Igneous intrusion

indian stratigraphy

1. Igneous intrusion

2. I gneous 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 the Bengaluru  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 )SiO 3 . 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- foliated metamorphic 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