江山—绍兴断裂带陈蔡岩群片麻岩SHRIMP锆石U-Pb年龄及其地质意义

浙江诸暨陈蔡地区发育一套副变质岩层，由中深度变质的片麻岩、片岩和大理岩组成，与双溪坞群为断层接触，其地质时代多被视为与江山—绍兴断裂带南东侧的华夏地块古老变质基底相同。陈蔡岩群地处江绍断裂带北东端，变质和变形较深，前人将其划归为华夏古陆的小块体，地层时代曾与福建古元古代麻源岩群或浙江八都岩群和江西中元古代铁砂街组对比。在陈蔡岩群下吴宅组含榴夕线黑云斜长片麻岩及含榴黑云斜长片麻岩中获得大量的锆石，具有典型的核、幔、边结构，核部年龄集中在新元古代，206Pb/238U年龄加权平均值为848±10Ma和845±9Ma，锆石后期增生边部年龄多为加里东期的年龄，206Pb/238U年龄加权平均值为431.4±7.8Ma。陈蔡岩群斜长片麻岩获得的锆石核部较单一的U-Pb年龄，反映出陈蔡岩群下吴宅组夕线石所代表的泥质成分所含的锆石单一，其锆石核部年龄应代表陈蔡岩群下吴宅组的原岩年龄；同时，认为陈蔡岩群既不能与福建古元古代的麻源岩群对比，也不能与江西地区中元古代铁砂街组对比。其原岩为稍晚于双溪坞群（905~877Ma）的新元古代地层（848~845Ma），从年代上倾向于江南造山带范畴。该年龄对江山—绍兴断裂带变质地层的区域对比和构造演化有重要意义。

SHRIMP zircon U-Pb dating of Neoproterozoic Chencai Complex in Jiangshan-Shaoxing fault zone and its implications

A series of parametamorphic rocks, as the oldest metamorphosed basement in the Jiangshan-Shaoxing fault zone, consist of gneiss, schist and marble of medium-deep metamorphism and are in fault contact with the Shuangxiwu Group in Chencai area, Zhuji County, Zhejiang Province. Because the Chencai Group which suffered deep metamorphism and deformation is developed in the southern part of the Jiangshan-Shaoxing fault zone, most Chinese geologists assign the group to the small patch of the Cathysia Plate and make correlation with Paleoproterozoic Mayuan Group in Fujian Province or the Badu Group in Zhejiang Province and the Mesoproterozoic Tieshajie Foramtion in Jiangxi Province. Lots of zircons were collected from garnet sillimanite biotite plagiogneiss and sillimanite garnet biotite gneiss in the Xiawuzhai Formation of the Chencai Group. The zircons have a typical core, mantle and edge structure, the age of the core is usually in the range of 848~845Ma and the age of the late accretion edge is Caledonian (431Ma). According to the zircon dating of the plagioclase gneiss of the Chencai Group, the authors consider that the Chencai Group can not only be correlated with the Paleoproterozoic Mayuan Group of Fujian Province, but also with Mesoproterozoic Tieshajie Formation of Jiangxi Province. The protolith age of the Chencai Group suggests Neoproterozoic (848~845Ma), which is younger than that of the Shuangxiwu Group (905~877Ma) in the stratigraphic column and the stratigraphic division, and the age belongs to the domain of the Jiangnan Orogenic Belt. However, this age has an important significance in the correlation of metamorphosed provinces in the JiangShan-ShaoXing fault zone and the study of its tectonic evolution.