关于内蒙与晋冀交界地区早前寒武纪灰片麻岩和孔兹岩的讨论

Discussion on the Early Precambrian Grey Gneiss and Khondalite in Nei Mongol Shanxi Hebei Junction Region, North China

晋冀内蒙边界地区的早前寒武纪岩石大致以大同－兴和一线为界分为南东部麻粒岩系和北西部孔兹岩系。麻粒岩系中称作灰片麻岩的浅色麻粒岩与世界各地太古代典型的灰片麻岩不同，其矿物组合含斜方辉石或二辉石，岩石化学成分高铁、高镁又高钙，不是典型的太古代灰片麻岩ＴＴＧ岩套，而应称作紫苏斜长花岗岩。如果原来是ＴＴＧ岩浆，则可能是因受表壳岩的混染所致。研究区内孔兹岩系的特征是堇青石片麻岩发育，沿韧性剪切带含堇青石的Ｓ－型花岗岩，以及作为熔融残留体的含堇青石麻粒岩也时有发现。三者的稀土元素分布型式表现出其间的成因关系。孔兹岩系中的夕线石多具近于平行（００１）的变形扭折带，在土贵乌拉还发现夕线石的（１１０）聚片双晶。这可能是夕线石双晶的首次发现。本文对夕线石作了详细测定。其＋２Ｖ＝２６°±０．５°．晶胞参数α＝β＝γ＝９０°，ａ＝７．５０，ｂ＝７．６５，ｃ＝５．８１，Ｖ＝３３０．０６３。

The Archaean rocks in junction region of Nei Mongoli and Shanxi and Hebei in the Central part of the northern margin granulite facies belt of North China Craton, can be divided into two series: granulite and khondalite series. The granulite series occurs in the southeastern part of the region,and consists of leucogranulite and dark granulite and a little supracrustal rocks. The khondalite series occurs in the northwestern part of the region, and includes the metasediments and S type granite. According to occurrence and the relationship of these two series, the southeastern series is older and the northwestern series is younger. But in fact there are some similar rocks in these two parts, only their difference is in kinds of dominant rocks. There are two rock kinds in granulites series: leucogranulite and mafic granulite. Most researchers believe that the lencogranulite is grey gniess. According to petrochemistry the leucogranulite is not typical grey gneiss but is enderbite, because it has much more FeO *+MgO content than the Archean TTG. In the light of mineral assemblage of Opx+Cpx the rock should be called enderbite. During emplacement the magma may be mixed with a lot of supracrustal rocks and early mafic rocks. This kind of rock should not be named gray gniess TTG. The khondalite series is characterized by containing cordierite gneisses, cordierite bearing S type granite,and cordierite bearing high Al granulite which is the residue of khondalite partial melting. The sillimanite with twinning was found in the high aluminium granulite at Togrog Ul. The mineral assemblage is garnet+sillimanite + cordierite with a minor spinel, rutile and ilmenite. Under microscope only in some grains the twinning lamella can be seen. In some other grains deformed kink band can seen. According to the optics, the powder X ray diffraction and the four circle diffractometer analysis the aluminum silicate mineral is sillimanite. Its cell parameters are as follows: α = β = γ =90°, a =7.50 , b = 7.65, V =330.06 3. On the cut section of nearly⊥ c axis we determined 2 V . The a verage of three times is=26±0.5°(+2 V ). We determined the twin plane is (110) in the light of the four circle diffraction and the oscillation method and Weissenberg photograph. The twinning axis is⊥(110). On the ⊥ c axis zero layer photograph indexed all diffraction spots are divisive, except 110, 220, 330, 440... It is enough to prove that those lamellae are sillimanite twinning. In some grains the deformed kink bands are nearly parallel to (001).