Abstract Zircon U‐Pb dating of three orthogneiss samples from the North Dabie terrane (NDT) is undertaken in order to reconstruct their formation and evolutionary histories, and also the crustal architecture of the Dabie orogen after Triassic subduction and exhumation. SHRIMP zircon U‐Pb dating, in combination with back scattered electron (BSE) imaging and Laser Raman spectrometry, provides accurate identification of the core, mantle and rim structure for zircon growth during protolith formation and overgrowth during subduction/exhumation and post‐collisional metamorphism. Concordant U‐Pb ages of 760–730 Ma and high Th/U ratios of >0.4 are obtained for relict oscillatory zoning fields of inherited cores that were not metamictized. These features suggest that these ages represent the time of magmatic protolith formation during the breakup of Rodinia. The overgrown mantle domains around the metamictized cores are clean with few mineral inclusions (e.g. quartz, garnet and apatite). Mantle domains have low Th/U ratios of <0.1 and yielded U‐Pb ages of 215–205 Ma, which are slightly younger than the known ages of peak ultrahigh‐pressure (UHP) metamorphism, suggesting that overgrowth took place during initial exhumation. The ages are similar to the time of retrograde metamorphism of the UHP orthogneisses in the Central Dabie terrane (CDT). Overgrown rims are also clean, with a few mineral inclusions of apatite and quartz. They yield two groups of U‐Pb ages, 138–137 Ma and 124–120 Ma. The former is considered to be the time of onset of orogenic extension and tectonic collapse, whereas the latter falls into the age range of widespread magmatism in the Dabie orogen, and is regarded as the time of extension climax that resulted in intensive anatexis of the crust. Whole‐rock Sr‐Nd isotope analyses of four orthogneisses show ε Nd (t) values of −1.2 to −15 and I Sr values >0.719, similar to the values obtained from UHP orthogneisses in the CDT. It is concluded that, as with the CDT, the orthogneisses with episodic zircon growths from the NDT should also be a part of the exhumed slice following the continental deep subduction. However, the orthogneisses in this study were buried at a lower level in the orogenic crust compared with those of the CDT prior to the Cretaceous magmatism. Therefore, the orthogneisses from the NDT were affected by the Cretaceous magmatism whereas the CDT orthogneisses were not affected.
The unearthing of a large number of red-yellow colored stones in Yuhui Site, Bengbu city, Anhui province of China, which was the only site of Longshan culture appointed by "Origin of the Chinese Civilization Project" in the Huaihe River basin, has drawn a widespread attention in archaeology. The present research was undertaken to examine elements, mineral composition and thermal history of one red-yellow color stone in different positions, classified according to specialized colors, by means of synchrotron radiation X-ray fluorescence (SR-XRF), X-ray diffraction (XRD), laser Raman spectroscopy (LRS), electron probe micro-analysis (EPMA) and thermoluminescence (TL). The results show that the main body of the color stone is limonite with a large amount of quartz inclusion. The yellow substances on the surface layer of the color stone were identified as goethite (alpha-FeOOH), and the red ones as red ochre (alpha-Fe2O3+clay+silica). The internal yellow brown substances inside the stone are mainly aphanitic goethite with a number of chromites and manganese dioxide, and also with small quantity of barite and bismuth. And the color stones are without historical firing treatment. These kinds of special polymetallic ores with unique nature and complex geologic history, which were unearthed in the late neolithic site with large numbers, may be closely related to some kind of primitive religious beliefs and special sacrificial practices.
The metamorphic rocks undergoing different grade metamorphism extensively spread in the Dabie orogen.The protolith's formation time and the time of metamorphism are very important for the studying of the evolution of the Dabie orogen.This paper summarizes the published zircon U- Pb ages for metamorphic rocks from the Dabie orogen and gives a preliminary geochronologic framework of the metamorphic rocks.
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