PLUGS AND PLUTONS

POOJA U MISHRA

SUBMITTED TO : DR. P.M. SOLANKI

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.

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.

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.

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

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

• 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.

• 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.

• 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.

Schlieren

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

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.

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.

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.

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.

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.

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.

Topographic map of Ambaji area

N

Ambaji ( Gabbar). Stock or boss

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

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

REFFRENCES

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

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