Yan Quanren

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/YbN=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/YbN=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 εNd(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.

Discovery of late Paleozoic fossils in tectonic blocks of the Dabao Formation in the South Qinling Orogenic Belt and their significance

The Dabao Formation in the South Qinling Orogenic Belt was previously regarded as Ordovician in age and consists of clastic matrix and blocks of siltstone, limestone, chert, and volcanic rocks. However, some Middle Devonian corals, conodonts, and other fossil fragments within the limestone blocks were discovered in recent field investigations, indicating that the Dabao Formation was formed during late Paleozoic. Combined with other regional geological data, the Dabao Formation in the Southern Qinling Orogenic Belt is considered to be a late Paleozoic or early Mesozoic accretionary complex.