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闪锌矿流体包裹体^(40)Ar-^(39)Ar法定年探讨——以广东凡口铅锌矿为例 被引量:30

Preliminary investigation of sphalerite^(40)Ar-^(39)Ar dating by crushing in vacuum:a case study from the Fankou Pb-Zn deposit,Guangdong province.
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摘要 本文首次把流体包裹体^(40)Ar-^(39)Ar 测年技术应用于闪锌矿,对凡口铅锌矿的闪锌矿进行了直接定年分析,获得了可靠的等时线年龄和坪年龄。闪锌矿中次生包襄体沿裂隙分布而易于提取,真空击碎的开始5个阶段以次生包裹体释气为主,其数据点在^(36)Ar/^(40)Ar-^(39)Ar/^(40)Ar 图上构成等时线,年龄为(233.6±7.4)Ma(1σ),对应的^(40)Ar/^(36)Ar 初始比值为703±27,表明次生包裹体含有过剩氩,其等时线年龄可能代表了成矿后的一期热液活动的时间。真空击碎第7至17阶段(末阶段)以原生包襄体释气为主,这些阶段构成了平坦的年龄坪,对应的数据点在^(36)Ar/^(40)Ar-^(39)Ar/^(40)Ar 图上构成线性关系很好的等时线,等时线年龄(265.8±10)Ma 与坪年龄(264.4±0.7)Ma 非常一致,代表了闪锌矿的成矿年代,它表明凡口铅锌矿的一期矿化作用发生在二叠纪中期:等时线截距对应的^(40)Ar/^(36)Ar 初始比值为291±2,表明原生包裹体不含过剩氩。闪锌矿流体包裹体^(40)Ar-^(39)Ar定年的成功将为金属硫化物矿床直接定年开辟一条有效的新途径。 In order to investigate the possibility of dating fluid inclusions in sphalerite using the ^40Ar-^39Ar technique by crushing in vacuum, a sphalerite sample (04FK-11Sp) from the Fankou Pb-Zn deposit was analyzed. The crushing experiment forms a fallingdown age spectrum. The apparent ages decline remarkably in the first five steps and a plateau yields in the last 11 steps(7 to 17). Two well-defined isochron lines are obtained on the inverse isochron diagram of ^36Ar/^40Ar vs. ^39Ar/^40Ar. The ^40Ar-^39Ar data of steps 1 to 5 define an isochron line with an age of ( 233.6 ± 7.4 ) Ma ( 1 σ) and an initial ^40 Ar/^36 Ar ratio of 703 ± 27. This isochron age is interpreted as the age of the secondary fluid inclusions formed by a post-mineralization hydrothermal event, and the high ^40Ar/^36Ar ratio of 703 ± 27 indicates that excess ^40Ar trapped into the sphalerite during the post-mineralization history associated with the fluid which formed the secondary fluid inclusions. Data points of steps 7 to 17 yield another well-defined isochron line with an isochron age of ( 265.8 ± 1.0) Ma and an initial ^40Ar/^36Ar ratio of 291 ± 2. This initial ratio, close to that of the modem atmosphere, indicates that no excess ^40Ar is present within the primary inclusions. The isochron age of (265.8 ± 1.0)Ma is well concordant with the plateau age of ( 264.4 ± 0.7) Ma ( ^39Ar released 49.2% ) ,implying that a mineralization epoch of the Fankou Pb-Zn deposit took place in the Middle Permian. This is the first time to distinguish the primary fluid inclusions from the secondary ones and obtain their real ages respectively. This preliminary study shows that sphalerite ^40Ar-^39Ar in vacuo crushing technique is an effective approach to obtain the sulphide mineralization ages.
出处 《岩石学报》 SCIE EI CAS CSCD 北大核心 2006年第10期2425-2430,共6页 Acta Petrologica Sinica
基金 国家自然科学基金(40472048)
关键词 ^40Ar-^39Ar法 闪锌矿 流体包裹体 反等时线 凡口铅锌矿 ^40Ar-^39Ar technique, Sphalerite, Fluid inclusion, Inverse isochron, Fankou Lead-Zinc deposit
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