AN EVALUATION OF THE RB VS (Y+NB) DISCRIMINATION DIAGRAM TO INFER TECTONIC SETTING OF SILICIC IGNEOUS ROCKS

The most commonly used tectonic discrimination diagrams for granites w ere introduced by Pearce et al. [Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic in terpretation of granitic rocks. J. Petrol. 25, 956-983.]. Since then, many studies have shown that some granites defy classification or thei r geochemical assignment does not fit with the geodynamic environment in which they are thought to have formed. In this paper we evaluate th e performance of the Pearce et al. tectonic discrimination method, spe cifically, the most widely-used Rb-(Y + Nb) diagram, using a new data base of over 250 occurrences worldwide, the tectonic settings of which are fairly well known. We conclude that a correlation of geochemistry and tectonic position exists, but that ambiguities and misclassificat ions arise from one or both of the following factors. First, complex o r polyphase orogeny can mix source rocks of different tectonic provena nce. This is common in continental arcs and collisional settings, whic h can be closely associated in space and time with extensional regimes . Second, differentiation can produce compositional trends which cross field boundaries, especially the VAG to WPG boundary. One can minimiz e this problem by using less felsic, noncumulate members of cogenetic series. We demonstrate the inherent weaknesses of trace element tecton ic discrimination diagrams. Such diagrams are of Little use if applied alone, but they can be valuable in combination with other methods suc h as dating and geologic assessment. (C) 1997 Elsevier Science B.V.