SHRIMP zircon U-Pb dating and lithogeochemical characteristics of diabase from Zhenping area in North Daba Mountain

Abstract：Mafic dike swarms, trending in NW-SE direction, are widely distributed in North Daba Mountain, intruding the Early Paleozoic strata. Previous studies focused on geochronology and geochemistry of Ziyang-Langao area; nevertheless, very insufficient research work was devoted to Zhenping area. The dating of SHRIMP zircon U-Pb for diabase samples from Zhenping area in North Daba Mountain shows the result that 12 points yield a group of weighted mean 206Pb/238U ages at (439±6)Ma(MSWD=1.7), consistent with the basic dike and volcanic rocks in the middle-northern area of North Daba Mountain, i.e., Ziyang-Langao area. Geochemical data show that basic dikes have high K2O and TiO2, with bimodal characteristics of rocks. They are enriched in large ion lithophile elements (LILE, e.g., Rb, Ba, Th, U) and light rare earth elements (LREE; ((La/Yb)N=11.18～15.67), and depleted in Sr, Y, Yb and heavy rare earths (HREE), suggesting geochemical characteristics of average mass fraction of continental rift basalt. The petrochemical diagrams of Zr/Y-Zr and Nb-Zr-Y also indicate that these rocks are continental rift basalt. Combined with previous studies, the authors hold that North Daba Mountain was a rift during the middle-late stage of Early Silurian, and the basic dikes and volcanic rocks were tensile product under the condition of continental rift.