serie NOVA TERRA nº 49

59 crust of Post Archean age, based on the similarity of the REE patterns to PAAS. The La N /Yb N ratio shows an average value of 5.26 (ranging between 2.59 and 8.07), which is lower than that of PAAS (La N / Yb N =9.08) due to a lower La concentration (average La=24.06 ppm, Yb=3.12 ppm). The abundance of La agrees with that reported by Bathia and Crook (1986) for igneous terrigenous sediments generated within a volcanic arc built on thinned continental crust, as discussed below. 4.2. Tectonic setting The geochemistry of major elements has been widely used to establish the tectonic setting of detrital sedimentary rocks ( Bathia, 1983; Roser and Korsch 1985, 1986, 1988; Bathia and Crook, 1986; Hegde and Chavadi, 2009 ). However, diagrams based on the abundance of elements, such as Na or K, must be treated with caution, because of their high mobility during depositional processes. Certain trace elements, such as REE, Cr, Co, Th, Sc, Y, La and Zr, are considered relatively immobile and consequently provide better discrimination of possible tectonic settings ( Taylor and McLennan, 1985 ). Various tectonic discrimination diagrams developed by Bathia and Crook (1986) are shown in Fig. 6 . These diagrams allow clear differentiation among the four tectonic settings considered to be the most common sites of greywacke deposition: (A) oceanic island arc, (B) continental island arc, (C) active continental margin, and (D) passive margin. In the Ti/Zr – La/Sc diagram ( Fig. 6 a), all the samples plot within the continental island arc fi eld. The La/Y – Sc/Cr diagram ( Fig. 6 b) shows greater scatter due to the variability of the La/Y ratio, the values of which range between 0.55 and 1.41. Nevertheless, all the samples plot in fi elds related to convergent plate tectonic settings. In the ternary diagrams, La – Th – Sc, Th – Co – Zr and Th – Sc – Zr ( Fig. 6 c, d and e, respectively), the samples are tightly grouped in the continental island arc fi elds, with only two samples falling within the fi eld for oceanic island arcs in one of the diagrams. This re fl ects the low La contents of the two samples relative to the others. According to Bathia and Crook (1986) , the most signi fi cant chemical signatures for characterizing greywackes deposited in a continental island arc setting are: La (25 ppm), Th (11 ppm), La/Sc (1.8), Th/Sc (0.85), Ti/Zr (20) and La/Th (2.3). These average values closely match those characteristic of the greywackes of the upper levels of the Órdenes Complex: La (24 ppm), Th (7.86 ppm), La/Sc (1.59), Th/Sc (0.51), Ti/Zr (25) and La/Th (3.1). Hence, accord- ing to the concentrations of trace elements with the highest discriminative power, the greywackes were deposited in a sedimen- tary basin related to a convergent dynamic regime, and in a tectonic setting designated as a continental island arc by Bathia and Crook (1986) . Finally, Fig. 7 a and b shows PAAS-normalized plots of the most signi fi cant elements for the discrimination of tectonic setting. The fi gures are plotted according to the criteria of Thompson (1982) . The patterns de fi ned by the metagreywackes are very similar to those typical of continental island arcs or active margins ( Winchester and Max, 1989 ). The plots are characterized by depletion in most of the large ion lithophile elements (LILE: Cs, Rb, Th, U, K 2 O, La, Ce, and P 2 O 5 ), which deviate slightly from one, whereas the high fi eld strength elements (HSFE: Zr, Hf, HREE, Sm, TiO 2 and Sc) Fig. 6. Trace elements tectonic setting discrimination diagrams for the metagreywackes. A — Oceanic island arc; B — continental island arc; C — active continental margins; D — passive margins. Diagrams are after Bathia and Crook (1986) . 346 J.M. Fuenlabrada et al. / Gondwana Research 17 (2010) 338 – 351