-like 

structure because of the intense condition of 

pressure due to elastic deformation during 

emplacement. In the western part of the study 

area, the intrusion of gabbroic rock units is 

associated with dunite in the Gar-Koh area. In 

this area the gabbroic outcrop covers 1-1.5 Km2, 

the rock units in this area are associated with 

concordant to dis-concordant layered of 

harzburgite, dunite and pegmatite vein within 

the gabbroic intrusion in the wehrlite rock units 

(Fig.4d-f).   

 

Figure 4 a-f. Field features of the Sorap Massif; Mantle 

rock units contact with Kuchakki Volcanic Group and 

Gabbroic rock units covered with sand dune (a and 

b). Gabbroic units consist of needle-like 

ferromagnesian minerals with light coarse grains of 

plagioclase (c). Ophiolitic rock (Dunite) is mined for 

chromite (d). Wehrlite is covered by the quaternary 

alluvial, sand dune and interlayered with gabbro (e 

and f). 

4. MATERIALS AND METHODS 

Detailed geological fieldwork of two weeks was 

conducted in the Sorap Massif in the Ras-Koh 

Ophiolitic belt.  During the fieldwork, the area 

was geologically mapped, described all-field 

features of the ophiolite, identified the 

lithological units and collected the twenty least 

altered samples for laboratory analysis. After 

that, the thin sections of eighteen least altered 

samples were prepared for petrographic study. 

The study of the petrographic thin section was 

carried out on Leica DM 750 digital polarized 

microscope in the Centre of Excellence 

Mineralogy, University of Balochistan (CEM-

UOB). 

5. RESULTS  

5.1.1. Norite 

Norite is mainly composed of plagioclase, 

orthopyroxene, amphibole, secondary minerals 

are sericite, chlorite and minor constituents of 

serpentine and accessory minerals are including 

epidote, apatite and opaque minerals. The 

texture is interlocking and subophitic, medium 

to coarse-grain of plagioclase, orthopyroxene, 

fine to medium grain amphibole with fine grain 

sericite and chlorite. Crystal shapes of medium 

to coarse grains have subhedral to euhedral 

crystal shape and fine grains have anhedral to 

partially subhedral crystal shape (Fig.5a-b).  

Plagioclase has medium to coarse grain texture, 

euhedral elongated laths crystal shape with 



 

 

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albite twining, 1st order interference color in 

cross-polarized light. In plane-polarized light, the 

plagioclase has colorless to white interference 

color with low to moderate relief. 

Orthopyroxenes have coarse grain texture, 

euhedral crystal shape with prismatic crystal 

structure and sample and lamellar twinning, 

highly pleochroism with 2nd to 3rd order 

interference color and right-angle well-

developed cleavage with parallel extension in 

cross-polarized light and plane-polarized light 

orthopyroxene have colorless to greenish 

interference color with moderate to high relief. 

Amphibole has fine to coarse texture, subhedral 

to euhedral crystal shape, 2nd order interference 

color and has a double set 56◦ to 126◦ angle of 

cleavages in cross-polarized light and plane-

polarized light the amphibole has high greenish 

to brownish pleochroic with moderate relief.    

Model constituents of norite is; plagioclase (44 – 

69.28%), orthopyroxene (18.3 – 28%) and 

Amphibole (5 – 10%).  

5.1.2. Gabbro Norite 

Gabbro norite mainly consists of plagioclase, 

equal constituents of orthopyroxene and 

clinopyroxene, amphibole with secondary 

minerals are sericite, chlorite and accessory 

minerals are epidote, apatite and opaque 

minerals. Gabbro norite has ophitic to sub-

ophitic and granular textures. Pyroxene grains 

are enclosed by plagioclase grains. In gabbro 

norite, the plagioclase grains are partially 

altered into sericite and pyroxene grains are 

intense to partially altered into the chlorite 

(Fig.5.c-d).  

