Granitoids Download

Transcript
Granitic Rocks
(Chapter 18)
Image: http://www.yosemite.ca.us/library/granite_crags/yosemite_watercolors.html
Granitoids
"Granitoids" (sensu lato): loosely applied to a wide range
of felsic plutonic rocks
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 postdate the thickening event by tens of
millions of years.
Image source: Winter, 2001
Granitoids
"Granitoids" (sensu lato): loosely applied to a wide range
of felsic plutonic rocks
A few broad generalizations:
2) Because the crust is solid in its normal state, a thermal
disturbance is required to form granitoids
3) Most granitoids are derived by crustal anatexis, but
that the mantle may also be involved as a source of heat
for crustal anatexis, or a source of melt.
Image source: Winter, 2001
Granitoids
Petrographic characteristics of granitoid rocks
Image source: Darrell Henry
Common minor minerals: apatite, zircon,
magnetite, ilmenite, monazite, titanite,
tourmaline, allanite, fluorite and pyrite
medium-to-coarsed grained rocks reflect slow cooling a volatiles.
• dominated by plagioclase
(generally first), quartz and Kfeldspar
• Hornblende (brown to green) and
biotite are the chief mafic minerals,
and Al-in-hornblende
geobarometer can yield P of
crystallization.
• Muscovite may be present as melt
phase or are a secondary mineral.
• Cpx may be found in the more
mafic granitoids
Granitoids
Petrographic characteristics of granitoid rocks
Backscattered electron image of a zircon
from the Strontian Granite, Scotland.
The grain has a rounded, un-zoned core
(dark) that is an inherited hightemperature non-melted crystal from
the pre-granite source.
The core is surrounded by a zoned
epitaxial igneous overgrowth rim,
crystallized from the cooling granite.
Image source: From Paterson et al. (1992),
Trans. Royal. Soc. Edinburgh. 83, 459-471.
Also Geol. Soc. Amer. Spec. Paper, 272, 459471.
Granitoids
Textures in granitoid rocks
At relatively low PH2O, a single
feldspar will crystallize and
then undergo further
exsolution (a, b) [hypersolvus]
Image source: John Winter, 2001
At relatively high
PH2O, two
feldspars will
crystallize with
possible further
exsolution of
each phase (c)
[subsolvus]
Granitoids
Petrographic characteristics of granitoid rocks
Myrmekites
• intergrowth of dendritic
quartz and plagioclase at Kfeldspar/plagioclase interface
• texture likely related to
subsolidus deformation
Myrmekite patch that appears to be replacing microcline. Faint twins in the
myrmekite clearly shows that the probably quartz "worms" are in a plagioclase
matrix. image source: Kurt Hollocher
Granitoids
Petrographic characteristics of granitoid rocks
Table 18-1. The Various Types of Enclaves
Name
Nature
Margin
Shape
Features
Xenolith
piece of country
rocks
sharp to
gradual
angular
to ovoid
contact metamorphic
texture and minerals
Xenocryst
isolated foreign
crystal
sharp
angular
corroded
reaction rim
Surmicaceous
Enclave
residue of melting
(restite)
Schlieren
disrupted enclave
gradual
oblate
coplanar orientation
Felsic Microgranular Enclave
disrupted
fine-grained margin
sharp to
gradual
ovoid
fine-granied
igneous texture
Mafic Microgranular Enclave
Blob of coeval
mafic magma
mostly
sharp
ovoid
fine-granied
igneous texture
Cumulate Enclave
(Autolith)
disrupted
cumulate
mostly
gradual
ovoid
coarse-grained
cumulate texture
After Didier and Barbarin (1991, p. 20).
sharp,
lenticular metamorphic texture
biotite rim
micas, Al-rich minerals
Granitoids
Granitoid Chemistry
biotite
muscovite
cordierite
andalusite
garnet
pyroxene
hornblende
biotite
aegirine
riebeckite
arfvedsonite
CaO
CaO
moles
CaO
K2O
K2O
Al2O3
K2O
Al2O3
Peraluminous
Metaluminous
Image source: John Winter, 2001
• most commonly calcalkaline
Al2O3
Na2O
Na2O
• Composition of
granitoid controlled by
composition of
source, pressure,
temperature, degree of
partial melting, and
the nature of
differentiation.
Na2O
Peralkaline
• variable aluminum
saturation that
generally depends on
the source of melting
Granitoids
Granitoid Chemistry
The fact that most
granitoids plot near the
low P ternary minimum
melts are most
consistent with melting
of a quartzofeldspathic
crustal parent.
Note the effects of
increasing pressure and
the An, B, and F
contents on the position
of the thermal minima.
Image source: John Winter, 2001
Granitoids
Granitoid Chemistry
• MORB-normalized spider
diagrams for the analyses in
Table 18-2
• The subduction zone
granitoids display the typical
decoupling of the LIL/HFS
elements.
• The plagiogranite is more
similar to patterns associated
with MORBs.
Image source: John Winter, 2001
Granitoids
Crustal melting (anatexis)
initiated due to increase in mantle-produced heat or thickened crust.
two possible ways to produce melts
H2O-saturated melting - produces minor
amounts of melt due to small amounts of
trapped water
Migmatite from the Hellroaring
Plateau in the Beartooth Mtns
(MT) with felsic granitoid melt
and restite of quartz +
plagioclase + K-feldspar +
biotite + garnet + sillimanite.
Image source: Darrell Henry
Dehydration melting – fluid from breakdown of
hydrous minerals: e.g.
