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东马努斯盆地普通火山岩、高镁安山岩及蚀变岩石地球化学特征研究
赵霞1,2
学位类型硕士
导师黄朋
2017-05-18
学位授予单位中国科学院大学
学位授予地点北京
学位专业海洋地质
关键词东马努斯盆地 高镁安山岩 岩浆物质来源 蚀变 元素迁移规律
摘要
本文使用水下机器人获取了东马努斯盆地的玄武质安山岩、英安岩和热液蚀变岩石样品,通过对样品进行全岩主微量、Sr-Nd-Pb同位素和斜长石电子探针分析,并结合前人数据,发现了该区一直被忽视的高镁安山岩,探究了岩浆物质来源及热液活动中元素的迁移规律。
东马努斯盆地的2个玄武质安山岩样品具有高镁含量,其MgO含量分别为5.3%和5.54%,FeOT含量分别为8.35%和8.22%,FeOT/MgO比值分别为1.575和1.484,Al2O3含量分别为15.77%和15.82%,CaO含量分别为9.22%和9.28%,属于高镁安山岩。
东马努斯盆地是一个活跃的弧后盆地,伴随着太平洋板块和所罗门板块双重俯冲的影响,俯冲物质来源存在争议。有学者认为俯冲物质来自于太平洋板块,也有学者认为来自于所罗门板块。本文首先通过岩浆中具有Pb和大离子亲石元素(K,Rb,Sr,Ba,和U)富集、高场强元素(Nb,Th,Ta和Ti)亏损的特征,证实了俯冲板块脱水作用产生了大量的含水物质注入到岩浆源区。再由87Sr/86Sr比值沿太平洋板块俯冲方向连续变化和238U过剩的现象证明岩浆受到了太平洋板块和所罗门板块双重俯冲的影响,并且,太平洋板块物质在东马努斯盆地滞留了很长一段时间,而所罗门板块物质是最近才加入的。高镁安山岩由地幔橄榄岩演化而来,地幔橄榄岩的同位素特征与马努斯MORB一致;俯冲板块物质包含了海底沉积物和蚀变洋壳,因此,东马努斯岩浆物质来源可能包括:马努斯MORB、太平洋和所罗门板块的海底沉积物和蚀变洋壳。如果这5个端元都对东马努斯岩浆有贡献,那么计算可得,太平洋蚀变洋壳与沉积物的比值为86.5:13.5,这二者的总含量与马努斯MORB的比值为8.2:91.8,所罗门蚀变洋壳与沉积物的比值为93.5:6.5,在马努斯MORB含量不变的情况下,每增加一份太平洋俯冲物质,需要增加9份所罗门俯冲物质来与之平衡。
文章还对蚀变岩石与新鲜火山岩之间的全岩主微量及斜长石电子探针数据进行对比,深入探究了蚀变火山岩的元素迁移特征及影响因素。结果显示,蚀变火山岩的硫化物以黄铜矿、方铅矿和闪锌矿为主,为富Cu型热液硫化物。岩石蚀变区的SiO2含量极高,为硅化蚀变。蚀变岩石的化学成分约为同区玄武质安山岩和英安岩成分总含量的平均值,原岩可能为安山岩。在热液蚀变的过程中,

蚀变岩石的质量增加了150%,其中元素Cu、Zn、Ga、Sr、Cd、Ba和Pb超强烈富集,Ti、V、Mn、Co、Ni、Mo和U强烈富集,Si和Fe中等富集;Sc、Cr、Nb、Ta、W和Bi轻微富集;Be和Ca超强烈亏损;Li、Na、Mg、K、Rb、REE和Y强烈亏损;Zr、Cs和Th轻微亏损;Al、P和Hf稳定不变。推测影响蚀变岩石元素迁移的因素有3种:成矿作用、交代作用(绿泥石化、硅化)和元素在流体中的活动性。
其他摘要
Basaltic andesite, dacite and hydrotherm altered volcanic rock were got from Eastern Manus Basin by Remote Operated Vehicle. By analyzing the bulk major and trace elements, Sr-Nd-Pb isotope compositions and the plagioclase in situ components of these rocks, and conbining with previous data, we found that there was High-Mg andesite in Eastern Manus Basin ignored. We also studied the magma source of fresh volcanic rocks and elemental migration mechanism of altered rock in Eastern Manus Basin.
