Petrology: Whole Rock Geochemical Discriminant Diagrams

You, too, can be a geochemist! You will never have to look at another rock or mineral again! Your only tenuous links to reality will be tables of numbers and vials of powder by post!

More seriously, geochemical discriminant diagrams can be useful for classifying glassy or fine-grained volcanics, and also for metamorphosed igneous rocks of all sorts. They can also help you start thinking about why the rocks are chemically the way they are. The diagrams below are those I have used, or tried to use, at one time or another. I had to figure out the discriminant line coordinates so that I could plot my data on them. It is a senseless waste of time for everyone else to have to do the same thing, so I present the coordinates for the benefit of all humanity. Let me know if you find this material useful.

Click on the citation for the discriminant lines shown in the adjacent pictures, click on the small image to get a bigger image. All discriminant line coordinates are given in Excel files with X and Y values, Z if necessary. Please contact me if you find any errors. I can add new discriminant diagrams on request, but please send the discriminant line coordinates and reference, or at least the reference. No guarantees as to speed of posting.

Rock classification

barker_1979Barker, 1979
An-Ab-Or diagram for tonalite, trondhmemite, granodiorite, granite.
ewart_1982Ewart, 1982
SiO2 vs. K2O for volcanic rocks.
lebas_etal_1986LeBas et al., 1986
SiO2 vs. Na2O+K2O for volcanic rocks.
streckeisen_1967Streckeisen, 1967
Q – Af – Pf diagram for classification of felsic and intermediate plutonic rocks.
winchester_floyd_1977aWinchester and Floyd, 1977
Immobile trace elements and SiO2 classification for igneous rocks, especially volcanics.

Rock series discriminants

brown_1982Brown, 1982
Alkali-lime index vs. SiO2, separating “normal” calc-alkaline andesites from more and less mature arc andesites.
floyd_winchester_1975cfloyd_winchester_1975afloyd_winchester_1975aFloyd and Winchester, 1975
Tholeiitic vs. alkaline series in oceanic and continental settings (not arcs).
frost_and_frost_2008afrost_and_frost_2008bfrost_and_frost_2008cfrost_and_frost_2008dFrost and Frost, 2008
Feldspathic igneous rock series (ferroan, magnesian, aluminous, alkaline, etc.).
Hansen et al., 2002
Discriminates between Archean TTG suite and more modern adakites on the one hand, and classical island arcs.
hastie_et_al_2007aHastie et al., 2007
Arc series: tholeiitic, calc-alkaline, and more alkaline series, and basaltic, andesitic, and rhyolitic compositions.
irvine_baragar_1971airvine_baragar_1971birvine_baragar_1971cIrvine and Baragar, 1971
Tholeiitic vs. calc-alkaline series discriminants.
miyashiro_1974Miyashiro, 1974
SiO2 vs. FeOtotal/MgO discriminant for arc tholeiites vs. arc calc-alkaline series.
miyashiro_shido_1975amiyashiro_shido_1975bmiyashiro_shido_1975cmiyashiro_shido_1975dmiyashiro_shido_1975emiyashiro_shido_1975fMiyashiro and Shido, 1975
Discriminants for arc tholeiite vs. arc calc-alkaline series.
Peccerillo and Taylor, 1976
SiO2 vs. K2O calc-alkaline vs. tholeiitic series.

Tectonic setting discriminants, mostly basalts

cabanis_lecolle_1989Cabanis and Lecolle, 1989
Nb – La – Y discriminant for arc, MORB/WP, and intermediate/contaminated rocks.
dilek_furnes_2009Dilek and Furnes, 2009
Ti-Zr discriminant for MORB, IAT, and boninites, modified after earlier workers.
dilek_et_al_2007Dilek et al., 2007
Cr vs Y discriminant for MORB, IAT, and boninites, modified after earlier workers.
hollocher_et_al_2012ahollocher_et_al_2012bHollocher et al., 2012
Discriminants for MORB, ocean island, and a variety of arc-type basalts.
meschede_1986Meschede, 1986
Y – Zr – Nb discriminant for arc, MORB, and within-plate basalts.
mullen_1983Mullen, 1983
P2O5 – MnO – TiO2 discriminant for ocean island, MORB, arc basalts, and boninites.
pearce_1975Pearce, 1975
Ti vs. Cr discriminant for ocean floor basalts vs. low-K arc tholeiites.
pearce_etal_1981Pearce, 1981
Zr – Ti discriminant for MORB, within-plate, and arc lavas. Apparently includes low-Ti lavas which are probably felsic.
pearce_1982apearce_1982bpearce_1982cPearce, 1982
Discriminants for mantle melting array vs. arc basalts.
pearce_1983apearce_1983bPearce, 1983
Th – Ta – Yb discriminant for mantle melting array vs. arc basalts.
pearce_cann_1973apearce_cann_1973bpearce_cann_1973cPearce and Cann, 1973
Immobile element discriminant for arc, within-plate, and ocean floor basalts.
pearce_norry_1979Pearce and Norry, 1979
Zr vs. Zr/Y discriminant for arc, ocean ridge, and within-plate basalts. Boninite field added.
shervias_1982Shervias, 1982
Ti – V discriminant diagram for tholeiitic to alkaline basalts, excluding calc-alkaline arc basalts. Boninite added.
wood_1980Wood, 1980
Ta – Th – Hf discriminant for arc, MORB, and within-plate basalts.

Tectonic setting discriminants, mostly felsic rocks

arth_1979Arth, 1979
Al2O3 vs Yb discriminant for oceanic vs. continental granitoids.
eby_1992aeby_1992beby_1992ceby_1992eEby, 1992
Y/Nb – Rb/Nb and other discriminants for A1 vs. A2-type anorogenic granites.
gorton_schandl_2000agorton_schandl_2000bGorton and Schandl, 2000
Th – Ta – Yb discriminants mostly for felsic to intermediate rocks.
pearce_etal_1984apearce_etal_1984bpearce_etal_1984cpearce_etal_1984dPearce, 1984
Discriminant diagrams for granitic rocks.
whalen_etal_1987_1awhalen_etal_1987_1bwhalen_etal_1987_1cwhalen_etal_1987_1dwhalen_etal_1987_2awhalen_etal_1987_2bWhalen et al., 1987
Discriminates A-type granites from all other granite types.