A rare composite xenolith and abundant cumulative pyroxenites obtained from the Mesozoic Fan-gcheng basalts on...
Hong-Fu Zhang·Eizo Nakamura·Katsura Kobayashi·Ji-Feng Ying·Yan-Jie Tang State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics,Chinese Academy of Sciences,P.O.Box 9825,100029 Beijing,China
This study carried out a comprehensive review of mantle-derived garnet peridotites on the basis of their garnet-spinel compositional relationships. The P-T estimates of the garnet peridotites of both orogenic and xenolithic derivations confirm previously established two garnet-spinel transition (GST) zones. Results of natural samples and experimental studies of all the garnet peridotites plot below the first GST at low P (<2 GPa). The second GST at high P (6–10 GPa), obtained from the experimental system (MgO-Cr2O3-SiO2), was firstly evidenced from natural samples in this study and may imply a challenge to our understanding of Cr-rich and Al-poor mineral phase transition under ultrahigh P condition. Based on the data and our estimates, we propose the existence of another GSTwithin the garnet peridotite discontinuous zone and a potential spinel-phase peridotite zone in the upper mantle.
Two types of melt pockets, closed melt pocket (CMP) and open melt pocket (OMP), are recognized from the peridotite xenoliths entrained in the Cenozoic kamafugites in western Qinling (秦岭), Central China. The Haoti (好梯) CMPs have a mineral assemblage of olivine+ clinopyroxene+amphibole+K-feldspar, whereas the Baiguan (白关) CMPs are composed of olivine+clinopyroxene+ilmenite+carbonate. The components of the OMPs are more complicated. In the Haoti OMPs, there are olivine, clinopyroxene, glass, low modal abundances of amphibole, K-feldspar (Kfs), ilmenite, sulfide, chlorite, perovskite, chromite and phlogopite. TheBaiguan OMPs contain olivine, clinopyroxene, glass, chlorite and chromite. Compositionally, olivines in the CMPs and OMPs are both apparently depleted in Ni, and those in the OMPs are also depleted in Fe and Mg, and enriched in Ca compared to the primary ones. Clinopyroxenes display large and system-atical compositional variations between the CMPs and OMPs, particularly in Al, Cr, Na, Ca and Ti. Glasses are generally depleted in Si compared to the worldwide glasses in melt pockets, although they still have large variations. Amphiboles and K-feldspars have relatively restricted compositional varia-tions. The petrographical observations and mineral chemistry suggest that the Haoti and Baiguan CMPs were generated by the in-situ decompression melting of orthopyroxenes, olivines and clinopy-roxenes, and by the addition of minor external K-rich and Ca-rich melt/fluids. The OMPs formed dur-ing the latest metasomatic event in the lithospheric mantle beneath the western Qinling.
苏本勋张宏福Patrick Asamoah Sakyi秦克章刘平平英基丰汤艳杰Sanjeewa P K Malaviarachchi肖燕赵新苗毛骞马玉光
This paper presents an overview of recent studies dealing with different ages of mantle peridotitic xenoliths and xenocrysts from the North China Craton, with aim to provide new ideas for further study on the destruction of the North China Craton. Re-Os isotopic studies suggest that the lithospheric mantle of the North China Craton is of Archean age prior to its thinning. The key reason why such a low density and highly refractory Archean lithospheric mantle would be thinned is changes in composition, thermal regime, and physical properties of the lithospheric mantle due to interaction of peridotites with melts of different origins. Inward subduction of circum craton plates and collision with the North China Craton provided not only the driving force for the destruction of the craton, but also continuous melts derived from partial melting of subducted continental or oceanic crustal materials that resulted in the compositional change of the lithospheric mantle. Regional thermal anomaly at ca. 120 Ma led to the melting of highly modified lithospheric mantle. At the same time or subsequently lithospheric exten- sion and asthenospheric upwelling further reinforced the melting and thinning of the lithospheric mantle. Therefore, the destruction and thinning of the North China Craton is a combined result of peridotite-melt interaction (addition of volatile), enhanced regional thermal anomaly (temperature increase) and lithospheric extension (decompression). Such a complex geological process finally produced a "mixed" lithospheric mantle of highly chemical heterogeneity during the Mesozoic and Cenozoic. It also resulted in significant difference in the composition of mantle peridotitic xenoliths between different regions and times.
