Comparison with Tibetan lavas

Ranges and means of major-elements and selected trace-element compositions of diorites (Mariano & Sial, 1990; Jardim de Sá, 1994; Neves & Vauchez, 1995; Mariano et al., 1996; Neves & Mariano, 1997; Neves et al., 2000; Mariano et al., 2001; Almeida et al., 2002; Campos et al., 2002; Hollanda et al., 2003) are compared to shoshonites from eastern Tibet (Turner et al., 1996) in Table 1. Selected major- and trace-element compositions for the diorites are presented in Table 2. The only noticeable differences in the major-element chemistry are that the shoshonites have slightly higher K2O (Figure 3a) and lower Al2O3 (Figure 3b). In both datasets, MgO ranges mainly from 2 to 5 wt%, but may reach up to 7 wt%. CaO and FeO correlate positively with MgO, respectively, reaching up to 8.0 and 10.6 wt%, in the Borborema Province, and 11 and 12 wt% in the shoshonites (Figure 3c and d). In the Peccerillo & Taylor (1976) diagram, the dioritic samples plot in the shoshonitic field and transitional between the high-K calc-alkalic and shoshonitic fields (Figure 3a). Selected trace element (Figure 4) show superposition of fields for diorites and shoshonites with the main difference due to the Rb enrichment (Figure 4a) of most of the shoshonitic samples.

Figure 3. Plots of K2O versus SiO2

Plots of K2O versus SiO2

Plots of K2O versus SiO2 (A) and Al2O3 (B), CaO (C) and FeO (D) versus MgO for samples of diorites from Borborema Province (crosses) and shoshonites from Tibetan plateau (open diamonds). The shoshonitic, high-K calc-alkalic and calc-alkalic and fields in (a) are from Peccerillo & Taylor (1976). See text and Fig. 1 for sources of data.


Figure 4. Selected trace element

Selected trace element

Selected trace element vs. MgO diagrams for samples of diorites from the Borborema Province (crosses) and shoshonites from Tibetan plateau (open diamonds).


The Borborema diorites and Tibetan shoshonites show comparable LILE and high-field strength elements (HFSE) concentrations, Sr, La and Nb, which are more abundant in the shoshonites (Table 1; Figure 5). The diorites have high light rare earth element (LREE) contents, with La/Yb ratios varying from 16 to 90, and negligible Eu anomalies. This results in fractionated REE patterns (Figure 5a), better marked from La to Dy, and with the heavy REEs showing a much flatter slope. These REE patterns are similar to those of Tibetan shoshonites (Figure 5b) except for their more fractionated patterns characterized by La/Yb ratios varying from 56 to 129.

The diorites are characterized by negative εNd values (-6.9 to -14.9), high initial 87Sr/86Sr ratios (0.7058-07102), and intermediate δ18O whole-rock values (between 7.5 and 8.2 ‰). Unradiogenic Nd and radiogenic Sr isotopic values are also characteristic of Tibetan shoshonites (Table 1).

Figure 5. Selected REE-patterns

Selected REE-patterns

Selected REE-patterns for dioritic rocks from Borborema Province (A) and shoshonites from Tibetan plateau (B). Normalizing factors from Evensen et al. (1978). See text and Fig. 1 for sources of data.