Abstract
Abstract Discrimination diagrams are being widely used for the past four decades to infer the tectonomagmatic affiliation of old rocks. There are several major- and trace-element composition based bivariate, ternary and multi-element discriminant function diagrams in the literature. Recent evaluation of the performance of these diagrams indicates that four sets of recent discriminant function based diagrams, in which the tectonic boundaries are determined from probability estimates for four tectonic settings of island arc, continental rift, ocean-island and mid-ocean ridge, have shown very high success rates. In this work, we have created an extensive geochemical database for on-land and off-shore basic rocks of northwest Mexico and applied these four sets of recent and highly successful tectonomagmatic discriminant function based diagrams to infer their tectonomagmatic origin. Each of these four sets of diagrams (two for major- and two for trace-elements) contained five diagrams. All the four sets of diagrams confirmed a continental rift setting for on-land basic rocks of 13 Ma age, major-element based diagrams also indicated a rift setting. However, one set of trace-element based discrimination diagrams indicated an arc to rift transition and the other set of trace- element based discrimination diagrams for the only sample from the Comondu arc, which has the required complete dataset, indicated an arc setting. All the four sets of discrimination diagrams indicated MORB setting (success rates 90%-98% and 100% for major-element and trace-element based diagrams, respectively) for off-shore rocks (most of the samples were obtained from DSDP drilled sites in the Gulf of California and some dredged rock samples from the Gulf of California) with very high success rates. All the inferred tectonic settings from these four sets of new discrimination diagrams, in general, are in conformity with those reported in the literature based on other methods. The results show that these discrimination diagrams may successfully discriminate the original tectonic setting of comparatively younger and older on-shore rocks as well as sea-water altered deep-sea rocks and dredged material.
Highlights
IntroductionTectonomagmatic discrimination diagrams are being widely used to infer the original tectonic setting of volcanic rocks (e.g., Pearce et al, 1977; Rollinson, 1993; Verma, 2010)
Tectonomagmatic discrimination diagrams are being widely used to infer the original tectonic setting of volcanic rocks (e.g., Pearce et al, 1977; Rollinson, 1993; Verma, 2010). These diagrams are developed on two major basic assumptions: (1) concentrations of the characteristic chemical elements being used in the discriminate diagrams show large differences in the rocks of different tectonic settings; and (2) these characteristic chemical elements of the rocks are relatively immobile from the period of rock formation to the present
In majority of the above mentioned diagrams, the tectonic discrimination boundaries were drawn by eye, i.e., were based on visual appearance of the clustering of data points of known tectonic settings, except in a few recent ones (Agrawal et al, 2004; Verma et al, 2006; Agrawal et al, 2008; Verma and Agrawal, 2011), in which the tectonic boundaries are determined based on probability estimates
Summary
Tectonomagmatic discrimination diagrams are being widely used to infer the original tectonic setting of volcanic rocks (e.g., Pearce et al, 1977; Rollinson, 1993; Verma, 2010). There are several major- and trace-element composition based diagrams for geochemical tectonic discrimination in the literature, which may be grouped as follows: (1) bivariate diagrams (Pearce and Gale, 1977; Pearce and Norry, 1979; Shervais, 1982; Pearce, 1982; Vasconcelos-F. et al, 1998, 2001); (2) ternary diagrams (Pearce and Cann, 1973; Pearce et al, 1977; Wood, 1980; Mullen, 1983; Meschede, 1986; Cabanis and Lecolle, 1989); (3) discriminant function based diagrams (Butler and Woronow, 1986; Pearce, 1976; Agrawal et al, 2004); and (4) multi-element discriminant function diagrams based on log-transformed ratios (Verma et al, 2006; Agrawal et al, 2008; Verma and Agrawal, 2011) All these discrimination diagrams have been developed based on a geochemical database of the volcanic rocks from known tectonic environments. In majority of the above mentioned diagrams, the tectonic discrimination boundaries were drawn by eye, i.e., were based on visual appearance of the clustering of data points of known tectonic settings, except in a few recent ones (Agrawal et al, 2004; Verma et al, 2006; Agrawal et al, 2008; Verma and Agrawal, 2011), in which the tectonic boundaries are determined based on probability estimates. Agrawal (1999) and Agrawal and Verma (2007) had already criticized the subjective use of eye for drawing tectonic field boundaries
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