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Serpentinites of the mantle wedge were identified from a composite oceanic–continental subduction zone that occurs as the Hong'an orogen in east–central China. They were comprehensively investigated by an integrated approach of whole-rock major and trace elements, mineral major and trace elements, whole-rock and olivine O isotopes, and phase equilibrium modeling as well as zircon U–Pb ages, trace elements, and Hf-O isotopes. These serpentinites show high Mg# and MgO/SiO2 ratios but low Al2O3/SiO2 ratios as well as low Ti contents, resembling those from the mantle wedge rather than the subducting oceanic slab. Petrographic observations were combined with CIPW norm calculations to suggest that the protoliths of these serpentinites are refractory peridotites in the mantle wedge. Whole-rock and mineral compositions indicate that these serpentinites experienced multiple episodes of metamorphism and metasomatism during the tectonic development from oceanic subduction in the Carboniferous to continental collision in the Triassic. The enrichments of Th, U, light rare earth elements and high field strength elements in the whole rock and the finding of zircon inside them indicate their formation through metasomatism of the mantle wedge peridotites by subducting crust-derived fluids. The ca. 310 Ma zircon domains exhibit positive ε Hf(t) values of 4.5 to 19.1 and variable δ18O values similar to those of oceanic-type eclogites in the Hong'an orogen but different trace element compositions, indicating their formation through metasomatism by fluids from the subducting Paleotethyan oceanic crust. The ca. 430 Ma zircons exhibit U–Pb ages, trace elements and Hf-O isotopes similar to those of the protolith magmatic zircons in the oceanic-type eclogites. The older zircon domains also exhibit similar element and isotope compositions to inherited zircons from the high- to ultrahigh-pressure metamorphic rocks in the Hong'an orogen. Thus, these relict zircons would be physically transferred into the serpentinites by metasomatic fluids originating from the subducting Paleotethyan oceanic crust. High Th/U ratios for the ca. 310 Ma zircons imply their formation through the metasomatic reaction rather than direct precipitation from the subducting oceanic crust-derived fluids. The metasomatic zircons of Triassic age show negative ε Hf(t) values of −16.0 to −2.7, indicating their formation through metasomatic overprinting by deeply subducting continental crust-derived fluids in the continental subduction channel during the Triassic continental collision. In this context, the mantle wedge peridotites were first metasomatized in the Carboniferous by the subducting oceanic crust-derived fluids. Afterwards, part of the metasomatized peridotites were off-scrapped into the oceanic subduction channel, where they were hydrated by further subducting oceanic crust-derived fluids to form antigorite serpentinite at forearc depths. The antigorite serpentinite was carried to deeper depths during subsequent continental subduction and underwent dehydration there. This gave rise to metamorphic olivine with extremely high Fo values of 96.7 to 97.6 and higher MnO but lower NiO contents than common mantle olivine. The coexistence of lizardite, brucite and magnesite in the serpentinites indicates their retrograde processes of serpentinization and carbonation on the subsurface. Therefore, the present observations and interpretations of orogenic serpentinites provide insights into the multistage fluid metasomatism at different depths during the tectonic transition from oceanic subduction to continental collision. [ABSTRACT FROM AUTHOR]

The Chang 6 tight sandstone reservoir of the Upper Triassic, developed in a large lacustrine shallow-water delta depositional environment, is the most important target for oil exploration and development in the basin. This study is undertaken with the purpose of investigating the influences of lithofacies, diagenetic fluid activity and diagenetic process on quality of the Chang 6 tight sandstone reservoir situated in the southeastern Ordos Basin, based on core observation and description, petrographic investigations, measurement of electron probe microanalysis (EMPA), homogenization temperature and laser Raman spectroscopy of fluid inclusions, laser microzone sampling carbon and oxygen isotope of carbonate cement, the X-ray diffraction (XRD), back scattered (BSE) image