Plagioclase has medium to coarse grain texture, 

subhedral elongated crystal shape with regular 

polysynthetic twinning and partially altered into 

sericite. Pyroxene grains are enclosed by the 

plagioclase have a double set of cleavage with 

an irregular fracture with anhedral to subhedral 

crystal shape with 2nd to 3rd order interference 

color and highly pleochroic with moderate to 

high relief. Pyroxene is intensely altered into 

chlorite.  

Model constituents of gabbro norite is; 

plagioclase (58.57 – 62.82%), orthopyroxene 

(11.66 – 25.71%), clinopyroxene (12.85 – 23.21) 

and Amphibole (6.66 – 12.14%).  

5.1.3. Gabbro  

Gabbro is mineralogically composed of 

high constituents of the primary minerals 

plagioclase, clinopyroxene and amphibole, 

secondary minerals are high constituents of 

sericite and chlorite with accessory minerals 

are including epidote, apatite and opaque 

minerals. Gabbro has sub-ophitic to 

interlocking texture between plagioclase 

and clinopyroxene grains. In a few thin 

sections, mediums to coarse grains laths of 

plagioclase and clinopyroxene are 

bounded by the fine grain chlorite and 

sericite in the groundmass.  The main 

texture types identified are poikilitic and 

granular. In poikilitic texture, the coarse 

grain clinopyroxene is enclosed by the fine 

to medium grains laths of plagioclase 

(Fig.5e-f).  

In gabbro, the plagioclase has medium to 

coarse grain texture, subhedral to euhedral 

crystal shape with pericline to albite twining, 1st 



 
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order interference in cross-polarized light and 

plane-polarized light. It has a colorless to white 

interference color with low to moderate relief. In 

a few thin sections, the plagioclase is intensely 

altered into secondary minerals sericite. 

Clinopyroxene has medium to coarse grain 

texture, subhedral to euhedral crystal shape 

with sample and a lamellar twin along the 

composition plane, 2nd to 3rd order interference 

color has pale green, pale brown, brown and 

pink color, perfect cleavage and irregular 

fracture with incline extinction along the 

cleavage angle in cross-polar light. Plane 

polarized light clinopyroxene has highly greenish 

pleochroic with moderate to high relief. 

Aegirine and augite are common members of 

clinopyroxene mainly occurred in this section.  

Model constituents of gabbro is; plagioclase 

(49.2 – 68.5%), clinopyroxene (15 – 30%) and 

Amphibole (5 – 10%).  

 

Figure 5.a-f. Petrographic features of the gabbroic 

rocks of Sorap Massif; Norite is mainly composed of 

plagioclase, orthopyroxene, amphibole and 

secondary minerals are sericites with interlocking and 

subophitic, medium to coarse grain texture (a and b). 

Gabbro norite mainly consists of plagioclase, equal 

constituents of orthopyroxene and clinopyroxene, 

amphibole and secondary minerals are sericite and 

chlorite with ophitic to sub-ophitic and granular 

texture (c and d). Gabbro is composed of high 

constituents of plagioclase, clinopyroxene and 

amphibole, secondary minerals sericite and chlorite 

with ophitic to the interlocking texture (e and f). 

 

5.2. Comparison of Sorap Massif Ras-Koh 

gabbros with Khanozai Muslim Bagh 

Ophiolites 

Gabbroic rock units in Sorap Massif in the Ras-

Koh Ophiolitic belt are basal parts of the crustal 

section of ophiolite and that are formed in the 

magma chamber by the processes of crustal 

fractionation. The Sorap Massif is a western 

segment of Ras-Koh Ophiolite and Sorap Massif 

has an immature sequence of intrusion of 

gabbroic rocks in its crustal section mainly 

composed of ultramafic to mafic cumulates 

including a harzburgite, mylonitic peridotite and 

dunite covered by the basaltic-andesitic 

volcanic rock units and quaternary alluvial 

sediments and sequence of pelagic sediments 

are absence in the Ras-Koh Ophiolitic belt. 

