Facies and distribution of metamorphic rocks beneath the Muslim Bagh ophiolite (NW Pakistan): tectonic implications.
The Muslim Bagh ophiolite Complex comprises two massifs; Jang Tor Ghar Massif (JTGM) and Saplai Tor Ghar Massif (STGM) and shows a nearly complete ophiolite sequence that is structurally underlain by metamorphic sole rocks. The metamorphic rocks are found around both the massifs of Muslim Bagh ophiolite but the best exposures are on the northwest side of JTGM and west of STGM. The sole rocks consist of mylonitic peridotite and sub-ophiolitic metamorphic rock series grading from (garnet-bearing) amphibolite facies to greenschist facies. A mylonitic texture in basal peridotite is the result of ductile lithospheric deformation while the sub-ophiolitic metamorphic rocks were formed during intra-oceanic subduction i.e. the first stage of the emplacement of Muslim Bagh ophiolite. The foliation plane of the metamorphic sole rocks on the northwestern base of the JTGM strikes northeastsouthwest with a dip ranging from 20o 75o to the east
while on the western side of STGM it trends northsouth and dips steeply towards east. Most of the lineations display northeasteast orientations which suggest that the ophiolite was obducted from west to east in the frame of today's geographic co-ordinates. During intra-oceanic subduction various parts of the subducted plate were accreted to the base of the overriding slab and subjected to a temperature between about 9001000 C forming mylonitic peridotite and then continued downwards to form the sub-ophiolitic metamorphic rocks.
Keywords: Intra-oceanic subduction; Metamorphic sole rocks; Mylonitic peridotites; Garnet amphibolites; Greenschists. 1. Introduction
The basal contact of many allochthonous ophiolite complexes are characterized by the presence of thin subjacent sheets of relatively high grade of metamorphic rocks. (e.g. Williams and Smith 1973; Coleman 1977; Searle and Malpas 1980) that lie a structural position below the ophiolite and above the underlying sedimentary and volcanic rocks (Gnos and Peter 1993).
Metamorphic rocks are exposed around the Muslim Bagh ophiolite with the best exposures found NW of Jang Tor Ghar Massif and west of Saplai Tor Ghar Massif. These rocks comprise mylonitic peridotite and sub-ophiolitic metamorphic rocks that range from garnetiferous amphibolites facies to amphibolite facies to greenschist facies and un- metamorphosed sediments. Hunting Survey Corporation (1960) Bilgrami (1964) and Van Vloten (1967) described the ophiolite as extrusive and intrusive complex and the metamorphic rocks as thermal aureole. Ahmad and Abbas (1979) described these metamorphic rocks as tectonic slivers. Munir and Ahmed (1985) were the first to report the petrology of the metamorphic rocks of Jang Tor Ghar Massif. The purpose of this study is to document the facies and distribution of metamorphic rocks beneath the Muslim Bagh ophiolite and discuss their origin and significance for the emplacement of ophiolite.
2. Regional geological setting
The rocks in the Muslim Bagh area are divided into three geo-tectonic terranes (Fig. 1). In the south the tectonically lowermost is the zone of the Indian passive continental margin. This zone consists mainly of limestone sandstone and mudstone ranging in age from Early Jurassic to Paleocene (Warraich et al. 1995). The Calcareous Zone is thrusted over by the Suture Zone; lying between the Indian plate and Afghan Block (Gansser 1964) and consists of the Muslim Bagh ophiolite (see next section) and the Bagh Complex. The Bagh Complex is an accretionary wedge complex comprising Triassic to Jurassic sedimentary rocks (Naka et al. 1996) Cretaceous igneous rocks (Sawada et al. 1992) Cretaceous pelagic to hemi-pelagic sediments and a small amount of mACopyrightlange (Mengal et al. 1994; Kojima et al. 1994). The Flysch Belt lies to the north of the Suture Zone and is composed of the Nisai Formation (Eocene; Allemann 1979) at its base that unconformably overlies the ophiolite.
These are succeeded by immature Oligocene turbidites of Khojak Formation (Qayyum et al. 1996) MiocenePliocene arenites and conglomerates of the Multana Formation and the Pleistocene lacustrine deposits of Bostan Formation (Fig. 1).
