| 内蒙古合教BIF型铁矿的形成时代、地球化学特征及地质意义 |
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| 引用本文： | 王佳营,李志丹,李光耀,文思博,谢瑜,张祺,张锋,丁宁.内蒙古合教BIF型铁矿的形成时代、地球化学特征及地质意义[J].地球科学,2020,45(6):2135-2151. |
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| 作者姓名： | 王佳营 李志丹 李光耀 文思博 谢瑜 张祺 张锋 丁宁 |
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| 作者单位： | 1.中国地质调查局天津地质调查中心, 天津 300170 |
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| 基金项目： | 中国地质调查项目12120113057300中国地质调查项目DD20160129中国地质调查项目DD20190119中国地质调查项目DD20201148国家自然科学基金项目41502082 |
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| 摘 要： | 内蒙古合教铁矿位于华北克拉通西部陆块北缘阴山地块，是固阳绿岩带内的一例具有中型规模的BIF型铁矿床.本文对矿区斜长角闪岩、铁矿石开展了年代学和岩石地球化学研究.对斜长角闪岩夹层进行LA-ICP-MS锆石U-Pb定年，锆石普遍发育振荡环带，Th/U比值均大于0.1（0.27~1.00），得到上交点年龄为2 549±29 Ma（MSWD=0.51），可大致代表合教BIF铁矿的形成时代，该时期是华北克拉通早寒武纪构造-变质-热事件和BIF（banded iron formation）形成最为强烈的时期（2.52~2.60 Ga）.斜长角闪岩原岩可能为玄武岩，表明合教铁矿为与火山活动关系密切的Algoma型BIF.斜长角闪岩稀土元素球粒陨石标准化配分曲线近于平坦，与E-MORB和弧后盆地玄武岩（BABB）曲线相似，原始地幔标准化蛛网图与BABB曲线相似，均存在Rb、Ba、Sr、K等大离子亲石元素的富集和Nb、Ta、U、Th等高场强元素的亏损，显示了岛弧岩浆岩的特征，结合前人提出的岛弧叠加地幔柱构造模式，认为合教斜长角闪岩原岩形成于弧后盆地构造环境，并有地幔柱的叠加作用，代表了合教BIF沉积时的构造环境.铁矿石LREE亏损，HREE富集（La/Yb）PAAS=0.29~0.50]，具有轻微的La正异常（La/La*=1.00~1.13），不明显的Ce负异常（Ce/Ce*=0.90~0.95），明显的Eu正异常（Eu/Eu*=1.54~2.27）和较明显的Y正异常（Y/Y*=1.07~1.42）.铁矿石的稀土配分曲线与固阳绿岩带科马提岩和海底热液海水混合物均极为相似，表明合教BIF的形成与海底热液活动有关，认为合教BIF型铁矿的Fe主要由海底高温热液淋滤科马提岩提供.
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| 关 键 词： | BIF型铁矿 锆石U-Pb年龄 地球化学 华北克拉通 合教 |
| 收稿时间： | 2019-05-22 |
Formation Age,Geochemical Signatures and Geological Significance of the Hejiao Iron Deposit,Inner Mongolia |
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| Abstract: | The Hejiao iron deposit in Inner Mongolia, which is located in the Yinshan block on the northern margin of western block of the North China craton, is a medium-sized BIF-type iron deposit in the Guyang greenstone belt. Geochronology and petrogeochemistry of amphibolite and iron ore were studied in this paper. LA-ICP-MS U-Pb dating of zircons from the amphibolites interlayer shows that cores of the zircons characterized by core-rim texture and with a Th/U ratio of higher than 0.1(0.27-1.00) have an upper intercept age of 2 549±29 Ma(MSWD=0.51), which is approximately the depositional age of the Hejiao BIF-type iron deposit, during which widespread tectonic-tectonothermal event and most BIFs occurrence took place at the Early Cambrian of North China craton. A protolith reconstruction shows that the protolith of amphibolites is basalt, indicating that Hejiao iron deposit belongs to Algoma-type BIF closely related to volcanic activities. The chondrite-normalized REE pattern of amphibolites is nearly flat, similar to those of E-MORB and back-arc basin basalts (BABB). Primitive mantle-normalized trace element pattern of amphibolites is similar to back-arc basin basalts (BABB), showing enrichment in LILEs such as Rb, Ba, Sr, K, and depletion in HFSEs such as Nb, Ta, U, Th, similar to the characteristics of island arc magmatic rocks. Combining the model of island arc-mantle plume interaction for the Guyang greenstone belt, it is speculated that the amohibolites were formed in a tectonic setting of back-arc basin, accompanied by mantle plume, which implies the tectonic environment of the Hejiao BIF. The characteristics of the iron ores include the depletion of LREE, enrichment of HREE(La/Yb)PAAS=0.29-0.50], slightly positive La anomaly(La/La*=1.00-1.13), strongly positive Eu anomaly (Eu/Eu*=1.54-2.27), positive Y anomaly(Y/Y*=1.07-1.42), and no distinct Ce anomalies(Ce/Ce*=0.90-0.95).Based on the similar signatures to the komatiite at bottom of the Guyang greenstone belt and the mixture of high-temperature hydrothermal fluid and seawater, it is inferred that Fe of the Hejiao BIF-type iron deposit was supplied through the high-temperature hydrothermal leaching of komatiites. |
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