| 其他摘要 | Experiment of dehydration melting of mafic rocks is an important method for studying the origin of intermediate-felsic magma (melt) in earth interior. Since the 1970s, this method was paid more and more attention. In the eastern North China craton, there are widespread Mesozoic adakitic rocks and their origins are hotly debated. Recent studies show that the Archean intermediate-mafic lower crust, Mesozoic adakitic rocks and the Hannuoba mafic ganulite xenoliths, which are all found in the Zhangjiakou area, might reflect the source and the melting products, respectively. This provides us a unique opportunity to use the experiment-petrologic method to study the origin of adakitic rocks in the eastern North China craton.
In this dissertation, partial melting experiments of massive and powdered amphibolite samples of the eastern North China craton are conducted simultaneously, under 1.5-2.0GPa and 800-1000℃. The main elements of each product are measured by electron microprobe and the trace elements of some melts are measured by LA-ICP-MS. We compare not only the melting features of massive and powdered samples, but also the differences and similarities between the run melts and the Mesozoic adakitic rocks of the eastern North China craton and between the experimental restites and the Hannuoba mafic ganulite xenoliths. Several conclusions are obtained as follows:
First, the fusing temperature of massive sample is lower than that of the powdered sample under the same pressure and temperature conditions and time duration. The massive sample partially melted under 1.5-2.0GPa, 800℃, no melts in the powdered samples were found after experiments at the same conditions. The partial melting took place at ±850℃ in powdered samples. In 850-1000℃, the contents of melts in massive sample are 5-17vol.％ higher than that in powdered sample at the same conditions. That is to say, the melting fraction in massive sample is higher than that in powdered sample under the same conditions, showing that the massive sample is much easier to reach the CMF (critical melt fraction) which indicates the melts could segregate to form magma. The results demonstrate that the formation of the magma in nature may be easier than that speculated through the experiments done with powdered samples previously. Moreover, under 1.5GPa and 950-1000℃, the geochemical characteristics of main and trace elements of melts in powdered and massive samples are broadly the same. It is considered they are equivalent in the petrologic applications.
Second, the compositions of experimental melts are from granitic to granodioritic at 1.5GPa. The major and trace element characteristics of 850-1000℃ melts are similar to those of the Mesozoic adakitic rocks in the eastern North China craton, and in particularly consistent with those of the three Mesozoic adakitic intrusions in the Zhangjiakou area. It suggests that partial melting of the amphibolite at 1.5GPa can produce the adakitic rocks in the eastern North China craton. The melt compositions of 2.0GPa runs are from granitic to trondhjemitic and have major and trace element characteristics different from those of the Mesozoic adakitic rocks in the eastern North China craton. This suggests that the adakitic rocks can’t be formed under this pressure.
Third, the experimental restites consist of Hb+Cpx+Gt±Pl, an assemblage of granulite or eclogite, contrast with the Cpx+Opx+Pl of the Hannuoba granulite xenoliths. Compositionally, the restites are richer in Fe and Al and poorer in Mg and Ca. Thus, we infer that the Hannuoba granulite xenoliths are the final products of the multiple episodes of magmatism from several source rocks at relative lower pressure. The experimental starting material and conditions can’t illustrate their complicated origin.
In addition, as some differences exist between the melts and the adakitic rocks, it is possible that there were some other sources, which were relatively richer in Mg and Ca, and poorer in Fe and Al, and relatively enriched in Th, U, Zr and Hf to some extent. They took partial melting together with amphibolite to produce the Mesozoic adakitic rocks in the eastern North China craton. |
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