3. Muslim Bagh Ophiolite
The Muslim Bagh ophiolite comprises two main blocks; Jang Tor Ghar Massif (JTGM) and Saplai Tor Ghar Massif (STGM) (Bilgrami 1964; Fig. 1). The two blocks are structurally related and belong to the same ophiolite nappe and overlie a zone of subophiolitic mACopyrightlange and sediments of the Bagh Complex (Mahmood et al. 1995; Naka et al. 1996; Fig. 1). The JTGM is mainly composed of foliated peridotite (harzburgite and dunite) and covers an area of about 150 km2. The STGM shows a nearly complete ophiolitic sequence ranging from foliated peridotite at the base through the Mantle-Crust Transition Zone to crustal rocks at the top. Many outcrops of dunite both in STGM and JTGM have a large concentration of chromite which are mined locally in the areas. The mantle rocks of Muslim Bagh ophiolite are intruded by a swarm of mafic dykes. These dykes vary from about 3-15 metres thick and trend NW-SE (Fig. 1).
The crustal section of the Muslim Bagh ophiolite consists of bands of dunite wehrlite pyroxenite at the base overlain by the layered and foliated gabbro. The gabbroic section is followed by a less-developed sheeted dyke complex which is intruded at the base and in the middle part by plagiogranite. The zircon found in the plagiogranite yield a U-Pb crystallization age of 80.2 1.5 Ma (Kakar et al. 2012).
The metamorphic rocks (also known as the Metamorphic Sole Rocks) are distributed around the Muslim Bagh ophiolite; in the northwestern part of JTGM west of STGM east of Nisai town and in the mACopyrightlange unit near the Bagh village (Fig. 1). These rocks are mainly amphibolites and greenschist facies metamorphic rocks. Thin slices of metamorphic rocks are found near the Nisai town and Bagh village both of them are not mapable on a reasonable scale (Fig. 1). They are thin blocks of amphibolite and greenschist facies rocks embedded in a mACopyrightlange unit. The metamorphic rocks on the northwestern part of The sole rocks of JTGM consist of a basal part of the foliated peridotite and the sub-ophiolitic metamorphic rocks; garnet amphibolite facies grades downwards into amphibolite facies and greenschist facies with calcitemarble inter-layers which exhibits decreasing metamorphic grade and decreasing intensity of deformation downward from the stratigraphic base of the ultramafic rocks. The basal part of the peridotite is mylonitic and banded. The metamorphic sole rocks beneath the STGM are not well exposed with the exception of the mylonitic peridotite. In the sub-ophiolitic metamorphic rocks; amphibolite and greenschist facies are thin and poorly exposed.
All the sub units of metamorphic sole rocks i.e. basal peridotite and the sub-ophiolitic metamorphic rocks like garnet amphibolite facies amphibolite facies to greenschist facies are separated by the thrust faults and indicating decreasing intensity of deformation downward from the stratigraphic base of the ophiolitic rocks. A number of measurements of the penetrative structures (foliation and lineation) are made at both the exposures of metamorphic sole rocks of Muslim Bagh ophiolite. These structures are shown on Figure 2. The foliation plane in the metamorphic sole rocks on the northwestern base of the JTGM strikes northeast with a dip ranging from 20o 75o to the east (Fig. 2). While the sole rocks on the western side of the STGM trend north to south and dip steeply towards the east. The majority of the lineations display NE-E orientations.
In Muslim Bagh the metamorphic rocks can be divided into sub-units as mylonitic peridotite garnet amphibolites facies amphibolites facies and green schist facies.
4.1. Mylonitic peridotite
The base of both the JTGM and STGM of Muslim Bagh ophiolite is marked by mylonitic peridotite. These rocks are banded (Fig. 3a) sheared and about 500 metres thick (Fig. 2). The banding is concentrated along the base of the peridotite sequence becoming less intense upwards and is readily distinguishable from earlier planer and linear fabrics recognized at higher levels in the sequence that are formed under mantle conditions (Nicolas1989). This is because this banding is due primarily to ophiolite emplacement (Boudier and Coleman 1981).
The mylonitic and banded harzburgite and dunite at the base of the ophiolite record plastic deformation developed possibly within the granulite facies (Boudier and Coleman 1981; Nicolas 1989). The porphyroclastic texture becomes increasingly ophiolite. A 900-1000oC temperature range is ascribed to this kind of texture (Mercier and Nicolas 1975). Mylonitic texture with complete recrystallization of olivine is common within a few metres from the contact of amphibolites.
