Tectonic settings and nature of the protoliths

As shown above, the protoliths of the studied eclogites are mainly basalts. It has been suggested that Zr and Nb are the most stable and near-immobile elements during eclogite-facies metamorphism (Becker et al., 2000); in the discrimination diagram for basalts based on Ti-Zr variations (Fig. 5.15, after Pearce, 1982), most eclogites fall into the volcanic-arc field, and only a few samples plot in the within-plate field; the fact that some samples from Bixiling

Fig. 5.14. Primitive mantle –normalized REE patterns in garnets from the Yanzihe granulite. (a) garnet with homogeneous major element compositions; (b) garnet with micro veins composed of Cl-rich amphiboles; pt1 and pt2 are close to the vein, whereas pt10 and pt11 are way from the vein; (c) garnet inclusion in quartz.

exceed the entire volcanic field supports their cumulate origin. The plot of Zr/Nb vs. Nb (Fig.

5.16) shows that all the studied eclogite samples have lower Zr/Nb ratios compared to MORB independent of Nb concentrations, indicative of island-arc basalts (McCulloch and Gamble, 1991). Figure 5.17 shows that the studied eclogite samples have highly variable Ti/V ratios ranging from 5 to 450; this range encompass all fields of basaltic rocks; however, most samples plot in the island arc tholetiite field, and clearly do not support an oceanic origin for the precursor rocks. Furthermore, low Cr and Ni in eclogites (except those in Bixiling) also contribute evidence for an island arc origin (Taylor, 1977).

The major and trace elements, and REE characteristics presented above indicate that the protoliths of the studied rocks from Dabie Shan have complex origins, and that these rocks derive from heterogeneous sources which are further modified by fractional crystallization. As mentioned above, eclogites from Bixiling and Lidu have negative Nb anomalies, whereas eclogites from Shuanghe, Shima and Raobazhai have positive Nb anomalies (Fig. 5.6). A negative Nb anomaly is the most characteristic feature of subduction zone volcanic rocks or typical continental crust. A recent study of Huang et al. (2000) has demonstrated that basaltic rocks with positive Nb anomalies were derived from within -plate processes with high Nb/Ta, whereas those with negative Nb anomalies were derived from a mantle which has been modified by subduction processes. Eu anomalies are small for most investigated samples.

Eclogites from Bixiling and Lidu show slight positive Eu anomalies, whereas those from Shima and Shuanghe display slight negative Eu anomalies. The only exception is the jadeite quartzite from Shuanghe, in which a distinct negative Eu anomaly can be observed (Fig. 5.6c). Mantle -derived rocks usually have no Eu anomalies, which means that primary mantle additions into the crust should induce no changes. In contrast, post-Archean sediments are well established as being characterized by significant negative Eu anomalies.

Based on the present data and previous studies from Dabie Shan area (Zhang Q. et al., 1995; Chavagnac and Jahn, 1996; You et al., 1996; Zhang R. et al., 1996; Liou et al., 1998;

Jahn et al., 1998, 1999b; Xiao et al., 2001), the geochemical characteristics of the studied samples can be summarized as follows:

(1) Eclogites and associated garnet peridotite from Bixiling and the eclogite from Raobazhai have geochemical characteristics of a mantle origin. The low SiO2 and high Ti and V contents in some samples from Bixiling are evidence of a cumulate origin. The negative Nb and slight positive Eu anomalies in the Bixiling eclogites are indicative of crustal contamination in the pre-metamorphic magmatic evolution (see also Chavagnac and Jahn, 1996), whereas eclogite in Raobazhai was less changed when induced into the crust.

(2) Low Zr/Nb ratios, Cr and Ni contents, REE patterns and tectonic environment discriminating diagrams suggest an island arc volcanic origin for most eclogites in the Dabie Shan area. Thus, the protoliths of the eclogites in Shima, Shuanghe and Lidu are more likely island arc volcanic rocks. Relatively young Tethys oceanic rocks have not been clearly identified in the studied samples.

(3) The jadeite quartzite in Shuanghe has distinctly low K, Rb, Ba and Sr contents that are very comparable with albitized siltstone (Kalsbeek, 1992). Its REE pattern is similar to that of upper continental sediments (Taylor and McLennan, 1985). The very distinctive negative Eu anomaly is strong evidence for its sedimentary origin.

(4) Granulite in Yanzihe has trace element patterns similar to upper continental crust (Weaver and Tarney, 1984) (Fig. 5.6f). However, the much lower trace element and REE concentrations, and alkalis contents indicate its protolith cannot be of upper crustal origin. It is possible that the granulite was partialy melted during metamorphism. Its negative initial εNd

value of – 6 (Table 5.1) is also indicative for such a partial melt process.

1000 10000 100000

10 100 1000

Zr (ppm)

Ti (ppm) Within-plate

Vocanic-arc

Fig. 5.15. Discrimination diagram for investigated samples based on Ti-Zr variations of whole rock (after Pearce, 1982). Symbols as in Fig. 5.2.

1 10 100 1000

0 10 20 30

Nb (ppm)

Zr/Nb

MORB Zr/Nb = 40

0 100 200 300 400 500 600 700

0 100 200 300 400

Ti(ppm)/100

V (ppm)

Ti/V=10 20 50

100 IAT

MORB +BAB

CFB

OIB+AB Fig. 5.16. Zr/Nb-Nb diagram for investigated samples (after McCulloch and Gamble, 1991). Symbols as in Fig.5.2.

Fig. 5.17. Ti-V discrimination diagram shows that the investigated samples are widely scattered, exceeding the entire field of basaltic rocks (after Shervais, 1982). IAT, island arc tholiite; MORB, mid-ocean ridge basalt; BABB, back-arc basin basalt, CFB, continental flood basalt; OIB, ocean island basalt; AB, alkaline basalt. Symbols as in Fig. 5.2.