东南大陆中生代玄武岩岩石系列及其构造意义

东南大陆中生代玄武岩和玄武安山岩占区内酸性岩浆作用的5-10％,并为钙碱系列和过渡系列。其矿物组合为斜长石+单斜辉石+角闪石±黑云母+透长石,玄武岩因LIL元素(K、Rb、Th、Ta、Ce)含量高,负Nb异常以及Fe、Mg、Cr、Ni、Co、Sc含量低而不同于产在活动大陆边缘环境的同类岩石,其LIL元素(K、Rb、Ce、Ba)与本区新生代玄武岩相似,但Fe、Mg、Cr、Ni、Co、Sc含量明显偏低。成分分带与酸性岩浆作用的分带性一致,都受壳、幔成分和构造等地质背景控制,产出构造环境为相对稳定的板内区。玄武质岩浆主要由酸性岩浆作用提供热源,地壳发生广泛熔融。从中生代至新生代,由下地壳至上地慢,由钙碱性和过渡系列至碱性系列,是扩张作用逐渐增强的结果。

CHARACTERISTICES OF MESOZOIC BASALTS AND THIER TECTONIC SIGNIFICANCE OF SOUTHEAST CHINA CONTINENTAL MARGIN

The paper deals with petrology and geochemistry (trace elements and REE) of calc-alkaline and trasnional basalts or basaltic andesties (SiO2=46-53 wt%) represented by a total of 20 speciments from the southeast China continental margin . Mineral assemblage of basalts is plagioclase + clinopyroxene + honblende + biotits + sanidine. The Mesozoic basalts in the area differ from those in the active continental margin setting in their increased abundance of some LIL-elements (K, Rb Th, Ta, Ce ) as well as negtive Nb anomaly and low abundance of most Fe-Mg elements (Cr, Ni, Co, Sc). Abundance of some LIL-elements (K, Rb , Ce, Ba) of the Mesozic basalts approximats to those of the Cenozoic alkaline basalts in the are, but abundance of Fe, Mg , Cr, Ni, Co, Sc of the Mesozoic basalts is clearly on the low side. Their EREE are high and are moderately fractionated. In the space, their chemical zoning is consistent with that of the silicic magmatism, which are controlled by the deep geological setting (e,g, compositions and structures of crust and mantle). Their source regions might be the high-speed layer of longitudinal wave in the lower curst. In some Zr, Ti, Y and their ratio relatively discrimination diagrams implicating tectonic setting , the plots of the Mesozic basalts lie in the WPB field of the stable tectonic movement.The Mesozoic basaltic magma provided the heat source to for silicic magmatism, causing extensive melting of crust, while itself was arrested in the depth of crust due to the resisting of plastic crust. Increased proportion of basic megma in later magmatism resulted to fromation of bimodal volcanic rocks, from the Mesozoic to Cenozoic era, because of gradually upwelling of mantle and progressively extending of crust, source regions of basalts were transferred from the lower crust to upper mantle, and the rock assmblege was from calc-alkaline and trasitional series through alkaline series to peralkaline series.