冀西北高压麻粒岩带构造环境的再思考

在崇礼岛弧型绿岩带与怀安古陆块之间，从怀安大虎沟，经宣化大东沟至赤城沤麻坑发育一条大体呈东西走向并向南倾斜的板状高压麻粒岩带。其中轻稀土亏损的洋中脊型拉斑玄武岩（ＴＨ１） 的存在显示部分洋壳残片的可能卷入。变质条件（８５０ ～９００ ℃，１３ ～１４ＧＰａ） 表明它属于俯冲杂岩。同时比怀安陆块（２ ９００ Ｍａ） 显著年轻的模式年龄（２ ７００ ～２ ６００ Ｍａ） 证明了它的增生性质。从综合地球化学、变质作用和年代学资料来看，它们不属于连续下地壳剖面的最下部，而更可能是新太古代侧向增生的弧后混杂岩带。

A new consideration about geotectonic setting of high-pressure granulite belt in northwestern Hebei Province

Between the Chongli arc type greenstone belt and the Huai'an ancient continent block, there develops a consecutive platy high pressure granulite belt.It strikes mostly to EW and dips to south.The belt extends from Dahugou in Huai'an County through Dadonggou in Xianhua County and then to Womakeng in Chicheng County. The existence of MORB type tholeiite depleted in LREE(TH1) demonstrates that some pieces of ocean crust may be involved there.Metamorphic condition (850～900 ℃,1 3～1 4 GPa) indicates that it belongs to a subducting complex.Its model age is 2 700～2 600 Ma,which is much younger than that of the Huai'an block(2 900 Ma).So this proves its accretion nature.Based on the data of geochemistry,metamorphism and geochronology acquired,it is suggested that they do not belong to the lowest part of the consecutive lower crust section,but belong to the lateral accretional back arc mélange in late Archaean.