钦杭结合带中段龙头寨群黄竹洞组地质时代的新证据:流纹岩夹层的SHRIMP锆石U-Pb年龄及地质意义

New geological age evidence of Huangzhudong Formation,Longtouzhai Group,northern Guangdong Province:SHRIMP zircons U-Pb age of volcanic interlayer and its geological significance

为进一步查明钦(州湾)-杭(州湾)结合带中段龙头寨群的地质时代及龙头寨群黄竹洞组与上覆南迳组的接触关系,在野外考察、代表性剖面测制的基础上,对粤北始兴司前龙头寨群黄竹洞组变质岩系顶部首次发现的火山岩夹层的2个弱变质流纹岩样品进行了SHRIMP锆石U-Pb年龄测定,获得^(206)Pb/^(238)U加权平均年龄分别为436.2±2.1Ma (n=15,MSWD=1.60)和438.7±2.1Ma(n=16,MSWD=1.17),指示黄竹洞组的地质时代属晚奥陶世-早志留世,为黄竹洞组的地质时代归属提供了证据。结合上覆南迳组碎斑熔岩SHRIMP锆石U-Pb年龄为443.6±5.4Ma,说明黄竹洞组火山岩夹层与南迳组火山岩的锆石U-Pb年龄在误差范围内一致,属同期火山活动的产物,为黄竹洞组与南迳组的接触关系属整合接触提供了时代约束。粤北龙头寨群黄竹洞组顶部火山岩及南迳组火山岩系的发现和地质时代的厘定,为钦-杭结合带中段晚奥陶世-早志留世龙头寨群属活动大陆边缘沉积提供了依据,暗示钦-杭结合带中段加里东造山带属俯冲-碰撞造山带。

To further constrain the geological age of Longtouzhai Group in the middle part of the Qinzhou bay-Hangzhou bay joint zone and the contact relationship between Huangzhudong Formation of Longtouzhai Group and Overlying Nanjing Formation,based on field survey and representative profile measurement,the age of two meta-rhyolites intercalation at the top of the metamorphic rock series of Huangzhudong Formation in the northern side of Hekou basin was conducted by using the SHRIMP zircon U-Pb dating.The two meta-rhyolites have zircon^(206)Pb/238 U weighted average mean ages of 436.2±2.1 Ma(n=15,MSWD=1.60)and 438.7±2.1 Ma(n=16,MSWD=1.17),respectively.The geological age of Huangzhudong Formation belongs to Late Ordovician-Early Silurian,which provides evidence for the attribution of the geological age of Huangzhudong Formation.They are consistent with the zircon U-Pb isotope ages of the porphyry lavas(443.6±5.4 Ma,n=13,MSWD=1.3)in the Nanjing Formation,illustrating the synchronized volcanic activities and it provides an era constraint for the contact relationship between Huangzhudong Formation and Nanjing Formation.The discovery of volcanic rocks at top of Huangzhudong Formation of Longtouzhai Group in northern Guangdong Province and the determination of geological age provide a basis for the active continental margin deposition of Longtouzhai Group in Late OrdovicianEarly Siliurian in the middle part of Qin-Hang joint zone,suggesting that the Caledonian orogenic belt in the middle part of the QinHang joint zone is a subduction-collision orogenic belt.