Mantle Replacement: Evidence from Comparison in Trace Elements between Peridotite and Diopside from Refractory and Fertile Mantle, North China
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摘要: 在华北地块东部, 河南鹤壁新生代玄武岩中的耐熔橄榄岩捕虏体被认为是克拉通地幔的残留; 山东山旺饱满橄榄岩捕虏体被认为是新生代玄武岩所捕获的新生地幔物质, 对它们全岩的常量、微量元素, 组成单矿物的常量元素和透辉石微量元素进行了对比.结果显示代表古老岩石圈的鹤壁克拉通型地幔和代表新生岩石圈的山旺“大洋型”地幔分别相当于原始地幔经历15 %~ 2 5 %和1%~ 5 %熔融的产物.它们在熔融之后又都遭受了硅酸盐质碳酸岩熔体的交代改造作用, 但前者明显强于后者.古老岩石圈橄榄岩的固相线温度受地幔熔/流体的长期交代而降低.由于早中生代时华北地块受扬子地块碰撞的地幔热扰动和软流圈上涌影响, 促使橄榄岩熔融.所融出的基性岩浆主要垫托在地壳底部, 形成壳-幔过渡带并实现大规模的壳-幔物质和热交换.第三纪以后的热沉降使抬升的软流圈物质冷却垫托在岩石圈底部构成新增生的岩石圈.因此, 发生于东部的中、新生代(相对于古生代) 岩石圈减薄不是软流圈抬升所引起的简单岩石圈厚度变小, 而是伴随着新生地幔物质对古老地幔的置换过程.Abstract: In the eastern part of the North China block, the refractory peridotite xenoliths in Cenozoic Hebi basalts (Henan Province) are regarded as the shallow relics of the cratonic mantle; while the fertile peridotite xenoliths in Cenozoic Shanwang basalts (Shandong Province) are regarded as the newly accreted mantle. In this paper, a comparison is made between the major and trace elements of the whole rocks and the major elements of the component minerals and the trace elements of the diopsides. The results show that Hebi "cratonic" peridotites and Shanwang "oceanic" ones, respectively, represent the products of 15%-25% and 1%-5% of fractional melting for the primary mantle. These products were metasomated by the silicate carbonatite melt. The mantle metasomatism in the former is stronger than that in the latter. The solidus temperature of the cratonic lithospheric peridotite fell due to the long-term metasomatism of the mantle molten/fluid bodies. The melt underplated at the basement of the lower crust, created the crust-mantle transition zone and resulting in the huge heat and material exchange. The ensuing decreased temperature after Eogene period resulted in the uplifted asthenospheric cooling and underplating at the lithospheric basement for the newly accreted lithosphere. Therefore, the Mesozoic-Cenozoic (relative to Paleozoic) lithospheric thinning did not initiate the simple decrease in lithospheric thickness arising from the asthenospheric upwelling. Instead, this thinning created the replacement process of the accompanying cratonic mantle by the newly accreted one.
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Key words:
- trace element /
- refractory peridotite /
- fertile peridotite /
- mantle replacement /
- North China block
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图 1 鹤壁耐熔地幔和山旺饱满地幔橄榄岩及其单斜辉石REE配分形式
Fig. 1. REE pattern of peridotites and clinopyroxenes from Hebi refractory and Shanwang fertile mantle
图 2 鹤壁和山旺橄榄岩及其单斜辉石微量元素蛛网图
HB.鹤壁耐熔地幔; SW.山旺饱满地幔; Cpx.单斜辉石; Peri.橄榄岩; 1%, 5%, 10%, 15%, 20%和25%分别代表由单斜辉石模拟的分离熔融程度
Fig. 2. Trace element spider figure of peridotites and clinopyroxenes from Hebi and Shanwang
表 1 鹤壁古老地幔与山旺新生地幔橄榄岩及其矿物的主元素成分对比
Table 1. Major element component comparison of peridotites and minerals between Hebi refractory and Shanwang fertile mantle
表 2 鹤壁古老地幔与山旺新生地幔橄榄岩及其单斜辉石微量元素对比
Table 2. Trace element component comparison of peridotites and clinopyroxenes between Hebi refractory and Shanwang fertile mantle 10-6
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