南秦岭竹山地区粗面质火山岩地球化学特征、LA-ICP-MS锆石U-Pb年龄及其大地构造意义

Geochemical characteristics and LA-ICP-MS zircon U-Pb age of the trachytic volcanic rocks in Zhushan area of Southern Qinling Mountains and their significance

南秦岭竹山地区广泛分布的北西—南东向展布的基性岩-粗面岩带,主要赋存于志留纪地层中。竹山地区粗面质火山岩地球化学研究表明,该区粗面质火山岩全碱Na_2O+K_2O含量较高,为碱性岩系列;大离子亲石元素(Ba、Th、U)和高场强元素(Nb、Ta等)相对富集,Sr、Ti、Yb等元素相对亏损;稀土元素总量(∑REE)较高,明显富集轻稀土元素,亏损重稀土元素;结合微量元素构造环境判别分析认为,该区粗面质火山岩形成于大陆裂谷环境。采用LA-ICP-MS方法测得该区粗面质火山碎屑岩中锆石的U-Pb年龄为430.6±2.7Ma,该年龄应代表粗面质火山岩主体的结晶年龄。研究结果表明,南秦岭竹山地区在早志留世发生了强烈的裂解活动,并形成一富硅质和炭质岩组合的深水盆地,该洋盆可能为南秦岭勉略洋向东的延伸。南秦岭竹山地区粗面质火山岩岩石化学及锆石U-Pb同位素定年研究,为了解南秦岭早古生代构造演化提供了重要依据。

Basic rock-trachyte belts, trending in NW-SE direction,are widely distributed in Zhushan area of Southern Qinling Mountains, intruding in Silurian strata. Previous researchers conducted detailed studies focused on basic rocks. Compared with the ba-sic rocks, the research on trachytic volcanic rocks seems very insufficient, and hence accurate data forming chronologic data are espe-cially lacking. The dating of trachytic volcanic rocks from Zhushan area in Southern Qinling Mountains shows that the trachytic vol-canic rocks are alkaline series rocks with relatively high Na2O＋K2O. Lithophile elements（Ba, Th, U）and high field strength elements （Nb, Ta）are relatively abundant, but the elements such as Sr, Ti and Yb are relatively poor. Rare earth elements（ΣREE, which are rich in light REE and deficient in heavy REE）are relatively high. Combined with the trace element characteristics, it is shown that the trachytic volcanic rocks were formed in a continental rifting environment. The crystallization of trachytic volcanic rocks took place at 430.6±2.7Ma, as shown by the LA-ICP-MS zircon U-Pb dating of trachytic pyroclastic rocks in Zhushan area. The studies show that a strong cleavage activity occurred in the period of early Silurian and formed a deep water basin with rich siliceous and car？ bonaceous rock combination in the Zhushan area of Southern Qinling Mountains. Combined with the discovery of ocean island ba-salts as well as island arc and oceanic basalts, the authors hold that this region might have developed a small finite ocean basin after Si-lurian, and the ocean basin was probably the eastward extension of the Mianlue Ocean in Southern Qinling Mountains. The study of the geochemical characteristics and zircon U-Pb age of the trachytic volcanic rocks provides an important basis for early Paleozoic tectonic evolution in Zhushan area of Southern Qinling Mountains.