Long-standing controversy persists over the presence and role of iron-rich melts in the formation of volcanic rock-hosted iron deposits. Conjugate iron-rich and silica-rich melt inclusions observed in thin-sections ar...Long-standing controversy persists over the presence and role of iron-rich melts in the formation of volcanic rock-hosted iron deposits. Conjugate iron-rich and silica-rich melt inclusions observed in thin-sections are considered as direct evidence for the presence of iron-rich melt, yet unequivocal outcrop-scale evidence of iron-rich melts are still lacking in volcanic rock-hosted iron deposits. Submarine volcanic rock-hosted iron deposits, which are mainly distributed in the western and eastern Tianshan Mountains in Xinjiang, are important resources of iron ores in China, but it remains unclear whether iron-rich melts have played a role in the mineralization of such iron ores. In this study, we observed abundant iron-rich agglomerates in the brecciated andesite lava of the Heijianshan submarine volcanic rock-hosted iron deposit, Eastern Tianshan, China. The iron-rich agglomerates occur as irregular and angular masses filling fractures of the host brecciated andesite lava. They show concentric potassic alteration with silicification or epidotization rims, indicative of their formation after the wall rocks. The iron-rich agglomerates have porphyritic and hyalopilitic textures, and locally display chilled margins in the contact zone with the host rocks. These features cannot be explained by hydrothermal replacement of wall rocks （brecciated andesite lava） which is free of vesicle and amygdale, rather they indicate direct crystallization of the iron-rich agglomerates from iron-rich melts. We propose that the iron-rich agglomerates were formed by open-space filling of volatile-rich iron-rich melt in fractures of the brecciated andesite lava. The iron-rich agglomerates are compositionally similar to the wall-rock brecciated andesite lava, but have much larger variation. Based on mineral assemblages, the iron-rich agglomerates are subdivided into five types, i.e., albite-magnetite type, albite-K-feldspar- magnetite type, K-feldspar-magnetite type, epidote-magnetite type and quartz-magnetite type, representing that products formed at different stages during the evolution of a magmatic-hydrothermal system. The albite-magnetite type represents the earliest crystallization product from a residual iron- rich melt; the albite-K-feldspar-magnetite and K-feldspar-magnetite types show features of magmatic- hydrothermal transition, whereas the epidote-magnetite and quartz-magnetite types represent products of hydrothermal alteration. The occurrence of iron-rich agglomerates provides macroscopic evidence for the presence of iron-rich melts in the mineralization of the Heijianshan iron deposit. It also indicates that iron mineralization of submarine volcanic rock-hosted iron deposits is genetically related to hydrothermal fluids derived from iron-rich melts.展开更多
黑尖山花岗闪长岩位于东天山造山带晚古生代弧盆系之小热泉子—大南湖古生代残留弧内,通过分析其野外地质特征、岩相学、岩石地球化学以及锆石U-Pb年代学特征,探讨了其岩石成因、构造背景,为东天山地区古洋盆闭合、碰撞造山时限等洋-陆...黑尖山花岗闪长岩位于东天山造山带晚古生代弧盆系之小热泉子—大南湖古生代残留弧内,通过分析其野外地质特征、岩相学、岩石地球化学以及锆石U-Pb年代学特征,探讨了其岩石成因、构造背景,为东天山地区古洋盆闭合、碰撞造山时限等洋-陆构造格局及其演化研究提供新的基础信息。电感耦合等离子体质谱(inductively coupled plasma mass spectrometry,LA-ICP-MS)锆石U-Pb同位素定年结果显示黑尖山花岗闪长岩形成年龄为(373.0±3.0)Ma。岩石地球化学研究表明,花岗闪长岩具有高硅、准铝质、中-高钾钙碱性系列的I型花岗岩特征;稀土元素总量变化范围为112.82×10^(-6)~129.91×10^(-6),且轻/重稀土的比值变化范围为8.73~11.26(平均值为9.64),指示轻稀土相对重稀土富集,弱Eu负异常(δEu=0.77~0.85);样品富集大离子亲石元素(LILE,如Rb、Ba、K、U),亏损高场强元素(HFSE,如Ta、Nb和Ti)及P、Ta、Nb负异常明显;显示岛弧岩浆岩的特征,形成于板块俯冲的构造环境。综合区域地质特征、岩石地球化学和岩浆源区特征,黑尖山花岗闪长岩是东天山小热泉子—大南湖岛弧内晚古生代大规模中酸性岩浆侵入作用的同期产物,表明东天山黑尖山地区晚泥盆世花岗闪长岩是古大洋(古北天山洋)板片向北俯冲的岛弧背景产物;进一步说明东天山黑尖山地区泥盆纪晚期古大洋仍未闭合。展开更多
