闽西南E?MORB型基性岩墙成因:来自地球化学、锆石U?Pb年代学及Sr?Nd同位素证据

闽西南地区发育富集洋脊玄武岩（E-MORB）地球化学特征的基性岩墙，这对研究晚中生代中国东南部的构造岩浆作用具有重要指示意义.利用岩石学、锆石U-Pb年代学、元素地球化学、同位素地球化学等方法对早白垩世闽西南基性岩墙进行研究，岩墙以辉绿岩和角闪辉长辉绿岩为主，属于中-低钾岩石系列，Mg#值为55.80~66.38.锆石U-Pb年龄为117.4±3.8 Ma，为早白垩世晚期岩浆活动的产物.样品富集Rb、Ba、U、K、LREE等元素，无明显Nb、Ta、Ti亏损，显示出E-MORB的地球化学特征；（87Sr/86Sr）_i=0.706 50~0.710 19、ε_Nd（t）=-0.9~4.0，同位素Sr中等富集、Nd弱亏损.成岩过程有少量橄榄石和单斜辉石的分离结晶作用，无明显地壳混染作用.由于太平洋板块受南岭E-W向巨厚岩石圈的阻碍，导致板片下插速率与邻区产生差异，局部撕裂形成板片窗，软流圈地幔物质沿“窗口”上涌并卷裹起板片上的海洋沉积物，在上升中发生交代作用形成具有E-MORB特征的地幔岩.在早白垩世晚期的大陆拉张-陆内初始裂谷背景下，伴随软流圈上涌富集地幔岩发生部分熔融，形成的基性岩浆上侵形成了闽西南基性岩墙. 

Genesis of E-MORB-Like Mafic Dykes in Southwestern Fujian Province,SE China: Evidence from Geochemistry,Zircon U-Pb Geochronology and Sr-Nd Isotope

E-MORB-like mafic dykes are exposed in Southwest Fujian Province and record key information of tectonic-magmatism in Southeast China during Late Cenozoic. A comprehensive research of petrology, zircon U-Pb dating, elemental geochemistry and isotope geochemistry was carried out. Mafic dykes are composed of dolerite and hornblende gabbro dolerite, and possess middle-low potassic features, with the Mg# values range from 55.80 to 66.38. Zircon U-Pb dating yield an age of 117.4±3.8 Ma, indicating that mafic dykes were emplaced at end of Early Cretaceous. Dykes enrich in Rb, Ba, U, K and LREE, without obvious depletion of Nb, Ta and Ti, which is consist with the E-MORB geochemical affinities. Samples have positive ε_Nd(t) (-0.9 to 4.0), and (87Sr/86Sr)_i values range from 0.706 50 to 0.710 19. Geochemical compositions show that the olivine and clinopyroxene fractionation have occurred and crustal contamination did not played an important role during the emplacement. We propose a slab window model to interpret the formation of mafic dykes. Subduction speed of Pacific plate beneath Nanling area was decreased by the overlying thickened lithosphere, which lead to the subduction velocity of the plate to be different from that of the adjacent area, and resulted in the formation of slab windows. Asthenospheric material could rise through slab windows, and carried and interacted with oceanic sediments, forming the E-MORB-like mantle rocks. With the upwelling of asthenosphere, E-MORB-like mantle materials will undergo partially melt and the resulting melts will emplace to form mafic dykes with a continental extensional-intracontinental rift setting at end of Early Cretaceous.