冀西北新太古代下部陆壳及其性质初探

Priviminary Research on Neo Archean Lower Continental Crust(Lower Crust) and Its Nature

按其地壳组成比例、强烈的紫苏花岗岩化和高压麻粒岩相变质作用，冀西北怀安一带存在一个发育基本完整的下部陆壳（下地壳）剖面，由于地体碰撞向南掀斜而出露地表。本文讨论了其地球化学与构造环境。稀土分配平坦型的拉斑玄武岩（ＴＨ１）类似于洋壳，但与ＭＯＲＢ显著不同，更象不成熟的大陆性火山弧。轻稀土富集型的拉斑玄武岩（ＴＨ２）成分分散，有的跨入钙碱性玄武岩区，又有大量中酸性火山岩－沉积岩伴生，分离结晶作用显著，接近于板内热点产物。两种反映截然不同构造环境的玄武岩又密切地伴生在一起，使本区表现了许多不同于一般陆壳的特殊性。与冀东相比，本区并以古地热梯度变陡（２０℃／ｋｍ）为特征。

According to constituent proportion of the crust, strong chanockitization and high pressure granulite facies metamorphism,we determined a lower continertal crust section in northwestern Hebei Province. It is exposed on the ground surface by terrane collision titling to south. This paper discusses the geochemistry and tectonic environment of the section.The flat REE pattern of tholeite(TH 1) suggests that it is similar to the nature of ocean crust,but obviously different from MORB, and much alike to immature continental volcanic arc. Coexisting with metamorphic mid acidic volcanic rocks and metasedimentary rocks, LREE rich type tholeite(TH 2) display clear differentiation crystallization and disperse composition. Some of its plots falls in calc alkaline basalt area, indicating it is similar to the nature of intraplate hot spot. Two kinds of tholeites,reflecting different tectonic environments, are closely associtated together, showing many specific characteristics different from general continental crust. Comparing to that in east Hebei Province, the geothermal gradient is steep(20℃/km) in this terrane. Perhaps it indicates that an essential origin of plate, like the modern plate, occurred about 2.8 Ga B.P. on the earth.