江西中西部地区白垩纪玄武质岩石的^(40)Ar/^(39)Ar年代学和地球化学研究

^(40)Ar/^(39)Ar geochronology and geochemistry of Cretaceous basaltic rocks for the central and northwestern Jiangxi Province

对江西中西部两处中生代玄武质岩石进行了40Ar/39Ar年代学和元素-同位素地球化学研究,赣西北上高和赣中螺丝山地区玄武质岩石分别给出了137Ma和90Ma的全岩40Ar/39Ar年龄。赣西北上高为亚碱性的玄武安山岩,而赣中螺丝山为碱性玄武岩/粗玄岩,两者均以富集LREE和LILE、无明显HFSE亏损为特征。前者初始87Sr/86Sr=0.70492～0.70575,εNd(t)=0.61～3.36,落在Samoa-OIB同位素组成范围;后者初始87Sr/86Sr=0.70323～0.70350,εNd(t)=6.19～6.93,与Hawaii-OIB的Sr-Nd同位素组成相似。与赣南早中生代(约173Ma)EMⅡ型玄武岩一起暗示该地区中生代岩浆源区有着自岩石圈向软流圈属性演化的时序迁移规律,表明该区自早中生代以来处于软流圈上隆的岩石圈伸展构造背景。

Ar/39Ar geochronological, elemental and SrNd isotopic results of the Cretaceous basaltic rocks are presented from the central and northwestern Jiangxi Province. A typical basaltic sample (01JF76) from Shanggao, northwestern Jiangxi Province yields a 40Ar/39Ar plateau age of (137.0±0.1) Ma, and a basalt from Luosishan, central Jiangxi Province gives a 40Ar/39Ar plateau age of (90.2±0.3) Ma. The volcanic rocks from Shanggao area belong to subalkaline basaltic andesites while those from Luosishan area to alkaline basalts. All samples exhibit a REEand LILEenriched, and insignificant HFSEdepleted pattern. The former has 87Sr/86Sr=0.704 92～0.705 75, and εNd(t)=0.61～3.36, similar to those of SamoaOIB. The latter exhibits 87Sr/86Sr=0.703 23～0.703 50, and εNd(t)=6.19～6.93, analogous to HawaiiOIB without significant crustal contamination. These geochemical characteristics, combined with those of the Early Mesozoic (～173 Ma) basaltic rocks from southern Jiangxi Province suggest a migrated evolution of magma source from EMⅡto OIB during 173～90 Ma. This provides further evidence for lithospheric extension/thinning in response to an upwelling asthenospheric mantle in the area during Mesozoic.