闽西晚中生代基性岩脉的地球化学研究

Geochemistry of the late Mesozoic mafic dikes in western Fujian Province

对闽西三个地区基性岩脉的地球化学研究发现,该区域的基性岩脉为亚碱性岩石系列,具有高Al、低Ti、Na2O>K2O的特征;相对富集大离子亲石元素,亏损高强场元素。半村辉绿岩脉和拔里角闪辉长岩脉以Nb、Ta、Ti负异常为特征,林子坟辉长辉绿岩脉具有Nb、Ta负异常和Ti弱正异常。半村和拔里的岩脉稀土元素总量高,明显富集轻稀土元素,(La/Yb)N=6.8～8.4;林子坟的岩脉稀土元素总量相对较低,轻稀土元素弱富集,(La/Yb)N=2.0。结合基性岩脉的地球化学特征和区域构造演化分析,初步认为半村和拔里的基性岩脉来自与俯冲作用有关的富大离子亲石元素流体交代的富集地幔;林子坟基性岩脉是软流圈地幔部分熔融的产物,地壳混染作用是其相对富集大离子亲石元素的原因。结合玄武岩构造环境判别图解可知,半村和拔里岩脉具有的大陆边缘弧的特征并不指示基性岩脉形成的构造环境,而是说明其源区受到俯冲板片派生出来的流体交代作用。半村和拔里岩脉形成于大陆板内拉张带,林子坟岩脉形成于板内裂谷环境。与华南其他地区的基性岩脉对比表明,虽然晚中生代华南地区处于拉张的构造背景下,但是岩脉形成的构造环境与微量元素地球化学特征均有明显的差异,反映了各地区构造环境演变和地幔演化的复杂性。

Mafic dikes are a special kind of rock series represented the pull apart role within the continental plate. It includes abundant information of mantle and is recognized as the time scale of the important tectonic conversion.From Jurassic to Cretaceous the tectonic stress took place a great conversion from extrusion to pull apart, accompanying with the intrusion of I type and type granites and the eruption of bimodal volcanic rocks and the formation of a great amount of mafic dikes in southern China. By the studies on mafic dikes in western Fujian, this paper discusses the diagenetic method and process and the source feature of mafic dikes in term of the petrology and geochemistry combined with the geotectonic background. The geochemical studies show that the late Mesozoic mafic dikes in western Fujian Province belong to subalkaline rock series with the characteristics of high Al, low Ti and Na2O > K2O. Compared to MORB, the mafic dikes are enriched in LILE and depleted in HFSE. The Bancun diabase dike and Bali bojite dike are characterized by the negative anomalies of Nb, Ta and Ti, whereas the Linzifen diabase dike shows the negative anomaly of Nb Ta and slightly positive anomaly of Ti. The ∑REE contents of the Bancun and Bali mafic dikes, with the obvious enrichment of LREE [(La/Yb)N=6.8～8.4], is higher than that of the Linzifen with the slight enrichment of LREE [(La/Yb)N=2.0]. The tectonic discriminant diagrams of Hf/3 Th Ta and Th/Zr Nb/Zr show that the Bancunand Bali mafic dikes might be formed in the continental pull apart belt, whereas the Lizhifen mafic dike might be formed in the continental rift or epicontinental rift. Based on above geochemical evidence and regional geotectonic evolution, it is indicated that the Bancun and Bali mafic dikes might be derived from subduction related metasomatic enriched mantle, however, the Linzifen mafic dike might be the partial melting product of the asthenosphere and the crustal contamination resulted in the enrichment of LILE. Combined with the basalt tectonic discrimination diagrams, the Bancun and Bali mafic dikes, with features of continent marginal arc magmatism, possess no meaning for identification of their tectonic environment, but imply the metasomatism of their source region caused by fluids derived from subducting plate. Therefore, the Bancun and Bali dikes are recognized as products formed in the pull apart belt with in continental plate; while the Linzifen dike in with in plate rift. In comparison with mafic dikes in other areas of South China, an apparent variation is found in the tectonicenvironment and trace element geochemistry of mafic dikes, although they were all formed at a same extensional tectonic environment in South China during late Mesozoic. It reflects the complexities of the tectonic environment evolution andmantle composition in various areas of South China.