山西地块G层铝土矿同位素年龄及其地质意义

ISOTOPIC AGE OF BED G BAUXITE OF SHANSI MASSIF AND ITS GEOLOGICAL SIGNIFICANCE

山西地块的G层铝土矿即产于该地块中奥陶统或上寒武统古侵蚀面之上 ,上石炭统半沟组之下的古风化壳型铝土矿。由于G层铝土矿下伏、上覆地层间隔时间长 ,所以其地质时代长期得不到解决。为此于世纪之交 ,作者等对G层铝土矿进行了Rb -Sr全岩同位素年龄测年及40 Ar/39/Ar快中子定年。获得Rb -Sr全岩等时线 3条 ,年龄值分别为 :31 8± 1 9Ma ,30 9 1± 1 0 5Ma及 30 9 5± 2 1Ma ;40 Ar/39/Ar高温坪年龄为 31 9 9± 3 0Ma ,二者基本一致 ,代表了古风化壳型G型铝土矿堆积 -沉积的成岩年龄 ,其年代地层相当于IUGS全球地层表石炭系上亚系 (UpperSubsys tem)的巴什基尔阶 (Rashkirian)。

The Bed G bauxite of Shanxi massif,a paleoresiduum bauxite underlying Bangou Formation of Upper Carboniferous,occurs on the fossil erosion surface of Middle Ordovician of Upper Cambrian.The geological time of Bed G bauxite have not been settled for a long time owing to the omission of the Upper Ordovician,Silurian,Devonian and Lower Carboniferous between its overlying and underlying strate.Therefore,the anthors had the Rb-Sr whole-rock isotopic dating and 40 Ar/ 39 Ar dating made for Bed G bauxite in advance to the 30th International Geological Congress.There lines of Rb-Sr whole-rock isochron were obtained in the area and the test results are respectively:318±19Ma,309 1±10 5Ma and 309 5±2 1Ma. 40 Ar/ 39 Ar high temperature plateau age is 319 9±3 0Ma.which is consistant with the above results,which represent the diagenetic age of Bed G Bauxite after accumulation-sedimentation.Its chronostratigraghy is corresponding to Bashkirian of the Upper Subsystem.Carboniferous in the IUGS Global Stratigraphic Chart.