江西庐山地区中生代飞来峰构造的厘定:同位素年代学和地质构造证据

Identification of the Mesozoic Klippe in Lushan Area,Jiangxi Province:Isotopic Chronological and Geological Evidence

通过对庐山地区前寒武纪地层系统的同位素年代学测定,建立庐山地区前寒武纪地层年代学框架,结合详细野外地质构造研究和显微构造分析,本文厘定了庐山地区中生代大型飞来峰构造。该飞来峰外来系统为汉阳峰一带出露的庐山垄群火山岩系,2个火山岩样品的锆石年龄分别为840±4 Ma和828±4 Ma,大致限定了该火山岩系的形成年龄;原地系统为汉阳峰山下广泛分布的星子群和板溪群,6个变沉积岩样品的碎屑锆石年代学数据指示原地系统地层的最大沉积年龄小于810~790 Ma,显示飞来峰外来系统和原地系统的层位关系已经发生倒置。庐山飞来峰下伏构造推覆面在其东南侧为韧性剪切带,发育糜棱岩和构造片岩,其中的矿物拉伸线理和长石碎斑系等运动学标志显示其逆冲推覆方向为由SW往NE。该飞来峰现今的长约12~15 km,宽约3~6 km,面积约为50 km^(2),最小推覆距离为15 km。根据推覆构造韧性剪切带内单矿物年代学推测其推覆时间在140 Ma左右,这既为华南地区燕山运动B幕时间提供了关键证据,也佐证华北、华南地区燕山运动B幕的一致性。庐山中生代飞来峰构造的厘定对中国东部燕山运动性质和板内造山形成机制具有重要科学意义。

Based on isotopic chronological dating of the Precambrian stratigraphic system and detailed field geological investigation and microstructural analysis,a Mesozoic klippe structure in the Lushan area was recognized.The allochthon consists mainly of the Lushanlong Group volcanic rocks that distribute from the top to hillside of the Hanyangfeng Mountain.Two volcanic samples were dated at 840±4 Ma and 828±4 Ma,respectively,which roughly limits the formation age of the Lushanlong Group.The autochthone consists of the Xingzi Group and the Banxi Group at the foot.The detrital zircon chronological results of six metasedimentary samples indicate that the maximum sedimentary age is less than 810 Ma to 790 Ma.The Lushan klippe is obviously different from the underlying beds in lithological composition and structural feature,showing as stratigraphic inversion.The gliding surface of the Lushan Klippe has ductile shear zone in the southeast where mylonite and tectonic schist are developed.The residual klippe is about 12 km to 15 km long and 3 km to 6 km wide,with an area of about 50 km^(2) and a minimum thrust distance of 15 km,which has the thrust direction of SW to NE.Geochronological dating of the ductile shear zone constrains the thrust moving time at ca.140 Ma,which provides key evidence for Phase B of the Yanshanian movement in South China,as well as support for the consistency of the Yanshanian movement in the North China-South China.This study not only provides detailed geochronological data for the strata in the Lushan area necessary to establish the regional framework of Precambrian stratigraphic chronology,but also provides valuable information for the study of the Yanshanian movement in the eastern China and the mechanism of intraplate orogenesis.