Igneous rock : granite

1. Granites & Granitoids
LECTURE (M.Sc Tech I year) 2008-2009
Dr. N.V.CHALAPATHI RAO
READER
DEPARTMENT OF GEOLOGY
BANARAS HINDU UNIVERSITY
VARANASI-221005 (U.P)
nvcr100@gmail.com
http://www.nvcraobravehost.com
http://www.bhu.ac.in/Geology/STAFF/NVC%20Rao.htm

2. Granites & Granitoids
Granites are the most abundant rocks in Earth’s continental
crust.
Granites or granitoids are the terms loosely applied to a
wide range of felsic plutonic rocks.
Granitoid is a field term.
The term embraces a wide variety of rocks viz., granites,
granodiorites, tonalites, alkali-feldspar granites etc.
Exact naming of granitoid is done by counting ‘mode’ of the
minerals under microscope and by expressing in terms of
Q (quartz), A (alkali feldspar) and P (Plagioclase)
(Streckeisen, 1972).
Entire gradation of various rock types is seen based on
mineral proportion.
Based on the proportion of feldspars (Ab, Plag & Or)
granitoids are also further classified.

3. Q-A-P CLASSIFICATION OF GRANITOIDS ( a part of QAPF
CLASSIFICATION)
All granitoid rocks have
Modal Qtz varying
from 20-60% & are named
Based on increasing proportion of
PF relative to KF

4. Terminology of common granitoids
• Quartzolite = 90% quartz
• Quartz + alkali feldspar (GRANITE sensu stricto)
• Hypabyssal equivalent of granite = GRANOPHYRE
• Fine grained granite = APLITE
• Qtz + K feldspar = Sodic plagioclase (ADAMELLITE)
• Qtz + Plagioclase > K-feldspar (GRANODIORITE)
• Qtz + Sodic Plagioclase >> K-feldspar (TONALITE)
• Alkali feldspar granite = contains only alkali fedspar
(alkali granites)

5. Rhyolite & Granite
• Rhyolite is a volcanic equivalent of granite –
both are manifestations of same magma.
• H.H. Read (1848) refused this. He believed
that granite is a plutonic rock and is not
related to volcanic rock.
• The reason being very few examples exist
showing intimate relationship between granite
& rhyolite.
• Eg. Nigerian granite province (granite cupolas
grade to rhyolite ash).

6. Mineralogy
• Granitoids are plutonic rocks of holocrystalline nature
and even grained.
• They comprise alkali-feldspar, plagioclase, quartz
and ferro-magnesian minerals (accesories).
• Alkali feldspar = orthoclase (mostly) or even
microcline (rarely)
• Ferro-magnesian minerals = biotite, hornblende and
augite
• In charnockite = hypersthene is an important mineral
(pyroxene)
• Other accessories are zircon, tourmaline, apatite,
ilmenite, magnetite, sphene, pyrite etc.
• In metamorphosed granitoids there may be kyanite,
andalusite, sillimanite, epidote etc depending on
the degree of metamorphism.

7. Texture
• Hypidiomorphic texture (predominantly of subhedral grains) also
called GRANITIC TEXTURE or GRANITOID TEXTURE.
• Most granitoids are HOMOPHANOUS or may show weak foliation
near margin of intrusions.
• PORPHYRITIC GRANITES- K feldspar phenocrysts or megacrysts
are formed during early magmatic crystallization
• HYPERSOLVUS granites- free of discrete plagoclase and contains
homogenous alkali feldspar (single feldspar)
• SUBSOLVUS granites- contains discrete plagioclase & alkali
feldspar (double feldspar)
• RAPAKIVI GRANITES- K feldspars show an overgrowth of
plagioclase feldspar (oligiclase)
• Rapakivi texture is a distinctive feature of hypersolvus granites
These are confined largely to Precambrian shield areas. These
textures are due to low-temperature recrystallization under
shallow, possibly sub-volcanic conditions.
• INTERGROWTH TEXTURES:
• Graphic- qtz & microcline
• Perthitic – Na-feldspar in K-feldspar
• Antiperthitic- K-feldspar in Na-feldspar
• Myrmekitic – wormy blebs of qtz in plagioclase

8. Classification of Granites
• Numerous attempts to classify diverse spectrum
of granitoid rocks.
• Simple mineralogical classification tells little
about their nature (genesis)
• Granitoid chemistry reflects that of source and
setting – led to diverse classsification schemes.
• Mineralogical classifications (IUGS classification)
• Chemical classification (alumina saturation, SIAM
classification etc)
• Tectonic classification (plate tectonic setting)

