1. Geol 2312 Igneous and Metamorphic Petrology
Lecture 17
Origin of Granites
From H.H. Read (1955)
March 6, 2009

2. Partial Melting vs. Fractional Crystallization The Sonju Lake – Finland Granite Connection
SLI
Finland Granite
The Problem: Even very efficient fractional crystallization will create only ~5% felsic magma

3. Partial Melting vs. Fractional Crystallization
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
Zoned zircon in a granite with older inherited (restite) core overgrown by new material from the felsic magma

4. Arc Plutonic Complexes- “Granite” Batholiths Feeder Chambers to Continental Arc Volcanics

5. Composite Emplacement of “Granitic” Batholiths
Tends toward more felsic compositions over time
Epizonal batholiths form mostly by roof collapse (stoping) or downdropping of the chamber floor

6. Geochemisty of Arc Plutonic Complexes Mimics Volcanic Compositions
Peruvian Coastal Batholith

7. Non-Genetic Classifications of Granitic Rocks
Chemistry-based
Mineralogy-based

8. Genetic Classification of Granitic Rocks Based on Source Rock/Mode of Origin

9. M-Type Granitoids Differentiates of Mafic Magmas

10. I-Type Granitoids Remelting of Mafic Underplated Crust

11. S-Type Granitoids Remelting of Sedimentary Rocks
Dehydration Melting of Hydrous Mineral-bearing Rocks

12. A-Type Granitoids Anorogenic Melting of Continental Interiors

14. Granites Created During Continent-Continent Collision (Orogenesis)

15. Post-Orogenic Grantoids Extensional Collapse
Post-Penokean granites