福建永泰云山碱性流纹岩的厘定及其地质意义  

Ascertainment of the Alkaline Rhyolites at Yunshan Mountain, Yongtai County, Fujian Province and Its Geological Significance

作　　者：邱检生[1] 王德滋[2] 周金城[2] 王汝成[1] 

机构地区：[1]南京大学成矿作用国家重点实验室,210093 [2]南京大学地球科学系,210093

出　　处：《地质论评》2000年第5期520-529,共10页Geological Review

基　　金：高等学校博士学科点专项科研基金;国家自然科学基金 (编号49632080)

摘　　要：福建永泰云山破火山石帽山群第三旋回顶部的霏细流纹岩含有霓石、钠铁闪石等碱性铁镁矿物。化学成分富硅、富碱,碱性指数(AKI值)大于0.90,Rb、Th、Nb、Zr、Ga等元素的含量高,Ga/Al值大;贫钙、镁、铝和过渡族元素,轻、重稀土比值较小,并具有显著的负铕异常。在K_2O-Na_2O及以Ga/Al值为基础的多种判别图上均投影在A型花岗岩区。上述特征与其邻近的魁歧典型碱性花岗岩相似,也可与国外有关典型碱性流纹岩相对比,据此将该层流纹岩厘定为碱性流纹岩。这一流纹岩与魁歧碱性花岗岩均形成于拉张的构造环境,两者的Nd同位素组成十分接近,ε_(Nd)(t)值分别为—3.62～—4.29和—3.45～—5.21,显示它们具有相同的岩浆源区,均为壳幔混源岩浆作用的产物。The Yunshan Mountain located in Yongtai County, Fujian Province is a late-Mesozoic caldera composed of rhyolitic lava, pyroclastic rocks and pyroclastic sedimentary rocks of the third cycle of the Shimaoshan Group. Detailed mineralogical, petrological and geochemical investigations have revealed that the felsitic rhyolites outcropping at the upper part of this cycle contain alkaline mafic minerals, such as aegirine and arfvedsonite. Chemically, they are rich in silica, alkali, with alkaline indexes (i. e. , AKI values) larger than 0. 90, and have relatively high contents of Rb, Th, Nb, Zr and Ga as well as high Ga/Al ratios (104×Ga/Al = 3. 24- 3. 74), but poor in calcium, magnesium, aluminum and siderophile elements. They also have low LREE/HREE ratios and display obvious negative europium anomalies (δEu= 0. 08-0. 10). On the K2O-Na2O diagrams and those discrimination diagrams based on Ga/Al ratios, they are all plotted into the A-type granite district. The above features are similar to those of the adjacent typical alkaline granites in Kuiqi, and are also comparable to those of the typical alkaline rhyolites abroad, thus it is concluded that these rhyolites can be ascribed to alkaline rhyolites. Investigation results also show that both the Yunshan alkaline rhyolites and the Kuiqi alkaline granites were all formed in a tensile tectonic environment, and have quite close εNd(t) values which range from -3. 62 to - 4. 29 and -3. 45 to -5. 21 respectively. These data suggest that they have the same magma source, both were originated from a crust-mantle mixing magma. The ascertainment of these alkaline rhyolites first proves that there exist alkaline rhyolites which have the same source with the alkaline granites in the coastal area of Zhejiang and Fujian Provinces. Besides, it also adds a new rock type to the late Yanshanian extensional rock association in this area. Therefore, it has important significance in further understanding the late-Mesozoic litho-spheric magmatism in that district.

关 键 词：碱性流纹岩 地球化学 地质意义 福建 厘定 

分 类 号：P588.1[天文地球—岩石学]