Petrographically the harzburgite from the mylonitic peridotite are darker in colour and less altered than associated dunite. They are porphyroclastic and mylonitic in texture. The original grains are fine grained and stretched into long thin streaks with large amounts of strain and recrystallization (Fig. 3b). These flow structures are observed almost in every thin section. The mineral composition of mylonitic peridotite is olivine pyroxene serpentine and opaque.
4.2. Garnet amphibolite facies
The garnet amphibolite facies rocks are only exposed below the JTGM and are about a metre thick. It is hard well foliated and has alternate bands of dark colour and light colour minerals that vary in thickness from 2-5 millimetres. Foliation and lineation in garnet amphibolite are parallel to sub-parallel to those found in the mylonitic peridotite. No relict igneous textures have been observed. Large garnet grains are present in garnet amphibolite facies rocks and range in size from about 2 to 3 millimetres (Fig. 3c).
The garnetiferous amphibolite consists of hornblende plagioclase and garnet with minor minerals like epidote quartz apatite and magnetite. The hornblende is poikiloblastic and crystals are lineated parallel to the rock foliation. In some thin sections foliation is well marked due to segregation into dark and light colour bands. The dark colour bands consist of hornblende while the light bands consist of plagioclase and quartz. The core of the garnet grains has numerous inclusions of quartz and probably magnetite which align to the rock foliation (Fig. 3d).
4.3. Amphibolite facies
The amphibolite facies rocks are present at both the exposures of metamorphic sole rocks but they are well developed in the JTGM and range in thickness from 300600 metres (Fig. 2) forming high mountain peaks while in STGM it is not well exposed. The amphibolites are dark grey to black foliated rocks composed of hornblende altered plagioclase epidote biotite and opaque. The amphibolites have a sharp upper contact with garnet amphibolite facies and grade downwards into greenschist facies rocks. In the transitional zone between amphibolite and greenschist facies there are various layers of variable thickness comprising different types of schist bands of quartzite and marble. The thickness of the quartzite/marble bands ranges from about 1 centimetre to 20 centimetres (Fig. 4a). The amphibolite facies is gradational over several tens of metres into greenschist facies by decrease in grain size and variation in colour.
It grades from prismatic hornblende into actinolite in the transitional zone to chlorite in the underlying green schist facies (Miyashiro 1994).
The major constituent minerals of the amphibolite facies rocks are; black to bluish green hornblende plagioclase epidote quartz biotite muscovite and other accessory minerals like garnet opaque chlorite. The amphibolite is porphyroblastic and contains porphyroblasts of hornblende. In some thin sections the hornblende is poikilitic and contains inclusions of quartz. In amphibolites the hornblendes are interlayered with plagioclase defining clear schistosity (Fig. 4b).
4.4. Greenschist facies
The greenschist facies rocks are about 100250 metres thick (Fig. 2). In the sole rocks of JTGM the greenschist facies have a gradational upper contact with amphibolite facies while the lower part is in thrust contact with the mACopyrightlange unit (Fig. 4c). The greenschist facies rocks show strong polyphase deformation. Banded quartzite comprises the bulk of greenschist facies. The other lithologies present include muscovite-sericite schist banded quartzite marble and garnetiferous phyllite. The marble and quartzite bands are common at various horizons.
The greenschist consists predominately of chlorite muscovite plagioclase and quartz with minor epidote garnet biotite and opaque (Fig. 4d). Chlorite is bright green to pale green in colour and pleochroic and occurs in composite crystals. The maximum interference colours are of low first order with abnormal shades of blue. The chlorite ranges in colour from green to olive green and are slightly pleochroic. Multiple twinning is common. Muscovite occurs as elongate flakes and makes the foliation.
