江山-广丰地区早白垩世晚期玄武岩的岩石地球化学及其构造意义

Geochemistry of basalts of late period of Early Cretaceous from Jiangshan-Guangfeng, SE China and its tectonic significance

江山-广丰地区所在的信江红色盆地与金衢红色盆地同属于NEE向赣杭断陷盆地带的组成部分。赣杭构造带自早白垩世晚期开始强烈拉张,红色盆地形成。红层下部夹有层状玄武岩,全岩K-Ar年龄为98～105Ma,属早白垩世晚期。岩石地球化学特征表明研究区有两类玄武岩:江山陈塘坞一带的拉斑玄武岩,礼贤、广丰一带的碱性橄榄玄武岩。Nd、Sr同位素组成分析显示,拉斑玄武岩类(87Sr/86Sr)i值为0.7054～0.7070,εNd(t)值为1.69～0.70(接近于0),表明源区为基本未受壳源物质混染的原始地幔,在弧后陆缘环境下快速拉张而上升,呈熔岩被形式分布于信江盆地东部;碱性橄榄玄武岩(87Sr/86Sr)i值为0.7076～0.7080,而εNd(t)值偏低(-6.43～-5.18),表明来源于已受壳源物质混染的富集地幔,在拉张加剧、裂陷加深的构造背景下喷发,相对集中分布于盆地中部,范围小于拉斑玄武岩。二者的差别主要是地幔源区受到不均匀混染所致。研究表明,早白垩世晚期至晚白垩世期间,太平洋板块俯冲的角度由缓变陡以及向东南方向的后退作用,对弧后陆缘的快速拉张并形成赣杭带红色断陷盆地及其玄武岩起着关键性作用。

The Xinjiang basin and JinhuaQuzhou basin, originating from late period of Early Cretaceous, are all the components of the NEEtrend Ganhang Fault Basin Zone of the Southeast China, and the JiangshanGuangfeng area is located in east end of the Xinjiang basin. Erupted basalts in this area have 98～105 Ma KAr isotopic age, and their geochemical characteristics show that these basalts are divided into two types: tholeiite and alkaline olivine basalt. The NdSr isotopic features show that all of these basalts have the trend of forming from enriched mantle source, and that the mantle nonequality mixing with the crustal materials has caused the difference. The tholeiites from Chentangwu area, Jiangshan City, have medium 87Sr/86Sr ratio (0.705 4～0.707 0) and lower εNd(t) (1.69～0.70), indicating that the magma of these basalts came from an unchanged mantle, they spreaded large area of the basin by extending on the continental margin. Those alkaline olivine basalts from Lixian Village and Guangfeng Town have relative high 87Sr/86Sr ratio (0.707 6～0.708 0) and relatively lower value of εNd(t) (-6.43～-5.18), indicating that these magma came from an enriched mantle by mixing the subducted crustal material, and they erupted and covered middle part of the basin. The results show that the formation and the evolution of the basalts of the Ganhang Fault Basin are intimately related with the subduction of the Pacific Plate during late period of Early Cretaceous to Late Cretaceous. The subduction angle of Pacific plate increased from a very low angle to a median angle and the subductive boundary migrated oceanward to the southeast, resulting in the extension of the SE China continental margin and all the phenomenons above.