IGPET LECTURE 4 TEXTURE.ppt igneous petrology notes

Igneous petrology notes and lecture

1. Magma & Igneous Textures
LECTURE-4 (M.Sc Tech) I year 008
Dr. N.V.CHALAPATHI RAO
READER
DEPARTMENT OF GEOLOGY
BANARAS HINDU UNIVERSITY
VARANASI-221005 (U.P)
[email protected]
http://www.nvcraobravehost.com
http://www.bhu.ac.in/Geology/STAFF/NVC%20Rao.htm

2. Magmas & Igneous rocks
•Magmas: complex liquids that vary greatly in composition
and properties.
•Temperatures : 1400
o
C (2500
o
F)
•Depths: 50km-200 km
•Why magma comes up?
•Due to lower density than the solid upper mantle, buoyancy
moves it up.
•What decides the magma becoming extrusive or intrusive
igneous rock?
•Race between upward movement & cooling.
•Pluton or intrusive rock- crystallizes before reaching
surface.
•Lava- molten magma or partially molten on surface.
•Extrusive or volcanic rock – forms when lava cools.

3. Importance of cooling rates
•Cooling rates directly proportional to grain size of
igneous rocks.
•Granite(plutonic rock)- qtz+ feldspar= visible to naked eye
•Rhyolite (volcanic rock) – qtz=feldspar= microscope
•Intrusive rocks cool slowly & crystallize slowly(lot of xls)
•Volcanic rocks cool fast and crystallize rapidly as extrusion
expose them to the surface of the cool Earth’s atmosphere
(very less xls).
•When volcanic rock +water= very rapid cooling and no
crystals are formed e.g. obsidian.
•Rocks can be classified (by some) based on grain size:
•Aphanitic- very fine grains
•Phaneritic- very coarse grains
•Porphyritic – combination of large & small xls.

4. Cooling & Compositions of Igneous rocks
•Melting of rocks occurs at many places in the Earth &
magma compositions reflect their sources.
•Two end members of magmas:
•Silicic Magmas >40-75 wt% SiO2 (deficient in MgO)
and also rich in Al2O3 (SIALIC magmas) or FELSIC
magmas. Also FeO and Fe2O3 in little amounts e.g.
granite, rhyolite
•Mafic Magmas < 50 wt% SiO2 (Mg, Fe rich) and
deficient in SiO2 . FeO & Fe2O3 are more than in silicic
magmas e.g. basalt, gabbro
•Ultramafic magmas: Extreme type of mafic magmas
(very rich in MgO) e.g. Komatiite, kimberlite
•Intermediate Magmas: compositions in between silicic
and mafic e.g. diorite, andesite.

5. Cooling & Compositions of Igneous rocks
•Rock of different compositions have different melting temperatures
as they have different minerals (different elements).
•Si & O promote melting since they have very stable molten
polymers (long chains of Si & O) which melt at lower temperatures
than mafic/ultramafic magmas.
•Temperatures in flowing lavas range from 900-1100
o
C with highest
temperatures corresponding to basaltic magmas and lowest
temperatures corresponding to andesitic or rhyolitic magmas.
•LIQUIDUS: As magma cools, the first crystals to form are at liquidus
temperatures; below which everything is a liquid.
•SOLIDUS: Last drop of melt crystallizes at solidus temperature;
below which everything is a solid.
•Normally many magmas crystallize at temperatures of less than
200
o
C than that of magma; some >1000
o
C; granites crystallize as
low as 700
o
C.

6. VOLATILES IN MAGMAS
•Magmas contain volatiles (gas, liquid or vapour).
•H
2
O, CO
2
are most common but others include
S, Cl, F etc.
•Consequently, although igneous minerals
crystallize at higher temperatures, they still
contain H
2
O, CO
2
and other gaseous
components.
•Volatiles will lower melting and crystallization
temperatures, they change magma viscosity.
•Silicic magmas are more viscous than basic
magmas.

