华北地块南缘太古宙灰色片麻岩及其成因

ARCHEAN GRAY GNEISSES IN THE SOUTHERN MARGIN OF THE NORTH CHINA CRATON AND THEIR GENESIS

华北地块南缘太古宙登封花岗-绿岩地体和太华高级片麻岩地体中的英云闪长-奥氏花岗质和花岗闪长质灰色片麻岩多占据背斜核部或穹窿部位,它们是多期侵入的复合岩体。灰色片麻岩与表壳岩中的拉斑玄武岩构成双峰式岩套,属高Al_2O_3型,随岩浆演化,表现了Na_2O/K_2O,K/Rb比值逐渐降低,Rb/Sr比值升高的地球化学趋势,其LREE富集,HREE亏损U(Ce/Yb)_N=10—25]和Eu呈正异常。经模拟计算,登封灰色片麻岩是由石英榴辉岩约10—35％部分熔融形成,而石英榴辉岩系绿岩带中拉斑玄武岩的转变产物。太华灰色片麻岩可能由富含REE的母岩的部分熔融形成。

A large volume of gray gneisses of tonalitic to trondhjemitic composition exist in the Dengfeng granite-greenstone belt and the Taihua high-grade gneisses terrane, the southern margin of the North China Craton. The gray gneisses frequently occupy the cores of domes or antiforms. They are either diapirie intrusion being responsible for the domes or intrusives into the cores of antiformal structu- res which predate the emplacement. Field relationship in the gneisses shows that they are composite bodies, characterized by polyphase plutonism and strongly deformed. Contact relationship with country rocks are gradational in most cases. Besides, the ductile thrusts are found as boundaries of some plutons. The gray gneisses forming the the bimodal suite with tholeiite in the supracrustal rocks belong to high-Al_2O_3 type. As magma evolves they roughly display a geochemical trend Of reduction in Na_2O/K_2O and K/Rb ratios,and of increase in Rb/Sr ratios. The REE features are similar to that of Amitsoq B-type gneisses, west Greenland, exhibiting enrichment of LREE and depletion of HREE[(Ce/Yb)_N=10-25], and positive Eu anomaly. REE of the Taihua gray gneisses is one time higher than of the Dengfeng gray gneisses and the(Ce/Yb)_N ratio is also higher than the latter. The REE modeling to the gray gneisses indicates that the Dengfeng gray gneisses were produced by ca. 10-35％ partial melting of quaftz-eclogite which was transformed from tholeiite in the greenston belt, the Taihua gray gneisses may by from partial melting of the REE-rich rocks.