PLUGS AND PLUTONS

1. PLUGS AND PLUTONS
POOJA U MISHRA
SUBMITTED TO :
DR. P.M. SOLANKI

2. PLUTONS
 A pluton is a rock body composed of intrusive igneous rocks
that has been formed or crystallized when magma slowly
cooled below the surface of the earth.
 The term pluton is commonly used for larger bodies ,
especially when the shape and structural relation are not
clear.
 Plutons are of all sizes and shapes from those a few feet long
to those that are exposed over hundreds of square miles.
Thus it includes, dikes , sills, laccoliths, lopoliths, batholiths,
and other igneous bodies.
 Plugs are volcanic vent or neck and they are the root of
volcanoes that have been eroded away. Having circular, sub-
circular or even irregular in shape with few sq. ft to miles.

3. Structural study of plutons involves a
consideration of their…
1. Internal structure.
2. Shape and size.
3. Structural and chronological relation to the
adjacent rocks.

4. 1. Internal structure.
The rocks in plutons may range from fine grained to very coarse
grained. It depends upon its cooling history and volatiles present
during consolidation, and crystallizing power of various mineral
species.
 Many igneous rocks are massive showing no preferred orientation
of mineral constituents.

5.  Some are characterized by foliation resulting from the parallel
arrangement of platy or ellipsoidal mineral grains.

6.  Banded or layered intrusive rocks are those that consist of
alternate layer of different mineral composition.

7. • Inclusions
We may find the fragments of older rock (angular sub-angular or
round)surrounded by igneous rock.
A xenolith is an inclusion that has been derived from some older
formation. Such as sandstone found in granite. Which is
genetically unrelated to igneous rock.

8. • An autolith is an inclusion of an older igneous rock, related to
younger igneous rock in which it occurs.
• An inclusion of diorite in granodiorite is autolith.
• Both belongs to parent magma.

9. • Segregation
 A rounded or irregular body with few inches to many feet in
diameter and in some cases hundreds of feet across, that has been
enriched in one or more of the mineral composing the igneous
rocks.
 e.g. the hornblende granite may contain clots that have much
hornblende than the surrounding granite.
• Schlieren
 A wavy streaky, irregular sheets which is lacking sharp contacts
with surrounding igneous rocks.
 it may be darker or lighter in which they occur.

10. Schlieren

11. 2. SHAPE AND SIZE OF PLUTONS
 The shape and size of small plutons may be observed directly.
 But for many larger plutons it is quite difficult to examine the
complete shape directly.
 Many dikes and sills are smaller than in which they occur.
 Although complete shape of entire original body can never be
determined because the part above of the surface is forever lost due
to erosion and below surface it is determined only by very expensive
programme of drilling and excavation

12. Batholiths
• A batholith is large emplacement of igneous intrusive
(plutonic) rock that forms from cooled magma deep in the
earth’s crust.
• It is formed as a result of magma forcing it’s way into rock
above. When it cools it forms granite and this can be exposed
by weathering or erosion.

13. Laccolith
• A laccolith is sheet intrusion that has been injected between two
layers of sedimentary rock. The pressure of the magma is high
enough that the overlying strata is forced upward, giving the
laccolith a dome or mushroom like form.

14. Lapolith
• A lopolith is a large igneous intrusion which is lenticular in
shape with a depressed central region. Lopoliths are generally
concordant with the intruded strata with dike or funnel-
shaped feeder bodies below the body.
• Essentially the opposite to laccoliths, with a flat top and
curved bottom.

15. Sill
• A sill is a tabular sheet intrusion that has intruded between older
layers of sedimentary rock, beds of volcanic lava or tuff, or even
along the direction of foliation in metamorphic rock. The term sill
is synonymous with concordant intrusive sheet. This means that
the sill does not cut across pre-existing rocks, in contrast to dikes
which do cut across older rocks.

16. Dikes
• Dikes are tabular body of igneous rock that across the
structure older formation
• Most dikes are formed due to injections of a magma into a
fracture.
• Types:- simple, multiple, composite and differentiated may
be applied to dikes.
• A simple dike is result of single intrusion of magma.
• Multiple dike is result of two or more intrusions same kind
of magma.
• Differentiated dike is one that was intruded as a
homogenous magma.
• Dikes may be small as fraction of inch to 1 – 20feet wide.
• The older fracture may be opened by tensional forces, in
which magma quietly enters the opening. And it fills a dike.

18. Ring Dike
 Ring dikes are oval or arcuate in plan in which the contacts are
steep, either vertical or very steep.
 The average diameter of ring dikes is 4-5 miles, and maximum 15
miles diameter are found.

20. Topographic map of Ambaji area
N

21. Ambaji ( Gabbar). Stock or boss

22. 3. Structural relation to host rocks.
 Many rocks into which plutons are injected are bedded or
foliated.
 A contact is said to be concordant if it is parallel to bedding or
foliations.
 It is said to be discordant if it cuts the bedding or foliation.
 In same way a plutons is said to be concordant or discordant
depending on the attitude of its contacts.
 In smaller bodies it is easy to observe directly but in larger
bodies indirect method is used such as strike and dip
observation

23. Age relative to adjacent rocks.
 An intrusive rock can be older or younger in age than the host
rocks.
 If intrusive rock is older then host rocks then it will rest on it
unconformably. The bedding above the unconformity is
roughly parallel to the contact.
 Scattered debris from intrusion may found in form of
conglomeratic layer above unconformity.
 An inclusion of country rock is found partially assimilated in
intrusion.
 As the intrusion is younger, sedimentary contact is indurated,
baked and intrusive margins are chilled and converted into
fine grained in texture

24. REFFRENCES
• M.P. BILLINGS.
• LECTURE NOTES.
• RESEARCH PAPERS FROM GEOSOCIETY.ORG

25. THANK YOU…