北山造山带破城山地区冰碛岩沉积时代、物源及构造意义:来自碎屑锆石U-Pb年代学及Lu-Hf同位素的约束

Sedimentary age,provenance and tectonic implications of the diamictite in Pochengshan area,Beishan:Constraints from detrital zircon U-Pb dating and Hf isotope

出露于北山造山带的洗肠井群为新元古代晚期的冰川沉积,记录了前寒武纪演化过程的重要信息。本文对北山破城山地区的洗肠井群进行了野外地质、碎屑锆石U-Pb年代学与Lu-Hf同位素研究。结果显示破城山地区的洗肠井群具有冰碛岩的特征,其中泥质粉砂岩和含冰碛砾石粉砂—细砂岩最年轻的碎屑锆石年龄分别为579±12 Ma和574±8 Ma,结合洗肠井群与上覆下寒武统双鹰山组为平行不整合接触关系,可精确限定洗肠井群的形成时代为埃迪卡拉纪晚期。洗肠井群碎屑岩具有单一的碎屑锆石年龄峰值(635~631 Ma),且该时期锆石的两阶段Hf模式年龄主要集中于1.52~0.60 Ga,显示中元古代地壳的再造与新生地壳的特征,其与天山造山带的塔里萨依组具有明显的相似性。洗肠井群碎屑锆石中还出现~0.9 Ga和~1.4 Ga的年龄,与北山造山带内部发育的中—新元古代的岩浆事件相当。且洗肠井群的冰碛砾石主要为白云岩、硅质白云岩和极少量的石英岩,与其下部呈断层接触的蓟县系平头山组和长城系古硐井群的岩性组合相一致,由此可见洗肠井群的碎屑物质很可能来源于北山造山带自身。同时认为洗肠井群冰碛岩为中高纬度地区冰川与水流冲刷共同作用的产物,而且在埃迪卡拉纪晚期北山与天山仍处于相同的构造位置,该时期它们已经与古老的克拉通分离,一侧可能受俯冲作用的影响。

The Xichangjing Group is related to late Neoproterozoic glaciation in the Beishan orogen,which is of great significance to understand the Precambrian tectonic evolution.The field investigation,geochronology,and zircon Lu-Hf isotope were conducted on the Xichangjing Group of the Pochengshan area.The results show that it is characterized by glacial sedimentation.The ages of the youngest detrital zircons are 579±12 Ma and 574±8 Ma for pelitic siltstone and silty-fine sandstone.Combined with the parallel unconformity between the Xichangjing Group and the overlying early Cambrian Shuangyingshan Formation,the sedimentary age of the Xichangjing Group is limited to the late Ediacaran.There is a single age peak(635~631 Ma)for detrital zircons of the clastic rocks from the Xichangjing Group,and during this period the crustal Hf model ages of zircons are mainly concentrated in 1.52~0.60 Ga,which shows the crustal growth and reworking of the Mesoproterozoic crust.The Xichangjing Group is obviously similar to the Talisayi Formation in the Tianshan orogen.Moreover,the Xichangjing Group has some detrital zircons associated with~0.9 Ga and~1.4 Ga magmatism in the Beishan orogen.Its gravel is mainly composed of dolomite,siliceous dolomite and a small amount of quartzite,which are consistent with the rock associations of the Pingtoushan Formation and Gudongjing Group.Therefore,it is believed that the detrital materials of the Xichangjing Group were derived from the Beishan orogen.In addition,the Xichangjing Group was formed by the synthetical effects of glacier and water erosion in middle-high latitude.We further suggest that the Beishan and Tianshan were located in the same tectonic location during late Ediacaran,which had been separated from an ancient craton,and they might have been subjected to the subduction.