Tectonic discrimination diagrams -

Fig. 3. Tectonic discrimination diagram of Rb versus Y + Nb from Pearce et al. (1984) and modified by Christiansen and Keith (1996) for syn-collision granites (syn-COLG), volcanic arc granites (VAG), within plate granites (WPG), and ocean ridge granites (ORG). The diagram suggests the granites and pegmatite were contaminated by a crustal component.

Discrimination diagrams have also been applied to the environment of deposition of sedimentary rocks. Potter et al. (1965) showed that a discriminant function based upon the trace elements B and V could be used to distinguish between freshwater and marine argillaceous sediments. More recently tectonic discrimination diagrams have also been extended to sedimentary rocks. Bhatia (1983) and Roser and Korsch (1988) have produced discriminant function diagrams which allow the identification of the provenance of sandstones according to their plate tectonic setting. 

Do tectonic discrimination diagrams still have a function ... 

Pearce, J., Harris, N.B.W., Tindle, A.D. 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology, 25, 956-983. 

The outputs are probabilities for each tectonically defined magma type including confidence values for data that are out of the typical range. The systan showed to overcome many of the ambiguities usually related to geochemical discrimination diagrams. 

There are a large number of discrimination diagrams applicable to basalts and basaldc andesites which use trace elements, major and minor elements and the mineral clinopyroxene. These are considered in turn. Table 5.1 classifies the different types of basalt according to tectonic setting and shows which diagrams might be useful in their identification. 

The quandary fiicing the user of discrimination diagrams is that tectonic environments do often carry a geochemical fingerprint but some of the fingerprints are not unique. Two possible solutions may be found and they are outlined here in... 

Pearce J.A. and Gale G.H., 1977, Identification of ore-deposition environment from trace element geochemistry of associated igneous host rocks. Geol Soc. Spec. Publ, 7, 14-24. Pearce JA., Harris N.B.W. and Tindle A.G., 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks, f. Petrol, 25, 956-983. 

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