These characteristics of an immature sequence 

of intrusion of gabbroic rock units in the crustal 

section are also recorded in a major ophiolitic 

belt around the world, i.e. ophiolitic belt in the 

Tethyan oceanic region including a Troodos 

Cyprus, Kizildag and Pozanti Karsanti Turkey, 

Pindos Greece, Semail Oman, Bela and Muslim 

Bagh Pakistan, Xigaze Tibet (5, 27, 28, 29, 30, 31, 

32, 15, 33).  



 

 

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The intrusion of gabbroic rock units in the crustal 

part of Sorap Massif in the Ras-Koh Ophiolite 

consists of a cyclic succession of ultra-mafic to 

mafic cumulates from bottom to top including a 

harzburgite, mylonitic peridotite, dunite, wehrlite 

and gabbro. Gabbroic rock units are classified 

into norite, gabbro norite and gabbro. 

Repeated series of each cyclic succession has 

different rock composition, structure and 

thickness because of the various phase of 

crystallization of minerals constituents due to 

differential supplied of magma in the magma 

chamber (34). The minerals constituents of 

gabbroic rock units of Sorap Massif in the Ras-

Koh Ophiolite enriched pyroxene and 

amphibole, that is crystallized in the intrusion of 

gabbroic rock units indicating an immature part 

of oceanic plate and dehydration part of 

subduction plate. These features are indicated 

the model of gabbroic intrusion of Sorap Massif 

in the Ras-Koh ophiolite is originated in the large 

magma chamber by the magmatic processes 

linked with subduction. 

According to Popal the intrusion in the crustal 

part of Khanozai Ophiolite mainly consist of 

cyclic series of ultra-mafic to mafic cumulates 

including dunite, pyroxenite, wehrlite and 

gabbro. The gabbroic rock units are classified 

into olivine gabbro, gabbronorite, olivine 

gabbronorite, norite and gabbro (35). The 

particular cycle of the reciprocated series has 

various rocks sequence, structures and 

thickness, that is indicating the numerous phases 

of crystallization of minerals constituents due to 

distinctive supplies of magma in the magma 

chamber (34). The mineral composition of 

gabbro is enriched in olivine, which is crystallized 

in the sequence of gabbroic rock units that are 

indicated the genesis of minerals constituents of 

gabbros. Mineral constituents of gabbro were 

formed by the numerous pluses of magma 

during the stable time of crustal growth (35). 

These features indicate that the Khanozai 

gabbroic may have originated from different 

intervals of time due to different episodes of 

magmatism. These characteristics of intrusion of 

the crustal section are also reported from the 

Muslim Bagh Ophiolite (14, 36). The features of 

Muslim Bagh and Khanozai gabbros indicating 

that they are not originated from the large 

magma chamber (37) and they may be formed 

by the magmatic processes such as the glacier 

model or sheeted sill model suggested for the 

ophiolitic gabbro’s (26, 38). 

 

CONCLUSION    

1. The Sorap massif of Ras-Koh Ophiolite is 

composed of mantle peridotites, ultra-

mafic cumulates and mafic cumulates.  

2. Petrographically, the crustal sequence of 

Sorap massif is classified as norite, gabbro-

norite and gabbro.  

3. Major constituents of gabbroic rock units 

are composed of orthopyroxene, 

clinopyroxene, amphibole and plagioclase 

associated with secondary minerals and 

minor constituents of accessory minerals.  

4. High constituents of ferromagnesian 

minerals including pyroxene and 

amphibole and low constituents of olivine in 

the gabbroic intrusion these are indicating 

the tectonic and petrogenesis of gabbroic 



 
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J. mt. area res., Vol. 6, 2021 

intrusion. The intrusion of gabbroic rock units 

is formed by the immature part of the 

oceanic plate and the dehydration part of 

oceanic plate subduction. 

Acknowledgments  

This research was financially supported by the 

Higher Education Commission, Pakistan under its 

National Research Program for Universities 

(NRPU) Project # 3593 to, M. Ishaq Kakar. 

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Received: 04 June 2021. Revised/Accepted: 29 June 2021. 

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