7. CRYSTALLIZATION OF MAGMAS
•Every mineral has a characteristic melting temperature. This leads to
an orderly & predictable process as magma solidifies.
•In case of mafic magmas (basalt) olivine and Ca-plagioclase, which
have high melting temperatures, will be first ones to crystallize.
•As temperature falls, some or all of olivine may disappear as it
becomes unstable.
•Olivine+SiO2 (in melt)= pyroxene
•At the same time, plagioclase (Ca-plagioclase (anorthite);
CaAl2Si2O8)+ Na,Si (in melt) = Na plagioclase (albite; NaAlSi3O8) +
Ca+Al (melt).
•With continued temperature falling, pyroxene becomes unstable and
reacts with melt to form amphibole, then mica, whereas plagioclase
comes more Na-rich.
•Inferences: mineralogy changes as crystallization proceeds
after crystallization is completed, minerals will reflect
overall magma composition.

8. CRYSTALLIZATION OF MAGMAS
•In case of SILICIC magmas, the first minerals to
crystallize are not olivine and anorthite but instead could
be amphibole, mica or feldspar.
•Reactions between minerals and melts take place as
cooling continues.
•K-Feldspar, muscovite and qtz are end products of
crystalliation of most silicic magmas.
•IMPORTANT INFERENCE IS WHEN A MAGMA
SOLIDIFIES the overall mineralogical assemblage is a
product of overall magma composition.

9. Bowen’s reaction series
•N.L.Bowen (1950) pioneered study of magmatic crystallization.
•Received Roebling medal from Mineralogical Society of America.
•Combining studies of naturally occurring rock types &
experimental studies, he developed an idealized model for
equilibrium crystallization in magmatic systems.
•BOWEN’S REACTION SERIES.
•It defines the order of crystallization of minerals in a cooling
magma.
•It has two branches – CONTINUOUS REACTION SERIES (RHS) &
DISCONTINUOUS REACTION SERIES (LHS)
•One mineral is present (Plagioclase) through out and changes its
composition.
•One mineral abruptly disappears (olivine) and new one appears.
•Residual minerals (left overs): muscovite, alkali feldspar and qtz

10. Bowen’s reaction series
Olivine Anorthite
Orthopyroxene bytwoniite
Clino pyroxene labradorite
amphibole andesine
biotite oligoclase
alkali feldspar
muscovite
quartz
Residual Phases
Low temperature
High Temperature
Cooling
(Si-poor, Mg & Fe-rich, low Al, low volatiles) minerals
(Si-rich, Mg & Fe-low, high Al, volatiles) minerals
Reflects the importance of silica role
How mineral chemistry excercises control on magma composition

11. Limitations of Bowen’s Reaction Series
•Only idealized system.
•If it goes on completion, then we will have only qtz, k-feldspar &
muscovite
•Hypothetical melt of 100% SiO2
•It gives 100% qtz; cannot crystallize any minerals of BRS because it
donot contain necessary elements! (other minerals are skipped).
•Hypothetical melt is 100% Forsterite (Mg2SiO4) composition.
•
It would yield only olivine and solidifies.
•COMPOSITION of MAGMA dictates
extent of crystallization.

12. IGNEOUS TEXTURES
•Texture: Geometrical aspects of minerals in a rock e.g.
size, shape , mutual relations etc.
•Can be seen only in microscope.
•Petrography is the study of thin sections.
•Igneous textures- two types
•Primary Textures:
•Occur during igneous crystallization and result from
interactions between minerals and melt.
•Secondary Textures:
•Alterations that take place after the rock is a completely
solid.

13. PRIMARY TEXTURES (CRYSTAL MELT
INTERACTIONS)
•Formation & growth of crystals, either from a
melt (igneous) or from a solid (metamorphism)
involve three principal processes
•viz., (I) initial nucleation of a crystal,
• (ii) subsequent crystal growth and
•(iii) Diffusion of chemical species (and heat)
through surrounding medium to and from the
surface of a growing crystal.

14. Nucleation, Crystal Growth & Diffusion
•Nucleation is a Critical initial step in crystal formation.
•A critical size of ‘embryonic cluster’ or ‘crystal nucleus’ must form. It requires
some degree of supersaturation or undercooling (cooling of a melt below
true crystallization temperature of a mineral).
•Crystals with simple structures nucleate more easily than those with
complex structures.g. olivine , magnetite or ilmenite crystallizes more easily
than a plagioclase.
•Crystal Growth involves involves growth of more faces, development of
complex silicates e.g. pyroxenes, amphiboles, micas.
•Diffusion growth of a mineral depletes the adjacent melt in the constituents
that the mineral preferentially incorporates. So, for growth to proceed new
material should DIFFUSE through melt, cross the depleteed zone and
reach crystal surface.
•All the above three influence the ultimate texture of resulting rock.
•Which ever rate is slowest, it will exercise most control on
crystallization.
•Apart from these, COOLING RATE is another important aspect.

