Reactivation of Lufilian Arc in Zambia: Zircon and Apatite Fission Track Chronology
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摘要: 锆石和磷灰石裂变径迹年代学对揭示构造热事件的形成及演化过程具有重要的研究意义.利用锆石和磷灰石裂变径迹测试及热史模拟探讨了卢弗里安弧构造带自泛非构造运动以来的构造热演化过程.谦比希铜矿床和恩昌加铜(钴)矿床位于卢弗里安弧铜-钴成矿带中的赞比亚境内.对采自该两个矿床中的5件新鲜岩石样品进行挑选,获得了5件锆石和4件磷灰石样品.首次获得了卢弗里安弧构造带中的裂变径迹年龄,5件锆石样品的年龄分别为265±22 Ma、230±10 Ma、228±9 Ma、225±9 Ma和221±10 Ma.4件磷灰石样品的年龄分别为145±10 Ma、133±10 Ma、130±10 Ma和92±9 Ma,径迹长度介于(11.4±2.4)~(11.8±2.4)μm.从热历史模拟结果可看出,从300~260 Ma,古地温持续降低至90 ℃左右;随后,缓慢降低至现今的地表温度.对比卢弗里安弧构造带、赞比西构造带和达马拉构造带中的年龄数据,研究表明非洲中南部地区二叠纪-白垩纪的构造活动是一个区域性的构造活动事件.Abstract: The fission track chronology of zircon and apatite are always used to reveal tectonic activities and evolutions. Based on zircon and apatite fission track analyses and thermal evolution history, in this paper, it discusses the tectonic thermal evolution process of Lufilian arc since the Pan-African tectonic movement. The Chambishi copper and the Nchanga copper and cobalt deposits occur in the Zambian Copper belt of the Lufilian arc. Five fresh rock samples were taken from these two deposits, from which 5 zircon samples and 4 apatite samples were obtained. For the first time we obtained the fission track ages in Lufilian arc. The results are as follow:the ages from the 5 zircon analysis are 265±22 Ma, 230±10 Ma, 228±9 Ma, 225±9 Ma and 221±10 Ma respectively and those of the 4 apatite samples are 145±10 Ma, 133±10 Ma, 130±10 Ma and 92±9 Ma respectively and the length is between 11.4±2.4 μm and 11.8±2.4 μm. According to the modeling results of ecological thermal evolution history, from 300 Ma to 260 Ma the paleo-geotemperature continued to decrease to about 90 ℃ and it subsequently slowed down to the present surface temperature. Comparing the ages of the Lufilian belt with the Zambezi and Damara belts, it is believed that the Permian-Jurassic tectonic activities were regional tectonic reactive events in the Central and Southern Africa.
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图 1 卢弗里安弧地区地质图及重要矿床位置
Fig. 1. Geological map of the Lufilian arc and major ore deposits
图 6 中南部非洲前寒武纪地质构造简图
Fig. 6. Precambrian tectonic sketch map in central-southern Africa
表 1 样品采样位置
Table 1. Location of samples
样品编号 经纬度 岩石类型 主要矿物组合 采样位置 对应地层单元 QJ2 28°1′59″E
12°39′5″S变质粉砂岩 石英、黑云母、白云母、绿帘石和方解石 谦比希西矿体 罗安群 QJ3 28°1′59″E
12°39′5″S变质长石石英砂岩 石英、钾长石、黑云母、绿泥石和方解石 谦比希西矿体 罗安群 EJ1 27°52′15″E
12°30′36″S变质砂岩 石英和长石,少量白云母 恩昌加矿体 罗安群 EJ2 27°52′15″E
12°30′36″S变质砂岩 石英和长石,少量黑云母、白云母、绿泥石、阳起石和粘土矿物 恩昌加矿体 罗安群 EJ3 27°52′15″E
12°30′36″S变质砂岩 石英、钾长石、黑云母、白云母、方解石和绿泥石 恩昌加矿体 罗安群 表 2 锆石裂变径迹分析结果
Table 2. Zircon fission track analytical results
样品编号 锆石颗粒数 ρs(105/cm2)(Ns) ρi(105/cm2)(Ni) ρd(105/cm2)(Nd) P(χ2)(%) 中心年龄t(Ma)(±1σ) QJ2-1 5 217.023
(960)50.865
(225)14.006
(8 055)25.9 265±22 QJ3-1 30 118.602
(7 170)33.099
(2 001)14.375
(8 055)85.0 230±10 EJ1-1 26 120.552
(5 937)36.225
(1 784)14.867
(8 055)40.1 221±10 EJ2-1 35 122.312
(8 789)36.837
(2 647)15.359
(8 055)36.1 228±9 EJ3-1 33 130.325
(7 251)40.997
(2 281)15.851
(8 055)53.8 225±9 注:ρs为自发径迹密度,ρi为诱发径迹密度,ρd为标准径迹密度,Ns为自发径迹数,Ni为诱发径迹数,Nd为标准径迹数. 表 3 磷灰石裂变径迹分析结果
Table 3. Apatite fission track analytical results
样品编号 磷灰石颗粒数 ρs(105/cm2)
(Ns)ρi(105/cm2)
(Ni)ρd(105/cm2)
(Nd)P(χ2)(%) 中心年龄t
(Ma)(±1σ)径迹长度L(μm)
(N)QJ2-2 35 1.283
(353)2.880
(792)14.640
(6 788)84.7 133±10 11.7±2.5
(99)QJ3-2 30 3.573
(842)7.423
(1 749)14.849
(6 788)2.6 145±10 11.8±2.4
(111)EJ2-2 34 3.244
(966)7.936
(2 363)15.163
(6 788)0 130±10 11.4±2.4
(104)EJ3-2 35 2.621
(455)7.569
(1 314)13.178
(6 788)0 92±9 11.5±2.0
(95)注:ρs为自发径迹密度,ρi为诱发径迹密度,ρd为标准径迹密度,Ns为自发径迹数,Ni为诱发径迹数,Nd为标准径迹数, N为径迹长度数. -
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