A geochemical study of element and Sr-Nd isotopes for eclogite and gneiss from CCSD core 734 to 933m.

Two continuous core segments composing of eclogite and gneiss from the main hole of CCSD project, at depths of 734. 21 to 737. 16m (02CCSD-Ⅰ) and 929. 67 to 932. 86m (02CCSD-Ⅱ) , respectively, are systematically analyzed for major and trace elements as well as Sr-Nd isotopes. The results show the following characteristic features: (1) the eclogites have variable chemical compositions with SiO2 concentrations of 46. 8 to 59. 8% ,and exhibit flat to LREE enrichment patterns, and enrichment or depletion in LILE as well as Nb and Ta negative anomalies in the primitive mantle-normalized spidergram; (2) the gneisses from both core segments show obviously different major and minor element compositions. The gneiss from the first core segment has relatively low SiO2 concentrations of 61. 3 to 66. 2% and high HREE contents, whereas the gneiss from the second core segment exhibits significantly high SiO2 concentrations of 73. 8 to 74. 1% and low HREE contents. Nevertheless, both of them are characterized by LREE and LILE enrichment as well as Eu and HFSE (Nb,Ta,Ti) negative anomalies; (3) both eclogite and gneiss from the first core segment display similar but relatively high eNd(750Ma) values of -3.6 to +0.5, but the gneiss from the second core segment has a significantly low eNd(750Ma) value of -8.7. Some of the samples have relatively high87Sr/86Sr ratios but very low Rb/Sr ratios, indicating preferential Rb loss relative to Sr. Some of the eclogites have very low contents of mobile elements ( Rb, Ba and K) , indicating the significant loss of these elements by dehydration metamorphism during continental subduction. In contrast, immobile elements such as Zr、Hf、Nb、Ta、 Ti 、Y、 REE and Ti show no evidence for significant loss, thus can be used to trace the geochemical nature of protoliths. Element and Nd isotope data indicate that the protolith of eclogites has an affinity to within-plate basalt; the protolith of gneiss from the first core segment is probably intermediate rock formed by crustal assimilation of basaltic magma during its emplacement, whereas the pritolith of gneiss from the second core segment is grantic rcok that formed by partial melting of old crustal materials. Therefore, the protoliths of both eclogites and gneisses are the products of Neoproterozoic bimodal magmatism that occurred in rifting tectonic zones along the northern margin of the South China Block, but experienced meteoric-hydrothermal alteration at high temperatures.