| 扬子地块北缘新元古代望江山层状岩体矿物成分和铂族元素特征:对岩浆演化过程和构造环境的制约 |
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| 引用本文： | 王岩,王梦玺,焦建刚.扬子地块北缘新元古代望江山层状岩体矿物成分和铂族元素特征:对岩浆演化过程和构造环境的制约[J].地质力学学报,2019,25(2):267-285. |
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| 作者姓名： | 王岩 王梦玺 焦建刚 |
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| 作者单位： | 长安大学地球科学与资源学院,陕西 西安,710064;长安大学地球科学与资源学院, 陕西 西安 710064;国土资源部岩浆作用成矿与找矿重点实验室, 陕西 西安 710064;长安大学地球科学与资源学院, 陕西 西安 710064;国土资源部岩浆作用成矿与找矿重点实验室, 陕西 西安 710064;教育部西部矿产资源与地质工程重点实验室, 陕西 西安 710064 |
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| 基金项目： | 陕西省自然科学基础研究计划项目一般项目（2018JM4009）；陕西省高校科协青年人才托举计划项目（20160213）；中央高校基本科研业务费专项（300102277103，310827171003） |
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| 摘 要： | 望江山层状岩体位于扬子地块北缘新元古代汉南杂岩带中，岩体从底部到顶部由超镁铁质岩过渡为中性岩:底部主要由辉石岩和橄长岩组成；中部为辉长苏长岩和辉长岩；上部为辉长岩和闪长岩。研究以中部岩相带橄榄辉长苏长岩、辉长苏长岩和辉长岩为对象，通过主要矿物的主微量元素和全岩主微量元素的分析，查明望江山岩体来源于尖晶石二辉橄榄岩组成的大陆下岩石圈地幔，并且地幔源区受到了来自俯冲板片流体的交代，岩体中部带的母岩浆为拉斑玄武质岩浆。钛铁矿—磁铁矿矿物对成分计算表明，母岩浆在形成时具有较高氧逸度。通过单斜辉石压力计得到岩体的侵位深度约为12.9~18 km。对岩体母岩浆橄榄石分离结晶过程的模拟计算表明，中部带橄榄石为母岩浆经过~28%分离结晶的产物。此外，铂族元素（PGE）组成暗示岩体并未经历过大规模的硫化物熔离，可能与缺乏地壳物质混染有关。岩体中单斜辉石与岛弧环境堆晶岩中单斜辉石成分相似，不同于裂谷环境中堆晶单斜辉石的成分；同时，全岩Th/Yb和Nb/Yb比值也与岛弧玄武岩比值相似，因此矿物和全岩成分均说明望江山层状岩体应形成于岛弧环境。研究认为扬子北缘在新元古代长期的俯冲过程中，大洋板片断离导致软流圈上涌，提供热源使交代大陆下岩石圈地幔部分熔融形成具有岛弧特征的镁铁质岩浆，在局部伸展环境中上升侵位形成汉南杂岩带中镁铁—超镁铁质层状岩体。
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| 关 键 词： | 矿物成分 铂族元素 硫化物熔离 望江山层状岩体 扬子地块北缘 |
| 收稿时间： | 2018/11/30 0:00:00 |
| 修稿时间： | 2019/1/13 0:00:00 |
MINERAL COMPOSITION AND PLATINUM-GROUP ELEMENTS OF THE NEOPROTEROZOIC WANGJIANGSHAN LAYERED INTRUSION AT THE NORTHERN MARGIN OF THE YANGTZE BLOCK: IMPLICATIONS FOR THE PROCESSES OF MAGMA EVOLUTION AND TECTONIC SETTING |
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| WANG Yan,WANG Mengxi and JIAO Jiangang.MINERAL COMPOSITION AND PLATINUM-GROUP ELEMENTS OF THE NEOPROTEROZOIC WANGJIANGSHAN LAYERED INTRUSION AT THE NORTHERN MARGIN OF THE YANGTZE BLOCK: IMPLICATIONS FOR THE PROCESSES OF MAGMA EVOLUTION AND TECTONIC SETTING[J].Journal of Geomechanics,2019,25(2):267-285. |
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| Authors: | WANG Yan WANG Mengxi and JIAO Jiangang |
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| Institution: | 1.School of Earth Science and Resources, Chang'an University, Xi'an 710064, Shannxi, China2.Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, Ministry of Land and Resources, Xi'an 710064, Shannxi, China3.Key Laboratory of Western China's Mineral Resources and Geological Engineering, Ministry of Education, Xi'an 710064, Shannxi, China |
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| Abstract: | The Wangjiangshan layered intrusion occurs in the Neoproterozoic Hannan arc at the northern margin of the Yangtze Block, and it is composed of ultramafic to intermediate rocks from the bottom to the top. The lower zone is mainly composed of pyroxenite and troctolite, the middle zone is olivine gabbro and gabbro, and the upper zone is gabbro and diorite. This study focuses on olivine noritegabbro, noritegabbro and gabbro in the middle zone. The compositions of olivine, pyroxene and bulk-rock suggest that the Wangjiangshan intrusion was derived from subcontinental lithospheric mantle which was mainly composed of spinel lherzolite and metasomatized by subducted slab-derived fluids. The parental magma of the middle zone was tholeiitic basalt with high oxygen fugacity, inferring from the in-situ compositions of ilmenite-magnetite mineral pair. The shallow magma chamber was about 12.9 to 18 km in depth in terms of clinopyroxene geobarometry. The modeling result shows that olivine from the middle zone was a product of~28% crystallization of the parental magma. In addition, the PGE pattern of the rocks from this study indicates a lack of large-scale sulfide segregation due to the absence of crustal contamination. The composition of clinopyroxene is akin to those from island arc, but different from clinopyroxene from the rifting. Moreover, rocks from the middle zone have bulk-rock Th/Yb and Nb/Yb ratios similar to ratios of island arc basalts. Therefore, the Wangjiangshan intrusion could be formed in island arc environment. We argue that the tearing and breakoff of subduction oceanic slab resulted in asthenosphere upwelling during the Neoproterozoic long-term subduction beneath the northern margin of Yangtze. This provided a heat source for the partial melting of the metasomatized subcontinental lithospheric mantle and subsequent formation of mafic magma with island arc characteristics, which ascended in an extensional environment to form mafic-ultramafic layered intrusions in the Hannan arc. |
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| Keywords: | mineral composition platinum-group elements sulfide segregation Wangjiangshan layered intrusion northern margin of the Yangtze Block |
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