The major occurrences of metamorphic sole rocks found beneath both the massifs of Muslim Bagh ophiolite i.e. JTGM and STGM are typical of those found beneath other ophiolites such as Semail Oman (e.g. Searle and Malpas 1980; Gnos and Peter 1993) Bay of Islands Newfoundland Canada (e.g. Williams and Smith 1973) ophiolites in Turkey (e.g. Lytwyn and Casey 1995; Collins and Robertson 1998; Celik and Delaloye 2006; Parlak et al. 2006; Celik 2007) ophiolites in Tibet (e.g. Wang et al. 2008 Guilmette et al. 2009). The Muslim Bagh sole rocks comprise mylonitic peridotite and sub-ophiolitic metamorphic rocks which display an inverted metamorphic sequence grading from garnet amphibolite facies directly beneath the mylonitic peridotite amphibolite facies to lower greenschist facies near the mACopyrightlange contact (Munir and Ahmed 1985; Mahmood et al. 1995; Fig. 2).
The metamorphic sole is possibly a composite of rocks developed at different pressures and temperatures and at different times along a thrust plane which were subsequently amalgamated into a single column beneath the advancing oceanic slab (Lytwyn and Casey 1995).
The Muslim Bagh Ophiolite yields zircon U-Pb crystallization age on plagiogranite as 80.2 1.5 Ma (Kakar et al. 2012). This age is the formation age of Muslim Bagh ophiolite. Hornblende was also dated from the amphibolites in subophiolitic metamorphic rocks that yielded a good K Ar age of 80.5 5.3 (Sawada et al. 1995) and an Ar-Ar age of 65.7 1.5 Ma (Mahmood et al. 1995). These ages are interpreted as the age of ophiolite emplacement. If the emplacement age (80.5 5.3) of the K-Ar dating is correct then we conclude that the Muslim Bagh ophiolite had formed at 80.2 1.5 Ma and emplaced during or immediately after their formation.
Mylonitic texture is observed in the basal peridotite whereas going upward asthenospheric microstructures predominate. The mylonitic textures are formed in response to ductile lithospheric deformation during the initiation of intra-oceanic subduction at temperatures from 900 1000 C (Boudier and Coleman 1981; Nicolas 1989; Mahmood et al. 1995; Elitok and Druppel 2008). The highest-grade rocks (garnet amphibolite) directly beneath the basal ophiolitic thrust were the first to accrete to the base of the ophiolite (e.g. Searle and Malpas 1980; Ghent and Stout 1981; Lytwyn and Casey 1995). The heat necessary to produce the first stage high- grade metamorphic event was probably supplied from the residual heat of hot peridotite of the obduction slab; presumably 9001000 C (e.g. Nicolas 1989; Mahmood et al. 1995) or 1100 1200 C (e.g. Elitok and Druppel 2008).
In short during intra-oceanic subduction various parts of the subducted plate are accreted to the base of the future Muslim Bagh Ophiolite and subjected to a range of metamorphic conditions at about 9001000 C (Nicolas 1989) forming mylonitic peridotite and continuing eventually downwards to high grade amphibolite conditions to form garnet amphibolite facies and amphibolite facies at a later stage of emplacement (overthrust) processes medium to lower grade metamorphic conditions produced greenschist facies (e.g. Mahmood et al. 1995; Fig. 5a). Finally the Muslim Bagh Ophiolite was obducted over the Indian Platform along with underlying Bagh Complex (e.g. Naka et al. 1996; Fig. 5b).
The above discussion leads to the following conclusions:
1. The Muslim Bagh ophiolite has typical metamorphic sole rocks beneath both the massifs. The sole rocks consist of a basal mylonitic peridotite and sub-ophiolitic metamorphic series displaying an inverted metamorphic sequence grading from garnet amphibolite facies to amphibolite facies to lower greenschist facies.
2. Mylonitic texture is observed in the basal peridotite which is formed in response to ductile lithospheric deformation during the initiation of intra-oceanic subduction at temperatures possibly ranging from 900 1000 C.
3. The sub-ophiolitic metamorphic rock series were formed during intra-oceanic subduction where various parts of the subducted plate are accreted to the base of the overriding plate and subjected to variable metamorphic conditions.
4. The kinematic analyses of the sole rocks suggest that the Muslim Bagh ophiolite emplaced from west to east in the present geographic frame of reference.
This work is part of the PhD study of the first author. Director Centre of Excellence in Mineralogy University of Balochistan is thanked for providing the field work facility. We are thankful to the reviewers for their constructive comments and suggestions.
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|Publication:||Journal of Himalayan Earth Sciences|
|Date:||Dec 31, 2014|
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