SUMMARY

Paleozoic volcanic rocks in the Sông Đà Structure: Mantle origin and dynamics

 

Nguyễn Hoàng, Nguyễn Đắc Lư, Nguyễn Văn Can

 

Paleozoic volcanic activity in association with the formation of the Sông Đà Rift Zone produced mainly alkaline and subalkaline mafic rocks and a minor amount of rhyolite to trachyte and transitional types. Sr, Nd and Pb isotopic composition of representative samples were analyzed. Initial isotopic values were obtained based on reported age of 283 Ma. ranging from 0.7036 – 0.7090, 0.5119 – 0.5124, và 18.32 – 23.50 for ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb, respectively. Except for one sample that has low Sr (0.7036) and high Nd (0.5124) all the initial Sr and Nd isotopic ratios are much higher than the Chondrite Uniform Reservoir (CHUR) calculated for 283 Ma. (⁸⁷Sr/⁸⁶Sr = 0.7041 and ¹⁴³Nd/¹⁴⁴Nd = 0.5123). Samples of high isotopic ratios accompanied by negative anomalies at high field strength elements such as Nb, Ta, and Zr were interpreted to be derived from mantle source(s) contaminated by crustal material introduced into the mantle via subduction zone. We suggest two dynamic models that may be driving force for the volcanic activity, (1) intraplate lithospheric extension, resulted from long-term plate compression; (2) back-arc spreading at a plate margin. Both of the dynamics may be related to the formation and closure of the Paleo-Tethys started about 400 Ma. ago.

 

Ngày nhận bài: 1-7-2003

Người biên tập: Trần Văn Trị