丹巴地区变质沉积岩Sm-Nd同位素研究

Sm Nd ISOTOPIC STUDY ON METASEDIMENTS IN THE AREA OF DANBA,CHINA

对丹巴地区广泛分布的志留系、泥盆系和三叠系地层进行了较系统的Ｓｍ－Ｎｄ同位素研究。这些变质沉积岩的地壳存留年龄为２２８４－１９７７Ｍａ，它介于扬子板块核部晚太古代地壳（２９００－２６００Ｍａ）和板块边缘中－新元古代地壳（１１８０－８１２Ｍａ）之间，表明丹巴地区的古生代地层可能主要是这两个物源区按一定比例均匀混合的产物。

The metamorphic rocks in the area of Danba are located at the western margin of the Yangtze plate.In addition,Jinning granitoids,Sillurian,Devonian and Triassic strata were widespead in Danba,and suffered metamorphism of green schist to amphibolite facies.In this paper systematic Sm Nd isotopic study has been conducted on these metasediments.In order to minimize the influence of Sm/Nd variation of samples (0.148-0.204) on the calculation of the crustal residence age,two stage model was used.Results show that the Sillurian,Devonian and Triassic strata have rather similar crustal residence ages,which are 2 284 2 053 Ma,2 278 1 977 Ma,and 2 267-2 142 Ma,respectively.But,the difference between the crustal residence ages and the formation ages of the strata is over 1 700 Ma.The points plotted in the tDM tstr diagram are spread far from the accordance line.These features indicate that:(1)the source region of the sediments in Danba was very stable during the period of sedimentation of about 2000 Ma,and had relatively homogeneous Nd isotopic compositions;(2) These Paleozoic sediments were recycling products of pre existing old crust with little mantleorigin materials Ever since the begining of the Jinning movement,the area of Danba had been a part of the margin of the Yangtze plate.In fact,the sediments in these two localities were similar both in living community and in petrograpgy,indication that the area of Danba received continuoulsy the sediments from the Yangtze plate Although there were Archaean and early Proterozoic strata in the North China plate,it is inadequate that the North China plate was the source of the sediments in the Danba area,because they were just separated by a wide Tethys ocean.Therefore,the Yangtze plate was probably the only source region of the Paleozoic sediments in the area of Danba.Chronological data show that the oldest strata in the source region outcropped in the area of Huanglin were located in the core of the Yangtze plate;the zircon age of the gneiss of the Kongling group is 2 900 2 600 Ma.At the western margin of the Yangtze plate there were mainly widespread products of middle late Proterozoic Jinning movement,dating 1 180-812 Ma.The crustal residence age of the Paleozoic strata lies between thosese of the two possible source regions.This indicates that the Paleozoic sediemnts in Danba were homogeneously mixing products of late Archaean strata in the core of the Yangtze plate and the middle late Proterozoic strata on the western margin of the Yangtze plate in certain degree.Therefore,the area of Danba has been a part of the Yangtze plate at least before Mesozoic.