Paleo-Asian Ocean Subduction Related Crust- Mantle Interaction And Deep Mantle Recycling Of Sedimentary Carbonate

The subduction of Paleo-Asian ocean induced one of the largest Phanerozoic accretionary orogen. It carried crust material back to the mantle and induced melt-peridotite reaction, which may play an important role in the destruction of the North China Craton. Meanwhile, the subduction of sedimentary carbonate also plays a critical role in deep carbon cycle, since it is the only way to bring the surface carbon into the deep earth. In this study, the Dalihu carbonatitic xenoliths from the southern part of the Xing-Meng Orogenic Belt and the Hannuoba clinopyroxene megacrysts from the north margin of the North China Craton were used to discuss the melt-peridotite reaction and deep mantle recycling of sedimentary carbonate induced by the subduction of Paleo-Asian ocean.The deep mantle recycling of sedimentary carbonate was recorded by the Dalihu carbonatitic xenoliths, carried by the Dalihu basalt, located in the southern part of the Xing-Meng Orogenic Belt. The Dalihu carbonatitic xenoliths, mainly composed of calcite, have sedimentary carbonate-like REE pattern and trace element pattern with significantly Ce anomaly and Sr anomaly. They also have very high 87Sr/86Sr ratio (87Sr/86Sr=0.70893-0.71054). These observations indicate that the Dalihu carbonatitic xenoliths originated from subducted sedimentary carbonate. The Dalihu carbonatitic xenoliths contain micro-diamond, indicating that depths>120 km were reached. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The Dalihu carbonatitic xenoliths have much higher Ni, Fe content than the sedimentary carbonate, indicating that they may suffer the mantle assimilation. The upper mantle minerals in the Dalihu carbonatitic xenoliths may be resulted from the intrusion of carbonatitic melt into the upper mantle pyroxenite.The Dalihu carbonatitic xenoliths contain a SiC dominated ultra-reduced mineral assemblage, including SiC, TiC, native metals (Si, Fe, Ni) and iron silicide. The Dalihu SiC was 20-50μm in size, blue to colorless in color, and usually identified in the micro- cavities within the carbonatitic xenolith. The Dalihu SiC grains have four types of polytypes, dominated by β-SiC (3C polytype) and 4H polytype followed by 15R and 6H. The Dalihu SiC has significant 13C-depleted isotopic composition (δ13C=-13.2%o to-22.8%o, average=-17.7%o) with obvious spatial variation. We provide a numerical modeling method to prove that the C isotope composition of the Dalihu SiC can be well-yielded by shallow degassing. Our modeling results showed that the very low δ13C value of the Dalihu SiC can be readily produced by the reaction between graphite and silicate with degassing, and the spatial variation in C isotope composition could have been formed in the progressive growth process of SiC. The Dalihu SiC occurs predominantly in micro-cavities that result from the exsolution of the volatile phase during the diapir rising process of carbonatitic melt. Filling of CO and/or CO2 in the micro-cavities could have buffered the reducing environment and separated SiC from the surrounding oxidizing phases. The fast cooling of host rock, which would leave insufficient time for the complete elimination of SiC, could have also contributed to the preservation of SiC.The subduction of Paleo-Asian ocean may induce melt-peridotite reaction under north margin of the North China Craton. It was recorded by the Hannuoba clinopyroxene megacrysts, carried by the Hannuoba basalt, located in the north margin of the North China Craton. The Hannuoba clinopyroxene megacrysts can be classified into two types based on the physical and chemical properties. The type 1 clinopyroxene megacryst has higher Cr and Nb contents and Mg# (82.9) than the type 2 clinopyroxene megacrysts as well as more evolved Sr and Nd isotopic compositions (87Sr/86Sr= 0.704520,143Nd/144Nd = 0.512350) than clinopyroxene megacrysts elsewhere. These characteristics suggest that the type 1 clinopyroxene megacryst could have been formed by a recycled crust-related melt-peridotite reaction and that the melt formed in the rutile unstable field. The type 2 clinopyroxene megacrysts exhibit good correlations between Mg# and major and trace element compositions. Type 2 Sr-Nd isotopic compositions cluster at the least evolved end of the Hannuoba basalt composition. These observations imply that the type 2 clinopyroxene megacrysts were crystallized from the host lava at high pressure. The type 1 clinopyroxene megacryst contains abundant coherent cryptocrystalline lamellae and orthopyroxene exsolutions. The bulk composition of the cryptocrystalline lamellae, composed of fine plagioclase and olivine, shows typical chemical features of garnet with a Sr isotopic composition similar to the clinopyroxene host. These observations indicate that the cryptocrystalline lamellae are decomposition products of garnet exsolutions in the clinopyroxene megacryst. P-T estimates suggest that garnet exsolution in the clinopyroxene megacryst could have occurred at 2.75 GPa and 1290℃. This garnet exsolution could be caused by increasing pressure or decreasing temperature, as indicated by experimental results. Although the temperature decreases during basalt eruption, the much quicker decrease in pressure will suppress the garnet exsolution in clinopyroxene. Therefore, we suggest that the type 1 clinopyroxene megacryst could have experienced pre-Mesozoic crustal uplifting and thickening at the north margin of the North China Craton.