皖南青阳-泾县地区中酸性脉岩锆石特征、U-Pb年龄及地质意义

文章在对皖南青阳-泾县地区与金-锑多金属矿化有关的中酸性脉岩进行系统调查的基础上,开展了锆石特征及U-Pb定年研究,识别出新生锆石、变质重结晶锆石、捕获锆石和继承锆石共4类锆石,并相应获得一系列年龄值。新生锆石和变质重结晶锆石年龄(125～124 Ma)代表中酸性脉岩形成时代,该年龄值与皖南地区燕山期晚阶段A型花岗岩(130～123 Ma)年龄基本一致,指示二者为同一期岩浆热事件的产物。微量元素特征显示中酸性脉岩源区存在石榴子石残留,而A型花岗岩源区存在斜长石残留,指示源区具双峰式岩浆组合特征,表明该时期皖南地区处于强烈的拉张构造背景下。捕获锆石给出148～142 Ma和136～132 Ma 2组年龄,对应于皖南燕山期早阶段和晚阶段早侵入岩的年龄,暗示脉岩出露区的深部存在隐伏的燕山早期岩浆岩,脉岩岩浆侵位过程中同化混染了早期岩浆岩。继承锆石给出的年龄与扬子板块地壳增生和再造事件中的3期模式增长的年龄相对应,反映与江南过渡带可能有相同的基底物质组成。中酸性脉岩中锆石具有较低的Ce4+/Ce3+比值,表明其岩浆结晶时具有相对低的氧逸度,有利于金、锑的硫化物的形成,进而发育金-锑矿化。区内多金属矿床(点)具有较为明显的分带性,近岩基的中酸性岩株及其接触带发育斑岩-矽卡岩型矿床,成矿元素以钨、钼、铜为主;远离岩基的岩枝或岩脉状中酸性脉岩中及旁侧断裂带内发育脉状矿化,成矿元素以金、银、锑为主,显示出区域成矿作用受断裂构造系统、岩浆活动期次和成矿元素活动性的共同制约。

Characteristics and U-Pb dating of zircons for intermediate-acidic dikes in Qingyang-Jingxian area of South Anhui Province and their geological significance

Based on the investigation of the intermediate-acidic dikes which are related to Au-Sb mineralization in Qingyang-Jingxian area in south Anhui Province, the authors studied the characteristics of the zircons in the intermediate-acidic dikes and carried out zircon U-Pb dating by LA-ICPMS. The zircons in the dikes show complex texture, genetic type and internal structure, and can be classified into four types:the newborn zircons, the metamorphic recrystallized zircons, the captured zircons and the inherited zircon. The intermediate-acidic dikes'' formation ages (125~124 Ma) were recorded by the ages of the newborn zircons and the metamorphic recrystallized zircons. This age is basically the same as the ages (130~123 Ma) of the A-type granite in Late Yanshanian period in south Anhui, which indicates that they were formed in the same magmatic thermal event. The magmatic source of the intermediate-acidic dikes had garnet residue, while the magmatic source of the A-type granites contained plagioclase remains, as shown by the evidence from microelement characterization. The magmatism had the characteristics of bimodal magmatic rocks, and indicates that the southern Anhui area was under an extensional tectonic setting in that period. The authors obtained two groups of ages about 148~142 Ma and 136~132 Ma by the captured zircons, which correspond to the two stages of magmatic activity in Yanshanian period in southern Anhui. It implies that there exist concealed Yanshanian magmatic rocks in the depth, and the early concealed magmatic rocks were assimilated and mixed by the magma of the dikes in the process of emplacement. The ages of the inherited zircons correspond to the ages of three stage pattern-growth of the Yangtze plate in crustal accretion and reworking events. This indicates that there may exist the same metamorphic basement under the Jiangnan transition zone. The low ratio of Ce₄₊/Ce₃₊ of zircons in the intermediate-acidic dikes shows a low oxygen fugacity, which indicates a reducing environment during the magma''s crystallization period, which was favorable for AuSb sulfide mineralization, and the formation of Au-Sb deposits as the IRGS (Reduced Intrusion-Related Gold System). The polymetallic deposits have obvious zonation in the study area. From the composite batholiths outward, the metallogenic elements change from W-Mo to Au-Sb. Regional mineralization was restricted by the combined action of the fault tectonic systems, the Yanshanian magmatic activity and the activity of the mineralization elements.