湘东北中生代基性岩脉微量元素地球化学特征及岩石成因

湘东北中生代基性岩脉可以分为煌斑岩和辉绿岩两类 ,前者形成于 136 .6 1Ma ,后者形成于 86 .18Ma ,与华南中生代主要拉张时期相对应。岩石富集LREE ,Eu负异常不明显 ,其形成主要受地幔部分熔融作用制约。早期煌斑岩类微量元素和Sr、Nd同位素总体上具有富集地幔 (EMⅡ型 )洋岛玄武岩 (OIB)特征 ,富集Nd、P、Cs,而K、Rb、Sr、U、Th等富集程度不明显 ,Ta、Nb略有富集 ,具有软流圈地幔上涌地幔热柱玄武岩岩浆源区性质 ,表现出软流圈地幔上涌部分熔融的成岩特点。晚期辉绿岩类表现出Ta、Nb、Ti亏损 ,但LILE并不富集 ,反映地壳混染程度的增强 ,具有大陆拉张带 (裂谷初期 )形成的玄武岩岩浆源区性质 ,为岩石圈地幔部分熔融形成。二者具有不同的岩浆源区性质 ,从早期到晚期岩浆源区向上迁移并致使部分陆壳物质混入 ,反映由热点式拉张到岩石圈伸展 减薄的作用过程。

TRACE ELEMENT GEOCHEMICAL CHARACTERISTICS AND GENESIS OF MESOZOIC MAFIC DIKES IN NORTHEAST HUNAN PROVINCE

Mesozoic mafic dikes occurring in Northeast Hunan Province can be divided into two rock types. One is diabase, which was formed at 86.18Ma, and the other is lamprophyre, which was formed at 136.61Ma.The chondrite-normalized REE patterns are characterized by LREE enrichment, with negative Eu anomalies. The La/Sm-La correlation shows that the mafic dikes were controlled by partial melting. The early lamprophyres are enriched in LILE, Nd, P, Cs and Ta, Nb, but less enriched in K, Rb, Sr, U and Th. The rocks are depleted in HFSE, which have the general characteristics of EMⅡ and OIB, indicating a basaltic magmatic source of mantle plume. The late diabases are depleted in Ta, Nb and Ti as well as in LILE. Ta, Nb depletion indicated the continental matter hydridization, demonstrating a basaltic magmatic source of continental extension belt (early rift). The lamprophyres and diabases have different magmatic sources. From early to late, the magmatic source migrated upward and made continental matter mixed, implicating a process of asthenosphere mantle uplift from early hot point to late lithospheric extension-thinning.