Geology and Petrography of Peridotites (Mantle Section) from Bela Ophiolite, Balochistan, Pakistan

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1 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 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 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 16

2 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. Page 17

3 Fig. 2 Overview map of the Bela oceanic lithosphere assemblage based onhunting Survey Corporation (1960) Ahsan et al. (1980) andgnos et al. (1998). Page 18

4 Peridotite (Ultramafic rocks) (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 Page 19

Table 1. Rock under lying and overlying rock unit of Bela ophiolite. Depleted Harzburgite The term depleted harzburgite characterized those harzburgite enriched in dunite.

Figure 6 depicts the out crop view of depleted harzburgite in the study area. Depleted harzburgite mainly consist up of orthopyroxene, spinel, and olivine.

The percentage of mineralogical assemblages are given in table-2 A) Mineralogical Study Harzburgite The harzburgite are mainly exposed in the upper unit (northern unit) of Bela ophiolite between

5 Table 1. Rock under lying and overlying rock unit of Bela ophiolite. 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 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. 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 s 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. 5 Field photograph showing close-up view of the normal Harzburgite in the Baran Lak area, Khuzdar District Fig. 7 Field photograph showing the dunite in the transition zone north of Lak Baran area Page 20

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.

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

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.

6 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 Dunite Harzburgite 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 Fig. 8 Photomicrograph of basal peridotite illustrating mylonitic texture 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 Page 21

7 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 N and 35 0 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 25 0 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 N and 40 0 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. 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 [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, [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, Hunting Survey Corporation (HSC), 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, [6] Sarwar G., (1992). Tectonic setting of the Bela ohiolites, southern Pakistan. Tectonophysics 207, [7] Zaigham N.A, and Malik K.A (1992). Upward continuity filtering of the Uthal gravity data. Acta Miner. Pakistan;6, [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, Received : 5 th May, 2017 Revised : 25 th May, 2017 Accepted : 5 th June, 2017 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. Page 22

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

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