The Kohistan Magmatic arc

1. THE KOHISTAN MAGMATIC ARC
The Kohistan and Ladakh terranes considered as intraoceanic island arcs developed in
response to northward subduction of the Neo-Tethyan oceanic lithosphere. It appears that the
arcs were welded to the Karakoram plate 85 to 95 Ma ago.
Volcanic sedimentary and magmatic sequences
Yasin Group: this group consists of sedimentary and volcanic rocks forming. a long and narrow
belt immediately to the south of the Shyok Suture.

2. The group has been divided into: (a) upper 2,000 m of red, purple, green and grey shales with
interbedded greenstones, probably tuffs, (b) middle 500 m of grey slates with some distal
turbidites, and (c) low,:[ >500 m volcanic-lithic conglomerate and sandstones, tuffs, slates and
rudist limestones.
ChaIt Volcanics: Underlying and locally interbedded with the Yasin Group, these yolcanics
form an arcuate belt more than 350 km in length. They have been referred to as the Greenstone
Complex. Locally, blocks of these rocks also occur within the Shyok Suture Melange (?
olistostrome).

3. The group contains Aptian-Albian fossils. Lateral lithological variations are common and some
sections are "devoid of the yolcanic components. The Hunza section contains mostly clastics
and volcaniclastics in the form of distal turbiditic slates with massive greywacke. The
Ishkoman section consists of slates with some quartzite coarsening to pebbly conglomerate
towards the Shyok Suture. In Chitral, the equivalent Purit Formation « 1 km thick) comprises
red calcareous shales overlying limestone. On the northeastern edge of the Nanga Parbat-
Haramosh Massif, arenaceous slates, conglomeratic schists, conglomerates, phyllite and Pakora
limestone of the Turmik. Formation are the possible easterly equivalent of the Yasin Group

4. The group probably have been deposited in an intra-arc and/or back-arc basin.
ChaIt Volcanics: Underlying and locally interbedded with the Yasin Group, these yolcanics
form an arcuate belt more than 350 km in length. They have been referred to as the Greenstone
Complex, Turmik Formation, Rakaposhi Volcanic Group, and ChaIt Volcanic Group. The
volcanics comprise a diverse group of rocks. They range from basalt to andesite, dacite and
rhyolite flows, conglomerate, breccia and tuff. They exhibit low grade metamorphism
characterised by chlorite, epidote, amphibole, sadie plagioclase, quartz, with or without
carbonate, iron oxide and mica. These minerals (± zeolite?) also occur in amygdules and
veins~ quartz veins are common in some places. Modal variations result in a variety of

5. colours: green, grey, brown, buff and white, locally mottled. There appears a regional variation
in the composition of the volcanics. Those of the eastern part (ChaIt, Jaglot, Turmick) consist
mainly of metmorphosed flows and tuffs of basaltic to andesitic composition with subordinate
dacite and rhyolite.
Utror, Drosh and Shamran Volcanics
The Utror Volcanics comprise over 3,000 m thick red, green, grey, white, at places mottled,
rocks.
Southeastern Kohistan Volcanics
Lithologies in the hanging wall of the Indus Suture in southeastern Kohistan belong to the Sapat
Mafic-Ultramafic Complex .

6. Between the latter and the Chilas Complex, the terrane is occupied by E to W -trending
stripes of volcanic and plutonic (diorite to granite) rocks. The volcanic stripes, named
differently by different authors, range from mafic to intermediate in composition and locally
contain small bodies of gabbro, tonalite, granodiorite and granite. The volcanics are locally
deformed internally and may display foliation. Pillow structures are preserved in some
andesitic(?) lavas near Dalpur (Buto Gah).

7. Skardu-Deosai Volcanics: The Skardu area of Ladakh-Kohistan arc contains three major
formations, intruded by a range of granitic and mafic rocks, have been described. The Bauma-
Harel Formaton in the north-eastern and northwestern part of the area adjacent to the Shyok
Suture consists of volcaniclastic metasediments (chlorite-epidote greenschist), interbedded
with slates, phyllites, minor calcareous rocks and multicoloured conglomerates. Containing
poorly preserved Cretaceous fossils, it has been correlated with ChaIt Volcanics and Gawuch
Formation further west. Coeval with, or younger than, this formation is the Katzarah
Formation covering large areas to its south.

8. folded high-grade (up to sillimanite-K-feldspar zone) gneisses, calc-silcates, marble and
metavolcanic amphibolites. To the south of Skardu occur the Burji Formation phyllite, slate,
chlorite-epidote metavolcanic and limestone containing upper Cretaceous fossils.
Kohistan Batholith:
This is a principal unit of the Kohistan magmatic arc and constitutes a 300 km long and up to
60 km broad pelt to the west of Nanga Parbat. It forms the western part of the great
Transhimalayan Batholith and continues on the eastern side of Nanga Parbat in Deosai,
Ladakh and beyond.

9. The Kohistan Batholith is composite and consists of numerous large to small plutons, plugs,
dykes and sheets emplaced over a time span of some 75 million years. A wide range of rocks
has been reported to constitute the batholith: gabbros, hornblendite, diorites, quartz diorite,
adamellite, granodiorite, granite, tonalite, trondhjemite, aplite, pegmatite, etc. Locally, as
many as five pulses of intrusions can be observed within a few tens of metres,commonly
showing a decrease in mafic constituents with time. Multiple intrusive activity is best seen in
the Indus Valley section where dykes and sheets of leucogranites, <1 to 10 m thick. occur in
swarms and networks, locally making up to a third of the exposures.

