山东诸城晚白垩世王氏群恐龙化石层碎屑锆石定年和物源示踪

Detrital Zircon Dating and Tracing the Provenances of Dinosaur Bone Beds from the Late Cretaceous Wangshi Group in Zhucheng,Shandong

山东东部诸城晚白垩世王氏群大面积、高密度集群埋藏的恐龙化石是近年研究的热点,但其沉积物源与精确沉积时限目前尚不明确。本文通过对山东诸城王氏群恐龙化石层中6个砂岩样品的碎屑锆石U-Pb定年表明,样品090414-24-D中最年轻的单颗粒锆石年龄(YSG)为77.3 Ma,应代表王氏群恐龙化石层的最大沉积时限(下限),结合前人对王氏群红土崖组的年代学研究表明,其沉积时限应老于73.5 Ma(上限),也说明赋存恐龙化石层的王氏群上部红土崖组为Campanian期沉积;碎屑锆石定年揭示物源的锆石年龄组成分布在100～130 Ma之间,6个样品的年龄主峰值多在110 Ma或120 Ma左右,与下伏的早白垩世青山群火山岩年龄主峰较为吻合。王氏群恐龙化石层砾岩的砾石成分和古水流研究显示,其中3组砾石统计表明砾岩层砾石成分主要由火山岩或火山碎屑岩组成,3组古水流数据表明,诸城地区王氏群红土崖组砾岩层的古流向以南和南东方向为主,尚有1组显示古流向为向北,表明王氏群物源主要来自盆地北侧莱阳和北西侧沂沭断裂带的青山群火山岩,其次为盆地南缘。本研究为诸城王氏群化石层沉积时限的限定和沉积物源的判断,以及胶莱盆地王氏群时期古地理格局的恢复等提供了重要的依据。

Objectives： The mass burial of dinosaur bone fossils in the Upper Cretaceous Wangshi Group in Zhucheng Basin has been a research focus in recent years. However, the provenance of the dinosaur bone fossils and the accurate depositional age of the bone beds remain ambiguous. We study the depositional age of the dinosaur bone beds and trace their provenances. Methods： We combined detrital zircon geochronology with gravel composition and palaeocun＇ent data to analyze sediments of dinosaur bone beds from the Hongtuya Formation of the Wangshi Group in the Zhucheng Basin. Data on gravel composition and palaeocurrents were collected from conglomerates in the dinosaur fossil layers and relative layers in the Hongtuya Formation. In total, 4 different data sets of palaeocurrent and 3 different data sets of gravel composition were collected. Detrital zircons were separated from 6 crushed sandstone samples collected from the dinosaur bone beds in the Hongtuya Formation, using electromagnetic and heavy liquids separation. Zircon grains were then hand-picked and mounted on adhesive tapes, enclosed in epoxy resin, and polished down to nearly half their thickness to expose internal structures. The internal structures of the zircon grains were then studied using cathodoluminescence （CL） imaging. The targeting of detrital zircons and CL imaging were carried out at the Beijing Sensitive High Resolution Ion Microprobe （SHRIMP） Center, Chinese Academy of Geological Sciences. U-Pb dating of zircons were conducted by LA-ICP-MS at the National Research Center for Geoanalysis, also at the Chinese Academy of Geological Sciences. Results： The youngest single grain age （YSG） of sample 090414-24-D was 77. 3 Ma, representing the maximum depositional age of the dinosaur bone beds. According to the chronological study of Hongtuya Formation of Wangshi group, we suggest that the deposition time of Hongtuya Formation should be older than 73.5 Ma. Dating results also revealed an age peak of -110 -120 Ma, which corresponds with the peak age of volcanic rocks of the Lower Cretaceous Qingshan Group. Through analysis of gravel composition and paleocurrents in the sediments of dinosaur bone beds, we found that three different data sets of the gravel composition of the conglomerates were volcanic rocks or pyroclastic rocks. Three different data sets of paleocurrents suggested that the main sediment source of the Wangshi Group dinosaur bone beds was from the north--northwest of the Basin. Only one group of data sets indicated a provenance south of the basin. Conclusions： （ 1 ） A previous study reported the age of the basalt located at the top of the Hongtuya Formation as 73.5 ±0.3 Ma （Yan Jun et al. , 2003＆）. Dinosaur bones were mostly buried in the lower Hongtuya Formation, so the depositional age of the dinosaur bone beds must be 〉 73 Ma. By assessing detrital zircons separated from 6 conglomerate samples from the Wangshi Group in Zhucheng, the present study found that the youngest single grain age （YSG） , 77.3 _＋ 1.54 Ma, represents the maximum depositional age of the dinosaur bone beds. Therefore, the dinosaur bone beds of the Late Cretaceous Wangshi Group in the Zhucheng Basin were deposited between 77.3 - 73.5 Ma. This finding also indicates that the Hongtuya Formation was deposited during the Campanian. （2） Dating results reveal that the main age composition of the detrital zircons was 100 - 130 Ma, which corresponds with the peak age of the volcanic rocks of the Lower Cretaceous Qingshan Group. Supplemented with research on gravel composition and palaeocurrents, we determined that the main provenance of the sediments over a large area and high density dinosaur bone beds in the Zhucheng Basin is the volcanic rocks of the Qingshan Group in the Yishu Fault Zone and Laiyang area, to the northwest and north of the Zhucheng Basin. （3） These findings provide an important framework for establishing a global comparison and geochronological framework of the Late Cretaceous dinosaur bone beds, and also provide further data for reconstructing the palaeogeography framework of the Jiaolai Basin in the Late Cretaceous Wangshi Group.