its all about the karakoram block

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Karakoram Block
In (l981) pointed 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. Searle (1991) has given detailed accounts of the petrography,
geochemistry and petrology of the igneous rocks, together with many radiometric dates, from
the Karakorams. In view of this readily available source of information, we provide only a
summarised account of the magmatism in the Karakoram block.
Jurassic volcanic underlying the Chitral Slates there is an up to 4 km broad belt of
volcanogenic rocks metamorphosed in greenschist facies (chlorite-epidote-albite-quartz). The
rocks are mostly intermediate in composition, with some thin acidic layers containing
abundant quartz. According to Calkins et al. (1981) they may consist of both flows and tuffs.
Named as Koghozi greenschist, these rocks become finer grained and thinner in the
southwest and northeast.
The Tupop Formation consists of conglomerates interbedded with sandstone and
unfossiliferous nodular limestone. The conglomerate contains up to 2 m blocks, some of
which are rhyolitic in composition. There is no further information published on these rocks.
This part of the Karakoram has been regarded to have been rifted away from Gondwana
during the Permian, but there seems a general lack of volcanic. The northeastern Karakoram
in Ladakh, on the other hand," contains of rift-related volcanic rocks. Khunjerab-Tirich Mir
Granitic Belt In western Chitral and the area to the south and west of Baroghil Pass, several
granitic intrusions have been mapped by Calkins et a1. (1981), Pudsey et a1. (1985),
Buchroithner (1980, 1985), and Leake et aI. (1989). these intrude a Jurassic-Devonian
tectonostratigraphic sequence thrust SSE, along the Reshun Fault, over the Cretaceous
Reshun Formation. Desio (1979) calls this fault as Chitral Fault and according to him it is
continuous with the Upper Hunza Fault. The latter is viewed by Desio as the northern
boundary of the Karakoram. The Reshun Thrust, therefore, serves as a major tectonic break
between the Khunjerab- Tirich Mir granitic belt to the north and the Karakoram granitic belt
to the south (Jan et al. 1981b). Eastwards, granitic rocks of comparable tectono-stratigraphic
position occur in Baroghil Pass, Sost, Giraf, Khunjerab and, possibly, beyond. The Wakhan
granites probably also belong to this group (Buchroithner and Scharbert 1979). Not much
petrological data have been published on the rocks of this belt. Of the several plutons in
western Chitral, those of TirichMir, Garam Chashma, and Kafiristan are the largest and of
batholithic dimensions. Poorly constrained by radiometric dates, Leake et a1. (1989) divide
these into two groups. The Tirich Mir and Kafiristan plutons consist of hornblende-biotite
granodiorite gneiss containing up to 5 cm long plagioclase crystals. Desio et a1. (1964) have
reported a 115 ± 4 Ma Rb-Sr biotite age for the Tirich Mir pluton. Slightly younger ages (93
to 86) have been reported for the Zebek intrusion to the northwest.
But in view of recent discovery of pre-Ordovician granites in Yarkhun Valley, it
would not be surprising if some other plutons in this belt are also of this older event. The

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Bumburet pluton, for example, is equated with those of Yarkhun (Le Fort et a1. 1994). The
second group of intrusions (e.g., Garam Chashma) are typical leucogranites (containing two
micas, tourmaline and garnet) associated with the highest grade of metamorphism. K-Ar
biotite ages of 48 ± 2 Ma for Kafiristan and 20 ± 1 and 19 ± 1 Ma for the Garam Chashma
pluton indicate younger cooling history (Searle 1991), but Leake et a1. (1989) thought that
the leucogranites are similar to the 28-12 Ma High Himalayan leu co granites (Le Fort 1981,
Searle and Fryer 1986). The Tirich Mir pluton has an associated band of gabbroic rocks along
the Tirich Fault. These have been described in a previous section. In Giraf area of upper
Hunza Valley, Desio and Martina (1972) described a 53 Ma (K-Ar) quartz syenitic and
monzonitic pluton comprising perthite phenocrysts in a matrix of feldspar, quartz,
hornblende, pyroxene and biotite. It is cut by dykes of granodiorite, aplite and porphyries.
The geochemical data for the granitic rocks from Khunjerab and Sost (Fig. 6.30)
suggest they are related to subduction processes (Searle 1991) and possibly linked with the
collision between the Karakoram plate and Kohistan arc (Debon et al. 1996). The upper
Hunza region plutons range from Mg-K metaluminous granitoids with biotite and amphibole
to two-micas peraluminous type. The metaluminous ones may have originated from a mantle
source with little crustal input, whereas the peraluminous granitoids are indicative of an
increasing involvement of the continental crust. For further details on geochemistry and
tectonic modelling of these granitoids, Debon et al. (1996) may be consulted. Karakoram
Granitic Belt also known as the Karakoram (Axial) Batholith, this belt extends for 600 km
from southwest ofDrosh 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. (Photos. 58 and 59).
Multiple intrusions can be observed in many places and, like the Kohistan Batholith,
young leucogranites may occur in swarms. Searle et al. (1989) and Crawford and Searle
(1992, 1993) have proposed a broad subdivision of the Batholith into pre-collision and post-
collision units. 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, Masherbrum and Sumayar units, Hunza and Korophon dykes, and
lamprophyres (Fig. 6.31). The Batholith consists of large plutonic units displaying major
differences in age, chemical and mineralogical composition, and tectonometamorphic history
(Debon et a1. 1987). Therefore, we follow Searle (1991) and describe it from the west,
central, northern and eastern sectors of the Karakoram Range in Pakistan.
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

