摘要
强烈的分离结晶作用会显著改变A型花岗岩的主要地球化学指标,模糊A型花岗岩与I、S型花岗岩之间的地球化学界限,如何准确判别成为一个难题,江西大乌山岩体提供了一个较为理想的研究实例。大乌山岩体位于华夏板块中部、南岭以北的赣中地区,由中粒黑云母二长花岗岩和细粒含白云母黑云母碱长花岗岩组成,锆石U-Pb定年结果显示,前者结晶年龄为156 Ma,后者为155~157 Ma。岩体具有低P2O5(0.05%~0.16%)、高SiO2(70.8%~74.9%)、高Rb/Sr(平均9.05)和Rb/Ba比值(平均2.38)的特征,弱过铝到强过铝质;Rb-Ba-Sr和Th/Nd-Th等元素含量关系特征显示高分异花岗岩的特点;较高的Ga/Al比值(2.89~3.48)和初始锆石饱和温度(839℃)以及填隙状黑云母所反映的贫水岩浆特征,均与A型花岗岩特性一致。高度的岩浆分异作用过程中,由于锆石、榍石等富锆矿物发生强烈分离结晶作用,结果使该岩体的(10000×Ga/Al)-(Zr+Nb+Ce+Y)以及Zr-SiO2两元相关演化趋势显著不同于分异的I、S型花岗岩;此外,该岩体的Y/Nb比值大于1.2。综合以上特征,可以判定大乌山岩体为高分异的A2型花岗岩。因此,在岩石学、矿物学和地球化学分析基础上,利用高分异的花岗岩在(10000×Ga/Al)-(Zr+Nb+Ce+Y)以及Zr-SiO2二元图解上所表现出的不同演化趋势是判别高分异花岗岩的成因类型比较有效的方法。该岩体具有较低的εNd(t)(-9.23^-14.6)和εHf(t)(-10.2^-6.5)值,两阶段Nd模式年龄为1.7~2.0 Ga,结合区域变质岩的资料,推断该岩体起源于类似于周潭群包含正-副变质岩的复合源区,在经历了印支期S型花岗质岩浆的抽离之后,在燕山早期拉张构造环境下,残留的较难熔的类似于闪长质的物质在上涌地幔物质加热的背景下进一步发生部分熔融而形成。
Strong magmatic differentiation by fractional crystallization could significantly modify the apparent geochemical features of A-type granites, resulting in an ambiguous boundary between A-type and I/S-type granites. Thus, distinguishing between these types of highly differentiated granites presents a significant challenge. The Dawushan granitic intrusive body in the central Jiangxi Province could be an illustrative case to demonstrate methods for discriminating a highly differentiated A-type granite. The Dawushan granite, composed of medium-grained biotite monzogranite(phase I) and fine-grained muscovite-bearing biotite alkali feldspar granite(phase II), is located in center of the Cathaysia Block and the northern Nanling Range. Zircon U-Pb dating shows that the formation ages of phase-I and-II are 156 Ma and 155~157 Ma respectively. The Dawushan granite is characterized by low P2O5(0.05%~0.16%), high SiO2(70.8%~74.9%) contents, high Rb/Sr(averaging 9.05) and Rb/Ba ratios(averaging 2.38), and is weakly to strongly peraluminous. Plots in the triangular diagram of Rb-Ba-Sr and bivariate diagram of Th/Nd-Th suggest that the Dawushan granite experienced a high degree of differentiation. Other characteristics, such as high Ga/Al ratio(2.89~3.48), zircon saturation temperature(839 ℃), and anhydrous feature indicated by interstitial biotite aggregation suggest that it is an A-type granitic intrusion. Considering strong fractional crystallization of minerals enriched in zirconium, such as zircon and sphene, the Dawushan granite shows a linear trend distinctively different from the I/S-type granite on the diagrams of 10000×Ga/Al vs. Zr+Nb+Ce+Y and Zr vs. SiO2. In addition, its Y/Nb ratios are greater than 1.2. Taken together, these characteristics suggest that the Dawushan granite belongs to a highly differentiated A2 granite rather than a differentiated I-or S-type granite. Therefore, on the basis of petrology, mineralogy and geochemistry analyses, the distinctive evolving trends shown on the diagrams of 10000×Ga/Al
作者
冯尚杰
陈卫锋
王洪作
凌洪飞
赵葵东
陈培荣
FENG Shangjie;CHEN Weifeng;WANG Hongzuo;LING Hongfei;ZHAO Kuidong;CHEN Peirongn(State Key Laboratory for Mineral Deposits Research,School of Earth Sciences and Engineering,Nanjing University,Nanjing,210023;CNNC Key Laboratory of Uranium Resources Exploration and Evaluation Technology,Beijing Research Institute of Uranium Geology,Beijing,100029;State Key Laboratory of Geological Processes and Mineral Resources,Faculty of Earth Resource,China University of Geosciences,Wuhan,430074)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2020年第4期1227-1247,共21页
Acta Geologica Sinica
基金
中国核工业地质局“高校铀矿地质科研项目”
国家重点研发计划“深地资源勘查开采”专项(编号2017YFC0602601)联合资助成果.
