关键词: 晚奥陶世, 钾质斑脱岩, SHRIMP测年, 成因环境, 金粟山组, 鄂尔多斯盆地南缘

Abstract:

The Late Ordovician Jinsushan Formation in southern Ordos basin is characterized by carbonate of deep marine facies，and contains several marked tuffaceous clay beds of orange color. Mineral and geochemical analysis show that the clay beds mainly consist of illite and mixed illite montmorillonite in composition，with some quartz，feldspar and zircon minerals that derived from intermediate to acidic magma, rich in K₂O, and are of typical K-bentonites. Analysis on trace elements and geochemical composition of the bentonites suggests that their source rock is likely derived from syn collision island arc volcanism. SHRIMP U-Pb dating on zircons from the bentonites has yielded three sets of ages: （451.5±4.9） to （452.1±5.1） Ma，（457.5±5.1）Ma and （465.8±8.3）Ma，with the former two identical to those of the well known Millbrig Kinnekulle and Dicke bentonites in Laurentia and Baltica. However，stepwise discriminant analysis on chemical composition of the bentonites and comparison with those from the Midcontinent of United States indicate that they are not congruent in chemical fingerprints，therefore may not have resulted from the same episodes of the giant eruptions. The tuffaceous materials in the Jinsushan bentonites were most likely from volcanic eruptions of the island arcs along north side of the Shang Dan oceanic basin.The strong basement subsidence at southern margin of the Ordos basin and the significant change in the sedimentary framework there in early Late Ordovician may be best interpreted as depositional responses to the tectonic extension and subsiding initiated in the back arc basin north to the Erlangping island arc.

Key words: Late Ordovician, K-bentonite；SHRIMP dating；tectonic environment；Jinsushan Formation；southern Ordos basin