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studied part of the batholith, with fragmentary information given in Pudsey and others (1985,
1986). 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 (Pudsey 1986) suggests that some units of the pluton were
emplaced in Paleozoic sediments before the formation of the Shyok Suture. However, there
are younger intrusions that apparently cut the suture. The fresh biotite granite of Bunizoni
may ~!-so be post-collisional. To the west of Ishkoman the Karakoram Batholith splits into
Darkot Pass (northern) and Ghamu Bar (southern) plutonic units. These have been studied in
some detail by Debon et al. (1987). The Darkot plutonic unit intrudes the upper Paleozoic
Darkot Group and consists of a wide range of rocks from quartz-monzodiorite to
leucogranite. The most abundant rocks are medium- to coarse-grained, porphyritic adamellite
and quartz-monzodiorite. Chemically these rocks compare well with subalkaline and calc-
alkaline granitoids from other areas (Fig. 6.32). The porphyritic type yields a 111 ± 6 Ma Rb-
Sr age whereas leucogranites have an isochron age of 109 ± 4 Ma (Debon et al. 1987).
The main intrusion may thus be AptianAlbian, but there is a great range in K -Ar
hornblende-biotite ages (199 to 44 Ma) and, according to Searle (1991), the Darkot Pass unit
may be Jurassic-Cretaceous. The Ghamu Bar pluton is also emplaced in the Darkot Group
and contains abundant septa of the sedimentary rocks. Much of the granitoid, particularly in
the Umalsit area,.is medium to fine-grained adamellite containing up to 5 x 18 cm megacrysts
ofK-feldspar. Northwards the porphyritic texture disappears progressively and the pluton
becomes a heterogeneous mixture of adamellite, granodiorite and leucogranites. The Ghamu
Bar pluton is traversed by mylonitic zones, and alteration is common. Major and RE element
geochemistry supports their calc- alkaline nature. Debon et al. (1987) report poorly
constrained Late Paleozoic Rb-Sr ages for the pluton. Along the Karam bar Valley, the
Karakoram Batholith comprises four plutonic complexes: (1) the mid-Cretaceous, calt-
alkaline Hunza plutonic unit, (2) a stock of subalkaline, porphyritic granitoids, (3) a
composite group of fine-grained granitoids, and (4) the Koz Sar alkaline complex. These
have been described in detail by Debon and Khan (1996). The alkaline rocks are unu~ual and
merit some comments. They constitute a ~5 x 20 km pluton and have a Rb-Sr isochron age of
88 ± 4 Ma. The most common members consist of metaluminous to slightly peralkaline
monzonite (± quartz), granite and leucogranite with Fe-rich mafic silicates including
clinopyroxene and calcic amphibole. The alkaline character of these rocks testifies to the
development of extensional tectonics, a process compatible with an oblique collision and/or
with the decrease of the convergence velocity. This late orogenic pluton appears to be
genetically related to the nearby subalkaline granitoids and is originated from the same
mantle source with a small crystal contribution (Debon and Khan 1996).
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) (Le Fort et al. 1983). It contains a range of

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rocks, from quartz diorite to granite, but granodiorite is predominant. The southern part is
well foliated and metamorphosed, with growth of garnet. Here also occurs a migmatite
consisting of several lithologies ranging from mafic to leucocratic. Large blocks of mafic
quartz diorites in granodiorite and interfingering relations between the two near Gulmit may
suggest that the quartz diorites were emplaced slightly earlier than the granodiorite. A dense
anastomosing network of late dykes consists of a co-magmatic biotite-aplite monzogranite,
two-mica granites, and garnet-muscovite pegmatitic leucogranites with and without aplitic
central part. Such dykes, as well as those of granodioritic composition, also occur in the
metasediments to the south of the main batholith (Searle 1991). In Nagar and Aliabad there
are two, over 100 m thick, sheets of garnet-muscovite leucogranite cutting regional foliation.
There a ~e granodiorite sheets at Hasanabad and Bar. These parallel the regional folition and
are probably related to the Hunza unit, but there also are younger granitic bodies near
Hasanabad. The Sumayar pluton intrudes staurolite-grade metasediments to the south of
Nagar. This 4 km wide body of leucogranite contains two micas, garnet and tourmaline, and
intrudes the deformation/metamorphic fabric of its host rocks. The Batura plutonic unit
consists of
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 (Searle et al. 1989,
1990). The orthogneiss contains K-feldspar megacryst, biotite and hornblende, and the
paragneisses comprise clinopyroxene-hornblende psammites and garnet-diopside marbles. V-
Pb zircon data show that the orthogneisses were intruded at 115120 Ma, whereas K-Ar
hornblende cooling ages are 111-94 ± 3 Ma. Leucogranitic pegmatite dykes, consisting of
quartz, two-feldspars, garnet, two-micas and tourmaline, are presumed to be pre-collisional,
with K-Ar biotite cooling ages ranging from 70 to 58 Ma (Searle 1991, Crawford and Searle
1992). Large-scale intrusions of quartz diorite, post-dating Lower Cretaceous host sediments,
are closely associated with the K2 Gneisses in Broad Peak-Gasherbrum area. These
porphyritic rocks have associated ash-flow/tuffs both of which are strongly altered. The
Broad Peak quartz diorites are very similar to the K2 Gneisses and have been correlated with
them and with the Muztagh Tower unit, all of which have typical calc-alkaline chemistry and
were probably related to an Andean-type magmatism.