广西大瑶山地区大进早古生代高分异A型花岗岩的厘定及成因  

Identification and Petrogenesis of Dajin Highly Fractionated A-type Granites Formed at Early Paleozoic in Dayaoshan Area of Guangxi, China

作　　者：贾小辉[1,2] 王晓地[1,2] 杨文强[1] JIA Xiao-hui;WANG Xiao-di;YANG Wen-qiang(Wuhan Center of Geological Survey,China Geological Survey,Wuhan 430223,Hubei,China;Research Center for Petrogenesis and Mineralization of Granitoid Rocks,China Geological Survey,Wuhan 430223,Hubei,China)

机构地区：[1]中国地质调查局武汉地质调查中心,湖北武汉430223 [2]中国地质调查局花岗岩成岩成矿地质研究中心,湖北武汉430223

出　　处：《地球科学与环境学报》2022年第2期171-190,共20页Journal of Earth Sciences and Environment

基　　金：国家自然科学基金项目(41302046);中国地质调查局地质调查项目(1212011085356,12120113063600,DD20221634)。

摘　　要：未分异A型花岗岩的判别指标、岩石成因及其构造意义已经取得了较大共识,而高分异A型花岗岩的判别指标仍未有统一的认识。广西大瑶山地区大进花岗岩形成于早古生代(约440 Ma),具有高Rb含量(>400×10^(-6))、Rb/Sr、Rb/Ba值及相对低的Zr/Hf值等特征;依据岩石具有高的初始锆石饱和温度(819℃~863℃),高Ga/Al值、Zr+Nb+Ce+Y含量及特定地球化学指标的演化趋势等相关证据,综合判定其为高分异A型花岗岩。元素地球化学及Nd-Hf同位素组成表明,大进花岗岩可能源自镁铁质下地壳物质的部分熔融,经历了分异结晶和少量地壳物质混染(AFC)过程。大进高分异A型花岗岩可能是后造山岩石圈拆沉的岩石学响应,其年龄代表了桂东北地区广西运动由造山挤压向后造山伸展的构造转换时间。The discriminant indexes, petrogenesis and tectonic significance of unfractionated A-type granites have been widely recognized, while there has not been a universal consensus on the discriminant criterion of highly fractionated A-type granites. LA-ICP-MS zircon U-Pb dating shows that Dajin granites form at Early Paleozoic(~440 Ma), and have high Rb contents(>400×10^(-6)), Rb/Sr and Rb/Ba values, and relatively low Zr/Hf values, illustrating that they are highly fractionated. According to the high zircon saturation temperature(819 ℃-863 ℃), and high Ga/Al(2.6-3.1), Zr+Nb+Ce+Y contents and the evolution trend of specific geochemical indexes, Dajin granites are comprehensively determined as highly fractionated A-type granites. Combining with geochemical and Nd-Hf isotopic compositions, it is suggested that Dajin granites are mostly likely generated by partial melting of the mafic lower crustal rocks, and experienced high degree fractional crystallization and contamination of a samall amount of the wall rock materials. As consequence to post-orogenic lithospheric delamination, the forming time of Dajin A-type granites represent the timing of geodynamic transition of Kwangsian orogeny from orogenic compression to post-orogenic extension in the northeastern Guangxi.

关 键 词：A型花岗岩 岩石成因 地球化学 早古生代 构造转换 广西运动 华南 

分 类 号：P581[天文地球—岩石学] P597[天文地球—地质学]