河南中部太古宙登封花岗—绿岩地体中灰色片麻岩地球化学特征及其成因

CHEMISTRY AND GENESIS OF GRAY GNEISSES IN THE DENGFENG GRANITE-GREENSTONE TERRANE CENTRAL-HENAN

河南中部太古宙花岗-绿岩地体中广泛发育英云闪长-奥长花岗质和花岗闪长质灰色片麻岩。与绿岩带中拉斑玄武岩构成双模式岩套的灰色片麻岩属太古宙常见的高Al_2O_3型片麻岩。随岩浆演化,它们显示了K/Rb、Na_2O/K_2O降低和Rb/Sr增高的地球化学趋热。灰色片麻岩的REE特征类似于西格陵兰阿米佐克B型片麻岩,表现了LREE富集、HREE亏损和正Eu异常的稀土分配模型并说明有石榴石作为残留相。经REE模拟计算表明,本区灰色片麻岩由石英榴辉岩经10—35%的部分熔融而成,石英榴辉岩是绿岩带中拉斑玄武岩在地幔深度的转变产物。

Gray gneisses of tonalitic to trondhjemitic and granodioritic composition are widely developed in the Dengfeng Archaean granite-greenstone terrane, forming a bi-model suite with the tholeiite in the greenstone belt.The gray gneisses belong to high-Al_2O_3 type. As the magma evolved, Na_2O/K_2O and K/Rb ratios were decreasing and the Rb/Sr ratio was increasing. The REE patterns are similar to those of Amitzoq B-type gneisses, 'West Greenland, exhibiting an enrichment of LREE and depletion of HREE, (Ce/Yb)_(cn) = 10—25, positive Eu anomaly. Garnets must have been the residual phases in the process of the magma formation. The REE modelling of the gray gneisses indicates, that the Dengfeng gray gneisses were produced by ca, 10—35% partial melting of quartz eclogite which was transformed from tholliete in the Dengfeng greenstone belt at depth of the mantle.