北大巴山镇坪地区辉绿岩锆石SHRIMP U-Pb 定年和岩石地球化学特征

摘要

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 ZiyangLangao 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.%北大巴山地区广泛分布的北西-南东向基性岩脉(墙)群,主要侵位于下古生界.前人对其中、北部紫阳-岚皋地区基性岩年代学和地球化学已有研究,但其南部镇坪地区基性岩却鲜有高精度年龄和地球化学数据报道.本文对镇坪地区辉绿岩锆石SHRIMP U-Pb定年结果为(439±6)Ma(n=9,MSWD=1.7),与紫阳-岚皋地区基性岩及火山岩形成时代一致.岩石富K2O、TiO2,具双峰式岩石组合特征.稀土元素总含量高,轻稀土元素相对富集,轻重稀土分异明显,配分模式为向右陡倾型;大离子亲石元素(Rb、Ba、Th、U)和高场强元素(Nb、Ta等)相对富集,Y、Yb、Sr相对亏损;具大陆裂谷玄武岩的地球化学特征.微量元素Zr/Y-Zr和2Nb-Zr/4-Y图解亦指示其具板内裂谷玄武岩特征.结合前人的研究结果表明,北大巴山地区在早志留世早期处于张裂状态,该区基性岩和火山岩为岩石圈处于拉张状态下大陆裂谷作用产物.