| 南岭九嶷山地区砂子岭岩体成因与构造属性:来自锆石U-Pb年代学、岩石地球化学及Sr、Nd、Hf同位素证据 |
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| 引用本文： | 李剑锋,付建明,马昌前,卢友月,程顺波,马丽艳,秦拯纬.南岭九嶷山地区砂子岭岩体成因与构造属性:来自锆石U-Pb年代学、岩石地球化学及Sr、Nd、Hf同位素证据[J].地球科学,2020,45(2):374-388. |
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| 作者姓名： | 李剑锋 付建明 马昌前 卢友月 程顺波 马丽艳 秦拯纬 |
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| 作者单位： | 1.中国地质调查局武汉地质调查中心, 湖北武汉 430205 |
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| 基金项目： | 中国地质调查局项目"南岭成矿带中西段地质矿产调查"121201009000150002 |
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| 摘 要： | 九嶷山地区砂子岭岩体作为南岭花岗岩带的有机组成部分，对其主要岩石类型开展了年代学研究，系统的LA-ICP-MS锆石定年结果表明，含斑中细粒花岗闪长岩成岩年龄为151.9±1.1 Ma、152.1±1.1 Ma，中细粒斑状二长花岗岩成岩年龄为154.1±1.2 Ma；确定其成岩年代为燕山早期，而不是以前普遍认为的印支期.岩石地球化学分析显示，砂子岭岩体具有富硅碱贫钙镁、K2O/Na2O为1.37~2.65、准铝-过铝质（0.93~1.09），FeO*/MgO比值大（5.43~15.33，平均7.14）等特点；岩石稀土含量介于186.75×10-6~413.17×10-6之间，明显高于世界花岗岩均值，稀土元素配分曲线呈右倾轻稀土富集型，具明显铕负异常，δEu值为0.095~0.224；岩石富集Ga、Y、Nb、Zr、Hf等大离子高场强元素及亏损Ni、Cr、Eu、Ti、V、P、Sr等元素，Ga/Al比值为245×10-6~582×10-6（平均值350×10-6）、Zr+Nb+Ce+Y为256.8×10-6~630.7×10-6（平均值441.95×10-6），显示A型花岗岩地球化学属性，形成于伸展构造体系的造山后环境.Sr、Nd、Hf同位素显示砂子岭岩体具较高Sr同位素初始值（ISr=0.716 03~0.718 17），较低的εNd（t）（-6.8~-7.4）、εHf（t）（4.8~-14.2）值特点；揭示其源区为地壳杂砂岩/泥质岩的部分熔融，成岩过程中有地幔物质的贡献；钕、铪模式年龄较接近，分别为1 498~1 546 Ma与1 061~1 756 Ma，暗示其源岩从地幔储库中脱离的时间为中元古代.结合南岭地区地质演化史，中生代九嶷山地区恰处于板块拼合带及太平洋板块弧后伸展的构造背景之下，具发生过岛弧岩浆作用、构造相对薄弱且存在大量具较高Lu-Hf、Sm-Nd同位素比值新生地壳物质的特点；地幔对流与软流圈上涌引发源区部分熔融形成具有类似同位素组成特征的A型花岗岩，即为砂子岭及九嶷山复式岩体的成因.
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| 关 键 词： | A型花岗岩 锆石U-Pb年代学 同位素 地球化学 构造背景 砂子岭 南岭 |
| 收稿时间： | 2019-05-25 |
Petrogenesis and Tectonic Setting of the Shaziling Pluton in Jiuyishan Area,Nanling: Evidence from Zircon U-Pb Geochronology,Petrogeochemistry, and Sr-Nd-Hf Isotopes |
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| Abstract: | A study of LA-ICP-MS zircon U-Pb dating for Shaziling pluton composed of granodiorites and monzogranites from Jiuyishan area considered as a part of Nanling granite belt was carried out. The results of chronology indicate that the Shaziling pluton was formed during the early Yanshanian (151.9±1.1-154.1±1.2 Ma) instead of the Indosinian. The analyses of geochemistry indicate that the Shaziling pluton was characterized by rich silicon-alkali and poor calcium-magnesium with K2O/Na2O ratios of 1.37-2.65, Al2O3 of 0.93-1.09 and FeO*/MgO ratios of 5.43-15.33 (average 7.14); The content of rare earth elements in the range of 186.75 to 413.17×10-6 is significantly higher than the those of world average granite, which shows the right-leaning distribution of enriched light rare earth elements with obvious negative anomaly of Eu and δEu values of 0.095-0.224. These rocks are enriched in large ion lithophile elements (LILEs, e.g., Ga, Y, Nb, Zr, and Hf) and relatively depleted in high field strength elements (HFSEs, e.g., Ni, Cr, Eu, Ti, V, P, and Sr), with Ga/Al ratios of (245~582)×10-6 (average 350×10-6) and Zr+Nb+Ce+Y of (256.8-630.7)×10-6 (average 441.95×10-6), similar to geochemical features of A-type granites, which indicates the Shaziling pluton formed in the post- orogenic environment of the extensional tectonic system. Sr, Nd and Hf isotopes show that the Shaziling pluton has higher initial Sr isotope values of 0.71603 to 0.71817, lower εNd(t) values of -6.8 to -7.4 and εHf(t) values of 4.8 to -14.2, revealing that the source area occurred partial melting of crustal graywacke/pelite with a contribution of mantle materials during the diagenesis. Nd and Hf are relatively close in mode ages, with 1 498-1 546 Ma and 1 061-1 756 Ma, respectively, suggesting that the source rocks separated from the mantle reservoir during the Mesoproterozoic. Combined with the geological evolution of Nanling area that was in the tectonic setting of plate juncture zone and post-arc extension of Pacific plate, magmatism occurred in the Mesozoic Jiuyishan area in which the structure is relatively weak and there are a lot of new crustal materials with higher Lu/Hf and Sm/Nd isotope ratios. The Shaziling and Jiuyishan complex massif, similar to isotopic composition of A-type granites was formed by partial melting of the source area caused by mantle convection and upwelling of asthenosphere. |
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