15. Nucleation, Crystal Growth & Diffusion
•Idealized rates of crystal
nucleation and growth as a
function of temperature below
the melting point. Slow cooling
results in only minor
undercooling (T
a
), so that rapid
growth and slow nucleation
produce fewer coarse-grained
crystals. Rapid cooling permits
more undercooling (T
b
), so that
slower growth and rapid
nucleation produce many fine-
grained crystals. Very rapid
cooling involves little if any
nucleation or growth (T
c
)
producing a glass.
INFERENCES:
1)At higher temp. the no. of
nuclei develop is low bt growth
of xtal is high.
2)As temp. dec. nuc. Inc. bt
growth dec.

16. Common Igneous Minerals

17. Genetic Classification of Igneous Rocks
Granite Gabbro
•
IntrusiveIntrusive: crystallized from slowly cooling
magma intruded within the Earth’s crust;
e.g. granite, gabbro.

18. Genetic Classification of Igneous Rocks
Rhyolite Basalt
•
ExtrusiveExtrusive: crystallized from rapidly cooling
magma extruded on the surface of the Earth
as lava, …

19. Genetic Classification of Igneous Rocks
•
ExtrusiveExtrusive: … or erupted as pyroclastic
material, i.e., fragmented pieces of magma
ejected and cooled in the air.
Pumice
Scoria
Ash

20. FelsicIntermediate Mafic
GraniteGranodioriteDiorite Gabbro
BasaltAndesiteDaciteRhyolite
Viscosity
Melting Temperature

21. - Covers 15,400 mi
2
!!
- Composition
- Granite
- Monzonite
- Granodiorite
- Diorite
- Even Gneiss
- All in one “magma chamber”

22. The Formation of
Magma Chambers
Partial meltingPartial melting
Less dense magmaLess dense magma
Magma risesMagma rises
Magma pools in Magma pools in
magma chambermagma chamber
Some minerals melt before others.
Results in mixture of melt and
solid.
Melt is less dense than solid. Low
density minerals tend to melt first.
Buoyant melt migrates through rock
pores and fractures.

23. Magma Differentiation
The process by which rocks of various
compositions can arise from a uniform
parent magma
Occurs because different minerals
crystallize at different temperatures
(i.e., the opposite of partial melting)

24. Different types of primary textures
•RAPAKIVI TEXTURE albiltic plagioclase overgrowths over
orthoclase (e.g. granites)
•SPHERULITIC TEXTURE in silicic volcaniclastics where needles of
qtz and alkali feldspar grow from a common centre.
•VARIOLITIC TEXTURE radiating plagioclase laths in some basalts.
•COMPOSITIONAL ZONING – occurs when a mineral changes its
composition during its growth.
•NORMAL ZONING: Anorthite rich core to albite rich rim
•REVERSE ZONING: Albite rich core to anorthite rich rim
(metamorphic plagioclase where growth is accompanied by rising
temperature).
•OSCILLATORY ZONING – Regular decrease of An content from
anorthite to albite in plagioclase.

25. ZONING Egs
•A Compositionally zoned hornblende phenocryst with pronounced
color variation visible in plane-polarized light. Field width 1 mm. b.
• B Zoned plagioclase

26. CRYSTALLIZATION SEQUENCE
•Early formed crystals are Euhedral (more
space)
•As more crystals are formed then mutual
interference impedes development of faces
and subhedral or anhedral crystals are
formed.
•Early ones have better forms and latest
ones are interstitial filling in spaces
between earlier ones.
•This is more or less a thumb rule but with
exceptions.

27. CRYSTALLIZATION SEQUENCE
•Igneous accessory minerals e.g. zircon, apatite,
titanite are euhedral eventhough they are
formed during last stages of crystallization.
•Inclusion relationship – another indicator of
sequence indicator and most reliable one.
•INCLUSION is the first to form and
‘ENVLOPING MINERAL’ is later formed. It
should be valid throughout the section!
•OPHITIC TEXTURE: refers to envelopment of
plagioclase laths by larger CPX (later formed).

