Journal
CHINESE SCIENCE BULLETIN
Volume 57, Issue 18, Pages 2289-2301Publisher
SCIENCE PRESS
DOI: 10.1007/s11434-012-5069-3
Keywords
high-silica adakite; Early Silurian; eclogite; melting during exhumation; west segment of the North Qilian Mountain
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Funding
- National Basic Research Program of China [2009CB825007]
- National Natural Science Foundation of China [40825007, 40821002]
- Chinese Geological Survey Projects [1212011121258]
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A trondhjemitic body occurs in the Aoyougou area, the western part of the North Qilian orogen. It is geochemically characterized by high SiO2, Na2O (high Na/K), elevated Sr/Y and (La/Yb)N, positive Sr anomaly, relatively enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted Nb, Ta, Ti, resembling the high-silica adakite. Zircon U-Pb SHRIMP dating yields a weighted mean age of 438 +/- 3 Ma. This age is significantly younger than eclogitization ages of 460-490 Ma in the North Qilian orogen, suggesting that formation of the adakite postdates the subduction of oceanic crust in association with closure of the ancient Qilian Ocean. Whole-rock Sr and Nd isotopic analyses give initial ratios of I (Sr)=0.7044-0.7047 and E > (Nd)(t)=3.0-4.1, indicating that they are derived from partial melting of the juvenile oceanic crust. In view of the tectonic evolution of the North Qilian orogen, the high-silica adakite was probably derived from decompression melting of the exhumed eclogite at the depth of similar to 60 km.
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