Mu + Pl + Qtz = Kfs + Al-sil + Melt
or
Bt + Pl + Al-sil + Qtz = Kfs + Grt + Melt
if produced melts are <30% the melt generally
stays with the source to produce a migmatite.
Granitoids
Crustal melting (anatexis)
(a) Simplified P-T phase
diagram for melting of
aluminous quartzofeldspathic
materials and
(b) quantity of melt generated
during the melting of
muscovite-biotite-bearing
crustal source rocks
Shaded areas in (a) indicate
melt generation.
Granitoids
Classifications of Granitoids – (genetic classification)
Table 18-3. The S-I-A-M Classification of Granitoids
SiO2
K2O/Na2O
Type
M
46-70%
low
Ca, Sr
high
I
53-76%
low
high in
mafic
rocks
S
65-74%
high
low
A/(C+N+K)*
low
Fe3+/Fe2+
Cr, Ni
low
18O
< 9‰
low
< 9‰
low
high
> 9‰
var
low
var
low
low: metal- moderate
uminous to
peraluminous
high
metaluminous
A
high
 77%
Na2O
high
* molar Al2O3/(CaO+Na2O+K2O)
low
var
peralkaline
87
Sr/86Sr
Misc
Petrogenesis
< 0.705
Low Rb, Th, U
Subduction zone
Low LIL and HFS or ocean-intraplate
Mantle-derived
< 0.705
high LIL/HFS
Subduction zone
med. Rb, Th, U
Infracrustal
hornblende
Mafic to intermed.
magnetite
igneous source
> 0.707 variable LIL/HFS Subduction zone
high Rb, Th, U
biotite, cordierite
Supracrustal
Als, Grt, Ilmenite sedimentary source
var
low LIL/HFS
Anorogenic
high Fe/Mg
Stable craton
high Ga/Al
Rift zone
High REE, Zr
High F, Cl
Data from White and Chappell (1983), Clarke (1992), Whalen (1985)
• I-type granitoids (igneous source) - partial melts of
mantle-derived mafic rocks (underplated basaltic
melts?); contain abundant hornblende and magnetite
Granitoids
Classifications of Granitoids – (genetic classification)
Table 18-3. The S-I-A-M Classification of Granitoids
SiO2
K2O/Na2O
Type
M
46-70%
low
Ca, Sr
high
I
53-76%
low
high in
mafic
rocks
S
65-74%
high
low
A/(C+N+K)*
low
Fe3+/Fe2+
Cr, Ni
low
18O
< 9‰
low
< 9‰
low
high
> 9‰
var
low
var
low
low: metal- moderate
uminous to
peraluminous
high
metaluminous
A
high
 77%
Na2O
high
* molar Al2O3/(CaO+Na2O+K2O)
low
var
peralkaline
87
Sr/86Sr
Misc
Petrogenesis
< 0.705
Low Rb, Th, U
Subduction zone
Low LIL and HFS or ocean-intraplate
Mantle-derived
< 0.705
high LIL/HFS
Subduction zone
med. Rb, Th, U
Infracrustal
hornblende
Mafic to intermed.
magnetite
igneous source
> 0.707 variable LIL/HFS Subduction zone
high Rb, Th, U
biotite, cordierite
Supracrustal
Als, Grt, Ilmenite sedimentary source
var
low LIL/HFS
Anorogenic
high Fe/Mg
Stable craton
high Ga/Al
Rift zone
High REE, Zr
High F, Cl
Data from White and Chappell (1983), Clarke (1992), Whalen (1985)
• S-type granitoids (sedimentary source) - partial melts of
aluminous sedimentary rocks; w/ Al-rich minerals (Alsilicates; cordierite, garnet), biotite (brown) and ilmenite
Granitoids
Classifications of Granitoids – (genetic classification)
Table 18-3. The S-I-A-M Classification of Granitoids
SiO2
K2O/Na2O
Type
M
46-70%
low
Ca, Sr
high
I
53-76%
low
high in
mafic
rocks
S
65-74%
high
low
A/(C+N+K)*
low
Fe3+/Fe2+
Cr, Ni
low
18O
< 9‰
low
< 9‰
low
high
> 9‰
var
low
var
low
low: metal- moderate
uminous to
peraluminous
high
metaluminous
A
high
 77%
Na2O
high
low
var
peralkaline
* molar Al2O3/(CaO+Na2O+K2O)
87
Sr/86Sr
Misc
Petrogenesis
< 0.705
Low Rb, Th, U
Subduction zone
Low LIL and HFS or ocean-intraplate
Mantle-derived
< 0.705
high LIL/HFS
Subduction zone
med. Rb, Th, U
Infracrustal
hornblende
Mafic to intermed.
magnetite
igneous source
> 0.707 variable LIL/HFS Subduction zone
high Rb, Th, U
biotite, cordierite
Supracrustal
Als, Grt, Ilmenite sedimentary source
var
low LIL/HFS
Anorogenic
high Fe/Mg
Stable craton
high Ga/Al
Rift zone
High REE, Zr
High F, Cl
Data from White and Chappell (1983), Clarke (1992), Whalen (1985)
• M-type granitoids (direct mantle source) - e.g.
plagiogranites in ophiolites
• A-type granitoids (on anorogenic settings) - peralkaline
melts in rifts
Granitoids
Classifications of Granitoids (tectonic setting)
Granitoids
Classifications of Granitoids (tectonic setting)