Zircon U-Pb and geochemical analyses for leucocratic intrusive rocks in pillow lavas in the Danfeng Group,north Qinling Mountains,China

Field observation showed that there are many irregular leucocratic intrusive rocks in pillow lavas in the Danfeng Group in the Xiaowangjian area, north Qinling orogenic belt. Photomicrographs indicated that the protoliths of those altered leucocratic intrusive rocks are dioritic rocks. Geochemical analyses showed that pillow lavas have a range of SiO₂ from 47.35% to 51.20%, low abundance of TiO₂ from 0.97% to 1.72%, and percentages of MgO (MgO^#=41–49). Chondrite-normalized REE patterns of pillow lavas are even, indicative of a weak differentiation between LREE and HREE (La/Yb_N=1.52–0.99). N-MORB-normalized trace element abundances showed that pillow lavas are enriched in incompatible elements (e.g., K, Rb, and Ba). Leucocratic intrusive rocks in pillow lavas have a wide range of SiO₂ from 53.85%–67.20%, low abundances of TiO₂ from 0.51%–1.10%, and MgO (MgO^#=40–51), and higher percentages of Al₂O₃ (13.32%–16.62%) and concentration of Sr (342–539 μg/g), ratios of Na₂O/K₂O (2–7) and Sr/Y (17–28). Chondrite-normalized REE patterns of leucocratic intrusive rocks showed highly differentiation between LREE and HREE (La/Yb_N=12.26–19.41). N-MORB-normalized trace element abundances showed that leucocratic intrusive rocks are enriched in incompatible elements (e.g., K, Rb, and Ba), and significantly depleted in HFSE (e.g., Nb, Ta, Zr and Ti), indicative of a relationship to subduction. Isotopically, leucocratic intrusive rocks have a similar ε _Nd(t) (+7.45–+13.14) to that of MORB (+8.8–+9.7), which indicates that those leucocratic intrusive rocks sourced from depleted mantle most likely. SHRIMP U-Pb analyses for zircon showed that those leucocratic intrusive rocks were formed at 442±7 Ma, yielding an age of subduction in the early Paleozoic in the north Qinling orogenic belt. Supported by the National Natural Science Foundation of China (Grant No. 40472119), the Programme of Excellent Young Scientists of the Ministry of Land and Resourcesof China, the Ministry of Science and Technology of China (Grant Nos. 2002CB412608 and 2006BABOlAll), and the China Geological Survey (Grant Nos. 1212010633601 and 1212010611807)