滇东北昭通地区寒武系筇竹寺组碎屑锆石年代学和地球化学及其地质意义

锆石是一种高度稳定矿物,一旦形成,将会保留形成时源区的信息。即使再次受到部分熔融或变质作用影响,所携带的源区信息也不易全部丢失,还能再次记录地质变化的信息。因此,碎屑锆石在沉积地层年代学、沉积物源及源区构造属性研究方面发挥着重要作用,其具体实践不仅可以丰富其地质年代学理论,对于重塑盆—山格局及古地理演化也具有重要意义。康滇古陆周缘筇竹寺组已有的研究主要从传统沉积学角度开展了岩相古地理分析,尚未开展碎屑锆石年代学及锆石原位微量元素分析。在古地理研究基础上,笔者等首次开展了康滇古陆东缘滇东北昭通地区筇竹寺组碎屑锆石年代学及锆石微量元素研究。研究表明,筇竹寺组碎屑锆石最新年龄为521 Ma,与中国地层表一致,代表了筇竹寺组底部沉积年龄和中国南方古生代最大海侵时限。筇竹寺组底部黑色泥岩碎屑锆石年龄具有2462~2511 Ma、1700~2061 Ma、550~1200 Ma 3个集中区域,与康滇地区Kenorland古陆、Columbia古陆、Rodinia古陆的聚合、裂解有着密切的关系。筇竹寺组碎屑锆石富集重稀土,一般具正Ce和负Eu异常,Th/U值大多大于0. 4,具岩浆锆石特征。其中,格林威尔造山运动及Rodinia古陆裂解产物为筇竹寺组碎屑锆石主要物源。灯影组是筇竹寺组源区最新层位,但岩性为白云岩不能提供碎屑锆石,其上梅树村组地层则缺失。该组碎屑锆石具有花岗岩、基性岩主要来源,且有钾镁煌斑岩、正长岩物源,并反映出多期S型花岗岩作用的深刻影响。

Detrital zircon chronology and element geochemistry of Qiongzgusi Formation in Zhaotong Area, Northeast Yunnan

Zircon is a highly stable mineral that, once formed, will retain information about the source region at the time of formation. Even if it is affected by partial melting or metamorphism again, the information carried by the source region is not easy to lose all, and the information of geological changes can be recorded again. Therefore, detrital zircon plays an important role in the study of sedimentary stratigraphic chronology, sediment sources and tectonic properties of source regions. Its practical practice can not only enrich its geochronology theory, but also have important significance for reshaping basin—mountain pattern and paleogeographic evolution. The existing studies of Qiongzhusi Formation in the margin of Kangdian ancient land mainly carried out the lithofacies paleogeography analysis from the traditional sedimentological perspective, but the detrital zircon chronology and in- situ trace element analysis have not yet been carried out.Methods: Based on the study of palaeogeography, the study of detritonal zircon chronology and trace elements of Qiongzhusi Formation in Zhaotong area of northeast Yunnan Province was carried out for the first time. Results:The results show that the latest age of the detrital zircons in the Qiongzhusi Formation is 521 Ma, which is consistent with the stratigraphic table in China, representing the sedimentary age at the bottom of the Qiongzhusi Formation and the maximum Paleozoic invasion time in southern China. The detrital zircon ages show three age peaks: 2462—2511 Ma, 1700—2061 Ma and 550—1200 Ma, which are closely related to the convergence and cracking of the Kenorland, Columbia and Rodinia paleolands in the Kangdian area. Detrital zircons in the Qiongzhusi Formation are rich in HREE, with positive Ce and negative Eu anomalies and Th/U ratios greater than 0.4, indicating the characteristics of magmatic zircons.Conclusions:The detrital zircons from the Qiongzhusi Formation were mainly derived from the Greenwell orogeny and the Rodinia paleocontinent. The Dengying Formation is the latest horizon in the source area of the Qiongzhusi Formation, but the lithology is dolomite, which cannot provide detrital zircon, and the upper Meishucun Formation is missing. The detrital zircons in Qionhzhusi Formation are mainly derived from granites and mafic rocks, as well as potassium—magnesium lamprophyre and syenite, reflecting the deep influence of S- type granite.