太古宙绿岩带岩石学和地球化学：实例与探讨

Petrology and geochemistry of Archean greenstone belts： A case study and discussion

绿岩带是太古宙大陆地壳重要的构造单元。 按照岩石组合特征， 绿岩带可划分为 3 个类型：1） 巴伯顿型， 主要由基性—超基性火山岩组成， 含少量酸性火山岩及沉积岩， 中性火山岩很不发育；2） 苏必利尔型， 主要由中性火山岩和中—基性火山岩组成， 含沉积岩； 3） 达尔瓦尔型， 以广泛发育的沉积岩为特征。 其中， 巴伯顿型绿岩带在世界范围内分布较广， 且组成较为复杂， 表现出一系列独特的岩石学和地球化学特征：1） 基性—超基性火山岩在绿岩带层序中占主导地位；2） 发育具有异常高的地幔潜能温度的科马提岩类；3） 存在太古宙亏损型和富集型玄武岩等。 华北克拉通清原地区的表壳岩虽然经历高级变质作用， 但仍 具有清晰的层序， 与巴伯顿型绿岩带岩石组合特征类似， 因此我们倾向于将其厘定为清原绿岩带。 清原绿岩带主体形成于 2.5 Ga， 与广泛分布的新太古代花岗质片麻岩形成时代一致， 并不存在大规模的中太古代地质体。 清原绿岩带的岩石学和地球化学研究表明新太古代晚期原始地幔柱模型可以较为合理的解释清原地区及华北克拉通东部陆块其它新太古代基底岩石的成因， 但太古宙原始地幔柱与显生宙地幔柱在某些方面有所不同。

The greenstone belt is an important tectonic unit in the Archean continental crust. The greenstone belt can be classified into three types according to their unique lithological units： 1） The Barberton type mainly consists of basic-ultrabasic volcanics, with minor felsic volcanics and secondary sediments, lack of andesites; 2） The Superior type is characterized by large amounts of andesites and basaltic andesites, with secondary sediments present. 3） The Dharwar type is characterized by the volumes of clastic sediments and carbonates. The Barberton type greenstone belt is extensively exposed on Earth, showing complex composition and a series of unique petrological and geochemical characteristics： e.g., 1） basic-ultrabasic volcanics are dominant in the sequence of the greenstone belt; 2） komatiites with anomalous potential temperature; 3） the basic volcanics can be divided into the enriched and the depleted basaltic rocks. Although the grade of metamorphism is significantly higher （upper amphibolite to granulite facies）, the Qingyuan greenstone belt is of clear sequence, analogous to the typical Barberton-type greenstone belt in lithological units. The majority of the Qingyuan greenstone belt were formed at 2.5 Ga, coeval with the extensively exposed late Neoarchean granitic gneisses, and no large scale of 〉2.7 Ga geological bodies exposed in the region. Petrological and geochemical data of the Qingyuan greenstone belt suggests that“ Archean proto-mantle plume model”is favored to account for the generation of the Late Neoarchean basement from Qingyuan area and other regions in the eastern block, North China Craton, not completely the same as the Post-Archean mantle plume in some aspect.