白银地区细碧石英角斑岩地球化学特征——兼谈白银板块构造环境

白银地区细碧石英角斑岩具有高钠低钙、富水的特点。地球化学成分与世界其他地区细碧角斑岩基本接近,而明显地区别于正常玄武岩。在里特曼岩系指数图上,细碧岩是一条斜率较大、碱性程度较高的曲线;石英角斑岩是一条斜率较小、碱性程度较低的曲线。从各种岩石的氧化物含量、比值和化学图解看出,其原岩为钙—碱系列岛弧玄武岩。据微量元素含量和图解、稀土元素标准化模式和稀土比值也得出与岛弧大陆拉斑玄武岩一致的结论。因此,该区为弧—沟系敛合型的岛弧及其活动的大陆边缘。

The Geochemical Features of Spilite-keratophyre in Baiyin Area and the Plate Tectonic Setting

Central Baiyin Ore zone is tectonically located in the east part of the North Qinlian Caledonian eugeosyncline, which is a convergent residual arc subducting to the south. The magmatic activities in this region were of frenquent and here develops mainly a suite of submarine spilite-keratophyre formation, which can be divided into 4 volcanic cycles showing the basic-acidic-basic eruptive sequence. The spilite-keratophyre formation in chemcal composition, characteristic index and standard mineral contents similar to that of the other areas in the world, but in these characteritics shows difference from the normal basalt. The spilite showing as a curve with larger slope and higher alkalinity is plotted in the upper part of the Rittmann index diagram, while the keratophyre in this diagram is shown as a curve with smaller slope and lower alkalinity. The data of trace elements, oxide contents and their ratios in various rocks indicate that both spilite and keratophyre primarily may be arc basalt of calcic-alkaline series.The REE data for these rocks all show LREE abundance with no Eu depletion, which is coincident essentially with the normalized pattern.The material sources belong to the syntexis transformation crust.The main oxides, trace elements and the REE geochemical feartures mentioned above indicated that tectoic setting of this region may belong to a trough-convergent arc and active continental margin. Having study the syntexis transformative crust and the K-spilite, we suggested, that the volcanic rocks were driven from a relative deep source and formed in a submarine volcanic arc setting away from the trough.