全吉地块前寒武纪地层学研究进展

Integrated Precambrian Stratigraphy of the Quanji Massif:A Review

青藏高原北部全吉地块的全吉群和小高炉群记录了“新元古代冰期事件”、“超大陆循环”和“大不整合事件”等一系列意义重大的地质事件,并以此作为探索全吉地块前寒武纪地质演化史的重要窗口。近年来,关于全吉地块的前寒武纪生物地层学、年代地层学和事件地层学研究取得了一系列重要进展:(1)在全吉地块发现的埃迪卡拉型宏体生物化石以及共生的管状动物化石,为探索现代型海洋生态系统起源提供了新的视角,同时将小高炉群的时代上限进一步限定为埃迪卡拉纪晚期(551~543 Ma)。(2)全吉地块缺失寒武系纽芬兰统,其埃迪卡拉纪—寒武纪界限的地层缺失大致为25~50 Ma,与华北克拉通西缘-西南缘的“大不整合事件”尺度具有相似性。(3)全吉群红藻山组与上覆地层小高炉群黑土坡组之间存在一个较大的不整合,年代地层学研究显示红藻山组的形成时代为古元古代末期(1640~1646 Ma),而不是之前认为的埃迪卡拉纪(635~539 Ma)。(4)全吉地块仅存在一期新元古代冰碛岩,即埃迪卡拉纪红铁沟组冰碛岩,其与华北克拉通西缘-西南缘分布的罗圈组和正目观组冰碛岩时代大致相当。(5)全吉地块与华北克拉通在埃迪卡拉纪—寒武纪过渡时期具有相似的构造-沉积演化史,二者在埃迪卡拉纪末期同处于中高纬度,而不是之前认为的低纬度地区。

The Quanji Group and Xiaogaolu Group in the Quanji Massif of the northern Tibetan Plateau recorded a series of important geological events,such as the Neoproterozoic glacial events,Supercontinent cycle and Great Unconformity(GU),thus offering an important window to review the Precambrian geological evolution of this tectonic unit.Recent investigations on the biostratigraphy,chronostratigraphy,and event stratigraphy indicate that:①The new finding Ediacara-type fossils and associated tubular organisms provide the new sight into our understanding on the origin of modern-looking marine ecosystem,and constraint the upper age limit of Xiaogaolu Group to the late Ediacaran(551~543 Ma).②The scale of Ediacaran-Cambrian unconformity in the Quanji massif has been suggested to be the approximately 25~50 Ma with missing stratigraphic successions of Terreneuvian Series(Cambrian),which is broadly correspond to the magnitude of GU at the western-southwestern margins of North China Craton.③There is a significant unconformity between the Hongzaoshan Formation(Quanji Group)and the overlying succession Heituopo Formation(Xiaogaolu Group).Chronostratigraphic investigation revealed that the age of Hongzaoshan Formation(Quanji Group)is the late Paleoproterozoic(1640~1646 Ma),rather than the Ediacaran period(635~539 Ma)as previously thought.④The Quanji massif contains a unique Neoproterozoic glaciation unit,the Hongtiedou diamictite,which is roughly equivalent to the Ediacaran glacial deposits of Zhengmuguan and Luoquan Formation from the western-southwestern margins of North China Craton.⑤Both of the Quanji massif and North China Craton have been proposed to be the midhigh latitude region during the late Ediacaran period,rather than the low latitude as previously thought.The common tectonostratigraphic characteristics suggest that the Quanji massif shares the closely tectonic-sedimentary evolution with North China Craton during the Ediacaran-Cambrian transition.