冀北赤城红旗营子群黑云斜长片麻岩的岩石学、地球化学及原岩特征

冀北赤城红旗营子群黑云斜长片麻岩具斑状变晶结构,变基质为中、细粒鳞片粒状变晶结构,片麻状构造,局部保留有变余砂状结构。岩石主要由斜长石(28%~32%)、石英(35%~40%)、黑云母(18%~25%)及少量石榴石(2%~8%)和石墨(1%~4%)等组成,其变质演化可能是一个降温、降压的过程。黑云斜长片麻岩样品具有较宽的常量元素变化范围,稀土元素含量不是很高且变化较大,ΣREE介于49.45×10-6~140.10×10-6之间,具有轻稀土富集、重稀土平坦的球粒陨石标准化稀土配分模式,轻、重稀土元素比值为5.23~9.16,(La/Yb)CN值为5.07~8.70。大多数样品具中等程度的负Eu异常和不太明显的负Ce异常,(Eu/Eu*)CN和(Ce/Ce*)CN值分别为0.63~0.81和0.82~1.01。岩石中Rb、Sr、Cs、Ba、Zr、Hf、Nb、Ta、Th和U等微量元素含量均低于上地壳的平均含量,但Sr/Ba和Th/U值高于上地壳的平均值。红旗营子群黑云斜长片麻岩的地球化学特征表明其原岩应为泥质(或含泥质)砂岩,可能形成于大陆岛弧环境。

Petrology, geochemistry and protolith of the biotite plagiogneiss from Hongqiyingzi Group in Chicheng County, northern Hebei Province

Biotite plagiogneiss samples from Hongqiyingzi Group in Chicheng County of northern Hebei Province have porphyroblastic texture with medium-fine grained lepido-granoblastic groundmass and gneissic structure.Blastopsammitic texture is preserved locally.The rocks mainly consist of plagioclase(28%～32%),quartz(35%～40%) and biotite(18%～25%),with minor garnet(2%～8%,as porphyroblast) and graphite(1%～4%).Garnet compositional characteristics reveal that the temperature and pressure fell contemporaneously during the main metamorphic evolution.All the biotite plagiogneiss samples have wider variation in major components.The total REE values are not very high and vary in the range of 49.45×10-6～140.10×10-6,displaying LREE-enriched and HREE-flattened patterns with chondrite-normalized(La/Yb)CN of 5.07～8.70 and LREE/HREE of 5.23～9.16.Most of these samples show intermediate negative Eu anomalies and weak negative Ce anomalies,the chondrite-normalized Eu/Eu* and Ce/Ce* values are separately 0.63～0.81 and 0.82～1.01,and the rocks have lower Rb,Sr,Cs,Ba,Zr,Hf,Nb,Ta,Th and U and higher Sr/Ba and Th/U ratios relative to the upper crust.The geochemical characteristics of the biotite plagiogneiss samples from Hongqiyingzi Group suggest that their protolith might have been pelitic (or pelitic-bearing) sandstone and was formed in a tectonic environment similar to the continental island arc.This recognition is important with regard to understanding the tectonic evolution of the central part of the northern margin of the North China Craton.