科研之友
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摘要
Abundant lherzolite, garnet pyroxenite and granulite xenoliths are found in the Neogene Hannuoba basalt of the North China craton. Garnet pyroxenites generally occur as veins/layers in spinel lberzolites. There is a gradual decrease in olivine and an increase in orthopyroxene mode going from the Iherzolite to the pyroxenite, suggesting that orthopyroxene may be forming at the expense of olivine. Garnet pyroxenites are enriched in the highly incompatible elements (e.g., Rb, K, Na, Sr, Ba, Nb and Ta) but have high and uniform Ni contents and Mg#s (83-90). This set of geochemical observations is paradoxical because the enrichments in highly incompatible elements signify derivation from a melt having either an evolved character or a significant fluid component, but the high Ni contents and high Mg#s suggest a much more primitive origin. A somewhat similar paradox is observed in the granulite xenoliths. Many of the granulite xenoliths have intermediate compositions, characterized by SiO2 > 50 wt.%, high Al2O3, Na2O, and Sr contents, low Y and heavy rare-earth contents, and high SrY, La/Yb and Na2O/K2O ratios. However, these intermediate granulites have unusually high Mg#s (54-71) and high Ni (21-147 ppm) contents for their SiO2 contents and would otherwise suggest that these granulites are more primitive than their SiO2 contents indicate. It has been hypothesized that continuous melt-rock reaction between a silicic melt and ultramafic country rock (lherzolite) can convert olivine to orthopyroxene, ultimately resulting in the formation of a high Mg# garnet pyroxenite, similar to what is seen in the Hannuoba garnet pyroxenite composite xenoliths. In addition, silicic melts that have reacted with mantle peridotite would be predicted to have anomalously high Mg#s and Ni contents (due to the strong buffering capacity of peridotite for Mg and Ni), producing melts having compositions similar to the intermediate-mafic granulite xenoliths in this study. It is thus possible that the Hannuoba garnet pyroxenites and intermediate-mafic granulites share a common petrogenetic origin. Such a link is further supported by the fact that the rare-earth element abundance patterns of melts calculated to be in equilibrium with the garnet pyroxenites roughly coincide with that of the intermediate granulites. It is concluded that the Hannuoba garnet pyroxenite-bearing composite xenoliths may present the first physical evidence for the hypothesized melt-rock reaction necessary for generating evolved magmas with high Mg# and hence, high-Mg# andesitic signature of the continental crust.
- 出版日期2005-5-30
- 单位中国地质大学（北京）; 西北大学
