小秦岭-熊耳山地区岩墙锆石SHRIMP年代学研究--秦岭造山带岩石圈拆沉的证据

Research on SHRIMP U-Pb chronology in Xiaoqinling-Xionger' shan area: the evidence of delamination of lithosphere in Qinling orogenic belt.

对小秦岭-熊耳山地区岩墙锆石开展了 SHRIMPⅡU-Pb 法年代学研究显示,岩墙岩浆源区的锆石主要表现为华北南缘陆壳基底中的继承性锆石特征。根据地质关系确定的侵位于中生代花岗岩中的岩墙,其锆石年龄定年结果主要表现为1843±10Ma 和768±15Ma 左右的年龄,多数锆石表现为该区域熊耳群火山活动事件1850Ma 左右的年龄,只有很少的锆石颗粒记录了中生代时期128Ma 左右的侵位年龄。总体看来,岩墙中锆石较全面地记录了区域构造演化的历史。锆石测年结果反映出华北陆块阜平运动(2500～2400Ma)、华北南缘熊耳群火山喷发事件(1850Ma)、震旦纪构造活动(850～700Ma)、扬子陆块俯冲拼贴于华北南缘的造山事件(约200Ma)和燕山期(约130Ma)的岩墙岩浆侵位活动。岩墙的 Sr、Nd、Pb 同位素示踪揭示了岩墙岩浆源区与扬子陆块的亲缘性,从而证明新元古代-晚古生代时期扬子陆块向华北陆块南缘俯冲,以及后来秦岭造山带在造山期后华北南缘下地壳基底拆沉的地球动力学过程。岩墙中锆石测年结果多显示为老的继承性锆石的原因可能是中生代时期生长的锆石颗粒较小,而继承性锆石颗粒较大,锆石单矿物分选过程中主要挑选出继承性锆石的缘故。

Chronology research by SHRIMP Ⅱ U-Pb dating on zircon from dikes of Xiaoqinling-Xiong＇ ershan area show that the zircons in dike magmas source mainly are inherited ones, which manifest chronologic characteristics of Southern edge of North China continental basement. The two dikes which have definite geological relation that invaded into granite of Mesozoic epoch may give the dating results of 1843±10Ma and 768±15Ma, and most zircon show the time of 1850Ma for a very large scale volcanic activity events of Xiong＇er group in the region, only few of zircon show the reasonable time of 128Ma in Mesozoic epoch. To view the chronologic data as a whole, the zircons in dikes comprehensively record the regional history of tectonic evolution. The results of zircon dating reflected the tectonic and magmas events in North China craton as Fuping movement （2500 -2400Ma） , volcanic eruption activity of Xionger＇ group in Southern edge of North China craton （1850Ma） , Paleo-Sinian tectonic movement （850 -700Ma） and succession events of orogen process （N200Ma） and dike magmas invading events（ - 130Ma） with Yangtze plate subducted and matched to Southern edge of North China craton. Research on tracing of Sr, Nd, Pb of dikes reveal that the magmas source of dikes have much affinity to Yangtze Plate, the fact testified the dynamic process of Yangtze plate subduction to Southern edge of North China craton from Neo-Priterozoic to Pilo-Paleozoic time and delamination of continental low Crust of Qinling orogenic belt in Southern edge of North China in post of orogen in the late. The reason for most of dating results of zircon in dikes show the characteristics of inherited one possibly is that the zircon grain grow small in Mesozoic epoch, but the inherit zircons grain are bigger than the zircon in Mesozoic, so that most of inherit zircon were selected during mineral sorting but Mesozoic zircon were difficult to be selected out.