Two pieces of basaltic andesite from Eastern Manus Basin have high Mg content. They have 5.3% and 5.54% of MgO, 8.35% and 8.22% of FeOT, 1.575 and 1.484 of FeOT/MgO, 15.77% and 15.82% of Al2O3, 9.22% and 9.28% of CaO, respectively. They are High-Mg andesite.
The Eastern Manus Basin is an active back-arc basin with the subduction of the Pacific and Solomon Plates. The magma source of Eastern Manus Rifts (EMR) lavas has been debating. Some scholars think that the subduction materials come from the Pacific Plate, others think that the subduction materials are from the Solomen Plate. Strong enrichments of Pb and LILEs (K, Rb, Sr, Ba and U) and depletions of HFSEs (Nb, Th, Ta and Ti) and REEs in EMR lavas suggest that numerous subduction components have been injected into the mantle source of EMR magmas. The subduction components come from the Pacific and Solomon Plates based on the slightly continuous change in the 87Sr/86Sr ratios along the Pacific Plate subducting direction and on the 238U excesses of the EMR lavas, which reveal that components from the Pacific Plate have remained in the mantle beneath the Eastern Manus Basin for a long time and the Solomon subducting components have been added recently. High-Mg andesite was developed from peridotite; the isotope composition of peridotite was as same as which of the Manus MORB. The subducted components include submarine sediments and altered oceanic crust. Thus, the five end members probably include sediments and basalts from the Solomon Plate, sediments and basalts from the Pacific Plate, and Manus MORB. As all 5 members are taken into consideration, the ratio of Pacific basalts versus sediments is 86.5:13.5, of Solomon basalts versus sediments is 93.5: 6.5, of Pacific components versus Manus MORB is 8.2:91.8. If the content of Manus MORB remains stable, the ratio of Solomon compo-

nents versus Pacific components is 9:1.
We analyzed the bulk major, trace elements and the plagioclase in situ components of these rocks and carried out some comparative studies of altered rock and fresh rocks to explore the characteristics and influence factors of element migration of the hydrothermal altered rock in Eastern Manus Basin. As results, the type of hydrothermal sulfide of altered rock is copper-rich sulfide which mainly composed of chalcopyrite, sphalerite and ilmenite. Silicified alteration is the main alteration type of this rock as the SiO2 content is extremely high in the altered area. The content of major and trace elements of the altered rock may be the average value of the total content of basaltic andesite and dacite from the Eastern Manus Basin. The protolith of the altered rock may be andesite. During the process of hydrothermal alteration, mass of the altered rock were increased by 150%. The altered rock most enriched elements such as Cu, Zn , Ga, Sr, Cd, Ba and Pb, more enriched Ti, V, Mn, Co, Ni, Mo and U, less enriched Si and Fe, least enriched Sc, Cr, Nb, Ta, W and Bi. Altered rock most depleted elements such as Be and Ca, more depleted Li, Na, Mg, K, Rb, REE and Y, least depleted Zr, Cs and Th. Al, P and Hf are immobile elements. Influence factors of element migration of the hydrothermal altered rock may include mineralization, metasomatism (such as chloritization and silicification) and the activity of elements in fluids.
学科领域地球科学 ; 地球化学 ; 地质学
语种中文
文献类型学位论文
条目标识符http://ir.qdio.ac.cn/handle/337002/136557
专题海洋地质与环境重点实验室
作者单位1.中科院海洋所
2.中国科学院大学
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赵霞. 东马努斯盆地普通火山岩、高镁安山岩及蚀变岩石地球化学特征研究[D]. 北京. 中国科学院大学,2017.
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