observation. Research result shows that three lithofacies are recognized. Each lithofacies has its initial petrological components that slightly differ from other ones, and this is in turn restricted the diagenetic evolution route and imposed diagenetic process and oil-charging intensity, which led to a different reservoir quality. Three carbonate cementation of microcrystalline calcite, sparry calcite I, sparry calcite II and ankerite cements enriched near the mudstone-sandstone interface, occurred in the Chang 6 reservoir which are unfavorable for the reservoir quality. The Chang 6 reservoir experienced three phases of diagenetic fluid activities. The fluid activity of the meteoric water occurred in the eodiagenetic stage offered a carbon source for the cementation of microcrystalline calcite (δ18O PDB : 13.3‰∼-10.0‰; δ13C PDB : 0.4‰∼2.1‰) and sparry calcite I (δ18O PDB : 19.8‰∼-13.4‰; δ13C PDB : 0.4‰∼0.8‰). The latter two fluids released by the thermal decarboxylation of organic matter during the two oil-charging stages, occurred in the mesodiagenetic stage, providing a carbon source for the formation of sparry calcite II (δ18O PDB : 21.8‰∼-18.7‰; δ13C PDB : 5.1‰∼-0.4‰) and ankerite (δ18O PDB : 21.5‰∼-16.8‰; δ13C PDB : 4.6‰∼-3.9‰). The dominant impact on reservoir quality is mechanical compaction, which destroys more intergranular pore than cementation, especially in the sandstone of lithofacies III (the porosity loss by compaction and cementation is 30.9% and 7.3%, respectively), compared to that of the lithofacies I sandstone (the porosity loss by compaction and cementation is 28.7% and 6.6%%, respectively) and the lithofacies II sandstone (the porosity loss by compaction and cementation is 28.8% and 8.5%, respectively). Laumontite cement formed during the eodiagenetic stage effectively resisted the intensity of mechanical compaction. The authigenic clay minerals in the three lithofacies are dominated by chlorite and illite, and most abundant in the lithofacies III. Primary intergranular pore can be effectively protected when the chlorite cement is less than 4%. The dissolution pores with average of 3.0%, 2.4% and 2.3% in the lithofacies I, lithofacies II and lithofacies III sandstone, respectively, formed during the mesodiagenetic stage improved the reservoir quality to a certain degree. Four models of diagenesis and pore evolution of the Chang 6 reservoir are summarized. The pattern I has the best reservoir quality due its fewer ductile component and more coarser grains, the weaker mechanical compaction and weaker cementation of laumontite and carbonate, as well as a stronger dissolution attributed to the intensified oil-charging during mesodiagenesis. • The coupled relationship of lithofacies, diagenesis, and fluid activity determines the quality of the reservoir. • The initial petrological composition controlled by lithofacies has a profound effect on the diagenetic evolution process. • The acidic diagenetic environment formed in two stages of oil-charging resulted in the dissolution of feldspar and laumontite. • Four models of diagenesis and pore evolution are summarized. [ABSTRACT FROM AUTHOR]

The study of continental deep-subduction has been one of the forefront and core subjects to advance the plate tectonics theory in the twenty-first century. The Dabie-Sulu orogenic belt in China crops out the largest lithotectonic unit containing ultrahigh-pressure metamorphic rocks in the world. Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in the Dabie-Sulu rocks. By taking these rocks as the natural laboratory, earth scientists have made seminal contributions to understanding of ultrahigh-pressure metamorphism and continental collision. This paper outlines twelve aspects of outstanding progress, including spatial distribution of the UHP metamorphic rocks, timing of the UHP metamorphism, time- scale of the UHP metamorphism, the protolith nature of deeply subducted continental crust, subduction erosion and crustal detachment during continental collision, the possible depths of continental subduction, fluid activity in the continental deep-subduction zone, partial melting during continental collision, element mobility in continental deep-subduction zone, recycling of subducted continental crust, geodynamic mechanism of postcollisional magmatism, and lithospheric architecture of collision orogen. Some intriguing questions and directions are also proposed for future studies. [ABSTRACT FROM AUTHOR]