Origin and Geological Significance of Diabase Dikes from Hongge Layered Intrusion in Panxi Region
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摘要: 为了了解峨眉山地幔柱岩浆系统的演化过程，对红格超大型钒钛磁铁矿矿床中辉绿岩脉进行了全岩主微量、矿物原位成分和同位素研究，并利用MELTS软件进行了岩浆过程模拟计算.研究发现，红格辉绿岩脉具有与峨眉山高钛玄武岩一致的稀土元素配分型式和Sr同位素组成，表明其起源于峨眉山地幔柱.辉绿岩含有斑晶和基质两个世代矿物，从斑晶核部到边部和基质，单斜辉石Mg#值较为连续变化，而斜长石An值具有明显的间断.MELTS模拟表明，这种成分变化难以用简单的岩浆上升侵位解释，反映了辉绿岩脉在岩浆演化过程中可能还与同期的正长质岩浆发生了混合.我们的研究表明，峨眉山地幔柱岩浆作用具有穿地壳、多期次、多阶段演化的特点，并且同源岩浆混合也是一个不可忽视的方面.Abstract: In order to understand the evolution of the magmatic system of the Emeishan mantle plume, whole-rock major and trace elements, in-situ mineral composition and isotope studies as well as MELTS simulation were carried out on the diabase dikes in the Hongge giant Fe-Ti-V oxide deposit. The study shows that the Hongge diabases have consistent REE patterns and Sr-isotope compositions with the Emeishan high-Ti basalts, indicating that the diabases were derived from the Emeishan mantle plume. The Hongge diabases contain two generations of minerals, i.e., phenocryst and matrix. It is found that Mg# value of clinopyroxene varied continuously from the phenocryst core to rim and matrix, while the An values of plagioclase show an obvious discontinuity. The MELTS simulation results show that the composition variation could not be explained by a simple process of magma ascending, reflecting the basaltic magma may have mixed with the synchronous syenitic magma during the evolution process. Our research suggests that the Emeishan mantle plume magmatism is characterized by trans-crustal, poly-phase and multi-stage evolution, and "self-mixing" between co-genetic magmas is also nonnegligible.
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Key words:
- diabase /
- layered intrusion /
- Panxi region /
- Emeishan large igneous province /
- petrology
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图 1 攀西地区峨眉山大火成岩省岩浆岩分布简图
Fig. 1. Distribution of igneous rocks of the Emeishan large igneous province in the Panxi region
图 2 红格镁铁‒超镁铁质层状岩体中辉绿岩脉野外照片
a.层状岩体被正长岩脉穿插，正长岩脉又被辉绿岩脉穿插；b.辉绿岩与正长岩呈相互混合的特征；c.辉绿岩被网脉状正长岩穿插为碎块状；d.辉绿岩被正长岩脉穿插破碎
Fig. 2. Field photographs of diabase dikes in the Hongge mafic-ultramafic layered intrusion
图 3 红格辉绿岩脉岩相显微照片
a.斑晶单斜辉石颗粒边部相对核部含有较多的铁钛氧化物微出溶体；b.斑晶斜长石颗粒边部含有微粒单斜辉石包裹体；c.多个斑晶斜长石颗粒集中出现，局部可见单斜辉石小颗粒呈串珠状地从边部穿插入斑晶斜长石之中；d.斜长石、单斜辉石、铁钛氧化物等微粒基质矿物相粒度相近，较为均匀分布，可见细柱状磷灰石颗粒
Fig. 3. Microphotographs of textures of the Hongge diabase dike
图 4 红格辉绿岩及相关岩石主量元素成分二元图解
数据来源：峨眉山火山岩（Qi et al.，2008）；攀枝花边缘相细粒辉长岩和含橄榄石斑晶辉长岩（Bai et al.，2019）；红格（炉库和白草矿区）正长岩脉和正长岩体（王汾连，2014）；红格（炉库矿区）辉绿岩脉（本研究）
Fig. 4. Binary diagrams of major elements of the Hongge diabase and related rocks
图 5 红格辉绿岩及相关岩石球粒陨石标准化稀土元素配分图（a、c）和原始地幔标准化微量元素蛛网图（b、d）
球粒陨石和原始地幔标准化值来自Sun and McDonough（1989）.其他数据来源：峨眉山高钛玄武岩（Qi et al.，2008）；攀枝花边缘相细粒辉长岩和含橄榄石斑晶辉长岩（Bai et al.，2019）；红格（炉库和白草矿区）正长岩脉和正长岩体（王汾连，2014）；红格（炉库矿区）辉绿岩脉（本研究）
Fig. 5. Chondrite-normalized REE patterns (a, c) and primitive mantle-normalized trace element spidergrams (b, d) of the Hongge diabase and related rocks
图 6 单斜辉石斑晶背散射电子图像（a）及元素面分布图（b、c、d）
Pl.斜长石；Cpx.单斜辉石；Mt.磁铁矿；Ilm.钛铁矿
Fig. 6. BSE image (a) and X-ray elemental intensity maps (b, c, d) of clinopyroxene phenocryst
图 7 红格辉绿岩脉单斜辉石和斜方辉石成分变化图解
Fig. 7. Composition variation of clinopyroxene and orthopyroxene within the Hongge diabase
图 8 斜长石斑晶背散射电子图像（a）及元素面分布图（b、c、d）
Pl.斜长石；Cpx.单斜辉石；Mt.磁铁矿；Ilm.钛铁矿
Fig. 8. BSE image (a) and X-ray elemental intensity maps (b, c, d) of plagioclase phenocryst
图 9 红格辉绿岩脉斜长石成分变化图解
Fig. 9. Composition variation diagram of plagioclase within the Hongge diabase
图 10 红格辉绿岩脉角闪石成分变化及分类图解
Fig. 10. Composition variation and classification of amphibole within the Hongge diabase
图 11 红格辉绿岩脉斜长石Sr同位素与该地区其他岩石比较
Sr同位素数据来源：峨眉山高钛玄武岩（Xu et al.，2001）、红格层状岩体（Zhong et al.，2003）、红格（炉库和白草矿区）正长岩体和正长岩脉（王汾连等，2015）、红格辉绿岩脉斜长石（本研究）
Fig. 11. Comparison of plagioclase Sr isotopic compositions of the Hongge diabase with other rocks in this area
图 12 MELTS模拟单斜辉石Mg#值（a）和斜长石An值（b）随岩浆演化而变异图解
MELTS模拟分为深部和浅部两个阶段.深部浆房的分离结晶：压力2 kbar，氧逸度QFM，初始成分见附表 8中原生岩浆a. 待岩浆演化至与An=54.8的斜长石平衡时，快速向上侵位，在浅部（1 kbar）就位.浅部岩浆房的分离结晶：初始成分见附表 8中演化岩浆b和d，压力1 kbar，氧逸度QFM
Fig. 12. MELTS modelled clinopyroxene Mg# value (a) and plagioclase An value (b) variation diagram with magma evolution
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