Yu Chen
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M. Sc. Thesis
Geochemical Evolution and Tectonic Significance of Boninites From the Dacha Daban Ophiolite, North Qilian Mountains, China.
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The Dacha Daban ophiolite, located in the middle part of the North Qilian Mountains, northwest China, is composed of a basal peridotite unit, a middle sequence of gabbro-diabase and an upper assemblage of pillow lavas. Two geochemical suites are recognized in the extrusive sequence. A boninitic suite is characterized by relatively high SiO2 (49-56 wt.%), MgO (4.5-15.6 wt.%), Cr and Ni and low TiO2 (0.23-0.48 wt.%), HREE and HFSE. The V/Zr ratio for these rocks averages 6.6 and the Ti/V ratio averages 63, whereas Mg#s [Mg/(Mg+Fe)] vary widely from 0.64-0.77. A tholeiitic suite has higher TiO2 (0.92-1.90 wt.%) and HREE and lower SiO2 (<51%) than the boninitic rocks. Both suites have similarly-shaped REE patterns with LREE depletion but the boninites have chondrite-normalized values less than seven whereas the tholeiites have values greater than 10.
Nd(t) values for the boninitic rocks range from +1.8 to +6.3 whereas those for the tholeiitic rocks range from +6.4 to +8.9 (calculated from t=500 m.y.). These values indicate that both suites were derived from a depleted mantle source. The variation in values of the boninitic rocks may be explained by the addition of a subduction component. The boninitic lavas are believed to have been derived from a strongly depleted mantle source possibly by 20-30% partial melting under hydrous conditions. Although enriched in LILE relative to MORB, these lavas show only slight enrichment in LREE suggesting that the subduction component was mainly MORB-derived fluid.
Based on the available geological and geochemical data, I suggest that the Dacha Daban ophiolite was formed in a supra-subduction zone environment.
This study is part of a larger project: "The study of the characteristics of the North Qilian ophiolites, evolution of orogenic belts, and deep geologic process" by Professor Qi Zhang of the Institute of Geology, Chinese Academy of Science, Beijing, China. The successful complete of this project has contributed to the study of the North Qilian belt, helped clarify the tectonic environment of the area and elucidated direction of ancient subduction in Northwest China.
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Supervisor: Paul Robinson