28. RESORPTION & DIFFERENTIAL MOVEMENT OF
CRYSTALS
•RESORPTION- refusion or dissolution of a mineral back into melt or
solution from which it is formed. Resorbed crystals have rounded
corners or embayed.
•SIEVE TEXTURE- advanced stage of resorption.
•FOLIATION- flow within a melt alignment of elongated or tabular
minerals.
•LINEATION- when micaceous minerals are involved.
•TRACHYTIC TEXTURE- lath shaped microlites (plagioclase) in a
volcanic rock are strongly aligned around phenocrysts.
•PILOTAXITIC or FELTY- random or non-aligned microlites.
•FLOW BANDING- mingling of two magmatic fluids.
•CUMULOPHYRIC TEXTURE- many clusters of adhering
phenocrysts
•GLOMEROPORPHYRITIC – clusters are of only one mineral.

29. Volcanic TEXTURES (Basalt)
•Phenocrysts
•Groundmass
•Microlites (groundmass crystals which are large)
•Crystallites (groundmass crystals which are small)
•Microphenocrysts (microlites larger than micrlites)
•OPHITIC texture- lath shaped plagioclase microphenocrysts
included in larger pyroxenes
•SUB-OPHITIC texture- smaller pyroxenes, parrtially enveloped by
pyroxenes)
•INTERGRANULAR TEXTURE- Plg & Pyx are subequal in size, with
glass as minor component in intergranular spaces
•INTERSERTAL TEXTURE- when glass is an important constituent
•HYALO OPHITIC- glass is plentiful
•HYALOPILITIC- Glass is most dominant & Plg and Pyx are very
minor.
•THERE IS A GRADATION OF THESE ABOVE TEXTURES!

30. \
•Figure 3-8. Ophitic texture. A single pyroxene envelops several well-developed
plagioclase laths. Width 1 mm. Skaergård intrusion, E. Greenland. © John
Winter and Prentice Hall.

31. •Sieve texture in a cumulophyric cluster of plagioclase phenocrysts.

32. •Resorbed and embayed olivine phenocryst.

33. •Trachytic texture in which
microphenocrysts of plagioclase are
aligned due to flow. Note flow around
phenocryst (P).
. Felty or pilotaxitic texture in which the . Felty or pilotaxitic texture in which the
microphenocrysts are randomly oriented. microphenocrysts are randomly oriented.
Basaltic andesiteBasaltic andesite

34. •Flow banding in andesite. Mt. Rainier,
WA.
Intergranular texture in basalt. Columbia Intergranular texture in basalt. Columbia
River Basalt Group, Washington. River Basalt Group, Washington.

35. CUMULATE TEXTURES – LAYERED IGNEOUS ROCKS
•Development of cumulate textures. a. Crystals accumulate by crystal settling or simply
form in place near the margins of the magma chamber. In this case plagioclase crystals
(white) accumulate in mutual contact, and an intercumulus liquid (pink) fills the interstices.
• b. Orthocumulate: intercumulus liquid crystallizes to form additional plagioclase rims plus
other phases in the interstitial volume (colored).

36. Cumulate textures
•. c. Adcumulates: Plagioclase fills most of the available space.
•d. Heteradcumulate: intercumulus liquid crystallizes to additional plagioclase rims, plus
other large minerals (hatched and shaded) that nucleate poorly and poikilitically envelop the
plagioclases

37. PYROCLASTIC TEXTURES
•These are due to EXPLOSIVE volcanic
activity
•Classification based on size & nature of
fragments (pyroclasts or tephra)
•LAPILLI, PISOLITIC TUFFS, EUTAXITIC
TEXTURES, FIAMME – terms related to
shapes of ash and beds.

38. SECONDARY TEXTURES
•Develop after igneous rock is entirely solid and are thus really
metamorphic in nature!
•Deuteric alteration (hydration – autometamorphism)
•Uralitzation- amphibole replacing pyroxene
•Biotitization- biotite from pyroxene or amphibole
•Chloritization- alteration of any mafic mineral to chlorite
•Sericitization- feldspar alters to sericite.
•Saussuritization- plagioclase alters to epidote.
•Symplectite- combined growth of two or more minerals as they
replace another.
•MYRMEKITIc- INTERGROWTH OF QTZ IN PLAGIOCLASE

39. •a. Pyroxene largely replaced by
hornblende. Some pyroxene
remains as light areas (Pyx) in the
hornblende core. Width 1 mm. b.
Chlorite (green) replaces biotite
(dark brown) at the rim and along
cleavages. Tonalite.
Pyx
Hbl
Bt
Chl