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秦岭地区老君山及熊耳岭花岗岩体,经我们测得Sr87/Sr86初始比值为0 7037—0.7059和0.7054,δO~18为7.81—9.25和7.00若按国内外花岗岩分类,自然归属于I型、磁铁矿系列和同熔型花岗岩范畴。而实际上应归属于S型、钛铁矿系列和改造型花岗岩范畴,如何解释这种“反常”现象呢?笔者认为这是由秦岭地壳组成特点决定的。秦岭地壳具有盖层和基底,较老的地层多为古洋壳。基底及古洋壳地层的岩石多数来自上地幔,这些岩石经交代、改造、重熔等形式生成的花岗岩,自然稳定同位素值具有低—偏中等值的特点。秦岭地区有八期花岗岩,是我国花岗岩广泛发育地区之一,大的花岗岩基,从理论上讲不可能来自上地幔,秦岭地区属大陆内部地壳厚度大,基底和古洋壳的物源多来自地幔,由此形成的壳源花岗岩,具有秦岭的特色,与华南地区壳源型花岗岩有所不同, 花岗岩的成因是个复杂问题,尚待深入研究。
The Laoshanju and Xiong’erling granite bodies in the Qinling Mountains show that the initial ratios of Sr87 / Sr86 are 0 7037-0.7059 and 0.7054, the δO ~ 18 are 7.81-9.25 and 7.00, respectively. If they are classified according to granite at home and abroad, they are naturally assigned to type I and magnetic Iron ore series and the same melting granite category. In fact, it belongs to the S-type, ilmenite series and remolded granite categories. How to explain this “anomaly” phenomenon? I believe this is determined by the characteristics of the Qinling crust. Qinling crust has a cover and basement, the older strata mostly paleo-oceanic crust. Most of the rocks in the basement and paleo-oceanic crust are from the upper mantle. The granite formed by the metasomatism, alteration and remelting of these rocks has the features of low-moderate middle value. There are eight stages of granite in the Qinling area. It is one of the granite widely developed areas in China. Large granitic basement is theoretically impossible to come from the upper mantle. The Qinling area belongs to the continental inner crust with a large thickness. The basement and paleo-oceanic crust come from Mantle, and the crust-derived granite thus formed has the characteristics of the Qinling Mountains, which is different from the crust-derived granite in southern China. The cause of granite is a complex issue yet to be further studied.