黑龙江省东南部张广才岭群新兴组的形成时代及物源

Formation Time and Provenance of the Xinxing Formation of Zhangguangcailing Group in Southeastern Heilongjiang Province

为确定张广才岭群新兴组的形成时代和物源,本文测定了其碎屑锆石和岩浆锆石的年龄。在新兴组中测得的两组碎屑锆石(LA-ICP-MS U-Pb)产生多组谐和年龄,BP18R15样品91个测点最小峰值(谐和)年龄为260Ma(41个点),BP18R83样品100个分析点最小峰值(谐和)年龄为253Ma(52个点)。锆石绝大多数呈自形—半自形晶,显示典型振荡岩浆生长环带或条痕状吸收,且Th/U值相对较高,暗示其岩浆成因。以此确定新兴组沉积下限为晚二叠世之后。结合侵入于新兴组的正长花岗岩年龄为210±1Ma,表明张广才岭群新兴组形成于晚二叠世—晚三叠世之间,而非前人所确定的新元古代。从新兴组中碎屑锆石的年龄频数可看出,新兴组的沉积物主要来源于周边晚古生代地质体以及次要的中元古代、新元古代、早古生代和早中生代地质体。前寒武纪碎屑锆石的存在暗示新兴组沉积时,区域地表或浅部曾存在一些具有类似年龄的前寒武纪残余地质体。新兴组几乎所有沉积物源来自松嫩-张广才岭地块,暗示松嫩-张广才岭地块与佳木斯-兴凯地块及华北地块的拼合应晚于晚二叠世。

In order to determine the formation time and provenance of the Xinxing Formation of Zhangguangcailing Group,this study carried out geochronological analyses on detrital zircons and magmatic zircons from the study area.Several groups of the concordant ages for two groups of detrital zircons from the Xinxing Formation were obtained in this study using LA-ICP-MS U-Pb dating method.The minimum peak（concordant）age for 91 points of sample BP18 R15 is 260 Ma（41 points）,while the minimum peak（concordant）age for 100 points of sample BP18 R83 is 253 Ma（52 points）.The minimum single point age was 192 Ma and 207 Ma.CL images show that most zircons are euhedral-subhedral in shape and present fine-scale oscillatory zoning,as well as high Th/U ratios,implying their magmatic origin.The study above suggests that the lower limit of the Xinxing Formation should be the late Permian.Combined with the age（210±1 Ma）of syenogranite intruding into the Xinxing Formation,this study believes that the Xinxing Formation of the Zhangguangcailing Group formed between late Permian and late Triassic,rather than the Neoproterozoic as previously believed.It can be concluded from the age frequency of detrital zircons from the Xinxing Formation that the sediments of the Xinxing Formation came mainly from the late Paleozoic geological bodies in surrounding areas,with minor from other geological bodies with time range from Mesoproterozoic,Neoproterozoic,and Early Paleozoic to Early Mesozoic.Existence of Precambrian detrital zircons implies that the Precambrian residual geological bodies might occur in regional surface or shallow crust during the deposition of the Xinxing Formation.Almost all of the sediments in the Xinxing Formation sourced from the SongnenZhangguangcailing massif,suggesting that the collage of Songnen-Zhangguangcai massif and JiamusiXingkai massif and north China massif should occur later than the late Permian.