东秦岭商丹地区刘岭群浅变质沉积岩系碎屑锆石U-Pb年龄及其地质意义

传统意义的刘岭群广泛分布于南秦岭北缘地区，由于特殊的大地构造位置，其一直被视为揭示秦岭造山带早古生代南秦岭构造带和北秦岭构造带汇聚过程的重要窗口.丹凤－商南－西坪地区的刘岭群较山阳－柞水地区的刘岭群变质程度深且变形复杂，分别被称为刘岭群浅变质沉积岩系和刘岭群沉积岩系以示区分.山阳－柞水地区的刘岭群沉积岩系据可靠的古生物化石证据被划归为中泥盆统或上泥盆统，而商丹地区的刘岭群浅变质沉积岩系则无可靠的时代证据，被划归为下古生界.通过对商丹地区浅变质沉积岩系中的石榴二云母石英片岩进行LA-ICP-MS锆石U-Pb同位素年龄研究，获得碎屑锆石年龄谱介于2 598~390 Ma，2个主要峰值年龄段分别为~1.0~0.6 Ga和~0.5~0.4 Ga，3个次要峰值年龄段分别为~2.6~2.3 Ga、~1.71~1.60 Ga和~1.3~1.2 Ga.综合本次及前人研究结果表明:刘岭群浅变质沉积岩系和沉积岩系物源具有双源性，南、北秦岭构造带均为其提供重要物源；商丹地区刘岭群浅变质沉积岩系最大沉积时限为~390 Ma，该地层最可能形成于中晚泥盆世；刘岭群沉积岩系和刘岭群浅变质沉积岩系可能为同时代沉积物，后期经历不同构造变形，形成于前陆盆地构造环境. 

Detrital Zircon U-Pb Ages and Geological Significance of Low-Metamorphosed Liuling Sedimentary Assemblage in the Shangdan Area,Eastern Qinling Orogenic Belt

The conventional Liuling Group is widespread in the northern part of South Qinling structure zone, which has been regarded as a key factor for revealing the convergence process between South Qinling structure zone and North Qinling structure zone in Early Paleozoic for its significant tectonic position. Compared to the Liuling Group exposed in Shanyang-Zhashui area, Liuling Group in Danfeng-Shangnan-Xiping area is more complicated in the degree of deformation and metamorphism, which is named as low-metamorphosed Liuling sedimentary assemblage and Liuling sedimentary assemblage in this paper, respectively.Liuling sedimentary assemblage in Shanyang-Zhashui area was defined as Middle-Upper Devonian based on reliable fossils, while there are none fossil evidences for low-metamorphosed Liuling sedimentary assemblage, defined as Lower Paleozoic.In this paper, LA-ICP-MS detrital zircon U-Pb ages of two garnet-bearing two-mica quartz schist samples in low-metamorphosed Liuling sedimentary assemblage were tested, results ranged from 2 598-390 Ma, yield two prominent age clusters at~1.0-0.6 Ga and~0.5-0.4 Ga, as well as three sub-prominent age clusters at~2.6-2.3 Ga, ~1.71-1.60 Ga and~1.3-1.2 Ga.Combined with regional geology, we suggest that:the Liuling sedimentary assemblage and low-metamorphosed Liuling sedimentary assemblage were mainly fed by South Qinling structure zone and North Qinling structure zone, own double-direction sources, they may have deposited at the same time in a forland basin, and experienced different degree of deformation and metamorphism lately.The maximum depositional age of low-metamorphosed Liuling sedimentary assemblage is~390 Ma, which most likely deposited in Middle-Late Devonian.