G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 14 Origin of Granites March 14, 2016 From H.H. Read (1955)

P ARTIAL M ELTING VS. F RACTIONAL C RYSTALLIZATION T HE S ONJU L AKE – F INLAND G RANITE C ONNECTION Finland Granite SLI The Problem: Even very efficient fractional crystallization will create only ~5% felsic magma

P ARTIAL M ELTING VS. F RACTIONAL C RYSTALLIZATION 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

A RC P LUTONIC C OMPLEXES - “G RANITE ” B ATHOLITHS F EEDER C HAMBERS TO C ONTINENTAL A RC V OLCANICS

Granitoids – all intermediate to felsic compositions plotting on a QAP(F) diagram Tends toward more granitic compositions over time Epizonal batholiths form mostly by roof collapse (stoping) or downdropping of the chamber floor C OMPOSITE E MPLACEMENT OF OF “G RANITOID ” B ATHOLITHS

G EOCHEMISTY OF A RC P LUTONIC C OMPLEXES M IMICS V OLCANIC C OMPOSITIONS Peruvian Coastal Batholith

N ON -G ENETIC C LASSIFICATIONS OF G RANITOID R OCKS Chemistry-based Mineralogy-based

E SSENTIAL M INERALOGY OF G RANITIC R OCKS L INKED TO THE QAP TERNARY EUTECTIC / MINIMUM Felsic Magma Generated from melting Crust Felsic Magma Generated from Mafic Magmas

G ENETIC C LASSIFICATION OF G RANITIC R OCKS B ASED ON S OURCE R OCK /M ODE OF O RIGIN

M-T YPE G RANITOIDS D IFFERENTIATES OF M AFIC M AGMAS

I-T YPE G RANITOIDS R EMELTING OF M AFIC U NDERPLATED C RUST

A-T YPE G RANITOIDS A NOROGENIC M ELTING OF C ONTINENTAL I NTERIORS M-Type

S-T YPE G RANITOIDS P ARTIAL M ELTING OF S EDIMENTARY R OCKS Dehydration Melting of Hydrous Mineral-bearing Metasedimentary Rocks High-grade metasedimentary rocks have little-no free water, so water-saturated melting curve (a-b-c) not as important as hydrous mineral melting curves Melt generation along the 40°C/km geotherm

T RACE E LEMENT G EOCHEMISTRY OF SIAM G RANITOIDS Strongly enriched in incompatible trace elements, especially LILE and LREE, U and Th. Negative anomalies in Sr and Ba (follows Ca) and Ti- Nb-Ta (except intraplate granitoids)

T RACE E LEMENT T ECTONIC D ISCRIMINATION D IAGRAMS FOR G RANITOIDS

G RANITES C REATED D URING C ONTINENT -C ONTINENT C OLLISION (O ROGENESIS )

P OST - O ROGENIC G RANTOIDS E XTENSIONAL C OLLAPSE Post- Penokean granites