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

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

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

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

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

Geochemisty of Arc Plutonic Complexes Mimics Volcanic Compositions Peruvian Coastal Batholith

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

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

M-Type Granitoids Differentiates of Mafic Magmas

I-Type Granitoids Remelting of Mafic Underplated Crust

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

A-Type Granitoids Anorogenic Melting of Continental Interiors

Granites Created During Continent-Continent Collision (Orogenesis)

Post-Orogenic Grantoids Extensional Collapse Post-Penokean granites