Geochemical characteristics of whole rock and minerals from the Late Cretaceous granitic rocks of the Rokko Mountains
Geochemical characteristics of whole rock and minerals from the Late Cretaceous granitic rocks of the Rokko Mountains
复制标题
六甲山晚白垩世花岗岩全岩及矿物地球化学特征
DOI：
10.2465/jmps.041120
复制
发表时间：
2007
影响因子：
0.7
通讯作者：
Toshio Arai
中科院分区：
文献类型：
--
作者：
Toshio Arai
六甲山脉主要由晚白垩世花岗岩组成。这些被认为是布引花岗闪长岩、八幡谷花岗岩、土桥石英闪长岩和六甲花岗岩。根据粒度和结构，布引花岗闪长岩可细分为斑状岩相和等粒状相，而垂直分区的六甲花岗岩岩体可分为细粒相、中粗粒相和显微相。细粒相出现在高海拔地区，中粗粒花岗岩出现在六甲山脉内的中低海拔地区。显微相侵入这两个品种。对花岗岩中的痕量稀土元素和矿物成分进行了分析。与中粗粒相相比，六甲花岗岩的细粒和显微相具有较高的 Rb、Nb 和 Y，以及较低的 Sr 和 Zr，并且在球粒陨石归一化 REE 模式中具有强烈的负 Eu 异常。在六甲花岗岩中，Rb 含量随着 Sr 的增加而降低。全岩化学表明，六甲花岗岩岩浆由于斜长石晶体分异而形成了分区岩浆室。主量元素的最小二乘质量平衡计算结果与Eu的分析数据非常吻合。六甲花岗质岩浆在岩浆室的顶部富含挥发性成分SiO2和K2O，而底部的化学成分则较为中间。细粒相An含量较低，细粒、显微相黑云母富Fe、贫镁。这些化学特征表明，这两个相是由比中粗粒相更加分化的岩浆形成的。
The Rokko Mountains consist primarily of late Cretaceous granitic rocks. These are recognized as Nunobiki Granodiorite, Yahatadani Granite, Dobashi Quartz Diorite, and Rokko Granite. Based on grain size and texture, the Nunobiki Granodiorite is subdivided into a porphyritic and equigranular facies, whereas the vertically zoned Rokko Granite pluton is differentiated into fine-grained, medium -to coarse-grained, and micrographic facies. The fine-grained facies occurs at high altitude, and the medium -to coarse-grained granite is found at an intermediate to low altitude within the Rokko Mountains. The micrographic facies intrudes both of these varieties. The granitic rocks were analyzed for trace and rare earth elements and mineral composition. The fine and micrographic facies of the Rokko Granite have higher Rb, Nb, and Y, and lower Sr and Zr when compared with the medium -to coarse-grained facies equivalents and have strong negative Eu anomalies in chondrite normalized REE patterns. In the Rokko Granite the Rb content decreases with an increase in Sr. Whole-rock chemistry indicates that the Rokko granitic magma formed a zoned magma chamber due to plagioclase crystal fractionation. Results of the least-square mass balance calculation for the major elements are in good agreement with the analysis data for Eu. The Rokko granitic magma was rich in the volatile components, SiO2, and K2O at the top of the magma chamber, whereas the base was more intermediate in chemical composition. The fine-grained facies has a low An content, and biotite in the fine and micrographic facies is rich in Fe and poor in Mg. These chemical characteristics indicate that two facies are formed from a more differentiated magma than the medium -to coarse-grained facies.