10. Mafic dykes:
The"Kohistan Batholith is intruded by low-medium-K basalt, basaltic andesite and
trachyandesite dykes, up to 4 m thick and locally forming swarms. These also cut the Chait
Volcanics, Jaglot Group rocks, and the fold strucutures and penetrative fabric associated with the
collision between the Kohistan arc and the Karakoram plate. They are considered to have
formed by partial melting of a mantle source metasomatised by subduction-related processes.
Chilas Complex: The Chilas Complex is a large body of mafic-ultramafic rocks extending from
Nanga Parbat to eastern Afghanistan. Similar rocks also occur on the eastern flank of Nanga
Parbat and in Kargil, Ladakh.

11. Jaglot Schist Group: The Yasin sediments and Chalt Volcanics are repeated to the sOlth by
the Gilgit anticline and Jaglot syncline (the Sai Nala and adjacent valleys, that the Chlt
Volcanics overlie a more than one kilometre thick sequence of metasediments and interbedded
metavolcanics that they termed as the Gilgit Group. Because of the volcanic.
Southern or Kamila Amphibolite Belt:
This is a complex belt of amphibolites and a variety of other rocks occupying the southern part
of the Kohistan arc. It consists predominantly of amphibolites and subordinately of
hornblendites, hornblende gneisses, diorites (± quartz), granitoids (including plagiogranite),
with minor

12. pegmatites and metasediments. The amphibolites can be divided into (a) fine-grained,
commonly banded, and (b) medium- to coarse-grained, homogeneous to gneissose, locally
banded/layered types. The former are considered as mainly volcanic and the latter plutonic in
origin.
Jijal Complex: Sitting on the Indus Suture along the southern fringe of the Kamila
amphibolite belt are a series of mafic-ultramafic, seemingly cumulus, complexes. Of these the
Sapat, Jijal, and Tora Tigga Complexes have been studied in some detail. The Sapat Complex
in southeastern Kohistan consists of basal ultramafic cumulates more than a kilometre thick.

13. Karakoram Block
Much of the magmatism in the Karakoram block is granitic in nature. Mafic plutons are rare
and lamprophyre dykes are small and sparse. There also are some poorly defined
Jurassi.:volcanic rock that the granitic rocks of the Karakoram block occur in two distinct
belts, the Karakoram granitic belt and the Khunjerab-Tirich Mir granitic belt. separated by a
major fault, the Reshun Fault.

14. Karakoram Granitic Belt
Also known as the Karakoram (Axial) Batholith, this belt extends for 600 km from southwest
Of Drosh to west of Pan gong Lake and attains a width of up to 30 km. This is a composite
batholith with many plutons, dykes and veins of variable composition emplaced over a period
exceeding 100 m.y. Multiple intrusions can be observed in many places and, like the Kohistan
Batholith, young leucogranites may occur in swarms. The pre-collision rocks include the Hushe
Gneisses and Kande plutonic unit (Jurassic), the Darkot Pass, Hunza, K2, Broad Peak, Muztagh
Tower units and early phases in Yasin Valley (Cretaceous). The post-collisional (Tertiary) ones
comprise Batura, Baltoro, Mango Gusar.

15. Masherbrum and Sumayar units, Hunza and Korophon dykes, and . lamprophyres. The
Batholith consists of large plutonic units displaying major differences in age, Western
Karakoram: The western-most exposure of the Karakoram Batholith, the KesuBunizom pluton,
extends from Mastuj to Kesu and beyond, a distance of more than 100 km. This is the least
studied part of the batholith, with fragmentary information given in Pudsey and others. The
batholith here consists of deformed (locally migmatitic) diorite, quartz diorite and granodiorite
intruded by younger, underformed granitic dykes and pegmatites. Ar-Ar hornblende age of 111
± 5 Ma suggests that some units of the pluton were emplaced in Paleozoic sediments before the
formation of the Shyok Suture. However, there are younger

16. intrusions that apparently cut the suture. The fresh biotite granite of Bunizoni may ~!-so be
post-collisional.
Hunza Karakoram:
Four distinct plutonic units constitute the batholith in the Hunza Valley section. These are the
Hunza Plutonic Complex, the Batura and Sumayar plutons and the leucocratic dykes. The
Hunza Plutonic Complex is reliably dated at 95 ± 5 Ma (D-Pb zircon) with two K-Ar dates
of 46 Ma (hornblende) and 26 Ma (biotite) . It contains a range of rocks, from quartz diorite to
granite, but granodiorite is predominant.

17. Northern Karakoram:
This includes the area adjacent to the Chinese border, hosting some of the highest mountain
peaks in the world. The K2 Gneiss consists of interlayered ortho- and paragneisses of the
middle crust upl ifted along the hanging wall of a large-scale thrust.
The Baltoro-Biafo-Hushe areas:
This area stretches from Snow Lake-Hispar in the west to the Siachen Glacier and Khapalu in
the east. Several large and small plutons have been reported from this area. East of Hushe
Valley the Karakoram Batholith is represented by the Kande plutonic unit. It is a composite
unit, the earliest phases of which are deformed diorites that may be coeval with

18. those of Hunza and Muztagh Tower. A regional. andalusite-grade metamorphism is associated
with this unit. There are tonalite to granodiorite lithologies similar to K2 and Masherbrum. The
youngest phase is a garnet-two-micas leucogranite forming a complex of dykes and veins.
These may be related to the Baltoro plutonic unit.