Magmatic Processes

8/14/2019 Magmatic Processes

1/17

Why does the mantle melt?Under static

conditions,

alltemperatures

will be

below the

rock melting

temperature.

If not,

melting and

melt escape

will extract

heat and

raise thelocal solidus,

until the

solidus is

above the

localtemperature.


2/17

Why the mantle melts: decompression

meltingConvective

rise of deep,

hot mantle

rock causes therising rocks to

cool

adiabatically at

~2C/kbar, and

to intersect the

rock solidus

line which has

a typical slope

of ~6C/kbar.

Melting

begins,absorbing heat

as melting

progresses.

Melt

eventuallyescapes.


3/17

Why the mantle melts: flux meltingIn subduction

zones,

aqueous

fluids and

water-bearing

magmas

escape from

the

subducting

oceanic

lithosphere.These rise

into the

overlying

mantle wedge

and act as aflux that

lowers the

melting

temperature

of the mantlewedge.


4/17

Why the crust melts: heating or flux meltingUnder static conditions the crust should not melt either.

Heating the crust:Addition of heat to the lower crust raises the geotherm until itcrosses the dry solidus.

Wetting the crust:Aqueous fluids lower the lower crust solidus to below the local

geotherm. Fluids may come from dehydration of hydrous minerals in crustal rock (e.g.,micas or amphiboles), or from crystallizing basalts in the deep crust.


5/17

Plutons all

solidify. If crystal

nucleation is

initially low,

phenocrysts can

grow from the

liquid.

All plutons

interact with

surrounding rocks

and so can havexenocrysts,

xenoliths, and

narrow dike or

sill extensions.

Phenocrysts, xenocrysts, xenoliths

Dike


6/17

Grain size variations caused

by differential cooling rates

at different distances from

the pluton margin

Chilled margins


7/17

Differential flow can

cause crystals and

xenoliths to become

parallel and to

migrate away fromthe walls.

Flow foliation and

differentiation


8/17

Flow foliation in large plutons

Marginal foliations developed within a pluton as a result of differential

flow along the contact. From Lahee (1961), Field Geology. McGraw

Hill. New York.


9/17

Thermal and

mechanical stresses

associated with pluton

emplacement canfracture shallow, brittle

rocks. Dense detached

blocks can sink into

the pluton depths.

With long exposure,

xenoliths can partially

melt, become

disaggregated, and

loose their identity,

thus contaminating the

magma. Exposed piles

of xenoliths on pluton

floors are rarely found.

Pluton rise: stopeing


10/17

Stopeing and xenoliths, Wichita Mtns., Oklahoma

Stopeing of diorite xenoliths into a grano-

diorite magma. Field trip led by members of

the University of Oklahoma, Norman,Geology Dept. Kurt Hollocher, 1975.


11/17

Ballooning involves

pluton swelling with

displacement of

ductile country rock.

Also illustrated here

is the filling of a

pluton by dikes. It

appears that, for

many plutons, dike

filling is the dominant

magma emplacement

mechanism. Maficdike swarms are

common, and felsic

dike swarms beneath

some felsic plutonscan be found.

Pluton rise: ballooning


12/17

Hot magma may

interact directlywith the wall

rock. Melted

wall rock can

mix with the rest

of the pluton.

The question

becomes how

much of the

pluton was from

below, and how

much was from

adjacent rock?

Pluton rise: melting and assimilation


13/17

Diapirism involves

density-driven rise of

buoyant magma

through denser

country rock. Saltdomes are perhaps

the best example of

diapirism.

Magmas are

generally not viscous

enough to be

emplaced by this

mechanism; it ismechanically easier

for the magma to

penetrate upward as

a thin dike ratherthan as a large blob.

Pluton rise: diapirism


14/17

Normal zoning of crystalsNormal zoning involves continuous composition change from a high-

temperature composition core to a low-temperature composition rim.

This example is of plagioclase.

C i ll d l


15/17

Convection-controlled crystal

dissolution and zoning

Zoned plagioclase with internal unconformities.

Augite with rounded, partly resorbed core.

Crystal growth and convection into different P,

T, or composition regions, can result inmultiple episodes of growth and dissolution.


16/17

Mineral growth and dissolution in magmas

Dissolution(resorption) of

phenocrysts or

xenocrysts

Regular growth

Resorbed quartz phenocryst

in a dacite porphyry.

Euhedral phenocrysts of

plagioclase and olivine in a

basalt.


17/17

Other mineral reaction relationships in magmas

Dehydration ofhydrous

minerals to an

anhydrous

pseudomorphic

assemblage.

Overgrowths ofone mineral on

another due to a

peritectic

reaction or

disequilibrium.

Quartz

xenocryst

rimmed

by augite

in an

olivine

basalt.

Quartz

xenocrystrimmed by

hornblende

in a syenite

porphyry.

Augiterimmed

and partly

replaced by

brown

hornblende

in a

gabbro.

Documents

Magmatic Processes