新元古代—寒武紀高精度同位素地質年代學李獻華

蘭中伍

摘 要：華北地臺周圍前寒武紀晚期的地層對比格架和演化歷史逐漸成為地質學界研究的焦點，受到了國內外地質學家廣泛的關注。其中，從薊縣剖面的侵入巖或者間層火山灰中測得的鋯石U-Pb年齡已經為華北地臺北緣中新元古代地層高精度年代格架提供了確鑿證據。這些年齡數據不僅解決了這套中新元古代地層的劃分和全球對比問題，而且為前寒武紀生物的起源和演化也提供了時間錨點。相比之下，由于缺乏可靠的同位素年齡制約，華北地臺南緣的汝陽群和洛峪群，蘇皖北部的徐淮群、遼南地區的金縣群、五行山群、細河群、永寧群的沉積時限和地層歸屬問題一直存在爭議。同樣，華南南華系（也即國際上的成冰系）的底界及年齡、長安冰期和古城冰期的年齡以及南華系的劃分及對比問題也一直未得到解決。華北地臺北緣和南緣這套中-新元古代地層蘊含著豐富的疑源類和宏觀藻類以及可能的后生動物實體化石和遺跡化石等生命信息，而且其沉積過程伴隨著哥倫比亞超大陸的裂解。華南成冰系地層的沉積伴隨著羅迪尼亞超大陸的裂解演化，冰期蘊含豐富的疑源類、宏觀藻類和多細胞動物化石，因此，對華北地臺周邊元古代地層以及華南成冰系地層進行精確的年代約束、重新厘定其地層劃分對比，將為哥倫比亞超大陸和羅迪尼亞超大陸的裂解演化以及早期生命的起源演化等重大科學問題提供新的年代學證據。

關鍵詞：華北 華南 元古代 地層 沉積年齡

Abstract：Late Precambrian stratigraphic correlation framework and evolution history around the areas bordering the North China Craton （NCC） has increasingly become the target of intense research that attracted comprehensive domestic and foreign attention. Intermittent reports of zircon U-Pb ages from intrusive rocks or interbedded volcanic tuff beds within critical stratigraphic horizons have provided solid evidence for establishing high precision chronostratigraphic framework for the Meso-Neoproterozoic strata in the Jixian Section， northern margin of NCC. These absolute ages not only have contributed to the classification of the Meso-Neoproterozoic strata in NCC and its global correlation， but also have provided key age constraints on the origination and evolution of Precambrian biota， especially on the macroscopic algae. In contrast， owing to lack of reliable radiometric age constraints， the depositional ages and stratigraphic attributions of the Ruyang and Luoyu groups in the southern margin of NCC， the Xuhuai Group in northern Jiangsu and Anhui provinces， the Jinxian， Wuhangshan， Xihe， and Yongning groups in southern Liaoning Province has long been disputed. Likewise， the age of the Nanhuan System （equivalent of Cryogenian） in South China is loosely controlled. The Proterozoic strata bordering the NCC contain abundant acritarchs， macroalgae， and possible metazoan fossils， their deposition being accompanied by the breakup of Columbian Supercontinent. Likewise， the formation of Nanhuan System was accompanied by the breakup of Rodian Supercontinent， and contains abundant acritarchs， macroalgae， and multicellular life fossils. Accordingly， precisely dating the depositional ages of the Proterozoic sedimentary successions distributed in the north and south China blocks will provide new， reliable age constraints on the breakup history of Columbian and Rodian supercontinents， and on the origin and evolution of early life.

Key Words：North China； South China； Proterozoic； Strata； Depositional age

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