Granitoid rocks
(Chapter 18)
last update:10/11/06
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 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
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Petrographic characteristics of granitoid rocks medium-to-coarsed grained rocks - reflect slow cooling a volatiles. dominated by plagioclase (generally first), quartz and K-feldspar Hornblende (brown to green) and biotite are the chief mafic minerals, and Al-in-hornblende geobarometer can yield the P of crystallization. Muscovite may be present as melt phase or are a secondary mineral. Cpx may be found in the more mafic granitoids |
Common minor minerals: apatite, zircon, magnetite, ilmenite, monazite, titanite, tourmaline, allanite, fluorite and pyrite
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Backscattered electron image of a zircon
from the Strontian Granite, Scotland. The grain has
a rounded, un-zoned core (dark) that is an inherited high-temperature
non-melted crystal from the pre-granite source. The core is
surrounded by a zoned epitaxial igneous overgrowth rim, crystallized from
the cooling granite.
From Paterson et al. (1992), Trans. Royal. Soc. Edinburgh. 83, 459-471. Also Geol. Soc. Amer. Spec. Paper, 272, 459-471. |
Textures in granites
Subsolvus granites - granites that crystallize at high P(H2O) where the solvus intersects the solidus generating two distinct feldspar fields
Hypersolvus granites - single feldspar granites that crystallize at low P(H2O) in a single feldspar field above the solvus.
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| Myrmekites | texture likely related to subsolidus deformation |
Granitoid Chemistry
 | in general, the composition of the
granitoid is controlled by the composition of the source, pressure,
temperature, degree of partial melting, and the nature of differentiation. |
| most commonly calc-alkaline |
| variable aluminum saturation that generally
depends on the source of melting |
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| Alumina saturation classes based on the molar proportions of Al2O3/(CaO+Na2O+K2O) ("A/CNK") after Shand (1927). Common non-quartzo-feldspathic minerals for each type are included. After Clarke (1992). Granitoid Rocks. Chapman Hall. |
The fact that most granitoids plot near the low P ternary minimum melts are most consistent with melting of a quartzofeldspathic crustal parent.
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The Ab-Or-Qtz system with the ternary cotectic curves and eutectic minima from 0.1 to 3 GPa. Included is the locus of most granite compositions from Figure 11-2 (shaded) and the plotted positions of the norms from the analyses in Table 18-2. Note the effects of increasing pressure and the An, B, and F contents on the position of the thermal minima. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. |
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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. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. |
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
Dehydration melting
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
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Migmatite from the Hellroaring Plateau in the Beartooth Mtns (MT) with felsic granitoid melt and restite of quartz + plagioclase + K-feldspar + biotite + garnet + sillimanite |
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(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. after Clarke (1992) Granitoid Rocks. Chapman Hall, London; and Vielzeuf and Holloway (1988) Contrib. Mineral. Petrol., 98, 257-276. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. |
Classifications of Granitoids
SIAM classification (genetic classification of Chappell and White)
| I-type granitoids
(igneous source) - partial melts of mantle-derived mafic rocks (underplated
basaltic melts?); contain abundant hornblende and magnetite |
| S-type granitoids
(sedimentary source) - partial melts of aluminous sedimentary rocks;
contain aluminous minerals (Al-silicates; cordierite, garnet), biotite
(brown) and ilmenite |
| M-type granitoids
(direct mantle source) - e.g. plagiogranites in ophiolites |
| A-type granitoids (on anorogenic settings) - peralkaline melts in rifts |
this classification is not always consistent
Tectonic setting classification of granitoids
