indian stratigraphy

1. Igneous intrusion

2. Igneous intrusions form when magma cools and
solidifies before it reaches the surface.
Three common types of intrusion are sills, dykes, and
batholiths.
Sills:
form when magma intrudes between the rock layers, forming
a horizontal or gently-dipping sheet of igneous rock. The
Whin Sill (top left image) in N. England provided a defensive
cliff-line on which the Romans built Hadrian’s Wall.
Dykes:
form as magma pushes up towards the surface through cracks
in the rock. Dykes are vertical or steeply-dipping sheets of
igneous rock. This example, (bottom left image) in the
Channel Islands, shows several criss-crossing dykes of
different ages.
Batholiths:
are large, deep-seated intrusions (sometimes called
Plutons) that form as thick, viscous magma slowly
makes its way toward the surface, but seldom gets
there! Dartmoor (bottom right) forms part of a large
batholith that extends under Cornwall and beyond.

3. Champion gneiss
 It is a sheared micaceous gneiss.
 It is characterized by the presence of opalescent
quartz grain with grayish tint.
 The gneiss is best developed along the eastern
border of the kolar-schist belt.
The Champion Gneiss occurring within the eastern part of the
belt is compositionally similar to the plutonic Dod Gneiss and
probably represents a near surface intrusion

4. Peninsular Gneiss
 They are a complex granite and granite-gneisses.
 They cover a large part of South India.
 It is younger than Champion gneiss
Peninsular Gneiss:
is a term coined to highlight the older gneissic complex of the metamorphics found all over the Indian Peninsula. This term was first
fashioned by W.F.Smeeth of the Mysore Geological Department in 1916 based on the first scientific study of this rock exposure.
The Geological Survey of India identified one of the best exposures of this rock mass, protruding high above the ground as a hillock
(pictured), dated 2.5 to 3.4 billion years, in theBengaluru city in India at the famous Lalbagh gardens and declared it as a National
Geological Monument to propagate the knowledge of the rock formation among the public, since Lalbagh is frequented by visitors
from all over the world. It is also called the Lalbagh rock.
As general information, gneiss is a coarse-grained high grade metamorphic rock formed at high pressures and temperatures in which
light and dark mineral constituents are segregated into visible bands.

5. Charnockites
• Course grained gneissose rocks rich in hypersthene.
• Their composition ranged from acid to ultrabasic.
• It possessed the characteristics of both the igneous and
metamorphic rock.
Charnockite is a granofels that contains orthopyroxene, quartz,
and feldspar.
Charnockite is frequently described as an orthopyroxene
granite. Granites are felsic rocks that usually contain no or very little
pyroxene. There is actually an entire array of rocks (mostly granitoids but
also syenite, monzonite, etc.) that may contain orthopyroxene plus quartz.
These rocks are collectively referred to as charnockitic rocks or
charnockitic suite. All of these rock names refer to igneous rocks which
makes it very logical to assume that charnockite is just an igneous rock
with a somewhat unusual composition.

6. Hypersthene is a common rock-forming inosilicate
mineral belonging to the group of orthorhombic
pyroxenes.
Its chemical formula is (Mg,Fe)SiO3. It is found in igneous
and some metamorphic rocks as well as in stony and iron
meteorites.
Mohs scale hardness: 5.5–6
Luster: vitreous to pearly
Crystal system: Orthorhombic
Color: gray, brown, or green

7.  Granofels is a term referring to medium- to
coarse-grained metamorphic rocks which have
a granoblastic texture (having poorly formed,
equi-granular crystal grains visible to the naked
eye) and lack distinct foliation or lineation. The
granofels term encompasses rocks which are
known under more specific names, such
as marble or hornfels.
Granoblastic is an adjective describing
an anhedral phaneritic equi-granular metamorphic
rock texture. Granoblastic texture is typical
of quartzite, marble and other non-foliatedmetamorphic
rocks without porphyroblasts.
Characteristics defining granoblastic texture include: grains
visible to the unaided eye, sutured boundaries and
approximately equidimensional grains. The grain boundaries
intersect at 120° triple junctions under ideal conditions.
Variation from the ideal results from stress produced
foliation during crystallization resulting in schistose textures.
A rock that has a granoblastic texture can be termed
a granofels.

8.  Euhedral crystals (also known
as idiomorphic or automorphic crystals) are
those that are well-formed, with sharp, easily
recognised faces.
 The opposite is anhedral (also known
as xenomorphic or allotriomorphic): a rock with an
anhedral texture is composed of mineral grains
that have no well-formed crystal faces or cross-
section shape in thin section.
 Anhedral crystal growth occurs in a competitive
environment with no free space for the formation of
crystal faces. An intermediate texture with some
crystal face-formation is termed subhedral.

9. Closepet Granite
 The granite occurs mainly in the mountain ranges of
Karnataka.
 width- 15-25 km, length-500 km from Closepet to Bellery
districts.
 It is course grained, porphyritic-granite that shows intrusive
relation to the Peninsular Gneiss.

10. Thank you
This is Joseph R Lalfakawma