9. Chemistry of Granitoids
• Siliceous SiO2 > 69 wt%
• Potassic K2O 2.5-5.5 wt%
• Sodic Na2O = 2.5 – 4.9 wt%
• Calcic CaO = 0.5 – 4.7 wt% (plagiogranites)
• Aluminous Al2O3 11-16 wt%

10. Granite classification based on alumina saturation (by Shand)

11. S-I-A- M Classification of Granites
• Chemical Classification
• Two types (Chappell & White, 1974):
• S-type- derived from melting of sedimentary or meta-
sedimentary source rocks. S-type granites have more
alumina than that can be accommodated in the
feldspars in norms. This is reflected in appearance of
corundum in norm.
• I type- derived from partial melting of mantle of
predominantly igneous source rocks (sub-crustal).
More sodic content and less alumina content.
(continental areas)
• I-type granite has normative diopside.
• S-type is richer in transition metals and in several
lithophile elements, such as Ba, Rb, Th and LREE.

12. S-I-A- M Classification of Granites
• What about granitoids which are occassionally
present in island arcs which have no sialic crust
beneath them?
• This led White (1979) to coin the term M – type
granitoid (direct mantle source). These are found ias
plagiogranites in ophiolites and in Ocean islands
(Iceland).
• Fourth classification is ANOROGENIC (A type)
granitoid.
• They are intruded into non-orogenic settings.
• There are conspicuous differences between S-I-A-M
granitoids and this is summarized in Table.

13. SIAM CLASSIFICATION OF GRANITES
Table 18-3. The S-I-A-M Classification of Granitoids
Type SiO2 K2O/Na2O Ca, Sr A/(C+N+K)* Fe3+
/Fe2+
Cr, Ni 18
O
87
Sr/86
Sr Misc Petrogenesis
M 46-70% low high low low low < 9‰ < 0.705 Low Rb, Th, U Subduction zone
Low LIL and HFS or ocean-intraplate
Mantle-derived
I 53-76% low high in low: metal- moderate low < 9‰ < 0.705 high LIL/HFS Subduction zone
mafic uminous to med. Rb, Th, U Infracrustal
rocks peraluminous hornblende Mafic to intermed.
magnetite igneous source
S 65-74% high low high low high > 9‰ > 0.707 variable LIL/HFS Subduction zone
high Rb, Th, U
metaluminous biotite, cordierite Supracrustal
Als, Grt, Ilmenite sedimentary source
A high Na2O low var var low var var low LIL/HFS Anorogenic
 77% high peralkaline high Fe/Mg Stable craton
high Ga/Al Rift zone
High REE, Zr
High F, Cl
* molar Al2O3/(CaO+Na2O+K2O) Data from White and Chappell (1983), Clarke (1992), Whalen (1985)

14. Tectonic Classification of Granites
• Tectonic classification (Winter, 2001)
• Granites occur in a three broad varieties of tectonic
settings:
• OROGENIC – mountain building associated with
compressive stresses associated with subduction.
• ANOROGENIC – magmatism within plate or at a
spreading plate margin.
• POST-OROGENIC – magmatism takes place after
the main orogenic event. ( because it needs a
precursor orogeny two diverse groups exist whether
they are orogenic or anorogenic). To avoid confusion,
the term TRANSITIONAL, meaning neither but
having aspects of both has been coined.

16. Tectonic classification of Granitoid rocks
• Oceanic orogenic Granitoids – occur where one oceanic plate is
subducted beneath another to form an ISLAND ARC. These are
typical M type granitoids. E.g. Papua New Guinea, Solomon islands
• Continental orogenic Granitoids – occur where oceanic plate
subducts beneath a continental plate (active continental margins).
Here huge granitic batholiths (Cordillera) of Western Americas (e.g.
Andes)are egs.
• Continental collisional granitoids – Collision of two continents
creates an impressive orogeny, dramatic thrusting , folding & crustal
thickening. E.g. Himalayan granites
• What are suture zones?
• Ancient continental collision zones as recognized today.
• Transitional (Post-orogenic ) granitoids:
• They are characteristically generated in an orogenic belt between
10Ma and 100 Ma after compressive deformation has ceased.
• They are commonly emplaced during a period of uplift or extensional
collapse that follows orogeny. (They are hybdrid in composition I to S
types) e.g. Late Caledonian plutons of Britain

