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赣中地区晚中生代高分异A型花岗岩的厘定及其成因研究
投稿时间:2019-07-18  修订日期:2019-11-15  点此下载全文
引用本文:
DOI:10.19762/j.cnki.dizhixuebao.2020105
摘要点击次数: 47
全文下载次数: 59
作者单位E-mail
冯尚杰 内生金属矿床成矿机制研究国家重点实验室 fengshangjie3@163.com 
陈卫锋 内生金属矿床成矿机制研究国家重点实验室,南京大学地球科学与工程学院  
王洪作 中核集团铀资源勘查与评价技术重点实验室核工业北京地质研究院  
凌洪飞 内生金属矿床成矿机制研究国家重点实验室  
赵葵东 内生金属矿床成矿机制研究国家重点实验室,南京大学地球科学与工程学院  
陈培荣 地质过程与矿产资源国家重点实验室中国地质大学(武汉)资源学院  
基金项目:中国核工业地质局“高校铀矿地质科研项目”和国家重点研发计划“深地资源勘查开采”专项(编号 2017YFC0602601)
中文摘要: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(70.8~70.8 %)、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、榍石等富锆矿物的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。
中文关键词:化学和Sr-Nd-Hf同位素成分,岩浆分异,A型花岗岩,大乌山花岗岩体,赣中地区
 
Identification and petrogenesis of a highly differentiated A-type granite formed in the late Mesozoic, central Jiangxi ProvinceFENG Shang-Jie1*,WANG Hong-Zuo2,1,CHEN Wei-Feng, ZHAO Kui-Dong3,CHEN Pei-Rong1,LING Hong-Fei1*
Author NameAffiliationE-mail
Feng Shangjie State Key Laboratory for Mineral Deposits Research fengshangjie3@163.com 
Chen Weifeng State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University  
Wang Hongzuo CNNC Key Laboratory of Uranium Resources Exploration and Evaluation Technology, Beijing Research Institute of Uranium Geology  
Ling Hong-Fei State Key Laboratory for Mineral Deposits Research  
Zhao Kuidong State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Resource, China University of Geosciences  
CHEN Peirong State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University  
Abstract:Strong magmatic differentiation by fractional crystallization could significantly modify apparent geochemical features of A-type granites, resulting in ambiguous boundary between A-type and I/S-type granites. Thus to distinguish these types of highly differentiated granites is a significant challenge. The Dawushan granitic intrusive body in the central Jiangxi Province could be an exemplificative case to demonstrate the methods for discriminating a highly differentiated A-type granite. The Dawushan granite, composed of medium-grained biotite monzo-granite (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 are156 Ma and 155~157 Ma, respectively. The Dawushan granite is characterized by low P2O5 (0.05~0.16%), high SiO2 (70.8~76.4 %) contents, high Rb/Sr (averaging 9.05) and Rb/Ba ratios (averaging 2.38), and weakly to strongly peraluminous feature. Plots in triangular diagram of Rb-Ba-Sr and bivariate diagram of Th/Nd-Th suggest the Dawushan granite experienced a highly differentiation process. 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 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 analyses of petrology, mineralogy and geochemistry, the distinctive evolving trends shown on the diagrams of 10000*Ga/Al vs. Zr+Nb+Ce+Y and Zr vs. SiO2 by I-&S-type and A-type granites can be effectively used for recovering and discriminating different primary rock types of highly fractionated granites. Isotopically, the Dawushan granite exhibits low εNd(t) (-9.23~-14.6) and εHf(t) values (-10.2~-6.50), and old two-stage Nd model ages of 1.7~2.0 Ga. Combined with the metamorphic rocks outcropped in this region, the Dawushan granite could be originated from a composite source consisting of orthometamorphic and parametamorphic rocks similar to the Zhoutan Group. In particular, such a source experienced an S-type melting extraction event during the Indosinian Period, leaving relatively refractory materials similar to diorite in composition. In the Yanshanian Period, the second melting event occurred in the residual source at higher temperatures with heat from the upwelling hot asthenospheric mantle under regional extension, forming the Dawushan A-type granite.
keywords:Chemical and Sr-Nd-Hf isotopic compositions, magmatic differentiation, A-type granite, Dawushan granite, central Jiangxi Province
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