Diagrama Ti - V: una nueva propuesta de discriminación para magmas básicos en cinco ambientes tectónicos

Resumen

We present new discrimination diagrams using Ti and V for basic magmas (hy-normative basalts, ne-normative basalts, and ne-normative basaltic trachyandesites), in which our aim was to distinguish between five different tectonic settings: continental arc (AC; arco continental); island arc (AI; arco de isla); rift (Rt), ocean island (IO; isla oceánica); y mid-ocean ridge (MOR). The database, on which this new proposal is based, was established from analytical data presented in 121 articles from international journals. Shervais (1982) proposed this kind of diagram using discrimination lines with equal values of the ratio (Ti/1,000)/V, but it is shown here to be inadequate for tectonomagmatic discrimination of basaltic magmas. In this new Ti – V diagram, we have proposed the discrimination lines on the basis of the tendencies and the fields occupied by the samples representing the different tectonic settings. We also present coordinates of these lines for an easy  reproduction of the new discrimination diagrams by potential users. As an example, one can observe that 99% of the samples from arcs fall in the field of AC and AI, which implies a great success in discriminating AC and AI from Rt and IO. On the other hand, there is a significant overlap between AC, AI and MOR fields. These diagrams have been validated using data for samples from several different sites with known tectonic settings. Finally, these new diagrams have been used to infer the tectonic settings of a series of rocks from tectonically complex or undefined areas. It is important to note that the use of these new diagrams requires that the proposed methodology be strictly followed, especially in calculating Fe₂O₃/FeO according to rock types before CIPW norm computations. A Fortran program for DOS or a VisualBasic program (SINCLAS) for Windows to perform this adjustment easily and correctly is available from one of the authors (S. P. Verma: spv@cie.unam.mx).