Download Origins Of The Himalayan Treasure Chest

Origins of the Himalayan Treasure Chest
A geologic
overview by
Bernhard
Grasemann
Institute for
Geological
Sciences,
University of
Vienna, Austria
and
Erich Draganits
Institute for
Engineering
Geology, Vienna
Technical
University,
Austria
Digital Topographic Model (GTOPO 30 - Global Topographic Data, U.S. Geological Survey)
of India and the Himalaya mountain range. The Himalayas stretch from Namche Barwa in the east (1),
where the Tsangpo River cuts through the range, to Nanga Parbat in the west (2), where the Indus cuts
through the mountains. The Karakoram Range runs north of the Indus and is separated from its westward
extension in the Hindu Kush by the Ishkoman River (3).
The Himalayas and their westward extension
into the Karakoram and Hindu Kush are the
highest mountains on earth, not to mention the
most topographically spectacular. Rising
abruptly from the borders of the Indus-Ganges
plains in the south, the Himalayas boast all 14 of
the world’s 8,000 plus meter peaks (Fig. 1).
Ongoing tectonic movement coupled with deep
valleys allow scientists insight into otherwise
hidden crustal processes, as well as providing
outcrops of some spectacular mineral deposits.
Those who endure the hardships and
privations of a journey into these regions are
rewarded with breathtaking landscapes,
adventurous experiences, boundless hospitality
22
and rare minerals. The section of the Himalayas
in northern Pakistan is especially famous for its
mineral riches. This area is relatively easy to
reach by way of the Karakoram Highway. In
addition to the region’s better-known minerals,
such as beryl (var. aquamarine, emerald),
corundum, topaz and tourmaline, one also finds
interesting and valuable species such as axinite,
fluorite and olivine. In general, occurrences of
collectible minerals are localized, with the
various mineralizations that are limited to certain
tectonic and lithologic units; therefore,
knowledge of the formation and tectonic events
of the Himalayas is essential to the successful
discovery of mineral deposits.
Origin of the Himalayas
The Himalayas and their northern and western
extensions in the Karakoram and Hindu Kush are
the result of the ongoing collision between the
Indian and Asian continents. The two began to
collide about 60 million years ago. In the
Paleozoic and Mesozoic, India, Africa, South
America, Australia and Antarctica were parts of
the Gondwana supercontinent. About 130 million
years ago, the Indian subcontinent separated from
Gondwanaland and pushed northward at a
velocity of roughly 20 centimeters per year (Fig.
2). As it moved, the Indian subcontinent pushed
the crust of the Tethys Ocean (situated between
India and Eurasia), under the southern edge of
Eurasia. There the Tethys oceanic crust partially
melted. By about 60 million years ago, the
oceanic crust of the Tethys had been pushed
entirely beneath Eurasia. No longer separated by
an ocean, India and Eurasia began to collide
along what is known as the Indus-Tsangpo suture
zone.
This colliding continues today and has created
thrust systems that divide the Himalaya,
Karakoram and Hindu Kush ranges into tectonic
units—units that can be traced for several
thousand kilometers. The collision with Eurasia
slowed the northward motion of India from its
original 20 centimeters per year to its current
speed of around 5 centimeters per year, with
India’s continental crust being subducted by
Eurasia. Parts of the Indian crust reached depths
of more than 80 kilometers into the Earth’s
mantle and thereby underwent ultra-high-pressure
metamorphism creating diamond and coesite.
Coesite, recognized petrographically and
confirmed by in situ Raman microprobe
spectroscopy, is reported from an eclogite
(metamorphic rock essentially made up of sodarich pyroxene and magnesium-rich garnet) from
the Kaghan Valley, Pakistan. The find represents
the first record of ultrahigh-pressure
metamorphism in the Himalayas. Formation
conditions of greater than 27 kilobars implied by
the presence of coesite are supported by garnetpyroxene-phengite barometry (27–29 kilobars,
690–750 °C) (O’Brien et al. 2001).
The Himalayas as we know them, with their
extreme topographic relief, formed only over the
last 8 or 9 million years. Because of their height,
these mountains influence global atmospheric
circulation and therefore to some extent also the
worldwide climate.
India-Asia
collision
Paleogeographic
reconstruction of
the collision
between India
and Eurasia
from their
position
71 million years
ago up to the
present.
Geologic Structure of the Himalaya
The Himalaya, Karakoram and Hindu Kush
mountains are composed of a series of diverse
geologic/tectonic macro-units (Figure 3), which
are separated by great fault systems. The
northernmost units are the Trans-Himalaya and
Karakoram batholiths (granitic plutons). These
granites between 130 and 30 million years ago.
Such granites are found principally in the
Kohistan and Ladakh batholiths as well as in the
Karakoram and in the continental suture zone
between the Indian and Eurasian continents.
Geochemical and isotope data show that these
granites were derived from melted oceanic crust
of the now extinct Tethys Ocean (Honegger et al.,
1982). Current examples of these same processes
can be seen in the granites of the Andes, which
are derived from the oceanic crust of the Pacific.
The Pacific oceanic crust is being subducted
beneath the continental crust of South America.