基金financially supported by the Geological Survey Program of China(grants No.K1410 and DD20160346)the National Natural Foundation of China(grants No.41672078 and 41402067)
文摘Long-standing controversy persists over the presence and role of iron-rich melts in the formation of volcanic rock-hosted iron deposits. Conjugate iron-rich and silica-rich melt inclusions observed in thin-sections are considered as direct evidence for the presence of iron-rich melt, yet unequivocal outcrop-scale evidence of iron-rich melts are still lacking in volcanic rock-hosted iron deposits. Submarine volcanic rock-hosted iron deposits, which are mainly distributed in the western and eastern Tianshan Mountains in Xinjiang, are important resources of iron ores in China, but it remains unclear whether iron-rich melts have played a role in the mineralization of such iron ores. In this study, we observed abundant iron-rich agglomerates in the brecciated andesite lava of the Heijianshan submarine volcanic rock-hosted iron deposit, Eastern Tianshan, China. The iron-rich agglomerates occur as irregular and angular masses filling fractures of the host brecciated andesite lava. They show concentric potassic alteration with silicification or epidotization rims, indicative of their formation after the wall rocks. The iron-rich agglomerates have porphyritic and hyalopilitic textures, and locally display chilled margins in the contact zone with the host rocks. These features cannot be explained by hydrothermal replacement of wall rocks （brecciated andesite lava） which is free of vesicle and amygdale, rather they indicate direct crystallization of the iron-rich agglomerates from iron-rich melts. We propose that the iron-rich agglomerates were formed by open-space filling of volatile-rich iron-rich melt in fractures of the brecciated andesite lava. The iron-rich agglomerates are compositionally similar to the wall-rock brecciated andesite lava, but have much larger variation. Based on mineral assemblages, the iron-rich agglomerates are subdivided into five types, i.e., albite-magnetite type, albite-K-feldspar- magnetite type, K-feldspar-magnetite type, epidote-magnetite type and quartz-magnetite type, representing that products formed at different stages during the evolution of a magmatic-hydrothermal system. The albite-magnetite type represents the earliest crystallization product from a residual iron- rich melt; the albite-K-feldspar-magnetite and K-feldspar-magnetite types show features of magmatic- hydrothermal transition, whereas the epidote-magnetite and quartz-magnetite types represent products of hydrothermal alteration. The occurrence of iron-rich agglomerates provides macroscopic evidence for the presence of iron-rich melts in the mineralization of the Heijianshan iron deposit. It also indicates that iron mineralization of submarine volcanic rock-hosted iron deposits is genetically related to hydrothermal fluids derived from iron-rich melts.
文摘黑尖山花岗闪长岩位于东天山造山带晚古生代弧盆系之小热泉子—大南湖古生代残留弧内,通过分析其野外地质特征、岩相学、岩石地球化学以及锆石U-Pb年代学特征,探讨了其岩石成因、构造背景,为东天山地区古洋盆闭合、碰撞造山时限等洋-陆构造格局及其演化研究提供新的基础信息。电感耦合等离子体质谱(inductively coupled plasma mass spectrometry,LA-ICP-MS)锆石U-Pb同位素定年结果显示黑尖山花岗闪长岩形成年龄为(373.0±3.0)Ma。岩石地球化学研究表明,花岗闪长岩具有高硅、准铝质、中-高钾钙碱性系列的I型花岗岩特征;稀土元素总量变化范围为112.82×10^(-6)~129.91×10^(-6),且轻/重稀土的比值变化范围为8.73~11.26(平均值为9.64),指示轻稀土相对重稀土富集,弱Eu负异常(δEu=0.77~0.85);样品富集大离子亲石元素(LILE,如Rb、Ba、K、U),亏损高场强元素(HFSE,如Ta、Nb和Ti)及P、Ta、Nb负异常明显;显示岛弧岩浆岩的特征,形成于板块俯冲的构造环境。综合区域地质特征、岩石地球化学和岩浆源区特征,黑尖山花岗闪长岩是东天山小热泉子—大南湖岛弧内晚古生代大规模中酸性岩浆侵入作用的同期产物,表明东天山黑尖山地区晚泥盆世花岗闪长岩是古大洋(古北天山洋)板片向北俯冲的岛弧背景产物;进一步说明东天山黑尖山地区泥盆纪晚期古大洋仍未闭合。
