黑龙江省东部早古生代地层的确定:地质与碎屑锆石U-Pb年代学证据

Determination of the Early Paleozoic Strata in Eastern Heilongjiang Province:Evidence from Field Geology and Detrital Zircon U-Pb Geochronology

通过研究黑龙江省东部早古生代地层晨明组和宝泉组碎屑锆石的U-Pb定年结果,讨论了晨明组和宝泉组的所属时代和物源.大多数锆石呈自形-半自形晶,显示出典型的岩浆生长环带或条痕状吸收,暗示其岩浆成因,极少数的锆石具有经变质作用形成的暗色增生边.定年结果表明：采自晨明组上部长石石英砂岩中碎屑锆石59个分析点产生了以下年龄组：561Ma、621Ma、683Ma、752Ma、803Ma、822Ma、851Ma、900Ma、922Ma、954Ma、1 781Ma、1 865Ma和1 933Ma,这表明晨明组沉积于561Ma之后.而以不整合接触关系覆盖于晨明组之上的宝泉组底部泥质板岩中碎屑锆石60个分析法点产生了425Ma、450Ma、485Ma、900Ma和1 750Ma年龄众数.由上述地质关系和年代学测定结果,结合晚古生代地层中碎屑锆石的年龄组合,可以判定位于黑龙江省东部地区晨明组的形成时代介于561～510Ma之间,这是首次在黑龙江省东部确定的具有确切年代学证据的早古生代地层.上述年龄组碎屑锆石的存在,表明其物源主要为该区新元古代和古元古代的地质体,这进一步揭示在早古生代期间,松嫩-张广才岭地块东缘可能存在前寒武纪残片.

This paper reports LA-ICP-MS zircon U-Pb dating results from the Chenming and Baoquan formations in the eastern Hei- longiiang Province in order to constrain their forming age and provenances. Most of detrital zircons are euhedral-subhedral in shape and display striped absorption or oscillatory zoning in CL images, implying their magmatie origin. The others show dark accretionary rim formed by metamorphism. The dating results on magrnatie zircons indicate that 59 detrital zircons from feldspathic quartz sandstones in the upper part of the Chenming Formation yielded age populations of 561 Ma, 621 Ma, 683 Ma, 752 Ma, 803 Ma, 822 Ma, 851 Ma, 900 Ma, 922 Ma, 954 Ma, 1 781 Ma, 1 865 Ma and 1 933 Ma. Based on detrital zircon dating results, we conclude that the sedimentary processes of the Chenming Formation could take place after 561 Ma In the study area, the Baoquan Formation overlain the Chermaing Formation with the unconformity relationship, 60 detrital zircons from argillic slates in the lower part of the Baoquan Formation yielded age populations of 425 Ma, 450 Ma, 485 Ma, 900 Ma and 1 750 Ma According to the above dating results and the field geological relationship, combined with the age populations of the detrial zircons from the Late Paleozoic strata, we consider that the Chenming Formation in the eastern Heilongjiang Province formed between 561 Ma to 510 Ma. The Early Paleozoic strata are firstly discovered with the exact geochronological evidence in the eastern Heilongjiang Province The above detrital zircon ages indicate that the provenances of the Chenming Formation could mainly come from the Neoproterozoic and Paleoproterozoic igneous rocks around the study area, implying that the minor Precambrian basement relic might be occurred in the eastern margin of the Songnen-Zhangguangcailing Massif in the Early Paleozoic.