The Trans-Himalaya unit is bounded in the
south by the Indus-Yarlung-Tsangpo suture zone,
which represents the actual collision front
23
Simplified geological map of the major tectonic units in the Himalayas
between India and Eurasia. All of the rocks south
of this zone were originally part of the Indian
subcontinent. The Indus-Yarlung-Tsangpo suture
zone is composed of very diverse, strongly
deformed rocks from the oceanic crust of the
ancient Tethys Ocean. These rocks include deepsea sediments such as flysch and radiolarites,
oceanic basalts, and even rocks that were derived
from Earth’s mantle!
South of the Trans-Himalaya unit is the High
Himalaya tectonic unit, which is generally
divided into two subunits: the Higher Himalayan
Crystalline and the Tethyan Himalaya. The
Tethyan Himalaya lies directly south of the
Indus-Yarlung-Tsangpo suture zone and is
composed of sediments from the previous
northern continental margin of India. These
sediments were deposited over a period of more
than 500 million years. They form a series of
sandstones, shales, limestones and dolomites that
are more than 10 kilometers thick, and that have
24
been folded by the continental collision.
The Higher Himalayan Crystalline subunit is
south of the Tethyan Himalaya. It is made up of
the sediments of the Tethyan Himalaya, which
were in part metamorphosed and were pushed
down more than 30 kilometers into the Earth’s
crust by the collision between India and Eurasia.
The most abundant rocks in the Higher
Himalayan Crystalline are mica schists, gneiss,
amphibolites, migmatites and two groups of
granite intrusions of different ages. The older
granites intruded the deeper series of the Tethyan
Himalaya about 480 million years ago; whereas
the younger granites rose into the already
metamorphosed rocks of the Higher Himalayan
Crystalline roughly 20 million years ago.
Numerous granite and pegmatite dikes formed
during the latter phase. The youngest of these
veins are only about 4 million years old. They
tend to occur in regions that are tectonically
active today (eg: Nanga Parbat).
The Lesser Himalaya units are south of the
Main Central Thrust, this thrust exhibits
displacements of more than 140 kilometers! The
Lesser Himalaya are composed mainly of weakly
metamorphosed Precambrian sedimentary rocks,
only minor portions of which can be
stratigraphically correlated to the High Himalaya.
Beginning about 9 or 10 million years ago, the
Lesser Himalaya were pushed over the SubHimalaya along the Main Boundary Thrust. The
Sub-Himalaya is the southernmost and youngest
tectonic unit in the Himalayas. The unit is
composed of sediments—mainly sandstones and
conglomerates—that were deposited by erosion
from the growing Himalayan ranges to the north.
Further south, along the Main Frontal Thrust,
the Sub-Himalaya is being pushed above the
young, undeformed sediments of the IndusGanges Plains. This fault zone is the youngest
overthrust belt of the India-Eurasia collision. It
is still active and is responsible for many of the
numerous earthquakes in this region.
western and eastern extensions. A wedge of
these schists, part of the strongly deformed
Tethys oceanic crust in the Indus-YarlungTsangpo suture zone, is being pushed over the
rocks of the Higher Himalaya along the Main
Mantle Overthrust. The combined metamorphism
of sedimentary and oceanic volcanic rocks,
together with alteration by mineralized water, is
likely responsible for the formation of emerald
(Kazmi & Snee, 1989).
Pink to deep red ruby comes mainly from the
metamorphic dolomite marble of the Hunza
Valley and from northern border of Pakistan, but
are also known from the Neelum Valley in (Indiacontrolled) Kashmir. The world famous
cornflower-blue sapphire from Kashmir is found
in strongly kaolinized pegmatite dikes, which are
hosted in mica schists of the High Himalaya.
This deposit is at elevations nearing 5,000 meters,
but it seems to be exhausted.
Mineral Occurrences
Mineral occurrences in the Karakoram and
Hindu Kush seem to be directly related to the
mountain-building processes. The great variety
in local country rocks, variations in element
transfers along fault zones, differences in
metamorphic alterations and geochemical
conditions lead to the diversity of mineralization.
Most of the minerals found in northern Pakistan
come from pegmatites. Tourmaline, beryl
(aquamarine) and apatite are good examples.
Corundum (var. ruby and sapphire) and beryl
(emerald) owe their formation mainly to
metamorphic and hydrothermal processes. The
processes of formation are not, however, always
sharply defined.
Aquamarine and tourmaline tend to be
relatively abundant in the young pegmatites in the
High Himalaya. They are especially widely
distributed in northern Pakistan, where they can
reach majestic sizes and are often associated with
apatite, topaz and garnet.
Yellowish brown to honey-yellow topaz are
found in the pegmatites around Skardu. Rare
pink to violet-red topaz comes from the Katlang
region in the Mardan district, from calcite and
quartz veins in carbonate rocks.
Pakistani emeralds are mainly found in the
talc-carbonate schists of the Swat valley and its
View to the south, toward Gumburajum Mountain
at the headwaters of the Kurgiakh River in Zanskar
(NW Himalayas). The rocks of the more than 1,000
meter high cliff belong to the Higher Himalaya
Crystalline. They are composed of numerous
granitic veins that intruded into metamorphosed
sediments about 22 million
years ago.
25