Changgui Gao

Chemical composition of the continental crust in the Qinling Orogenic Belt and its adjacent North China and Yangtze cratons

The North China and Yangtze (South China) cratons are the most important tectonic units of China. The two cratons finally coalesced through collision during the Late Paleozoic to Middle Triassic with the formation of the E-W-trending Qinling-Dabie Mountains. We report abundances of thirteen major, sixteen trace, and fourteen rare earth elements of the sedimentary cover and upper crust of the Qinling Orogenic Belt (including the North and South Qinling) and the adjacent southern margin of the North China Craton and northern margin of the Yangtze Craton as well as the study area as a whole. The estimates are based on systematic sampling and analyses of >4500 individual rock samples taken over an area of 153200 km2. An attempt is also made to estimate deep and total crustal compositions of the tectonic units from an exposed crust cross section and exposed amphibolite and granulite facies rocks. The proposed granodioritic to quartz-dioritic total crustal composition for the study area is consistent with the regional mean crustal P-wave velocity (6.06 km s−1) and high surface heat flow estimates (72–109 mW m−2), and yields the key element ratios: ThK = 3.5 − 3.8 × 10 −4; SmNd = 0.20–0.21; and GaAl = 0.21 − 0.23 × 10−3, that are identical to those generally accepted. The dioritic to granodioritic deep crustal compositions agree with the characteristic low Vp(5.7–6.6 km s−1) of the regional middle-lower crust and with the step-like, sharp contrast in velocity (>1.0 km s−1) between the lower crust and upper mantle of the Qinling region which points out that mafic underplates/restites are probably not an important constitute of the present-day Qinling lower crust except for the North Qinling Belt. There are clear cratonic-orogenic distinctions in crustal, particularly upper crustal, trace element compositions but not in major and rare earth element compositions. The results imply a poorly differentiated crust, in terms of chemical composition, and contrasting patterns of heat-production distribution with depth for the Qinling region. The upper and deeper crusts exhibit broadly similar compositions except for carbonate components (CaO and CO2). Average compositions of granites are characterized by being low in SiO2 (69.41–70.29%), K2O/Na2O (0.90–1.05), and initial 87Sr86Sr (mostly 0.704–0.714) and less prominant negative Eu anomalies (EuEu∗ = 0.53–0.64) compared to granites in typical collision orogenic belts, the average worldwide granite and average S-type granite.

First direct evidence of sedimentary carbonate recycling in subduction-related xenoliths

Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ18OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. 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 assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.

SiC-dominated ultra-reduced mineral assemblage in carbonatitic xenoliths from the Dalihu basalt, Inner Mongolia, China

Abstract Sic and associated ultra-reduced minerals were reported in various geological settings, however, their genesis and preservation mechanism are poorly understood. Here, we reported a Sic-dominated ultra-reduced mineral assemblage, including Sic, Tic, native metals (Si, Fe, and Ni) and iron silicide, from carbonatitic xenoliths in Dalihu, Inner Mongolia. All minerals were identified in situ in polished/thin sections. Sic is 20–50 μm in size, blue to colorless in color, and usually identified in the micro-cavities within the carbonatitic xenolith. Four types of Sic polytypes were identified, which are dominated by β-SiC (3C polytype) and 4H polytype followed by 15R and 6H. These Sic are featured by 13C-depleted isotopic compositions (δ13C = –13.2 to –22.8‰, average = –17.7‰) with obvious spatial variation. We provided a numerical modeling method to prove that the C isotopic composition of the Dalihu SiC can be well-yielded by degassing. Our modeling results showed that degassing reaction between graphite and silicate can readily produce the low δ13C value of SiC, and the spatial variations in C isotopic composition could have been formed in the progressive growth process of SiC. The detailed in situ occurring information is beneficial for our understanding of the preservation mechanism of the Dalihu ultra-reduced phase. The predominant occurrence of SiC in micro-cavities implies that exsolution and filling of CO2 and/or CO in the micro-cavities during the diapir rising process of carbonatitic melt 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.