17. Tectonic classification of Granitoid rocks
• Continental Anorogenic Granitoids: Here magmatism occurs
in settings that are not genetically related with compressive
orogeny.
• One such occurrence is at Yellowstone-snake river plain in USA,
which is above a hotspot.
• Here mantle plume supplies heat for crustal anatexis and
generation of locally voluminous rhyolites (at surface) and
deeper granitods.
• Greatest anorogenic magmatic events occurred during
Proterozoic where a suite of 6000 km long belt extending from
Southern California to labrador , across Greenland to Baltic
sheild (all part of a supercontinent at that time) and include
ANORTHOSITE-MANGERITE-CHARNOCKITE-GRANITE
(AMCG suite).
• Charnockites are high temperature, nearly anhydrous , rocks
and can be either of igneous or metamorphic origin.
• Oceanic Anorogenic Granitoids: Plagiogranites of
ophiloites e.g Troodoos & Oman ophiolites.

18. Geochemical Discrimination Plots of granites

19. Pegmatites
• Pegmatites are a extraordinary group of rocks which are very
coarse grained, that form dykes and pod like segregations in
many granitic plutons.
• They form at last stages of crystallization , after main mass of
magma has crystallized and a hydrous fluid has formed a separate
phase.
• Gabbroic pegmatites are very rare – anhydrous nature of minerals.
• Nepheline syenite pegmatites are rare.
• Individual crystals reach enormous sizes , 2-3.5 m x 9 mts (4 m
thick micas!)
• Their bulk composition is similar to granites but certain elements
are extraordinarily enriched.
• Li, Cs, Be, Sn, Nb, Zr, U, Th, Bo, P and F.
• Spodumene, beryl, pollusite etc are important minerals.
• Rare elements partition more into aqueous phase than into
silicate phase. This is the reason for their abundance. They
also donot substitute isomorphously into earlier formed
minerals.

20. Petrogenesis of Granite
• Granites can be formed by differentiation
of
• Lunar basalts
• Achondrites (meteoritic basalts)
• Oceanic basalts
• Continental basalts
Experiments have established that first
liquids that are formed by melting of
pelitic sedimentary rocks and & most
igneous rocks under high pressures
approach granite compositions!!

21. The Great Granite Controversy
• During 1940’s two distinct thoughts emerged regarding genesis
of granitoids.
• Based on EXPERIMENTAL & FIELD OBSERVATIONS,
petrologists polarised into two schools of thought –
• MAGMATIC ORIGIN OF GRANITES
• METASOMATIC TRANSFORMATION (GRANITISATION)
ORIGIN
• Bowen’s studies showed that a basaltic magma can give rise to
acid magma (granitic).
• Extensive field studies , in high grade terranes, showed that
extensive metasomatic transformation was capable of bringing
about compositional changes insitu of a variety of pre-existing
rocks to create granites (H.H.Read).
• Many workers believed there are more than one origin for
granites & the term ‘GRANITES & GRANITES’ was coined.

22. Evidences in favour of magmatic emplacement
• Structure:
• Dykes, joint systems, planar & linear structures
(flowage), xenoliths & thermal aureoles
• Petrography
• Chllied facies, definite rystallization sequence
of minerals, , exsolution (perthites)
• Geochemistry
• Homogenity in composition

23. Evidences in favour of granitization
(transformation in situ)
• Structure:
• Gradationsal contact between country rock & granite
gneiss
• Continuation of regional structure
• Petrography
• Absence of crystallization order; replacement of minerals
e.g. Hbl by biotite)
• Geochemistry
• Chemical analyses of country rocks showed tranfer of
elements
• Trace element geochemistry of granites and country rocks
were similar

24. A few broad generalizations…
1) Most granitoids of significant volume occur in
areas where the continental crust has been
thickened by orogeny, either continental arc
subduction or collision of sialic masses. Many
granites, however, may post-date the thickening
event by tens of millions of years.
2) Because the crust is solid in its normal state,
some thermal disturbance is required to form
granitoids
3) Most workers are of the opinion that the majority
of granitoids are derived by crustal anatexis, but
that the mantle may also be involved. The
mantle contribution may range from that of a
source of heat for crustal anatexis, or it may be
the source of material as well.