江西会昌岽背玄武岩地球化学和Sr-Nd同位素特征及其地质意义

摘要

江西会昌岽背地区晚中生代玄武岩的SiO_2含量为49.06%～50.14%,K_2O含量为1.26%～1.63%.其微量元素显示出大离子亲石元素富集,高场强元素亏损,尤以明显的Nb、Ta亏损为特征;REE总量为(135.5～146.8)×10~(-6),(La/Yb)_N≈10,表现出LREE富集的稀土元素配分模式,且无明显的Eu异常;Sr-Nd同位素具有高(~(87)Sr/~(86)Sr)i(0.70678～0.70695)和低εNd(t)(-2.46～-2.18)的特征.据以上特征并结合区域地质背景,反映出岽背玄武岩的源区可能为EMⅡ型富集型大陆岩石圈地幔,其形成的构造环境为活动大陆边缘.较低的Nb/U比值(7～8)暗示该玄武质岩浆在形成过程中遭受了一定程度的大陆地壳混染或与俯冲过程有关的物质的加入.岽背玄武岩具有陆缘弧型地球化学特征,指示会昌地区在晚白垩世晚期(约85 Ma)处于活动大陆边缘的构造环境,该玄武岩的形成可能与燕山晚期古太平洋板块向欧亚大陆的俯冲消减作用关系密切.底侵作用可能为会昌地区燕山晚期成矿岩浆的生成提供了流体和热,有利于铜多金属矿的形成.%The Late Mesozoic Dongbei basalts, located in Huichang area of Jiangxi Province, have SiO_2 contents of 49.06% ～50.14% and K_2O 1.26% ～1.63%. They are rich in large-ion lithosphile dements and poor in high field strength elements, and have ∑REE of (135.5～146.8) × 10~(-6), (La/Yb)_N of ca.10 and LREE enrich-ment, with strong negative Nb and Ta anomalies and no obvious Eu anomalies in the MORB-normalized spider diagram. The Sr-Nd isotopic compositions show that Dongbei basalts have high initial ~(87)Sr/~(86)Sr ratios of 0.70678～0.70695 and low εNd(t) of -2.46～ -2.18. These geochemical characteristics indicate that the Dongbei basalts might have been derived from the source of EM Ⅱ -type lithospheric mantle under the tectonic setting of an active continental margin. The low values of Nb/U (7～8) suggest that the basalts probably expe-rienced the addition of some subduction-related materials and/or a certain degree of crust contamination. The characteristics of the Dongbei basalts are similar to those of the basalts with the tectonic settings of an active con-tinent margin, indicating that Huichang area lays in an active epicontinental environment during the late stage of Late Cretaceous (ca. 85 Ma). The Dongbei basalts might have had an intimate contact with the paleo-Pacific plate subduction underneath the Eurasia plate in the late Yanshanian period. The basaltic underplating may have provided fluid components and heat for the metallogenic magma of Huichang area during the late Yanshanian pe-riod, which contributed to the formation of copper deposits.